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CdTe solar cells with improved CdS films fabricated by the writing method
Solar Energy Materials 23 (1991) 371-379 North-Holland
Sola, Energy Materials
CdS/CdTe solar cells with improved CdS films fabricated by the writing method T. Arita, A. H a n a f u s a , N. Ueno, Y. Nishiyama, S. Kitamura and M. M u r o z o n o Matsushita Batteo, h~dustrial Co., Ltd., Matsushita-cho, Moriguchi, Osaka 570, Japan A new deposition technique called "the writing method" is examined for fabrication of CdS window layer in C d S / C d T e solar cells by the screen-printing and sintering method. Obtained CdS film is very smooth with very few pinholes. This technique aiso posscs~e~ high patterning precision and realizes the active/total area ratio of 0.8 by reducing the separatiov, of CdS stripes. As a result of the optimization of patterning dimensions, the convt.rsion efficiency of 7.8~ - in 1200 cm 2 large area cell (9 ~c'~ in acti,.e area) is obtained.
1, Introduction
Cadmium telluride is a promising base material for a solar cell due to its nearly optimum energy band gap (!.45 eV) and high absorption coefficient. The latter makes it possible to produce solar cells from polycrystalline thin film with shorter diffusicm length of minority c~rrier. CdS/CdTe polycrystalline solar cells can be produced by various techniques,.:, vacuum deposition, closed-space sublimation (C~3S), sputtering, screen-printing and sintering method, and so on. The screenprinted CdS/CdTe solar cell was firstly fabricated by successive repeating of "screen-prialfi.",g and heating in a belt furnace" method on a borosilicate glass [i]. This technique can render large ceil area at relatively low cost. We have developed this CdS/CdTe solar cell for indoor use [_2,3] and is now mass-production stage for the pocketable calculator market [1,3,4]. Cell 9erformav_ce of COTe basis solar cell has been improved up to date. Albright et al. reported the conversion efficiency of 7.3% with an aperture area of 773 cm 2 [5], We have achieved high conversion efficiency of il.3% in small area CdS/CdTe solar cell (1.07 cm2~ and 6.2% in large area (1200 cm 2) by screen-printing and sintering technique [6]. These performances, however, are limited not only by relatively low open circuit voltage and fill factor but also short circuit current originated from low active- ve~-sus total- area ratio fabricated by the screen-printing method, it is also a problem in screen-printed CdS layer which has the inhomogeneity of the thickness and high pinhole density. In order to overcome these problems, we have examined a new techniql~e of thick film deposition called "lhe writing method" and applied for CdS layer fabrication. In this report, experimental procedure and performances of CdS/CdTe solar cells fabric~',ted on this CdS film are summarized. 0165-1633/91/$03.5(I ~3 1¢~91 - Elsevier Science Publishers B.V. All righl~ reserved
372
T. Arita et aL / CdS / Cdfe solar cells
2. Exl~erimenta| results and discussion 2.1. C d S fibn fi~brication
Fig. 1 shows a conceptional illustration of the writing method. In this method, the CdS paste is deposit,-:d on a glass substrate from a nozzle by air pressure and CdS film is formed. The film thickness can be controlled by changing the nozzle height, which is adjusted by the height of the diamond stylus. CdS line width is determined by the slit width. The CdS film pattern is formed under computer ~,OlltlOl.
