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High efficiency a-Si solar cells with a superlattice structure p-layer and stable a-Si solar cells with reduced SiH2 bond density
Journal of Non-CrystallineSolids 97&98 (1987) 289-292 North-Holland, Amsterdam
289
Section 8 : Device Physics (Focused session 6)
HIGH EFFICIENCY a-Si SOLAR CELLS WITH A SUPERLATTICE STRUCTURE P-LAYER AND STABLE a-Si SOLAR CELLS WITH REDUCED Si-H 2 BOND DENSITY
Y. KUWANO, H. TARUI, T. TAKAHAMA, M. NISHIKUNI, Y. HISHIKAWA, N. NAKAMURA, S. TSUDA, S. NAKANO, and M. OHNISHI* Research Center, Applied Research Center , SANYO Electric Co., Ltd. 1-18-13, Hashiridani, Hirakata City, Osaka, Japan
In order to improve the conversion efficiency of a-Si solar cells, a superlattice structure p-layer and the use of B(CHR) 3 as a dopant gas have been investigated for the first time. The collectlon efficiency spectrum of a glass/TCO/p-superlattice structure(a-SiC/a-Si)/in/Metal cell shows a remarkable increase in the short wavelength region. This result suggests that the superlattice structure p-layer is an active photovoltaic layer. It is also found that the photoconductivity of p-type a-Si:H films increases by using B(CHq)R as a dopant gas. A conversion efficiency of I0.0 % (module efficiency 9.02-%) is obtained for a i0 cm x i0 cm integrated single Junction a-Si solar cell submodule using B(CH3) 3. As for reliability it is found that the light induced degradation of a-Si solar cells can be reduced by simultaneous reduction of the impurity concentrations in a-Si with the super chamber (separated UHV reaction chamber) and the Si-H 2 bond density.
i. INTRODUCTION The conversion efficiency of a-Si solar cells was improved to 11.7 % for a glass/textured TCO/pin/Ag structure with a high-quality i-layer fabricated by a super-chamber method and a highly conductive p-layer fabricated by the photo-CVD method. 1 required.
Still, further improvement in conversion efficiency is
Also, the problem of light-induced degradation still remains.
In
this paper, first, characteristics of solar cells using a superlattice structure p-layer fabricated by a photo-CVD method are investigated.
Second,
high quality p-layer using B(CH3) 3 as a dopant gas is referred to. Finally, an investigation on light-induced degradation, especially with regard to the influence of hydrogen and impurities, is described. 2. SOLAR CELLS WITH THE a-Si/a-SiC SUPERLATTICE STRUCTURE P-LAYER The superlattice structure p-layer films were fabricated by a photo-CVD method 2. method.
The i-layer and the n-layer were fabricated by a glow discharge Typical collection efficiency spectra of a-Si solar cells are shown in
Fig. 1 (broken lines).
The collection efficiency at short wavelengths is
remarkably high for the cell with the superlattice structure p-layer.
The
experimental results are investigated by an optical analysis for the multi-layered structure.
The calculated collectSon efficiency spectra of solar
0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Y. Kuwano et a L / High efficiency a-Si solar cells
290
cells are also shown in Fig. 1 (solid lines), where P is the inner quantum efficiency of the p-layer.
The
collection efficiency spectrum agrees with the measured spectrum when the p-layer is assumed to be a photogenerating layer with P=0.7, which
Measured : superls~ice
188
suggests that the superlattice structure p-layer is an active photovoltaic layer, and is effective for improving the collection efficiency at short
58
= o
wavelengths. Fig. 2 shows DICE (dynamic inner
%8
collection efficiency) spectra of a-Si
Wavelength (nm)
solar cells at the forward bias of 0.55V 3.
DICE values at the p/i
interface are significantly high when the superlattice structure is used for the p-layer,
Fig. i Cell structure and collection efficiency spectra of superlattice structure p-layer solar cells
which also indicates the photovoltaic effect of the p-layer.
N-type superlattice structure a-Si films are also investigated.
Figure 3
shows the conductivity and the Eop t (optical bandgap) of n-type a-Si/a-SiC superlattice structure films as a function of the well layer thickness.
