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Advances in High-Efficiency III-V Multijunction Solar Cells

Advances in High-Efficiency III-V Multijunction Solar Cells Hindawi Publishing Corporation Advances in OptoElectronics Volume 2007, Article ID 29523, 8 pages doi:10.1155/2007/29523 Research Article Advances in High-Efficiency III-V Multijunction Solar Cells Richard R. King, Daniel C. Law, Kenneth M. Edmondson, Christopher M. Fetzer, Geoffrey S. Kinsey, Hojun Yoon, Dimitri D. Krut, James H. Ermer, Raed A. Sherif, and Nasser H. Karam Spectrolab, Inc., 12500 Gladstone Avenue, Sylmar, CA 91342, USA Received 25 May 2007; Accepted 12 September 2007 Recommended by Armin G. Aberle The high efficiency of multijunction concentrator cells has the potential to revolutionize the cost structure of photovoltaic elec- tricity generation. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide- band-gap tunnel junctions capable of operating at high concentration, metamorphic buffers to transition from the substrate lattice constant to that of the epitaxial subcells, concentrator cell AR coating and grid design, and integration into 3-junction cells with current-matched subcells under the terrestrial spectrum have resulted in new heights in solar cell performance. A metamorphic Ga In P/Ga In As/ Ge 3-junction solar cell from this research has reached a record 40.7% efficiency at 240 suns, under 0.44 0.56 0.92 0.08 2 ◦ the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm ,25 C), and experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell developed to date. Solar cells with more junctions offer the potential for still higher efficiencies to be reached. Four-junction cells limited by radiative recombination can reach over 58% in principle, and practical 4-junction cell efficiencies over 46% are possible with the right combination of band gaps, taking into account series resistance and gridline shadowing. Many of the optimum band gaps for maximum energy conversion can be accessed with metamorphic semiconductor materials. The lower current in cells with 4 or more junctions, resulting in lower I R resistive power loss, is a particularly significant advan- tage in concentrator PV systems. Prototype 4-junction terrestrial concentrator cells have been grown by metal-organic vapor-phase epitaxy, with preliminary measured efficiency of 35.7% under the AM1.5 direct terrestrial solar spectrum at 256 suns. Copyright © 2007 Richard R. King et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION the National Renewable Energy Laboratory (NREL). Many of the high efficiency device structures developed in the exper- iments leading to these record performance cells have now In the past decade, terrestrial concentrator multijunction III- been incorporated in production III-V multijunction cells, V cells have embarked upon a remarkable ascent in solar increasing the average efficiency of these mass-produced so- conversion efficiency. The realization that very high conver- lar cells as well, while other experimental device improve- sion efficiencies can be achieved with advanced multijunc- ments will be implemented in production in the coming tion solar cells in practice, not just in theory, has prompted a months and years. This paper discusses the science behind resurgence of research in multijunction cells and commer- the 40.7% metamorphic and 40.1% lattice-matched cells, the cial interest in concentrator III-V photovoltaics. This pa- opportunity to reach new levels of photovoltaic (PV) system per discusses recent advances in multijunction cell research cost-effectiveness with production III-V concentrator cells that have led to experimental metamorphic (MM), or lattice- that make use of these advances, and possibilities for the next mismatched, solar cells with 40.7% efficiency under the con- generations of terrestrial concentrator cells with efficiencies centrated terrestrial spectrum [1, 2]. This is the first solar cell of 45%, or even 50%. to reach over 40% efficiency, and is the highest solar conver- sion efficiency yet achieved for any type of photovoltaic de- vice. Experimental lattice-matched (LM) cells have also bro- 2. METAMORPHIC SOLAR CELLS ken the 40% milestone, with 40.1% efficiency demonstrated for an LM 3-junction cell. Both of these cell-efficiency results Perhaps the essential distinguishing feature of III-V multi- have been independently verified by cell measurements at junction cells is the very wide range of subcell and device 2 Advances in OptoElectronics 3-junction Eg 1 /Eg 2 /0.67 eV cell efficiency 3-junction Eg 1 /Eg 2 /0.67 eV cell efficiency 2.1 2.1 2 ◦ 2 ◦ 240 suns (24.0W/cm ), AM1.5D (ASTM G173-03), 25 C 240 suns (24.0W/cm ), AM1.5D (ASTM G173-03), 25 C Ideal efficiency – radiative recombination limit Series resistance and grid shadowing included 2 2 LM LM 1.9 1.9 MM MM 40.7% 40.7% 40.1% 40.1% 1.8 1.8 49% 54% 1.7 1.7 52% 47% 1.6 1.6 50% 45% 48% 43% 1.5 1.5 46% 41% 39% 44% 37% 1.4 1.4 35% 42% 40% 38% 1.3 1.3 11.11.21.31.41.51.6 11.11.21.31.41.51.6 E = subcell 2 bandgap (eV) E = subcell 2 bandgap (eV) g 2 g 2 (a) (b) 3-junction E /E /0.67 eV cell efficiency g 1 g 2 2.1 2 ◦ 240 suns (24.0W/cm ), AM1.5D (ASTM G173-03), 25 C Normalized to experimental 3J cell V and J oc sc LM 1.9 MM 40.7% 40.1% 1.8 42% 1.7 40% 1.6 38% 1.5 36% 34% 1.4 32% 30% 1.3 11.11.21.31.41.51.6 E = subcell 2 bandgap (eV) g 2 Disordered GaInP top subcell Ordered GaInP top subcell (c) Figure 1: Calculated iso-efficiency contours for 3-junction terrestrial concentrator cells with variable top and middle subcell band gaps for the terrestrial solar spectrum at 240 suns: (a) theoretical efficiency based on radiative recombination [1]; (b) including the effects of grid resistance and shadowing using the metal grid design of the record 40.7%-efficient cell; and (c) additionally including empirically determined average quantum efficiency of 0.925, and 3-junction cell V 233 mV lower than the ideal voltage based on radiative recombination alone, oc giving an experimentally grounded prediction of practical, state-of-the-art, 3J cell efficiencies, as a function of subcell E . Subcell 1 and 2 band gap pairs of GaInP and GaInAs at the same lattice constant are shown for both disordered and ordered GaInP. The measured efficiencies and band gap combinations for the record 40.7% MM and 40.1% LM cells are plotted, at 240 and 135 suns, respectively, showing the theoretical advantage of the metamorphic design, now realized in practice. structure band gaps that can be grown with high crystal dle (subcell 2) band gap E [1]. Figure 1(a) plots contours g 2 quality, and correspondingly high minority-carrier recombi- of ideal efficiency based on the diode characteristics of sub- nation lifetimes. This is true for lattice-matched multijunc- cells limited only by the fundamental mechanism of radiative tion cells, but the flexibility in band gap selection takes on recombination, and on the shape of the terrestrial solar spec- a whole new dimension when metamorphic semiconductors trum. The cell model is discussed in greater detail in [10]. are used, providing freedom from the constraint that all sub- Efficiencies up to 54% can be seen to be possible in princi- cells must have the same crystal lattice constant. The area of ple at this concentration for 3-junction cells in the radiative metamorphic solar cell materials has attracted interest from recombination limit, increasing to over 58% for 4-junction photovoltaic research groups around the globe [1–11]. terrestrial concentrator cells [10]. The theoretical benefits of flexibility in subcell band gap In 3-junction GaInP/GaInAs/Ge metamorphic solar selection are made apparent in Figure 1(a), which plots iso- cells, the GaInP and GaInAs subcells can be grown on a efficiency contours for 3-junction terrestrial concentrator metamorphic buffer such that these two subcells are lattice- cells as a function of top (subcell 1) band gap E and mid- matched to each other, but are both lattice-mismatched to g 1 E = subcell 1 (top) bandgap (eV) g 1 E = subcell 1 (top) bandgap (eV) g 1 E = subcell 1 (top) bandgap (eV) g 1 Richard R. King et al. 3 Contact Contact AR n -GaInAs AR n -Ga(In)As n-AlInP window n-GaInP emitter n-AlInP window n-GaInP emitter p-GaInP base GaInP top cell p-GaInP base p-AlGaInP BSF ++ p-AlGaInP BSF p -TJ ++ Wide-bandgap ++ n -TJ p -TJ tunnel junction ++ n-GaInP window n -TJ n-GaInAs emitter n-GaInP window n-Ga(In)As emitter Ga(In)As p-GaInAs base middle cell p-Ga(In)As base p-GaInP BSF p-GaInP BSF p-GaInAs ++ p -TJ Tunnel junction Step-graded ++ n -TJ Buffer ++ p -TJ Buffer region n-Ga(In)As buffer ++ n -TJ Nucleation Nucleation n -Ge emitter n -Ge emitter p-Ge base and substrate Ge bottom cell p-Ge base and substrate Contact Contact Lattice-mismatched Lattice-matched (LM) or metamorphic (MM) Figure 2: Schematic cross-sectional diagrams of lattice-matched (LM) and metamorphic (MM) GaInP/GaInAs/Ge 3-junction cell configu- rations, corresponding to the LM 40.1% and MM 40.7%-efficient concentrator cells. the Ge growth substrate and subcell. The band gap combi- cluded, essentially identical to that measured experimentally nations that are possible with GaInP and GaInAs subcells at for the 40.7% cell at 240 suns. same lattice constant, but with varying lattice mismatch to In Figure 1(c), additional real-life effects are included by the Ge substrate, are shown in Figure 1(a). The cases with using empirical values for the active-area external quantum a disordered group-III sublattice in the GaInP subcell, giv- efficiency (EQE), and for the decrease in 3-junction cell V oc ing higher band gap at the same GaInP composition, and from Shockley-Read-Hall (SRH) recombination in addition with an ordered (low E ) group-III sublattice in the GaInP to radiative recombination. The record 40.7%-efficiency 3- subcell, are both plotted. Metamorphic cells can be seen to junction MM cell has an average active-area external quan- bring the cell design closer to the region of E , E space that tum efficiency of 0.925, and actual V that is 233 mV lower g 1 g 2 oc has the highest theoretical efficiencies. The lower band gaps than the ideal V in the radiative limit. This is equivalent oc of MM subcells can use a larger part of the solar spectrum, to 78 mV per subcell on average, though since the GaInAs that is wasted as excess photogenerated current in the Ge bot- middle subcell V is often close to the radiative limit, the oc tom cell in most lattice-matched 3-junction cells. In the past, difference between actual V and ideal radiative V is more oc oc recombination at dislocations in MM materials have often heavily distributed in the top and bottom subcells. With the thwarted this promise of higher theoretical efficiency. How- addition of these last real-life effects, the calculated contours ever, for the recent metamorphic 40.7%-efficient and lattice- in Figure 1(c) show a good estimate of the efficiencies that matched 40.1%-efficient cell results, plotted in Figure 1, the can be achieved in practical, state-of-the-art, 3-junction cells density and activity of dislocations have been controlled suf- as a function of band gap. The measured efficiencies of the ficiently to show the efficiency advantage of the MM design, plotted 40.7% MM and 40.1% LM record cells correspond to not just theoretically but now also experimentally. the efficiency contours in Figure 1(c), but are also plotted in Figures 1(b) and 1(c) take this analysis a bit farther. The Figures 1(a) and 1(b) for reference. It should be noted that efficiency contours in Figure 1(b) take into account the shad- unlike Figure 1(a), the present, state-of-the-art, practical ef- owing and specific series resistance associated with the metal ficiencies of the contours in Figure 1(c) are not fundamental grid pattern used on the 40.7% record cell. The fill factor cal- limits, and can be made higher by finding ways to reduce the culated for the 3-junction cell with the band gap combina- nonfundamental EQE and V losses that have been included oc tion of the MM 40.7% cell is 87.5% with series resistance in- in Figure 1(c). Tunnel junction Bottom cell Bottom cell Top cell Top cell Wide-E tunnel Middle cell Tunnel junction -E Wide tunnel Middle cell 4 Advances in OptoElectronics 8880 HIPSS PV Performance Characterization Team 1.2 Ge Spectrolab Metamorphic 0.8 GaInP/GaInAs/Ge cell Graded V = 2.911 V oc buffer 8820 2 J = 3.832 A/cm 0.6 sc FF = 87.50% V = 2.589 V mp Ga In As MC 8800 0.4 0.92 0.08 Efficiency = 40.7% ± 2.4% GaInP TC 240 suns (24.0W/cm ) intensity 0.2 (115) 2 0.2669 cm designated area glancing ◦ 25 ± 1 C, AM1.5D, low-AOD spectrum exit XRD 00.51 1.52 2.53 −2500 −2480 −2460 −2440 −2420 −1 ˚ Voltage (V) Qx (tilt) A Line of 100% Figure 5: Illuminated I-V curve for the record 40.7% metamor- relaxation Line of 0% phic 3-junction cell, independently verified at NREL. This is the first relaxation photovoltaic cell of any type to reach over 40% solar conversion ef- Figure 3: High-resolution X-ray diffraction reciprocal space map of ficiency. a metamorphic 3-junction cell structure, showing a metamorphic buffer with almost no residual strain, and a GaInP top cell that is pseudomorphic with respect to the Ga In As middle cell. 0.92 0.08 ciprocal space map (RSM) shown in Figure 3.The buffer can be seen to be nearly 100% relaxed, with very little residual strain to drive the formation of dislocations in the active up- 100 100 per subcells. 90 90 The shift in the quantum efficiency of the 3 subcells in 80 80 GaInP/GaInAs/Ge 3-junction cells, as a result of the higher 70 70 indium composition and lower band gap of the metamor- 60 60 phic GaInP and GaInAs subcells, is shown in Figure 4 [1]. In 50 50 this way the MM cells are able to capture some of the current 40 40 density that would otherwise be wasted in the Ge subcell. The 30 30 quantum efficiencies are overlaid on the AM0, and terrestrial AM1.5G and AM1.5D, low-AOD solar spectra, to show the 20 20 current densities available in the response range of each sub- 10 10 cell. 0 0 300 500 700 900 1100 1300 1500 1700 1900 Wavelength (nm) 3. HIGH-EFFICIENCY MULTIJUNCTION CELLS AM1.5D, low-AOD EQE, lattice-matched Band gap engineering of subcells in 3-junction solar cells, AM1.5G, ASTM G173-03 EQE, metamorphic made possible by metamorphic semiconductor materials, has AM0, ASTM E490-00a now resulted in higher measured efficiencies for metamor- Figure 4: External quantum efficiency for GaInP, GaInAs, and Ge phic cells than in even the best lattice-matched cells. Exper- subcells of LM and MM 3-junction cells, showing extension of the iments on step-graded buffers, used to transition from the lower-E MM GaInP and GaInAs responses to longer wavelengths, substrate to the subcell lattice constant, have been used to allowing them to use more of the solar spectrum [1]. control the classic problem of dislocations in the active cell regions due to the lattice mismatch. The band gap-voltage offset (E /q) − V is a key indicator of the quality and sup- g oc Schematic diagrams of LM and MM cells are shown in pression of SRH recombination in semiconductors of vari- Figure 2, showing the step-graded metamorphic buffer used able band gap, where lower offset values are desired, since in the MM case to transition from the lattice constant of the it is a measure of the separation between electron and hole substrate to that of the upper subcells. The lattice constants quasi-Fermi levels and the conduction and valence band and strain in the various MM 3-junction cell layers are im- edges [8–10]. Metamorphic 8%-In GaInAs single-junction aged in the high-resolution X-ray diffraction (HRXRD) re- cellswerebuilt andtestedwithaband gap-voltage offset Current density per unit −1 2 Qy (strain) A wavelength (mA/(cm μm)) External quantum efficiency (%) Current (A) Richard R. King et al. 5 0.8 1.16 have the highest rate of increase. These high III-V cell effi- ciencies have translated to concentrator PV module efficien- 1.14 0.7 cies over 30%, more than double the ∼15% module efficien- 1.12 0.6 cies that are more typical of flat-plate silicon modules. This high efficiency is extremely leveraging for PV system eco- 0.1 0.5 nomics [12], as it reduces all area-related costs of the module. 0.08 0.4 Production multijunction concentrator cells with efficiency 0.06 0.3 in the 40% range could cause the market growth for concen- trator PV to explode, with multi-GW/year production levels. 0.04 0.2 In Figure 8, the measured efficiency, V ,and fillfactor oc 0.02 0.1 are plotted as a function of incident intensity, or concentra- 0 0 tion ratio, for the record 40.7% MM and 40.1% LM cells, as 00.51 1.52 2.53 well as for an additional MM cell with good performance at Voltage (V) high intensities. It is interesting to note that the efficiencies Concentrator cells 1-sun cells of both the record MM and LM cells track very closely at the Latt.