View source for Physics:Solar cell

{{Short description|Photodiode used to produce power from light on a large scale}}

[[File:Solar cell.png|thumb|A conventional crystalline silicon solar cell (as of 2005). Electrical contacts made from busbars (the larger silver-colored strips) and fingers (the smaller ones) are printed on the silicon wafer.]]
[[File:Photovoltaic cell.svg|thumb|upright|Symbol of a Photovoltaic cell.]]

A '''solar cell''' or '''photovoltaic cell''' ('''PV cell''') is an electronic device that converts the energy of [[Physics:Light|light]] directly into [[Physics:Electricity|electricity]] by means of the [[Chemistry:Photovoltaic effect|photovoltaic effect]].<ref name="chemistryexplained.com">[http://www.chemistryexplained.com/Ru-Sp/Solar-Cells.html Solar Cells]. chemistryexplained.com</ref> It is a form of photoelectric cell, a device whose electrical characteristics (such as [[Physics:Electric current|current]], [[Physics:Voltage|voltage]], or [[Physics:Electrical resistance and conductance|resistance]]) vary when it is exposed to light. Individual solar cell devices are often the electrical building blocks of [[Physics:Solar panel|photovoltaic modules]], known colloquially as "solar panels". The common single-junction [[Chemistry:Silicon|silicon]] solar cell can produce a maximum [[Physics:Open-circuit voltage|open-circuit voltage]] of approximately 0.5 to 0.6 [[Volt|volt]]s.<ref>{{cite web|url=http://solarbotics.net/starting/200202_solar_cells/200202_solar_cell_use.html|title=Solar cells – performance and →use|website=solarbotic s.net}}</ref>

Photovoltaic cells may operate under [[Astronomy:Sunlight|sunlight]] or artificial light. In addition to producing energy, they can be used as a [[Physics:Photodetector|photodetector]] (for example [[Physics:Infrared detector|infrared detector]]s), detecting light or other [[Physics:Electromagnetic radiation|electromagnetic radiation]] near the visible range, or measuring light intensity.

The operation of a PV cell requires three basic attributes:
* The absorption of light, generating [[Physics:Exciton|exciton]]s ([[Physics:Bound state|bound]] [[Physics:Electron|electron]]-[[Physics:Electron hole|hole]] pairs), unbound electron-hole pairs (via excitons), or plasmons.
* The separation of [[Physics:Charge carrier|charge carrier]]s of opposite types.
* The separate extraction of those carriers to an external circuit.

In contrast, a [[Engineering:Solar thermal collector|solar thermal collector]] supplies [[Physics:Heat|heat]] by [[Physics:Absorption (electromagnetic radiation)|absorbing sunlight]], for the purpose of either direct heating or indirect electrical power generation from heat. A "photoelectrolytic cell" ([[Chemistry:Photoelectrochemical cell|photoelectrochemical cell]]), on the other hand, refers either to a type of photovoltaic cell (like that developed by [[Biography:Edmond Becquerel|Edmond Becquerel]] and modern [[Physics:Dye-sensitized solar cell|dye-sensitized solar cell]]s), or to a device that [[Chemistry:Water splitting|splits water]] directly into [[Chemistry:Hydrogen|hydrogen]] and [[Chemistry:Oxygen|oxygen]] using only solar illumination.

Photovoltaic cells and solar collectors are the two means of producing [[Engineering:Solar power|solar power]].


== Applications ==
Assemblies of solar cells are used to make [[Physics:Solar panel|solar modules]] that generate electrical power from [[Astronomy:Sunlight|sunlight]], as distinguished from a "solar thermal module" or "solar hot water panel". A solar array generates [[Engineering:Solar power|solar power]] using [[Physics:Solar energy|solar energy]].

=== Vehicular applications ===
[[File:GM_Sunraycer.JPG|thumb|The Sunraycer vehicle developed by GM (General Motors)]]

Application of solar cells as an alternative energy source for vehicular applications is a growing industry. Electric vehicles that operate off of [[Physics:Solar energy|solar energy]] and/or sunlight are commonly referred to as solar cars.{{citation needed|date=April 2023}} These vehicles use [[Physics:Solar panel|solar panel]]s to convert absorbed light into electrical energy that is then stored in [[Physics:Electric battery|batteries]].{{citation needed|date=April 2023}} There are multiple input factors that affect the output power of solar cells such as [[Physics:Temperature|temperature]], material properties, weather conditions, [[Astronomy:Solar irradiance|solar irradiance]] and more.<ref>{{Cite journal |last1=Al-Ezzi |first1=Athil S. |last2=Ansari |first2=Mohamed Nainar M. |date=2022-07-08 |title=Photovoltaic Solar Cells: A Review |journal=Applied System Innovation |language=en |volume=5 |issue=4 |pages=67 |doi=10.3390/asi5040067 |issn=2571-5577 |doi-access=free }}</ref>

The first instance of photovoltaic cells within vehicular applications was around midway through the second half of the 1900's. In an effort to increase publicity and awareness in solar powered transportation Hans Tholstrup decided to set up the first edition of the [[Physics:World Solar Challenge|World Solar Challenge]] in 1987.{{citation needed|date=November 2023}} It was a 3000&nbsp;km race across the Australian outback where competitors from industry research groups and top universities around the globe were invited to compete.{{citation needed|date=November 2023}} [[Company:General Motors|General Motors]] ended up winning the event by a significant margin with their [[Engineering:Sunraycer|Sunraycer]] vehicle that achieved speeds of over 40&nbsp;mph.{{citation needed|date=November 2023}} Contrary to popular belief however solar powered cars are one of the oldest alternative energy vehicles.<ref>{{Cite conference |location=Capri, Italy |last=Connors |first=John |date=21-23 May 2007 |title=On the Subject of Solar Vehicles and the Benefits of the Technology |book-title=2007 International Conference on Clean Electrical Power |pages=700–705 |doi=10.1109/ICCEP.2007.384287}}</ref>

Current solar vehicles harness energy from the [[Astronomy:Sun|Sun]] via [[Physics:Solar panel|Solar panel]]s which are a collected group of solar cells working in tandem towards a common goal.<ref name="Arulious-2021">{{Cite journal |last1=Arulious |first1=Jora A |last2=Earlina |first2=D |last3=Harish |first3=D |last4=Sakthi Priya |first4=P |last5=Inba Rexy |first5=A |last6=Nancy Mary |first6=J S |date=2021-11-01 |title=Design of solar powered electric vehicle |journal=Journal of Physics: Conference Series |volume=2070 |issue=1 |pages=012105 |issn=1742-6588 |doi=10.1088/1742-6596/2070/1/012105 |bibcode=2021JPhCS2070a2105A |doi-access=free}}</ref> These solid-state devices use [[Physics:Quantum|quantum]] mechanical transitions in order to convert a given amount of solar power into electrical power.<ref name="Arulious-2021" /> The [[Physics:Electricity|electricity]] produced as a result is then stored in the vehicle's [[Physics:Electric battery|battery]] in order to run the motor of the vehicle.<ref name="Arulious-2021" /> Batteries in solar-powered vehicles differ from those in standard [[Earth:Internal combustion engine|ICE]] cars because they are fashioned in a way to impart more power towards the electrical components of the vehicle for a longer duration.<ref>{{Cite journal |last1=Nivas |first1=Mandakuriti |last2=Naidu |first2=Rambilli Krishna Prasad Rao |last3=Mishra |first3=Debani Prasad |last4=Salkuti |first4=Surender Reddy |date=March 2022 |title=Modeling and analysis of solar-powered electric vehicles |journal=International Journal of Power Electronics and Drive Systems  |volume=13 |issue=1 |pages=480–487 |issn=2722-256X |doi=10.11591/ijpeds.v13.i1.pp480-487 |doi-access=free|url=https://zenodo.org/record/6615186 }}</ref>

=== Cells, modules, panels and systems ===
{{Main|Physics:Photovoltaic system}}
[[File:From a solar cell to a PV system.svg|thumb|upright=1.35|From a solar cell to a PV system. Diagram of the possible components of a [[Physics:Photovoltaic system|photovoltaic system]]]]

Multiple solar cells in an integrated group, all oriented in one plane, constitute a [[Physics:Solar panel|solar photovoltaic panel or module]]. Photovoltaic modules often have a sheet of glass on the sun-facing side, allowing light to pass while protecting the semiconductor [[Engineering:Wafer (electronics)|wafers]]. Solar cells are usually connected in [[Engineering:Series and parallel circuits|series]] creating additive voltage. Connecting cells in parallel yields a higher current.

However, problems in paralleled cells such as shadow effects can shut down the weaker (less illuminated) parallel string (a number of series connected cells) causing substantial power loss and possible damage because of the reverse bias applied to the shadowed cells by their illuminated partners.{{citation needed|date=July 2020}}

Although modules can be interconnected to create an array with the desired peak DC voltage and loading current capacity, which can be done with or without using independent MPPTs ([[Physics:Maximum power point tracking|maximum power point trackers]]) or, specific to each module, with or without module level power electronic (MLPE) units such as [[Physics:Solar micro-inverter|microinverters]] or [[Physics:Power optimizer#Isolating panels|DC-DC optimizers]]. Shunt [[Engineering:Diode|diode]]s can reduce shadowing power loss in arrays with series/parallel connected cells.

{| class="wikitable" style="text-align: center; margin: 4px auto 1px auto;"
|+ Typical PV system prices in 2013 in selected countries (US$/W)
|-
!
! style="width:80px;"| [[Engineering:Solar power in Australia|Australia]]
! style="width:80px;"| China
! style="width:80px;"| France
! style="width:80px;"| Germany
! style="width:80px;"| Italy
! style="width:80px;"| Japan
! style="width:80px;"| United Kingdom
! style="width:80px;"| United States
|-
| style="text-align:left;"|Residential
|1.8
|1.5
|4.1
|2.4
|2.8
|4.2
|2.8
|4.9
|-
| style="text-align:left;"|Commercial
|1.7
|1.4
|2.7
|1.8
|1.9
|3.6
|2.4
|4.5
|-
| style="text-align:left;"|Utility-scale
|2.0
|1.4
|2.2
|1.4
|1.5
|2.9
|1.9
|3.3
|-
| colspan="9" style="font-size:0.85em; padding:5px 6px; text-align:left; background:#f2f2f2;"| Source: ''[[Organization:International Energy Agency|IEA]] – Technology Roadmap: Solar Photovoltaic Energy report'', 2014 edition<ref name=IEA-roadmap-PV-2014>
{{cite web
 |title       = Technology Roadmap: Solar Photovoltaic Energy
 |url         = http://www.iea.org/publications/freepublications/publication/TechnologyRoadmapSolarPhotovoltaicEnergy_2014edition.pdf
 |publisher   = IEA
 |access-date  = 7 October 2014
 |archive-url  = https://web.archive.org/web/20141001012612/http://www.iea.org/publications/freepublications/publication/TechnologyRoadmapSolarPhotovoltaicEnergy_2014edition.pdf
 |archive-date = 1 October 2014
 |year        = 2014
 |url-status     = live
}}</ref>{{rp|15}}<br />Note: ''DOE – Photovoltaic System Pricing Trends'' reports lower prices for the U.S.<ref name="sunshot-2014">{{cite web |url=http://www.nrel.gov/docs/fy14osti/62558.pdf |title=Photovoltaic System Pricing Trends – Historical, Recent, and Near-Term Projections, 2014 Edition |publisher=NREL |page=4 |date=22 September 2014 |archive-url=https://web.archive.org/web/20150226074251/http://www.nrel.gov/docs/fy14osti/62558.pdf |archive-date=26 February 2015 |url-status=live  }}</ref>
|}
By 2020, the United States cost per watt for a utility scale system had declined to $0.94.<ref name=nrel-june2021>
{{cite web
|title=Documenting a Decade of Cost Declines for PV Systems
|url = https://www.nrel.gov/news/program/2021/documenting-a-decade-of-cost-declines-for-pv-systems.html
|publisher = National Renewable Energy Laboratory (NREL)
|access-date = 3 June 2021
}}</ref>

== History ==
{{For timeline}}
The [[Chemistry:Photovoltaic effect|photovoltaic effect]] was experimentally demonstrated first by French physicist [[Biography:Edmond Becquerel|Edmond Becquerel]]. In 1839, at age 19, he built the world's first photovoltaic cell in his father's laboratory. [[Biography:Willoughby Smith|Willoughby Smith]] first described the "Effect of Light on Selenium during the passage of an Electric Current" in a 20 February 1873 issue of Nature. In 1883 Charles Fritts built the first solid state photovoltaic cell by coating the [[Physics:Semiconductor|semiconductor]] [[Chemistry:Selenium|selenium]] with a thin layer of [[Chemistry:Gold|gold]] to form the junctions; the device was only around 1% efficient.<ref>{{cite journal |last1=Marques Lameirinhas |first1=Ricardo A. |last2=N. Torres |first2=João Paulo |last3=de Melo Cunha |first3=João P. |title=A photovoltaic technology review: history, fundamentals and applications |journal=Energies |date=2022 |volume=15 |issue=5 |page=1823 |doi=10.3390/en15051823|doi-access=free }}</ref> Other milestones include:
* 1888 – Russian physicist Aleksandr Stoletov built the first cell based on the outer [[Physics:Photoelectric effect|photoelectric effect]] discovered by [[Biography:Heinrich Hertz|Heinrich Hertz]] in 1887.<ref>{{cite book|last=Gevorkian |first=Peter |title=Sustainable energy systems engineering: the complete green building design resource|year=2007 |publisher=McGraw Hill Professional |isbn=978-0-07-147359-0 |url={{google books|id=i8rcHsOfPScC|p=498|plainurl=y}} }}</ref>
* 1904 – [[Biography:Julius Elster|Julius Elster]], together with [[Biography:Hans Friedrich Geitel|Hans Friedrich Geitel]], devised the first practical photoelectric cell.<ref>{{cite web |title=Julius (Johann Phillipp Ludwig) Elster: 1854 - 1920 |website=Adventures in Cybersound |url=http://www.acmi.net.au/AIC/ELSTER_BIO.html |access-date=2016-10-15 |url-status=dead |archiveurl=https://web.archive.org/web/20110308055542/http://www.acmi.net.au/AIC/ELSTER_BIO.html |archivedate=2011-03-08}}</ref>
* 1905 – [[Biography:Albert Einstein|Albert Einstein]] proposed a new quantum theory of light and explained the [[Physics:Photoelectric effect|photoelectric effect]] in a landmark paper, for which he received the Nobel Prize in Physics in 1921.<ref>[http://nobelprize.org/nobel_prizes/physics/laureates/1921/ "The Nobel Prize in Physics 1921: Albert Einstein"], Nobel Prize official page</ref>
* 1941 – [[Biography:Vadim Lashkaryov|Vadim Lashkaryov]] discovered ''p''-''n''-junctions in [[Chemistry:Copper(I) oxide|Cu<sub>2</sub>O]] and [[Chemistry:Silver sulfide|Ag<sub>2</sub>S]] protocells.<ref>{{cite journal |author=Lashkaryov, V. E. |date=2008 |title=Investigation of a barrier layer by the thermoprobe method |journal=Ukr. J. Phys. |volume=53 |issue=Special Issue |pages=53–56 |issn=2071-0194 |url= http://ujp.bitp.kiev.ua/files/journals/53/si/53SI11p.pdf |url-status=dead |archive-date=28 September 2015 |archive-url=https://web.archive.org/web/20150928014344/http://ujp.bitp.kiev.ua/files/journals/53/si/53SI11p.pdf}} Translated and reprinted from ''Izv. Akad. Nauk SSSR'', Ser. Fiz. '''5''', No. 4–5, pp. 442–446 (1941)</ref>
* 1946 – Russell Ohl patented the modern junction semiconductor solar cell,<ref>"Light sensitive device" {{US patent|2402662}} Issue date: June 1946</ref> while working on the series of advances that would lead to the [[Engineering:Transistor|transistor]].
* 1948 - ''Introduction to the World of Semiconductors'' states [[Biography:Kurt Lehovec|Kurt Lehovec]] may have been the first to explain the photo-voltaic effect in the peer reviewed journal ''Physical Review''.<ref>{{Cite journal |title=The Photo-Voltaic Effect |first=K. |last=Lehovec |date=15 August 1948 |journal=Physical Review |volume=74 |issue=4 |pages=463–471 |doi=10.1103/PhysRev.74.463 |bibcode=1948PhRv...74..463L}}</ref><ref>{{cite book |author=Lau, W.S. |date=October 2017 |title=ULSI Front-End Technology: Covering from the First Semiconductor Paper to CMOS FINFET Technology |chapter-url=https://www.worldscientific.com/doi/pdf/10.1142/9789813222168_0001 |chapter=Introduction to the World of Semiconductors |page=7 |doi=10.1142/10495|isbn=978-981-322-215-1 }}</ref>
* 1954 – The first practical photovoltaic cell was publicly demonstrated at Bell Laboratories.<ref>{{cite journal |title=April 25, 1954: Bell Labs Demonstrates the First Practical Silicon Solar Cell |journal=APS News |volume=18 |issue=4 |date=April 2009 |publisher=American Physical Society |url=http://www.aps.org/publications/apsnews/200904/physicshistory.cfm }}</ref> The inventors were Calvin Souther Fuller, [[Biography:Daryl Chapin|Daryl Chapin]] and [[Biography:Gerald Pearson|Gerald Pearson]].<ref>{{cite book|first=K. A.|last=Tsokos|title=Physics for the IB Diploma Full Colour|url={{google books |plainurl=y |id=uWVQrIZqz_MC}}|date=28 January 2010|publisher=Cambridge University Press|isbn=978-0-521-13821-5}}</ref>
* 1958 – Solar cells gained prominence with their incorporation onto the Vanguard I satellite.

=== Space applications ===
[[File:Explorer 6 paddles up.jpg|thumb|upright=1.2|[[Organization:NASA|NASA]] used solar cells on its spacecraft from the very beginning. For Example, Explorer 6, launched in 1959, had four arrays that folded out once in orbit. They provided power for months in space.]]
Solar cells were first used in a prominent application when they were proposed and flown on the Vanguard satellite in 1958, as an alternative power source to the [[Physics:Primary cell|primary battery]] power source. By adding cells to the outside of the body, the mission time could be extended with no major changes to the spacecraft or its power systems. In 1959 the United States launched [[Astronomy:Explorer 6|Explorer 6]], featuring large wing-shaped solar arrays, which became a common feature in satellites. These arrays consisted of 9600 Hoffman solar cells.

By the 1960s, solar cells were (and still are) the main power source for most Earth orbiting satellites and a number of probes into the solar system, since they offered the best [[Engineering:Power-to-weight ratio|power-to-weight ratio]]. However, this success was possible because in the space application, power system costs could be high, because space users had few other power options, and were willing to pay for the best possible cells. The space power market drove the development of higher efficiencies in solar cells up until the [[Organization:National Science Foundation|National Science Foundation]] "Research Applied to National Needs" program began to push development of solar cells for terrestrial applications.

In the early 1990s the technology used for space solar cells diverged from the silicon technology used for terrestrial panels, with the spacecraft application shifting to [[Chemistry:Gallium arsenide|gallium arsenide]]-based III-V semiconductor materials, which then evolved into the modern III-V multijunction photovoltaic cell used on spacecraft.