It is
found that wide-bandgap n-type films with high photoconductivity are obtained by the superlattice structure. 3. HIGH QUALITY P-TYPE a-SiC:H FILMS USING B(CH3) 3 Next, high-quality a-SiC:H films with a new doping gas of B(CH3) 3 are
I
J
P
% o
~" O'ph
o
16 s
T
1
2.2 .IB .G .4 .2
~ I ~ ~c--"-L
Conventional ( p layer
I Superlattiee p layer
8
:>,
-~ 16' c o o
Z
~
o LU
2.0
1'o' 2'5 ~'o 16o Well l a y e r t h i c k n e s s
DEPTH OF I - L R Y E R
2.1
Eopt
(,~,)
(R)
Fig. 2 DICE spectra of a-Si solar cells
Fig. 3 Conductivity and of n-type superlattlce E°Pt structure a-Si
Y. Kuwano et a L / High efficiency a-Si solar cells
i o~
'E o
~%
°°-'~-....~ o
1 O" >. .>
O'ph
o
~ I00
O'ph
AM-1 lOOmW/cm2 ~tegratedtype] Modules i z e (l~S) lOcmx lOcm Voe (V) Is= (mA) F.F. Pmax (mW)
IE \
10 s
so
a-SiC by B(CH3)3 \ ------ a-SiC by B2HB
--
o o
12. 29
I07, 5 0. 683
902 (intrinsic,%) i 0 . 0
1:8 119 210 211 Optical bandgap (eV)
0
f
II0 Voltage (V)
Fig. 5 Illuminated I-V characteristics of an a-Si solar cell submodule
Fig. 4 Photoconductivity of a-SiC films as a function of optical bandgap investigated for the first time.
291
It is found that the photoconductlvity of
a-SiC films is significantly improved by using B(CH3) 3 compared with that using B2H6, as shown in Fig. 4.
The increase in the photoconductivity is probably
due to the reduction of B-B bonds in a-Si films.
A conversion efficiency of
i0.0 % (module efficiency 9.02 %) has been obtained for a textured TCO/high quality p/high quality i/n/Ag structure as shown in Fig. 5 for a i0 cm x i0 cm integrated a-Si solar cell submodule by using B(CH3) 3 for the doping of the p-layer as well as the high quality i-layer and a laser patterning method. 4. ROLE OF HYDROGEN IN THE LIGHT-INDUCED EFFECTS OF LOW-IMPURITY a-Si:H Recently it was shown that factors associated with light-induced defects around the midgap have influence on light-induced degradation in low impurity films 3.
Therefore, we investigate the influence of hydrogen, which is
related to the structural flexibility and the light-induced defects around the 0 0.7
beforeexposure
---'%"--g~^
o
(AM-I 500mW/cm=,Shrs)
,~.
o.s
I
i0=,
I
0.151
i0==
0.10 0.05
0.05
0
,,,, 1 01
The changes i n f i l l
factor as a fanetlon of SIH 2 bond density
Annesling temp. O 90"C A 1 10"C V 130"C [] 1 5 0 " C /
0.15
o.1o
Density of Si-H= bond(cm-3) Fig. 6
ii i, i,
X 170"C
a f t ~ L~. 0 . 6
u-I Anneslin~ temp. O 90"C Zl I 1 0 ' C u-i V 1 3 0 " C u-I 150"C
102
Annealing time (min)
0s
ol
,I
01
~
0 2 1 ~,-3 , , ,,,,id" i , ,,,,,, 102 103
Annealing time (min)
Fig. 7 Recovery of the fill factor by thermal annealing
292
Y. Kuwano et al. / High efficiency a-Si solar cells
mldgap.
Fig. 6 shows the fill factor of a-Si solar cells before and after
light exposure as a function of the SIH 2 bond density in the i-layer.
Figure 6
indicates that the reduction of the SIH 2 bond density is effective for both reducing the light-lnduced degradation and improving the conversion efficiency. In order to clarify the role of SiH 2 bonds in the light induced effect, experiments on the recovery processes of a-Si solar cells by thermal annealing are carried out (Fig. 7).