-matched Metamorphic Latt.-matched Metamorphic same concentration, but the measurements were able to be V 3.054 2.911 V oc 2.622 2.392 V extended to a higher concentration for the MM cell. Fill fac- J /inten. sc 0.1492 0.1596 A/W 0.1437 0.1599 A/W tors for both types of cell are quite high at about 88% in the mp 2.755 2.055 V 2.589 V 2.301 0.819 FF 0.881 0.875 0.850 100–200 sun range. The open-circuit voltage V increases at oc 1.0 suns 1.0 Conc. 135 240 suns rates of approximately 210 mV/decade and 190 mV/decade 4.0 4.0cm Area 0.2547 0.267 cm 32.0% for the MM and LM record cells, respectively, in the 100–200 Eff. 40.1% 40.7% 31.3% ◦ suns range. Thus the MM subcells increase in voltage some- Total area efficiency at 25 C Designated area efficiency at 25 C AM1.5D, low-AOD spectrum AM1.5G 1 sun = 0.100 W/cm what more rapidly with excess carrier concentration than in the LM case, as one would expect if defects in the MM mate- Figure 6: Comparison of the light I-V characteristics of the 40.1% rials are becoming less active at mediating recombination at lattice-matched and 40.7% metamorphic 3-junction concentrator higher injection levels. From the slopes of V versus concen- oc cells, and earlier record one-sun cells [1]. The higher current and tration (current density) we can extract values of the diode lower voltage of the metamorphic design is evident. ideality factor n. Subtracting off the 59 mV/decade increase for the Ge subcell, which has diode ideality factor very close to unity, gives an average n for the upper two subcells of 1.26 of 0.42 V at one sun, essentially the same as GaAs control in the MM case and 1.10 in the LM case in the same con- cells, reflecting the long minority-carrier lifetimes that can centration range, with decreasing n as the incident intensity be achieved in metamorphic materials. increases. An extensive experimental campaign was carried out on GaInP/GaInAs/Ge terrestrial concentrator cells, using a vari- 4. FOUR-JUNCTION SOLAR CELLS ety of metamorphic and lattice-matched 3-junction cell con- figurations, wide-band-gap tunnel junctions and other high- A 4-junction (Al)GaInP/ AlGa(In)As/ Ga(In)As/ Ge terres- efficiency semiconductor device structures, current matching trial concentrator solar cell [10] is shown in Figure 9,where conditions, cell sizes, grid patterns, and fabrication processes, the parentheses indicate optional elements in the subcell resulting in new understanding of the limiting mechanisms composition. This type of cell divides the photon flux avail- of terrestrial multijunction cells, and new heights in perfor- able in the terrestrial solar spectrum above the band gap of mance. Figure 5 plots the measured illuminated I-V curve for the GaInAs subcell 3 into 3 pieces, rather than 2 pieces in the record efficiency 40.7% metamorphic GaInP/GaInAs/Ge the case of a 3-junction cell. As a result, the current density 3-junction cell at 240 suns [1], under the standard spectrum of a 4-junction cell is roughly 2/3 that of a corresponding 2 2 for concentrator solar cells (AM1.5D, low-AOD, 24.0 W/cm , 3-junction cell, and the I R resistive power loss is approxi- 25 C). This is the first solar cell to reach over 40% efficiency, mately (2/3) = 4/9, or less than half that of a 3-junction cell. and is the highest solar conversion efficiency yet achieved for Figure 9 shows a lattice-matched 4-junction cell, with all the any type of photovoltaic device. A lattice-matched 3-junction subcells at the lattice constant of the Ge substrate, but lattice- cell has also achieved over 40% efficiency, with 40.1% mea- mismatched versions of the 4-junction cell are also possible, 2 ◦ sured at 135 suns (AM1.5D, low-AOD, 13.5 W/cm ,25 C). giving greater flexibility in bandgap selection. These efficiencies have been independently verified by mea- Iso-efficiency contours for 4-junction terrestrial concen- surements at the National Renewable Energy Laboratory trator cells, under the AM1.5D (ASTM G173-03) solar spec- (NREL). Light I-V characteristics of both the record MM and trum at 500 suns (50.0 W/cm ), are plotted in Figure 10 as LM devices are compared in Figure 6 [1]. a function of the band gaps of subcells 2 and 3. Ideal 4- The highest cell efficiencies from a number of pho- junction cell efficiency is plotted in Figure 10(a),and prac- tovoltaic technologies by year since 1975 are plotted in tical cell efficiency, consistent with the measured 3J cell effi- Figure 7, showing the most recent 40.7%-efficient cell result. ciency, in Figure 10(b). The band gap of subcell 1 is held at It is interesting to note that III-V multijunction concentrator 1.9 eV, corresponding to GaInP at the Ge lattice constant with cells not only are the highest efficiency technology, but also a disordered group-III sublattice, and subcell 4 (the bottom Current density/incident intensity (A/W) Current density/incident intensity (A/W) 6 Advances in OptoElectronics 40.7% Best research-cell efficiencies 40% Boeing- Boeing- Spectrolab Spectrolab NREL 36 (metamorphic) (inverted, Spectrolab semi-mismatched) Japan Energy NREL/ NREL Spectrolab NREL UNSW UNSW Spire UNSW 24 NREL UNSW Cu(In, Ga)Se UNSW Stanford Georgia 14x concentration Spire FhG-ISE UNSW Tech Georgia 20 ARCO Sharp Tech Varian NREL NREL NREL Westing. Sharp NREL NREL house (large area) Univ. Stuttgart University No. Carolina NREL So. Florida AstroPower (45 μm thin-film State Univ. NREL NREL (small area) transfer) NREL ARCO Euro-ClS Solarex Kodak (CdTe/ClS) Boeing United Solar Boeing EPFL Photon AMETEK Energy Matsushita Kaneka Boeing Solarex United Solar Monosolar Kodak (2 μm on glass) RCA NREL Konarka Groningen Univ. Linz Boeing University EPFL 4 of Maine University RCA Siemens RCA RCA Linz RCA RCA University RCA Linz 1975 1980 1985 1990 1995 2000 2005 2010 Year Multijunction concentrators Emerging PV Three-junction (2-terminal, monolithic) Dye cells Two-junction (2-terminal, monolithic) Organic cells (various technologies) Thin film technologies Crystalline Si cells Cu(In, Ga)Se Single crystal CdTe Multicrystalline Amorphous Si:H (stabilized) Thick Si film Nano-, micro-, poly- Si Multijunction polycrystalline Figure 7: Plot of record cell efficiencies for a range of photovoltaic technologies, showing the recent advance to 40.7% efficiency for III-V multijunction cells. Chart courtesy of Bob McConnell, NREL. subcell) is fixed at the 0.67-eV band gap of Ge, for this anal- the terrestrial AM1.5D (ASTM G173-03) spectrum, the cur- ysis [10]. rent density of each subcell can also be determined. The cur- The diamond in the plots indicates the 1.62/1.38 eV band rent densities are 9.24, 9.24, 9.58, and 21.8 mA/cm2 for sub- gap combination of the AlGaInAs subcell 2 and GaInAs sub- cells 1, 2, 3, and 4, respectively, such that the subcells are very cell 3 of a 4-junction cell described later in the paper (see the close to being current matched for this 4J cell. quantum efficiency measurements in Figure 11). Ideal effi- Illuminated light I-V curves are shown in Figure 12 for ciencies of over 58%, and practical cell efficiencies of 47% are a 4-junction (Al)GaInP/ AlGa(In)As/ Ga(In)As/ Ge solar cell possible for 4-junction terrestrial concentrator cells with a measured at 256 suns [10], and for a similar solar cell with band gap combination of 1.90/1.43/1.04/0.67 eV. These band only the upper3junctionsactive(inactiveGe).The open- gaps are accessible with metamorphic materials and the use circuit voltage of the 4-junction cell is 4.364 V, compared of transparent graded buffer layers, for example, in inverted to 3.960 V for the cell with an inactive subcell 4, indicating metamorphic cell designs [6, 8, 10, 11]. It is worth noting the Ge bottom cell accounts for about 400 mV of the V at oc that these practical 4J cell efficiencies are about 5 absolute ef- this concentration. Independently confirmed measurements ficiency points over those for 3-junction cells. Four-junction of the 40.1% lattice-matched and 40.7% metamorphic 3- cells benefit from reduced resistive power losses as described junction cells are also shown for comparison. Preliminary above, and for this band gap combination, also benefit from measured efficiency for this as yet nonoptimized 4J cell is more efficient use of the terrestrial solar spectrum. 