Novel materials for high-efficiency III–V multi-junction solar cells

Available online at www.sciencedirect.com

Solar Energy 82 (2008) 173–180 www.elsevier.com/locate/solener

Novel materials for high-efficiency III–V multi-junction solar cells Masafumi Yamaguchi a,*, Ken-Ichi Nishimura a, Takuo Sasaki a, Hidetoshi Suzuki a, Kouji Arafune a, Nobuaki Kojima a, Yoshio Ohsita a, Yoshitaka Okada b, Akio Yamamoto c, Tatsuya Takamoto d, Kenji Araki e a

Toyota Technological Institute, 2-12 Hisakata, Tempaku, Nagoya 468-8511, Japan b University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan c Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan d Sharp Corporation, 282-1 Hajikami, Shinjo-cho, Nara 639-2198, Japan e Daido Steel Corporation, 2-30 Daido-cho, Minami-ku, Nagoya 457-8545, Japan

Received 18 April 2007; received in revised form 15 June 2007; accepted 15 June 2007 Available online 30 July 2007 Communicated by: Associate Editor T.M. Razykov

Abstract As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm2. In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells.  2007 Elsevier Ltd. All rights reserved. Keywords: Solar cells; High efficiency; III–V compounds; Multi-junction; Concentrator; New III–V-nitride materials; Quantum wells; Quantum dots

1. Introduction Dissemination of PV systems has been advanced and solar cell module productions have also been significantly increased in Japan as a result of R&D programs. Fig. 1 shows cumulated installed capacity of PV systems in Japan by year. The total installed capacity of PV systems in 2005 reached 289.9 MW and the cumulative installed capacity recorded 1421.9 MW (over 1 GW level). We have also the target of cumulative capacity of PV systems by 2010 is

*

Corresponding author. Tel.: +81 52 809 1875; fax: +81 52 809 1879. E-mail address: [email protected] (M. Yamaguchi).

0038-092X/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2007.06.011

4.82 GW and the target by 2030 is 100 GW (Kurokawa and Aratani, 2004) that is equivalent to 10% of Japanese electricity consumption. Such a rapid growth in PV system installation in Japan needs production of large-scale PV systems that means necessity for development of higher efficiency solar cell modules. In addition, we will have to realize module efficiency of 40% and cell efficiency of 50% until 2030 by using concentrator multi-junction (MJ) solar cells and modules according to the Japanese 2030PV roadmap. Table 1 shows efficiency targets of various solar cells and modules in the Japanese 2030PV roadmap. We have started research studies on MJ solar cells for space applications in 1982 (Yamaguchi et al., 1987). As

Cumulative Installed Capacity (GW)

174

M. Yamaguchi et al. / Solar Energy 82 (2008) 173–180 Cumulated

Cumulated (20%Growth)

Cumulated (30%Growth)

NSS Milestone

Scenario 1

Scenario 2

2. High efficiency MJ concentrator cells and modules We have proposed wide bandgap InGaP double hetero (DH) structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell (Takamoto et al., 2003). Fig. 2 shows a structure and concentrator ratio dependence of efficiency for a high-efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell. InGaP and InGaAs layers were grown on a fragile Ge substrate with thickness of 150 lm. Cell size was 7 mm · 9 mm with 7 mm square aperture area. As a result of such proposals and grid design of concentrator cells, we have successfully fabricated high efficiency concentrator InGaP/ InGaAs/Ge 3-junction solar cells designed for 500-sun application. The efficiencies by in-house measurement are 39.2% at 200-suns and 38.9% at 489-suns as shown in Fig. 2 (Takamoto et al., 2005). The official value is 37.4% at 489-suns because the present cells are designed for space use. We have also developed a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules (Araki et al., 2003). We have demonstrated 27.6% efficiency with 400-suns concentrator InGaP/InGaAs/Ge 3-junction solar cell modules with an area of 7,056 cm2 and 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm2. Fig. 3 shows 28% efficient 400-suns concentrator module with area of 7056 cm2 and dome-shaped Fresnel lens. Table 2 shows uncorrected peak efficiencies of 400-suns and 550-suns concentrator cell modules (Araki et al., 2005). Our results were confirmed by the Fraunhofer ISE and NREL. Almost 1-year observation showed a seasonal fluctuation inherent to MJ cells and stable energy output. By 1980 h of sunshine duration, the total concentrator solar

1000

100

10

1

0.1

0.01 1995

2000

2005

2010

2015

2020

2025

2030

Year

Fig. 1. Cumulated installed capacity of PV systems in Japan by year.

