View source for Physics:Solar cell efficiency

{{Short description|Ratio of energy extracted from sunlight in solar cells}}
<noinclude> </noinclude>
[[File:Best-research-cell-efficiencies-rev220126 pages-to-jpg-0001.jpg|thumb|Reported timeline of research solar cell energy conversion efficiencies since 1976 (National Renewable Energy Laboratory)|538x538px]]
'''Solar cell efficiency''' refers to the portion of energy in the form of sunlight that can be converted via photovoltaics into electricity by the [[Physics:Solar cell|solar cell]].

The efficiency of the solar cells used in a [[Physics:Photovoltaic system|photovoltaic system]], in combination with latitude and climate, determines the annual energy output of the system. For example, a solar panel with 20% efficiency and an area of 1&nbsp;m<sup>2</sup> will produce 200&nbsp;kWh/yr at Standard Test Conditions if exposed to the Standard Test Condition solar irradiance value of 1000&nbsp;W/m<sup>2</sup> for 2.74 hours a day. Usually solar panels are exposed to sunlight for longer than this in a given day, but the solar irradiance is less than 1000&nbsp;W/m<sup>2</sup> for most of the day. A solar panel can produce more when the sun is high in the sky and will produce less in cloudy conditions or when the sun is low in the sky. The sun is lower in the sky in the winter. 

In a high yield solar area like central Colorado, which receives annual [[Astronomy:Solar irradiance|insolation]] of 2000&nbsp;kWh/m<sup>2</sup>/year,<ref name="nrel.gov">{{cite web |url=http://www.nrel.gov/gis/images/map_pv_national_lo-res.jpg |title=Photovoltaic Solar Resource of the United States |author=Billy Roberts |date=20 October 2008 |website=[[Organization:National Renewable Energy Laboratory|National Renewable Energy Laboratory]] |access-date=17 April 2017}}</ref> such a panel can be expected to produce 400&nbsp;[[Kilowatt-hour|kWh]] of energy per year. However, in Michigan, which receives only 1400&nbsp;kWh/m<sup>2</sup>/year,<ref name="nrel.gov" /> annual energy yield will drop to 280&nbsp;kWh for the same panel. At more northerly European latitudes, yields are significantly lower: 175&nbsp;kWh annual energy yield in southern England under the same conditions.<ref>{{cite web|url=http://www.inference.org.uk/withouthotair/c6/page_40.shtml |title=Sustainable Energy - without the hot air |author=David J. C. MacKay |website=inference.org.uk |access-date=20 November 2017 |quote=Solar photovoltaics: data from a 25-m2 array in Cambridgeshire in 2006|}}</ref>

[[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.  The absorption and collection efficiencies of a solar cell depend on the design of transparent conductors and active layer thickness.<ref name=kumar2017 />]]

Several factors affect a cell's conversion efficiency value, including its [[Physics:Reflectance|reflectance]], [[Physics:Thermodynamic efficiency limit|thermodynamic efficiency]], [[Physics:Theory of solar cells#Charge carrier separation|charge carrier separation]] efficiency, charge carrier collection efficiency and [[Physics:Thermal conduction|conduction]] efficiency values.<ref name="Energy Efficiency and Renewable Energy">{{cite web|title=Photovoltaic Cell Conversion Efficiency Basics|url=http://energy.gov/eere/energybasics/articles/photovoltaic-cell-conversion-efficiency-basics|publisher=U.S. Department of Energy|access-date=6 September 2014}}</ref><ref name=kumar2017>{{Cite journal|last=Kumar|first=Ankush|date=2017-01-03|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> Because these parameters can be difficult to measure directly, other parameters are measured instead, including [[Engineering:Quantum efficiency|quantum efficiency]], [[Physics:Open-circuit voltage|open-circuit voltage]] (V<sub>OC</sub>) ratio, and {{Slink||Fill factor}}.  Reflectance losses are accounted for by the quantum efficiency value, as they affect "external quantum efficiency." Recombination losses are accounted for by the quantum efficiency, V<sub>OC</sub> ratio, and fill factor values. Resistive losses are predominantly accounted for by the fill factor value, but also contribute to the quantum efficiency and V<sub>OC</sub> ratio values.
In 2019, the world record for solar cell efficiency at 47.1% was achieved by using [[Physics:Multi-junction solar cell|multi-junction]] [[Physics:Concentrator photovoltaics|concentrator]] solar cells, developed at National Renewable Energy Laboratory, Golden, Colorado, USA.<ref>{{Cite journal |last1=Geisz |first1=John F. |last2=France |first2=Ryan M. |last3=Schulte |first3=Kevin L. |last4=Steiner |first4=Myles A. |last5=Norman |first5=Andrew G. |last6=Guthrey |first6=Harvey L. |last7=Young |first7=Matthew R. |last8=Song |first8=Tao |last9=Moriarty |first9=Thomas |date=April 2020 |title=Six-junction III–V solar cells with 47.1% conversion efficiency under 143 Suns concentration |url=http://www.nature.com/articles/s41560-020-0598-5 |journal=Nature Energy |language=en |volume=5 |issue=4 |pages=326–335 |doi=10.1038/s41560-020-0598-5 |osti=1659948 |s2cid=216289881 |issn=2058-7546}}</ref>

