VALENCY ELECTRON CONTROL IN A GLOW DISCHARGE PRODUCED a-SiC : H AND ITS APPLICATION TO a-Si SOLAR CELL

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VALENCY ELECTRON CONTROL IN A GLOW DISCHARGE PRODUCED a-SiC : H AND ITS

APPLICATION TO a-Si SOLAR CELL

Y. Tawada, M. Kondo, H. Okamoto, Y. Hamakawa

To cite this version:

Y. Tawada, M. Kondo, H. Okamoto, Y. Hamakawa. VALENCY ELECTRON CONTROL IN A GLOW DISCHARGE PRODUCED a-SiC : H AND ITS APPLICATION TO a-Si SOLAR CELL.

Journal de Physique Colloques, 1981, 42 (C4), pp.C4-471-C4-474. �10.1051/jphyscol:1981499�. �jpa-

00220956�

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JOURNAL DE PHYSIQUE

Colloque C4, suppZ6ment au nO1O, Tome 42, octobre 1981 page C4-471

VALENCY ELECTRON CONTROL I N A GLOW DISCHARGE PRODUCED a - S i C : H AND I T S A P P L I C A T I O N TO a - S i SOLAR CELL

Y. ~awada; M. Kondo, H. Okamoto and Y. Hamakawa

Faculty of Engineering Sc<ence, Osaka University, Toyonaka, Osaka 560, Japan

Abstract.- A clear valency electron controlability has been found in hydroqen- ated amorphous silicon carbide produced by the plasma deposition of [~i~~(l-,) +CH4 (X) ] gas mixture. A series of experimental investigation on electrical, optical and optoelectronic properties in the amorphous silicon carbide has been made. Emplying a-SiC:H as a wide gap window material in p-i-n a-Si solar cell, more than 7.5% conversion efficiency has been obtained with Jsc=13.45mA/

cm2, Voc=O. 909volts and FF=O. 61 7.

Introduction.- Valency control of glow discharge produced amorphous silicon was firstly described by Spear and LeComber(l), and a great deal of attentions has been focused upon its application to electronic devices. Particularly, an excellent photoconductive property with high optical absorption for the visible light in these materials is successfully matched with strong social needs for the low cost solar ce1 l(2). Through efforts to improve the conversion efficiency, a remarkable improvement has been seen, that is, 6.1% with the inverted p-i-n ce11(3), 6.3% with a-Si :F:H MIS ce1

l(4)

and 7.14% with a-SiC:H/a-Si :H heterojunction ce1 l(5) are worthy of not ice.

The first investigation of plasma deposited a-SiC:H film has been made by Anderson and Spear(6), and they demonstrated that optical band gap of undoped a-SiC:H continuously increases with increasing carbon content. However, no information about the valency electron control on this material has been reported yet. We have conducted a series of experimental trials on the wide band gap amorphous materials with a variety of new element combinations. Recently, we have found a rather good valency electron controlability with substitutional impurity doping in hydrogenated a-Sic produced by plasma decomposition of [S~H~(~-,)+CH~(,)

l

gas mixture(5,7,8).

In this paper, we present the details of impurity doping Into the hydrogenated a-Sic by the p1 asma decompos it ion of [S i

H4

l -x)+CH4 (xk+PH? or B2H6] and the i r effects on electrical, optical and optoelectron~c propert~ S o doped a-SiC:H. Utilizing this film as a window side p-layer, we have developed a new type solar cell having more than 7.5% conversion efficiency.

Sample preparation.- Films of a-SiC:H and a-Si:H were prepared in the plasma decomposition system as described

in

our previous paper(7). We adopted rnethane(~~4) as a carbon source instead of ethylene(C2H4) as had been used by Anderson and Spear(6) to promote tetrahedral bonding of carbon in a-SiC:H. a-SiC:H was deposited on Corning #7059 glass for electrical, optical and optoelectronic measurements, and on high resistivity c-Si for I R spectrum measurement by decomposition of [SiH4(1-

X ) +

CHI,(~)]. Substrate temperature and gas pressure were about 250°C and 2-5Torr, respectively.