Table 1 Efficiency targets of various modules (cells) in the Japanese PV2030 Road Map Cell type Thin-bulk multi-c-Si Thin-film Si CIS type Super-high efficiency Dye-sensitized

2010 16(20) 12(15) 13(19) 28(40) 6(10)

2020 19(25) 14(18) 18(25) 35(45) 10(15)

2030 22(25) 18(20) 22(25) 40(50) 15(18)

one of the Sunshine Program in Japan, an R&D project for super high-efficiency MJ cells was started based on our results in 1990 (Yamaguchi and Wakamatsu, 1996). Since 2001, the R&D project for super high-efficiency concentrator MJ cells and modules for terrestrial applications was initiated in Japan (Yamaguchi, 2002). This paper presents our approaches for high-efficiency III–V compound MJ and concentrator solar cells.

40 39

Efficiency (%)

38 37 36 35 34 33 32 31 30 1

10

100

1000

Concertration Ratio Fig. 2. A structure and concentration ratio dependence of efficiency for a high efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell.

M. Yamaguchi et al. / Solar Energy 82 (2008) 173–180

175

Efficiency (%)

Theory (Conc.) Realized (Conc.) 65 60 55 50 45 40 35 30 25 20

1

2

3

4

5

Theory (1-sun) Realized (1-sun)

6

7

8

9

10

11

Number of Junction Fig. 4. Conversion efficiency potential of multi-junction solar cells.

3.1. Lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cells Conversion efficiency of InGaP/(In)GaAs/Ge based multi-junction solar cells has been improved up to 31– 32% at AM1.5G by considering the lattice matching between sub cell layers and Ge substrate. However, because the present band-gap InGaP/InGaAs combination is far from the optimum combination for the AM1.5 spectra, the lattice-mismatched system should be studied in order to realize high efficiencies. Efficiencies up to 39% are possi-

39

0.014 0.012

38

0.01

37

0.008

36

0.006 0.004

35

0.002 34

0

33

-0.002

Lattice mismatching

0.016

2

In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied at Toyota Tech. Inst. (TTI), Tsukuba Univ. and Fukui Univ. We have obtained promising results as described below. Fig. 4 shows conversion efficiency potential of multi-junction solar cells.

Δa/a

0 0. 02 0. 04 0. 06 0. 08 0. 1 0. 12 0. 14 0. 16 0. 18

3. New approaches on novel materials and structures for super high-efficiency MJ cells

Conversion efficiency (%)

cell module energy generation was 277 kW h/m2 that is 1.6 times higher than typical crystaline Si flat-plate solar cell module (14.1%). The annual efficiency was 20.5% that is almost two times higher than flat-plate system.

η(%)

40

0.

Fig. 3. Twenty-eight percent of efficient concentrator module with domeshaped Fresnel lens.

In composition x of InxGa1-xAs

Fig. 5. Conversion efficiency potential of lattice-mismatched InGaP/ InGaAs/Ge 3-junction solar cells.

ble under ideal band-gap combination with In composition 0.16 of InxGa1xAs middle cell. Fig. 5 shows conversion eficiency potential of lattice-mismatched InGaP/InGaAs/ Ge 3-junction solar cells. The lattice-mismatched InGaP/ InGaAs/Ge system should be studied in order to realize high efficiencies of about 39% at 1-sun AM1.5G. However, strain-induced dislocations and defects are thought to degrade solar cell properties in lattice-mismatched system. In this study, we have studied the effects of graded buffer layer and thermal cycle annealing (TCA) upon solar cell properties, dislocation behavior and other

Table 2 Uncorrected peak efficiencies of 400-suns and 550-suns concentrator cell modules Concentration

Area (cm2)

Site

Ambient (C)

Uncorrected efficiency (peak value) (%)