==Factors affecting energy conversion efficiency==
The factors affecting [[Physics:Energy conversion efficiency|energy conversion efficiency]] were expounded in a landmark paper by [[Biography:William Shockley|William Shockley]] and Hans Queisser in 1961.<ref name=shockley>{{cite journal | author1 = Shockley William | author2 = Queisser Hans J | year = 1961 | title = Detailed Balance Limit of Efficiency of p-n Junction Solar Cells | url = http://jap.aip.org/resource/1/japiau/v32/i3/p510_s1 | archive-url = https://archive.today/20130223070842/http://jap.aip.org/resource/1/japiau/v32/i3/p510_s1 | url-status = dead | archive-date = 2013-02-23 | journal = Journal of Applied Physics | volume = 32 | issue =  3| pages = 510–519 | doi = 10.1063/1.1736034 | bibcode = 1961JAP....32..510S }}</ref> See [[Physics:Shockley–Queisser limit|Shockley–Queisser limit]] for more detail.

===Thermodynamic efficiency limit and infinite-stack limit===
{{Main|Physics:Thermodynamic efficiency limit}}
[[File:ShockleyQueisserFullCurve.svg|thumb|The Shockley–Queisser limit for the efficiency of a single-junction solar cell under unconcentrated sunlight at 273 K. This calculated curve uses actual solar spectrum data, and therefore the curve is wiggly from IR absorption bands in the atmosphere. This efficiency limit of ~34% can be exceeded by multijunction solar cells.]]

If one has a source of heat at temperature {{mvar|T<sub>s</sub>}} and cooler heat sink at temperature {{mvar|T<sub>c</sub>}}, the maximum theoretically possible value for the ratio of work (or electric power) obtained to heat supplied is {{math|1-''T<sub>c</sub>''/''T<sub>s</sub>''}}, given by a [[Physics:Carnot heat engine|Carnot heat engine]]. If we take 6000&nbsp;K for the temperature of the sun and 300&nbsp;K for ambient conditions on earth, this comes to 95%. In 1981, Alexis de Vos and Herman Pauwels showed that this is achievable with a stack of an infinite number of cells with band gaps ranging from infinity (the first cells encountered by the incoming photons) to zero, with a voltage in each cell very close to the open-circuit voltage, equal to 95% of the band gap of that cell, and with 6000&nbsp;K blackbody radiation coming from all directions. However, the 95% efficiency thereby achieved means that the electric power is 95% of the '''''net''''' amount of light absorbed – the stack '''''emits''''' radiation as it has non-zero temperature, and this radiation has to be subtracted from the incoming radiation when calculating the amount of heat being transferred and the efficiency. They also considered the more relevant problem of maximizing the power output for a stack being illuminated from all directions by 6000&nbsp;K blackbody radiation. In this case, the voltages must be lowered to less than 95% of the band gap (the percentage is not constant over all the cells). The maximum theoretical efficiency calculated is 86.8% for a stack of an infinite number of cells, using the incoming concentrated sunlight radiation.<ref name=Vos80>{{cite journal | last1 = De Vos | first1 = A. | year = 1980| title = Detailed balance limit of the efficiency of tandem solar cells | journal = Journal of Physics D: Applied Physics | volume = 13 | issue = 5| pages = 839–846 | doi = 10.1088/0022-3727/13/5/018 | bibcode = 1980JPhD...13..839D }}</ref> When the incoming radiation comes only from an area of the sky the size of the sun, the efficiency limit drops to 68.7%.<ref>{{Cite journal|title=On the Thermodynamic Limit of Photovoltaic Energy Conversion|author1=A. De Vos  |author2=H. Pauwels |name-list-style=amp |doi=10.1007/BF00901283|journal=Appl. Phys.|volume=25|issue=2  |pages=119–125|year=1981 |bibcode = 1981ApPhy..25..119D |s2cid=119693148 }}</ref>