Valency electron control o f a-SiC:H.- The optical band ga E of a-SiC:H films was determined from the straight line intercept of (&)-lP2 $%:US ho curve at high absorption region(a=lo4/cm) following the analysis of Davis and Mott(9). It seems unreasonable to measure the photoconductivity of a-SiC:H having a wide band gap

*

on leave from Kanegafuchi Chemical Industry, Yoshida-cho 1-2-80, Kobe 652, Japan.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981499

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C4-472 JOURNAL DE PHYSIQUE

ranging from 1.76 t o 2.2eV under t h e same monochromatic i l l u m i n a t i o n . Therefore, f o r v a r i o u s c o m p o s i t i o n a l a-SiC:H, p h o t o c o n d u c t i v i t y was measured under AM-1 ( 1 0 0 m ~ / c m ~ ) i l l u m i n a t i o n and monochromatic i l l u m i n a t i o n a t which a-SiC:H has

104/cm o p t i c a l a b s o r p t i o n c o e f f i c i e n t . The m o n o - F chromatic p h o t o c o n d u c t i v i t y was normalized t o r l p ~ ^UE proposed by Zanzucchi e t a l . (10). Fig. l shows

Cl$-

t h e p h o t o c o n d u c t i v i t y 0 h under AM-l i l l u m i n a t i o n

6

versus t h e normalized p R o t o c o n d u c t i v i t y ^{Q ~ T}f o r ^b v a r i o u s compositional a-SiC:H which has an o p t i c a l

band gap from 1.76 t o 2.2eV. As i s seen i n t h i s 1 6 ~ - f i g u r e , t h e r e e x i s t s an obvious r e l a t i o n between

AM-l p h o t o c o n d u c t i v i t y and normalized photocon-

d u c t i v i t y . From these r e s u l t s , i t i s recognized -8

t h a t AM-l p h o t o c o n d u c t i v i t y can be commonly used 10169 'i7 1b6 '65 as a q u i c k j u d g i n g method f o r o p t o e l e c t r o n i c ^{1 / ~ 7}(cm21~)

p r o p e r t i e s o f a-SiC:H h a v i n g an o p t i c a l band gap F k . 1 AM-1 photoconductivity ad versus

from 1 .76eV t o 2.2eV. QUT for various compositional a-S~C:H.

Fig.2 shows t h e gaseous c o m p o s i t i o n a l de-

pendence o f o p t i c a l band gap Egopt and AM-l photo- ¹ ¹A.o:unLoped ¹ ¹ ^I ^l ^l ¹ c o n d u c t i v i t y opt, o f a-SiC:H. O p t i c a l band gap ^A^.^.^; B doped o f undoped and boron doped a-SiC:H increases w i t h

i n c r e a s i n g methane f r a c t i o n o f ( X ) , b u t AM-l p h o t o c o n d u c t i v i t y o f undoped one s i g n i f i c a n t l y

decreases by adding small amount o f methane. On 2.1- t h e o t h e r hand, boron doped one shows one o r f o u r l a r g e r magnitude o f p h o t o c o n d u c t i v i t y as compared

-

w i t h undoped one. These improved p h o t o c o n d u c t i -

v i t i e s o f boron doped a-SiC:H a r e w o r t h n o t i n g . _LE

Fig.3 shows S i - H s t r e t c h i n g mode a b s o r p t i o n s

L?'.'- -

o f a-SiC:H d e p o s i t e d a t Ts=250°C. As can be seen from t h i s f i g u r e , I R a b s o r p t i o n c o e f f i c i e n t

o f 2000cm-~ band decreases and I R a b s o r p t i o n c o e f - o ^j f i c i e n t o f 2090cm-I band increases a l o n g w i t h t h e 1.7-

increase o f methane f r a c t i o n o f ^{( X ) .} Comparing

:

-168 t h i s I R s p e c t r a w i t h t h e s i g n i f i c a n t p h t o c o n d u c t i -

v i t y decrease o f undoped a-SiC:H as shown i n ^O-Q.

Fig.2, t h e r e might be c l o s e l y r e l a t e d problem I I I I I I I I 0

' 1 16'

between t h e increase o f 2090cm-~ band and t h e de- 0.5 1.0

'IH4(1 -x)+CH,(x) X - crease o f p h o t o c o n d u c t i v i t y i n undoped a-SiC:H.

Wieder e t a1

.

^{( l}^l⁾ i d e n t i f i e d t h e 2090cm-1 band as Fig. 2 Optical gap Egopt and ^AM-1 a r i s i n g from t h e v i b r a t i o n a l s t r e t c h o f mono- photoconductivity of undoped and boxwn h y d r i d e Si-H bonds a t t a c h e d w i t h one, two o r

doped a-5iC:H.

t h r e e carbons, r e s p e c t i v e l y ,

S i

S

ⁱ

F

S i - k i - H , C-Si-H, C - S i - H .