35.7% at 256 suns. Four-junction cells designed for the terrestrial solar spec- trum and the high current densities of concentrator opera- 5. SUMMARY tion have been grown by metal-organic vapor-phase epitaxy (MOVPE), processed into devices, and tested. The external Multijunction GaInP/GaInAs/Ge solar cells have been quantum efficiency of one such 4J cell is plotted in Figure 11 demonstrated with 40.7% efficiency using metamorphic versus photon energy. The band gaps of each subcell can be semiconductor technology, and 40.1% for lattice-matched determined from the quantum efficiency data, and the ex- cells [1]. These are the first solar cells to reach over the mile- tracted values are listed in the legend. By convoluting with stone efficiency of 40%, and have the highest solar conversion Efficiency (%) Revised 11-21-06 Richard R. King et al. 7 42 100 4-junction 1.9eV/E /E /0.67 eV cell efficiency 1.7 g 2 g 3 40.7% 2 ◦ 500 suns (50 W/cm ), AM1.5D (ASTM G173-03), 25 C 40.1% LM Eff. 40 MM Ideal efficiency – radiative recombination limit 1.6 38.5% at 630 suns 1.5 FF 58% 34 88 1.4 56% 1.3 V × 10 oc 54% 1.2 28 50% 38% 46% 26 76 1.1 34% 42% 0.80.91 1.11.21.31.41.5 1 10 100 1000 E = subcell 3 bandgap (eV) g 3 Concentration (suns) (1 sun = 0.100 W/cm ) (a) MM, 40.7% max LM, 40.1% max 4-junction 1.9eV/Eg 2 /Eg 3 /0.67 eV cell efficiency 1.7 MM, high concentration 2 ◦ 500 suns (50 W/cm ), AM1.5D (ASTM G173-03), 25 C Normalized to measured 3J cell efficiency 1.6 Figure 8: Efficiency, V , and FF of record performance 40.7% oc metamorphic and 40.1% lattice-matched 3-junction cells as a func- 1.5 47% tion of incident intensity. An additional cell is shown which main- 1.4 tains an efficiency of 38.5% over 600 suns, and 36.9% over 950 suns. 46% 1.3 44% 42% 1.2 40% 38% 1.1 34% 30% Contact AR AR 0.80.91 1.11.21.31.41.5 Cap E = subcell 3 bandgap (eV) g 3 (Al)GaInP cell 1 1.9eV (b) Wide-E tunnel junction Figure 10: Contour plots of (a) ideal efficiency and (b) efficiency AlGa(In)As cell 2 1.6eV normalized to experiment, for 4-junction solar cells, with variable subcell 3 and subcell 2 band gaps. Wide-E tunnel junction Ga(In)As cell 3 1.4eV Tunnel junction Ga(In)As buffer Nucleation Ge cell 4 and substrate 0.67 eV Back contact 50 Figure 9: A 4-junction (Al)GaInP/ AlGa(In)As/ Ga(In)As/Ge ter- restrial concentrator solar cell cross-section. efficiencies of any type of photovoltaic device to date. These 0.51 1.52 2.53 3.5 very high experimental cell efficiencies have begun to be Photon energy (eV) translated to production solar cells as well. GaInP subcell 1 (top cell) GaInAs subcell 3 The dependence of 3- and 4-junction terrestrial concen- 1.38 eV 1.86 eV trator cell efficiency on the band gaps of subcells 1, 2, and AlGaInAs subcell 2 Ge subcell 4 3 is calculated, and presented in contour plots of both ideal 1.62 eV 0.7eV efficiency and practical cell efficiency. Ideal cell efficiencies are over 58%, and practical efficiencies of 47% are achiev- Figure 11: External quantum efficiency of a 4-junction (Al)GaInP/ able for 4-junction concentrator cells [10]. The low resistive AlGa(In)As/ Ga(In)As/ Ge terrestrial concentration cell. Efficiency (%) and V × 10 (V) oc Fill factor (%) Quantum efficiency (%) E = subcell 2 bandgap (eV) E = subcell 2 bandgap (eV) g 2 g 2 8 Advances in OptoElectronics REFERENCES 1.16 [1] R.R.King,D.C.Law,K.M.Edmondson,etal., “40% ef- 1.14 ficient metamorphic GaInP/GaInAs/Ge multijunction solar 1.12 cells,” Applied Physics Letters, vol. 90, no. 18, Article ID 183516, 0.1 3 pages, 2007. [2] R.R.King,D.C.Law,K.M.Edmondson,etal., “Metamorphic 0.08 concentrator solar cells with over 40% conversion efficiency,” 0.06 in Proceedings of the 4th International Conference on Solar Con- centrators (ICSC-4), El Escorial, Spain, March 2007. 0.04 [3] R. R. King, M. Haddad, T. Isshiki, et al., “Metamorphic 0.02 GaInP/GaInAs/Ge solar cells,” in Proceedings of the 28th IEEE Photovoltaic Specialists Conference (PVSC-28), pp. 982–985, 00.51 1.52 2.53 3.54 4.5 Anchorage, Alaska, USA, September 2000. Voltage (V) [4]F.Dimroth,U.Schubert, andA.W.Bett, “25.5% efficient Ga In P/Ga In As tandem solar cells grown on GaAs 3J concentrator cells 4J concentrator cells 0.35 0.65 0.83 0.17 Latt.-matched Metamorphic 4J cell Inactive Ge (3J) substrates,” IEEE Electron Device Letters,vol. 21, no.5,pp. V 3.960 V oc 2.911 V 3.054 4.364 209–211, 2000. J /inten. 0.0923 A/W sc 0.1571 A/W 0.0923 0.1492 [5] T. Takamoto, T. Agui, K. Kamimura, et al., “Multifunction mp 2.755 2.589 V 3.572 V 3.949 solar cell technologies—high efficiency, radiation resistance, FF 0.881 0.875 0.882 0.886 Conc. 135 240 suns and concentrator applications,” in Proceddings of the 3rd World 256 254 suns Area 2 0.255 0.267 cm 0.256 0.256 cm Conference on Photovoltaic Energy Conversion (WCPEC-3), Eff. 40.1% 40.7% 35.7% 32.3% vol. 1, pp. 581–586, Osaka, Japan, May 2003. Aperture area efficiency, 25 C Aperture area efficiency, 25 C [6] M. W. Wanlass, S. P. Ahrenkiel, R. K. Ahrenkiel, et al., “Lattice- AM1.5D, low-AOD spectrum AM1.5D ASTM G173-03 Preliminary measurement Independently confirmed measurement mismatched approaches for high-performance, III-V photo- voltaic energy converters,” in Proceedings of the 31st IEEE Pho- tovoltaic Specialists Conference (PVSC-31), pp. 530–535, Lake Figure 12: Illuminated I-V characteristics of an unoptimized 4- Buena Vista, Fla, USA, January 2005. junction terrestrial concentrator cell with 35.7% efficiency, and V oc [7] A. W. Bett, C. Baur, F. Dimroth, and J. Schone, ¨ “Metamorphic over 4.3 volts. I-V curves for the record 40.7%-efficient metamor- GaInP-GaInAs layers for photovoltaic applications,” in Mate- phic and 40.1% lattice-matched 3-junction cells are also shown. rials Research Society Symposium Proceedings, vol. 836, p. 223, Boston, Mass, USA, November-December 2005. [8] R.R.King,D.C.Law,C.M.Fetzer, et al., “Pathwaysto40% efficient concentrator photovoltaics,” in Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition power loss that results from the high-voltage, low-current (EU PVSEC-20), pp. 118–123, Barcelona, Spain, June 2005. design of cells with 4 or more junctions is a powerful ad- [9] R. R. King, D. C. Law, K. M. Edmondson, et al., “Metamor- vantage in concentrator applications. New 4-junction terres- phic and lattice-matched solar cells under concentration,” in trial concentrator cell architectures have been demonstrated, Proceedings of IEEE of the 4th World Conference on Photovoltaic with 35.7% measured efficiency [10]. The recent realization Energy Conversion (WCPEC-4), vol. 1, pp. 760–763, Waikoloa, of very high-efficiency III-V multijunction cells has posi- Hawaii, USA, May 2006. tioned concentrator PV technology such that it may well have [10] R. R. King, R. A. Sherif, D. C. Law, et al., “New horizons in a game-changing effect on the economics of PV electricity III-V multijunction terrestrial concentrator cell research,” in Proceedings of the 21st European Photovoltaic Solar Energy Con- generation in the near future. Terrestrial concentrator cells ference and Exhibition (EU PVSEC-21), p. 124, Dresden, Ger- with 3, 4, or more junctions, coupled with advances in meta- many, September 2006. morphic materials that have resulted in record solar cell ef- [11] J. F. Geisz, S. Kurtz, M. W. Wanlass, et al., “High-efficiency ficiency of 40.7% today, offer the promise to increase effi- GaInP/ GaAs/ InGaAs triple-junction solar cells grown in- ciency and lower the cost of terrestrial photovoltaic concen- verted with a metamorphic bottom junction,” Applied Physics trator systems still further, to 45%, and perhaps even to 50% Letters, vol. 91, Article ID 023502, 3 pages, 2007. efficiency. [12] R. A. Sherif, R. R. King, N. H. Karam, and D. R. Lilling- ton, “The path to 1 GW of concentrator photovoltaics us- ing multijunction solar cells,” in Proceedings of the 31st IEEE ACKNOWLEDGMENTS Photovoltaic Specialists Conference (PVSC-31), pp. 17–22, Lake Buena Vista, Fla, USA, January 2005. The authors would like to thank Robert McConnell, Martha Symko-Davies, Fannie Posey-Eddy, Keith Emery, James Kiehl, Tom Moriarty, Sarah Kurtz, Kent Barbour, Hector Co- tal, Mark Takahashi, Andrey Masalykin, and the entire mul- tijunction solar cell team at Spectrolab. This work was sup- ported in part by the Department of Energy through the NREL High-Performance PV program (NAT-1-30620-01), and by Spectrolab. 