Effects of the surface morphology of transparent electrode on film deposition and photovoltaic performance of A-SI:H solar cells

198

Journal of Non-CrystallineSolids 115 (1989) 198-200 North-Holland

EFFECTS OF THE SURFACE MORPHOLOGY OF TRANSPARENT ELECTRODE ON FILH DEPOSITION AND PHOTOVOLTAIC PERFORMANCE OF A-SI:H SOLAR CELLS

H. Sakai, T. Yoshida, S. Fujikake, Y. Ichikawa, A. Ueda, O. Ishiwata and M.Nagano Fuji Electric Corporate Research and Development, Ltd. 2-2-i Nagasaka, Yokosuka, Kanagawa240-01, Japan The r e l a t i o n between the o p e n - c i r c u i t voltage (Voc) and the surface morphology of transparent e l e c t r o d e s (TE) in a-Si:H based s o l a r c e l l was studied. I t was found that use of TE's with higher Haze r a t i o s led to lowering the Voc, and the drop in Yoc occurred severely when the thickness of i - l a y e r was less than 200nm. The cause of the Voc drop was investigated by transmission electron microscopy and c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s under the dark condition. These r e s u l t s indicated t h a t defects a r i s i n g from"grain boundaries" of a-Si:H existed in the i layer near the TE/a-Si i n t e r f a c e , and these acted as a recombination c e n t e r for c a r r i e r s . 1. INTRODUCTION Recently, we have succeeded in developing a

s u b s t r a t e s , where CI i n t e r f a c e layer.

s t a b l e two-stacked tandem solar cell with a conversion e f f i c i e n c y of more than 10%; in this tandem c e l l a r e l a t i v e l y thin i (a-Si:H) layer (30Ohm) was used for the bottom c e l l and an

All the a-Si based layers were deposited by the conventional capacitively-coupled r f glow discharge decomposition. The film deposition c o n d i t i o n s for p, CI, i and n layers were the same for these sample c e l l s ; the bandgaps of p, CI, i and n layers were 2.0eV, 1.0eV, 1.75eV and 1.7eV, r e s p e c t i v e l y .

i n i t i a l degradation of about 10% was a t t a i n e d in conversion e f f i c i e n c y . 1 For f u r t h e r improvement in conversion e f f i ciency of such t h i n tandem solar c e l l s , textured

is

the constant bandgap

Sn02 t r a n s p a r e n t electrodes (TE) with higher l i g h t s c a t t e r i n g r a t i o s are strongly required to enhance the o p t i c a l confinement effect. However, we found that a p p l i c a t i o n of TE's with higher

3. RESULTS AND DISCUSSION Figure 1 shows the Voc as a function, of i layer thickness for Substrates A,B and C. The Yoc of the c e l l s with Substrate C is s i g n i f i -

s c a t t e r i n g r a t i o s r e s u l t e d in decrease in the o p e n - c i r c u i t voltage (Voc). In t h i s paper, we studied the r e l a t i o n between the surface morphology of TE's and the p h o t o v o l t a i c c h a r a c t e r i s t i c s of a-Si solar c e l l s in d e t a i l to understand the cause of the Voc drop in tandem s o l a r c e l l s . The mechanism of the Voc drop was i n v e s t i g a t e d by the current-voltage c h a r a c t e r i s t i c s under the dark condition and by t r a n s m i s s i o n e l e c t r o n microscopy (TEM).

c a n t l y low as compared with the other c e l l s , in p a r t i c u l a r when the i layer thickness is less than 20Ohm. This r e s u l t is very crucial for the tandem c e l l because the i layer thickness of the top c e l l is lOOnm or less. Thus, we conclude from t h i s r e s u l t that the Voc drop in tandem s o l a r c e l l s with highly textured TE's is mainly due to that in the top c e l l .

2. SAMPLE PREPARATION We provided four kinds of substrates, A, B, C and D shown in Table 1. A-Si solar c e l l s with a

glass/SnO2/p(a-SiC)/CI(a-SiC)/i(a-Si)/n(a-Si)/ Hetal

structure

was

fabricated

on

these

0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

TABLE 1 Haze r a t i o and sheet r e s i s t a n c e of 8ubstrates A, B, C and D. Sheet reslstance(~/[]) A B C D

1.3 5.6 12 27

10 lO 12

H. Sakai et al. / Surface morphology of transparent electrode

i

.

.

.

.

199

.

0.9,~

AC') @

0.9~ 0.9C

z~ C(O) "~ m._AA.B

&

-

a-Si

"~0 8~ >.

o11o- I

0.86

o/

0.8Z, ,

h

l

"-

' 50OO

Sn02

Thickness ofi-loyer(A) FIGURE 1 Voc as a function of i - l a y e r thickness for s u b s t r a t e A, B and C.

FIGURE 2 A TEM image of the a Si solar c e l l deposited on a highly t e x t u r e d s u b s t r a t e (Substrate D). ( m a g n i f i c a t i o n x50000)

We considered that

the Voc drop was r e l a t e d

to the g e n e r a t i o n of some defect during the a-Si

g r ai n s.

film deposition.

To c l a r i f y the o r i g i n of this

phenomenon,

focused

c l e a r l y in the i layer region near the TE/a-Si i n t e r f a c e . I t is r e a d i l y supposed that these

we

our

study

on

the

o b s e r v a t i o n of the cross s e c t io n of a-Si layer de po s i t ed on various SnO2 TE's by TEM. 3.1. TEM Image of a-Si Solar Cells Figure solar

2 shows

cell

substrate

the

deposited (Substrate

SnO2 g rai n s .

TEM image of the a Si on

a

D) with

highly

textured

rectangle-shaped

We can find white s t r i p e s in the a-

Si layer. These white s t r i p e s correspond to the region where d e n s i t y is lower than the other region.