-

t c c

^E

It may be p o s s i b l e t h a t t h e decrease o f

I a ) C H & 0 % (b) CH' l0 X

-

^{( C )} ^CH&^{2 0 %}

p h o t o c o n d u c t i v i t y i n undoped a-SiC:H i s ^(dl ^30"

( e ) CH' 7 0 % ( e )

caused by monohydride Si-H bonds a t t a c h e d

w i t h carbons. 5

Fig.4 shows a summary o f t h e e f f e c t s

5

o f i m p u r i t y doping on t h e b a s i c p r o p e r t i e s 1

-

o f a-SiC:H prepared by t h e plasma decompo- ^u s i t i o n o f [SiH,$(0.8)+CH~(0.2

1.

As i s Z

1,

seen i n t h i s f i g u r e , t h e dar c o n d u c t i v i t y

$

~ d a t room temperature i s on t h e o r d e r o f

H

_'_l

Fig. 3 IR stretching mode absorption of a-SiC:H 0

-

fib deposited a t l's=250°C. ^2m ²¹⁰³ ^MOO ¹⁹⁰⁰

WAVE NUMBER 1cm-'1

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Fig.4 Effects of impurity doping on the basic Fig.5 Effects of impurity doping on the baisic proparties of a-SiC:H prepared by the pZasma properties of a-SiC:H p r e p m d by the ptaama decomposition of [SiH4 (O. +CH4 (O. 2, 1. decomposition of [SiH4 (o. +CH4 (O.

1

1 0 - ~ ( Q c m ) - l f o r 5% diborane doping and on t h e o r d e r o f 1 0 - ~ ( Q c m ) - l f o r 0.1% phosphine doping. I n c o n t r a s t w i t h these, t h e d a r k c o n d u c t i v i t y o f undoped a-SiC:H i s on t h e o r d e r o f 1 0 - ~ ~ ( Q c m ) - 1 . The a c t i v a t i o n energy AE c l e a r l y changes from 1.08eV o f undoped a-SiC:H t o 0.4eV f o r d i b o r a n e doping and t o 0.2eV f o r phosphine doping.

The o p t i c a l band gap Egopt o f doped a-SiC:H i s almost c o n s t a n t except f o r s u f f i c i e n t - l y h i g h boron doping.

Fig.5 shows t h e e f f e c t s o f doping on t h e b a s i c p r o p e r t i e s o f a-SiC:H produced by decomposition o f [SiH~,(~.5)+CH4(0.5)] gas m i x t u r e . A r a t h e r c l e a r valency e l e c t r o n c o n t r o l a b i l i t y can be a l s o seen i n t h i s case. The d a r k c o n d u c t i v i t y ad i s on t h e o r d e r o f 1 0 - 5 ( 0 c m ) - ~ f o r 1% d i b o r a n e doping and on t h e o r d e r o f 10-3(Qcm)-1

f o r 0.2% phosphine doping. While, t h e dark c o n d u c t i v i t y o f undoped a-SiC:H i s so small t o measure a t room temperature. The minimum a c t i v a t i o n energy i s 0.4eV and 0.3eV f o r p- and n-type, r e s p e c t i v e l y . I n t h i s case, p h o t o c o n d u c t i v i t y recovery by doping i s r a t h e r l a r g e , b u t t h e p h o t o c o n d u c t i v i t y i s one o r d e r s m a l l e r than t h e case o f x=0.2. O p t i c a l band gap o f t h i s case i s i n f l u e n c e d by i m p u r i t y doping and decreases as t h e increase o f doping f r a c t i o n f o r b o t h diborane and phosphine doping.

Performance of

a-SiC:H/a-Si :H

heterojunction solar cell .-

Valency e l e c t r o n con- t r o l a b i l i t y o f wide gap a-SiC:H shows us t h a t t h i s m a t e r i a l becomes a v e r y u s e f u l window m a t e r i a l f o r b o t h p - i - n and i n v e r t e d p - i - n c e l l s t r u c t u r e . U t i l i z i n g a-SiC:H

f i l m s as a h e t e r o j u n c t i o n window, we have developed a new t y p e o f a-Si s o l a r c e l l w i t h t h e c o n s t r u c t i o n o f glass/Sn07 (25Q&)/p a-SiC:H/i-n a-Si :H/Al. The l a y e r

t h i c k n e s s o f p a-SiC:H, i a-Si :H and _SMLL

AREA SOLAR CELL(^,&) ^IA)"s'C:H~o-SLzH

n a-Si :H a r e 10081, 5000& and 500W, CELL NP.56.316-1

r e s p e c t i v e l y . J-V c h a r a c t e r i s t i c s DRTE 01.4.17

measurements were c a r r i e d o u t under ^Voc-0.909[*h-13.45 lmR/.rZl ^(V1

AM-l s o l a r s i m u l a t o r . The s o l a r FF -61.7 1x1

s i m u l a t o r w a s c a l i b r a t e d f i r s t l y b y EFF-7.55 ( 2 1

measuring t h e performance o f an a - ~ i :H -E ^'O' I P ~ ~ - I O O . O ( ~ Y / . . ~ I RRER-0.033 f..z~

c e l l under sun l i g h t (near AM-l sun ^t- l i g h t a t Toyonaka, Osaka, Japan, i n 7 . 5