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Hindawi Publishing Corporation Advances in OptoElectronics Volume 2007, Article ID 29523, 8 pages doi:10.1155/2007/29523 Research Article Advances in High-Efficiency III-V Multijunction Solar Cells Richard R. King, Daniel C. Law, Kenneth M. Edmondson, Christopher M. Fetzer, Geoffrey S. Kinsey, Hojun Yoon, Dimitri D. Krut, James H. Ermer, Raed A. Sherif, and Nasser H. Karam Spectrolab, Inc., 12500 Gladstone Avenue, Sylmar, CA 91342, USA Received 25 May 2007; Accepted 12 September 2007 Recommended by Armin G. Aberle The high efficiency of multijunction concentrator cells has the potential to revolutionize the cost structure of photovoltaic elec- tricity generation. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide- band-gap tunnel junctions capable of operating at high concentration, metamorphic buffers to transition from the substrate lattice constant to that of the epitaxial subcells, concentrator cell AR coating and grid design, and integration into 3-junction cells with current-matched subcells under the terrestrial spectrum have resulted in new heights in solar cell performance. A metamorphic Ga In P/Ga In As/ Ge 3-junction solar cell from this research has reached a record 40.7% efficiency at 240 suns, under 0.44 0.56 0.92 0.08 2 ◦ the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm ,25 C), and experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell developed to date. Solar cells with more junctions offer the potential for still higher efficiencies to be reached. Four-junction cells limited by radiative recombination can reach over 58% in principle, and practical 4-junction cell efficiencies over 46% are possible with the right combination of band gaps, taking into account series resistance and gridline shadowing. Many of the optimum band gaps for maximum energy conversion can be accessed with metamorphic semiconductor materials. The lower current in cells with 4 or more junctions, resulting in lower I R resistive power loss, is a particularly significant advan- tage in concentrator PV systems. Prototype 4-junction terrestrial concentrator cells have been grown by metal-organic vapor-phase epitaxy, with preliminary measured efficiency of 35.7% under the AM1.5 direct terrestrial solar spectrum at 256 suns. Copyright © 2007 Richard R. King et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION the National Renewable Energy Laboratory (NREL). Many of the high efficiency device structures developed in the exper- iments leading to these record performance cells have now In the past decade, terrestrial concentrator multijunction III- been incorporated in production III-V multijunction cells, V cells have embarked upon a remarkable ascent in solar increasing the average efficiency of these mass-produced so- conversion efficiency. The realization that very high conver- lar cells as well, while other experimental device improve- sion efficiencies can be achieved with advanced multijunc- ments will be implemented in production in the coming tion solar cells in practice, not just in theory, has prompted a months and years. This paper discusses the science behind resurgence of research in multijunction cells and commer- the 40.7% metamorphic and 40.1% lattice-matched cells, the cial interest in concentrator III-V photovoltaics. This pa- opportunity to reach new levels of photovoltaic (PV) system per discusses recent advances in multijunction cell research cost-effectiveness with production III-V concentrator cells that have led to experimental metamorphic (MM), or lattice- that make use of these advances, and possibilities for the next mismatched, solar cells with 40.7% efficiency under the con- generations of terrestrial concentrator cells with efficiencies centrated terrestrial spectrum [1, 2]. This is the first solar cell of 45%, or even 50%. to reach over 40% efficiency, and is the highest solar conver- sion efficiency yet achieved for any type of photovoltaic de- vice. Experimental lattice-matched (LM) cells have also bro- 2. METAMORPHIC SOLAR CELLS ken the 40% milestone, with 40.1% efficiency demonstrated for an LM 3-junction cell. Both of these cell-efficiency results Perhaps the essential distinguishing feature of III-V multi- have been independently verified by cell measurements at junction cells is the very wide range of subcell and device 2 Advances in OptoElectronics 3-junction Eg 1 /Eg 2 /0.67 eV cell efficiency 3-junction Eg 1 /Eg 2 /0.67 eV cell efficiency 2.1 2.1 2 ◦ 2 ◦ 240 suns (24.0W/cm ), AM1.5D (ASTM G173-03), 25 C 240 suns (24.0W/cm ), AM1.5D (ASTM G173-03), 25 C Ideal efficiency – radiative recombination limit Series resistance and grid shadowing included 2 2 LM LM 1.9 1.9 MM MM 40.7% 40.7% 40.1% 40.1% 1.8 1.8 49% 54% 1.7 1.7 52% 47% 1.6 1.6 50% 45% 48% 43% 1.5 1.5 46% 41% 39% 44% 37% 1.4 1.4 35% 42% 40% 38% 1.3 1.3 11.11.21.31.41.51.6 11.11.21.31.41.51.6 E = subcell 2 bandgap (eV) E = subcell 2 bandgap (eV) g 2 g 2 (a) (b) 3-junction E /E /0.67 eV cell efficiency g 1 g 2 2.1 2 ◦ 240 suns (24.0W/cm ), AM1.5D (ASTM G173-03), 25 C Normalized to experimental 3J cell V and J oc sc LM 1.9 MM 40.7% 40.1% 1.8 42% 1.7 40% 1.6 38% 1.5 36% 34% 1.4 32% 30% 1.3 11.11.21.31.41.51.6 E = subcell 2 bandgap (eV) g 2 Disordered GaInP top subcell Ordered GaInP top subcell (c) Figure 1: Calculated iso-efficiency contours for 3-junction terrestrial concentrator cells with variable top and middle subcell band gaps for the terrestrial solar spectrum at 240 suns: (a) theoretical efficiency based on radiative recombination [1]; (b) including the effects of grid resistance and shadowing using the metal grid design of the record 40.7%-efficient cell; and (c) additionally including empirically determined average quantum efficiency of 0.925, and 3-junction cell V 233 mV lower than the ideal voltage based on radiative recombination alone, oc giving an experimentally grounded prediction of practical, state-of-the-art, 3J cell efficiencies, as a function of subcell E . Subcell 1 and 2 band gap pairs of GaInP and GaInAs at the same lattice constant are shown for both disordered and ordered GaInP. The measured efficiencies and band gap combinations for the record 40.7% MM and 40.1% LM cells are plotted, at 240 and 135 suns, respectively, showing the theoretical advantage of the metamorphic design, now realized in practice. structure band gaps that can be grown with high crystal dle (subcell 2) band gap E [1]. Figure 1(a) plots contours g 2 quality, and correspondingly high minority-carrier recombi- of ideal efficiency based on the diode characteristics of sub- nation lifetimes. This is true for lattice-matched multijunc- cells limited only by the fundamental mechanism of radiative tion cells, but the flexibility in band gap selection takes on recombination, and on the shape of the terrestrial solar spec- a whole new dimension when metamorphic semiconductors trum. The cell model is discussed in greater detail in [10]. are used, providing freedom from the constraint that all sub- Efficiencies up to 54% can be seen to be possible in princi- cells must have the same crystal lattice constant. The area of ple at this concentration for 3-junction cells in the radiative metamorphic solar cell materials has attracted interest from recombination limit, increasing to over 58% for 4-junction photovoltaic research groups around the globe [1–11]. terrestrial concentrator cells [10]. The theoretical benefits of flexibility in subcell band gap In 3-junction GaInP/GaInAs/Ge metamorphic solar selection are made apparent in Figure 1(a), which plots iso- cells, the GaInP and GaInAs subcells can be grown on a efficiency contours for 3-junction terrestrial concentrator metamorphic buffer such that these two subcells are lattice- cells as a function of top (subcell 1) band gap E and mid- matched to each other, but are both lattice-mismatched to g 1 E = subcell 1 (top) bandgap (eV) g 1 E = subcell 1 (top) bandgap (eV) g 1 E = subcell 1 (top) bandgap (eV) g 1 Richard R. King et al. 3 Contact Contact AR n -GaInAs AR n -Ga(In)As n-AlInP window n-GaInP emitter n-AlInP window n-GaInP emitter p-GaInP base GaInP top cell p-GaInP base p-AlGaInP BSF ++ p-AlGaInP BSF p -TJ ++ Wide-bandgap ++ n -TJ p -TJ tunnel junction ++ n-GaInP window n -TJ n-GaInAs emitter n-GaInP window n-Ga(In)As emitter Ga(In)As