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Hydrogen passivation of defects in polycrystalline silicon solar cells
Renewable Energy, Vol. 6, No. 3, pp. 303-305, 1995 Elsevier Science Ltd printed in Great Britain 0960-1481/95 $9.50 + 000
Pergamon 0960-1481(95)00017-8
HYDROGEN PASSIVATION OF DEFECTS IN POLYCRYSTALLINE SILICON SOLAR CELLS. B.M.
ABDURAKHMANOV
and R.R.
BILYALOV
Institute of Electronics, Academgorodok, 700143 Tashkent, Uzbekistan
ABSTRACT. The investigations of hydrogen passivation of defects in polycrystalline silicon produced by the Czochralski method have been carried on. The results presented give evidence that it is advisable to use this material to create cheap effective solar cells. KEY WORDS Polycrystalline
silicon,
hydrogen
passivation
INTRODUCTION Now more prospective material for producing cheap and effective solar cells is polycrystalline silicon. The only obstacle to its wide application in the production of solar cells is the relatively low efficiency of photoelectric transformation due to the high recombination activity of the material. Hydrogen passivation is rather successfully used to reduce the recombination activity. A great number of papers have recently appeared, where the results of hydrogen passlvation of solar cells manufactured on the basis of cast poly-Si of the type SILSO and POLYX (Muller et al., 1986; Martinuzzi #t al., 1987) or EFG-ribbon poly-Si (Hanoka et al., 1983; Lewalsky et @i., 1987) are published. Attention was not paid to poly-Si with large grains produced by the Czochralski method. The indicated Poly-Si can be specially produced in the form of one ingot or several ones being pulled simultaneously from the melt in a composition of which there is a waste of the mono-Si production such as ends of ingots, crucible remainders, ingot fragments and nonstandard products of the monoSi production. It should be specially noted that it is possible to produce this material simultaneously with a mono-crystalline non-dislocation ingot of a large diameter where its bottom polycrystalline part is of a result of an intentional move of the process into a mode of pulling a polycrystal. It is evident that this type of poly-Si meets one of the main requirements of helioengineering, namely the low cost of initial material for the production of solar cells. The purpose of this paper is the comparison of the efficiency of hydrogen passivation of defect in poly-Si produced by the Czochralski method and the cast one (Abdurakhmanov et al., 1992; A b d u r a k h m a n o v et al., 1993). 303
304
B . M . ABDURAKHMANOV and R. R. BILYALOV
EXPERIMENTAL
TECHNIQUES
