PHOTO-INDUCED MICROCRYSTALLINE Inx(Si0.1Se0,9)1-x FILM-ITO SOLAR CELL

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PHOTO-INDUCED MICROCRYSTALLINE Inx(Si0.1Se0,9)1-x FILM-ITO SOLAR CELL

T. Matsushita, M. Okuda, H. Naito, A. Suzuki, T. Nakau

To cite this version:

T. Matsushita, M. Okuda, H. Naito, A. Suzuki, T. Nakau. PHOTO-INDUCED MICROCRYS-

TALLINE Inx(Si0.1Se0,9)1-x FILM-ITO SOLAR CELL. Journal de Physique Colloques, 1982, 43

(C1), pp.C1-277-C1-282. �10.1051/jphyscol:1982138�. �jpa-00221795�

JOURNAL DE PHYSIQUE

CoZZoque ^C1, supplgment au nOIO, Tome 4 3 , octobre 1 9 8 2 page C i - 2 7 7

PHOTO-INDUCED MICROCRYSTALLINE l n , ( s i O . I s e o n g ) l-x FILM -IT0 SOLAR CELL

T. Matsushita, M. Okuda*, H. Naito*

,

A. Suzuki and T. Nakau*

Co ZZege of Engineering, Osaka Industrial. University, Nakagaito, Daito, Osaka, Japan 574

"College of Engineering, University of Osaka Prefecture, Mozu, Sakai, Osaka, Japan 591

Resum@.- Ce papier decriB l e s proprietes Plectriques e t optiques des couches Inx (SiO, lSeO, 9)1

-x

e t 1 ' e f f e t photovol taPque de l a s t r u c t u r e ITO- Inx(Si0,1Se0,9)1-x

-

Au

.(Oc

^x

²

0 , 5 ) . Les couches photoinduites microcris- t a l l ines Inos1 (Si0.1Se0.9)0. deposees sur des supports IT0 obtenus s o i t par evaporation s o i t puiverisation

o n t ete

examinees par rnicroscopie electronique

a

balayage. Dans l e cas d'ITO pulv&ris&s, la v i t e s s e de c r i s - t a l l i s a t i o n e t a i t plus grande e t l e rayon de l a zone c r i s t a l l i s e e plus pe- t i t que pour l e s IT0 @vapor@s. La reponse spectrale du systeme avec

x

= 0 , l apres r e c u i t optique a mis en @vidence une amelioration remarquable

0

de l a phototension de 3000

a

7000 A e t l e rendement de conversion a a t t e i n t 3,53 % (12 rn\/cmE).

Abstract.- This paper describes the e l e c t r i c a l and optical properties of Inx(Si0,1Se0,9),-x films, and the photovoltaic e f f e c t f o r IT0

-

I n x ( ~ i O s l Se0.9)1-x

-

AU s t r u c t u r e ( O ~ x ~ 0 . 5 ) . The photo-induced microcrystall ine In0., ( S i O . 1Se0,9)0,9 films deposited on both the evaporated and the sputt- ered IT0 layers were examined by scanning electron microscopy(SEM)

.

^{For the}

case of the sputtered ITO, the c r y s t a l l i z a t i o n velocity was greater and the radius of the c r y s t a l l i z e d region was smaller than t h a t f o r the evaporated ITO. The spectral response of the system with

x

= 0.1 a f t e r the optical annealing showed the remarkable enhancement of the photovoltage from 3000 t o 7000 and the conversion efficiency reached 3.53 % ( I 2 mW/cm 2 ) .

