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Concerns about the changing environment and fossil fuel depletion have prompted much controversyand scrutiny. One way to address these issues is to use concentrating photovoltaics (CPV) as analternate source for energy production. Multijunction solar cells built from III–V semiconductors arebeing evaluated globally in CPV systems designed to supplement electricity generation for utilitycompanies. The high efficiency of III–V multijunction concentrator cells, with demonstrated efficiencyover 40% since 2006, strongly reduces the cost of CPV systems, and makes III–V multijunction cellsthe technology of choice for most concentrator systems today. In designing multijunction cells,consideration must be given to the epitaxial growth of structures so that the lattice parameter betweenmaterial systems is compatible for enhancing device performance. Low resistance metal contacts arecrucial for attaining high performance. Optimization of the front metal grid pattern is required tomaximize light absorption and minimize I2R losses in the gridlines and the semiconductor sheet.Understanding how a multijunction device works is important for the design of next-generationhigh efficiency solar cells, which need to operate in the 45%–50% range for a CPV system to makebetter economical sense. However, the survivability of solar cells in the field is of chief concern,and accelerated tests must be conducted to assess the reliability of devices during operation inCPV systems. These topics are the focus of this review.1 IntroductionThe rising cost of producing electricity from fossil fuels isfavorably shifting energy production to renewable resourcessuch as solar photovoltaics (PV), for example.1Increasingadvances in solar cell efficiency and lowering cost endeavors,along with clean energy initiatives are the primary factors for theimpetus in solar energy production to date. In 2007, the worldproduction of solar cells reached 3.8 GW with Japan being theleading producer.2The rest of the world with rapidly increasingdevelopment capabilities to manufacture solar cells in massquantities contributed to over 70% of the total PV generated in2007. Market trends indicate that solar cell manufacturingcapacities have been climbing annually at an average greaterthan 40% over last 10 years with a 50% leap in 2007 compared toDr Cotal received his PhD inSolid State Physics fromOklahoma State University. Asa Postdoctoral Fellow he workedon InP-, GaAs- and Si-basedsolar cells at the Naval ResearchLaboratory. He leads Spec-trolab’s terrestrial developmentprograms for the design, fabri-cation and testing of III–Vmultijunction solar cells. Hisinterests are in device physics,device and heat transfermodeling, SMT of concentratorcells and terrestrial PV designsfor use in specialized applications. He won R&D 100 (2001 &2007) and Scientific American 50 (2002) recognition for hisoutstanding work in the development of the Triple JunctionTerrestrial Concentrator Solar Cell Technology.Dr Fetzer is manager of theMOVPE research and develop-ment group responsible for thedevelopment of metal–organicvapor phase epitaxial depositionprocesses for advanced solarcells and photonic devices. Hereceived his BS in Physics andPhD in Materials Science andEngineering from University ofUtah. As an Associate TechnicalFellow at the Boeing Companyhe has developed advancedepitaxial layers of III–Vcompound semiconductors.Before joining Spectrolab in 2001, Dr Fetzer performed researchunder Dr G. B. Stringfellow at the University of Utah exploring theuse of surfactant-mediated epitaxy to control ternary alloymicrostructure and the underlying surface physics.Spectrolab, Inc, Sylmar, California, 91342, USA174 |Energy Environ. Sci., 2009,2, 174–192This journal isªThe Royal Society of Chemistry 2009REVIEW www.rsc.org/ees| Energy & Environmental Sciencethe previous year. Ostensibly, growth in the PV industry isprojected to continue growing through 2010 including PVinstallations.Components such as modules, inverter, tracker, installationand other balance of systems (BOS) are what typically define aninstalled PV system. The burgeoning demand of global PVinstallations is driven by the factors mentioned above; however,not all solar cells produced each year have in the past beenconverted to installed PV power. The disparities between PVproduced and installed are due to the lack of more aggressivesolar Renewable Portfolio Standards, the deficiency of moremarket incentives, the lack of implementing product andbuilding regulations, and issues with the supply chain of opticalcomponents.3–7Despite this, global PV installations are rising. In2007, they rose by an average of 46% compared to 2006 witha finishing year-end total in the range of 2287–2826 MWaccording to various reports with one rendition evident inFig. 1.8–10Most of the power came from Europe, demonstratingdominance once again in this area. Over 1300 MW of this,however, was installed in Germany which is still holding a largesection of the industry’s market share for installed PV. Theglobal cumulative installed PV electricity in 2007 was over 9 GWwhich is still an insignificant amount when comparing to theestimated total power consumption in the world of 18 TW (1.8 1010kW).11,12The total cumulative power from installed, grid-tied, concentrating photovoltaics (CPV) in 2007 was over1.5 MW mostly based on single crystal silicon (Si) solar cells andmultijunction cells from III–V semiconductors.13–18Joseph Boisvert was born inMalden, Massachusetts. Hereceived his BS from theMassachusetts Institute ofTechnology in 1980 and MS andPhD degrees from the Univer-sity of Illinois at Urbana-Champaign in 1984. Dr Boisvertis presently a Senior Scientist/Engineer at Spectrolab, Inc.,a Boeing Company, where he isresponsible for developing III–Vphotodetectors and solar cells.He has 24 years of experience inthe research and development ofsolid-state photodetectors and solar cells, including device andprocess design, modeling, test and data analysis. Dr. Boisvert is anAssociate Technical Fellow of the Boeing Company.Dr Richard R. King is BoeingTechnical Fellow and PrincipalScientist responsible for Photo-voltaic Cell R&D at Spectrolab.He received his BS, MS andPhD degrees at StanfordUniversity. His photovoltaicsresearch over the last 20 yearsincludes work on III–V meta-morphic materials and devices,4-, 5-, and 6-junction cells, andminority-carrier recombinationat heterointerfaces. Dr King ledSpectrolab’s development ofhigh-efficiency III–V multi-junction cells, resulting in the first solar cells of any type to reachover 40% efficiency, and was recognized with R&D 100 awards in2001 and 2007, and a Scientific American 50 award in 2002. DrKing was inducted into the Space Technology Hall of Fame in2004, and has 11 patents and over 90 publications on photovoltaicsand device physics.Mr Hebert received his BS inMechanical Engineering and BAin Philosophy from the Univer-sity of Notre Dame, and hisMS in Mechanical Engineeringfrom Colorado State University.Mr Hebert performedmanufacturing energy efficiencyaudits at the Industrial Assess-ment Center at Colorado Statewhile performing his research on
high speed deposition of CdTe
solar cells. Mr Hebert is
a Senior Staff Engineer at
Spectrolab. He has nine years
experience of performing characterization and qualification testing
on multijunction space cells and has received four Hughes Tech-
nical Excellence Awards, a patent and a trade secret. Mr Hebert is
currently leading qualification of the first generation multijunction
terrestrial cells.
Dr N. H. Karam is Vice Presi-
dent of Advanced Technology
Products and has the overall
responsibility for Boeing -
Spectrolab’s internally and
externally funded R&D projects.
He has over 20 years of experi-
ence in materials research,
photovoltaic and optoelectronic
devices, and MOCVD reactor
design. He is currently actively
involved in the development of
advanced space solar cells
through Spectrolab’s jointly
funded Dual Use Science &
Technology program with the Air Force Research Laboratory. Dr
Karam and his Advanced Technology Products Team recently won
the R&D 100 and the Scientific American 50 awards for
contributions in the field of energy generation.
This journal is
ª
The Royal Society of Chemistry 2009
Energy Environ. Sci.
, 2009,
2
, 174–192 | 175
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