CdS paste is made of high-purity CdS (5N) powder (particle size of 2 to 3/.tm) mixed with CdCI, as a flux and appropriate amount of propylene glycol as a bini:ier, which is almost the same process employed in screen printing already r r~ eporte,a, td. The printed paste is dried at 120 °C for 1 hour. Then the substrate is housed in an ~ t m m a sintering boat [8], and sintered in N, atmosphere at 690 °C using a belt furnace. Fig. 2 shows SEM images of the sintered Cd$ film surface fabricated by the screen printing (a), and the writing method (b) respectively. It is seen in fig. 2a that there exist a big periodic undulation corresponding to a screen mesh and lots of i i l | [~,.pllbe Ill~ll p:.,m._~ . .3., . . w ,.:~t. occupy 1 to 2 percent of the printed area. On the other hand, in fig. 2b, there are very few pinholes and the film surface is quite flat. Fig. 3 shows cross sectional view of the films and their Dektak traces. As can be seen, the film thickness fabricated by the screen-printing method varies widely, but the film thickness by the writing method is aniform. As a result of this improved uniformity of the thickness, junction area decreases by several percent. 2...). Cell f a b d c a t i o n Fig. 4 shows a schematic illustration of the C d S / C d T e solar cell employed in this experiment. A CdS film is formed by the writing of screen-printing method. In the writing method, typical nozzle slit is 3.7 × 0.15 mm and speed of the nozzle is 10 cm/s. Other films are formed by screen-printing method [6]. A CdS window layer is directly formed on the glass substrate, because the CdS film is so thick that a transparent electrode ,~ucb as SnO~ is not used between the glass and the CdS film. A CdTe tilm is fabricated on the CdS film, and then a carbon electrode is
i diamond ~1111/1t/,"/
t
i [J slit nozzle ,.;i/) . paste / / //,',~ / / / / / ! ~
glass substrate
Fig. 1. (onceplional illustrationof the writingmethod.
T. Arita et aL / C,.¢~/ ('dTe solar cells
373
0.5ram Fig. 2. SEM images of the sintered CdS film surface fabricated by the screen printi~tg {a} and the writing method (b) respectively.
formed on the CdTe film. For production of 30 × 40 c m 2 submodule, 84 unit cells are connected in series through {Ag + In) electr{:de. Fig. 5 shows performances of 10 × i0 cm 2 solar cells whose CdS window layer is fabricated by the writing method as a function of the CdS film thickness. Active area of the cell is 65 cm 2. Each plot is normalized by the performance va!ue of the
I
J
L_~
imm
i
li~m
Fig. 3. Cross-sectional view of ~he CdS films and their Dektak traces: {a} by the screen printing method: {b) bv the writing method.
374
T. Arita e" aL / CdS / t ~,'Te solar cells
Ag+ln
/
~,!-~ N~{'
electrode
.
ca'S'film
[ / ~
8!zss .o
FJ~. -,. Schematic illustration of CdS/CdTe solar cell. • i,r
.i
solar cell with 30 /.tin CdS film fabricated by the screen-printing. It, and ~,~ decrease a.nd FF increases with the increase of the COS film thickness. "fhe maximum conversion efficiency is obtained at CdS film thickness of 30 p.m. Compared with the cell performance by the screen-printing method, every photo-
li t 0'9'f J. 1
o-o--oO o '
.
.
.
.
.
.
.
.
.
.
.
.
.
o.1.
c_; co
",. I~
-o
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o
c~
I. I C23 ~
~0
I. 0
~
~---0
.............. 6 / - 0 9 - ....................
0.9
°/
0.8 • i. i
,-~----00~,
~ 1.0 -~O.g
....... o/~
°.st 0 CdS
,
- ........ o ~ - . - - . - -
,
,
,
,
10
20
40
<10
film
thickness
.-,
\ I o/
__,
50
60 (/,m)
Fig, 5. Cell performance as d funclion of CdS film !kickness.
" 375
1: Arita et aL / CdS / CdTe sohar cells
40~ t
CdS Film Thickness
301 :
/sT~
~-
//39um
Z= 2 0 I-
___
/ /,3z/ o
~-10 [ 0 1 _
400
\ ! 6,.,~. .
I
. . . .
500
i
_
600
I
700
.
i
800
900
Vla;'e I " n g t . (nm) Fig. 6. Reflectance spectra of ('dS/CdTe so~ar cells with different CdS film !hickness.
voltaic parameter is improved by a few percent. The improvement of 1... and I,~,~ seems to due to the increase of active area by extinction of pinholes and the decrease of junctitJ~ area by using fiat CdS films as explained in figs. 2 and 3. Fig. 6 shows a reflectance spectra of C d S / C d T e solar cells with different CdS film thickness. As increasing the CdS film thickness, reflectance increases. This causes the reduction of 1~. Fig. 7 sho,,vs cross sectional SEM images of CdS/CdTe sol3.r cell with 15 ~ m CcS film (a) and 35 tzm CdS film (b). The thin CdS film consists of single grains, On the other hand, the thick CdS film is formed from 2 or
201Jm Fig, 7. Cross-sectional SErv~ i~iages of CdS/CdTe ,~o!ar cells with 15 tzm CdS film (a) and 35 #m (_'dS film (b).