1. Introduction.- L i t t l e information of the amorphous S i chalcogenide films i s known about the e l e c t r i c a l and optical p r o p e r t i e s ' ' 2 ) . T t i s d i f f i c u l t t o investi- gate the amorphous Si-Se system a s a device material, although t h i s system i s expec- ted f o r a good photoconductive material. The chemical decomposition by interaction with water vapor occurs a t the bulk surface and t h i s system leaves a surface compo- sed of Si02, Se, Hz and pungent H2Se vapor. B u t we have empirically known t h a t thin film samples prepared by evaporation seem t o be chemically s t a b l e in a i r . We have already reported the photovoltaic e f f e c t f o r the construction of _-, Au

-

chalcogenide _.,

\

film

-

Sn02 layer in the Si-Se systemL' and in other systems such as In-Se systemJ'.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982138

C1-278 JOURNAL DE PHYSIQUE

I t i s , t h e r e f o r e , i n t e r e s t i n g t o study t h e p h y s i c a l p r o p e r t i e s and t h e p h o t o v o l t a i c p r o p e r t i e s o f t h e In-Si-Se system i n o r d e r t o o b t a i n a low c o s t s o l a r c e l l . Also, t h e study o f t h e e f f e c t o f annealing on In-Si-Se f i l m s i s e s s e n t i a l t o an under- standing o f t h e mechanism o f p h o t o - m i c r o c r y s t a l l i z a t i o n . I n t h i s paper, we describe e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f amorphous and m i c r o c r y s t a l l i n e Inx(SiO.l

Se0.9)1-x f i l m s and t h e p h o t o v o l t a i c e f f e c t f o r Au

-

Inx(Si0,1Se0,9)1-x f i l m s

-

^{I T 0}

s t r u c t u r e i n t h e composition range from x = 0 t o 0.5. Emphasis i s placed on how t h e p h o t o c r y s t a l l i z a t i o n o f amorphous I n 0 ~ 1 ( S i 0 ~ 1 S e 0 ~ 9 ) 0 ~ 9 f i l m s i s a f f e c t e d w i t h t h e c h a r a c t e r i s t i c s o f t h e s u b s t r a t e I T 0 and whether o r n o t t h e compositional change i n t h e c r y s t a l l i z e d r e g i o n i s recognized.

2. Experimental.- The f a b l i c a t i o n o f t h e p h o t o v o l t a i c c e l l i s as f o l l o w s :

Amorphous I n x ( S i O . lSeO.g)l-x f i l m was deposited by vacuum evaporation onto t h e I T 0

0

(.sheet r e s i s t a n c e 20 ohm/sq, 3 mm wide, 2000 A t h i c k s t r i p e ) coated g l a s s substrate.

D e p o s i t i o n r a t e i s 50 i / s e c ; pressure i s 2 x Torr; s u b s t r a t e temperature i s 30-C. F i n a l l y t h e sandwich s t r u c t u r e was completed by crosswise evaporation o f 500

t h i c k , 3 mm wide Au electrode. The t h i c k n e s s o f amorphous I n x ~ S i 0 , 1 S e 0 ~ 9 ) 1 - x f i l m

0

ranged 2000

-

5000 A. The e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f Inx(Si0,1Se0,9)1-x f i l m s must be known p r i o r t o t h e measurement o f photovoltage. The thermal a c t i v a - t i o n energy

AE

i s determined by t h e i n v e r s e temperature dependence o f c o n d u c t i v i t y according t o t h e r e l a t i o n

w cc

exp(- A E / k T ) . The m o b i l i t y gaps Ecl a r e determined e x p e r i m e n t a l l y as an energy o f t h e peak value o f p h o t o c o n d u c t i v i t y ~ The i n t e r c e p t s

7 1 0

o f Id h

L') "'

^vs h 3 curves extraporated t o

d

= 0 a r e taken as t h e value o f t h e o p t i c a l energy gaps E~~~ ; where d i s t h e a b s o r p t i o n c o e f f i c i e n t , h i s the P l a n k ' s c o n s t s n t and L, i s t h e 1 ig h t frequency. Experimental r e r u l t s o f 9 P