In recent years, research has moved towards designing and manufacturing lightweight, flexible, and highly efficient solar cells. Terrestrial solar cell technology generally uses photovoltaic cells that are laminated with a layer of glass for strength and protection. Space applications for solar cells require that the cells and arrays are both highly efficient and extremely lightweight. Some newer technology implemented on satellites are multi-junction photovoltaic cells, which are composed of different PN junctions with varying bandgaps in order to utilize a wider spectrum of the sun's energy. Additionally, large satellites require the use of large solar arrays to produce electricity. These solar arrays need to be broken down to fit in the geometric constraints of the launch vehicle the satellite travels on before being injected into orbit. Historically, solar cells on satellites consisted of several small terrestrial panels folded together. These small panels would be unfolded into a large panel after the satellite is deployed in its orbit. Newer satellites aim to use flexible rollable solar arrays that are very lightweight and can be packed into a very small volume. The smaller size and weight of these flexible arrays drastically decreases the overall cost of launching a satellite due to the direct relationship between payload weight and launch cost of a launch vehicle.<ref>{{Cite web|url=http://www.nasa.gov/mission_pages/station/structure/elements/solar_arrays.html|title=International Space Station Solar Arrays|last=Garcia|first=Mark|date=31 July 2017|website=NASA|access-date=10 May 2019}}</ref>

In 2020, the US Naval Research Laboratory conducted its first test of solar power generation in a satellite, the Photovoltaic Radio-frequency Antenna Module (PRAM) experiment aboard the [[Astronomy:Boeing X-37|Boeing X-37]]<!--#OTV-6 (USSF 7)-->.<ref>{{cite web |last1=David |first1=Leonard |title=Air Force's X-37B robotic space plane wings past 500 days in Earth orbit |url=https://www.livescience.com/x-37b-space-plane-500-days-in-orbit-otv-6-mission |publisher=LiveScience |access-date=6 November 2021 |language=en |date=4 October 2021}}</ref><ref>{{cite news |last1=David |first1=Leonard |title=Space solar power's time may finally be coming |url=https://www.space.com/space-solar-power-research-advances |access-date=6 November 2021 |work=Space.com |date=3 November 2021 |language=en}}</ref>

=== Improved manufacturing methods ===
Improvements were gradual over the 1960s. This was also the reason that costs remained high, because space users were willing to pay for the best possible cells, leaving no reason to invest in lower-cost, less-efficient solutions. The price was determined largely by the [[Engineering:Semiconductor industry|semiconductor industry]]; their move to [[Engineering:Integrated circuit|integrated circuit]]s in the 1960s led to the availability of larger [[Physics:Boule (crystal)|boules]] at lower relative prices. As their price fell, the price of the resulting cells did as well. These effects lowered 1971 cell costs to some $100 per watt.{{sfn|Perlin|1999|p= 50}}

In late 1969 Elliot Berman joined Exxon's task force which was looking for projects 30 years in the future and in April 1973 he founded Solar Power Corporation (SPC), a wholly owned subsidiary of Exxon at that time.{{sfn|Perlin|1999|p=53}}<ref name="williams 2005">
{{cite book
 | first = Neville | last = Williams
 | title = Chasing the Sun: Solar Adventures Around the World
 | url = https://archive.org/details/chasingsunsolara0000will | url-access = registration | year = 2005
 | publisher = New Society Publishers
 | page = [https://archive.org/details/chasingsunsolara0000will/page/84 84]
 | isbn = 9781550923124}}
</ref><ref name=jones>
{{cite book
 | first1 = Geoffrey | last1 = Jones
 | first2 = Loubna | last2 = Bouamane 
 | title = "Power from Sunshine": A Business History of Solar Energy
 | url = http://www.hbs.edu/faculty/Publication%20Files/12-105.pdf
 | year = 2012
 | publisher = Harvard Business School
 | pages = 22–23
 }}
</ref> The group had concluded that electrical power would be much more expensive by 2000, and felt that this increase in price would make alternative energy sources more attractive. He conducted a market study and concluded that a price per watt of about $20/watt would create significant demand.{{sfn|Perlin|1999|p=53}} The team eliminated the steps of polishing the wafers and coating them with an anti-reflective layer, relying on the rough-sawn wafer surface. The team also replaced the expensive materials and hand wiring used in space applications with a [[Engineering:Printed circuit board|printed circuit board]] on the back, [[Physics:Acrylate polymer|acrylic]] plastic on the front, and [[Chemistry:Silicone|silicone]] glue between the two, "potting" the cells.{{sfn|Perlin|1999|p=54}} Solar cells could be made using cast-off material from the electronics market. By 1973 they announced a product, and SPC convinced Tideland Signal to use its panels to power navigational [[Earth:Buoy|buoy]]s, initially for the U.S. Coast Guard.<ref name="williams 2005" />

=== Research and industrial production ===
Research into solar power for terrestrial applications became prominent with the U.S. National Science Foundation's Advanced Solar Energy Research and Development Division within the "Research Applied to National Needs" program, which ran from 1969 to 1977,<ref>[https://www.nsf.gov/about/history/nsf50/nsf8816.jsp The National Science Foundation: A Brief History], Chapter IV, NSF 88-16, 15 July 1994 (retrieved 20 June 2015)</ref> and funded research on developing solar power for ground electrical power systems. A 1973 conference, the "Cherry Hill Conference", set forth the technology goals required to achieve this goal and outlined an ambitious project for achieving them, kicking off an applied research program that would be ongoing for several decades.<ref>{{cite conference |author=Herwig, Lloyd O. |book-title=National center for photovoltaics (NCPV) 15th program review meeting |volume=462 |page=785 |doi=10.1063/1.58015 |bibcode=1999AIPC..462..785H |title=Cherry Hill revisited: Background events and photovoltaic technology status|year=1999}}</ref> The program was eventually taken over by the Energy Research and Development Administration (ERDA),<ref>{{cite conference |author1=Deyo, J. N. |author2=Brandhorst, H. W. Jr. |author3=Forestieri, A. F. |title=Status of the ERDA/NASA photovoltaic tests and applications project |url=https://ntrs.nasa.gov/search.jsp?R=19770006594 |conference=12th IEEE Photovoltaic Specialists Conf. |date=15–18 Nov 1976}}</ref> which was later merged into the U.S. Department of Energy.

Following the [[Finance:1973 oil crisis|1973 oil crisis]], oil companies used their higher profits to start (or buy) solar firms, and were for decades the largest producers. Exxon, ARCO, Shell, Amoco (later purchased by BP) and Mobil all had major solar divisions during the 1970s and 1980s. Technology companies also participated, including General Electric, Motorola, IBM, Tyco and RCA.<ref name="Information1979">{{cite magazine|title=The multinational connections-who does what where|magazine=New Scientist|volume=84|issue=1177|url={{google books |plainurl=y |id=x3w5ZfGUS_0C}}|date=18 October 1979 |publisher=Reed Business Information|issn=0262-4079}}</ref>

== Declining costs and exponential growth ==
{{multiple image
|direction = vertical | width = 250

| image1=Price history of silicon PV cells since 1977.svg |caption1=Price per watt history for conventional ([[Chemistry:Crystalline silicon|c-Si]]) solar cells since 1977

| image2=1975 – Price of solar panels as a function of cumulative installed capacity.svg |caption2=[[Swanson's law]]–stating that solar module prices have dropped about 20% for each doubling of installed capacity—defines the "learning curve" of [[Chemistry:Photovoltaics|solar photovoltaics]].<ref name=OWID_PanelCostAndCapacity_2021>{{cite web |title=Solar (photovoltaic) panel prices vs. cumulative capacity |url=https://ourworldindata.org/grapher/solar-pv-prices-vs-cumulative-capacity |website=OurWorldInData.org |archive-url=https://archive.today/20230929225159/https://ourworldindata.org/grapher/solar-pv-prices-vs-cumulative-capacity |archive-date=29 September 2023 |date=2023 |url-status=live }} OWID credits source data to: Nemet (2009); Farmer & Lafond (2016); International Renewable Energy Agency (IRENA).</ref>

| image3=PV cume semi log chart 2014 estimate.svg |caption3=[[Physics:Growth of photovoltaics|Growth of photovoltaics]] – Worldwide total installed PV capacity

| image4=CO2 mitigation and price.jpg |caption4=Energy volume of silicon solar cells and oil harnessed by human beings per dollar; Carbon intensity of some key electricity generation technologies.<ref>{{Cite journal |last1=Yu |first1=Peng |last2=Wu |first2=Jiang |last3=Liu |first3=Shenting |last4=Xiong |first4=Jie |last5=Jagadish |first5=Chennupati |last6=Wang |first6=Zhiming M. |date=1 December 2016 |title=Design and fabrication of silicon nanowires towards efficient solar cells |journal=Nano Today |volume=11 |issue=6 |pages=704–737 |doi=10.1016/j.nantod.2016.10.001 |url=http://discovery.ucl.ac.uk/1536157/1/NANOTODAY_2016_Accepted%20version.pdf}}</ref>
}}
Adjusting for inflation, it cost $96 per watt for a solar module in the mid-1970s. Process improvements and a very large boost in production have brought that figure down more than 99%, to 30¢ per watt in 2018 
<ref>
{{cite web
|title=U.S. Solar Photovoltaic System Cost Benchmark: Q1 2018
|url = https://www.nrel.gov/docs/fy19osti/72399.pdf
|type=pdf
|publisher = National Renewable Energy Laboratory (NREL)
|access-date = 3 June 2021
|page=26
}}
</ref>
and as low as 20¢ per watt in 2020.
<ref>
{{cite web
|title=U.S. Solar Photovoltaic System and Energy Storage Cost Benchmark: Q1 2020
|url = https://www.nrel.gov/docs/fy21osti/77324.pdf
|type=pdf
|publisher = National Renewable Energy Laboratory (NREL)
|access-date = 3 June 2021
|page=28
}}
</ref>
[[Swanson's law]] is an observation similar to Moore's Law that states that solar cell prices fall 20% for every doubling of industry capacity. It was featured in an article in the British weekly newspaper [[Finance:The Economist|The Economist]] in late 2012.<ref name="Econ1">{{cite news|url=https://www.economist.com/news/21566414-alternative-energy-will-no-longer-be-alternative-sunny-uplands| title =Sunny Uplands: Alternative energy will no longer be alternative| newspaper=The Economist| access-date = 28 December 2012| date=21 November 2012}}</ref> Balance of system costs were then higher than those of the panels. Large commercial arrays could be built, as of 2018, at below $1.00 a watt, fully commissioned.<ref name=nrel-june2021/>

As the semiconductor industry moved to ever-larger [[Physics:Boule (crystal)|boules]], older equipment became inexpensive. Cell sizes grew as equipment became available on the surplus market; ARCO Solar's original panels used cells {{convert|2|to|4|in|mm|-1}} in diameter. Panels in the 1990s and early 2000s generally used 125&nbsp;mm wafers; since 2008, almost all new panels use 156&nbsp;mm cells. The widespread introduction of flat screen televisions in the late 1990s and early 2000s led to the wide availability of large, high-quality glass sheets to cover the panels.

During the 1990s, polysilicon ("poly") cells became increasingly popular. These cells offer less efficiency than their monosilicon ("mono") counterparts, but they are grown in large vats that reduce cost. By the mid-2000s, poly was dominant in the low-cost panel market, but more recently the mono returned to widespread use.

Manufacturers of wafer-based cells responded to high silicon prices in 2004–2008 with rapid reductions in silicon consumption. In 2008, according to Jef Poortmans, director of [[Company:IMEC|IMEC]]'s organic and solar department, current cells use {{convert|8|-|9|g|oz}} of silicon per watt of power generation, with wafer thicknesses in the neighborhood of 200 microns. [[Chemistry:Crystalline silicon|Crystalline silicon]] panels dominate worldwide markets and are mostly manufactured in China and Taiwan. By late 2011, a drop in European demand dropped prices for crystalline solar modules to about $1.09<ref name="247wallst.com">[http://247wallst.com/2011/10/06/solar-stocks-does-the-punishment-fit-the-crime-fslr-spwra-stp-jaso-tsl-ldk-tan/ Solar Stocks: Does the Punishment Fit the Crime?]. 24/7 Wall St. (6 October 2011). Retrieved 3 January 2012.</ref> per watt down sharply from 2010. Prices continued to fall in 2012, reaching $0.62/watt by 4Q2012.<ref>{{cite web |last=Parkinson|first=Giles |title=Plunging Cost of Solar PV (Graphs) |publisher=Clean Technica |date=7 March 2013 |url=http://cleantechnica.com/2013/03/07/plunging-cost-of-solar-pv-graphs/ |access-date=18 May 2013}}</ref>

Solar PV is growing fastest in Asia, with China and Japan currently accounting for half of [[Physics:Growth of photovoltaics|worldwide deployment]].<ref name="iea-pvps-snapshot-1992-2014">
{{cite web
 |url         = http://www.iea-pvps.org/fileadmin/dam/public/report/technical/PVPS_report_-_A_Snapshot_of_Global_PV_-_1992-2014.pdf
 |title       = Snapshot of Global PV 1992–2014
 |publisher   = International Energy Agency – Photovoltaic Power Systems Programme
 |date        = 30 March 2015
 |archive-url  = https://web.archive.org/web/20150407023056/http://www.iea-pvps.org/index.php?id=92&eID=dam_frontend_push&docID=2430
 |archive-date = 7 April 2015
 |url-status     = live
}}
</ref> Global installed PV capacity reached at least 301 gigawatts in 2016, and grew to supply 1.3% of global power by 2016.<ref>{{cite web|title=Solar energy – Renewable energy – Statistical Review of World Energy – Energy economics – BP |website=bp.com |url=http://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy/renewable-energy/solar-energy.html |access-date=2 September 2017|url-status=dead|archive-date=23 March 2018|archive-url= https://web.archive.org/web/20180323025347/https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy/renewable-energy/solar-energy.html}}</ref>

It was anticipated that electricity from PV will be competitive with wholesale electricity costs all across Europe and the energy payback time of crystalline silicon modules can be reduced to below 0.5 years by 2020.<ref>{{Cite journal|last1=Mann|first1=Sander A. |last2=de Wild-Scholten|first2=Mariska J.|last3=Fthenakis|first3=Vasilis M.|last4=van Sark|first4=Wilfried G.J.H.M. |last5=Sinke |first5=Wim C.|date=1 November 2014|title=The energy payback time of advanced crystalline silicon PV modules in 2020: a prospective study|journal=Progress in Photovoltaics: Research and Applications |volume=22 |issue=11 |pages=1180–1194 |doi=10.1002/pip.2363 |issn=1099-159X |hdl=1874/306424 |s2cid=97151576 |hdl-access=free}}</ref>

Falling costs are considered one of the biggest factors in the rapid growth of renewable energy, with the cost of solar photovoltaic electricity falling by ~85% between 2010 (when solar and wind made up 1.7% of global electricity generation) and 2021 (where they made up 8.7%).<ref>{{cite web |last1=Jaeger |first1=Joel |title=Explaining the Exponential Growth of Renewable Energy |url=https://www.wri.org/insights/growth-renewable-energy-sector-explained |access-date=8 November 2021 |language=en |date=20 September 2021}}</ref> In 2019 solar cells accounted for ~3 % of the world's electricity generation.<ref name="techrev">{{cite web |title=Solar panels are a pain to recycle. These companies are trying to fix that. |website=MIT Technology Review |language=en |url=https://www.technologyreview.com/2021/08/19/1032215/solar-panels-recycling/ |access-date=8 November 2021}}</ref>

=== Subsidies and grid parity ===
Solar-specific feed-in tariffs vary by country and within countries. Such tariffs encourage the development of solar power projects. Widespread grid parity, the point at which photovoltaic electricity is equal to or cheaper than [[Physics:Mains electricity|grid power]] without subsidies, likely requires advances on all three fronts. Proponents of solar hope to achieve grid parity first in areas with abundant sun and high electricity costs such as in {{wipe|California}} and {{wipe|Japan}}.<ref>{{cite web |url=http://www.bp.com/genericarticle.do?categoryId=9013609&contentId=7005395 |title=BP Global – Reports and publications – Going for grid parity |access-date=4 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20110608081013/http://www.bp.com/genericarticle.do?categoryId=9013609&contentId=7005395 |archive-date=8 June 2011  }}. Bp.com. Retrieved 19 January 2011.</ref> In 2007 BP claimed grid parity for Hawaii and other islands that otherwise use [[Chemistry:Diesel fuel|diesel fuel]] to produce electricity. George W. Bush set 2015 as the date for grid parity in the US.<ref>[https://web.archive.org/web/20131127115307/http://www.bp.com/liveassets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/frontiers/STAGING/local_assets/pdf/bpf19_14-18_solar.pdf BP Global – Reports and publications – Gaining on the grid]. Bp.com. August 2007.</ref><ref>[https://web.archive.org/web/20131029191313/http://www.bp.com/popupimage.do?img_path=liveassets%2Fbp_internet%2Fglobalbp%2Fglobalbp_uk_english%2Freports_and_publications%2Ffrontiers%2FSTAGING%2Flocal_assets%2Fimages%2Ffr19solar_parity570x417.jpg%20&alt_tag=Graphic%20about%20grid%20parity%2C%20when%20the%20cost%20of%20solar%20energy%20equals%20that%20of%20grid%20electricity The Path to Grid Parity]. bp.com</ref> The Photovoltaic Association reported in 2012 that Australia had reached grid parity (ignoring feed in tariffs).<ref>Peacock, Matt (20 June 2012) [http://www.abc.net.au/news/2011-09-07/solar-industry-celebrates-grid-parity/2875592 Solar industry celebrates grid parity], ABC News.</ref>

The price of solar panels fell steadily for 40 years, interrupted in 2004 when high subsidies in Germany drastically increased demand there and greatly increased the price of purified silicon (which is used in computer chips as well as solar panels). The recession of 2008 and the onset of Chinese manufacturing caused prices to resume their decline. In the four years after January 2008 prices for solar modules in Germany dropped from €3 to €1 per peak watt. During that same time production capacity surged with an annual growth of more than 50%. China increased market share from 8% in 2008 to over 55% in the last quarter of 2010.<ref>Baldwin, Sam (20 April 2011) [https://web.archive.org/web/20130512121859/http://energy.colostate.edu/docs/BaldwinTalkColoradoStateUniversity2011-04-20final.pdf Energy Efficiency & Renewable Energy: Challenges and Opportunities]. Clean Energy SuperCluster Expo Colorado State University. U.S. Department of Energy.</ref> In December 2012 the price of Chinese solar panels had dropped to $0.60/Wp (crystalline modules).<ref>{{cite web |url=http://www.enfsolar.com/news/Small-Chinese-Solar-Manufacturers-Decimated-in-2012 |title=Small Chinese Solar Manufacturers Decimated in 2012 |website=ENF Solar Trade Platform and Directory of Solar Companies |date=8 January 2013 |publisher=ENF Ltd. |access-date=1 June 2013}}</ref> (The abbreviation Wp stands for watt peak capacity, or the maximum capacity under optimal conditions.<ref>{{cite web|title=What is a solar panel and how does it work? |website=Energuide.be |publisher=Sibelga |url=http://www.energuide.be/en/questions-answers/what-is-a-solar-panel-and-how-does-it-work/136/ |access-date=2017-01-03}}</ref>)

As of the end of 2016, it was reported that [[Finance:Spot contract|spot prices]] for assembled solar ''panels'' (not cells) had fallen to a record-low of US$0.36/Wp. The second largest supplier, Canadian Solar Inc., had reported costs of US$0.37/Wp in the third quarter of 2016, having dropped $0.02 from the previous quarter, and hence was probably still at least breaking even. Many producers expected costs would drop to the vicinity of $0.30 by the end of 2017.<ref>{{cite news|last1=Martin|first1=Chris |date=30 December 2016|title=Solar Panels Now So Cheap Manufacturers Probably Selling at Loss|website=Bloomberg View |url=https://www.bloomberg.com/news/articles/2016-12-30/solar-panels-now-so-cheap-manufacturers-probably-selling-at-loss| publisher=Bloomberg LP|access-date=3 January 2017}}</ref> It was also reported that new solar installations were cheaper than coal-based thermal power plants in some regions of the world, and this was expected to be the case in most of the world within a decade.<ref>{{cite news| last1=Shankleman|first1=Jessica|last2=Martin|first2=Chris| title=Solar Could Beat Coal to Become the Cheapest Power on Earth| url=https://www.bloomberg.com/news/articles/2017-01-03/for-cheapest-power-on-earth-look-skyward-as-coal-falls-to-solar| access-date=3 January 2017| work=Bloomberg View| publisher=Bloomberg LP| date=3 January 2017}}</ref>

== Theory ==
[[File:Solartce3.gif|thumb|Schematic of charge collection by solar cells. Light transmits through transparent conducting electrode creating electron hole pairs, which are collected by both the electrodes.<ref name=kumar2017 />]]
[[File:Silicon Solar cell structure and mechanism.svg|thumb|Working [[Physics:Theory of solar cells|mechanism]] of a solar cell]]
{{Main|Physics:Theory of solar cells}}
A solar cell is made of [[Physics:List of semiconductor materials|semiconducting materials]], such as [[Chemistry:Silicon|silicon]], that have been fabricated into a [[Physics:P–n junction|p–n junction]]. Such junctions are made by [[Physics:Doping (semiconductor)|doping]] one side of the device p-type and the other n-type, for example in the case of silicon by introducing small concentrations of [[Chemistry:Boron|boron]] or [[Chemistry:Phosphorus|phosphorus]] respectively.

