HIGH EFFICIENCY, LARGE-AREA PHOTOVOLTAIC DEVICES USING AMORPHOUS Si : F : H ALLOY

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HIGH EFFICIENCY, LARGE-AREA

PHOTOVOLTAIC DEVICES USING AMORPHOUS Si : F : H ALLOY

A. Madan, W. Czubatyj, J. Yang, J. Mcgill, S. Ovshinsky

To cite this version:

A. Madan, W. Czubatyj, J. Yang, J. Mcgill, S. Ovshinsky. HIGH EFFICIENCY, LARGE-AREA

PHOTOVOLTAIC DEVICES USING AMORPHOUS Si : F : H ALLOY. Journal de Physique Collo-

ques, 1981, 42 (C4), pp.C4-463-C4-466. �10.1051/jphyscol:1981497�. �jpa-00220954�

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HIGH EFFICIENCY, LARGE-AREA PHOTOVOLTAIC DEVICES USING AMORPHOUS S i : F : H ALLOY

A. Madan, W. Czubatyj, J . Yang, J . McGill and S.R. Ovshinsky

Energy Conversion Devices, Inc., 1675 West MapZe Road, Troy, Michigan 48084, U. S.A.

Abstract.- Overall conversion efficiency of 6.6% has been obtained f o r a photovoltaic device over an a c t i v e area 0.73 cm2 using amorphous Si:F:H alloy i n a 141s configuration.

1ntroduction.- The p o s s i b i l i t y of low-cost thin film photovoltaic c e l l s using amorphous s i l i c o n (a-Si) based alloys has generated an intense amount of i n t e r e s t in the past few years. Previously, we have reported on the properties of a-Si :F:H a l l o y (1,2) as a useful photovoltaic material. We a l s o reportea an overall device conversion efficiency of 6.3% over an a c t i v e area of 0.042 cm2 ( 3 ) . In t h i s paper we consider the level of reproducibility t h a t has been obtained and i a t e s t data on MIS (metal -insulator-semiconductor) devices which have yielded conversion e f f i

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ciencies of 6.6% over a much larger a c t i v e area.

Photovoltaic Properties of a-Si:F:H Alloy.- In previous publications we have reported t h a t an a-Si:F:H alloy with a low density of s t a t e s and a high photoconduc- t i v i t y can be fabricated using the radio frequency glow discharge of mixed SiF4 and H2 gases ( 1 , 2 ) . We have a l s o shown t h a t t h i s type of material can be doped e a s i l y by introducing small amounts of PH or AsH3 in the gas phase t o obtain conductivi- t i e s ( f o r the n+ layer)

-

¹⁰^{( Q}^cm?-1.

Typical MIS device struc$ures were fabricated a s follows: a t h i n , highly con- ductive n+ layer ( - 200-500 A). was deposited. onto a r e f l e c t i n g bottom contact such as MO, Cr, e t c ; next,

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⁵⁰⁰⁰A of a c t i v e photoconductive a-Si:F:H layer was deposited using a volume gas r a t i o of SiF4/H2

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^511. (The photoconductivity under AM-1 excitation of t h i s component i s t y p i c a l l y in the range 10-4

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1 0 ~ 3 ( R cm)-l, which provides f o r a low s e r i e s resistance in operation.) Then a 20 A thick layer of oxide such a s Nb205 was thermally evaporated and contact was made t o the device usingo70 f o f high work function Au:Pd (90:lO) o r P t metal. Finally, a layer of 350 A thick ZnS served as an a n t i r e f l e c t i o n coating.

Fig. 1 : Forward and reverse J-V c h a r a c t e r i s t i c s of a typical MIS device.

W a1 0.3 0.4 OS M

V VOLTS

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981497

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C4-464 JOURNAL DE PHYSIQUE

Figure 1 shows a typical dark current densi ty-v01 tage (J-V) c h a r a c t e r i s t i c s f o r forward and reverse bias. We note t h a t the r e c t i f i c a t i o n r a t i o a t 0.5V bias i s about 105. The dark diode i d e a l i t y f a c t o r f o r the device i s n = 1.2. We have previously shown ( 4 ) t h a t the value of n i s dependent on the oxide layer thickness ( i . e . , f o r 6 = 0 (corresponding t o a native oxide) n = 1.05 and f o r 6 = 30 A: n = 1.2) and can be interpreted in terms of a low surface s t a t e density.

The c e l l response under simulated AM-1 illumination exhibits the following c h a r a c t e r i s t i c s : Voc = 0.8V, Jsc = 12.9 mA cm-2, FF = 0.61, yielding an overall conversion efficiency of 6.3% over an a c t i v e area of 0.042 cm2.

The presence of the i n s u l a t o r enhances the open c i r c u i t voltage, V,,, due t o t h e suppression of the majority c a r r i e r (electron) current without any e f f e c t on the minority c a r r i e r (holes) density which c o n s t i t u t e s the short-circuit current.

This i s shown in Figure 2 ( a ) and (b) where Jsc and AVoc, the enhancement in V,,, a r e plotted against the insulator thickness, 6. We should note t h a t f o r 6

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³⁰^A,

AVoc = 250 mV.

