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High efficiency a-Si:H alloy cell deposited at high deposition rate
Journal of Non-Crystalline Solids 164-166 (1993) 689-692 North-Holland
IOURNAL or
~I,L]]~
S0~
High efficiency a-Si:H alloy cell deposited at high deposition rate K.Saito,M.Sano,K.Ogawa and I.Kajita E project,Canon,4-1-1 Kizugawadai,Kizu-cho, Soraku-gun,Kyoto,Japan We have made an a-Si:H alloy triple cell, which has intrinsic layers, prepared by low pressure microwave plasma CVD of Sill 4 or Sill 4 mixed with GeH 4. The deposition rate is about one hundred angstrom/sec. The efficiency of the triple cell has reached 11.6% under AM1.5 100mW/cm 2. Using the intrinsic layer deposited at 7.5 angstrom/sec in the top cell,the efficiency of the triple cell has reached 12.3% under AM1.5 100mW/cm 2. The Urbach energy of the intrinsic a-Si:H layer deposited at 75 angstrom/sec is about 50meV, and the Urbach energy of the intrinsic a-SixGe 1.x:H layer which has 100 angstrom/sec deposition rate is about 50meV. 1. introduction Amorphous silicon(a-Si:H) alloy solar cells have been expected to be candidates for the low cost solar cell. But the electric power cost of an a-Si:H alloy solar cell is higher than that of thermal or nuclear power generation[I]. In order to reduce the electric power cost of a-Si:H alloy solar cells, it is necessary to increase the deposition rate of a-Si:H alloys and to increase the source gas utilization efficiency, The experiment on high rate deposition of a-Si alloy has been tried with microwave(MW) PCVD[2], very high frequency(VHF) PCVD[3] and PCVD using higher silanes[4]. In such experiments, the deposition rate was under 20 angstrom/sec. Higher deposition rates of intrinsic(i) layer decrease the efficiency of the solar cell. So far, no one has made a high efficiency a-Si:H alloy cell with an i layer deposited at about one hundred angstrom/sec, In order to increase the deposition rate, the source gas has to be decomposed in a higher electromagnetic energy. There are many active species in the deposition chamber, so the species are polymerized in the gas phase and on the substrate surface,
We speculate that the high deposition rate a-Si:H alloy film deposited in the high pressure contains polymers which have poor electrical properties. We think it is important that the a-Si:H alloy is deposited in low pressure without the polymerization reaction. In this paper, we will report on studies of the a-Si:H alloy cell which has an intrinsic layer,and which has a deposition rate about one hundred angstrom/sec, deposited at low pressure. 2. Experimental a-Si:H and a-SixGel.x:H intrinsic layer were prepared by low pressure MW PCVD of Sill 4 or Sill 4 mixed with GeH 4 respectively. The pressure in the deposition chamber was between 0.1 and 30mTorr. The microwave frequency was about 2.45GHz. The microwave power into deposition chamber was between 100 and 1000W. Stainless steel substrates coated with Ag/ZnO were used for cell preparation, a-Si:H single cells were deposited on specular substrates, a-SixGel.x:H single cells and triple cells were deposited on textured substrates, p+ and n+ layers for the cells were prepared by rf PCVD.
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
690
K. Saito et al. I a-Si:H alloy cell deposited at high deposition rate
Urbach energies of a-Si:H and a-SixGel.x:H
6
were measured with the constant photocurrent method(CPM) for a-Si:H single cell and a-SixGe 1.x:H single cell respectively.
5 ~. •, ~ 4 X3
IV characteristics of the triple cells were measured by a solar simulator under AM1.5
e-
,.,~= o 2
100mW/cm 2 at25"C.
1
3. Results 3.1. a-Si:H single cell Fig.1 shows the tail state absorption spectrum of an a-Si:H single cell which has an i layer prepared by low pressure MW PCVD of Sill 4 at
0 o.5
the rate of 75 angstrom/sec. The Urbach energy is about 50meV calculated from the tail state absorption spectrum curve of the a-Si:H cell in Fig.l. In spite of the high rate deposition of a-Si:H, the Urbach energy of the a-Si:H is almost comparable with the Urbach energy of the a-Si:H deposited by RF PCVD at low
f r • o • ~.~o..Oe'~ -
1
1.5 2 2.5 h ~(oV) Fig.1 The tail state absorption spectrum of a-Si:H, Urbach energy:~50meV GeH 4 at a deposition rate of 100 angstrom/sec. The Urbach energy is about 50meV calculated from the tail state absorption spectrum curve of a-SixGel.x:H cell in Fig.2. This Urbach energy of the MW PCVD a-SIxGel.x:H is as small as
rates,
the Urbach energy of low rate RF PCVD a.SixGel.x:H"
This low pressure MW PCVD can utilize Sill 4 gas much more efficiently than RF PCVD. In this MW PCVD, the decomposition efficiency of Sill 4 gas is almost 100% which is estimated
Also in this low pressure MW PCVD the decomposition efficiency of Sill 4 mixed with GeH 4 is almost 100% estimated from the
from the deposition rate. In order to increase the deposition rate of a-Si:H, it is important to decrease the pressure in the chamber and as a result to prevent the polymerization reaction in the gas phase and on the substrate surface.