p-GaInAs base middle cell p-Ga(In)As base p-GaInP BSF p-GaInP BSF p-GaInAs ++ p -TJ Tunnel junction Step-graded ++ n -TJ Buffer ++ p -TJ Buffer region n-Ga(In)As buffer ++ n -TJ Nucleation Nucleation n -Ge emitter n -Ge emitter p-Ge base and substrate Ge bottom cell p-Ge base and substrate Contact Contact Lattice-mismatched Lattice-matched (LM) or metamorphic (MM) Figure 2: Schematic cross-sectional diagrams of lattice-matched (LM) and metamorphic (MM) GaInP/GaInAs/Ge 3-junction cell configu- rations, corresponding to the LM 40.1% and MM 40.7%-efficient concentrator cells. the Ge growth substrate and subcell. The band gap combi- cluded, essentially identical to that measured experimentally nations that are possible with GaInP and GaInAs subcells at for the 40.7% cell at 240 suns. same lattice constant, but with varying lattice mismatch to In Figure 1(c), additional real-life effects are included by the Ge substrate, are shown in Figure 1(a). The cases with using empirical values for the active-area external quantum a disordered group-III sublattice in the GaInP subcell, giv- efficiency (EQE), and for the decrease in 3-junction cell V oc ing higher band gap at the same GaInP composition, and from Shockley-Read-Hall (SRH) recombination in addition with an ordered (low E ) group-III sublattice in the GaInP to radiative recombination. The record 40.7%-efficiency 3- subcell, are both plotted. Metamorphic cells can be seen to junction MM cell has an average active-area external quan- bring the cell design closer to the region of E , E space that tum efficiency of 0.925, and actual V that is 233 mV lower g 1 g 2 oc has the highest theoretical efficiencies. The lower band gaps than the ideal V in the radiative limit. This is equivalent oc of MM subcells can use a larger part of the solar spectrum, to 78 mV per subcell on average, though since the GaInAs that is wasted as excess photogenerated current in the Ge bot- middle subcell V is often close to the radiative limit, the oc tom cell in most lattice-matched 3-junction cells. In the past, difference between actual V and ideal radiative V is more oc oc recombination at dislocations in MM materials have often heavily distributed in the top and bottom subcells. With the thwarted this promise of higher theoretical efficiency. How- addition of these last real-life effects, the calculated contours ever, for the recent metamorphic 40.7%-efficient and lattice- in Figure 1(c) show a good estimate of the efficiencies that matched 40.1%-efficient cell results, plotted in Figure 1, the can be achieved in practical, state-of-the-art, 3-junction cells density and activity of dislocations have been controlled suf- as a function of band gap. The measured efficiencies of the ficiently to show the efficiency advantage of the MM design, plotted 40.7% MM and 40.1% LM record cells correspond to not just theoretically but now also experimentally. the efficiency contours in Figure 1(c), but are also plotted in Figures 1(b) and 1(c) take this analysis a bit farther. The Figures 1(a) and 1(b) for reference. It should be noted that efficiency contours in Figure 1(b) take into account the shad- unlike Figure 1(a), the present, state-of-the-art, practical ef- owing and specific series resistance associated with the metal ficiencies of the contours in Figure 1(c) are not fundamental grid pattern used on the 40.7% record cell. The fill factor cal- limits, and can be made higher by finding ways to reduce the culated for the 3-junction cell with the band gap combina- nonfundamental EQE and V losses that have been included oc tion of the MM 40.7% cell is 87.5% with series resistance in- in Figure 1(c). Tunnel junction Bottom cell Bottom cell Top cell Top cell Wide-E tunnel Middle cell Tunnel junction -E Wide tunnel Middle cell 4 Advances in OptoElectronics 8880 HIPSS PV Performance Characterization Team 1.2 Ge Spectrolab Metamorphic 0.8 GaInP/GaInAs/Ge cell Graded V = 2.911 V oc buffer 8820 2 J = 3.832 A/cm 0.6 sc FF = 87.50% V = 2.589 V mp Ga In As MC 8800 0.4 0.92 0.08 Efficiency = 40.7% ± 2.4% GaInP TC 240 suns (24.0W/cm ) intensity 0.2 (115) 2 0.2669 cm designated area glancing ◦ 25 ± 1 C, AM1.5D, low-AOD spectrum exit XRD 00.51 1.52 2.53 −2500 −2480 −2460 −2440 −2420 −1 ˚ Voltage (V) Qx (tilt) A Line of 100% Figure 5: Illuminated I-V curve for the record 40.7% metamor- relaxation Line of 0% phic 3-junction cell, independently verified at NREL. This is the first relaxation photovoltaic cell of any type to reach over 40% solar conversion ef- Figure 3: High-resolution X-ray diffraction reciprocal space map of ficiency. a metamorphic 3-junction cell structure, showing a metamorphic buffer with almost no residual strain, and a GaInP top cell that is pseudomorphic with respect to the Ga In As middle cell. 0.92 0.08 ciprocal space map (RSM) shown in Figure 3.The buffer can be seen to be nearly 100% relaxed, with very little residual strain to drive the formation of dislocations in the active up- 100 100 per subcells. 90 90 The shift in the quantum efficiency of the 3 subcells in 80 80 GaInP/GaInAs/Ge 3-junction cells, as a result of the higher 70 70 indium composition and lower band gap of the metamor- 60 60 phic GaInP and GaInAs subcells, is shown in Figure 4 [1]. In 50 50 this way the MM cells are able to capture some of the current 40 40 density that would otherwise be wasted in the Ge subcell. The 30 30 quantum efficiencies are overlaid on the AM0, and terrestrial AM1.5G and AM1.5D, low-AOD solar spectra, to show the 20 20 current densities available in the response range of each sub- 10 10 cell. 0 0 300 500 700 900 1100 1300 1500 1700 1900 Wavelength (nm) 3. HIGH-EFFICIENCY MULTIJUNCTION CELLS AM1.5D, low-AOD EQE, lattice-matched Band gap engineering of subcells in 3-junction solar cells, AM1.5G, ASTM G173-03 EQE, metamorphic made possible by metamorphic semiconductor materials, has AM0, ASTM E490-00a now resulted in higher measured efficiencies for metamor- Figure 4: External quantum efficiency for GaInP, GaInAs, and Ge phic cells than in even the best lattice-matched cells. Exper- subcells of LM and MM 3-junction cells, showing extension of the iments on step-graded buffers, used to transition from the lower-E MM GaInP and GaInAs responses to longer wavelengths, substrate to the subcell lattice constant, have been used to allowing them to use more of the solar spectrum [1]. control the classic problem of dislocations in the active cell regions due to the lattice mismatch. The band gap-voltage offset (E /q) − V is a key indicator of the quality and sup- g oc Schematic diagrams of LM and MM cells are shown in pression of SRH recombination in semiconductors of vari- Figure 2, showing the step-graded metamorphic buffer used able band gap, where lower offset values are desired, since in the MM case to transition from the lattice constant of the it is a measure of the separation between electron and hole substrate to that of the upper subcells. The lattice constants quasi-Fermi levels and the conduction and valence band and strain in the various MM 3-junction cell layers are im- edges [8–10]. Metamorphic 8%-In GaInAs single-junction aged in the high-resolution X-ray diffraction (HRXRD) re- cellswerebuilt andtestedwithaband gap-voltage offset Current density per unit −1 2 Qy (strain) A wavelength (mA/(cm μm)) External quantum efficiency (%) Current (A) Richard R. King et al. 5 0.8 1.16 have the highest rate of increase. These high III-V cell effi- ciencies have translated to concentrator PV module efficien- 1.14 0.7 cies over 30%, more than double the ∼15% module efficien- 1.12 0.6 cies that are more typical of flat-plate silicon modules. This high efficiency is extremely leveraging for PV system eco- 0.1 0.5 nomics [12], as it reduces all area-related costs of the module. 0.08 0.4 Production multijunction concentrator cells with efficiency 0.06 0.3 in the 40% range could cause the market growth for concen- trator PV to explode, with multi-GW/year production levels. 0.04 0.2 In Figure 8, the measured efficiency, V ,and fillfactor oc 0.02 0.1 are plotted as a function of incident intensity, or concentra- 0 0 tion ratio, for the record 40.7% MM and 40.1% LM cells, as 00.51 1.52 2.53 well as for an additional MM cell with good performance at Voltage (V) high intensities. It is interesting to note that the efficiencies Concentrator cells 1-sun cells of both the record MM and LM cells track very closely at the Latt.