===Ultimate efficiency===
Normal photovoltaic systems however have only one [[Physics:P–n junction|p–n junction]] and are therefore subject to a lower efficiency limit, called the "ultimate efficiency" by Shockley and Queisser. Photons with an energy below the band gap of the absorber material cannot generate an [[Physics:Theory of solar cells#Photogeneration of charge carriers|electron-hole pair]], so their energy is not converted to useful output, and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through [[Physics:Phonon|phonon]] interactions as the kinetic energy of the carriers slows to equilibrium velocity. Traditional single-junction cells with an optimal [[Physics:Band gap|band gap]] for the solar spectrum have a maximum theoretical efficiency of 33.16%, the [[Physics:Shockley–Queisser limit|Shockley–Queisser limit]] .<ref>{{Cite journal|title = Tabulated Values of the Shockley–Queisser Limit for Single Junction Solar Cells|journal = Solar Energy|date = 2016-02-08|pages = 139–147|volume = 130|doi = 10.1016/j.solener.2016.02.015 |language = EN|first = Sven|last = Rühle|bibcode = 2016SoEn..130..139R}}</ref>

Solar cells with multiple band gap absorber materials improve efficiency by dividing the solar spectrum into smaller bins where the thermodynamic efficiency limit is higher for each bin.<ref>{{Cite journal|title=Limiting efficiencies for multiple energy-gap quantum devices|author1=Cheng-Hsiao Wu  |author2=Richard Williams |name-list-style=amp |doi=10.1063/1.331859|journal=J. Appl. Phys.|volume=54|issue=11  |page=6721 |year=1983|bibcode = 1983JAP....54.6721W }}</ref>

===Quantum efficiency===
{{Main|Engineering:Quantum efficiency}}
As described above, when a photon is absorbed by a solar cell it can produce an electron-hole pair. One of the carriers may reach the p–n junction and contribute to the current produced by the solar cell; such a carrier is said to be ''collected''. Or, the carriers [[Physics:Carrier generation and recombination|recombine]] with no net contribution to cell current.

Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers) when the cell is operated under short circuit conditions. The "external" quantum efficiency of a [[Chemistry:Silicon|silicon]] solar cell includes the effect of optical losses such as transmission and reflection.

In particular, some measures can be taken to reduce these losses. The reflection losses, which can account for up to 10% of the total incident energy, can be dramatically decreased using a technique called texturization, a light trapping method that modifies the average light path.<ref>{{cite journal | doi=10.1016/0927-0248(92)90126-A | volume=26 | issue=1–2 | title=The surface texturization of solar cells: A new method using V-grooves with controllable sidewall angles | journal=Solar Energy Materials and Solar Cells | pages=71–78| date=March 1992 | last1=Verlinden | first1=Pierre | last2=Evrard | first2=Olivier | last3=Mazy | first3=Emmanuel | last4=Crahay | first4=André }}</ref>

Quantum efficiency is most usefully expressed as a ''spectral'' measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others, spectral measurements of quantum efficiency can yield valuable information about the quality of the semiconductor bulk and surfaces. Quantum efficiency alone is not the same as overall energy conversion efficiency, as it does not convey information about the fraction of power that is converted by the solar cell.

===Maximum power point===
[[File:Dust on the a solar panel.jpg|thumb|250px|Dust often accumulates on the glass of solar modules - highlighted in this negative image as black dots - which reduces the amount of light admitted to the solar cells]]

A solar cell may operate over a wide range of [[Physics:Voltage|voltage]]s (V) and [[Physics:Current (electricity)|currents]] (I). By increasing the resistive load on an irradiated cell continuously from zero (a ''[[Physics:Short circuit|short circuit]]'') to a very high value (an ''open circuit'') one can determine the maximum power point, the point that maximizes V&times;I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. (The output power is zero in both the short circuit and open circuit extremes).

The maximum power point of a solar cell is affected by its temperature. Knowing the technical data of certain solar cell, its power output at a certain temperature can be obtained by <math>P(T)=P_{STC} + \frac{dP}{dT}(T_{cell} - T_{STC})</math>, where <math>P_{STC}</math> is the power generated at the standard testing condition; <math>T_{cell}</math> is the actual temperature of the solar cell.