In operation, [[Physics:Photon|photon]]s in [[Astronomy:Sunlight|sunlight]] hit the solar cell and are absorbed by the semiconductor.  When the photons are absorbed, electrons are [[Physics:Excited state|excited]] from the [[Physics:Valence and conduction bands|valence band to the conduction band]] (or from [[Chemistry:HOMO and LUMO|occupied to unoccupied molecular orbitals]] in the case of an [[Physics:Organic solar cell|organic solar cell]]), producing [[Physics:Semiconductor#Charge carriers (electrons and holes)|electron-hole pairs]]. 
If the electron-hole pairs are created near the junction between p-type and n-type materials the local electric field sweeps them apart to opposite electrodes, producing an excess of electrons on one side and an excess of holes on the other. When the solar cell is unconnected (or the external [[Engineering:Electrical load|electrical load]] is very high) the electrons and holes will ultimately restore equilibrium by [[Physics:Diffusion current|diffusing]] back across the junction against the field and [[Physics:Relaxation|recombine]] with each other giving off heat, but if the load is small enough then it is easier for equilibrium to be restored by the excess electrons going around the external circuit, doing useful work along the way.

An array of solar cells converts solar energy into a usable amount of [[Physics:Direct current|direct current]] (DC) electricity. An inverter can convert the power to [[Physics:Alternating current|alternating current]] (AC).

The most commonly known solar cell is configured as a large-area [[Physics:P–n junction|p–n junction]] made from silicon. Other possible solar cell types are organic solar cells, dye sensitized solar cells, perovskite solar cells, quantum dot solar cells etc. The illuminated side of a solar cell generally has a [[Physics:Transparent conducting film|transparent conducting film]] for allowing light to enter into the active material and to collect the generated charge carriers. Typically, films with high transmittance and high electrical conductance such as [[Chemistry:Indium tin oxide|indium tin oxide]], conducting polymers or conducting nanowire networks are used for the purpose.<ref name=kumar2017>{{Cite journal|last=Kumar|first=Ankush|date=3 January 2017|title=Predicting efficiency of solar cells based on transparent conducting electrodes|journal=Journal of Applied Physics|volume=121|issue=1|pages=014502|doi=10.1063/1.4973117|issn=0021-8979|bibcode=2017JAP...121a4502K}}</ref>

== Efficiency ==
[[File:ShockleyQueisserFullCurve.svg|thumb|The Shockley-Queisser limit for the theoretical maximum efficiency of a solar cell. Semiconductors with band gap between 1 and 1.5eV (827 nm to 1240 nm; near-infrared) have the greatest potential to form an efficient single-junction cell. (The efficiency "limit" shown here can be exceeded by multijunction solar cells.)]]
{{Main|Physics:Solar cell efficiency}}
Solar cell efficiency may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of these individual metrics.

The [[Physics:Energy conversion efficiency|power conversion efficiency]] of a solar cell is a parameter which is defined by the fraction of incident power converted into electricity.<ref>{{Cite web|url=http://www.pveducation.org/pvcdrom/solar-cell-efficiency|title=Solar Cell Efficiency {{!}} PVEducation|website=www.pveducation.org|access-date=31 January 2018|archive-date=31 January 2018|archive-url=https://web.archive.org/web/20180131050352/http://pveducation.org/pvcdrom/solar-cell-efficiency|url-status=dead}}</ref>

A solar cell has a voltage dependent efficiency curve, temperature coefficients, and allowable shadow angles.

Due to the difficulty in measuring these parameters directly, other parameters are substituted: thermodynamic efficiency, quantum efficiency, [[Engineering:Quantum efficiency|integrated quantum efficiency]], [[Physics:Open-circuit voltage|V<sub>OC</sub>]] ratio, and fill factor. Reflectance losses are a portion of quantum efficiency under "[[Engineering:Quantum efficiency#Types of quantum efficiency|external quantum efficiency]]". Recombination losses make up another portion of quantum efficiency, V<sub>OC</sub> ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also make up minor portions of quantum efficiency, V<sub>OC</sub> ratio.

The ''fill factor'' is the ratio of the actual maximum obtainable [[Physics:Power|power]] to the product of the [[Physics:Open-circuit voltage|open-circuit voltage]] and short-circuit current. This is a key parameter in evaluating performance. In 2009, typical commercial solar cells had a fill factor > 0.70. Grade B cells were usually between 0.4 and 0.7.<ref>{{cite web| title=T.Bazouni: What is the Fill Factor of a Solar Panel| url=http://www.solarfreaks.com/what-is-the-fill-factor-of-a-solar-panel-t138.html| access-date=17 February 2009| url-status=dead| archive-url=https://web.archive.org/web/20090415162123/http://www.solarfreaks.com/what-is-the-fill-factor-of-a-solar-panel-t138.html| archive-date=15 April 2009}}</ref> Cells with a high fill factor have a low [[Physics:Equivalent series resistance|equivalent series resistance]] and a high [[Engineering:Shunt (electrical)|equivalent shunt resistance]], so less of the current produced by the cell is dissipated in internal losses.

Single p–n junction crystalline silicon devices are now approaching the theoretical limiting power efficiency of 33.16%,<ref>{{Cite journal|title = Tabulated Values of the Shockley-Queisser Limit for Single Junction Solar Cells |journal = Solar Energy|date = 8 February 2016|pages = 139–147|volume = 130|doi = 10.1016/j.solener.2016.02.015 |first = Sven|last = Rühle|bibcode = 2016SoEn..130..139R}}</ref> noted as the [[Physics:Shockley–Queisser limit|Shockley–Queisser limit]] in 1961. In the extreme, with an infinite number of layers, the corresponding limit is 86% using concentrated sunlight.<ref name=Vos80>{{Cite journal | doi = 10.1088/0022-3727/13/5/018| title = Detailed balance limit of the efficiency of tandem solar cells| journal = Journal of Physics D: Applied Physics| volume = 13| issue = 5| page = 839| year = 1980| last1 = Vos | first1 = A. D. | bibcode = 1980JPhD...13..839D| s2cid = 250782402}}</ref>

[[File:NREL PV Cell Record Efficiency Chart.png|thumb|upright=1.35|Reported timeline of research solar cell energy conversion efficiencies (National Renewable Energy Laboratory)]]
In 2014, three companies broke the record of 25.6% for a silicon solar cell. Panasonic's was the most efficient. The company moved the front contacts to the rear of the panel, eliminating shaded areas. In addition they applied thin silicon films to the (high quality silicon) wafer's front and back to eliminate defects at or near the wafer surface.<ref>Bullis, Kevin (13 June 2014) [http://www.technologyreview.com/news/528351/record-breaking-solar-cell-points-the-way-to-cheaper-power Record-Breaking Solar Cell Points the Way to Cheaper Power]. ''MIT Technology Review''</ref>

In 2015, a 4-junction GaInP/GaAs//GaInAsP/GaInAs solar cell achieved a new laboratory record efficiency of 46.1% (concentration ratio of sunlight = 312) in a French-German collaboration between the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE), CEA-LETI and SOITEC.<ref>{{Cite journal |last1=Dimroth |first1=Frank |last2=Tibbits |first2=Thomas N.D. |last3=Niemeyer |first3=Markus |last4=Predan |first4=Felix |last5=Beutel |first5=Paul |last6=Karcher |first6=Christian |last7=Oliva |first7=Eduard |last8=Siefer |first8=Gerald |last9=Lackner |display-authors=0 |first9=David |year=2016 |title=Four-Junction Wafer Bonded Concentrator Solar Cells |journal=IEEE Journal of Photovoltaics |volume=6 |issue=1 |pages=343–349 |s2cid=47576267 |url=http://publica.fraunhofer.de/bitstreams/aabc6d11-d9f6-4f1e-8bc6-569ce93d630d/download |doi=10.1109/jphotov.2015.2501729|url-access= }}</ref>

In September 2015, Fraunhofer ISE announced the achievement of an efficiency above 20% for [[Engineering:Epitaxial wafer|epitaxial wafer]] cells. The work on optimizing the atmospheric-pressure [[Chemistry:Chemical vapor deposition|chemical vapor deposition]] (APCVD) in-line production chain was done in collaboration with NexWafe GmbH, a company spun off from Fraunhofer ISE to commercialize production.<ref name=NexWafe>{{cite web| title=20% Efficient Solar Cell on EpiWafer| url=https://www.ise.fraunhofer.de/en/press-and-media/press-releases/press-releases-2015/nexwafe| first1=Stefan| last1=Janz| first2=Stefan| last2=Reber| publisher=Fraunhofer ISE |date=14 September 2015| access-date=15 October 2015}}</ref><ref name="DrießenAmiri2016">{{cite journal |last1=Drießen |first1=Marion |last2=Amiri |first2=Diana |last3=Milenkovic |first3=Nena |last4=Steinhauser |first4=Bernd |last5=Lindekugel |first5=Stefan |last6=Benick |first6=Jan |last7=Reber |first7=Stefan |last8=Janz |first8=Stefan |title=Solar Cells with 20% Efficiency and Lifetime Evaluation of Epitaxial Wafers |journal=Energy Procedia |volume=92 |year=2016 |pages=785–790 |issn=1876-6102 |doi=10.1016/j.egypro.2016.07.069| doi-access=free|url=http://publica.fraunhofer.de/bitstreams/85e223be-f1eb-46a7-9f89-e33164a7ab89/download }}</ref>

For triple-junction thin-film solar cells, the world record is 13.6%, set in June 2015.<ref>{{cite web|title = Solar cell sets world record with a stabilized efficiency of 13.6%|url = http://phys.org/news/2015-06-solar-cell-world-stabilized-efficiency.html |date=4 June 2015 |author=Zyg, Lisa |work=Phys.org}}</ref>

In 2016, researchers at Fraunhofer ISE announced a GaInP/GaAs/Si triple-junction solar cell with two terminals reaching 30.2% efficiency without concentration.<ref>[https://nationalsolarclean.com/ 30.2% Efficiency – New Record for Silicon-based Multi-junction Solar Cell]. (9 November 2016). Retrieved 15 November 2016.</ref>

In 2017, a team of researchers at [[Organization:National Renewable Energy Laboratory|National Renewable Energy Laboratory]] (NREL), [[Organization:École Polytechnique Fédérale de Lausanne|EPFL]] and CSEM ({{wipe|Switzerland}}) reported record one-sun efficiencies of 32.8% for dual-junction GaInP/GaAs solar cell devices. In addition, the dual-junction device was mechanically stacked with a Si solar cell, to achieve a record one-sun efficiency of 35.9% for triple-junction solar cells.<ref>{{Cite journal |last1=Essig |first1=Stephanie |last2=Allebé |first2=Christophe |last3=Remo |first3=Timothy |last4=Geisz |first4=John F. |last5=Steiner |first5=Myles A. |last6=Horowitz |first6=Kelsey |last7=Barraud |first7=Loris |last8=Ward |first8=J. Scott |last9=Schnabel |first9=Manuel |date=September 2017 |title=Raising the one-sun conversion efficiency of III–V/Si solar cells to 32.8% for two junctions and 35.9% for three junctions |journal=Nature Energy |volume=2 |issue=9 |pages=17144 |doi=10.1038/nenergy.2017.144 |issn=2058-7546 |bibcode=2017NatEn...217144E |s2cid=115327057}}</ref>
{{Further|Physics:Timeline of solar cells}}

== Materials ==
[[File:Global photovoltaics market share by technology 1980-2021.svg|thumb|Global photovoltaics market share by technology 1980-2021.<ref name="Fraunhofer ISE 2022">
 {{cite web
  |title=Photovoltaics Report
  |url=https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Photovoltaics-Report.pdf
  |publisher=Fraunhofer ISE
  |date=22 September 2022
  |archive-url=https://web.archive.org/web/20220923220758/https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Photovoltaics-Report.pdf
  |archive-date=23 September 2022
  |url-status=live
 }}</ref>{{rp|24,25}}]]

Solar cells are typically named after the semiconducting material they are made of. These [[Physics:List of semiconductor materials|materials]] must have certain characteristics in order to absorb [[Astronomy:Sunlight|sunlight]]. Some cells are designed to handle sunlight that reaches the Earth's surface, while others are optimized for [[Physics:Solar panels on spacecraft|use in space]]. Solar cells can be made of a single layer of light-absorbing material ([[Physics:P–n junction|single-junction]]) or use multiple physical configurations (multi-junctions) to take advantage of various absorption and charge separation mechanisms.

Solar cells can be classified into first, second and third generation cells. The first generation cells—also called conventional, traditional or [[Engineering:Wafer (electronics)|wafer]]-based cells—are made of [[Chemistry:Crystalline silicon|crystalline silicon]], the commercially predominant PV technology, that includes materials such as polysilicon and [[Earth:Monocrystalline silicon|monocrystalline silicon]]. Second generation cells are thin film solar cells, that include [[Chemistry:Amorphous silicon|amorphous silicon]], [[Physics:Cadmium telluride photovoltaics|CdTe]] and [[Physics:Copper indium gallium selenide solar cell|CIGS]] cells and are commercially significant in utility-scale [[Physics:Photovoltaic power station|photovoltaic power station]]s, building integrated photovoltaics or in small [[Engineering:Stand-alone power system|stand-alone power system]]. The [[Engineering:Third-generation photovoltaic cell|third generation of solar cells]] includes a number of thin-film technologies often described as emerging photovoltaics—most of them have not yet been commercially applied and are still in the research or development phase. Many use organic materials, often organometallic compounds as well as inorganic substances. Despite the fact that their efficiencies had been low and the stability of the absorber material was often too short for commercial applications, there is research into these technologies as they promise to achieve the goal of producing low-cost, high-efficiency solar cells.<ref>{{Cite journal |last1=Marques Lameirinhas |first1=Ricardo A. |last2=Torres |first2=João Paulo N. |last3=de Melo Cunha |first3=João P. |date=January 2022 |title=A Photovoltaic Technology Review: History, Fundamentals and Applications |journal=Energies |language=en |volume=15 |issue=5 |pages=1823 |doi=10.3390/en15051823 |issn=1996-1073|doi-access=free }}</ref> As of 2016, the most popular and efficient solar cells were those made from thin wafers of silicon which are also the oldest solar cell technology.<ref>{{Cite journal |last1=Ranabhat |first1=Kiran |last2=Patrikeev |first2=Leev |last3=Antal'evna-Revina |first3=Aleksandra |last4=Andrianov |first4=Kirill |last5=Lapshinsky |first5=Valerii |last6=Sofronova |first6=Elena |date=2016 |title=An introduction to solar cell technology |journal=Istrazivanja I Projektovanja Za Privredu |language=en |volume=14 |issue=4 |pages=481–491 |doi=10.5937/jaes14-10879 |s2cid=114726770 |issn=1451-4117|doi-access=free }}</ref>

=== Crystalline silicon ===
{{Main|Chemistry:Crystalline silicon}}
By far, the most prevalent bulk material for solar cells is [[Chemistry:Crystalline silicon|crystalline silicon]] (c-Si), also known as "solar grade silicon".<ref>{{cite journal |last1=Zanatta |first1=A.R. | title= The Shockley-Queisser limit and the conversion efficiency of silicon-based solar cells |journal=Results Opt. |date=December 2022 |volume=9 |pages=100320–7pp |doi=10.1016/j.rio.2022.100320 |s2cid=253359097 }}</ref> Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, [[Engineering:Ribbon|ribbon]] or [[Engineering:Wafer (electronics)|wafer]]. These cells are entirely based around the concept of a [[Physics:P–n junction|p–n junction]]. Solar cells made of c-Si are made from [[Engineering:Wafer (electronics)|wafers]] between 160 and 240&nbsp;micrometers thick.

==== Monocrystalline silicon ====
{{Main|Earth:Monocrystalline silicon}}
[[File:Sono Sion Front Back.jpg|thumb|The roof, bonnet and large parts of the outer shell of the [[Engineering:Sion (electric car)|Sion]] are equipped with highly efficient monocrystalline silicon cells]]

[[Earth:Monocrystalline silicon|Monocrystalline silicon]] (mono-Si) solar cells feature a single-crystal composition that enables electrons to move more freely than in a multi-crystal configuration. Consequently, monocrystalline solar panels deliver a higher efficiency than their multicrystalline counterparts.<ref>{{cite web |url=https://www.targray.com/solar/pv-modules/monocrystalline |title=Monocrystalline Solar Modules |access-date=2020-08-27}}</ref> The corners of the cells look clipped, like an octagon, because the wafer material is cut from cylindrical ingots, that are typically grown by the [[Physics:Czochralski process|Czochralski process]]. Solar panels using mono-Si cells display a distinctive pattern of small white diamonds.