deposition rate. 7 6 ..., 5 ~ 4 '~" 3 o
3.2. a - S i x G e l . x : H single cell Fig.2 shows the tail state absorption spectrum of a-SixGel.x:H single cell which has a flat band gap a-SixGel.x:H i layer. The flat band gap a-SixGe 1.x:H i layer was prepared by low pressure MW PCVD of Sill 4 mixed with 37%
. . . . . ~,~p~r .
- 2 1 0 0.5
;..., ,-.• 1
1.5
2
h v (eV) Fig.2 The tail state absorption spectrum of a-SixGe 1.x:H, Urbach energy:~50meV
K. Saito et al. / a-Si:H alloy cell deposited at high deposition rate
3.3.a-Si:H/a-SixGe 1.x:H/a-sixGel.x:H triple cell Fig.3 shows the triple cell structure having a-Si:H top cell, a-SixGel.x:H middle cell and a-SixGe 1.x:H bottom cell.
~
~
IT0 p+
'(
e 4 I ~, 2 E 0 "3
MW a-Si:H
Voc: 2.312V Jsc: 7.20mA/cm2
-2
F.F.: 0.698 EFF: 11.61
-4
area:0.25cm2
4 " " " ' = -1
0
n+ p+
MW a-SixGel-x:H
691
1
/
2
3
v(~ Fig.4 The IV curve of the triple cell which has a high rate a-Si:H top cell(75A/sec) 4
n+
p+ MW a-SixGel-x:H
2
~'
0 "
~ -2.
voc: 2.30V Jsc: 7.38mA/cm2
=8 !
F,F,: 0,72S EFF: 12.3%
~
~
~" -4 •
O/Ag/SUS Fig.3 The triple cell structure
. area:0.25cm2
• ~mm
-8 -1
0
V(V)
2
3
We have made two types of triple cells; one has a high rate a-Si:H top cell (75angstrom/sec), the other has a low rate top a-Si:H cell (7.5angstrom/sec).a-SixGe 1.x:H middle and
Fig.5 The IV c u r v e of the triple cell which has a low rate a-Si:H top cell (7.5 A/sec)
bottom cells have a high deposition rate i layer
4. Conclusion
(100angstrom/sec).
We have made a triple cell in which i layers
Fig.4 shows the IV curve of the triple cell which has a high rate a-Si:H top cell. The efficiency of the triple cell which has a high rate a-Si:H top cell is 11.6%. Fig.5 shows the IV curve of the triple cell which has a low rate a-Si:H top cell. The efficiency of the triple cell which has a low rate a-Si:H top cell is 12.3 %. As a result, even though i layers were deposited at high deposition rate, the efficiency of the triple cell is high.
were deposited at about 100 angstrom/sec by low pressure microwave plasma CVD. The efficiency of the triple cell is 11.6%. The efficiency of the triple cell which has a low rate a-Si:H top cell (7.5 angstrom/sec) is 12.3%. We have achieved high deposition rate and high source gas utilization in the solar cell preparation, while maintaining high solar cell efficiency.
692
K. Saito et al. / a-Si:H alloy cell deposited at high deposition rate
Acknowledgement
References
We would like to express our thanks to members of E project Research and Development Department 12 for the solar cell preparation, measurement and useful discussions. We wish to thank Mr. E.Kondo for his unceasing interest, support and encouragement.