The i n i t i a l m a t e r i a l was that of p - t y p e w i t h a specific resistance of 1-2 O h m and a m e a n g r a i n size of i-i0 mm, an area of the samples was 4x2 cm 2. The p / n - j u n c t i o n was f o r m e d at a d e p t h of 1 m k m by p h o s p h o r diffusion. The c o n t a c t s w e r e c o a t e d by v a c u u m s p u t t e r i n g Ti-Ni. The a n t i r e f l e c t i o n c o a t i n g was not coated. H y d r o g e n a t i o n was m a d e by l o w - e n e r g y ion i m p l a n t a t i o n (1.2 keY; 0.25 mA/cm2; 20 min) u s i n g the K a u f f m a n source at a t e m p e r a t u r e of 300°C (Bilyalov et al., 1990). B e f o r e and a f t e r h y d r o g e n passivation the current-voltage characteristics were measured under a solar s i m u l a t o r in the c o n d i t i o n s of AM 1.5 at i00 M W t / c m 2. W h i l e m e a s u r i n g the t e m p e r a t u r e was m a i n t a i n e d at a level of 25°C. The k i n e t i c s of g r a i n size and d i s l o c a t i o n p a s s i v a t i o n was d e f i n e d by the L B I C - m e t h o d . The m o n o c h r o m a t i c b e a m w i t h a d i a m e t e r of i0 m k m and a wave l e n g t h of 0.9 m k m was u s e d as a l i g h t probe. The d i f f u s i o n l e n g t h and the rate of s u r f a c e r e c o m b i n a t i o n w e r e d e t e r m i n e d by m e a s u r i n g n o r m a l i z e d photocurrent. RESULTS As a r e s u l t of h y d r o g e n p a s s i v a t i o n the m i n o r i t y c a r r i e r d i f f u s i o n length (Ln)iS i n c r e a s e d by 1.5-2.5 f r o m 20-40 m k m to 50-60 m k m for the C z o c h r a l s k i p o l y - S i and by 1.5-2 for the cast one. T h e i n c r e a s e in L n for the material being s t u d i e d e s s e n t i a l l y d e p e n d s on w h a t p a r t of the ingot the samples were cut. The b o t t o m p a r t of the p o l y - S i ingot g r o w n by the C z o c h r a l s k i method is m o r e d e f e c t i v e and there are a g r e a t n u m b e r of i m p u r i t i e s of t r a n s i t i v e m e t a l s in it. T h e y d e c r e a s e l i f e t i m e of the c h a r g e c a r r i e r s as a r e s u l t of w h i c h the i n i t i a l v a l u e of L n is less than that for the top p a r t of the ingot. The i n c r e a s e in L n c o r r e l a t e s w i t h that in spectral r e s p o n c e w i t h i n the long w a v e r e g i o n of 0.6-1.2 mkm. The d i f f e r e n c e in the m a x i m a l v a l u e s of spectral responce should be s p e c i a l l y noted. It is 0.85 m k m for the C z o c h r a l s k i p o l y - S i and 0.8 m k m for the cast one. The r e s u l t s of m e a s u r i n g the p h o t o v o l t a i c p a r a m e t e r s b e f o r e and after h y d r o g e n p a s s i v a t i o n are g i v e n in Table. It is seen that a f t e r h y d r o g e n a t i o n the p a r a m e t e r s of s o l a r cells on the base of the m a t e r i a l b e i n g s t u d i e d become c o m p a r a b l e w i t h those on the base of the cast poly-Si. Table.
P a r a m e t e r s of solar cells f r o m p o l y - S i g r o w n by the C z o c h r a l s k i m e t h o d (A) and the cast one (B) before and a f t e r passivation
Isc
Uoc
ff
eff
N
before a~ter mA/cm k
before
after
before
after
before
after
V
A
i0.0
18.0
0.5
0.53
0.55
0.55
3.25
5.57
B
18.2
20.1
0.55
0.57
0.7
0.7
7.0
8.0
The d i f f e r e n c e in the e f f i c i e n c y of t r a n s f o r m a t i o n is a p p a r e n t l y due d i f f e r i n g the d e f e c t s t r u c t u r e in the C z o c h r a l s k i and cast poly-Si.