Since performing t h e above work ( 3 ) , Gutkowicz-Krusin e t a1 ( 5 ) have suggested t h a t t h e introduction of the insulator could a l s o enhance the J

,

primarily a t the blue end of the spectrum, due t o the reduction i n the thermal dfffusion of electrons against t h e e l e c t r i c f i e l d . The above data, shown i n Figure 2(b) ,indicates t h a t J,= i s possibly enhanced. Figure 3 shows the spectral response of a c e l l with and without t h e insulator. We note t h a t the spectral response of the c e l l i s improved toward t h e blue end of the spectrum when an insulator i s introduced in agreement with Gutkowicz-Krusin e t a l ' s suggestion.

Fig. 2 : ( a ) Short-circuit current density as a function of oxide thickness ( 6 ) .

(b):

3peti-circui t v01 tage enhancement as a h n c t i o n of oxide thickness ( 6 ) .

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f i l l f a c t o r i s t y p i c a l l y 0.55

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0.6, Voc 2 0.78

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0.88, J ^C 12

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14 mA cm-2 w i t h A.R. coating; t h e d e v i c e e f f i c i e n c y , as shown i n F i g u r e 4 t b ) , i s between 3-4% w i t h - o u t A.R. coating, and 5.5%

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6.6% w i t h A.R. coating.

I:

% .

U U P I O ) U U O .

WAVELENGTH ("m)

Fig. 3 : R e l a t i v e c o l l e c t i o n e f f i c i e n c y f o r devices w i t h and w i t h o u t t h e i n - s e r t i o n o f an i n s u l a t i n g l a y e r .

h

A M c a * n n a

w m r M

A A A A A

S ^{A A}

81

^. . . . I

6 RUN NQ

1448 U 5 S U S 6 U63 U66 1473

RUN NQ

Fig. 4 : ( a ) Devi open-circui t v01 FF. and s h o r t - c i

ce parameters such as ( b ) : Device e f f i c i e n c i e s w i t h and tage, Voc, fill f a c t o r , w i t h o u t a n t i r e f l e c t i o n (A. R.) c o a t i n g . r c u i t c u r r e n t d e n s i t y

,

Jscy as a f u n c t i o n o f r u n number f o r devices f a b r i c a t e d under o s t e n s i b l y i d e n t i c a l c o n d i t i o n s .

I n F i g u r e 5, we show t h e o v e r a l l conversion e f f i c i e n c y f o r l a r g e r area devices.

The t o t a l area o f t h e device i s 0.81 cm2 and has u t i l i z e d a g r i d p a t t e r n (Ag), and t h e a c t i v e area i s 0.73 cm2, which was used i n t h e c a l c u l a t i o n t o o b t a i n t h e o v e r a l l conversion e f f i c i e n c y . We n o t e t h a t t h e e f f i c i e n c i e s , w i t h a n t i r e f l e c t i o n coating,

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C4-466 JOURNAL DE PHYSIQUE

f o r these large-area devices i s t h e same as f o r the small areas. The best efficiency obtained i s 6.6% with the following c h a r a c t e r i s t i c s , Voc = 0.88V, Jsc = 13.1 mA cm-2, FF = 0.57., Recently, Hamakawa (6) has reported t h a t a conversion efficiency of 7.1% over an area 0.033 cm2 has been obtained, using glass/ITO/Si:C/a-Si:H/n+/Al s t r u c t u r e i n which the f i l l f a c t o r was 0.65. I t i s therefore i n t e r e s t i n g t o note t h a t with a similar f i l l f a c t o r , the active area efficiency f o r our best device would be 7.5%. Indeed, when the illumination i n t e n s i t y i s decreased, the f i l l fac- t o r in our devices does improve which suggests a sheet resistance e f f e c t coming in- to play. Therefore, the lower f i l l f a c t o r i n our devices i s not a materials problem, but a simple technological problem which should be e a s i l y remedied.

ACTIVE AREA- cm1

RVWNO.

u s o ( M l 4 a o Y 0 0 1 5 0 0 M O l b p

F i g . 5 : Device efficiency f o r large-area (0.73 cm2) c e l l S .

Conclusion.- In summary, we have shown t h a t high efficiency, large-area c e l l s can be reproducibily made using a-Si:F:H alloy.

Acknowledgements.- We wish t o thank Ronald Himmler, Lynn Bement, Robin S t i e r s , Orest Iwasiuk, and Larry Christian f o r t h e i r help in measurements and sample pre- paration. The authors also appreciate stimulating discussions with Professors M. Shur and M. Shaw. We a l s o wish t o thank ARC0 f o r i t s assistance and cooperation during t h e course of t h i s work.

1. OVSHINSKY, S.R. and MADAN, A., Nature 276 (1978) 482.

2. MADAN, A., OVSHINSKY, S.R., and BENN, E., Phil. Mag. B 40 (1979) 259.

3. MADAN, A . , MCGILL, J . CZUBATYJ, W . , YANG, J . , and OVSHIWSKY, S.R., Appl. Phys.

Lett. 37 (1980) 826.

4. MADAN, A . , MCGILL, J . , OVSHINSKY, S.R., CZUBATYJ, W . , YANG, J . , and SHUR, M.S., in Proc. of t h e Society of Photo-Optical Instrumentationa Engineers 248 (1980) 26.

5. GUTKOWICZ-KRUSIN, D., WRONSKI, C . R . , and TIEDJE, T . , Appl. Phys. Lett. 38 (1981) 87.

6. HAMAKAWA, Y . , Private Communication.