Observations on the relationship between structure and electrical performance in silicon ribbon solar cells

Mat. Res. Bull. Vol. 9, pp. 1421-1426, 1974. Printed in the United States.

Perffamon P r e s s , Inc.

OBSERVATIONS ON THE RELATIONSHIP BETWEEN STRUCTURE AND ELECTRICAL PERFORMANCE IN SILICON RIBBON SOLAR CELLS

H. B. S e r r e z e , J. C. Swartz, G. Entine and K. V. Ravi Tyco Laboratories, Inc. Waltham, M a s s a c h u s e t t s 02154 U.S.A.

(Received August 28, 1974; Communicated by J. J. Tietjen) ABSTRACT A c o r r e l a t i o n between the p r e s e n c e of growth twins and the conv e r s i o n efficiencies of s o l a r c e l l s made from silicon ribbons grown by the edge-defined, f i l m - f e d growth (EFG) technique has been observed. Ribbon s a m p l e s with no twins furnish c e l l s of the highest c o n v e r s i o n efficiency (~ 9% a i r m a s s zero) w h e r e a s those with twins have significantly degraded efficiency.

Introduction The i m p o r t a n c e of photovoltaic c o n v e r s i o n of solar e n e r g y into e l e c t r i c a l energy is well recognized. Silicon s o l a r c e l l s have been used for this p u r p o s e for many years. However, widespread use of solar c e l l s for t e r r e s t r i a l applications r e q u i r e s that the cost of solar c e l l s and, in particular, the cost of silicon in all its various f o r m s be significantly reduced. The r e c e n t advent (I, 2) of the edge-defined, f i l m - f e d growth (EFG) of silicon ribbons has p e r m i t t e d the f i r s t step to be taken in this direction. In addition to the cost factors involved in photovoltaic devices, the efficiency of the c e l l s in converting solar e n e r g y to e l e c t r i c a l power is of m a j o r importance. In the past, many efficiency degradation m e c h a n i s m s have been p r o p o s e d to be o p e r a t i v e in silicon solar c e l l s (3). Most, if not all, of t h e s e factors a r e r e l a t e d to m a t e r i a l quality p a r a m e t e r s such as the impurity content and type, the p r e s e n c e of c r y s t a l l o g r a p h i c defects, etc. In this paper w e present s o m e preliminary results on the structure of silicon ribbons grown by the E F G technique and the relationship between structure and the properties of solar cells m a d e from these ribbons. 1421

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Experimental Procedure Silicon ribbons (typical dimensions 0.3 x 25 x 250 mm) I-8~2-cm, p-type (boron doped) were grown by the EFG technique. The ribbons were all grown from the same machine using the same starting materials and growth techniques. The only controlled difference betweenruns was the orientation of the seed crystal The followingorientations were used: (110) < 111>, (II0) < I12>, (111) 15° off< 112>, and (III) 17" off< 112>. Subsequentto growth, the ribbons were lightly etched in a mixture of nitric, hydrofluoricand acetic acids and macrophotographs of the entire surface were obtained. Selected regions of the ribbon were also preferentially etchedwith Sirtl etch (4) to reveal dislocations and low angle grain boundaries. The ribbons w e r e cut into approximately 1 by 2 c m blanks for p r o c e s s i n g into s o l a r cells. Following another short etch, standard phosphorus diffusion techniques w e r e used to diffuse an n - l a y e r into the p-type ribbon base with a junction depth of ~ 0 . 5 /~m. E l e c t r i c a l contact was made using evaporated and s i n t e r e d A1-Ag c o m p o s i t e contacts. No a n t i - r e f l e c t i o n coatings w e r e applied. S o l a r ' c e l l I-V c u r v e s w e r e obtained under approximate AM0 (air m a s s zero) illumination conditions using a 650-W q u a r t z - i o d i n e lamp and 3 inch water filter. Intensity calibration of the source was p e r f o r m e d using a c a l i b r a t e d c o m m e r c i a l Si s o l a r celL Following the e l e c t r i c a l analysis of the cells, s e l e c t e d devices w e r e stripped of contacts, Sirtl etched, and photographed. E x p e r i m e n t a l Results The m o s t obvious defects in all of the ribbons w e r e twin boundaries. The twins nucleated p r i m a r i l y at the seed junction, subsequent nucleation being r e l a t i v e r a r e . With a seed orientation of (110) < 112> the twins a r e p a r a l l e l to the growth d i r e c t i o n as illustrated in Fig. 1. For the other o r i entations at least two twin planes a r e active so that twin i n t e r s e c t i o n s occur as i l l u s t r a t e d in Fig. 2. In one of the (110) < 111> ribbons the initial twins ran out so that a large t w i n - f r e e region (with the seed orientation) was p r o duced. The dislocation distribution was quite heterogeneous in the twinned crystals; single slip o c c u r r e d in s o m e twins, o t h e r s w e r e relatively dislocation-free. Except for a few longitudinal subgrain boundaries, the dislocation distribution in the single c r y s t a l s was relatively homogeneous with a density on the o r d e r of 10b/cm z. Table I c o m p a r e s the p r o p e r t i e s of a total of 28 solar c e l l s made f r o m the t h r e e c l a s s e s of ribbon samples: single crystal, p a r a l l e l twinned, and nonparallel twinned. Not unexpectedly, the single c r y s t a l s make the best s o l a r cells. Fig. 3 shows the AM0 I-V c u r v e for one of the single