Direct normal irradiation (W/m2)

400· 400· 400· 400· 550· 550·

7.056 7.056 1.176 1.176 5.445 5.445

Inuyama, Japan Manufacturer Toyohashi, Japan Independent Fraunhofer ISE, Germany Independent NREL, USA Independent Inuyama, Japan Manufacturer Toyohashi, Japan Independent

29 7 19 29 33 28

27.6 25.9 27.4 24.9 28.9 27.0

810 645 839 940 741 777

176

M. Yamaguchi et al. / Solar Energy 82 (2008) 173–180

physical properties in lattice-mismatched InxGa1xAs middle cells. Effects of thermal cycle annealing (TCA) upon reduction in dislocation density measured by the electron beaminduced current method as a function of cycle number N are shown in Fig. 6. In Fig. 6, theoretical results for reduction in dislocation density by TCA described below are also shown. Significant reduction in misfit dislocation density in the lattice mismatched InGaAs cells has been observed after thermal cycle annealing, and as the cycle number is increased the reduction is greater. High quality InGaAs films with a dislocation density of 3–5 · 105 cm2 with high minority-carrier lifetime of about 8 ns have been obtained using only thermal cycle annealing with a TCA temperature of 800 C. As a result of dislocation density reduction by TCA, improvements in minority-carrier lifetime (measured by the photoluminescence decay method) and Voc of InGaAs middle cells were obtained (Sasaki et al., 2006). We have assumed that dislocation motion in the latticed mismatched materials is the same with that in GaAs films heteroepitaxially grown on Si substrates (Yamaguchi et al., 1988). Dislocation annihilations, such as movement toward edges and interfaces, dislocation coalescence, and dislocation re-emission due to thermal annealing, were considered in the analysis as follows: K1

D ! annihilation

ð1Þ

K2

ð2Þ

D þ D ! D2

where D, D2 are densities of dislocations and coalescenced dislocations, respectively, and K1, K2 are the corresponding rate constants. The reaction equations for dislocations are given by dD=dt ¼ K 1 D  K 2 D2

ð3Þ

2

dD2 =dt ¼ K 2 D =2

ð4Þ

The boundary conditions are as follows:

Density of dislocation (cm-2)

Dðt ¼ 0Þ ¼ D0 ;

D2 ðT ¼ 0Þ ¼ 0

The resulting solutions are given by D ¼ 1=½ð1=D0 þ K 2 =K 1 Þ expðK 1 tÞ  K 2 =K 1  D2 ¼

ðD20 K 2 =K 1 Þ½1

2

 ðexpðK 1 tÞÞ 

ð6Þ ð7Þ

Because dislocation coalescence is thought to be the main mechanism on dislocation annihilation in the thermally cycle annealed materials, K1D  K2D2 were assumed. The resulting solution is given by D ¼ D0 =½1 þ D0 K 2 t

ð8Þ

In the analysis, reaction constant K2 was also assumed to be proportional to dislocation velocity. In particular, the velocity of b-dislocations in GaAs bulk crystals (Choi et al., 1977) under a stress of r was used as follows: m ¼ 9:86  109 r1:6 expð1:35eV=kTÞðcm=sÞ K 2 ¼ m=h ¼ am

ð9Þ ð10Þ

where m is dislocation velocity and, a is a constant, determined to be 7.2 · 108 cm from experimental results for GaAs-on-Si (Yamaguchi et al., 1988). The stress as a driving force of dislocation motion is thought to be derived from misfit stress at around annealing temperature. In this case, stress value of 107 dyn/cm2 was used as the stress value. Fig. 6 also shows calculated results for changes in dislocation density for InGaAs solar cells using thermal cycling annealing at the annealing temperature of 800 C. Good agreement between experimental values and calculated results validates the analytical model. Although we have realized 31.7% efficiency with latticemismatched InGaP/InGaAs/Ge 3-junction cell without TCA (Voc = 0.74 V), we couldn’t obtain over 33% at 1-sun by using higher Voc InGaAs middle cells. We expect more than 33% by using 0.775 V middle cell and 35% if we realize 0.83 V middle cell. For concentrator cells, we expect 40% by using 0.775 V middle cell and 42% if we realize 0.83 V middle cell as shown in Fig. 7.