==== Epitaxial silicon development ====
[[Engineering:Epitaxial wafer|Epitaxial wafer]]s of crystalline silicon can be grown on a monocrystalline silicon "seed" wafer by [[Chemistry:Chemical vapor deposition|chemical vapor deposition]] (CVD), and then detached as self-supporting wafers of some standard thickness (e.g., 250&nbsp;µm) that can be manipulated by hand, and directly substituted for wafer cells cut from monocrystalline silicon ingots. Solar cells made with this "[[Engineering:Saw#Terminology|kerf]]less" technique can have efficiencies approaching those of wafer-cut cells, but at appreciably lower cost if the CVD can be done at [[Physics:Atmospheric pressure|atmospheric pressure]] in a high-throughput inline process.<ref name=NexWafe/><ref name="DrießenAmiri2016"/> The surface of epitaxial wafers may be textured to enhance light absorption.<ref name=pyramid>{{cite journal| doi=10.1021/acs.nanolett.6b01240 | pmid=27525513 | title=Ultrathin Epitaxial Silicon Solar Cells with Inverted Nanopyramid Arrays for Efficient Light Trapping | first1= Alexandre| last1= Gaucher| first2= Andrea| last2= Cattoni| first3= Christophe| last3= Dupuis| first4= Wanghua| last4= Chen| first5= Romain| last5= Cariou|last6 = Foldyna| first6=Martin | last7=Lalouat | first7=Loı̈c | last8=Drouard | first8=Emmanuel | last9=Seassal | first9=Christian | last10=Roca i Cabarrocas | first10=Pere | last11=Collin | first11=Stéphane | journal= Nano Letters| year= 2016| volume= 16| issue=9 | pages= 5358–64| bibcode=2016NanoL..16.5358G | s2cid=206734456 | url=https://hal-polytechnique.archives-ouvertes.fr/hal-01401081/file/Gaucher2016.pdf }}</ref><ref name="ChenCariou2016">{{cite journal | last1=Chen | first1=Wanghua | last2=Cariou | first2=Romain | last3=Foldyna | first3=Martin | last4=Depauw | first4=Valerie | last5=Trompoukis | first5=Christos | last6=Drouard | first6=Emmanuel | last7=Lalouat | first7=Loic | last8=Harouri | first8=Abdelmounaim | last9=Liu | first9=Jia | last10=Fave | first10=Alain | last11=Orobtchouk | first11=Régis | last12=Mandorlo | first12=Fabien | last13=Seassal | first13=Christian | last14=Massiot | first14=Inès | last15=Dmitriev | first15=Alexandre | last16=Lee | first16=Ki-Dong | last17=Cabarrocas | first17=Pere Roca i | title=Nanophotonics-based low-temperature PECVD epitaxial crystalline silicon solar cells | journal=Journal of Physics D: Applied Physics | volume=49 | issue=12 | year=2016 | pages=125603 | issn=0022-3727 | doi=10.1088/0022-3727/49/12/125603| bibcode=2016JPhD...49l5603C | s2cid=125317340 }}</ref>

In June 2015, it was reported that [[Physics:Heterojunction|heterojunction]] solar cells grown epitaxially on n-type monocrystalline silicon wafers had reached an efficiency of 22.5% over a total cell area of 243.4&nbsp;cm<math>^2</math>.<ref name="KobayashiWatabe2015">{{cite journal|last1=Kobayashi|first1=Eiji|last2=Watabe|first2=Yoshimi|last3=Hao|first3=Ruiying|last4=Ravi|first4=T. S.|title=High efficiency heterojunction solar cells on n-type kerfless mono crystalline silicon wafers by epitaxial growth|journal=Applied Physics Letters|volume=106|issue=22|year=2015|pages=223504|issn=0003-6951|doi=10.1063/1.4922196|bibcode=2015ApPhL.106v3504K}}</ref>

==== Polycrystalline silicon ====
{{Main|Earth:Polycrystalline silicon}}
[[Earth:Polycrystalline silicon|Polycrystalline silicon]], or multicrystalline silicon (multi-Si) cells are made from cast square ingots—large blocks of molten silicon carefully cooled and solidified. They consist of small crystals giving the material its typical [[Earth:Crystallite|metal flake effect]]. Polysilicon cells are the most common type used in photovoltaics and are less expensive, but also less efficient, than those made from monocrystalline silicon.

==== Ribbon silicon ====
Ribbon silicon is a type of polycrystalline silicon—it is formed by drawing flat thin films from molten silicon and results in a polycrystalline structure. These cells are cheaper to make than multi-Si, due to a great reduction in silicon waste, as this approach does not require [[Engineering:Saw|saw]]ing from ingots.<ref>{{cite book|url=http://www.ece.gatech.edu/research/UCEP/papers/3world/STRING%20RIBBON%20SILICON%20SOLAR%20CELLS%20WITH%2017.8%25%20EFFICIENCY.pdf|title=String ribbon silicon solar cells with 17.8% efficiency|author=Kim, D.S.|journal=Proceedings of 3rd World Conference on Photovoltaic Energy Conversion, 2003|date=18 May 2003|pages= 1293–1296|volume=2|isbn=978-4-9901816-0-4
|display-authors=0}}</ref> However, they are also less efficient.

==== Mono-like-multi silicon (MLM) ====
This form was developed in the 2000s and introduced commercially around 2009. Also called cast-mono, this design uses polycrystalline casting chambers with small "seeds" of mono material. The result is a bulk mono-like material that is polycrystalline around the outsides. When sliced for processing, the inner sections are high-efficiency mono-like cells (but square instead of "clipped"), while the outer edges are sold as conventional poly. This production method results in mono-like cells at poly-like prices.<ref>Wayne McMillan, [http://btimaging.com/Downloads/bt_PV_eng-Advertorial3_Final.pdf "The Cast Mono Dilemma"] {{webarchive|url=https://web.archive.org/web/20131105134839/http://www.btimaging.com/downloads/bt_pv_eng-advertorial3_final.pdf |date=5 November 2013 }}, BT Imaging</ref>

=== Thin film ===
{{Main|Engineering:Thin-film solar cell}}
Thin-film technologies reduce the amount of active material in a cell. Most designs sandwich active material between two panes of glass. Since silicon solar panels only use one pane of glass, thin film panels are approximately twice as heavy as crystalline silicon panels, although they have a smaller ecological impact (determined from life cycle analysis).<ref>{{Cite book| doi = 10.1115/SED2002-1051| chapter = Net Energy Analysis for Sustainable Energy Production from Silicon Based Solar Cells| title = Solar Energy| page = 181| year = 2002| last1 = Pearce| first1 = J.| last2 = Lau| first2 = A.| isbn = 978-0-7918-1689-9| chapter-url = http://www.cede.psu.edu/users/alau/ases02_net_energy_pv.pdf}}{{dead link|date=May 2020|bot=medic}}{{cbignore|bot=medic}}</ref> <!-- The majority of film panels have 2–3 percentage points lower conversion efficiencies than crystalline silicon.<ref>Datasheets of the market leaders: ''First Solar'' for thin film, ''Suntech'' and ''SunPower'' for crystalline silicon</ref> [[Chemistry:Cadmium telluride|Cadmium telluride]] (CdTe), [[Chemistry:Copper indium gallium selenide|copper indium gallium selenide]] (CIGS) and [[Chemistry:Amorphous silicon|amorphous silicon]] (a-Si) are three thin-film technologies often used for outdoor applications. As of December 2013, CdTe cost per installed watt was $0.59 as reported by First Solar. CIGS technology laboratory demonstrations reached 20.4% conversion efficiency as of December 2013. The lab efficiency of GaAs thin film technology topped 28%.{{citation needed|date=June 2014}} The [[Engineering:Quantum efficiency|quantum efficiency]] of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon. Most recently, CZTS solar cell emerge as the less-toxic thin film solar cell technology, which achieved ~12% efficiency. --><ref>{{Cite journal|title = Thin Film Solar Cells: Research in an Industrial Perspective|journal = Ambio|date = March 2012|issn = 0044-7447|pmc = 3357764|pmid = 22434436|pages = 112–118|volume = 41|issue = 2|doi = 10.1007/s13280-012-0265-6|first = Marika|last = Edoff| bibcode=2012Ambio..41S.112E }}</ref>

==== Cadmium telluride ====
{{Main|Physics:Cadmium telluride photovoltaics}}
Cadmium telluride is the only thin film material so far to rival crystalline silicon in cost/watt. However cadmium is highly toxic and [[Chemistry:Tellurium|tellurium]] (anion: "telluride") supplies are limited. The [[Chemistry:Cadmium|cadmium]] present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during fires in residential roofs.<ref name="Fthenakis 303–334">{{Cite journal
| last = Fthenakis
| first = Vasilis M.
| title = Life cycle impact analysis of cadmium in CdTe PV production
| journal = Renewable and Sustainable Energy Reviews
| volume = 8
| issue = 4
| pages = 303–334
| year= 2004
| url = http://www.clca.columbia.edu/papers/Life_Cycle_Impact_Analysis_Cadmium_CdTe_Photovoltaic_production.pdf
| doi = 10.1016/j.rser.2003.12.001}}</ref> A square meter of CdTe contains approximately the same amount of Cd as a single C cell nickel-cadmium battery, in a more stable and less soluble form.<ref name="Fthenakis 303–334" />

==== Copper indium gallium selenide ====
{{Main|Physics:Copper indium gallium selenide solar cell}}
Copper indium gallium selenide (CIGS) is a [[Physics:Direct and indirect band gaps|direct band gap]] material. It has the highest efficiency (~20%) among all commercially significant thin film materials (see [[Physics:Copper indium gallium selenide solar cell|CIGS solar cell]]). Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. Recent developments at [[Company:IBM|IBM]] and Nanosolar attempt to lower the cost by using non-vacuum solution processes.<ref>[https://www-03.ibm.com/press/us/en/pressrelease/24464.wss "IBM and Tokyo Ohka Kogyo Turn Up Watts on Solar Energy Production"], IBM</ref>

==== Silicon thin film ====
Silicon thin-film cells are mainly deposited by [[Chemistry:Chemical vapor deposition|chemical vapor deposition]] (typically plasma-enhanced, PE-CVD) from [[Chemistry:Silane|silane]] gas and [[Chemistry:Hydrogen|hydrogen]] gas. Depending on the deposition parameters, this can yield [[Chemistry:Amorphous silicon|amorphous silicon]] (a-Si or a-Si:H), [[Physics:Protocrystalline|protocrystalline]] silicon or [[Physics:Nanocrystalline silicon|nanocrystalline silicon]] (nc-Si or nc-Si:H), also called microcrystalline silicon.<ref>{{Cite journal | doi = 10.1016/S0927-0248(02)00436-1| title = Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry| journal = Solar Energy Materials and Solar Cells| volume = 78| issue = 1–4| page = 143| year = 2003| last1 = Collins | first1 = R. W. | last2 = Ferlauto | first2 = A. S. | last3 = Ferreira | first3 = G. M. | last4 = Chen | first4 = C. | last5 = Koh | first5 = J. | last6 = Koval | first6 = R. J. | last7 = Lee | first7 = Y. | last8 = Pearce | first8 = J. M. | last9 = Wronski | first9 = C. R. 
}}</ref>

Amorphous silicon is the most well-developed thin film technology to-date. An amorphous silicon (a-Si) solar cell is made of non-crystalline or microcrystalline silicon. Amorphous silicon has a higher bandgap (1.7&nbsp;eV) than crystalline silicon (c-Si) (1.1&nbsp;eV), which means it absorbs the visible part of the solar spectrum more strongly than the higher power density [[Physics:Infrared|infrared]] portion of the spectrum. The production of a-Si thin film solar cells uses glass as a substrate and deposits a very thin layer of silicon by [[Chemistry:Plasma-enhanced chemical vapor deposition|plasma-enhanced chemical vapor deposition]] (PECVD).

Protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open-circuit voltage.<ref>{{Cite journal| doi=10.1063/1.2714507| title=Optimization of open circuit voltage in amorphous silicon solar cells with mixed-phase (amorphous+nanocrystalline) p-type contacts of low nanocrystalline content| journal=Journal of Applied Physics| volume=101| issue=11| pages=114301–114301–7| year=2007| last1=Pearce| first1=J. M.| last2=Podraza| first2=N.| last3=Collins| first3=R. W.| last4=Al-Jassim| first4=M. M.| last5=Jones| first5=K. M.| last6=Deng| first6=J.| last7=Wronski| first7=C. R.| url=http://me.queensu.ca/people/pearce/publications/documents/t14.pdf| bibcode=2007JAP...101k4301P| archive-url=http://archive.wikiwix.com/cache/20090613201701/http://me.queensu.ca/people/pearce/publications/documents/t14.pdf| archive-date=13 June 2009| url-status=dead}}</ref> Nc-Si has about the same bandgap as c-Si and nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nc-Si.

==== Gallium arsenide thin film ====
The semiconductor material [[Chemistry:Gallium arsenide|gallium arsenide]] (GaAs) is also used for single-crystalline thin film solar cells. Although GaAs cells are very expensive{{Citation needed|date=June 2023}}, they hold the world's record in efficiency for a single-junction solar cell at 28.8%.<ref>{{cite book |doi=10.1109/PVSC.2012.6317891 |chapter=The opto-electronic physics that broke the efficiency limit in solar cells |title=2012 38th IEEE Photovoltaic Specialists Conference |year=2012 |last1=Yablonovitch |first1=Eli |last2=Miller |first2=Owen D. |last3=Kurtz |first3=S. R. |isbn=978-1-4673-0066-7 |at=001556 |s2cid=30141399}}</ref> Typically fabricated on crystalline silicon wafer<ref>{{Cite web |last= |title=Thin-Film Trick Makes Gallium Arsenide Devices Cheap |url=https://spectrum.ieee.org/thinfilm-trick-makes-gallium-arsenide-devices-cheap |access-date=2023-06-26 |website=IEEE Spectrum |language=en}}</ref> with a 41% fill factor, by moving to porous silicon fill factor can be increased to 56% with potentially reduced cost.  Using less active GaAs material by fabricating nanowires is another potential pathway to cost reduction.<ref>{{Cite web |last=Gemini |first=Redaksjonen |date=2021-11-05 |title=New ultra-high material efficient, low-cost solar cells — using nanowires |url=https://norwegianscitechnews.com/2021/11/new-ultra-high-material-efficient-low-cost-solar-cells-using-nanowires/ |access-date=2023-06-26 |website=Norwegian SciTech News |language=en-US}}</ref> GaAs is more commonly used in multijunction photovoltaic cells for concentrated photovoltaics (CPV, HCPV) and for [[Physics:Solar panels on spacecraft|solar panels on spacecraft]], as the industry favours efficiency over cost for [[Physics:Space-based solar power|space-based solar power]]. Based on the previous literature and some theoretical analysis, there are several reasons why GaAs has such high power conversion efficiency. First, GaAs bandgap is 1.43ev which is almost ideal for solar cells. Second, because Gallium is a by-product of the smelting of other metals, GaAs cells are relatively insensitive to heat and it can keep high efficiency when temperature is quite high. Third, GaAs has the wide range of design options. Using GaAs as active layer in solar cell, engineers can have multiple choices of other layers which can better generate electrons and holes in GaAs.

=== Multijunction cells ===
{{Main|Physics:Multi-junction solar cell}}
[[File:Dawn solar panel.jpg|thumb|[[Engineering:Dawn (spacecraft)|Dawn]]'s 10&nbsp;[[Kilowatt|kW]] triple-junction gallium arsenide solar array at full extension]]

Multi-junction cells consist of multiple thin films, each essentially a solar cell grown on top of another, typically using metalorganic vapour phase epitaxy. Each layer has a different band gap energy to allow it to absorb [[Physics:Electromagnetic radiation|electromagnetic radiation]] over a different portion of the spectrum. Multi-junction cells were originally developed for special applications such as [[Astronomy:Satellite|satellite]]s and [[Astronomy:Space exploration|space exploration]], but are now used increasingly in terrestrial [[Physics:Concentrator photovoltaics|concentrator photovoltaics]] (CPV), an emerging technology that uses lenses and curved mirrors to concentrate sunlight onto small, highly efficient multi-junction solar cells. By concentrating sunlight up to a thousand times, ''High concentration photovoltaics (HCPV)'' has the potential to outcompete conventional solar PV in the future.<ref name=Fraunhofer-PR-2014>
{{cite web
 |title       = Photovoltaics Report
 |url         = http://www.ise.fraunhofer.de/en/downloads-englisch/pdf-files-englisch/photovoltaics-report-slides.pdf
 |publisher   = Fraunhofer ISE
 |access-date  = 31 August 2014
 |archive-url  = https://web.archive.org/web/20140809192020/http://www.ise.fraunhofer.de/en/downloads-englisch/pdf-files-englisch/photovoltaics-report-slides.pdf
 |archive-date = 9 August 2014
 |date        = 28 July 2014
 |url-status     = live
 |df          = dmy-all
}}</ref>{{rp|21,26}}

Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide (GaAs), and germanium (Ge) p–n junctions, are increasing sales, despite cost pressures.<ref>{{cite journal|last=Oku|first=Takeo|author2=Kumada, Kazuma |author3=Suzuki, Atsushi |author4= Kikuchi, Kenji |title=Effects of germanium addition to copper phthalocyanine/fullerene-based solar cells|journal=Central European Journal of Engineering|date=June 2012|volume=2|issue=2|pages=248–252|doi=10.2478/s13531-011-0069-7|bibcode=2012CEJE....2..248O|s2cid=136518369|doi-access=free}}</ref> Between December 2006 and December 2007, the cost of 4N gallium metal rose from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000–1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.{{citation needed|date=June 2014}}

A triple-junction cell, for example, may consist of the semiconductors: GaAs, [[Chemistry:Germanium|Ge]], and [[Physics:Indium gallium phosphide|{{chem|GaInP|2}}]].<ref>[http://www.spectrolab.com/DataSheets/TerCel/tercell.pdf Triple-Junction Terrestrial Concentrator Solar Cells]. (PDF) Retrieved 3 January 2012.</ref> Triple-junction GaAs solar cells were used as the power source of the Dutch four-time [[Physics:World Solar Challenge|World Solar Challenge]] winners Nuna in 2003, 2005 and 2007 and by the Dutch solar cars Solutra (2005), Twente One (2007) and 21Revolution (2009).{{citation needed|date=June 2014}} GaAs based multi-junction devices are the most efficient solar cells to date. On 15 October 2012, triple junction metamorphic cells reached a record high of 44%.<ref>Clarke, Chris (19 April 2011) [http://www.kcet.org/news/rewire/solar/photovoltaic-pv/san-jose-solar-company-breaks-efficiency-record-for-pv.html San Jose Solar Company Breaks Efficiency Record for PV]. Optics.org. Retrieved 19 January 2011.</ref>

==== GaInP/Si dual-junction solar cells ====
In 2016, a new approach was described for producing hybrid photovoltaic wafers combining the high efficiency of [[Physics:Multi-junction solar cell|III-V multi-junction solar cells]] with the economies and wealth of experience associated with silicon. The technical complications involved in growing the III-V material on silicon at the required high temperatures, a subject of study for some 30 years, are avoided by epitaxial growth of silicon on GaAs at low temperature by [[Chemistry:Plasma-enhanced chemical vapor deposition|plasma-enhanced chemical vapor deposition]] (PECVD).<ref name="CariouChen2016">{{cite journal|last1=Cariou|first1=Romain|last2=Chen|first2=Wanghua|last3=Maurice|first3=Jean-Luc|last4=Yu|first4=Jingwen|last5=Patriarche|first5=Gilles|last6=Mauguin|first6=Olivia|last7=Largeau|first7=Ludovic|last8=Decobert|first8=Jean|last9=Roca i Cabarrocas|first9=Pere|title=Low temperature plasma enhanced CVD epitaxial growth of silicon on GaAs: a new paradigm for III-V/Si integration|journal=Scientific Reports|volume=6|year=2016|pages=25674|issn=2045-2322|doi=10.1038/srep25674|pmid=27166163|pmc=4863370|bibcode=2016NatSR...625674C}}</ref>