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SILICON RIBBON SOLAR CELLS

FIG. 1

FIG. 2

Example of a silicon ribbon possessing parallel twins. The seed junction is at the top of the ribbon.

1

/kc

i

i

i

mA/cm

2

Example of a silicon ribbon possessing intersectin~ twins. The (If0) < I I I > seed junction is at the top of the ribbon.

i

Prr, i x = g 5 m W / c m •¢ Z 3 4

1423

(a)

z

CFF=075

5-578 T: Z8"C AMO e l 4 0 r o W / NOA - R C O A T I N G

1

CELL R I B B O N iJ-e 7-~-3 AREA = 24 cm 2

cm 2 )

IC

Voc,O~5}'~.. v o.i

o.z

o~

0.4

o.~

v CVOLT$.~

FIG. 3 AM0 I-V c u r v e for a single c r y s t a l EFG silicon ribbon solar c e l l

06

FIG. 4 C r y s t a l s t r u c t u r e of 1 by 2 c m solar c e l l s made from (a) t w i n - f r e e , (b) p a r a l l e l twinned, and (c) nonp a r a l l e l twinned EFG silicon ribbon. The fracture in cell (a) o c c u r r e d subsequent to cell testing.

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"SILICON RIBBON SOLAR CELLS

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c r ~ s t a l cells. Assuming that the m a x i m u m power output could be i n c r e a s e d 30~'o by a suitable a n t i - r e f l e c t i o n (A-R) coating, its c o n v e r s i o n efficiency would be about 9% AM0.

TABLE I Summary of Silicon Ribbon Solar Cells

No. of C r y s t a l Quality Cells , . m

,,

Open-Circuit Voltage (volts)

Curve Fill Factor

Reverse Leakage Current

Estimated AM0 Efficiency*

,

Single c r y s t a l

3

0.55

- 0.57

0.70 - 0.75

< 0.7 m A / c m 2

8-

Parallel twins

9

0.51

- 0.55

0.60 - 0.75

< 1 mA/cm2

6 - 6%

16

0.20

- 0.55

0.25 - 0.70

< 50 m A / c m 2

2 - 8%

Non-parallel twins

lo%

*This value is e s t i m a t e d on the basis of being able to r e a l i z e a 30% power i n c r e a s e with a suitable a n t i - r e f l e c t i o n coating.