ð5Þ

10000000 EXP

THEORY

1000000

100000 0

1

2

3

Number of TCA Fig. 6. Effects of thermal cycle annealing (TCA) upon reduction in dislocation density measured by the electron beam-induced current method in lattice mismatched InGaAs cells as a function of cycle number N in comparison with those of calculated results.

Fig. 7. Voc of InGaAs middle cells and 3-junction cell efficiency vs. bandgap of InGaAs.

M. Yamaguchi et al. / Solar Energy 82 (2008) 173–180

3.2. High quality (In)GaAsN material for 4- and 5-junction applications Although we have attained 31.7% at 1-sun AM1.5 G with InGaP/InGaAs/Ge 3-junction cell, it is necessary to develop new material such as InGaAsN (Kurtz et al., 2005) with a bandgap energy of 1.04 eV in order to realize 4- or 5-junction cells with efficiencies of more than 40% or 50% as shown in Fig. 4. In addition, 1 eV energy gap material should be lattice-matched to Ge and GaAs. InGaAsN is one of appropriate materials for 4- or 5-junction solar cell configuration because this material has 1 eV band gap and is lattice-matched to GaAs and Ge. However, present InGaAsN single-junction solar cells have been inefficient because of low minority-carrier lifetime due to N-related recombination centers such as N–H–VGa and (N–N)As and low electron mobility due to alloy scattering and non-homogeneity of N. Fig. 8 shows changes in minority-carrier (electron) diffusion length of GaAsN films as a function of N concentration (Kurtz et al., 2005) and calculated acceptor concentration dependenth of minority-carrier diffusion length in GaAsN with changing minority-carrier mobility l. Acceptor concentration Na dependence of minority-carrier diffusion length L were calculated by the following equations: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi L ¼ lskT =q ð11Þ s ¼ 1=BN a

ð12Þ

Electron Diffusion Length (nm)

where s is minority-carrier lifetime, k is Boltzamann constant, q is electronic charge, B is radiative recombination coefficient and B = 2 · 1010 cm3/s in GaAs was used in the calculation. The minority-carrier diffusion length in GaAsN is found to decrease very quickly with the addition of nitrogen as shown in Fig. 8. Nitrogen in GaAsN seems to act as a shallow acceptor impurity and the larger change

10000

177

in electron diffusion length likely reflects the larger change in minority-carrier mobility. InGaAsN material has substantial difficulty, so-called large miscibility gap, due to covalent radius difference of N and As, and difference of binding energy of Ga–N and In–N bonds. Previously, the metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) methods have been widely studied to grow InGaAsN thin films. However, the optical and electrical properties of them have been drastically deteriorated with the incorporation of nitrogen (N). In the case of MOCVD, high growth pressure induces high residual impurity concentration such as C and high growth temperature induces compositional fluctuation of N in InGaAsN films. Therefore, the lower growth temperature is required. With MBE technique, no carbon contamination is observed, but the crystal quality is deteriorated by the damage of N2 plasma as a nitrogen source. To resolve such problems, we have proposed chemical beam epitaxy (CBE) (Yamaguchi et al., 1994) as a new growth technique for InGaAsN materials (Lee et al., 2005; Nishimura et al., 2005). CBE incorporates both the beam nature of MBE and use of all vapor sources and is thought to be high-productivity growth method because of effective source consumption. All sources including N source are supplied to the surface of substrate using metal organic gases and decompose on the growing surface under low pressure (102 Pa). GaAsN epitaxial thin films were grown on semi-insulating GaAs (0 0 1) oriented 2 tilt toward [0 1 0] by CBE. Triethylgallium (TEGa- [Ga(C2H5)3]), trisdimethylaminoarsenic (TDMAAs[As(N(CH3)2)3]), and monomethylhydrazine (MMHy[(CH3)N2H3] were used as the Ga, As and N sources, respectively. Fig. 9 shows the X-ray diffraction (XRD) full width at half maximum (FWHM) of GaAsN (0 0 4) rocking curves as a function of N composition. XRD-FWHM values increases with increasing growth temperature, while the N composition increases. At lower temperature region (<360 C), XRD-FWHM values increase rapidly due to the increasing of lattice-mismatch with increasing N incorporation. The minimum XRD-FWHM of 35.4 arcsec with N composition 0.9%