A high quality, monocrystalline silicon solar cell, at 25&nbsp;°C cell temperature, may produce 0.60&nbsp;[[Volt|V]] open-circuit (''V<sub>OC</sub>''). The cell temperature in full sunlight, even with 25&nbsp;°C air temperature, will probably be close to 45&nbsp;°C, reducing the open-circuit voltage to 0.55&nbsp;V per cell. The voltage drops modestly, with this type of cell, until the short-circuit current is approached (''I<sub>SC</sub>''). Maximum power (with 45&nbsp;°C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0.43&nbsp;V in this case) and 90% of the short-circuit current. This output can be up to 70% of the ''V<sub>OC</sub> x I<sub>SC</sub>'' product. The short-circuit current (''I<sub>SC</sub>'') from a cell is nearly proportional to the illumination, while the open-circuit voltage (''V<sub>OC</sub>'') may drop only 10% with an 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2&nbsp;''V<sub>OC</sub>'' at 1/2&nbsp;''I<sub>SC</sub>''. The usable power output could thus drop from 70% of the ''V<sub>OC</sub> x I<sub>SC</sub>'' product to 50% or even as little as 25%. Vendors who rate their solar cell "power" only as ''V<sub>OC</sub> x I<sub>SC</sub>'', without giving load curves, can be seriously distorting their actual performance.

The maximum power point of a photovoltaic varies with incident illumination. For example, accumulation of dust on photovoltaic panels reduces the maximum power point.<ref>{{Cite journal|author=A. Molki|doi=10.1088/0031-9120/45/5/F03|title=Dust affects solar-cell efficiency|journal=Physics Education|volume=45|issue=5|pages=456–458|year=2010|bibcode = 2010PhyEd..45..456M }}</ref> For systems large enough to justify the extra expense, a maximum power point tracker tracks the instantaneous power by continually measuring the voltage and [[Physics:Electric current|current]] (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is ''always'' transferred, regardless of the variation in lighting.

===Fill factor===
Another defining term in the overall behaviour of a solar cell is the '''''fill factor''''' (''FF''). This factor is a measure of quality of a solar cell. This is the available ''power'' at the ''maximum power point'' (''P<sub>m</sub>'') divided by the ''open circuit voltage'' (''V<sub>OC</sub>'') and the ''short circuit current'' (''I<sub>SC</sub>''):

:<math>FF = \frac{P_{m}}{V_{OC} \times I_{SC}} = \frac{\eta \times A_c \times G}{V_{OC} \times I_{SC}}.</math>

The fill factor can be represented graphically by the IV sweep, where it is the ratio of the different rectangular areas.<ref>"Part II – Photovoltaic Cell I-V Characterization Theory and LabVIEW Analysis Code". Part II – Photovoltaic Cell I-V Characterization Theory and LabVIEW Analysis Code - National Instruments, 10 May 2012, ni.com/white-paper/7230/en/.</ref>

The fill factor is directly affected by the values of the cell's series, [[Engineering:Shunt (electrical)|shunt resistances]] and diodes losses. Increasing the shunt resistance (R<sub>sh</sub>) and decreasing the [[Physics:Equivalent series resistance|series resistance]] (R<sub>s</sub>) lead to a higher fill factor, thus resulting in greater efficiency, and bringing the cell's output power closer to its theoretical maximum.<ref name=nelson>{{Cite book|title = The Physics of Solar Cells|author = Jenny Nelson|publisher = Imperial College Press|year = 2003|isbn = 978-1-86094-340-9|url=http://www.solarfreaks.com/download/file.php?id=281}}</ref>

Typical fill factors range from 50% to 82%. The fill factor for a normal silicon PV cell is 80%.

== Comparison ==

{{Main|Chemistry:Photovoltaics}}

Energy conversion efficiency is measured by dividing the electrical output by the incident light power. Factors influencing output include spectral distribution, spatial distribution of power, temperature, and resistive load. [[Organization:International Electrotechnical Commission|IEC]] standard 61215 is used to compare the performance of cells and is designed around standard (terrestrial, temperate) temperature and conditions (STC): [[Physics:Irradiance|irradiance]] of 1&nbsp;kW/m<sup>2</sup>, a spectral distribution close to solar radiation through AM (airmass) of 1.5 and a cell temperature 25&nbsp;°C.  The resistive load is varied until the peak or maximum power point (MPP) is achieved.  The power at this point is recorded as [[Watt-peak]] (Wp).  The same standard is used for measuring the power and efficiency of PV modules.

Air mass affects output. In space, where there is no atmosphere, the spectrum of the sun is relatively unfiltered. However, on earth, air filters the incoming light, changing the solar spectrum. The filtering effect ranges from [[Physics:Air mass (solar energy)|Air Mass]] 0 (AM0) in space, to approximately Air Mass 1.5 on Earth. Multiplying the spectral differences by the quantum efficiency of the solar cell in question yields the efficiency. Terrestrial efficiencies typically are greater than space efficiencies. For example, a silicon solar cell in space might have an efficiency of 14% at AM0, but 16% on earth at AM 1.5. Note, however, that the number of incident photons in space is considerably larger, so the solar cell might produce considerably more power in space, despite the lower efficiency as indicated by reduced percentage of the total incident energy captured.