The twinned c e l l s have noticeably p o o r e r c h a r a c t e r i s t i c s (see Table I). In c o m p a r i n g the p a r a l l e l and n o n - p a r a l l e l twinned s a m p l e s t h e r e appears to be a significant difference in reproducibility. Finally, Fig. 4 shows the s t r u c t u r e of t h r e e typical cells r e v e a l e d by Sirtl etching, k is perhaps r e m a r k a b l e that the solar cell efficiency of s a m p l e s such as Figs. 4b and c can be t h r e e fourths of that of the t w i n - f r e e cells. Discussion The influence of c r y s t a l l o g r a p h i c defects on the e l e c t r i c a l c h a r a c t e r i s t i c s of s e m i c o n d u c t o r devices has been a subject of c o n s i d e r a b l e activity and i n t e r e s t (5). However, the factors limiting the efficiencies of silicon solar c e l l s a r e less well investigated. Various suggestions have been advanced in the l i t e r a t u r e to r e l a t e c o n v e r s i o n efficiencies of s o l a r cells to d e f e c t - i m p u r i t y factors in the m a t e r i a l The p r e l i m i n a r y investigation p r e s e n t e d h e r e indicates a tentative c o r r e l a t i o n between the p r e s e n c e of twins and poor c o n v e r s i o n efficiencies. In most c a s e s the twins g e n e r a t e d in the c r y s t a l s a r e quite thin and t h e r e f o r e r e s e m b l e stacking faults in the lattice. The e l e c t r i c a l effects a s s o c i a t e d with stacking faults in silicon

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SILICON RIBBON SOLAR CELLS

have been r e c o g n i z e d (6, 7). c u r r e n t s , noise, etc.

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T h e s e include f a c t o r s such as e x c e s s r e v e r s e

The ribbons c u r r e n t l y grown by the EFG technique also display the p r e s e n c e of f a i r l y high d e n s i t i e s of d i s l o c a t i o n s ( ~ 105/cm2). N e v e r t h e l e s s , c e l l s of a quality approaching those m a d e f r o m C z o c h r a l s k i grown c r y s t a l s have been obtained in such ribbons when twins a r e absent f r o m the m a t e r i a l . F u r t h e r work is n e c e s s a r y to d e t e r m i n e w h e t h e r the p r e s e n c e of twins deg r a d e s c e l l p e r f o r m a n c e d i r e c t l y or if twin nucleation g e n e r a t e s s t r e s s e s of a sufficient magnitude as to nucleate dislocations which function as local r e g i o n s of low m i n o r i t y c a r r i e r lifetime. The equally important effects of t r a n s i t i o n m e t a l i m p u r i t i e s and t h e i r i n t e r a c t i o n with c r y s t a l l o g r a p h i c defects in the ribbon a r e undoubtedly of c o n s e q u e n c e in d e t e r m i n i n g the efficiency of silicon ribbon s o l a r ceils. Conclusions A tentative c o r r e l a t i o n be tw een the p r e s e n c e of twins in EFG silicon ribbons and the c o n v e r s i o n e f f i c i e n c i e s of s o l a r c e l l s built f r o m th ese ribbons has been e s t a b l i s h e d . T w i n - f r e e c r y s t a l s r e s u l t in the best quality s o l a r c e l l s w h e r e a s i n t e r s e c t i n g twins appear to introduce the most a d v e r s e effects. Acknowledgements The a ut hor s wish to acknowledge the e x p e r i m e n t a l a s s i s t a n c e of Mr. R. King and Mr. D. Bliss. Part of this work was supported by c o n t r a c t s f r o m the National Science Foundation and the Jet Propulsion L ab o rato ry . References .

FL E. Bates, F. H. Cocks, and A. I. Mlavsky, Proc. Ninth Photovoltaic Spe c i al i s t s Conf., p. 386 (19"/2).

.

FL E. Bates, D. N. Jewett, and V. E. White, Proc. Tenth Photovoltaic Specialists Conf., p. 197 (1973). M.

,

Wolf, E n e r g y C o n v e r s i o n 11, 63

(1971).

4.

E. Sirtl and A. Adler, Z. Metallk. 52, 529 (1961).

5.

See, for example, S e m i c o n d u c t o r Silicon 1973, ed. H. R. Huff and R. R. Burgess. The Electroclaem. Soc., Princeton, New J e r s e y , pp. 625-736. .

F. Barson, M. S. Hess, and M. M. Roy, J. E l e c t r o c h e m . Soc. 116, 304 (1969).

.

K. V. Ravi, C. J. Varker, and C. E. Volk, J. Electrochem. Soc. 120, 533 (1973).