1000

100

10 1.00E+18

Experimental values μ=30cm2/Vs μ=300cm2/Vs μ=3000cm2/Vs 1.00E+19

1.00E+20

1.00E+21 -3

Acceptor Concentration(cm )

Fig. 8. Changes in minority-carrier (electron) diffusion length of GaAsN films as a function of N concentration and calculated acceptor concentration dependenth of minority-carrier diffusion length in GaAsN with changing minority-carrier mobility l.

Fig. 9. N concentration dependence of XRD (X-ray diffraction)-FWHM (full width at half-maximum) for GaAsN films grown by the CBE and the other methods.

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M. Yamaguchi et al. / Solar Energy 82 (2008) 173–180

obtained at 380 C temperature is close with that of GaAs substrate. All XRD- FWHM values obtained in this study were lower than those obtained in GaAsN films grown by the other methods such as MOCVD (DMHy), MBE (N2), and CBE (N2) techniques. We have also successfully obtained the higher electron mobility (2000 cm2/V s) in GaAsN films compared to those by the conventional MOCVD and MBE methods. For hole mobility in GaAsN films, we have obtained 200 cm2/V s, which is similar with that of p-GaAsN films (N  1%) reported until now. High electron mobility obtained in this study implies that the compositional fluctuation of N can be controlled by low temperature CBE growth. However, the concentrations of impurities (H, C) are still over 1018 cm3 at low growth temperature. Further study for lowering impurity concentration is needed.

p -GaAs p-AlGaAs p-GaAs

MgF2 AR 105nm External Quantum Efficiency (%)

Front Contact: Au-Zn electrode

Since the minority-carrier diffusion length of this alloy is very short, we have investigated the effect of InGaAsN intrinsic layer in p-GaAs/i-n-InGaAsN heterojunction solar cells fabricated by atomic hydrogen-assisted molecular beam epitaxy (MBE). Fig. 10 shows a structure and external quantum efficiencies (EQEs) of p-GaAs/i-n-InGaAsN heterojunction solar cells with varying intrinsic layer thickness, made by using atomic-H assisted RF-MBE. A structure and external quantum efficiencies (EQEs) of pGaAs/i-n-InGaAsN heterojunction solar cells with varying intrinsic layer thickness, made by using atomic-H assisted RF-MBE. In order to increase photo current generation in poor quality InGaAsN sub-cells, p-i-n structure was used. As shown in the figure, the insertion of i-layer results in the overall improvements of EQE. As a result, it is thought that minority-carrier diffusion length is equiva-

50nm 30nm 0.25μm 0 ∼1 m

i-InGaAsN n-InGaAsN 1.0 ∼ 0.4 m + n -GaAs buffer 0.5 m + n -GaAs substrate Back Contact: In electrode

100

Intrinsic layer width 0nm 300nm 600nm 1000nm

80 60 40 20 0

600

800

1000

Wavelength (nm) Fig. 10. A structure and external quantum efficiencies (EQEs) of p-GaAs/i-n-InGaAsN heterojunction solar cells with varying intrinsic layer thickness, made by using atomic-H assisted RF-MBE.

Intrinsic layer width 600nm 25 2

Current Density (mA/cm )

Projected PV curve

20 Intrinsic layer width 600nm

15

Jsc=22.57(mA/cm2) Voc=0.73(V) Jmax=20.37(mA/cm2) Vmax=0.55(V) FF=0.68 n-value=1.88 η=11.27(%)

10 5 0 0.0

0.5

1.0

Voltage (V) i-layer width (nm) Efficiency (% )

0

300

AM1.5 600

1000

7.05 10.17 11.27 8.38

Fig. 11. Current–voltage curve of a p-GaAs/i-n InGaAsN hetero-junction cell with intrinsic layer width of 600 nm.