, a

E, EQ and E : ~ ~ a r e shown i n Table I . The annealing process was performed under i l l u m i n a t ~ o n ( . 1 0 0 -

r,

mt\l/cmLj f o r 15 min, a t 70 C which was t h e optimum c o n d i t i o n f o r t h e m i c r o c r y s t a l l i- z a t i o n . The photo-annealed m i c r o c r y s t a l 1 in e Inoal ( S i 0 ~ l S e o ~ 9 ) 0 ~ 9 f i l m s deposited on both t h e evaporated and the s p u t t e r e d I T 0 l a y e r s were examined by scanning e l e c t r o n microscopy(SEb7). The photovoltage measurement was c a r r i e d o u t by i r r a d i a t i n g t h e c e l l through t h e transparent I T 0 l a y e r u s i n g t h e AFI-1 l i g h t source. The cleaved cross s e c t i o n o f t h e p h o t o v o l t a i c c e l l was observed by SEM.

Table I.- The data o f c o n d u c t i v i t y , a c t i v a - t i o n energy and energy gaps obtained from t h e e l e c t r i c a l and t h e o p t i c a l measurements i n amorphous I n x ( S i O . Se0,9)1-x f i l m s .

x 0

0.1

0.2

0.3 0.4 0.5

at 30"C(S/cm) E (eV) Eg (eV1 ' : E (ev)

4.9 x 10-l1 1.20 2.63 1.85 as deposited

c7.4 x (0.951 (2.05) (1.52) after annealins

7.1 x 10-l1 1.27 2.68 1.86 as deposited

(4.8 x (0.93) (2.03) (1.43) after annealing

-- - - -

2.3 x 1.19 2.65 1.86 as deposited

(3.5 (0.82) (1.97) (1.42) after annealing

4.3 x 10-lo 0.60 2.30 1.67 as deposited

3.9 x lo-' 0.44 2.18 1.35 as deposited

1.8 x 0.39 1.93 0.62 as deposited

3. Results.-

3.1 Spectral response.- I t i s e v i d e n t from Table I t h a t t h e dark c o n d u c t i v i t y , t h e a c t i v a t i o n energy and t h e energy gaps do n o t change e s s e n t i a l l y w i t h composi, t i o n s o f x = 0.1 and 0.2, b u t these decrease w i t h i n c r e a s i n g t h e I n c o n t e n t f o r

~ 2 0 . 3 . Tn a d d i t i o n t o these r e s u l t s , the samples o f x = 0.1 and 0.2 showed t h e l a r g e s t . value o f t h e p e r s i s t e n t p h o t o c o n d u c t i v i t y below 250 K i n t h e In-Si-Se system4'. The s p e c t r a l response o f t h e system w i t h d i f f e r e n t x values i s shown i n Pm.T t i s recognized t h a t t h e p h o t o v o l t a i c e f f e c t o f t h e samples o f x = 0.1 and

-

0.2 i s remarkably enhanced by t h e annealing process described above. F i g u r e 2 g i v e s t h e open c i r c u i t photovoltage Voc f o r t h e cases o f b o t h "as deposited" samples and

" a f t e r forming" ones which process was described below. The f a c t t h a t t h e maximum open c i r c u i t v o l t a g e i s observed f o r t h e samples o f x = 0.1 and 0.2 i n b o t h cases o f "as deposited" and " a f t e r forming" i s w e l l corresponding t o t h e c h a r a c t e r i s t i c s

P i g . :

Spectral response o f t h e photo- c u r r e n t f o r (a) "as deposited" and ( b )

" a f t e r nnealing". Large enhanced e f f e c t of t h e photocurrent i s caused a f t e r t h e anneal i ng

.

o f s p e c t r a l response shown i n Fig. 7 (a) and (b). I n t h e f o l l o w i n g , t h e e x p e r i - mental r e s u l t s , which were c a r r i e d o u t f o r t h e I n 0 , 1 ( S i 0 ~ , S e 0 , 9 ) 0 ~ 9 f i l m , w i l l be shown.