Si single-junction solar cells have been widely studied for decades and are reaching their practical efficiency of ~26% under 1-sun conditions.<ref>{{cite journal|doi=10.1109/JPHOTOV.2014.2350695|title=Toward the Practical Limits of Silicon Solar Cells|journal=IEEE Journal of Photovoltaics|volume=4|issue=6|pages=1465–1469|year=2014|last1=Smith|first1=David D.|last2=Cousins|first2=Peter|last3=Westerberg|first3=Staffan|last4=Jesus-Tabajonda|first4=Russelle De|last5=Aniero|first5=Gerly|last6=Shen|first6=Yu-Chen|s2cid=33022605}}</ref> Increasing this efficiency may require adding more cells with bandgap energy larger than 1.1 eV to the Si cell, allowing to convert short-wavelength photons for generation of additional voltage. A dual-junction solar cell with a band gap of 1.6–1.8 eV as a top cell can reduce thermalization loss, produce a high external radiative efficiency and achieve theoretical efficiencies over 45%.<ref>{{cite journal|last1=Almansouri|first1=Ibraheem|last2=Ho-Baillie|first2=Anita|last3=Bremner|first3=Stephen P.|last4=Green|first4=Martin A.|year=2015|title=Supercharging Silicon Solar Cell Performance by Means of Multijunction Concept|journal=IEEE Journal of Photovoltaics|volume=5|issue=3|pages=968–976|doi=10.1109/JPHOTOV.2015.2395140|s2cid=8477762}}</ref> A tandem cell can be fabricated by growing the GaInP and Si cells. Growing them separately can overcome the 4% lattice constant mismatch between Si and the most common III–V layers that prevent direct integration into one cell. The two cells therefore are separated by a transparent glass slide so the lattice mismatch does not cause strain to the system. This creates a cell with four electrical contacts and two junctions that demonstrated an efficiency of 18.1%. With a fill factor (FF) of 76.2%, the Si bottom cell reaches an efficiency of 11.7% (± 0.4) in the tandem device, resulting in a cumulative tandem cell efficiency of 29.8%.<ref>{{cite journal|doi= 10.1109/JPHOTOV.2016.2549746|title=Realization of GaInP/Si Dual-Junction Solar Cells with 29.8% 1-Sun Efficiency|journal=IEEE Journal of Photovoltaics|volume=6|issue=4|pages=1012–1019|year=2016|last1=Essig|first1=Stephanie|last2=Steiner|first2=Myles A.|last3=Allebe|first3=Christophe|last4=Geisz|first4=John F.|last5=Paviet-Salomon|first5=Bertrand|last6=Ward|first6=Scott|last7=Descoeudres|first7=Antoine|last8=Lasalvia|first8=Vincenzo|last9=Barraud|first9=Loris|last10=Badel|first10=Nicolas|last11=Faes|first11=Antonin|last12=Levrat|first12=Jacques|last13=Despeisse|first13=Matthieu|last14=Ballif|first14=Christophe|last15=Stradins|first15=Paul|last16=Young|first16=David L.|osti=1329999 |doi-access=free}}</ref> This efficiency exceeds the theoretical limit of 29.4%<ref>{{cite journal|doi=10.1109/JPHOTOV.2013.2270351|title=Reassessment of the Limiting Efficiency for Crystalline Silicon Solar Cells|journal=IEEE Journal of Photovoltaics|volume=3|issue=4|pages=1184–1191|year=2013|last1=Richter|first1=Armin|last2=Hermle|first2=Martin|last3=Glunz|first3=Stefan W.|s2cid=6013813}}</ref> and the record experimental efficiency value of a Si 1-sun solar cell, and is also higher than the record-efficiency 1-sun GaAs device. However, using a GaAs substrate is expensive and not practical. Hence researchers try to make a cell with two electrical contact points and one junction, which does not need a GaAs substrate. This means there will be direct integration of GaInP and Si.

== Research in solar cells ==

=== Perovskite solar cells ===
{{Main|Physics:Perovskite solar cell}}
[[Physics:Perovskite solar cell|Perovskite solar cell]]s are solar cells that include a [[Chemistry:Perovskite|perovskite]]-structured material as the active layer. Most commonly, this is a solution-processed hybrid organic-inorganic tin or lead halide based material. Efficiencies have increased from below 5% at their first usage in 2009 to 25.5% in 2020, making them a very rapidly advancing technology and a hot topic in the solar cell field.<ref>{{Cite web |url=https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200922.pdf |title=Best Research-Cell Efficiencies |url-status=live |access-date=28 September 2020 |archive-date=27 November 2020 |archive-url=https://web.archive.org/web/20201127183812/https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200922.pdf}}</ref> Researchers at [[Organization:University of Rochester|University of Rochester]] reported in 2023 that significant further improvements in cell efficiency can be achieved by utilizing [[Physics:Purcell effect|Purcell effect]].<ref>{{cite web |url=https://www.rochester.edu/newscenter/metal-perovskite-solar-cells-vs-silicon-efficiency-550212/ |title=Perovskites, a 'dirt cheap' alternative to silicon, just got a lot more efficient |author=<!--Not stated--> |date=February 16, 2023 |access-date=June 3, 2023 }}</ref>

Perovskite solar cells are also forecast to be extremely cheap to scale up, making them a very attractive option for commercialisation. So far most types of perovskite solar cells have not reached sufficient operational stability to be commercialised, although many research groups are investigating ways to solve this.<ref>{{cite journal|last1=Kosasih|first1=Felix Utama|last2=Ducati|first2=Caterina|title=Characterising degradation of perovskite solar cells through in-situ and operando electron microscopy |journal=Nano Energy|date=May 2018|volume=47|pages=243–256 |doi=10.1016/j.nanoen.2018.02.055 |url=https://www.repository.cam.ac.uk/handle/1810/275845|url-access=}}</ref> Energy and environmental sustainability of perovskite solar cells and tandem perovskite are shown to be dependent on the structures.<ref>{{Cite journal |last1=Tian|first1=Xueyu|last2=Stranks |first2=Samuel D.|last3=You|first3=Fengqi|date=July 2020|title=Life cycle energy use and environmental implications of high-performance perovskite tandem solar cells|journal=Science Advances|language=en|volume=6|issue=31 |pages=eabb0055 |doi=10.1126/sciadv.abb0055|pmid=32789177|pmc=7399695 |bibcode=2020SciA....6...55T |issn=2375-2548|doi-access=free}}</ref><ref>{{Cite journal |last1=Gong |first1=Jian |last2=Darling |first2=Seth B. |last3=You |first3=Fengqi |date=2015-07-03 |title=Perovskite photovoltaics: life-cycle assessment of energy and environmental impacts |journal=Energy & Environmental Science |language=en |volume=8 |issue=7 |pages=1953–1968 |doi=10.1039/C5EE00615E |issn=1754-5706}}</ref><ref>{{Cite journal |last1=Tian |first1=Xueyu |last2=Stranks |first2=Samuel D. |last3=You |first3=Fengqi |date=2021-06-24 |title=Life cycle assessment of recycling strategies for perovskite photovoltaic modules |journal=Nature Sustainability |volume=4 |issue=9 |language=en |pages=821–829 |doi=10.1038/s41893-021-00737-z |bibcode=2021NatSu...4..821T |s2cid=235630649 |issn=2398-9629}}</ref> Photonic front contacts for light management can improve the perovskite cells' performance, via enhanced broadband absorption, while allowing better operational stability due to protection against the harmful high-energy (above Visible) radiation.<ref>{{Cite journal |last1=Haque |first1=Sirazul |last2=Mendes |first2=Manuel J. |last3=Sanchez-Sobrado |first3=Olalla |last4=Águas |first4=Hugo |last5=Fortunato |first5=Elvira |last6=Martins |first6=Rodrigo |date=2019-05-01 |title=Photonic-structured TiO2 for high-efficiency, flexible and stable Perovskite solar cells |journal=Nano Energy |language=en |volume=59 |pages=91–101 |doi=10.1016/j.nanoen.2019.02.023 |s2cid=139461077 |issn=2211-2855|doi-access=free }}</ref> The inclusion of the toxic element [[Chemistry:Lead|lead]] in the most efficient perovskite solar cells is a potential problem for commercialisation.<ref>{{Cite journal |last1=Li |first1=Junming |last2=Cao |first2=Hai-Lei |last3=Jiao |first3=Wen-Bin |last4=Wang |first4=Qiong |last5=Wei |first5=Mingdeng |last6=Cantone |first6=Irene |last7=Lü |first7=Jian |last8=Abate |first8=Antonio |date=2020-01-21 |title=Biological impact of lead from halide perovskites reveals the risk of introducing a safe threshold|journal=Nature Communications |language=en |volume=11 |issue=1 |pages=310 |doi=10.1038/s41467-019-13910-y |pmid=31964862 |pmc=6974608 |bibcode=2020NatCo..11..310L |issn=2041-1723}}</ref>

=== Bifacial solar cells ===
{{Main|Physics:Bifacial solar cells}}

[[File:Noto Bifacial Plant.png|thumb|Bifacial solar cell plant in Noto (Senegal), 1988 - Floor painted in white to enhance albedo.]]
With a transparent rear side, bifacial solar cells can absorb light from both the front and rear sides. Hence, they can produce more electricity than conventional monofacial solar cells. The first patent of bifacial solar cells was filed by Japanese researcher Hiroshi Mori, in 1966.<ref>{{Cite web |date=3 October 1961|author=Mori Hiroshi |title=Radiation energy transducing device|url=https://patents.google.com/patent/US3278811 |website=Google Patents}}</ref> Later, it is said that Russia was the first to deploy bifacial solar cells in their space program in the 1970s.{{Citation needed|date=May 2018}} In 1976, the [http://www.ies.upm.es/ Institute for Solar Energy] of the Technical University of Madrid, began a research program for the development of bifacial solar cells led by Prof. Antonio Luque. Based on 1977 US and Spanish patents by Luque, a practical bifacial cell was proposed with a front face as anode and a rear face as cathode; in previously reported proposals and attempts both faces were anodic and interconnection between cells was complicated and expensive.<ref>{{patent|ES|453575 (A1)|A. Luque: "Procedimiento para obtener células solares bifaciales" filing date 5 May 1977}}</ref><ref>{{patent|US|4169738 (A)|A. Luque: "Double-sided solar cell with self-refrigerating concentrator" filing date 21 November 1977}}</ref><ref>{{cite journal |last1=Luque |first1=A. |last2=Cuevas |first2=A. |last3=Eguren |first3=J. |title=Solar-Cell Behavior under Variable Surface Recombination Velocity and Proposal of a Novel Structure |journal=Solid-State Electronics |date=1978 |volume=21 |issue=5 |pages=793–794 |doi=10.1016/0038-1101(78)90014-X |bibcode=1978SSEle..21..793L}}</ref> In 1980, Andrés Cuevas, a PhD student in Luque's team, demonstrated experimentally a 50% increase in output power of bifacial solar cells, relative to identically oriented and tilted monofacial ones, when a white background was provided.<ref>{{Cite journal |last1=Cuevas |first1=A. |last2=Luque |first2=A. |last3=Eguren |first3=J. |last4=Alamo |first4=J. del |title=50 Per cent more output power from an albedo-collecting flat panel using bifacial solar cells |journal=Solar Energy |date=1982 |volume=29 |issue=5 |pages=419–420 |doi=10.1016/0038-092x(82)90078-0 |bibcode=1982SoEn...29..419C}}</ref> In 1981 the company Isofoton was founded in Málaga to produce the developed bifacial cells, thus becoming the first industrialization of this PV cell technology. With an initial production capacity of 300&nbsp;kW/yr of bifacial solar cells, early landmarks of Isofoton's production were the 20kWp power plant in San Agustín de Guadalix, built in 1986 for Iberdrola, and an off grid installation by 1988 also of 20kWp in the village of Noto Gouye Diama (Senegal) funded by the Spanish international aid and cooperation programs.

Due to the reduced manufacturing cost, companies have again started to produce commercial bifacial modules since 2010. By 2017, there were at least eight certified PV manufacturers providing bifacial modules in North America. The International Technology Roadmap for Photovoltaics (ITRPV) predicted that the global market share of bifacial technology will expand from less than 5% in 2016 to 30% in 2027.<ref>{{Cite web|title=International Technology Roadmap for Photovoltaic (ITRPV) – Home |website=www.itrpv.net|url=http://www.itrpv.net/|access-date=20 February 2018}}</ref>

Due to the significant interest in the bifacial technology, a recent study has investigated the performance and optimization of bifacial solar modules worldwide.<ref name="Sun-2018">{{Cite journal |last1=Sun |first1=Xingshu |last2=Khan |first2=Mohammad Ryyan |last3=Deline |first3=Chris |last4=Alam |first4=Muhammad Ashraful |title=Optimization and performance of bifacial solar modules: A global perspective |journal=Applied Energy |year=2018 |volume=212 |pages=1601–1610 |arxiv=1709.10026 |s2cid=117375370 |doi=10.1016/j.apenergy.2017.12.041|bibcode=2018ApEn..212.1601S }}</ref><ref>{{Cite journal|last1=Khan|first1=M. Ryyan |last2=Hanna |first2=Amir |last3=Sun |first3=Xingshu |last4=Alam |first4=Muhammad A. |title=Vertical bifacial solar farms: Physics, design, and global optimization |journal=Applied Energy |volume=206 |pages=240–248 |year=2017 |arxiv=1704.08630 |bibcode=2017ApEn..206..240K |s2cid=115039440 |doi=10.1016/j.apenergy.2017.08.042}}</ref> The results indicate that, across the globe, ground-mounted bifacial modules can only offer ~10% gain in annual electricity yields compared to the monofacial counterparts for [[Earth:Albedo|a ground albedo coefficient]] of 25% (typical for concrete and vegetation groundcovers). However, the gain can be increased to ~30% by elevating the module 1&nbsp;m above the ground and enhancing the ground albedo coefficient to 50%. Sun ''et al.'' also derived a set of empirical equations that can optimize bifacial solar modules analytically.<ref name="Sun-2018" /> In addition, there is evidence that bifacial panels work better than traditional panels in snowy environments as bifacials on dual-axis trackers made 14% more electricity in a year than their monofacial counterparts and 40% during the peak winter months.<ref>Burnham, Performance of Bifacial Photovoltaic Modules on a Dual-Axis Tracker in a High-Latitude, High-Albedo Environment, 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), Chicago, IL, USA, 2019, pp. 1320-1327.</ref>

[https://nanohub.org/tools/pub An online simulation tool] is available to model the performance of bifacial modules in any arbitrary location across the entire world. It can also optimize bifacial modules as a function of tilt angle, azimuth angle, and elevation above the ground.<ref>{{Cite journal |last1=Zhao |first1=Binglin |last2=Sun |first2=Xingshu |last3=Khan |first3=Mohammad Ryyan |last4=Alam |first4=Muhammad Ashraful |date=19 February 2018 |title=Purdue Bifacial Module Calculator |website=nanoHUB |doi=10.4231/d3542jb3c}}</ref>

=== Intermediate band ===
{{Main|Physics:Intermediate band photovoltaics}}

'''Intermediate band photovoltaics''' in solar cell research provides methods for exceeding the [[Physics:Shockley–Queisser limit|Shockley–Queisser limit]] on the efficiency of a cell. It introduces an intermediate band (IB) energy level in between the valence and conduction bands. Theoretically, introducing an IB allows two [[Physics:Photons|photons]] with energy less than the bandgap to excite an electron from the valence band to the conduction band. This increases the induced photocurrent and thereby efficiency.<ref>{{cite journal |doi=10.1103/PhysRevLett.78.5014|title=Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels |journal=Physical Review Letters |volume=78 |issue=26 |pages=5014–5017 |year=1997 |last1=Luque |first1=Antonio |last2=Martí |first2=Antonio |bibcode=1997PhRvL..78.5014L}}</ref>

Luque and Marti first derived a theoretical limit for an IB device with one midgap energy level using [[Detailed balance|detailed balance]]. They assumed no carriers were collected at the IB and that the device was under full concentration. They found the maximum efficiency to be 63.2%, for a bandgap of 1.95eV with the IB 0.71eV from either the valence or conduction band.
Under one sun illumination the limiting efficiency is 47%.<ref>{{cite book |author=Okada, Yoshitaka |author2=Tomah Sogabe |author3=Yasushi Shoji|doi=10.1039/9781849739955-00425|isbn= 978-1-84973-995-5|chapter=Ch. 13: Intermediate Band Solar Cells|title= Advanced Concepts in Photovoltaics |editor=Arthur J. Nozik |editor2=Gavin Conibeer |editor3=Matthew C. Beard|volume=11|place= Cambridge, UK|publisher= Royal Society of Chemistry|year= 2014|pages= 425–54|series=Energy and Environment Series}}</ref> Several means are under study to realize IB semiconductors with such optimum 3-bandgap configuration, namely via materials engineering (controlled inclusion of deep level impurities or highly-mismatched alloys) and nano-structuring (quantum-dots in host hetero-crystals).<ref>{{Cite journal |last1=Ramiro |first1=Iñigo |last2=Martí |first2=Antonio |date=July 2021 |title=Intermediate band solar cells: Present and future |journal=Progress in Photovoltaics: Research and Applications |language=en |volume=29 |issue=7 |pages=705–713 |doi=10.1002/pip.3351 |s2cid=226335202 |issn=1062-7995|url=https://oa.upm.es/64919/2/IBSC_present_and_future_PIP3351.pdf |url-access= }}</ref>

=== Liquid inks ===
In 2014, researchers at [[Organization:California NanoSystems Institute|California NanoSystems Institute]] discovered using [[Chemistry:Kesterite|kesterite]] and [[Chemistry:Perovskite|perovskite]] improved [[Engineering:Electric power conversion|electric power conversion]] efficiency for solar cells.<ref>{{Cite news |url=http://phys.org/news/2014-09-liquid-inks-solar-cells.html |title=Researchers use liquid inks to create better solar cells |website=Phys.org |date=17 September 2014 |first=Shaun |last=Mason}}</ref>

=== Upconversion and downconversion===
[[Physics:Photon upconversion|Photon upconversion]] is the process of using two low-energy (''e.g.'', infrared) photons to produce one higher energy photon; '''downconversion''' is the process of using one high energy photon (''e.g.'', ultraviolet) to produce two lower energy photons. Either of these techniques could be used to produce higher efficiency solar cells by allowing solar photons to be more efficiently used. The difficulty, however, is that the conversion efficiency of existing [[Physics:Phosphor|phosphor]]s exhibiting up- or down-conversion is low, and is typically narrow band.