1.Y.Hamakawa, 22nd I E E E Photovoltaic Specialist Conference, (1991)1199 2.T.Watanabe,K.Azuma,M.Tanaka,M.Nakatani, T.Sonobe and T.Simada, Jap.J.AppI.Phys. Vol.27(1988)1126 3.A.Shah,J.Dutta,N.Wyrsch,K.Prasad,H.Curtins, F.Finger, A.Howling and Ch.Hollenstein, Mat.Res.Symp.Proc. Vol.258(1992)15 4.P.K.Bhat, C.Marshall, J.Sandwish, H.Chatham, R.E.I.Schro and A.Madan, 20th IEEE Photovoltaic Specialists Conference,(1988)91
IOURNAL or
~I,L]]~
S0~
High efficiency a-Si:H alloy cell deposited at high deposition rate K.Saito,M.Sano,K.Ogawa and I.Kajita E project,Canon,4-1-1 Kizugawadai,Kizu-cho, Soraku-gun,Kyoto,Japan We have made an a-Si:H alloy triple cell, which has intrinsic layers, prepared by low pressure microwave plasma CVD of Sill 4 or Sill 4 mixed with GeH 4. The deposition rate is about one hundred angstrom/sec. The efficiency of the triple cell has reached 11.6% under AM1.5 100mW/cm 2. Using the intrinsic layer deposited at 7.5 angstrom/sec in the top cell,the efficiency of the triple cell has reached 12.3% under AM1.5 100mW/cm 2. The Urbach energy of the intrinsic a-Si:H layer deposited at 75 angstrom/sec is about 50meV, and the Urbach energy of the intrinsic a-SixGe 1.x:H layer which has 100 angstrom/sec deposition rate is about 50meV. 1. introduction Amorphous silicon(a-Si:H) alloy solar cells have been expected to be candidates for the low cost solar cell. But the electric power cost of an a-Si:H alloy solar cell is higher than that of thermal or nuclear power generation[I]. In order to reduce the electric power cost of a-Si:H alloy solar cells, it is necessary to increase the deposition rate of a-Si:H alloys and to increase the source gas utilization efficiency, The experiment on high rate deposition of a-Si alloy has been tried with microwave(MW) PCVD[2], very high frequency(VHF) PCVD[3] and PCVD using higher silanes[4]. In such experiments, the deposition rate was under 20 angstrom/sec. Higher deposition rates of intrinsic(i) layer decrease the efficiency of the solar cell. So far, no one has made a high efficiency a-Si:H alloy cell with an i layer deposited at about one hundred angstrom/sec, In order to increase the deposition rate, the source gas has to be decomposed in a higher electromagnetic energy. There are many active species in the deposition chamber, so the species are polymerized in the gas phase and on the substrate surface,
We speculate that the high deposition rate a-Si:H alloy film deposited in the high pressure contains polymers which have poor electrical properties. We think it is important that the a-Si:H alloy is deposited in low pressure without the polymerization reaction. In this paper, we will report on studies of the a-Si:H alloy cell which has an intrinsic layer,and which has a deposition rate about one hundred angstrom/sec, deposited at low pressure. 2. Experimental a-Si:H and a-SixGel.x:H intrinsic layer were prepared by low pressure MW PCVD of Sill 4 or Sill 4 mixed with GeH 4 respectively. The pressure in the deposition chamber was between 0.1 and 30mTorr. The microwave frequency was about 2.45GHz. The microwave power into deposition chamber was between 100 and 1000W. Stainless steel substrates coated with Ag/ZnO were used for cell preparation, a-Si:H single cells were deposited on specular substrates, a-SixGel.x:H single cells and triple cells were deposited on textured substrates, p+ and n+ layers for the cells were prepared by rf PCVD.
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
690
K. Saito et al. I a-Si:H alloy cell deposited at high deposition rate
Urbach energies of a-Si:H and a-SixGel.x:H
6
were measured with the constant photocurrent method(CPM) for a-Si:H single cell and a-SixGe 1.x:H single cell respectively.
5 ~. •, ~ 4 X3
IV characteristics of the triple cells were measured by a solar simulator under AM1.5
e-
,.,~= o 2
100mW/cm 2 at25"C.
1
3. Results 3.1. a-Si:H single cell Fig.1 shows the tail state absorption spectrum of an a-Si:H single cell which has an i layer prepared by low pressure MW PCVD of Sill 4 at
0 o.5
the rate of 75 angstrom/sec. The Urbach energy is about 50meV calculated from the tail state absorption spectrum curve of the a-Si:H cell in Fig.l. In spite of the high rate deposition of a-Si:H, the Urbach energy of the a-Si:H is almost comparable with the Urbach energy of the a-Si:H deposited by RF PCVD at low
f r • o • ~.~o..Oe'~ -
1
1.5 2 2.5 h ~(oV) Fig.1 The tail state absorption spectrum of a-Si:H, Urbach energy:~50meV GeH 4 at a deposition rate of 100 angstrom/sec. The Urbach energy is about 50meV calculated from the tail state absorption spectrum curve of a-SixGel.x:H cell in Fig.2. This Urbach energy of the MW PCVD a-SIxGel.x:H is as small as
rates,
the Urbach energy of low rate RF PCVD a.SixGel.x:H"
This low pressure MW PCVD can utilize Sill 4 gas much more efficiently than RF PCVD. In this MW PCVD, the decomposition efficiency of Sill 4 gas is almost 100% which is estimated
Also in this low pressure MW PCVD the decomposition efficiency of Sill 4 mixed with GeH 4 is almost 100% estimated from the
from the deposition rate. In order to increase the deposition rate of a-Si:H, it is important to decrease the pressure in the chamber and as a result to prevent the polymerization reaction in the gas phase and on the substrate surface.