-matched Metamorphic Latt.-matched Metamorphic same concentration, but the measurements were able to be V 3.054 2.911 V oc 2.622 2.392 V extended to a higher concentration for the MM cell. Fill fac- J /inten. sc 0.1492 0.1596 A/W 0.1437 0.1599 A/W tors for both types of cell are quite high at about 88% in the mp 2.755 2.055 V 2.589 V 2.301 0.819 FF 0.881 0.875 0.850 100–200 sun range. The open-circuit voltage V increases at oc 1.0 suns 1.0 Conc. 135 240 suns rates of approximately 210 mV/decade and 190 mV/decade 4.0 4.0cm Area 0.2547 0.267 cm 32.0% for the MM and LM record cells, respectively, in the 100–200 Eff. 40.1% 40.7% 31.3% ◦ suns range. Thus the MM subcells increase in voltage some- Total area efficiency at 25 C Designated area efficiency at 25 C AM1.5D, low-AOD spectrum AM1.5G 1 sun = 0.100 W/cm what more rapidly with excess carrier concentration than in the LM case, as one would expect if defects in the MM mate- Figure 6: Comparison of the light I-V characteristics of the 40.1% rials are becoming less active at mediating recombination at lattice-matched and 40.7% metamorphic 3-junction concentrator higher injection levels. From the slopes of V versus concen- oc cells, and earlier record one-sun cells [1]. The higher current and tration (current density) we can extract values of the diode lower voltage of the metamorphic design is evident. ideality factor n. Subtracting off the 59 mV/decade increase for the Ge subcell, which has diode ideality factor very close to unity, gives an average n for the upper two subcells of 1.26 of 0.42 V at one sun, essentially the same as GaAs control in the MM case and 1.10 in the LM case in the same con- cells, reflecting the long minority-carrier lifetimes that can centration range, with decreasing n as the incident intensity be achieved in metamorphic materials. increases. An extensive experimental campaign was carried out on GaInP/GaInAs/Ge terrestrial concentrator cells, using a vari- 4. FOUR-JUNCTION SOLAR CELLS ety of metamorphic and lattice-matched 3-junction cell con- figurations, wide-band-gap tunnel junctions and other high- A 4-junction (Al)GaInP/ AlGa(In)As/ Ga(In)As/ Ge terres- efficiency semiconductor device structures, current matching trial concentrator solar cell [10] is shown in Figure 9,where conditions, cell sizes, grid patterns, and fabrication processes, the parentheses indicate optional elements in the subcell resulting in new understanding of the limiting mechanisms composition. This type of cell divides the photon flux avail- of terrestrial multijunction cells, and new heights in perfor- able in the terrestrial solar spectrum above the band gap of mance. Figure 5 plots the measured illuminated I-V curve for the GaInAs subcell 3 into 3 pieces, rather than 2 pieces in the record efficiency 40.7% metamorphic GaInP/GaInAs/Ge the case of a 3-junction cell. As a result, the current density 3-junction cell at 240 suns [1], under the standard spectrum of a 4-junction cell is roughly 2/3 that of a corresponding 2 2 for concentrator solar cells (AM1.5D, low-AOD, 24.0 W/cm , 3-junction cell, and the I R resistive power loss is approxi- 25 C). This is the first solar cell to reach over 40% efficiency, mately (2/3) = 4/9, or less than half that of a 3-junction cell. and is the highest solar conversion efficiency yet achieved for Figure 9 shows a lattice-matched 4-junction cell, with all the any type of photovoltaic device. A lattice-matched 3-junction subcells at the lattice constant of the Ge substrate, but lattice- cell has also achieved over 40% efficiency, with 40.1% mea- mismatched versions of the 4-junction cell are also possible, 2 ◦ sured at 135 suns (AM1.5D, low-AOD, 13.5 W/cm ,25 C). giving greater flexibility in bandgap selection. These efficiencies have been independently verified by mea- Iso-efficiency contours for 4-junction terrestrial concen- surements at the National Renewable Energy Laboratory trator cells, under the AM1.5D (ASTM G173-03) solar spec- (NREL). Light I-V characteristics of both the record MM and trum at 500 suns (50.0 W/cm ), are plotted in Figure 10 as LM devices are compared in Figure 6 [1]. a function of the band gaps of subcells 2 and 3. Ideal 4- The highest cell efficiencies from a number of pho- junction cell efficiency is plotted in Figure 10(a),and prac- tovoltaic technologies by year since 1975 are plotted in tical cell efficiency, consistent with the measured 3J cell effi- Figure 7, showing the most recent 40.7%-efficient cell result. ciency, in Figure 10(b). The band gap of subcell 1 is held at It is interesting to note that III-V multijunction concentrator 1.9 eV, corresponding to GaInP at the Ge lattice constant with cells not only are the highest efficiency technology, but also a disordered group-III sublattice, and subcell 4 (the bottom Current density/incident intensity (A/W) Current density/incident intensity (A/W) 6 Advances in OptoElectronics 40.7% Best research-cell efficiencies 40% Boeing- Boeing- Spectrolab Spectrolab NREL 36 (metamorphic) (inverted, Spectrolab semi-mismatched) Japan Energy NREL/ NREL Spectrolab NREL UNSW UNSW Spire UNSW 24 NREL UNSW Cu(In, Ga)Se UNSW Stanford Georgia 14x concentration Spire FhG-ISE UNSW Tech Georgia 20 ARCO Sharp Tech Varian NREL NREL NREL Westing. Sharp NREL NREL house (large area) Univ. Stuttgart University No. Carolina NREL So. Florida AstroPower (45 μm thin-film State Univ. NREL NREL (small area) transfer) NREL ARCO Euro-ClS Solarex Kodak (CdTe/ClS) Boeing United Solar Boeing EPFL Photon AMETEK Energy Matsushita Kaneka Boeing Solarex United Solar Monosolar Kodak (2 μm on glass) RCA NREL Konarka Groningen Univ. Linz Boeing University EPFL 4 of Maine University RCA Siemens RCA RCA Linz RCA RCA University RCA Linz 1975 1980 1985 1990 1995 2000 2005 2010 Year Multijunction concentrators Emerging PV Three-junction (2-terminal, monolithic) Dye cells Two-junction (2-terminal, monolithic) Organic cells (various technologies) Thin film technologies Crystalline Si cells Cu(In, Ga)Se Single crystal CdTe Multicrystalline Amorphous Si:H (stabilized) Thick Si film Nano-, micro-, poly- Si Multijunction polycrystalline Figure 7: Plot of record cell efficiencies for a range of photovoltaic technologies, showing the recent advance to 40.7% efficiency for III-V multijunction cells. Chart courtesy of Bob McConnell, NREL. subcell) is fixed at the 0.67-eV band gap of Ge, for this anal- the terrestrial AM1.5D (ASTM G173-03) spectrum, the cur- ysis [10]. rent density of each subcell can also be determined. The cur- The diamond in the plots indicates the 1.62/1.38 eV band rent densities are 9.24, 9.24, 9.58, and 21.8 mA/cm2 for sub- gap combination of the AlGaInAs subcell 2 and GaInAs sub- cells 1, 2, 3, and 4, respectively, such that the subcells are very cell 3 of a 4-junction cell described later in the paper (see the close to being current matched for this 4J cell. quantum efficiency measurements in Figure 11). Ideal effi- Illuminated light I-V curves are shown in Figure 12 for ciencies of over 58%, and practical cell efficiencies of 47% are a 4-junction (Al)GaInP/ AlGa(In)As/ Ga(In)As/ Ge solar cell possible for 4-junction terrestrial concentrator cells with a measured at 256 suns [10], and for a similar solar cell with band gap combination of 1.90/1.43/1.04/0.67 eV. These band only the upper3junctionsactive(inactiveGe).The open- gaps are accessible with metamorphic materials and the use circuit voltage of the 4-junction cell is 4.364 V, compared of transparent graded buffer layers, for example, in inverted to 3.960 V for the cell with an inactive subcell 4, indicating metamorphic cell designs [6, 8, 10, 11]. It is worth noting the Ge bottom cell accounts for about 400 mV of the V at oc that these practical 4J cell efficiencies are about 5 absolute ef- this concentration. Independently confirmed measurements ficiency points over those for 3-junction cells. Four-junction of the 40.1% lattice-matched and 40.7% metamorphic 3- cells benefit from reduced resistive power losses as described junction cells are also shown for comparison. Preliminary above, and for this band gap combination, also benefit from measured efficiency for this as yet nonoptimized 4J cell is more efficient use of the terrestrial solar spectrum. 