One upconversion technique is to incorporate [[Chemistry:Lanthanide|lanthanide]]-doped materials ([[Chemistry:Erbium|{{chem|Er|3+}}]], [[Chemistry:Ytterbium|{{chem|Yb|3+}}]], [[Chemistry:Holmium|{{chem|Ho|3+}}]] or a combination), taking advantage of their [[Physics:Luminescence|luminescence]] to convert [[Physics:Infrared radiation|infrared radiation]] to visible light. Upconversion process occurs when two [[Physics:Infrared|infrared]] photons are absorbed by rare-earth [[Physics:Ion|ion]]s to generate a (high-energy) absorbable photon. As example, the energy transfer upconversion process (ETU), consists in successive transfer processes between excited ions in the near infrared. The upconverter material could be placed below the solar cell to absorb the infrared light that passes through the silicon. Useful ions are most commonly found in the trivalent state. {{chem|Er|+}} ions have been the most used. {{chem|Er|3+}} ions absorb solar radiation around 1.54&nbsp;µm. Two {{chem|Er|3+}} ions that have absorbed this radiation can interact with each other through an upconversion process. The excited ion emits light above the Si bandgap that is absorbed by the solar cell and creates an additional electron–hole pair that can generate current. However, the increased efficiency was small. In addition, fluoroindate glasses have low [[Physics:Phonon|phonon]] energy and have been proposed as suitable matrix doped with {{chem|Ho|3+}} ions.<ref>{{cite journal|author1=Hernández-Rodríguez, M.A. |author2=Imanieh, M.H. |author3=Martín, L.L. |author4= Martín, I.R. |title=Experimental enhancement of the photocurrent in a solar cell using upconversion process in fluoroindate glasses exciting at 1480nm|journal=Solar Energy Materials and Solar Cells|date=September 2013 |volume=116 |pages=171–175 |doi=10.1016/j.solmat.2013.04.023}}</ref>

=== Light-absorbing dyes ===
[[Physics:Dye-sensitized solar cell|Dye-sensitized solar cell]]s (DSSCs) are made of low-cost materials and do not need elaborate manufacturing equipment, so they can be made in a DIY fashion. In bulk it should be significantly less expensive than older solid-state cell designs. DSSC's can be engineered into flexible sheets and although its conversion efficiency is less than the best thin film cells, its price/performance ratio may be high enough to allow them to compete with fossil fuel electrical generation.

Typically a [[Chemistry:Ruthenium|ruthenium]] metalorganic dye (Ru-centered) is used as a [[Physics:Monolayer|monolayer]] of light-absorbing material, which is adsorbed onto a thin film of [[Physics:Titanium dioxide nanoparticle|titanium dioxide]]. The dye-sensitized solar cell depends on this mesoporous layer of nanoparticulate [[Chemistry:Titanium dioxide|titanium dioxide]] (TiO<sub>2</sub>) to greatly amplify the surface area (200–300 m<sup>2</sup>/g {{chem|TiO|2}}, as compared to approximately 10 m<sup>2</sup>/g of flat single crystal) which allows for a greater number of dyes per solar cell area (which in term in increases the current). The photogenerated electrons from the light absorbing dye are passed on to the n-type {{chem|TiO|2}} and the holes are absorbed by an [[Physics:Electrolyte|electrolyte]] on the other side of the dye. The circuit is completed by a [[Chemistry:Redox|redox]] couple in the electrolyte, which can be liquid or solid. This type of cell allows more flexible use of materials and is typically manufactured by screen printing or [[Physics:Ultrasonic nozzle|ultrasonic nozzle]]s, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from [[Chemistry:Chemical decomposition|degradation]] under heat and UV light and the cell casing is difficult to seal due to the solvents used in assembly. Due to this reason, researchers have developed solid-state dye-sensitized solar cells that use a solid electrolyte to avoid leakage.<ref>{{Cite journal|last1=Wang |first1=Peng |last2=Zakeeruddin |first2=Shaik M. |last3=Moser |first3=Jacques E. |last4=Nazeeruddin |first4=Mohammad K. |last5=Sekiguchi |first5=Takashi |last6=Grätzel |first6=Michael |date=June 2003 |title=A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte |journal=Nature Materials |language=en |volume=2 |issue=6 |pages=402–407 |doi=10.1038/nmat904 |pmid=12754500 |bibcode=2003NatMa...2..402W |s2cid=27383758 |issn=1476-4660|doi-access=free }}</ref> The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations.<ref>[http://www.g24i.com Dye Sensitized Solar Cells]. G24i.com (2 April 2014). Retrieved 20 April 2014.</ref>

=== Quantum dots ===
{{Main|Physics:Quantum dot solar cell}}
[[Physics:Quantum dot solar cell|Quantum dot solar cell]]s (QDSCs) are based on the Gratzel cell, or [[Physics:Dye-sensitized solar cell|dye-sensitized solar cell]] architecture, but employ low [[Physics:Band gap|band gap]] [[Physics:Semiconductor|semiconductor]] [[Physics:Nanoparticle|nanoparticle]]s, fabricated with crystallite sizes small enough to form quantum dots (such as [[Chemistry:Cadmium sulfide|CdS]], [[Chemistry:Cadmium selenide|CdSe]], [[Chemistry:Stibnite|{{chem|Sb|2|S|3}}]], [[Chemistry:Lead(II) sulfide|PbS]], etc.), instead of organic or organometallic dyes as light absorbers. Due to the toxicity associated with Cd and Pb based compounds there are also a series of "green" QD sensitizing materials in development (such as CuInS<sub href="Refractive index">2,</sub> CuInSe<sub href="multiple exciton generation">2</sub> and CuInSeS).<ref>{{Cite journal|date=1 October 2016|title=Quantum dot sensitized solar cell: Recent advances and future perspectives in photoanode|journal=Solar Energy Materials and Solar Cells|volume=155|pages=294–322|doi=10.1016/j.solmat.2016.05.062|issn=0927-0248|last1=Sharma|first1=Darshan|last2=Jha|first2=Ranjana|last3=Kumar|first3=Shiv}}</ref> QD's size quantization allows for the band gap to be tuned by simply changing particle size. They also have high [[Physics:Refractive index|extinction coefficients]] and have shown the possibility of [[Physics:Multiple exciton generation|multiple exciton generation]].<ref>{{cite journal|doi=10.1126/science.1209845|title=Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell|year=2011|last1=Semonin|first1=O. E.|last2=Luther|first2=J. M.|last3=Choi|first3=S.|last4=Chen|first4=H.-Y.|last5=Gao|first5=J.|last6=Nozik|first6=A. J.|last7=Beard|first7=M. C.|journal=Science|volume=334|issue=6062|pages=1530–3|pmid=22174246 |bibcode=2011Sci...334.1530S|s2cid=36022754}}</ref>

In a QDSC, a mesoporous layer of [[Chemistry:Titanium dioxide|titanium dioxide]] nanoparticles forms the backbone of the cell, much like in a DSSC. This {{chem|TiO|2}} layer can then be made photoactive by coating with semiconductor quantum dots using [[Chemistry:Chemical bath deposition|chemical bath deposition]], [[Engineering:Electrophoretic deposition|electrophoretic deposition]] or successive ionic layer adsorption and reaction. The electrical circuit is then completed through the use of a liquid or solid redox couple. The efficiency of QDSCs has increased<ref>{{cite journal|doi=10.1021/ar200315d|pmid=22493938|title=Boosting the Efficiency of Quantum Dot Sensitized Solar Cells through Modulation of Interfacial Charge Transfer|year=2012|last1=Kamat|first1=Prashant V.|journal=Accounts of Chemical Research|volume=45|issue=11|pages=1906–15}}</ref> to over 5% shown for both liquid-junction<ref>{{cite journal|doi=10.1021/ja211224s|title=Mn-Doped Quantum Dot Sensitized Solar Cells: A Strategy to Boost Efficiency over 5%|year=2012|last1=Santra|first1=Pralay K.|last2=Kamat|first2=Prashant V.|journal=Journal of the American Chemical Society|volume=134|issue=5|pages=2508–11|pmid=22280479}}</ref> and solid state cells,<ref>{{cite journal|doi=10.1021/jz100308q|title=Sb2S3-Based Mesoscopic Solar Cell using an Organic Hole Conductor|year=2010|last1=Moon|first1=Soo-Jin|last2=Itzhaik|first2=Yafit|last3=Yum|first3=Jun-Ho|last4=Zakeeruddin|first4=Shaik M.|last5=Hodes|first5=Gary|last6=GräTzel|first6=Michael|journal=The Journal of Physical Chemistry Letters|volume=1|issue=10|page=1524}}</ref> with a reported peak efficiency of 11.91%.<ref>{{cite journal|pmid=26962680|doi=10.1021/jacs.6b00615|title=Zn–Cu–In–Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%|journal=Journal of the American Chemical Society|volume=138|issue=12|pages=4201–4209|year=2016|last1=Du|first1=Jun|last2=Du|first2=Zhonglin|last3=Hu|first3=Jin-Song|last4=Pan|first4=Zhenxiao|last5=Shen|first5=Qing|last6=Sun|first6=Jiankun|last7=Long|first7=Donghui|last8=Dong|first8=Hui|last9=Sun|first9=Litao|last10=Zhong|first10=Xinhua|last11=Wan|first11=Li-Jun}}</ref> In an effort to decrease production costs, the Prashant Kamat research group<ref>[http://nd.edu/~kamatlab/research_solarCells.html Solar Cell Research || The Prashant Kamat lab at the University of Notre Dame]. Nd.edu (22 February 2007). Retrieved 17 May 2012.</ref> demonstrated a solar paint made with {{chem|TiO|2}} and CdSe that can be applied using a one-step method to any conductive surface with efficiencies over 1%.<ref>{{cite journal|doi=10.1021/nn204381g|title=Sun-BelievableSolar Paint. A Transformative One-Step Approach for Designing Nanocrystalline Solar Cells|year=2012|last1=Genovese|first1=Matthew P.|last2=Lightcap|first2=Ian V.|last3=Kamat|first3=Prashant V.|journal=ACS Nano|volume=6|pages=865–72|pmid=22147684|issue=1}}</ref> However, the absorption of quantum dots (QDs) in QDSCs is weak at room temperature.<ref name="Yu-2017">{{Cite journal|last1=Yu|first1=Peng|last2=Wu|first2=Jiang|last3=Gao|first3=Lei|last4=Liu|first4=Huiyun|last5=Wang|first5=Zhiming|date=1 March 2017|title=InGaAs and GaAs quantum dot solar cells grown by droplet epitaxy|journal=Solar Energy Materials and Solar Cells|volume=161|pages=377–381|doi=10.1016/j.solmat.2016.12.024|url=https://discovery.ucl.ac.uk/1539114/1/Accepted%20QDSC%20droplet%20epitaxy.pdf|url-access=}}</ref> The [[Physics:Plasmonic solar cell|plasmonic nanoparticles]] can be utilized to address the weak absorption of QDs (e.g., nanostars).<ref>{{Cite journal|last1=Wu|first1=Jiang|last2=Yu|first2=Peng|last3=Susha|first3=Andrei S.|last4=Sablon|first4=Kimberly A.|last5=Chen|first5=Haiyuan|last6=Zhou|first6=Zhihua|last7=Li|first7=Handong|last8=Ji|first8=Haining|last9=Niu|first9=Xiaobin|date=1 April 2015|title=Broadband efficiency enhancement in quantum dot solar cells coupled with multispiked plasmonic nanostars|journal=Nano Energy|volume=13|pages=827–835|doi=10.1016/j.nanoen.2015.02.012|s2cid=98282021 }}</ref> Adding an external infrared pumping source to excite intraband and interband transition of QDs is another solution.<ref name="Yu-2017" />

=== Organic/polymer solar cells ===
[[Physics:Organic solar cell|Organic solar cell]]s and polymer solar cells are built from thin films (typically 100&nbsp;nm) of [[Chemistry:Organic semiconductor|organic semiconductor]]s including polymers, such as polyphenylene vinylene and small-molecule compounds like copper phthalocyanine (a blue or green organic pigment) and [[Chemistry:Fullerene|carbon fullerenes]] and fullerene derivatives such as PCBM.

They can be processed from liquid solution, offering the possibility of a simple roll-to-roll printing process, potentially leading to inexpensive, large-scale production. In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. Current cell efficiencies are, however, very low, and practical devices are essentially non-existent.

Energy conversion efficiencies achieved to date using conductive polymers are very low compared to inorganic materials. However, Konarka Power Plastic reached efficiency of 8.3%<ref>[http://www.pv-tech.org/news/nrel_validates_konarkas_8.3_power_plastic_efficiency_record Konarka Power Plastic reaches 8.3% efficiency]. pv-tech.org. Retrieved 7 May 2011.</ref> and organic tandem cells in 2012 reached 11.1%.{{citation needed|date=June 2014}}

The active region of an organic device consists of two materials, one electron donor and one electron acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an [[Physics:Exciton|exciton]], separating when the exciton diffuses to the donor-acceptor interface, unlike most other solar cell types. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance.<ref>{{Cite journal| doi=10.1016/S1369-7021(07)70276-6| title=Polymer-based solar cells| journal=Materials Today| volume=10| issue=11| page=28| year=2007| last1=Mayer| first1=A.| last2=Scully| first2=S.| last3=Hardin| first3=B.| last4=Rowell| first4=M.| last5=McGehee| first5=M.| doi-access=free}}</ref>

In 2011, MIT and Michigan State researchers developed solar cells with a power efficiency close to 2% with a transparency to the human eye greater than 65%, achieved by selectively absorbing the ultraviolet and near-infrared parts of the spectrum with small-molecule compounds.<ref>{{Cite journal| doi=10.1063/1.3567516| title = Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications| journal=Applied Physics Letters| volume=98| issue=11| page=113305| year=2011| last1=Lunt| first1=R. R.| last2=Bulovic| first2=V.| bibcode=2011ApPhL..98k3305L| doi-access=free| hdl=1721.1/71948| hdl-access=free}}</ref><ref>{{cite news |last=Rudolf|first=John Collins|date=20 April 2011 |title=Transparent Photovoltaic Cells Turn Windows into Solar Panels |website=green.blogs.nytimes.com |url=http://green.blogs.nytimes.com/2011/04/20/transparent-photovoltaic-cells-turn-windows-into-solar-panels/}}</ref> Researchers at UCLA more recently developed an analogous polymer solar cell, following the same approach, that is 70% transparent and has a 4% power conversion efficiency.<ref>{{cite web |url=http://www.enviro-news.com/news/ucla-scientists-develop-transparent-solar-cell.html|title=UCLA Scientists Develop Transparent Solar Cell|publisher=Enviro-News.com |date=24 July 2012|url-status=dead|archive-url=https://web.archive.org/web/20120727005130/http://www.enviro-news.com/news/ucla-scientists-develop-transparent-solar-cell.html|archive-date=27 July 2012}}</ref><ref>{{Cite journal| doi=10.1002/adma.201103404| pmid=22057647| title=Practical Roadmap and Limits to Nanostructured Photovoltaics| journal=Advanced Materials| volume=23| issue=48| pages=5712–27| year=2011| last1=Lunt| first1=R. R.| last2=Osedach| first2=T. P.| last3=Brown| first3=P. R.| last4=Rowehl| first4=J. A.| last5=Bulović| first5=V.| bibcode=2011AdM....23.5712L| hdl=1721.1/80286| s2cid=13511794| hdl-access=free}}</ref><ref>{{Cite journal| doi=10.1063/1.4738896| title=Theoretical limits for visibly transparent photovoltaics| journal=Applied Physics Letters| volume=101| issue=4| page=043902| year=2012| last1=Lunt| first1=R. R.| bibcode=2012ApPhL.101d3902L}}</ref> These lightweight, flexible cells can be produced in bulk at a low cost and could be used to create power generating windows.

In 2013, researchers announced polymer cells with some 3% efficiency. They used block copolymers, self-assembling organic materials that arrange themselves into distinct layers. The research focused on P3HT-b-PFTBT that separates into bands some 16 nanometers wide.<ref>{{Cite journal |last1=Guo |first1=C. |last2=Lin |first2=Y. H. |last3=Witman |first3=M. D. |last4=Smith |first4=K. A. |last5=Wang |first5=C. |last6=Hexemer |first6=A. |last7=Strzalka |first7=J. |last8=Gomez |first8=E. D. |last9=Verduzco |first9=R. |title=Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase Separation |journal=Nano Letters |volume=13 |issue=6 |pages=2957–63 |year=2013 |pmid=23687903 |bibcode=2013NanoL..13.2957G |url= https://figshare.com/articles/Conjugated_Block_Copolymer_Photovoltaics_with_near_3_Efficiency_through_Microphase_Separation/2406439 |doi=10.1021/nl401420s}}</ref><ref>{{cite web |title=Organic polymers create new class of solar energy devices |date=31 May 2013 |publisher=Kurzweil Accelerating Institute |url=http://www.kurzweilai.net/organic-polymers-create-new-class-of-solar-energy-devices |access-date=1 June 2013}}</ref>

=== Adaptive cells ===
Adaptive cells change their absorption/reflection characteristics depending on environmental conditions. An adaptive material responds to the intensity and angle of incident light. At the part of the cell where the light is most intense, the cell surface changes from reflective to adaptive, allowing the light to penetrate the cell. The other parts of the cell remain reflective increasing the retention of the absorbed light within the cell.<ref name=tr1407>Bullis, Kevin (30 July 2014) [http://www.technologyreview.com/news/529476/adaptive-material-could-cut-the-cost-of-solar-in-half Adaptive Material Could Cut the Cost of Solar in Half]. ''MIT Technology Review''</ref>

In 2014, a system was developed that combined an adaptive surface with a glass substrate that redirect the absorbed to a light absorber on the edges of the sheet. The system also includes an array of fixed lenses/mirrors to concentrate light onto the adaptive surface. As the day continues, the concentrated light moves along the surface of the cell. That surface switches from reflective to adaptive when the light is most concentrated and back to reflective after the light moves along.<ref name=tr1407/>

=== Surface texturing ===
[[File:SolarImpulse HB-SIA landing Brussels Airport 3-crop.jpg|thumb|upright=1.1|[[Engineering:Solar Impulse|Solar Impulse]] aircraft are Swiss-designed single-seat monoplanes powered entirely from photovoltaic cells]]

For the past years, researchers have been trying to reduce the price of solar cells while maximizing efficiency. [[Engineering:Thin-film solar cell|Thin-film solar cell]] is a cost-effective second generation solar cell with much reduced thickness at the expense of light absorption efficiency. Efforts to maximize light absorption efficiency with reduced thickness have been made. Surface texturing is one of techniques used to reduce optical losses to maximize light absorbed. Currently, surface texturing techniques on silicon photovoltaics are drawing much attention. Surface texturing could be done in multiple ways. Etching single crystalline silicon substrate can produce randomly distributed square based pyramids on the surface using anisotropic etchants.<ref>{{cite journal|last=Campbell|first=Patrick|author2=Green, Martin A. |title=Light Trapping Properties of Pyramidally textured surfaces|journal=Journal of Applied Physics|date=Feb 1987|volume=62|issue=1| pages=243–249|doi=10.1063/1.339189|bibcode=1987JAP....62..243C}}</ref> Recent studies show that c-Si wafers could be etched down to form nano-scale inverted pyramids. Multicrystalline silicon solar cells, due to poorer crystallographic quality, are less effective than single crystal solar cells, but mc-Si solar cells are still being used widely due to less manufacturing difficulties. It is reported that multicrystalline solar cells can be surface-textured to yield solar energy conversion efficiency comparable to that of monocrystalline silicon cells, through isotropic etching or photolithography techniques.<ref>{{cite journal|last=Zhao|first=Jianhua|author2=Wang, Aihua |author3=Green, Martin A.|title=19.8% efficient "honeycomb" textured multicrystalline and 24.4% monocrystalline silicon solar cells|journal=Applied Physics Letters|date=May 1998|volume=73|issue=14| pages=1991–1993|doi=10.1063/1.122345|bibcode=1998ApPhL..73.1991Z}}</ref><ref>{{cite journal|last=Hauser|first=H.|author2=Michl, B.|author3=Kubler, V.|author4= Schwarzkopf, S.|author5= Muller, C.|author6= Hermle, M.|author7= Blasi, B.|title=Nanoimprint Lithography for Honeycomb Texturing of Multicrystalline Silicon|journal=Energy Procedia|date=2011|volume=8|pages=648–653|doi=10.1016/j.egypro.2011.06.196|doi-access=free}}</ref> Incident light rays onto a textured surface do not reflect back out to the air as opposed to rays onto a flat surface. Rather some light rays are bounced back onto the other surface again due to the geometry of the surface. This process significantly improves light to electricity conversion efficiency, due to increased light absorption. This texture effect as well as the interaction with other interfaces in the PV module is a challenging optical simulation task. A particularly efficient method for modeling and optimization is the [[Physics:OPTOS formalism|OPTOS formalism]].<ref>{{Cite journal|last1=Tucher|first1=Nico|last2=Eisenlohr|first2=Johannes|last3=Gebrewold|first3=Habtamu|last4=Kiefel|first4=Peter|last5=Höhn|first5=Oliver|last6=Hauser|first6=Hubert|last7=Goldschmidt|first7=Jan Christoph|last8=Bläsi|first8=Benedikt|date=11 July 2016|title=Optical simulation of photovoltaic modules with multiple textured interfaces using the matrix-based formalism OPTOS|journal=Optics Express|volume=24|issue=14|pages=A1083–A1093|doi=10.1364/OE.24.0A1083|pmid=27410896|bibcode=2016OExpr..24A1083T|doi-access=free}}</ref> In 2012, researchers at MIT reported that c-Si films textured with nanoscale inverted pyramids could achieve light absorption comparable to 30 times thicker planar c-Si.<ref>{{cite journal|last=Mavrokefalos|first=Anastassios|author2=Han, Sang Eon. |author3=Yerci, Selcuk |author4= Branham, M.S.|author5= Chen, Gang. |title=Efficient Light Trapping in Inverted Nanopyramid Thin Crystalline Silicon Membranes for Solar Cell Applications|journal=Nano Letters|date=June 2012|volume=12|issue=6| pages=2792–2796|doi=10.1021/nl2045777|bibcode=2012NanoL..12.2792M |pmid=22612694|hdl=1721.1/86899|s2cid=18134294 |hdl-access=free}}</ref> In combination with anti-reflective coating, surface texturing technique can effectively trap light rays within a thin film silicon solar cell. Consequently, required thickness for solar cells decreases with the increased absorption of light rays.