1". Arita et al. / CdS / CdTe ~',,far cells
376
.,-..4
O
o
/
$..,
+...a
J~
./
r oo
o.......~ f I
0 Fie.. 8. Content ¢ff Ci
l0
q
I
I
I
, ...I.._.._,....._
20 30 4(t 5(I G0 CdS film thickncss (~m)
O
"-
in the CdS film as a function of CdS film thickness measured by ionchromatography.
3 piled up grains, and thcre are many pores in the CdS film, which causes multi-step cptical scattering and consequently the increase of reflection loss. We have reported that residual CdCI 2 in the CdS film affects the cell performance [6]. Fig. 8 ~hows content of Ci- in the CdS film as a function of CdS film thickness measured by ionchromatography. The CI- is resolved from CdCI 2 in hot watcr, so it reflects the quantity of residuai CdC!, in the CdS film. As can be seen in fig. 8, with the increase of CdS film thickness, the content of C!- increases, which implies the increase of thc ~ota! amount 6f CdCI_, per unit area. As
i~~ '
~:i,~-~:~"
111~
?.
~0~ ~ ~ ~-~
50n~,~ m Fig.
9. S E M
images
of a
seperation par| of CdS films prepared by the screen prinlhi~ (a) and the writing method ( b ) .
T. Arita et aL / CdS / CdTe sc!er ,.,,ft..,..
ulE~
377
_/0~0~0~0~0 ~0
R~GE
10~ - 0" - ,.--4 r~
1 0:
10 ~ . ,...4 ..4-.a C~ ,-....-4
lO:r
1
I
0
o.I 0.2 c,.a o.,1 Distance between CdS stripes(mm)
Fig. 10. Insulation resistance be'~wem, CdS stripes.
reported, the excess CdCl 2 favors formation of a thick C d S - C d T e mixed crystal layer. This mixed crystal layer degrades the CdS/COTe junction property and spectral response, wlaich makes Voc and l~c low. For production of large-area high efficiency C d S / C d T e solar cell. it is also important to reduce cell separation space, so called dead space. Fig. 9 shows SEM images of the seperation part of CdS films prepared by the screen-printing (a) and the writing method (b). In the screen-printing methed, it is difficult to make t_'_.e
distance shorter than several hundred microns between CdS stripes without contact as indicated in fig. 9a. On the tJ~.b.cr ~ialld. in the writin,,, method, it is easy to make less than a hundred microns. Fig. 10 shows insulation resistance between CdS stripes shc~wn in fig. 9. In the screen-printing method, the insulation resistance decreases gradu.a!!y with the decrease oz" distance, while in the writing method, it is
All!, 5 lOOmL'cm:e
°50 2qO 1
-
1200cm Voc isc FF Eff: Eff~
I1
~-'.
lO0
0
65. 22zm,', 0.647 9. 896 7.8~
\ ~. \
.\ !
a
0
~0
~0
60
VOLTAGE ~,\J
Fig. 1i. !--V characterislics of 30 x 40 cm: submod~de.
37g
T, Arim et al, / CdS / CdTe soh~r ceils
more than 2 Mf~ even at 9.15 ram. As a result, 80% of active/total area ratio in 30 x 40 cm-' submodule was achieved. Fig. 11 shows ! - V characteristics of the best 30 x 40 cm 2 submodule obtained so far. A unit cell number of the submodule is 84. The photovoltaic characteristics are as follows; t~c = 65.3 V, I,~ = 222 mA, FF = 0.647 and conversion efficiency = 7.8% (9.8% in active area) under AM1.5 100 m W / c m 2 ilh:mination. In the process of cell fabrication, heat treatment in the H_,-N., gas has strong effect for reduction of CdS film resistivity [9]. The photovoitaic performance of large-area submodule will be further impro'ved by optimizing condition of heat treatment in }:t2-N 2 gas and unit cell number.