M. Yamaguchi et al. / Solar Energy 82 (2008) 173–180

E (eV)

3.5

3.0

absorption PL peak [Shan] [Pereira]

2.5

Eg

InN

Eg

InN

=0.77eV, b=1.43eV =1.9eV, b=2.63eV

2.0

1.5

1.0

In Ga N, 295K 1-x

0.50

0

0.2

0.4

0.6

x

0.8

1

x Fig. 12. Composition dependence of bandgaps of In1xGaxN alloys.

lently improved from less than 100 nm to 600 nm by inserting i-layer. Fig. 11 shows current–voltage curve of a pGaAs/i-n-InGaAsN heterojunction solar cell with i-layer thickness of 600 nm. With an optimized i-layer thickness of 600 nm, 11.27% efficiency has been obtained (Miyashita et al., 2006). 3.3. Preliminary study on new InGaN materials for 10junction cells As a result of the re-measurement of the bandgap of InN, InGaN material system is known to offer a substantial potential to develop super high-efficiency solar cells because this material system ranges the major bulk of the solar spectrum from 3.4 eV to 0.7 eV as shown in Fig. 12 (Davydov et al., 2002; Walukiewicz et al., 2004). Since a continuum of bandgaps can be obtained by changing compositions of In and Ga, 10-junction solar cells with conversion efficiency of more than 50% will be able to fabricate. However, experimental minority-carrier lifetime in these materials is currently dominated by material quality. Achieving p-type conductivity in InGaN alloys is difficult partly due to high background electron concentration. In this study, the best data for MOVPE samples with lowest residual carrier concentration of 4.5 · 1018 cm3 and highest electron mobility of 1100 cm2/V s have been obtained (Hashimoto et al., 2005).

179

dark current was reduced and external quantum efficiencies (EQE) in the wavelength regions of 650–850 nm and the longer wavelength region above 900 nm are improved compared to those of 2-step PM-QWs and conventional QWs. 3-step InGaAs/GaAs PM-MQW solar cell has shown projected efficiency of 18.27% that is higher than 16.56% of conventional QW solar cell. Quantum dot (QD) solar cells have also attracted attention as possible photon utilization. By taking advantage of spontaneous self-assembly or self-organization mechanism of coherent 3D islanding during growth, known as Stranski–Krastanow (S–K) growth mode in lattice-mismatched epitaxy, In(Ga)As QDs with dot diameter of 63–70 nm and size uniformity of 12–14% have successfully been fabricated on GaAs or InP (311)B substrate. As a preliminary result, 7.65% efficiency has been obtained with InAs quantum dot cells (Okada et al., 2005). 4. Summary We have developed high efficiency (38.9% at 489-suns AM1.5G) InGaP/InGaP/Ge 3-junction solar cells and large-area (5.445 cm2) 3-junction concentrator cell modules with an efficiency of 28.9%. In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) preliminary study on new InGaN materials for 10-junction cells, (5) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (6) 7.65% efficiency InAs quantum dot cells. Acknowledgements This work is partially supported by the Ministry of Education, Culture, Sports, Science and Technology as a part of the Private University Academic Frontier Center Program ‘‘Super-High Efficiency Photovoltaic Research Center’’. This work is also partially supported by the New Energy and Industrial Technology Development Organization as a part of the New Sunshine Program under the Ministry of Economy, Trade and Industry, Japan.

3.4. Novel quantum well and dot structures References The multi-quantum well (MQW) solar cells have attracted attention for high efficiency, however, the reported results are lower than the calculated values. In order to achieve higher carrier collection efficiencies out of QWs thereby minimizing recombination losses, potentially modulated (PM) MQW structure has been proposed (Okada and Shiotsuka, 2005). By using 3-step PM-QWs,

Araki, K., Kondo, M., Uozumi, H., Yamaguchi, M., 2003. Development of a Robust and high efficiency concentrator receiver. In: Proceedings of the Third World Conference on Photovoltaic Energy Conversion, Osaka, Japan, pp. 630. Araki, K., Uozumi, H., Egami, T., Hiramatsu, M., Miyazaki, Y., Kemmoku, Y., Akisawa, A., Ekins-Daukes, N.J., Lee, H.S., Yamaguchi, M., 2005. Development of concentrator modules with dome-shaped

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