=== Encapsulation ===
Solar cells are commonly encapsulated in a transparent polymeric resin to protect the delicate solar cell regions for coming into contact with moisture, dirt, ice, and other conditions expected either during operation or when used outdoors. The encapsulants are commonly made from [[Chemistry:Polyvinyl acetate|polyvinyl acetate]] or glass. Most encapsulants are uniform in structure and composition, which increases light collection owing to light trapping from total internal reflection of light within the resin. Research has been conducted into structuring the encapsulant to provide further collection of light. Such encapsulants have included roughened glass surfaces,<ref>{{cite book|doi=10.1109/PVSC.2010.5614568|chapter=Light management for reduction of bus bar and gridline shadowing in photovoltaic modules|title=2010 35th IEEE Photovoltaic Specialists Conference|pages=000979|year=2010|last1=Jaus|first1=J.|last2=Pantsar|first2=H.|last3=Eckert|first3=J.|last4=Duell|first4=M.|last5=Herfurth|first5=H.|last6=Doble|first6=D.|isbn=978-1-4244-5890-5|s2cid=30512545}}</ref> diffractive elements,<ref>{{Cite journal|last1=Mingareev|first1=I.|last2=Berlich|first2=R.|last3=Eichelkraut|first3=T. J.|last4=Herfurth|first4=H.|last5=Heinemann|first5=S.|last6=Richardson|first6=M. C.|date=6 June 2011|title=Diffractive optical elements utilized for efficiency enhancement of photovoltaic modules|journal=Optics Express|volume=19|issue=12|pages=11397–404|doi=10.1364/OE.19.011397|pmid=21716370|bibcode=2011OExpr..1911397M|url=https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=2671&context=facultybib2010|doi-access=free}}</ref> prism arrays,<ref>{{Cite journal|last1=Uematsu|first1=T|last2=Yazawa|first2=Y|last3=Miyamura|first3=Y|last4=Muramatsu|first4=S|last5=Ohtsuka|first5=H|last6=Tsutsui|first6=K|last7=Warabisako|first7=T|date=1 March 2001|title=Static concentrator photovoltaic module with prism array|journal=Solar Energy Materials and Solar Cells|series=PVSEC 11 – PART III|volume=67|issue=1–4|pages=415–423|doi=10.1016/S0927-0248(00)00310-X}}</ref> air prisms,<ref name="Chen-2016">{{Cite journal|last1=Chen|first1=Fu-hao|last2=Pathreeker|first2=Shreyas|last3=Kaur|first3=Jaspreet|last4=Hosein|first4=Ian D.|date=31 October 2016|title=Increasing light capture in silicon solar cells with encapsulants incorporating air prisms to reduce metallic contact losses|journal=Optics Express|volume=24|issue=22|pages=A1419–A1430|doi=10.1364/oe.24.0a1419|pmid=27828526|bibcode=2016OExpr..24A1419C|doi-access=free}}</ref> v-grooves,<ref>{{Cite journal|last1=Korech|first1=Omer|last2=Gordon|first2=Jeffrey M.|last3=Katz|first3=Eugene A.|last4=Feuermann|first4=Daniel|last5=Eisenberg|first5=Naftali|date=1 October 2007|title=Dielectric microconcentrators for efficiency enhancement in concentrator solar cells|journal=Optics Letters|volume=32|issue=19|pages=2789–91|doi=10.1364/OL.32.002789|pmid=17909574|bibcode=2007OptL...32.2789K}}</ref> diffuse elements, as well as multi-directional waveguide arrays.<ref>{{Cite book|last1=Hosein|first1=Ian D.|last2=Lin|first2=Hao|last3=Ponte|first3=Matthew R.|last4=Basker|first4=Dinesh K.|last5=Saravanamuttu|first5=Kalaichelvi|date=3 November 2013|title=Enhancing Solar Energy Light Capture with Multi-Directional Waveguide Lattices|doi=10.1364/OSE.2013.RM2D.2|journal=Renewable Energy and the Environment|pages=RM2D.2|isbn=978-1-55752-986-2}}</ref> Prism arrays show an overall 5% increase in the total solar energy conversion.<ref name="Chen-2016" /> Arrays of vertically aligned broadband waveguides provide a 10% increase at normal incidence, as well as wide-angle collection enhancement of up to 4%,<ref>{{Cite journal|last1=Biria|first1=Saeid|last2=Chen|first2=Fu Hao|last3=Pathreeker|first3=Shreyas|last4=Hosein|first4=Ian D.|date=22 December 2017|title=Polymer Encapsulants Incorporating Light-Guiding Architectures to Increase Optical Energy Conversion in Solar Cells|journal=Advanced Materials|volume=30|issue=8|pages=1705382|doi=10.1002/adma.201705382|pmid=29271510|s2cid=3368811 }}</ref> with optimized structures yielding up to a 20% increase in short circuit current.<ref>{{Cite journal|last1=Biria|first1=Saeid|last2=Chen|first2=Fu-Hao|last3=Hosein|first3=Ian D.|title=Enhanced Wide-Angle Energy Conversion Using Structure-Tunable Waveguide Arrays as Encapsulation Materials for Silicon Solar Cells|journal=Physica Status Solidi A|issue=2|pages=1800716|doi=10.1002/pssa.201800716|year=2019|volume=216|bibcode=2019PSSAR.21600716B|s2cid=125253775}}</ref> Active coatings that convert infrared light into visible light have shown a 30% increase.<ref>{{Cite journal|last1=Huang|first1=Zhiyuan|last2=Li|first2=Xin|last3=Mahboub|first3=Melika|last4=Hanson|first4=Kerry M.|last5=Nichols|first5=Valerie M.|last6=Le|first6=Hoang|last7=Tang|first7=Ming L.|last8=Bardeen|first8=Christopher J.|date=12 August 2015|title=Hybrid Molecule–Nanocrystal Photon Upconversion Across the Visible and Near-Infrared|journal=Nano Letters|volume=15|issue=8|pages=5552–5557|doi=10.1021/acs.nanolett.5b02130|pmid=26161875|bibcode=2015NanoL..15.5552H|doi-access=free|url=https://escholarship.org/content/qt1bb3z0sc/qt1bb3z0sc.pdf?t=ozic6b}}</ref> Nanoparticle coatings inducing plasmonic light scattering increase wide-angle conversion efficiency up to 3%. Optical structures have also been created in encapsulation materials to effectively "cloak" the metallic front contacts.<ref>{{Cite journal|last1=Schumann|first1=Martin F.|last2=Langenhorst|first2=Malte|last3=Smeets|first3=Michael|last4=Ding|first4=Kaining|last5=Paetzold|first5=Ulrich W.|last6=Wegener|first6=Martin|date=4 July 2017|title=All-Angle Invisibility Cloaking of Contact Fingers on Solar Cells by Refractive Free-Form Surfaces|journal=Advanced Optical Materials|volume=5|issue=17|pages=1700164|doi=10.1002/adom.201700164|s2cid=102931532 }}</ref><ref>{{Cite journal|date=1 August 2018|title=Freeform surface invisibility cloaking of interconnection lines in thin-film photovoltaic modules|journal=Solar Energy Materials and Solar Cells|volume=182|pages=294–301|doi=10.1016/j.solmat.2018.03.034|last1=Langenhorst|first1=Malte|last2=Schumann|first2=Martin F.|last3=Paetel|first3=Stefan|last4=Schmager|first4=Raphael|last5=Lemmer|first5=Uli|last6=Richards|first6=Bryce S.|last7=Wegener|first7=Martin|last8=Paetzold|first8=Ulrich W.|s2cid=102944355}}</ref>

===Autonomous maintenance===
Novel self-cleaning mechanisms for solar panels are being developed. For instance, in 2019 via [[Engineering:Etching (microfabrication)#Wet etching|wet-chemically etched]] [[Physics:Nanowire|nanowire]]s and a hydrophobic coating on the surface water droplets could remove 98% of dust particles, which may be especially relevant for applications in the desert.<ref name="phys-20191209">{{cite news |author=American Associates, Ben-Gurion University of the Negev |title=Researchers develop new method to remove dust on solar panels|url=https://phys.org/news/2019-12-method-solar-panels.html |date=9 December 2019 |work=[[Organization:Ben-Gurion University of the Negev|Ben-Gurion University of the Negev]] |access-date=3 January 2020 }}</ref><ref>{{cite journal |last1=Heckenthaler |first1=Tabea |last2=Sadhujan |first2=Sumesh |last3=Morgenstern |first3=Yakov |last4=Natarajan |first4=Prakash |last5=Bashouti |first5=Muhammad |last6=Kaufman |first6=Yair |title=Self-Cleaning Mechanism: Why Nanotexture and Hydrophobicity Matter |journal=Langmuir |date=3 December 2019 |volume=35 |issue=48 |pages=15526–15534 |doi=10.1021/acs.langmuir.9b01874 |pmid=31469282 |s2cid=201673096 |issn=0743-7463}}</ref>

== Manufacture ==
[[File:FX-77.JPG|thumb|upright|Early solar-powered calculator]]
Solar cells share some of the same processing and manufacturing techniques as other semiconductor devices. However, the strict requirements for cleanliness and quality control of semiconductor fabrication are more relaxed for solar cells, lowering costs.

[[Earth:Polycrystalline silicon|Polycrystalline silicon]] wafers are made by wire-sawing block-cast silicon ingots into 180 to 350 micrometer wafers. The wafers are usually lightly p-type-doped. A surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p–n junction a few hundred nanometers below the surface.

Anti-reflection coatings are then typically applied to increase the amount of light coupled into the solar cell. [[Chemistry:Silicon nitride|Silicon nitride]] has gradually replaced titanium dioxide as the preferred material, because of its excellent surface passivation qualities. It prevents carrier recombination at the cell surface. A layer several hundred nanometers thick is applied using [[Chemistry:Plasma-enhanced chemical vapor deposition|plasma-enhanced chemical vapor deposition]]. Some solar cells have textured front surfaces that, like anti-reflection coatings, increase the amount of light reaching the wafer. Such surfaces were first applied to single-crystal silicon, followed by multicrystalline silicon somewhat later.

A full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "bus bars" are screen-printed onto the front surface using a [[Chemistry:Silver|silver]] paste. This is an evolution of the so-called "wet" process for applying electrodes, first described in a US patent filed in 1981 by Bayer AG.<ref>Fitzky, Hans G. and Ebneth, Harold (24 May 1983) {{US patent|4385102}}, "Large-area photovoltaic cell"</ref> The rear contact is formed by screen-printing a metal paste, typically aluminium. Usually this contact covers the entire rear, though some designs employ a grid pattern. The paste is then fired at several hundred degrees Celsius to form metal electrodes in [[Physics:Ohmic contact|ohmic contact]] with the silicon. Some companies use an additional electroplating step to increase efficiency. After the metal contacts are made, the solar cells are interconnected by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of [[Physics:Tempered glass|tempered glass]] on the front, and a [[Chemistry:Polymer|polymer]] encapsulation on the back.

Different types of manufacturing and recycling partly determine how effective it is in decreasing emissions and having a positive environmental effect.<ref name="techrev"/> Such differences and effectiveness could be quantified<ref name="techrev"/> for production of the most optimal types of products for different purposes in different regions across time.

=== Manufacturers and certification ===
[[File:SolarCellProduction-E.PNG|thumb|upright=1.25|Solar cell production by region<ref name="pvnews">[http://www.greentechmedia.com/research/report/pv-news Pv News November 2012]. Greentech Media. Retrieved 3 June 2012.</ref>]]

[[Organization:National Renewable Energy Laboratory|National Renewable Energy Laboratory]] tests and validates solar technologies. Three reliable groups certify solar equipment: UL and IEEE (both U.S. standards) and [[Organization:International Electrotechnical Commission|IEC]].

Solar cells are manufactured in volume in Japan, Germany, China, Taiwan, Malaysia and the United States, whereas Europe, China, the U.S., and Japan have dominated (94% or more as of 2013) in installed systems.<ref name="iet.jrc.ec.europa.eu">Jäger-Waldau, Arnulf (September 2013) [http://iet.jrc.ec.europa.eu/remea/pv-status-report-2013 PV Status Report 2013] {{Webarchive|url=https://web.archive.org/web/20180609195200/https://iet.jrc.ec.europa.eu/remea/pv-status-report-2013 |date=9 June 2018 }}. European Commission, Joint Research Centre, Institute for Energy and Transport.</ref> Other nations are acquiring significant solar cell production capacity.

Global PV cell/module production increased by 10% in 2012 despite a 9% decline in solar energy investments according to the annual "PV Status Report" released by the European Commission's Joint Research Centre. Between 2009 and 2013 cell production has quadrupled.<ref name="iet.jrc.ec.europa.eu"/><ref>[http://ec.europa.eu/dgs/jrc/downloads/jrc_20130930_newsrelease_pv_status.pdf PV production grows despite a crisis-driven decline in investment]. European Commission, Brussels, 30 September 2013</ref><ref>[http://iet.jrc.ec.europa.eu/remea/pv-status-report-2013 PV Status Report 2013 | Renewable Energy Mapping and Monitoring in Europe and Africa (REMEA)] {{Webarchive|url=https://web.archive.org/web/20180609195200/https://iet.jrc.ec.europa.eu/remea/pv-status-report-2013 |date=9 June 2018 }}. Iet.jrc.ec.europa.eu (11 April 2014). Retrieved 20 April 2014.</ref>

==== China ====
Since 2013 China has been the world's leading installer of solar photovoltaics (PV).<ref name="iet.jrc.ec.europa.eu"/> As of September 2018, sixty percent of the world's solar photovoltaic modules were made in China.<ref>{{Cite web|url=https://www.bbc.com/future/article/20180822-why-china-is-transforming-the-worlds-solar-energy|title=How China's giant solar farms are transforming world energy|last=Baraniuk|first=Chris|website=www.bbc.com|language=en|access-date=2019-10-24}}</ref> As of May 2018, the largest photovoltaic plant in the world is located in the Tengger desert in China.<ref name="ieefa.org">{{Cite web|url=http://ieefa.org/ieefa-report-advances-in-solar-energy-accelerate-global-shift-in-electricity-generation/|title=IEEFA Report: Advances in Solar Energy Accelerate Global Shift in Electricity Generation|date=2018-05-21|website=Institute for Energy Economics & Financial Analysis|language=en-US|access-date=2019-10-24}}</ref> In 2018, China added more photovoltaic installed capacity (in GW) than the next 9 countries combined.<ref name="iea-pvps.org">{{Cite web |url=http://www.iea-pvps.org/fileadmin/dam/public/report/statistics/IEA-PVPS_T1_35_Snapshot2019-Report.pdf |title=2019 Snapshot of Global PV Markets |url-status=dead |archiveurl=https://web.archive.org/web/20190421164151/http://www.iea-pvps.org/fileadmin/dam/public/report/statistics/IEA-PVPS_T1_35_Snapshot2019-Report.pdf |archivedate=2019-04-21}}</ref> As of 2022, China's share in the manufacturing of solar panels exceeded 80% across all manufacturing stages.<ref>{{Cite web |title=Executive summary – Solar PV Global Supply Chains – Analysis |url=https://www.iea.org/reports/solar-pv-global-supply-chains/executive-summary |access-date=2022-12-12 |website=IEA |language=en-GB}}</ref>

==== Malaysia ====
{{main|Physics:Photovoltaics manufacturing in Malaysia}}
In 2014, Malaysia was the world's third largest manufacturer of [[Chemistry:Photovoltaics|photovoltaics]] equipment, behind {{wipe|China}} and the [[Social:European Union|European Union]].<ref name="NYT">{{cite news | url=https://www.nytimes.com/2014/12/12/business/energy-environment/solar-rises-in-malaysia-during-trade-wars-over-panels.html?_r=0 | title=Solar Rises in Malaysia During Trade Wars Over Panels|work={{wipe|The New York Times}}| date=12 December 2014}}</ref>

==== United States ====
Solar energy production in the U.S. has doubled from 2013 to 2019.<ref>{{Cite web|url=https://e360.yale.edu/digest/solar-energy-capacity-in-u-s-cities-has-doubled-in-the-last-6-years|title=Solar Energy Capacity in U.S. Cities Has Doubled in the Last 6 Years|website=Yale E360|language=en-US|access-date=2019-10-24}}</ref> This was driven first by the falling price of quality silicon,<ref>[http://cleantechnica.com/2013/03/07/plunging-cost-of-solar-pv-graphs/ Plunging Cost Of Solar PV (Graphs)]. CleanTechnica (7 March 2013). Retrieved 20 April 2014.</ref><ref>[http://www.downtoearth.org.in/content/falling-silicon-prices-shakes-solar-manufacturing-industry Falling silicon prices shakes up solar manufacturing industry]. Down To Earth (19 September 2011). Retrieved 20 April 2014.</ref><ref>{{Cite web|url=https://www.statista.com/statistics/301564/us-silicon-price-by-type/|title=Silicon price by type U.S. 2018|website=Statista|language=en|access-date=2019-10-24}}</ref> and later simply by the globally plunging cost of photovoltaic modules.<ref name="ieefa.org"/><ref>{{Cite web|url=https://news.energysage.com/solar-panel-efficiency-cost-over-time/|title=How Solar Panel Cost & Efficiency Change Over Time {{!}} EnergySage|date=2019-07-04|website=Solar News|language=en-US|access-date=2019-10-24}}</ref> In 2018, the U.S. added 10.8GW of installed solar photovoltaic energy, an increase of 21%.<ref name="iea-pvps.org"/>

'''Latin America''' : Latin America has emerged as a promising region for solar energy development in recent years, with over 10 GW of installations in 2020. The solar market in Latin America has been driven by abundant solar resources, falling costs, competitive auctions and growing electricity demand. Some of the leading countries for solar energy in Latin America are Brazil, Mexico, Chile and Argentina. However, the solar market in Latin America also faces some challenges, such as political instability, financing gaps and power transmission bottlenecks.