3. Conclusions Experimental approaches for the performance impre',ement of C d S / C d T e solar cells have been performed. In order to improve tqe CdS fihn quality, a new fabrication technique called the writing method was, examined. Using this m e t h ~ , we have successfully obtained pinhole-less CdS films with flat :~urface. Every photovoitaic parameter is improved in a solar ce~] mbricated on this CdS films. Tt~e active/to~al area ratio of 0.8 was also achieved by reducing the distance between CdS strii~es by this method. As a result, conversion efficiency of 7.8% for 30 × 40 cm- submodule was obtaiaed under AM_,.5 100 r a W / c m " illumination.
Acknowledgements Th~s work was supported by the New Energ3' and lndtistrial Technology Development Organization undea- the c,_,n~hine _P~oject of the Ministry. The authors wi.,;h to thank Professor Y. Hamakawa of Osaka University and Associate Professor H. T~kakura of Toyama Prefectural University for their useful advice. The authors also wish to thaak the people concerned in the R & D department of the Sunshine Project at NEDO, and J. Miyoshi, Director of Matsushita Battery Industrial C e . Ltd.
References [i] S. lkt:~ami, Solar Cells :,,"~(1988) 89. [2] N. Suyama, N. Ueno, K. Omura, l-i. Takada. T. Hibino and M. Murozono. in: Proc. 18~h IEEE Photovoltaic Sp¢ci:~lists Conf., New Orleai,:s, LA, 1987, p. 1471P, [3] N. Suyama, N. Ueno, K. Omura, H. Takad ~, S. Kitamma. Nat. Tech. Rep., Matsushila Teehno Research, Osaka. 32 (1986) 667. [4] M. Murozono, Y. Umeo ~mt.~ H, Ogawa, Progress in Batteries & Solar Cells .8 (I~,',19}324. [5j S.P, AIbright. J.F. Jordan, B. Ackerman and R.R. Chambelin, Solar Ce!ls 27 (1989) 77. [6] N. Suyama, T. Arila, Y. Nishiyama, N. Ueno, S. Kitamura and M. Murozono, in: Proc. 21th IEEE Photovoltaic Specialists Conf., Kissimmee, FL, 199(}. to be published.
7". Arita et al. / CdS / C.,4Te solar cells
379
[7] N. Ueno, Y. Nishiyama, T. Arita, M. Suyama, Y. Kita and M. ~4~_,rozono, Int. PVSEC-4, Sydney, i 089, p 481. [8] H. Uda, K. Kuribayashi, ¥. Komatsu, A. Nakano and S. Ikegami, Jpn. J. Appl. Phys. 22 (1983) 1832. [9] N. S,~),ama, T. Arita, Y. Nishiyama, N. Ueno. S. Kitamura and M. Murozono, Int. PVSEC-5, Kyoto, Jo.pan, 1990~ "to be published.
Sola, Energy Materials
CdS/CdTe solar cells with improved CdS films fabricated by the writing method T. Arita, A. H a n a f u s a , N. Ueno, Y. Nishiyama, S. Kitamura and M. M u r o z o n o Matsushita Batteo, h~dustrial Co., Ltd., Matsushita-cho, Moriguchi, Osaka 570, Japan A new deposition technique called "the writing method" is examined for fabrication of CdS window layer in C d S / C d T e solar cells by the screen-printing and sintering method. Obtained CdS film is very smooth with very few pinholes. This technique aiso posscs~e~ high patterning precision and realizes the active/total area ratio of 0.8 by reducing the separatiov, of CdS stripes. As a result of the optimization of patterning dimensions, the convt.rsion efficiency of 7.8~ - in 1200 cm 2 large area cell (9 ~c'~ in acti,.e area) is obtained.