J u l y , 1980) and d e t e r m i n i n g t h e s o l a r ^(B)

energy power by means o f NASA-cal i - ^CELL^NO.^P-10-2s

ORTE 81. 3. 3

b r a t e d c-Si s o l a r c e l l and p y r o h e l i o - p W..-O,BOI IVI

meter. The a-Si:H s o l a r c e l l was r~h-11.02 (.RI..z?

then placed i n s i m u l a t o r , and t h e FF 164.7 1x1

EFF-5.71 (21

l i g h t i n t e n s i t y was a d j u s t e d t o g i v e PL~-IOO.OI.YI..ZI

t h e same s h o r t c i r c u i t c u r r e n t as RRER-0.033 1 . ~ 2 1

measured under s u n l i q h t . The s e n s i -

t i v e area o f small size(3.3mm2) s o t a r BUTPUT VOLTRGE ( V 1 c e l l was d e f i n e d t o be about 5%

Fig. 6 J- V characteristics of a-SiC:B/a-Si:E hetem- l a r g e r t h a n t h e d e p o s i t e d area o f

a1 umi num bottom electrode(3.14mm2) .-

.

bv , j ~ c t i o n sozap ceZt a d ordinary p-i-n a-Si:H t a k i n g account o f t h e e x p e r i m e n t a l l y hanojunction sotar c e t t .

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JOURNAL DE PHYSIQUE

conf i rmed edge e f f e c t s t o t h e photo- 1 7 . 5

,

Summar -

Summaries o f t h e r e s u l t s a r e shown below:

' 6 C : H was prepared by plasma decomposition o f [SiH4(l-x)+CHb(x)] gas m i x t u r e . (2) A sharp decrease o f p h o t o c o n d u c t i v i t y i s observed i n undoped a-SIC:H. On t h e

o t h e r hand, boron doped a-SiC:H e x h i b i t s a c l e a r recovery o f p h o t o c o n d u c t i v i t y , and one o r f o u r o r d e r s l a r g e r magnitude p h o t o c o n d u c t i v i t y can be seen as compared w i t h undoped a-SiC:H.

(3) a-SiC:H shows a r a t h e r good doping e f f i c i e n c y both f o r donors and a c c e p t o r s o f i m p u r i t i e s . From these r e s u l t s , a-SiC:H might become a very u s e f u l window m a t e r i a l b o t h f o r p - i - n and i n v e r t e d p - i - n c e l l c o n f i g u r a t i o n s .

(4) U t i l i z i n g boron doped a-SiC:H f i l m as a p - s i d e window m a t e r i a l , more than 7.5%

conversion e f f i c i e n c y has been o b t a i n e d .

(5) Comparing a-SiC:H/a-Si:H h e t e r o j u n c t i o n s o l a r c e l l w i t h o r d i n a r y p - i - n a-Si:H homojunction s o l a r c e l l , t h e performance o f t h i s c e l l i s improved by 22% i n J s c , 13.5% i n Voc and 32% i n Q.

(6) Improvements o f J s c and Voc m i g h t be caused by a decrease o f u n a v a i l a b l e o p t i c a l a b s o r p t i o n i n p - l a y e r and an increase o f d i f f u s i o n p o t e n t i a l i n t h e p - i - n j u n c t i o n , r e s p e c t i v e l y .

c u r r e n t .

Fig.6 shows J-V c h a r a c t e r i s t i c s o f ^{1 5 . 0}

Acknow1edwments.- The a u t h o r s w i s h t o thank P r o f . A. H i r a k i and Dr. T. lmura o f Osaka U n i v e r s i t y and M r . Kubo o f Kanegafuchi Chemical I n d u s t r y f o r h e l p f u l d i s c u s s i o n s . Technical a s s i s t a n c e by M r . C. Sada i s a l s o acknowledged. T h i s work i s p a r t i a l l y supported by "Special Research P r o j e c t on Amorphous M a t e r i a l s and Physics sponsored by t h e M i n i s t r y o f Education" and "Sunshine P r o j e c t s o l a r p h o t o v o l t a i c D i v i s i o n 1 ' .