We modify the theory of a recovery process proposed
by Eser et al. 4,5 as follows,
K1 FF-FF
0 =C
+
R_-~
Here, R : order of reaction, E
r
Er (logt -
k-----~lOgl0e) . . . .
(i)
: activation energy of the recovery,
FFo, KI,C : constant Equation (i) shows good agreement with the experimental results as shown by the llnes in Fig. 7.
As a result, it is found that the R value becomes small
when the SiH 2 bond density is large.
This result indicates that the SiH 2 bonds
induce a structural change around the sites related to the llght induced effect, which suggests that the role of SI-H 2 bond on the light induced effect is related to the structure of the silicon network of a-Sl. 4. CONCLUSIONS Investigations are carried out for high-efficiency, solar cells.
high-reliabillty a-Sl
In order to improve the conversion efficiency, a new type of
solar cell using a superlattice structure p-layer fabricated by a photo-CVD method are analyzed. photovoltaic layer,
It is found that the p-layer acts as an active p-type a-SiC:H films using B(CH3) 3 as a dopant gas are
also studied for the first time.
It is found that highly photoconductive
p-type a-SIC:H films are obtained by using B(CH3) 3.
A conversion efficiency of
i0.0 % (module efficiency 9.02 %) is obtained for a i0 cm x i0 cm integrated a-Si solar cell by using B(CH3) 3.
As for stability, recovery experiments
indicate that an improvement in the stability of a-Si:H films by the reduction of SIH 2 bond density is related to the modification of the silicon network. ACKNOWLEDGEMENT This work is supported by NEDO as a part of the Sunshine Project under the Ministry of International Trade and Industry. REFERENCES i) S. Nakano et al. : MRS Symposia Proceedings, 70, 511 (1986). 2) S. Tsuda et al. : Proc. 18th IEEE Photovol. Spec. Conf. 1295 (1985). 3) Y. Kuwano et al. : International Conf. Stability of Amorphous Silicon Alloy Materials and Devices, Palo Alto (1987). 4) Z. E. Smith and S. Wagner : J. Non-Cryst. Solids, 77&78, 1461 (1985). 5) E. Eser : J. Appl. Phys. 59-10 (1986).
289
Section 8 : Device Physics (Focused session 6)
HIGH EFFICIENCY a-Si SOLAR CELLS WITH A SUPERLATTICE STRUCTURE P-LAYER AND STABLE a-Si SOLAR CELLS WITH REDUCED Si-H 2 BOND DENSITY
Y. KUWANO, H. TARUI, T. TAKAHAMA, M. NISHIKUNI, Y. HISHIKAWA, N. NAKAMURA, S. TSUDA, S. NAKANO, and M. OHNISHI* Research Center, Applied Research Center , SANYO Electric Co., Ltd. 1-18-13, Hashiridani, Hirakata City, Osaka, Japan
In order to improve the conversion efficiency of a-Si solar cells, a superlattice structure p-layer and the use of B(CHR) 3 as a dopant gas have been investigated for the first time. The collectlon efficiency spectrum of a glass/TCO/p-superlattice structure(a-SiC/a-Si)/in/Metal cell shows a remarkable increase in the short wavelength region. This result suggests that the superlattice structure p-layer is an active photovoltaic layer. It is also found that the photoconductivity of p-type a-Si:H films increases by using B(CHq)R as a dopant gas. A conversion efficiency of I0.0 % (module efficiency 9.02-%) is obtained for a i0 cm x i0 cm integrated single Junction a-Si solar cell submodule using B(CH3) 3. As for reliability it is found that the light induced degradation of a-Si solar cells can be reduced by simultaneous reduction of the impurity concentrations in a-Si with the super chamber (separated UHV reaction chamber) and the Si-H 2 bond density.
i. INTRODUCTION The conversion efficiency of a-Si solar cells was improved to 11.7 % for a glass/textured TCO/pin/Ag structure with a high-quality i-layer fabricated by a super-chamber method and a highly conductive p-layer fabricated by the photo-CVD method. 1 required.
Still, further improvement in conversion efficiency is
Also, the problem of light-induced degradation still remains.