1, Introduction
Cadmium telluride is a promising base material for a solar cell due to its nearly optimum energy band gap (!.45 eV) and high absorption coefficient. The latter makes it possible to produce solar cells from polycrystalline thin film with shorter diffusicm length of minority c~rrier. CdS/CdTe polycrystalline solar cells can be produced by various techniques,.:, vacuum deposition, closed-space sublimation (C~3S), sputtering, screen-printing and sintering method, and so on. The screenprinted CdS/CdTe solar cell was firstly fabricated by successive repeating of "screen-prialfi.",g and heating in a belt furnace" method on a borosilicate glass [i]. This technique can render large ceil area at relatively low cost. We have developed this CdS/CdTe solar cell for indoor use [_2,3] and is now mass-production stage for the pocketable calculator market [1,3,4]. Cell 9erformav_ce of COTe basis solar cell has been improved up to date. Albright et al. reported the conversion efficiency of 7.3% with an aperture area of 773 cm 2 [5], We have achieved high conversion efficiency of il.3% in small area CdS/CdTe solar cell (1.07 cm2~ and 6.2% in large area (1200 cm 2) by screen-printing and sintering technique [6]. These performances, however, are limited not only by relatively low open circuit voltage and fill factor but also short circuit current originated from low active- ve~-sus total- area ratio fabricated by the screen-printing method, it is also a problem in screen-printed CdS layer which has the inhomogeneity of the thickness and high pinhole density. In order to overcome these problems, we have examined a new techniql~e of thick film deposition called "lhe writing method" and applied for CdS layer fabrication. In this report, experimental procedure and performances of CdS/CdTe solar cells fabric~',ted on this CdS film are summarized. 0165-1633/91/$03.5(I ~3 1¢~91 - Elsevier Science Publishers B.V. All righl~ reserved
372
T. Arita et aL / CdS / Cdfe solar cells
2. Exl~erimenta| results and discussion 2.1. C d S fibn fi~brication
Fig. 1 shows a conceptional illustration of the writing method. In this method, the CdS paste is deposit,-:d on a glass substrate from a nozzle by air pressure and CdS film is formed. The film thickness can be controlled by changing the nozzle height, which is adjusted by the height of the diamond stylus. CdS line width is determined by the slit width. The CdS film pattern is formed under computer ~,OlltlOl.
CdS paste is made of high-purity CdS (5N) powder (particle size of 2 to 3/.tm) mixed with CdCI, as a flux and appropriate amount of propylene glycol as a bini:ier, which is almost the same process employed in screen printing already r r~ eporte,a, td. The printed paste is dried at 120 °C for 1 hour. Then the substrate is housed in an ~ t m m a sintering boat [8], and sintered in N, atmosphere at 690 °C using a belt furnace. Fig. 2 shows SEM images of the sintered Cd$ film surface fabricated by the screen printing (a), and the writing method (b) respectively. It is seen in fig. 2a that there exist a big periodic undulation corresponding to a screen mesh and lots of i i l | [~,.pllbe Ill~ll p:.,m._~ . .3., . . w ,.:~t. occupy 1 to 2 percent of the printed area. On the other hand, in fig. 2b, there are very few pinholes and the film surface is quite flat. Fig. 3 shows cross sectional view of the films and their Dektak traces. As can be seen, the film thickness fabricated by the screen-printing method varies widely, but the film thickness by the writing method is aniform. As a result of this improved uniformity of the thickness, junction area decreases by several percent. 2...). Cell f a b d c a t i o n Fig. 4 shows a schematic illustration of the C d S / C d T e solar cell employed in this experiment. A CdS film is formed by the writing of screen-printing method. In the writing method, typical nozzle slit is 3.7 × 0.15 mm and speed of the nozzle is 10 cm/s. Other films are formed by screen-printing method [6]. A CdS window layer is directly formed on the glass substrate, because the CdS film is so thick that a transparent electrode ,~ucb as SnO~ is not used between the glass and the CdS film. A CdTe tilm is fabricated on the CdS film, and then a carbon electrode is
i diamond ~1111/1t/,"/
t
i [J slit nozzle ,.;i/) . paste / / //,',~ / / / / / ! ~
glass substrate
Fig. 1. (onceplional illustrationof the writingmethod.
T. Arita et aL / C,.¢~/ ('dTe solar cells
373
0.5ram Fig. 2. SEM images of the sintered CdS film surface fabricated by the screen printi~tg {a} and the writing method (b) respectively.
formed on the CdTe film. For production of 30 × 40 c m 2 submodule, 84 unit cells are connected in series through {Ag + In) electr{:de. Fig. 5 shows performances of 10 × i0 cm 2 solar cells whose CdS window layer is fabricated by the writing method as a function of the CdS film thickness. Active area of the cell is 65 cm 2. Each plot is normalized by the performance va!ue of the
I
J
L_~
imm
i
li~m
Fig. 3. Cross-sectional view of ~he CdS films and their Dektak traces: {a} by the screen printing method: {b) bv the writing method.