LARGE AREA SOLAR CELL( l .0cll?)

and LE COMBER P.G.. Phil. Mag.

33

(1976) 935.

Surface Sci. (1979) 444.

a t y p i c a l a-SiC:H/a-Si :H h e t e r o j u n c t i o n

-

s o l a r c e l l and an o r d i n a r y p - i - n a-Si :H

2

^12.; CELL N O . 56-423-2

homojunction s o l a r c e l l. The conversion

E

O R T E 8 1 . 4.24 Voc- 8 6 5 . 6 ImVI

e f f i c i e n c y o f 7.55% w i t h ~ ~ ~ = 1 3 . 4 5 m ~ / c m ~ ,

2

1 0 . 0 1-c- 12.95lmR/c.121

Voc=O. 909vol t s and FF=O. 61 7 has been

-

^FF

-

6 0 . 5 0 1 ~ 1

o b t a i n e d by an a-SiC:H/a-Si:H c e l l . On +

EFF= 6 . 7 8 1 7 1

t h e o t h e r hand, t h e conversion e f f i c i e n c y 7 . 5 . nu o f an o r d i n a r y p - i - n a-Si:H i s 5.7% w i t h

g

J s c = l 1.02m~/cm2, Voc=0.801volts and 2 U 5 . 0 v ~ . h = l825.21OHtil

FF=o.647. The performance o f t h i s a-SiC:H, V ~ . X - 6 3 1 . Z l n V 1

/a-Si:H h e t e r o j u n c t i o n s o l a r c e l l i s 2 1m.x- 1 0 . 7 5 l n R 1 (L

c l e a r l y improved by 22% i n JSC, 13.5% i n

2

2.5

Voc and 32% i n rl as compared wi t h an o r d i

-

^Q

nary p - i - n a-Si:H homojunction s o l a r c e l l .

.

^{\ .} , T h i s remarkable improvement o f J s c and 0 . 0 0 . 2 0.4 0 . 6 0 . 8 1 . 0

Voc i s caused by wide band gap o f a-SiC:H, _OUTPUT VClLTRGE ( V 1 t h a t i s , a decrease o f u n a v a i l a b l e o p t i c a l

a b s o r p t i o n i n p - l a y e r and an i n c r e a s e of- F i g . 7 J - V characteristics of a k r g e -a(l.0cm2) d i f f u s i o n p o t e n t i a l i n p - i - n j u n c t i o n , a-sic:~/a-Si: H heterojmctim solar ce 22.

r e s p e c t i v e l y . As f o r a l a r g e area s o l a r

c e l l ( l .0cm2), a conversion e f f i c i e n c y o f 6.78% has been o b t a i n e d w i t h ~,,=12.95m~/cm~, Voc=0.866volts and FF=0.605 as shown i n Fig.7. The d e t a i l e d r e s u l t s o f a-SiC:H/

a-Si:H h e t e r o j u n c t i o n s o l a r c e l l w i l l be r e p o r t e d i n t h e separated paper.

CARLSON O.E., Solar ~ n e r ~ ~ ~ t . 3 (1980) 503.

MADAN A., MC GILL J., CZUBATJYI W., YANG J. and OVSHINSKY S.R., Appl

.

Phys. Lett.

2

(1980) 826.

TAWADA Y., OKAMOTO H. and HAMAKAWA Y., Appl. Phys. Lett.

39

(1981 ) i n press.

( 6 ) ANDERSON D.A., and SPEAR W.E., Phil. Mag.

2

(1977) 1.

(7) TAWADA Y., YAMAGUCHI T., NONOMUM S., HOTTA S . , OKAMOTO H. and HAMAKAWA Y., Jpn. J. Appl. Phys.

S U P D ~ . 20-2 (1981) 213.

(8) TAWADA ~ K O N D O M., OKAMOTO H. and HAMAKAWA Y., Proc. 15th I E E E Photovoltaic Specialists Conf., Florida(l981) to be published.

(9) DAVIS E.A. and MOTT N.F., Phil. Mag. 22 (1970) 903.

(10) ZANZUCCHI P.J.. WRONSKI C.R. and CARLSON D.E., J. Appl

.

^Phys.^Q(1977) 5227.

(11) WIEDER H., CARDONA M. and GUARNIERI C.R.. Phys. Stat. Sol. (b)

92

(1979) 99.