3.2 Forming e f f e c t . - F i g u r e 3 shows t h e amblent temperature dependence o f t h e photovoltage f o r t h e sample o f x = 0.1.

Tn t h e f i r s t r i s i n g process(at 4"C/min.

r a t e ) , the open c i r c u i t v o l t a g e Voc decreases u n t i l l t h e ambient tempera- t u r e reaches 70°C(A+B) and a f t e r t h a t , Voc continues t o increase w i t h tempera- t u r e , l e a d i n g t o a s a t u r a t e d value(B+C).

Fig. 2 : V a r i a t i o n o f t h e open c i r c u i t

-

voltage, obtained as a f u n c t i o n o f x ( f o r t h e case o f evaporated ITO).

6OOr

0 3 k 40 50 60 70 80 90 ,433 110 1201 Ambient temperature T('o F i g . 3 : R e l a t i o n of t h e photovoltage

-

vs t h e ambient temperature f o r x = 0.1.

The forming e f f e c t i s induced through t h e c o o l i n g p e r i o d between C and 0.

C 1-280 JOURNAL DE PHYSIQUE

Film Film

Au, \ ^{I TOJE vapo) \I TO(Evapo1}

&

F i g . 4 : A SEM

and ( b ) t h e forming one under i l l u m i -

I

nation(100 m ~ / c m 2 ) f o r 15 min. a t

As deposited After forming

^70°C.

( a ) ( b )

I n t h e subsequent c o o l i n g process, Vnr increases from C t o D. T h i s i s a s o - c a l l e d

"

-

forming process, by which a r e p r o d u c i b l e temperature dependence o f V,, between D and C has been obtained. The forming e f f e c t i n t h e s t r u c t u r e u s i n g

the

evaporated I T 0 i s l a r g e r than t h a t f o r t h e s p u t t e r e d ITO. F i g u r e 4 shows SEM micrographs o f a cleaved cross s e c t i o n o f t h e I n 0 m 1 ( S i 0 ~ 1 S e 0 , 9 ) 0 ~ 9 s o l a r c e l l : ( a ) t h e as deposited c e l l and ( b ) t h e forming one under i l l u m i n a t i o n . The SEM micrograph i n Fig.4 (b) r e v e a l s t h e c y l i n d r i t i c a l c r y s t a l l i n e . Large s t r u c t u r a l change i s recognized t o be caused by t h e forming process. The s p e c t r a l response o f t h e photovoltage was enhan- ced i n t h e wide wavelength range from 3000 t o 7000 A a f t e r t h e forming process.

The value o f photovol tage increased a1 so a f t e r t h e forming process, corresponding t o " a f t e r forming" curve o f F i g . 2 o r t o D p o i n t o f Fig.3. Furtheremore, t h e power conversion e f f i c i e n c y i n t h e sample o f x = 0.1 reached 3.53, 2.37 and 1.9 % f o r t h e AM-1 12, 40 and 100 mw/cmZ; t h a t i s t h e e f f i c i e n c y decreased w i t h i n c r e a s i n g t h e i l l u m i n a t i o n i n t e n s i t y . T h i s behavior o r i g i n a t e s i n t h e presence o f a h i g h s e r i e s r e s i s t a n c e i n t h e h e t e r o s t r u c t u r e s o l a r c e l l which was measured t o be about 50 ohm under i l l u m i n a t i o n i n t e n s i t y o f 100 mW/cm 2

.