374
T. Arita e" aL / CdS / t ~,'Te solar cells
Ag+ln
/
~,!-~ N~{'
electrode
.
ca'S'film
[ / ~
8!zss .o
FJ~. -,. Schematic illustration of CdS/CdTe solar cell. • i,r
.i
solar cell with 30 /.tin CdS film fabricated by the screen-printing. It, and ~,~ decrease a.nd FF increases with the increase of the COS film thickness. "fhe maximum conversion efficiency is obtained at CdS film thickness of 30 p.m. Compared with the cell performance by the screen-printing method, every photo-
li t 0'9'f J. 1
o-o--oO o '
.
.
.
.
.
.
.
.
.
.
.
.
.
o.1.
c_; co
",. I~
-o
0.7
o
c~
I. I C23 ~
~0
I. 0
~
~---0
.............. 6 / - 0 9 - ....................
0.9
°/
0.8 • i. i
,-~----00~,
~ 1.0 -~O.g
....... o/~
°.st 0 CdS
,
- ........ o ~ - . - - . - -
,
,
,
,
10
20
40
<10
film
thickness
.-,
\ I o/
__,
50
60 (/,m)
Fig, 5. Cell performance as d funclion of CdS film !kickness.
" 375
1: Arita et aL / CdS / CdTe sohar cells
40~ t
CdS Film Thickness
301 :
/sT~
~-
//39um
Z= 2 0 I-
___
/ /,3z/ o
~-10 [ 0 1 _
400
\ ! 6,.,~. .
I
. . . .
500
i
_
600
I
700
.
i
800
900
Vla;'e I " n g t . (nm) Fig. 6. Reflectance spectra of ('dS/CdTe so~ar cells with different CdS film !hickness.
voltaic parameter is improved by a few percent. The improvement of 1... and I,~,~ seems to due to the increase of active area by extinction of pinholes and the decrease of junctitJ~ area by using fiat CdS films as explained in figs. 2 and 3. Fig. 6 shows a reflectance spectra of C d S / C d T e solar cells with different CdS film thickness. As increasing the CdS film thickness, reflectance increases. This causes the reduction of 1~. Fig. 7 sho,,vs cross sectional SEM images of CdS/CdTe sol3.r cell with 15 ~ m CcS film (a) and 35 tzm CdS film (b). The thin CdS film consists of single grains, On the other hand, the thick CdS film is formed from 2 or
201Jm Fig, 7. Cross-sectional SErv~ i~iages of CdS/CdTe ,~o!ar cells with 15 tzm CdS film (a) and 35 #m (_'dS film (b).
1". Arita et al. / CdS / CdTe ~',,far cells
376
.,-..4
O
o
/
$..,
+...a
J~
./
r oo
o.......~ f I
0 Fie.. 8. Content ¢ff Ci
l0
q
I
I
I
, ...I.._.._,....._
20 30 4(t 5(I G0 CdS film thickncss (~m)
O
"-
in the CdS film as a function of CdS film thickness measured by ionchromatography.
3 piled up grains, and thcre are many pores in the CdS film, which causes multi-step cptical scattering and consequently the increase of reflection loss. We have reported that residual CdCI 2 in the CdS film affects the cell performance [6]. Fig. 8 ~hows content of Ci- in the CdS film as a function of CdS film thickness measured by ionchromatography. The CI- is resolved from CdCI 2 in hot watcr, so it reflects the quantity of residuai CdC!, in the CdS film. As can be seen in fig. 8, with the increase of CdS film thickness, the content of C!- increases, which implies the increase of thc ~ota! amount 6f CdCI_, per unit area. As
i~~ '
~:i,~-~:~"
111~
?.
~0~ ~ ~ ~-~
50n~,~ m Fig.
9. S E M
images
of a
seperation par| of CdS films prepared by the screen prinlhi~ (a) and the writing method ( b ) .