deposition rate. 7 6 ..., 5 ~ 4 '~" 3 o
3.2. a - S i x G e l . x : H single cell Fig.2 shows the tail state absorption spectrum of a-SixGel.x:H single cell which has a flat band gap a-SixGel.x:H i layer. The flat band gap a-SixGe 1.x:H i layer was prepared by low pressure MW PCVD of Sill 4 mixed with 37%
. . . . . ~,~p~r .
- 2 1 0 0.5
;..., ,-.• 1
1.5
2
h v (eV) Fig.2 The tail state absorption spectrum of a-SixGe 1.x:H, Urbach energy:~50meV
K. Saito et al. / a-Si:H alloy cell deposited at high deposition rate
3.3.a-Si:H/a-SixGe 1.x:H/a-sixGel.x:H triple cell Fig.3 shows the triple cell structure having a-Si:H top cell, a-SixGel.x:H middle cell and a-SixGe 1.x:H bottom cell.
~
~
IT0 p+
'(
e 4 I ~, 2 E 0 "3
MW a-Si:H
Voc: 2.312V Jsc: 7.20mA/cm2
-2
F.F.: 0.698 EFF: 11.61
-4
area:0.25cm2
4 " " " ' = -1
0
n+ p+
MW a-SixGel-x:H
691
1
/
2
3
v(~ Fig.4 The IV curve of the triple cell which has a high rate a-Si:H top cell(75A/sec) 4
n+
p+ MW a-SixGel-x:H
2
~'
0 "
~ -2.
voc: 2.30V Jsc: 7.38mA/cm2
=8 !
F,F,: 0,72S EFF: 12.3%
~
~
~" -4 •
O/Ag/SUS Fig.3 The triple cell structure
. area:0.25cm2
• ~mm
-8 -1
0
V(V)
2
3
We have made two types of triple cells; one has a high rate a-Si:H top cell (75angstrom/sec), the other has a low rate top a-Si:H cell (7.5angstrom/sec).a-SixGe 1.x:H middle and
Fig.5 The IV c u r v e of the triple cell which has a low rate a-Si:H top cell (7.5 A/sec)
bottom cells have a high deposition rate i layer
4. Conclusion
(100angstrom/sec).
We have made a triple cell in which i layers
Fig.4 shows the IV curve of the triple cell which has a high rate a-Si:H top cell. The efficiency of the triple cell which has a high rate a-Si:H top cell is 11.6%. Fig.5 shows the IV curve of the triple cell which has a low rate a-Si:H top cell. The efficiency of the triple cell which has a low rate a-Si:H top cell is 12.3 %. As a result, even though i layers were deposited at high deposition rate, the efficiency of the triple cell is high.
were deposited at about 100 angstrom/sec by low pressure microwave plasma CVD. The efficiency of the triple cell is 11.6%. The efficiency of the triple cell which has a low rate a-Si:H top cell (7.5 angstrom/sec) is 12.3%. We have achieved high deposition rate and high source gas utilization in the solar cell preparation, while maintaining high solar cell efficiency.
692
K. Saito et al. / a-Si:H alloy cell deposited at high deposition rate
Acknowledgement
References
We would like to express our thanks to members of E project Research and Development Department 12 for the solar cell preparation, measurement and useful discussions. We wish to thank Mr. E.Kondo for his unceasing interest, support and encouragement.
1.Y.Hamakawa, 22nd I E E E Photovoltaic Specialist Conference, (1991)1199 2.T.Watanabe,K.Azuma,M.Tanaka,M.Nakatani, T.Sonobe and T.Simada, Jap.J.AppI.Phys. Vol.27(1988)1126 3.A.Shah,J.Dutta,N.Wyrsch,K.Prasad,H.Curtins, F.Finger, A.Howling and Ch.Hollenstein, Mat.Res.Symp.Proc. Vol.258(1992)15 4.P.K.Bhat, C.Marshall, J.Sandwish, H.Chatham, R.E.I.Schro and A.Madan, 20th IEEE Photovoltaic Specialists Conference,(1988)91