Novel materials for high-efficiency III–V multi

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.
178
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
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(Okada and Shiotsuka, 2005). By using 3-step PM-QWs,
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M. Yamaguchi et al. / Solar Energy 82 (2008) 173–180
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