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 44.0% with multiple-junction production cells and 44.4% with multiple dies assembled into a hybrid package.<ref>{{cite web
| url = http://cleantechnica.com/2012/10/15/solar-junction-powers-up-cpv-with-new-conversion-efficiency-record/|title = Solar Junction Breaks Its Own CPV Conversion Efficiency Record | access-date = 2013-12-18 | date = 2013-12-18}}</ref><ref>{{cite web
| url = http://cleantechnica.com/2013/06/23/solar-cell-efficiency-world-record-set-by-sharp-44-4/
| title = Solar Cell Efficiency World Record Set By Sharp — 44.4% 
| date = 28 July 2013
| access-date = 28 July 2013
}}</ref> Solar cell energy conversion efficiencies for commercially available ''multicrystalline Si'' solar cells are around 14–19%.<ref>{{cite web
|url = http://publica.fraunhofer.de/documents/N-73521.html|title = Silicon Solar Cells with Screen-Printed Front Side Metallization Exceeding 19% Efficiency| year=2007 | last1=Schultz | first1=O. | last2=Mette | first2=A. | last3=Preu | first3=R. | last4=Glunz | first4=S. W. }}</ref> The highest efficiency cells have not always been the most economical – for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide produced at low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while delivering only about four times the output.

However, there is a way to "boost" solar power. By increasing the light intensity, typically photogenerated carriers are increased, increasing efficiency by up to 15%. These so-called "[[Physics:Concentrator photovoltaics|concentrator systems]]" have only begun to become cost-competitive as a result of the development of high efficiency GaAs cells. The increase in intensity is typically accomplished by using concentrating optics. A typical concentrator system may use a light intensity 6–400 times the sun, and increase the efficiency of a one sun GaAs cell from 31% at AM 1.5 to 35%.

A common method used to express economic costs is to calculate a price per delivered [[Kilowatt-hour|kilowatt-hour]] (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. Commercially available solar cells (as of 2006) reached system efficiencies between 5 and 19%.

Undoped crystalline silicon devices are approaching the theoretical limiting efficiency of 29.43%.<ref name="Richter2013">{{Cite journal|author1=A. Richter |author2=M. Hermle |author3=S.W. Glunz |title = Reassessment of the limiting efficiency for crystalline silicon solar cells|journal = IEEE Journal of Photovoltaics|volume = 3|issue = 4|pages = 1184–1191|date = October 2013|doi = 10.1109/JPHOTOV.2013.2270351|s2cid=6013813 }}</ref> In 2017, efficiency of 26.63% was achieved in an amorphous silicon/crystalline silicon heterojunction cell that place both positive and negative contacts on the back of the cell.<ref>{{Cite journal|author1=K. Yoshikawa |author2=H. Kawasaki |author3=W. Yoshida  |name-list-style=amp |title=Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26%|journal=Nature Energy|volume=2|issue=5 |pages=17032|year=2017|doi=10.1038/nenergy.2017.32|bibcode=2017NatEn...217032Y}}</ref><ref>{{cite web
| url = https://www.kaneka.co.jp/en/service/news/nr201708252/
|title = New World Record Established for Conversion Efficiency in a Crystalline Silicon Solar Cell
| access-date = 2018-03-15
| date = 2017-08-25}}</ref>

=== Energy payback ===


The energy payback time is defined as the recovery time required for generating the energy spent for manufacturing a modern photovoltaic module. In 2008, it was estimated to be from 1 to 4&nbsp;years<ref name="NRELPV">{{cite web|url=http://www.nrel.gov/docs/fy05osti/37322.pdf|title=What is the Energy Payback for PV?|date=December 2004|access-date=20 December 2008}}</ref><ref>{{Cite journal|author1=M. Ito |author2=K. Kato |author3=K. Komoto |title=A comparative study on cost and life-cycle analysis for 100 MW very large-scale PV (VLS-PV) systems in deserts using m-Si, a-Si, CdTe, and CIS modules|journal=Progress in Photovoltaics: Research and Applications| volume=16|pages=17–30| year=2008| doi=10.1002/pip.770|s2cid=97914857 |display-authors=0}}</ref> depending on the module type and location. With a typical lifetime of 20 to 30 years, this means that modern solar cells would be net energy producers, i.e., they would generate more energy over their lifetime than the energy expended in producing them.<ref name="NRELPV"/><ref>{{cite web|url=http://alpha.chem.umb.edu/chemistry/ch471/evans%20files/Net_Energy%20solar%20cells.pdf|title=Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells|access-date=2011-09-13}}</ref><ref>{{Cite journal| title=Can Solar Cells Ever Recapture the Energy Invested in their Manufacture?| first=Richard| last= Corkish| journal=Solar Progress| volume=18| issue= 2| pages= 16–17| year=1997| url=http://www.csudh.edu/oliver/smt310-handouts/solarpan/pvpayback.htm}}</ref> Generally, [[Engineering:Thin-film solar cell|thin-film]] technologies—despite having comparatively low conversion efficiencies—achieve significantly shorter energy payback times than conventional systems (often < 1&nbsp;year).<ref>{{Cite journal|author1=K. L. Chopra |author2=P. D. Paulson |author3=V. Dutta  |name-list-style=amp |title=Thin-film solar cells: An overview Progress in Photovoltaics|journal=Research and Applications|volume=12|issue=23 |pages=69–92|year=2004|doi=10.1002/pip.541|s2cid=39250492 }}</ref>