'''Middle East and Africa''' : The Middle East and Africa has also experienced significant growth in solar energy deployment in recent years, with over 8 GW installations in 2020. The solar market in the Middle East and Africa has been driven by the low-cost generation of solar energy, the diversification of energy sources, the fight against climate change and rural electrification are motivated. Some of the notable countries for solar energy in the Middle East and Africa are Saudi Arabia, United Arab Emirates, Egypt, Morocco and South Africa. However, the solar market in the Middle East and Africa also faces several obstacles, including social unrest, regulatory uncertainty and technical barriers.<ref>{{Cite web |date=2023-11-02 |title=Weltweite Solarenergiemärkte: Wachstum, Trends und Herausforderungen - Arbitrage Solar |url=https://arbitragesolar.de/weltweite-solarenergiemarkte-wachstum-trends-und-herausforderungen/ |access-date=2023-11-02 |language=en-US}}</ref>

=== Materials sourcing ===
Like many other energy generation technologies, the manufacture of solar cells, especially its rapid expansion, has many environmental and supply-chain implications. Global mining may adapt and potentially expand for sourcing the needed minerals which vary per type of solar cell.<ref>{{cite journal |last1=Herrington |first1=Richard |title=Mining our green future |journal=Nature Reviews Materials |date=June 2021 |volume=6 |issue=6 |pages=456–458 |doi=10.1038/s41578-021-00325-9 |bibcode=2021NatRM...6..456H |s2cid=235128115 |language=en |issn=2058-8437|doi-access=free }}</ref><ref>{{cite web |last1=Kemp |first1=Deanna |last2=Bainton |first2=Nick |title=More clean energy means more mines – we shouldn't sacrifice communities in the name of climate action |url=https://theconversation.com/more-clean-energy-means-more-mines-we-shouldnt-sacrifice-communities-in-the-name-of-climate-action-170938 |website=The Conversation |date=4 November 2021 |access-date=9 November 2021 |language=en}}</ref> Recycling solar panels could be a source for materials that would otherwise need to be mined.<ref name="techrev"/>

== Disposal ==
Solar cells degrade over time and lose their efficiency. Solar cells in extreme climates, such as desert or polar, are more prone to degradation due to exposure to harsh UV light and snow loads respectively.<ref>{{cite journal |last1=Jordan |first1=Dirk C. |last2= Kurtz |first2= Sarah R. |date= June 2012 |title= Photovoltaic Degradation Rates – An Analytical Review |url= https://www.nrel.gov/docs/fy12osti/51664.pdf |journal=Progress in Photovoltaics: Research and Applications |access-date= 6 March 2019}}</ref> Usually, solar panels are given a lifespan of 25–30 years before they get decommissioned.<ref>[https://news.energysage.com/how-long-do-solar-panels-last/ How long do solar panels last?]. CleanTechnica (4 February 2019). Retrieved 6 March 2019.</ref>

The International Renewable Energy Agency estimated that the amount of solar panel electronic waste generated in 2016 was 43,500–250,000 metric tons. This number is estimated to increase substantially by 2030, reaching an estimated waste volume of 60–78 million metric tons in 2050.<ref>[https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2016/IRENA_IEAPVPS_End-of-Life_Solar_PV_Panels_2016.pdf End-of-Life Management: Solar Photovoltaic Panels]. International Renewable Energy Agency (June 2016). Retrieved 6 March 2019.</ref>

=== Recycling ===
The most widely used solar cells in the market are crystalline solar cells. A product is truly recyclable if it can harvested again. In the 2016 Paris Agreement, 195 countries agreed to reduce their carbon emissions by shifting their focus away from fossil fuels and towards renewable energy sources. Owing to this, Solar will be a major contributor to electricity generation all over the world. So, there will be a plethora of solar panels to be recycled after the end of their life cycle. In fact, many researchers around the globe have voiced their concern about finding ways to use silicon cells after recycling.<ref>{{Cite journal |last1=Doi |first1=Takuya |last2=Tsuda |first2=Izumi |last3=Unagida |first3=Hiroaki |last4=Murata |first4=Akinobu |last5=Sakuta |first5=Koichi |last6=Kurokawa |first6=Kosuke |date=March 2001 |title=Experimental study on PV module recycling with organic solvent method |url=http://dx.doi.org/10.1016/s0927-0248(00)00308-1 |journal=Solar Energy Materials and Solar Cells |volume=67 |issue=1–4 |pages=397–403 |doi=10.1016/s0927-0248(00)00308-1 |issn=0927-0248}}</ref><ref>{{Cite book |last1=Yamashita |first1=Katsuya |last2=Miyazawa |first2=Akira |last3=Sannomiya |first3=Hitoshi |title=2006 IEEE 4th World Conference on Photovoltaic Energy Conference |chapter=Reserch and Development on Recycling and Reuse Treatment Technologies for Crystalline Silicon Photovoltaic Modules |date=2006 |pages=2254–2257 |chapter-url=http://dx.doi.org/10.1109/wcpec.2006.279621 |publisher=IEEE |doi=10.1109/wcpec.2006.279621|isbn=1-4244-0016-3 |s2cid=933430 }}</ref><ref>{{Cite journal |last1=Micheli |first1=Leonardo |last2=Fernández |first2=Eduardo F. |last3=Muller |first3=Matthew |last4=Smestad |first4=Greg P. |last5=Almonacid |first5=Florencia |date=August 2020 |title=Selection of optimal wavelengths for optical soiling modelling and detection in photovoltaic modules |url=http://dx.doi.org/10.1016/j.solmat.2020.110539 |journal=Solar Energy Materials and Solar Cells |volume=212 |pages=110539 |doi=10.1016/j.solmat.2020.110539 |s2cid=218900846 |issn=0927-0248|arxiv=2005.13020 }}</ref><ref>{{Cite book |last1=Eberspacher |first1=C. |last2=Fthenakis |first2=V.M.  |title=Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference - 1997 |chapter=Disposal and recycling of end-of-life PV modules |date=1997 |pages=1067–1072 |chapter-url=http://dx.doi.org/10.1109/pvsc.1997.654272 |publisher=IEEE |doi=10.1109/pvsc.1997.654272|isbn=0-7803-3767-0 |s2cid=118374147 }}</ref>

Additionally, these cells have hazardous elements/compounds, including [[Chemistry:Lead|lead]] (Pb), [[Chemistry:Cadmium|cadmium]] (Cd) or cadmium sulfide (CdS), selenium (Se), and barium (Ba) as dopants aside from the valuables silicon (Si), aluminum (Al), silver (Ag), and copper (Cu). The harmful elements/compounds if not disposed of with the proper technique can have severe harmful effects on human life and wildlife alike.<ref name="Rahman">{{Cite journal |last1=Rahman |first1=Md Mokhlesur |last2=Mateti |first2=Srikanth |last3=Sultana |first3=Irin |last4=Hou |first4=Chunping |last5=Falin |first5=Alexey |last6=Cizek |first6=Pavel |last7=Glushenkov |first7=Alexey M. |last8=Chen |first8=Ying |date=2021-05-05 |title=End-of-Life Photovoltaic Recycled Silicon: A Sustainable Circular Materials Source for Electronic Industries |journal=Advanced Energy and Sustainability Research |volume=2 |issue=11 |doi=10.1002/aesr.202100081 |s2cid=235568140 |issn=2699-9412|doi-access=free |hdl=10536/DRO/DU:30152718 |hdl-access=free }}</ref>

RECYCLING

There are various ways c-Si can be recycled. Mainly thermal and chemical separation methods are used. This happens in two stages<ref>{{Cite journal |last1=Klugmann-Radziemska |first1=Ewa |last2=Ostrowski |first2=Piotr |last3=Drabczyk |first3=Kazimierz |last4=Panek |first4=Piotr |last5=Szkodo |first5=Marek |date=December 2010 |title=Experimental validation of crystalline silicon solar cells recycling by thermal and chemical methods |url=http://dx.doi.org/10.1016/j.solmat.2010.07.025 |journal=Solar Energy Materials and Solar Cells |volume=94 |issue=12 |pages=2275–2282 |doi=10.1016/j.solmat.2010.07.025 |issn=0927-0248}}</ref>

* PV solar cell separation: in thermal delamination, the [[Chemistry:Ethylene-vinyl acetate|ethylene vinyl acetate]] (EVA) is removed and materials such as glass, Tedlar®, aluminium frame, steel, copper and plastics are separated;
* cleansing the surface of PV solar cells: unwanted layers (antireflection layer, metal coating and p–n semiconductor) are removed from the silicon solar cells separated from the PV modules; as a result, the silicon substrate, suitable for re-use, can be recovered.

CONVERSION

A research study was conducted by scientists to see how efficiently the solar panels were made from nanosilicon and nanosilicon/graphite hybrids.<ref name="Rahman"/> The experiment techniques consist of

1.    <u>Recovery of PV Cells from End-of-Life PV Module</u> – This is a patented technique where the solar panels are deconstructed and each material is cleaned separately.

2.    <u>Purification of Broken PV Cells</u> – 40 g of broken PV cells were placed in a glass bottle of 500ml which contained 20% KOH (potassium oxide). Heat treatment of this aqueous solution was done at 80 °C for 0.5 h. All Al metal and other impurities were dissolved in a 20% KOH solution, and the solid PV silicon was deposited as sediment. The solid PV was dried in a vacuum and 32 g of impurity-free PV recycled silicon was obtained.

3.    <u>Conversion of Purified PV Recycled Silicon into Nanosilicon and Nanosilicon/Graphite Hybrid Production</u> - A large-scale planetary ball mill (PULVERISETTE P5 5/4 classic line) was used. Impurity-free PV recycled cells/silicon were loaded inside a stainless-steel milling container together with five hardened steel balls (diameter of 25.4 mm). The sample was milled at a rotation speed of 160 rpm for 15 h at room temperature under an argon atmosphere of 300 kPa. During high-energy ball milling, particle size was reduced to nanometer level (<100 nm). The same process was used to produce a PV nano-Si/graphite hybrid except for commercial graphite powder (Product-282863, Sigma-Aldrich, powder <20 μm, synthetic) which was added with eight hardened steel balls. The mixture was milled at a rotation speed of 160 rpm for 20 h at room temperature under an argon atmosphere of 300 kPa. A hybrid of PV nano-Si/graphite with a weight ratio of 5 wt% PV nano-Si and 95 wt% graphite was obtained.

The obtained PV nano-Si/graphite electrode showed excellent cyclic stability with high-capacity retention even after long-term 600 cycles. These results proved that silicon can be easily converted into nano-Si/graphite hybrids and harvested into PV modules and can work with the same efficiency as a c-Si module.

CHALLENGES

There are a lot of different PV modules in the market which have different compositions. So, it is difficult to have a common PV cell breakdown process. Also, recyclers have to do quality control which is not possible if different PV modules have to be recycled. There are also various applications of pure Si outside of the Solar industry and the recyclers might be tempted to sell there if they get a higher value for the product.<ref>{{Cite book |last1=Tao |first1=Meng |last2=Fthenakis |first2=Vasilis |last3=Ebin |first3=Burcak |last4=Butler |first4=Evelyn |last5=Sinha |first5=Parikhit |last6=Corkish |first6=Richard |last7=Wambach |first7=Karsten |last8=Simon |first8=Ethan |title=2020 47th IEEE Photovoltaic Specialists Conference (PVSC) |chapter=Major Challenges and Opportunities in Silicon Solar Panel Recycling |date=2020-06-14 |pages=0292–0294 |chapter-url=http://dx.doi.org/10.1109/pvsc45281.2020.9300650 |publisher=IEEE |doi=10.1109/pvsc45281.2020.9300650|isbn=978-1-7281-6115-0 |s2cid=230994749 }}</ref>

Other questions that need to be answered are<ref>{{Cite journal |last1=Tao |first1=Meng |last2=Fthenakis |first2=Vasilis |last3=Ebin |first3=Burcak |last4=Steenari |first4=Britt-Marie |last5=Butler |first5=Evelyn |last6=Sinha |first6=Parikhit |last7=Corkish |first7=Richard |last8=Wambach |first8=Karsten |last9=Simon |first9=Ethan S. |date=2020-07-22 |title=Major challenges and opportunities in silicon solar module recycling |url=http://dx.doi.org/10.1002/pip.3316 |journal=Progress in Photovoltaics: Research and Applications |volume=28 |issue=10 |pages=1077–1088 |doi=10.1002/pip.3316 |s2cid=225438845 |issn=1062-7995}}</ref>

* To whom do the recyclers sell the recovered modules, components, and/or materials?
* What are the costs for different recycling scenarios?
* Location of recycling facilities?
* Would mobile recycling facilities make more sense over centralized ones?
* What infrastructure should be established for waste module collection?
* On the policy side, the main questions are the following:
* Who should pay for waste module recycling?

The First Solar panel recycling plant opened in Rousset, France in 2018. It was set to recycle 1300 tonnes of solar panel waste a year, and can increase its capacity to 4000 tonnes.<ref>If Solar Panels Are So Clean, Why Do They Produce So Much Toxic Waste?. Forbes (23 May 2018). Retrieved 6 March 2019.</ref><ref>[https://www.reuters.com/article/us-solar-recycling/europes-first-solar-panel-recycling-plant-opens-in-france-idUSKBN1JL28Z Europe's First Solar Panel Recycling Plant Opens in France]. Reuters (25 June 2018). Retrieved 6 March 2019.</ref><ref>[https://elecsome.com/ solar panel upcycling solutions in Australia]. Retrieved 30 November 2019.</ref> If recycling is driven only by market-based prices, rather than also environmental regulations, the economic incentives for recycling remain uncertain and as of 2021 the environmental impact of different types of developed recycling techniques still need to be quantified.<ref name="techrev" />

== See also ==
{{div col}}
* [[Physics:Anomalous photovoltaic effect|Anomalous photovoltaic effect]]
* [[Engineering:Autonomous building|Autonomous building]]
* [[Chemistry:Black silicon|Black silicon]]
* [[Physics:Electromotive force#Solar cell|Electromotive force (Solar cell)]]
* [[Physics:Energy development|Energy development]]
* [[Sustainable development]]
* Flexible substrate
* Green technology
* [[Physics:Hot spot (photovoltaics)|Hot spot (photovoltaics)]]
* [[Physics:Inkjet solar cell|Inkjet solar cell]]
* [[Physics:List of types of solar cells|List of types of solar cells]]
* [[Physics:Maximum power point tracking|Maximum power point tracking]]
* Metallurgical grade silicon
* [[Engineering:Microgeneration|Microgeneration]]
* Nanoflake
* [[Chemistry:Photovoltaics|Photovoltaics]]
* [[Physics:P–n junction|P–n junction]]
* [[Physics:Plasmonic solar cell|Plasmonic solar cell]]
* [[Engineering:Printed electronics|Printed electronics]]
* [[Engineering:Quantum efficiency|Quantum efficiency]]
* [[Engineering:Renewable energy|Renewable energy]]
* [[Physics:Roll-to-roll processing|Roll-to-roll processing]]
* Shockley-Queisser limit
* [[Physics:Solar cell research|Solar cell research]]
* ''[[Physics:Solar Energy Materials and Solar Cells|Solar Energy Materials and Solar Cells]]'' (journal)
* Solar module quality assurance
* Solar roof
* Solar shingles
* [[Physics:Solar tracker|Solar tracker]]
* [[Physics:Spectrophotometry|Spectrophotometry]]
* Standardization#Environmental protection
* [[Physics:Theory of solar cells|Theory of solar cells]]
* Thermophotovoltaics
* [[Engineering:Variable renewable energy|Variable renewable energy]]
{{div col end}}

== References ==
{{Reflist|30em}}

== Bibliography ==
{{refbegin}}
* {{cite book|url={{google books |plainurl=y |id=xHFK9cM77a8C |p=50}}|title=From space to Earth: the story of solar electricity|last=Perlin|first=John|publisher=Earthscan|year=1999|isbn=978-0-937948-14-9|page=50}}
{{refend}}

== External links ==
{{Commons|solar cell}}
* [https://web.archive.org/web/20131011055159/http://www.pvlighthouse.com.au/index.aspx PV Lighthouse Calculators and Resources for photovoltaic scientists and engineers]
* [http://www.pveducation.org/pvcdrom Photovoltaics CDROM online] {{Webarchive|url=https://web.archive.org/web/20140415091528/http://www.pveducation.org/pvcdrom |date=15 April 2014 }}
* [http://www.madehow.com/Volume-1/Solar-Cell.html Solar cell manufacturing techniques]
* {{curlie|Science/Technology/Energy/Renewable/Solar/|Renewable Energy: Solar}}
* [https://web.archive.org/web/20071221234020/http://www.soton.ac.uk/~solar/ Solar Energy Laboratory] at [[Organization:University of Southampton|University of Southampton]]
* [https://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/ NASA's Photovoltaic Info]
* {{Cite journal | doi = 10.1002/pip.1021| title = Solar cell efficiency tables (version 36)| year = 2010| last1 = Green | first1 = M. A. | last2 = Emery | first2 = K. | last3 = Hishikawa | first3 = Y. | last4 = Warta | first4 = W. | journal = Progress in Photovoltaics: Research and Applications| volume = 18| issue = 5| page = 346| doi-access = free }}
* [https://books.google.com/books?id=4OIDAAAAMBAJ&pg=-PA112 "Electric Energy From Sun Produced by Light Cell" ''Popular Mechanics'', July 1931] article on various 1930s research on solar cells
*{{cite journal |doi=10.1088/2515-7655/ab2338 |title=Emerging inorganic solar cell efficiency tables (Version 1) |date=2019 |last1=Wong |first1=Lydia H. |last2=Zakutayev |first2=Andriy |last3=Major |first3=Jonathan D. |last4=Hao |first4=Xiaojing |last5=Walsh |first5=Aron |last6=Todorov |first6=Teodor K. |last7=Saucedo |first7=Edgardo |journal=Journal of Physics: Energy |volume=1 |issue=3 |page=032001 |bibcode=2019JPEn....1c2001W |s2cid=250871748 |doi-access=free |hdl=10044/1/70500 |hdl-access=free }}

{{Galvanic cells}}


{{DEFAULTSORT:Solar cell}}
[[Category:Energy conversion]]
[[Category:Solar cells| ]]
[[Category:Energy harvesting]]
[[Category:American inventions]]
[[Category:Physical chemistry]]

{{Sourceattribution|Solar cell}}