In
this paper, first, characteristics of solar cells using a superlattice structure p-layer fabricated by a photo-CVD method are investigated.
Second,
high quality p-layer using B(CH3) 3 as a dopant gas is referred to. Finally, an investigation on light-induced degradation, especially with regard to the influence of hydrogen and impurities, is described. 2. SOLAR CELLS WITH THE a-Si/a-SiC SUPERLATTICE STRUCTURE P-LAYER The superlattice structure p-layer films were fabricated by a photo-CVD method 2. method.
The i-layer and the n-layer were fabricated by a glow discharge Typical collection efficiency spectra of a-Si solar cells are shown in
Fig. 1 (broken lines).
The collection efficiency at short wavelengths is
remarkably high for the cell with the superlattice structure p-layer.
The
experimental results are investigated by an optical analysis for the multi-layered structure.
The calculated collectSon efficiency spectra of solar
0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Y. Kuwano et a L / High efficiency a-Si solar cells
290
cells are also shown in Fig. 1 (solid lines), where P is the inner quantum efficiency of the p-layer.
The
collection efficiency spectrum agrees with the measured spectrum when the p-layer is assumed to be a photogenerating layer with P=0.7, which
Measured : superls~ice
188
suggests that the superlattice structure p-layer is an active photovoltaic layer, and is effective for improving the collection efficiency at short
58
= o
wavelengths. Fig. 2 shows DICE (dynamic inner
%8
collection efficiency) spectra of a-Si
Wavelength (nm)
solar cells at the forward bias of 0.55V 3.
DICE values at the p/i
interface are significantly high when the superlattice structure is used for the p-layer,
Fig. i Cell structure and collection efficiency spectra of superlattice structure p-layer solar cells
which also indicates the photovoltaic effect of the p-layer.
N-type superlattice structure a-Si films are also investigated.
Figure 3
shows the conductivity and the Eop t (optical bandgap) of n-type a-Si/a-SiC superlattice structure films as a function of the well layer thickness.
It is
found that wide-bandgap n-type films with high photoconductivity are obtained by the superlattice structure. 3. HIGH QUALITY P-TYPE a-SiC:H FILMS USING B(CH3) 3 Next, high-quality a-SiC:H films with a new doping gas of B(CH3) 3 are
I
J
P
% o
~" O'ph
o
16 s
T
1
2.2 .IB .G .4 .2
~ I ~ ~c--"-L
Conventional ( p layer
I Superlattiee p layer
8
:>,
-~ 16' c o o
Z
~
o LU
2.0
1'o' 2'5 ~'o 16o Well l a y e r t h i c k n e s s
DEPTH OF I - L R Y E R
2.1
Eopt
(,~,)
(R)
Fig. 2 DICE spectra of a-Si solar cells
Fig. 3 Conductivity and of n-type superlattlce E°Pt structure a-Si
Y. Kuwano et a L / High efficiency a-Si solar cells
i o~
'E o
~%
°°-'~-....~ o
1 O" >. .>
O'ph
o
~ I00
O'ph
AM-1 lOOmW/cm2 ~tegratedtype] Modules i z e (l~S) lOcmx lOcm Voe (V) Is= (mA) F.F. Pmax (mW)
IE \
10 s
so
a-SiC by B(CH3)3 \ ------ a-SiC by B2HB
--
o o
12. 29
I07, 5 0. 683
902 (intrinsic,%) i 0 . 0
1:8 119 210 211 Optical bandgap (eV)
0
f
II0 Voltage (V)
Fig. 5 Illuminated I-V characteristics of an a-Si solar cell submodule
Fig. 4 Photoconductivity of a-SiC films as a function of optical bandgap investigated for the first time.
291
It is found that the photoconductlvity of
a-SiC films is significantly improved by using B(CH3) 3 compared with that using B2H6, as shown in Fig. 4.
The increase in the photoconductivity is probably
due to the reduction of B-B bonds in a-Si films.