A study published in 2013 which the existing literature found that energy payback time was between 0.75 and 3.5 years with thin film cells being at the lower end and multi-si-cells having a payback time of 1.5–2.6 years.<ref>{{cite journal | last1 = Peng | first1 = Jinqing | last2 = Lu | first2 = Lin | last3 = Yang | first3 = Hongxing | year = 2013 | title = ''Review on lifecycle assessment of energy payback and greenhouse gas emission of solar photovoltaic systems'' | journal = Renewable and Sustainable Energy Reviews | volume = 19 | pages = 255–274 | doi = 10.1016/j.rser.2012.11.035 | hdl = 10397/34975 | hdl-access = free }}</ref> A 2015 review assessed the energy payback time and EROI of solar photovoltaics. In this meta study, which uses an insolation of 1,700&nbsp;kWh/m<sup>2</sup>/year and a system lifetime of 30 years, mean harmonized EROIs between 8.7 and 34.2 were found. Mean harmonized energy payback time varied from 1.0 to 4.1 years.<ref>{{cite journal | last1 = Bhandari | first1 = Khagendra P. | last2 = Jennifer | first2 = M.Collier | last3 = Ellingson | first3 = Randy J. | last4 = Apul | first4 = Defne S. | year = 2015 | title = ''Energy payback time (EPBT) and energy return on energy invested (EROI) of solar photovoltaic systems: A systematic review and meta-analysis'' | journal = Renewable and Sustainable Energy Reviews | volume = 47 | pages = 133–141 | doi = 10.1016/j.rser.2015.02.057 }}</ref> [[Chemistry:Crystalline silicon|Crystalline silicon]] devices achieve on average an energy payback period of 2 years.<ref name="NRELPV"/><ref name="sciencedaily-highest-efficiency">{{cite web|url= https://www.sciencedaily.com/releases/2008/10/081023100536.htm |title=Highest silicon solar cell efficiency ever reached |date=24 October 2008 |publisher=ScienceDaily |access-date=9 December 2009}}</ref>

Like any other technology, solar cell manufacture is dependent on the existence of a complex global industrial manufacturing system. This includes the fabrication systems typically accounted for in estimates of manufacturing energy; the contingent mining, refining and global transportation systems; and other energy intensive support systems including finance, information, and security systems. The difficulty in measuring such energy overhead confers some uncertainty on any estimate of payback times.<ref>Trainer, FE (2007) "Renewable Energy Cannot Sustain a Consumer Society"</ref>

== Technical methods of improving efficiency ==

=== Choosing optimum transparent conductor ===
The illuminated side of some types of solar cells, thin films, have a transparent conducting film to allow 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 indium tin oxide, conducting polymers or conducting nanowire networks are used for the purpose. There is a trade-off between high transmittance and electrical conductance, thus optimum density of conducting nanowires or conducting network structure should be chosen for high efficiency.<ref name="kumar2017"/>

=== Promoting light scattering in the visible spectrum ===
Lining the light-receiving surface of the cell with nano-sized metallic studs can substantially increase the cell efficiency. Light reflects off these studs at an oblique angle to the cell, increasing the length of the light path through the cell. This increases the number of photons absorbed by the cell and the amount of current generated.<ref name="Hindu Article">{{cite news|last1=Mukunth|first1=Vasudevan|title=Improving the efficiency of solar panels|url=http://www.thehindu.com/sci-tech/science/improving-the-efficiency-of-solar-panels/article5265330.ece|access-date=6 August 2016|work=The Hindu|date=24 October 2013}}</ref>