The white

stripes

originated

from the

edge in the v a l l e y s between the Sn02 grains, and

Hence,

the

white

stripes

boundaries usually become c e n t e r for the c a r r i e r s .

a

appear

more

recombination

I f the white s t r i p e s work as a recombination cen t er , the c a r r i e r t r an sp o r t in the a-Si c e l l s , e s p e c i a l l y with thinner i layers, would be a f f e c t e d s t r o n g l y . To study t h i s, we measured the c u r r e n t v o l t a g e (I-V) c h a r a c t e r i s t i c s these s o l a r c e l l s under the dark condition.

of In

previous papers, we have reported that the dark I-V curve could be a good measure to evaluate the Voc and d e f e c t s in the i layer

grow into the a-Si layers. However, they disappear at lOOnm-2OOnm from the TE/a-Si boundary. These white s t r i p e s were also observed in the TEM image of a-Si f i l m deposited on Substrate C as

shown

in

Figure3 (a).

On the

contrary,

no

v i s i b l e white s t r i p e was observed for the c e l l s deposited

on Substrates A and B.

~:~ ° ~ " ~ ? ? : : :

From these

- a-Si-

i~}#i

results, the white s t r i p e s w e r e likely to appear in the Sn02 TE's with narrow rectangular and V-shaped valleys. Figure

3 (b)

shows a

picture of

the a-Si

film with 50nm thick deposited on Substrate C. As seen from t h i s

figure,

" SnO2"

the surface of the

t h r u s t through the a-Si layer. At present, we i n t e r p r e t these white s t r i p e s

(b) FIGURE 3 TEM images of a-Si films with 500nm thick (a) an d 50nm thick (b) deposited on Substrate C.

as

(magnification

a Si

film s t i l l

morphology of

keep the roughness along the

the Sn02 and the white s t r i p e s

"grain boundaries" of

a-Si

grown on

Sn02

(a)

x50000)

200

H. Sakai et al./ Surface morphology of transparent electrode

16'

/

s

I0

from . . . .

f

(b) c /

results

g2 O.& 06 08 Voltoge(V)

I~

stripes

is

considered

to

be

in decrease

in the defect density.

On

since the i n t r i n s i c defects due

to dangling bonds increase with i layer thickness, the deference in I-V curves for the

&b'

0

white

the other hand,

A.B

ulO

the

reduced; because the s t r i p e s disappears in the i layer region near the i / n i n t e r f a c e , and this

0.2 0.~ 0.6 QII 1.0 1.:'

t2

Voltoge(V)

FIGURE 4 Dark I-V c h a r a c t e r i s t i c s of c e l l s deposited on Substrates A, B and C with an a-Si film t hick n es s es of 5Ohm(a) and 50Ohm(b).

t h r ee c e l l s 4-(b).

becomes small

as

shown in Figure

From the above r e s u l t s and discussion, concluded t h a t the white s t r i p e s appeared

we in

the i layer of the c e l l s deposited on Substrate C a f f e c t as d e f e c t s which enhance the recombinat i o n of c a r r i e r s in a-Si based p-i n diodes.

3.2. Dark I-V C h a r a c t e r i s t i c s Figures 4-(a) and (b) show the dark I-V characteristics

of

the

cells

deposited

on

Sub-

s t r a t e s , A, B and C, with an i layer thicknesses of 50nm and 50Ohm, r e s p e c t i v e l y . As seen from Figure

4-(a),

the

dark

current

of

the

cell

deposited on S u b s tr a te C is large in comparison with those of the c e l l s deposited on Substrates A and E. In c o n t r a s t , characteristics

of

the d i f f e r e n c e in the I-V

the

50Ohm c e l l s

is

not so

large as shown in Figure4-(b). The dark forward c u r r e n t of a Si based p - i - n

4. CONCLUSIONS 1) The e x i s t e n c e of the white s t r i p e s was found in the TEM image of the a-Si layers deposited on well textured substrates with r e c t a n g u l a r or V shaped valleys. 2) I t was found that the photovoltaic performance of the s o l a r c e l l s with thin i - l a y e r s s t r o n g l y depended on the presence of the white s t r i p e s , which a f f e c t e d as d e f e c t s enhancing the recombination of the p - i - n diodes.

diode is mainly c o n t r o l l e d by the recombination current TE's

at

are

the

p/i

i n t e r f a c e 2.

used,

the

area

increases. area

could

Thus, be a

the

of

increase

possibility

When textured

the p / i

junction

in the junction to

increase

the

dark current of the cells deposited on S ubs t rat e C. In t h i s case, increase in the dark current

is expected to be in proportion to the

area of p / i of i layer showed that

i n t e r f a c e and should be independent thickness. the dark

Our r e s u l t s , however, current had a strong

thickness dependence as shown in f i g u r e 4. I f the white s t r i p e s act as defects, the r e s u l t s can be explained q u a l i t a t i v e l y . When the i layer thickness is thin(Figure 4 - ( a ) ) , S ubs t rat e C c e l l , which has many s t r i p e s i layer, has a l a r g e r recombination than the other c e l l s . When i t is thick 4-(b)), however, the d e f e c t density

in the current (Figure arising

ACKNOWLEDGMENTS This work was supported by the New Energy and Industrial Technology Development Organization as a part of the Sunshine Project of the Ministry of International Trade and Industry. REFERENCES 1. H. Sakai e t a l . , Prec. of MRS symp., Diego, 1989) (to be published)

(San

2. T. Yoshida et al., Proc. of 20th IEEE P h o t o v o l t a i c Speci. Conf., (Las Vegas, 1988) pp. 335-339.