These conversion e f f i c i e n c i e s a r e more than one thousand as l a r g e as t h a t obtained before t h e forming process. Now, we determined t h e m i n o r i t y c a r r i e r l i f e t i m e from t h e o p e n - c i r c u i t photovoltage decay

r \

i n amorphous c e l l s and m i c r o c r y s t a l 1 in e ones3'. The obtained m i n o r i t y c a r r i e r 1 if e - times a r e 6.12 and 104 p s e c f o r t h e as deposited c e l l and f o r t h e annealed one under i l l u m i n a t i o n r e s p e c t i v e l y . Next, a slope o f a p l o t o f 1/C 2 vs V

-

^gives

-

t h e value o f t h e acceptor c o n c e n t r a t i o n which was measured t o be 7.6 x 10" and 8.6 x 1016 l / c m f o r t h e as deposited c e l l and f o r t h e annealed one under i l l u m i n a t i o n 3 r e s p e c t i v e l y . I t i n d i c a t e s t h a t t h e annealing process under i l l u m i n a t i o n causes t h e r e d u c t i o n o f t h e e f f e c t i v e d e n s i t y o f recombination center.

4. Discussion and summary.-

In

order t o c l a r i f y the mechanism o f t h e forming e f f e c t as shown i n Fig.3, t h e c r y s t a l l i z a t i o n process was observed a t t h e v a r i o u s stage of t h e annealing process under i l l u m i n a t i o n and i n darkness by o p t i c a l microscope,

i f t h e r e a r e d i f f e r e n c e s between t h e 1 TO(Evapo) ITO(Sputter) i l l u m i n a t i o n e f f e c t and o n l v t h e -tinated :Iluminat ed h e a t i n g e f f e c t i n darkness. I t was

recognized from these r e s u l t s t h a t t h e c r y s t a l l i z a t i o n r a t e induced by

i l l u m i n a t i o n was g r e a t e r than t h a t

.

f o r h e a t i n g i n darkness6). F i g u r e 5

shows the c r y s t a l 1 iz a t i o n process ^L

under i l l u m i n a t i o n f o r amorphous Inosl (Si0.1Se0.g)0,9 f i l m s deposited b o t h (a) on the evaporated I T 0 and (b) on t h e s p u t t e r e d ITO. I t has been c l e a r l y by SEM t h a t t h e surface o f t h e s p u t t - ered I T 0 i s more smooth than t h a t o f the evaporated ITO; namely, t h e nucle- a t i o n f o r c r y s t a l l i z a t i o n induced by i l l u m i n a t i o n i s smaller and t h e number o f t h e n u c l e a t i o n i s muchlarger f o r the case o f t h e s p u t t e r e d I T 0 than t h a t f o r t h e evaporated ITO. These surface c h a r a c t e r i s t i c s o f both IT0 a r e w e l l r e f l e c t e d i n Fig.5. Now, as shown i n Fig.3, t h e forming e f f e c t i s

more remarkable i n the evaporated I T 0 (3) l i P 1 1 n

than i n t h e s p u t t e r e d ITO. Uhen com-

pared Fig.3 w i t h Fig.5, i t seems t h a t ( a ) (b)

the l a r g e r t h e c r y s t a l l i z e d r e g i o n i n

the f i l m s i s , t h e l a r g e r t h e a b i l i t y F i g . 5 : Observation o f t h e c r y s t a l l i z a - generating the P~~~~~~~ tage is * Figure

Grocess

^{under i} 11 umination(tungsten 5 i s t h e photographs u s i n g t r a n s m i t t i n g

l i g h t t o observe t h e c r y s t a l l i z e d region. lamp 100 mW/cm 2 ) f o r amorphous Inosl(

Therefore, i t i s recognized t h a t t h e nu-

c l e a t i o n f o r c r y s t a l l i z a t i o n i s formed Si0.1Se0.9)0.9 deposited both ( a ) a t t h e i n t e r f a c e between t h e s u b s t r a t e on the evaporated I T 0 and ( b ) on t h e IT0 and t h e f i l m , and grows upward t o s p u t t e r e d ITO.

t h e f r e e surface side. I n order t o i n -

v e s t i g a t e t h e aspect of surface and cross s e c t i o n i n d e t a i l , s t r u c t u r a l change due t o t h e annealing was observed by SEM F i u r e 6 and 7 e x h i b i t a s t r u c t u r a l change due t o the annealing e f f e c t o f Inosl i~*film deposited on both t h e eva- porated and t h e s p u t t e r e d ITO. For t h e case u s i n g the s p u t t e r e d I T 0 o f Fig.7, t h e c y l i n d r i t i c a l c r y s t a l l i n e s a r e e a s i l y induced by t h e annealing e f f e c t , and t h e rugged surface appears, which i s w e l l corresponding t o t h e o p t i c a l observation o f Fig.5 (b)

.