The main materials used for the nano-studs are [[Chemistry:Silver|silver]], [[Chemistry:Gold|gold]], and [[Chemistry:Aluminium|aluminium]]. Gold and silver are not very efficient, as they absorb much of the light in the visible spectrum, which contains most of the energy present in sunlight, reducing the amount of light reaching the cell.<ref name="Hindu Article"/> Aluminium absorbs only ultraviolet radiation, and reflects both visible and infra-red light, so energy loss is minimized. Aluminium can increase cell efficiency up to 22% (in lab conditions).<ref name="Scientific Reports Paper">{{cite journal|last1=Hylton|first1=Nicholas|last2=Li|first2=X. F|last3=Giannini|first3=K. H.|last4=Lee|first4=N. J|last5=Ekins-Daukes|first5=N. J.|last6=Loo|first6=J.|last7=Vercruysse|first7=D.|last8=Van Dorpe|first8=P.|last9=Sodabanlu|first9=H.|last10=Sugiyama|first10=M.|last11=Maier|first11=S. A.|title=Loss mitigation in plasmonic solar cells: aluminium nanoparticles for broadband photocurrent enhancements in GaAs photodiodes|journal=Scientific Reports|volume=3|pages=2874|date=7 October 2013|doi=10.1038/srep02874|pmid=24096686|pmc=3791440|bibcode=2013NatSR...3E2874H}}</ref>

=== Radiative cooling ===
An increase in solar cell temperature of approximately 1&nbsp;°C causes an efficiency decrease of about 0.45%. To prevent this, a transparent [[Chemistry:Silicon dioxide|silica]] crystal layer can be applied to solar panels. The silica layer acts as a [[Astronomy:Black body|thermal black body]] which emits heat as [[Physics:Infrared|infrared radiation]] into space, cooling the cell up to 13&nbsp;°C.<ref>{{Cite journal|last1=Zhu|first1=Linxiao|last2=Raman|first2=Aaswath P.|last3=Fan|first3=Shanhui|date=2015-10-06|title=Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody|journal=Proceedings of the National Academy of Sciences|language=en|volume=112|issue=40|pages=12282–12287|doi=10.1073/pnas.1509453112|issn=0027-8424|pmc=4603484|pmid=26392542|bibcode=2015PNAS..11212282Z|doi-access=free}}</ref>

=== Anti-reflective coatings and textures ===
Antireflective coatings could result in more destructive interference of incident light waves from the sun.<ref>Gee, Justin. "How to Make Solar Panels More Efficient in 2018 | EnergySage". EnergySage Solar News Feed, EnergySage, 19 Sept. 2017, news.energysage.com/how-to-make-solar-panels-more-efficient/.</ref> Therefore, all sunlight would be transmitted into the photovoltaic. Texturizing, in which the surface of a solar cell is altered so that the reflected light strikes the surface again, is another technique used to reduce reflection. These surfaces can be created by etching or using lithography. Adding a flat back surface in addition to texturizing the front surface helps to trap the light within the cell, thus providing a longer optical path.

=== Rear surface passivation ===

[[Engineering:Surface passivation|Surface passivation]] is critical to solar cell efficiency.<ref>{{cite book |last1=Black |first1=Lachlan E. |title=New Perspectives on Surface Passivation: Understanding the Si-Al2O3 Interface |date=2016 |publisher=Springer |isbn=9783319325217 |url=https://core.ac.uk/download/pdf/156698511.pdf}}</ref> Many improvements have been made to the front side of mass-produced solar cells, but the aluminium back-surface is impeding efficiency improvements.<ref>"Rear-Surface Passivation Technology for Crystalline Silicon Solar Cells: A Versatile Process for Mass Production". Ieee, IEEE, 2012, www.osapublishing.org/DirectPDFAccess/F1E0036E-C63D-5F6F-EA52FF38B5D1786D_270075/oe-21-S6-A1065.pdf?da=1&id=270075&seq=0&mobile=no.</ref> The efficiency of many solar cells has benefitted by creating so-called passivated emitter and rear cells (PERCs). The chemical deposition of a rear-surface dielectric passivation layer stack that is also made of a thin [[Chemistry:Silica|silica]] or [[Chemistry:Aluminium oxide|aluminium oxide]] film topped with a [[Chemistry:Silicon nitride|silicon nitride]] film helps to improve efficiency in [[Chemistry:Silicon|silicon]] solar cells. This helped increase cell efficiency for commercial [[Physics:Czochralski process#Production of Czochralski silicon|Cz-Si]] wafer material from just over 17% to over 21% by the mid-2010s,<ref>{{cite book |last1=Black |first1=Lachlan E. |title=New Perspectives on Surface Passivation: Understanding the Si-Al2O3 Interface |date=2016 |publisher=Springer |isbn=9783319325217 |pages=1–2 |url=https://core.ac.uk/download/pdf/156698511.pdf}}</ref> and the cell efficiency for quasi-mono-Si to a record 19.9%.