35.7% at 256 suns. Four-junction cells designed for the terrestrial solar spec- trum and the high current densities of concentrator opera- 5. SUMMARY tion have been grown by metal-organic vapor-phase epitaxy (MOVPE), processed into devices, and tested. The external Multijunction GaInP/GaInAs/Ge solar cells have been quantum efficiency of one such 4J cell is plotted in Figure 11 demonstrated with 40.7% efficiency using metamorphic versus photon energy. The band gaps of each subcell can be semiconductor technology, and 40.1% for lattice-matched determined from the quantum efficiency data, and the ex- cells [1]. These are the first solar cells to reach over the mile- tracted values are listed in the legend. By convoluting with stone efficiency of 40%, and have the highest solar conversion Efficiency (%) Revised 11-21-06 Richard R. King et al. 7 42 100 4-junction 1.9eV/E /E /0.67 eV cell efficiency 1.7 g 2 g 3 40.7% 2 ◦ 500 suns (50 W/cm ), AM1.5D (ASTM G173-03), 25 C 40.1% LM Eff. 40 MM Ideal efficiency – radiative recombination limit 1.6 38.5% at 630 suns 1.5 FF 58% 34 88 1.4 56% 1.3 V × 10 oc 54% 1.2 28 50% 38% 46% 26 76 1.1 34% 42% 0.80.91 1.11.21.31.41.5 1 10 100 1000 E = subcell 3 bandgap (eV) g 3 Concentration (suns) (1 sun = 0.100 W/cm ) (a) MM, 40.7% max LM, 40.1% max 4-junction 1.9eV/Eg 2 /Eg 3 /0.67 eV cell efficiency 1.7 MM, high concentration 2 ◦ 500 suns (50 W/cm ), AM1.5D (ASTM G173-03), 25 C Normalized to measured 3J cell efficiency 1.6 Figure 8: Efficiency, V , and FF of record performance 40.7% oc metamorphic and 40.1% lattice-matched 3-junction cells as a func- 1.5 47% tion of incident intensity. An additional cell is shown which main- 1.4 tains an efficiency of 38.5% over 600 suns, and 36.9% over 950 suns. 46% 1.3 44% 42% 1.2 40% 38% 1.1 34% 30% Contact AR AR 0.80.91 1.11.21.31.41.5 Cap E = subcell 3 bandgap (eV) g 3 (Al)GaInP cell 1 1.9eV (b) Wide-E tunnel junction Figure 10: Contour plots of (a) ideal efficiency and (b) efficiency AlGa(In)As cell 2 1.6eV normalized to experiment, for 4-junction solar cells, with variable subcell 3 and subcell 2 band gaps. Wide-E tunnel junction Ga(In)As cell 3 1.4eV Tunnel junction Ga(In)As buffer Nucleation Ge cell 4 and substrate 0.67 eV Back contact 50 Figure 9: A 4-junction (Al)GaInP/ AlGa(In)As/ Ga(In)As/Ge ter- restrial concentrator solar cell cross-section. efficiencies of any type of photovoltaic device to date. These 0.51 1.52 2.53 3.5 very high experimental cell efficiencies have begun to be Photon energy (eV) translated to production solar cells as well. GaInP subcell 1 (top cell) GaInAs subcell 3 The dependence of 3- and 4-junction terrestrial concen- 1.38 eV 1.86 eV trator cell efficiency on the band gaps of subcells 1, 2, and AlGaInAs subcell 2 Ge subcell 4 3 is calculated, and presented in contour plots of both ideal 1.62 eV 0.7eV efficiency and practical cell efficiency. Ideal cell efficiencies are over 58%, and practical efficiencies of 47% are achiev- Figure 11: External quantum efficiency of a 4-junction (Al)GaInP/ able for 4-junction concentrator cells [10]. The low resistive AlGa(In)As/ Ga(In)As/ Ge terrestrial concentration cell. Efficiency (%) and V × 10 (V) oc Fill factor (%) Quantum efficiency (%) E = subcell 2 bandgap (eV) E = subcell 2 bandgap (eV) g 2 g 2 8 Advances in OptoElectronics REFERENCES 1.16 [1] R.R.King,D.C.Law,K.M.Edmondson,etal., “40% ef- 1.14 ficient metamorphic GaInP/GaInAs/Ge multijunction solar 1.12 cells,” Applied Physics Letters, vol. 90, no. 18, Article ID 183516, 0.1 3 pages, 2007. [2] R.R.King,D.C.Law,K.M.Edmondson,etal., “Metamorphic 0.08 concentrator solar cells with over 40% conversion efficiency,” 0.06 in Proceedings of the 4th International Conference on Solar Con- centrators (ICSC-4), El Escorial, Spain, March 2007. 0.04 [3] R. R. King, M. Haddad, T. Isshiki, et al., “Metamorphic 0.02 GaInP/GaInAs/Ge solar cells,” in Proceedings of the 28th IEEE Photovoltaic Specialists Conference (PVSC-28), pp. 982–985, 00.51 1.52 2.53 3.54 4.5 Anchorage, Alaska, USA, September 2000. Voltage (V) [4]F.Dimroth,U.Schubert, andA.W.Bett, “25.5% efficient Ga In P/Ga In As tandem solar cells grown on GaAs 3J concentrator cells 4J concentrator cells 0.35 0.65 0.83 0.17 Latt.-matched Metamorphic 4J cell Inactive Ge (3J) substrates,” IEEE Electron Device Letters,vol. 21, no.5,pp. V 3.960 V oc 2.911 V 3.054 4.364 209–211, 2000. J /inten. 0.0923 A/W sc 0.1571 A/W 0.0923 0.1492 [5] T. Takamoto, T. Agui, K. Kamimura, et al., “Multifunction mp 2.755 2.589 V 3.572 V 3.949 solar cell technologies—high efficiency, radiation resistance, FF 0.881 0.875 0.882 0.886 Conc. 135 240 suns and concentrator applications,” in Proceddings of the 3rd World 256 254 suns Area 2 0.255 0.267 cm 0.256 0.256 cm Conference on Photovoltaic Energy Conversion (WCPEC-3), Eff. 40.1% 40.7% 35.7% 32.3% vol. 1, pp. 581–586, Osaka, Japan, May 2003. Aperture area efficiency, 25 C Aperture area efficiency, 25 C [6] M. W. Wanlass, S. P. Ahrenkiel, R. K. Ahrenkiel, et al., “Lattice- AM1.5D, low-AOD spectrum AM1.5D ASTM G173-03 Preliminary measurement Independently confirmed measurement mismatched approaches for high-performance, III-V photo- voltaic energy converters,” in Proceedings of the 31st IEEE Pho- tovoltaic Specialists Conference (PVSC-31), pp. 530–535, Lake Figure 12: Illuminated I-V characteristics of an unoptimized 4- Buena Vista, Fla, USA, January 2005. junction terrestrial concentrator cell with 35.7% efficiency, and V oc [7] A. W. Bett, C. Baur, F. Dimroth, and J. Schone, ¨ “Metamorphic over 4.3 volts. I-V curves for the record 40.7%-efficient metamor- GaInP-GaInAs layers for photovoltaic applications,” in Mate- phic and 40.1% lattice-matched 3-junction cells are also shown. rials Research Society Symposium Proceedings, vol. 836, p. 223, Boston, Mass, USA, November-December 2005. [8] R.R.King,D.C.Law,C.M.Fetzer, et al., “Pathwaysto40% efficient concentrator photovoltaics,” in Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition power loss that results from the high-voltage, low-current (EU PVSEC-20), pp. 118–123, Barcelona, Spain, June 2005. design of cells with 4 or more junctions is a powerful ad- [9] R. R. King, D. C. Law, K. M. Edmondson, et al., “Metamor- vantage in concentrator applications. New 4-junction terres- phic and lattice-matched solar cells under concentration,” in trial concentrator cell architectures have been demonstrated, Proceedings of IEEE of the 4th World Conference on Photovoltaic with 35.7% measured efficiency [10]. The recent realization Energy Conversion (WCPEC-4), vol. 1, pp. 760–763, Waikoloa, of very high-efficiency III-V multijunction cells has posi- Hawaii, USA, May 2006. tioned concentrator PV technology such that it may well have [10] R. R. King, R. A. Sherif, D. C. Law, et al., “New horizons in a game-changing effect on the economics of PV electricity III-V multijunction terrestrial concentrator cell research,” in Proceedings of the 21st European Photovoltaic Solar Energy Con- generation in the near future. Terrestrial concentrator cells ference and Exhibition (EU PVSEC-21), p. 124, Dresden, Ger- with 3, 4, or more junctions, coupled with advances in meta- many, September 2006. morphic materials that have resulted in record solar cell ef- [11] J. F. Geisz, S. Kurtz, M. W. Wanlass, et al., “High-efficiency ficiency of 40.7% today, offer the promise to increase effi- GaInP/ GaAs/ InGaAs triple-junction solar cells grown in- ciency and lower the cost of terrestrial photovoltaic concen- verted with a metamorphic bottom junction,” Applied Physics trator systems still further, to 45%, and perhaps even to 50% Letters, vol. 91, Article ID 023502, 3 pages, 2007. efficiency. [12] R. A. Sherif, R. R. King, N. H. Karam, and D. R. Lilling- ton, “The path to 1 GW of concentrator photovoltaics us- ing multijunction solar cells,” in Proceedings of the 31st IEEE ACKNOWLEDGMENTS Photovoltaic Specialists Conference (PVSC-31), pp. 17–22, Lake Buena Vista, Fla, USA, January 2005. The authors would like to thank Robert McConnell, Martha Symko-Davies, Fannie Posey-Eddy, Keith Emery, James Kiehl, Tom Moriarty, Sarah Kurtz, Kent Barbour, Hector Co- tal, Mark Takahashi, Andrey Masalykin, and the entire mul- tijunction solar cell team at Spectrolab. This work was sup- ported in part by the Department of Energy through the NREL High-Performance PV program (NAT-1-30620-01), and by Spectrolab. 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