A compositional a n a l y s i s using t h e e l e c t r o n probe microanalysis(EPMA) was performed on t h e c r y s t a l 1 i z e d and t h e amorphous regions shown i n Fig.6 and 7.

For t h e case o f evaporated ITO, the weight percent o f Se increased from 82.6 t o 90.3 % w i t h i n c r e a s i n g t h e annealing time from 5 t o 15 minutes. On t h e o t h e r hand, the weight percent of Se, f o r t h e case of s p u t t e r e d ITO, a l s o increased from 87.1 t o 89.2 % w i t h i n c r e a s i n g t h e annealing t i m e from 5 t o 15 minutes. I t i s , t h e r e f o r e deduced t h a t the degree o f c r y s t a l l i z a t i o n o f Se i n c r e a s i n g w i t h t h e i l l u m i n a t i o n time i s r e s p o n s i b l e f o r t h e progress o f c r y s t a l l i z a t i o n i n t h e f i l m .

I n summary, t h e photovol t a i c c e l l c o n s i s t i n g o f t h e m i c r o c r y s t a l 1 in e Inx(SiO.

Se0.9)1,x f i l m

-

I T 0 s t r u c t u r e reached t h e maximum conversion e f f i c i e n c y 3.53 % a t room temperature(AM-1 12 mW/cm 2 ) . Therefore, we regard the m i c r o c r y s t a l l i n e I n x ( Si0.1Se0,9)1-x s o l a r c e l l as promising f o r i t s p r a c t i c a l a p p l i c a t i o n s .

C1-282 JOURNAL DE PHYSIQUE

Fig. 6 : A SEM micrographs e x h i b i t i n g t h e annealing e f f e c t o f Ino,1(Si0,1Se0,g)0~9

-

f i l m deposited on t h e evaporated ITO. The l e f t i s t h e surface, t h e r i g h t two t h e

,.

cross sections. The anneal i n g process was performed under i 11 umination(100 mw/cmL) f o r 15 min. a t 70°C.

Fig. 7 : A SEM micrographs e x h i b i t i n g t h e annealing e f f e c t o f I n o s 1 (Si0~1Se0,9)0,9

-

f i l m deposited on t h e s p u t t e r e d

ITO.

The l e f t i s t h e surface, the r i g h t two t h e cross sections. The annealing process was performed under illumination(.lOO mW/cm 2 ) f o r 15 min. a t 70°C.

References.- 1) Petersen K.E., B i r k h o l z U., and Adler D., Phys. Rev.

88

(1973) 1453. 2) Matsushita T., Okuda M., Suzuki A., N a i t o H., and Nakau T., Jpn. J . Appl.

Phys.

20

(1981) Supple. 20-2, 147. 3) Matsushita T., Suzuki A,, Okuda M., and Sakai T., Jpn. 3. Appl. Phys.

19

(1980) Supple. 19-2, 123. 4) Suzuki A., Matsushita T., Okuda M., N a i t o H., and Nakau T., Jpn. 3. Appl. Phys.

21

(1982) 407. 5) Mahan J.E., Ekstedt T.W., Frank R.I., and Kaplow R., I.E.E.E.

-

ED-26 (1979) 733.

6) Matsushita T., Suzuki A., Okuda I,?,, and Nang T.T., T h i n S o l i d ~ i l m s

58

⁽¹⁹⁷⁹⁾

413.