A conversion efficiency of
i0.0 % (module efficiency 9.02 %) has been obtained for a textured TCO/high quality p/high quality i/n/Ag structure as shown in Fig. 5 for a i0 cm x i0 cm integrated a-Si solar cell submodule by using B(CH3) 3 for the doping of the p-layer as well as the high quality i-layer and a laser patterning method. 4. ROLE OF HYDROGEN IN THE LIGHT-INDUCED EFFECTS OF LOW-IMPURITY a-Si:H Recently it was shown that factors associated with light-induced defects around the midgap have influence on light-induced degradation in low impurity films 3.
Therefore, we investigate the influence of hydrogen, which is
related to the structural flexibility and the light-induced defects around the 0 0.7
beforeexposure
---'%"--g~^
o
(AM-I 500mW/cm=,Shrs)
,~.
o.s
I
i0=,
I
0.151
i0==
0.10 0.05
0.05
0
,,,, 1 01
The changes i n f i l l
factor as a fanetlon of SIH 2 bond density
Annesling temp. O 90"C A 1 10"C V 130"C [] 1 5 0 " C /
0.15
o.1o
Density of Si-H= bond(cm-3) Fig. 6
ii i, i,
X 170"C
a f t ~ L~. 0 . 6
u-I Anneslin~ temp. O 90"C Zl I 1 0 ' C u-i V 1 3 0 " C u-I 150"C
102
Annealing time (min)
0s
ol
,I
01
~
0 2 1 ~,-3 , , ,,,,id" i , ,,,,,, 102 103
Annealing time (min)
Fig. 7 Recovery of the fill factor by thermal annealing
292
Y. Kuwano et al. / High efficiency a-Si solar cells
mldgap.
Fig. 6 shows the fill factor of a-Si solar cells before and after
light exposure as a function of the SIH 2 bond density in the i-layer.
Figure 6
indicates that the reduction of the SIH 2 bond density is effective for both reducing the light-lnduced degradation and improving the conversion efficiency. In order to clarify the role of SiH 2 bonds in the light induced effect, experiments on the recovery processes of a-Si solar cells by thermal annealing are carried out (Fig. 7).
We modify the theory of a recovery process proposed
by Eser et al. 4,5 as follows,
K1 FF-FF
0 =C
+
R_-~
Here, R : order of reaction, E
r
Er (logt -
k-----~lOgl0e) . . . .
(i)
: activation energy of the recovery,
FFo, KI,C : constant Equation (i) shows good agreement with the experimental results as shown by the llnes in Fig. 7.
As a result, it is found that the R value becomes small
when the SiH 2 bond density is large.
This result indicates that the SiH 2 bonds
induce a structural change around the sites related to the llght induced effect, which suggests that the role of SI-H 2 bond on the light induced effect is related to the structure of the silicon network of a-Sl. 4. CONCLUSIONS Investigations are carried out for high-efficiency, solar cells.
high-reliabillty a-Sl
In order to improve the conversion efficiency, a new type of
solar cell using a superlattice structure p-layer fabricated by a photo-CVD method are analyzed. photovoltaic layer,
It is found that the p-layer acts as an active p-type a-SiC:H films using B(CH3) 3 as a dopant gas are
also studied for the first time.
It is found that highly photoconductive
p-type a-SIC:H films are obtained by using B(CH3) 3.
A conversion efficiency of
i0.0 % (module efficiency 9.02 %) is obtained for a i0 cm x i0 cm integrated a-Si solar cell by using B(CH3) 3.
As for stability, recovery experiments
indicate that an improvement in the stability of a-Si:H films by the reduction of SIH 2 bond density is related to the modification of the silicon network. ACKNOWLEDGEMENT This work is supported by NEDO as a part of the Sunshine Project under the Ministry of International Trade and Industry. REFERENCES i) S. Nakano et al. : MRS Symposia Proceedings, 70, 511 (1986). 2) S. Tsuda et al. : Proc. 18th IEEE Photovol. Spec. Conf. 1295 (1985). 3) Y. Kuwano et al. : International Conf. Stability of Amorphous Silicon Alloy Materials and Devices, Palo Alto (1987). 4) Z. E. Smith and S. Wagner : J. Non-Cryst. Solids, 77&78, 1461 (1985). 5) E. Eser : J. Appl. Phys. 59-10 (1986).