Concepts of the rear surface passivation for silicon solar cells has also been implemented for CIGS solar cells.<ref>{{cite journal |title=Employing Si solar cell technology to increase efficiency of ultra-thin Cu(In, Ga)Se2 solar cells |journal=Progress in Photovoltaics: Research and Applications |volume=22 |issue=10 |pages=1023–1029 |doi=10.1002/pip.2527|pmid=26300619 |pmc=4540152 |year=2014 |last1=Vermang |first1=Bart |last2=Wätjen |first2=Jörn Timo |last3=Fjällström |first3=Viktor |last4=Rostvall |first4=Fredrik |last5=Edoff |first5=Marika |last6=Kotipalli |first6=Ratan |last7=Henry |first7=Frederic |last8=Flandre |first8=Denis }}</ref> The rear surface passivation shows the potential to improve the efficiency. Al<sub>2</sub>O<sub>3</sub> and SiO<sub>2</sub> have been used as the passivation materials. Nano-sized point contacts on Al<sub>2</sub>O<sub>3</sub> layer<ref>{{cite journal |title=A morphological and electronic study of ultrathin rear passivated Cu(In, Ga) Se2 solar cells |journal=Thin Solid Films |volume=671 |pages=77–84 |doi=10.1016/j.tsf.2018.12.028 |bibcode=2019TSF...671...77B |year=2019 |last1=Bose |first1=S. |last2=Cunha |first2=J.M.V. |last3=Borme |first3=J. |last4=Chen |first4=W.C. |last5=Nilsson |first5=N.S. |last6=Teixeira |first6=J.P. |last7=Gaspar |first7=J. |last8=Leitão |first8=J.P. |last9=Edoff |first9=M. |last10=Fernandes |first10=P.A. |last11=Salomé |first11=P.M.P. |hdl=10773/30445 |s2cid=139582764 |url=https://zenodo.org/record/4479818 }}</ref> and line contacts on SiO2 layer<ref>{{cite journal |title=Optical Lithography Patterning of SiO2 Layers for Interface Passivation of Thin Film Solar Cells |journal=RRL Solar |volume=2 |issue=12 |pages=1800212 |doi=10.1002/solr.201800212 |year=2018 |last1=Bose |first1=Sourav |last2=Cunha |first2=José M. V. |last3=Suresh |first3=Sunil |last4=De Wild |first4=Jessica |last5=Lopes |first5=Tomás S. |last6=Barbosa |first6=João R. S. |last7=Silva |first7=Ricardo |last8=Borme |first8=Jérôme |last9=Fernandes |first9=Paulo A. |last10=Vermang |first10=Bart |last11=Salomé |first11=Pedro M. P. |hdl=10773/30564 |s2cid=139388117 |url=https://zenodo.org/record/1451691 }}</ref> provide the electrical connection of CIGS absorber to the rear electrode [[Chemistry:Molybdenum|Molybdenum]]. The point contacts on the Al<sub>2</sub>O<sub>3</sub> layer are created by e-beam lithography and the line contacts on the SiO<sub>2</sub> layer are created using [[Engineering:Photolithography|photolithography]]. Also, the implementation of the passivation layers does not change the morphology of the CIGS layers.

[[File:Tot heat flux z.png|thumb]]

=== Thin film materials ===
[[Physics:Thin film|Thin film]] materials show a lot of promise for solar cells in terms of low costs and adaptability to existing structures and frameworks in technology.<ref>Da, Yun, and Yimin Xuan. "Role of Surface Recombination in Affecting the Efficiency of Nanostructured Thin-Film Solar Cells .” Osapublishing, 2013, www.osapublishing.org/DirectPDFAccess/F1E0036E-C63D-5F6F-EA52FF38B5D1786D_270075/oe-21-S6-A1065</ref> Since the materials are so thin, they lack the optical absorption of bulk material solar cells. Attempts to correct this have been tried, more important is thin film surface recombination. Since this is the dominant recombination process of nanoscale thin-film solar cells, it is crucial to their efficiency. Adding a passivating thin layer of silicon dioxide could reduce recombination.

== See also ==


* Environmental impact of the energy industry
* [[Physics:Efficient energy use|Energy efficiency]]

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

==External links==
{{Commons|solar cell}}
* {{curlie|Science/Technology/Energy/Renewable/Solar/Solar_Electric/|Solar electric}}
*{{cite web|url=https://www.researchgate.net/publication/239441364|title=How Can We Increase the Efficiency of Solar Panels?}}
*{{cite web|url=https://solwiser.com/solar-panel-efficiency/|title=Factors That Affect Solar Panel Efficiency|date=18 July 2021 }}


[[Category:Solar cells]]
[[Category:Photovoltaics]]

{{Sourceattribution|Solar cell efficiency|1}}