T. Arita et aL / CdS / CdTe sc!er ,.,,ft..,..
ulE~
377
_/0~0~0~0~0 ~0
R~GE
10~ - 0" - ,.--4 r~
1 0:
10 ~ . ,...4 ..4-.a C~ ,-....-4
lO:r
1
I
0
o.I 0.2 c,.a o.,1 Distance between CdS stripes(mm)
Fig. 10. Insulation resistance be'~wem, CdS stripes.
reported, the excess CdCl 2 favors formation of a thick C d S - C d T e mixed crystal layer. This mixed crystal layer degrades the CdS/COTe junction property and spectral response, wlaich makes Voc and l~c low. For production of large-area high efficiency C d S / C d T e solar cell. it is also important to reduce cell separation space, so called dead space. Fig. 9 shows SEM images of the seperation part of CdS films prepared by the screen-printing (a) and the writing method (b). In the screen-printing methed, it is difficult to make t_'_.e
distance shorter than several hundred microns between CdS stripes without contact as indicated in fig. 9a. On the tJ~.b.cr ~ialld. in the writin,,, method, it is easy to make less than a hundred microns. Fig. 10 shows insulation resistance between CdS stripes shc~wn in fig. 9. In the screen-printing method, the insulation resistance decreases gradu.a!!y with the decrease oz" distance, while in the writing method, it is
All!, 5 lOOmL'cm:e
°50 2qO 1
-
1200cm Voc isc FF Eff: Eff~
I1
~-'.
lO0
0
65. 22zm,', 0.647 9. 896 7.8~
\ ~. \
.\ !
a
0
~0
~0
60
VOLTAGE ~,\J
Fig. 1i. !--V characterislics of 30 x 40 cm: submod~de.
37g
T, Arim et al, / CdS / CdTe soh~r ceils
more than 2 Mf~ even at 9.15 ram. As a result, 80% of active/total area ratio in 30 x 40 cm-' submodule was achieved. Fig. 11 shows ! - V characteristics of the best 30 x 40 cm 2 submodule obtained so far. A unit cell number of the submodule is 84. The photovoltaic characteristics are as follows; t~c = 65.3 V, I,~ = 222 mA, FF = 0.647 and conversion efficiency = 7.8% (9.8% in active area) under AM1.5 100 m W / c m 2 ilh:mination. In the process of cell fabrication, heat treatment in the H_,-N., gas has strong effect for reduction of CdS film resistivity [9]. The photovoitaic performance of large-area submodule will be further impro'ved by optimizing condition of heat treatment in }:t2-N 2 gas and unit cell number.
3. Conclusions Experimental approaches for the performance impre',ement of C d S / C d T e solar cells have been performed. In order to improve tqe CdS fihn quality, a new fabrication technique called the writing method was, examined. Using this m e t h ~ , we have successfully obtained pinhole-less CdS films with flat :~urface. Every photovoitaic parameter is improved in a solar ce~] mbricated on this CdS films. Tt~e active/to~al area ratio of 0.8 was also achieved by reducing the distance between CdS strii~es by this method. As a result, conversion efficiency of 7.8% for 30 × 40 cm- submodule was obtaiaed under AM_,.5 100 r a W / c m " illumination.
Acknowledgements Th~s work was supported by the New Energ3' and lndtistrial Technology Development Organization undea- the c,_,n~hine _P~oject of the Ministry. The authors wi.,;h to thank Professor Y. Hamakawa of Osaka University and Associate Professor H. T~kakura of Toyama Prefectural University for their useful advice. The authors also wish to thaak the people concerned in the R & D department of the Sunshine Project at NEDO, and J. Miyoshi, Director of Matsushita Battery Industrial C e . Ltd.
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379
[7] N. Ueno, Y. Nishiyama, T. Arita, M. Suyama, Y. Kita and M. ~4~_,rozono, Int. PVSEC-4, Sydney, i 089, p 481. [8] H. Uda, K. Kuribayashi, ¥. Komatsu, A. Nakano and S. Ikegami, Jpn. J. Appl. Phys. 22 (1983) 1832. [9] N. S,~),ama, T. Arita, Y. Nishiyama, N. Ueno. S. Kitamura and M. Murozono, Int. PVSEC-5, Kyoto, Jo.pan, 1990~ "to be published.