to
Hydrogen passivation of defects
305
In the cast one the efficiency of passivation is due to passivation of defects inside the grains. The passivation of vacancy defects is especially effective (Saidov et al., 1989a). In addition, the electroactive complexes of carbon and transitive metals are well passivated (Saidov et al., 1989b). The efficiency of the grain boundary passivation is less than that in the Czochralski poly-Si. In the Czochralski poly-Si the concentration of undesired impurities little exceeds their concentration in mono-Si. Therefore the main defects in this material are the grain boundaries and the dislocation pile-up. In spite of the considerable concentration of dislocations they are well passivated by hydrogen because the most part of them is l i n e a r dislocations. The recombination rate is decreased from 3.6"106 to 4.4"105 cm/s. As shown by the LBIC-analysis the passivation of the grain boundaries occurs during the first i0 minutes of the hydrogenation after that the linear grain boundaries are passivated. Only the boundaries between the grains oriented in different directions remain unpassivated. Thus, the hydrogenation results in the considerable decrease of the recombination activity of the Czochralski poly-Si. Therefore this material is more suitable for manufacturing cheap and effective solar cells. REFERENCES Abdurakhmanov, B.M., T.H.Achilov, A.L.Kadirov, Sh.T.Kasimov, M.S.Saidov, V.Y.Tadjiev, M.Halikov, S.Hoshimov and O.I.Chechetka (1992). Production technology of secondary cast polycrystalline silicon and solar cells on its base. G~liotehnika, 28, No.4, 8-14. Abdurakhmanov, B.M., N.I.Abrosimova, R.R.Bilyalov and M.S.Saidov (1993). Photovoltaic properties of polycrystalline cast silicon. Geliotehnika, 29, No.l, 9-12. Bilyalov, R.R., M.S.Saidov and V.P.Chirva (1990). Hydrogen passivation of polycrystalline silicon solar cells. Geliotehnika, 2__6_, No.4, 36-39. Hanoka, J.I., C.H.Seager, D.J.Sharp and J.K.G.Panitz (1983). Hydrogen passivation of defects in silicon ribbon grown by the edge-defined filmfed growth process. Appl.Phys.Lett., 42, No.7, 618-620. Lewalski, N., R.Schindler and B.Voss (1987). Characterization of hydrogen passivated polycrystalline silicon solar cells. In: Proc.19th IEEE Pho~QvQltaic Spec. Conf, New Orlean, 1059-1062. Muller, J., Y.Ababou, A.Barhdadi, E.Courcelle, S.Unamuno, D.Salles and P.Siffert (1986). Passivation of polycrystalline silicon solar cells by low-energy hydrogen ion implantation. Solar Cells, 17, No.2-3, 201231. Martinuzzi, S., H.EI.Chitani, L.Ammor, M.Pasquinelli and H.Poitevin (1987). Improvement of polycfystalline silicon cells by backside hydrogenations. In: Proc.19th IEEE Photovoltaic Spec. Conf, New Orlean, 1069-1074. Saidov, H.S., B.M.Abdurakhmanov, R.R.Bilyalov and V.P.Chirva (1989). The effect of radiation by hydrogen plasma on IR-absorbtion of polycrystalline silicon. Dokladi Academii Nauk UzSSR, 8, 24-26. Saidov, M.S., B.M.Abdurakhmanov, R.R.Bilyalov and V.P.Chirva (1989). The EPR-study of polycrystalline silicon films passivated by hydrogen plasma. Dokladi Academii Nauk UzSSR, !, 25-27.
Pergamon 0960-1481(95)00017-8
HYDROGEN PASSIVATION OF DEFECTS IN POLYCRYSTALLINE SILICON SOLAR CELLS. B.M.
ABDURAKHMANOV
and R.R.
BILYALOV
Institute of Electronics, Academgorodok, 700143 Tashkent, Uzbekistan
ABSTRACT. The investigations of hydrogen passivation of defects in polycrystalline silicon produced by the Czochralski method have been carried on. The results presented give evidence that it is advisable to use this material to create cheap effective solar cells. KEY WORDS Polycrystalline
silicon,
hydrogen
passivation
INTRODUCTION Now more prospective material for producing cheap and effective solar cells is polycrystalline silicon. The only obstacle to its wide application in the production of solar cells is the relatively low efficiency of photoelectric transformation due to the high recombination activity of the material. Hydrogen passivation is rather successfully used to reduce the recombination activity. A great number of papers have recently appeared, where the results of hydrogen passlvation of solar cells manufactured on the basis of cast poly-Si of the type SILSO and POLYX (Muller et al., 1986; Martinuzzi #t al., 1987) or EFG-ribbon poly-Si (Hanoka et al., 1983; Lewalsky et @i., 1987) are published. Attention was not paid to poly-Si with large grains produced by the Czochralski method. The indicated Poly-Si can be specially produced in the form of one ingot or several ones being pulled simultaneously from the melt in a composition of which there is a waste of the mono-Si production such as ends of ingots, crucible remainders, ingot fragments and nonstandard products of the monoSi production. It should be specially noted that it is possible to produce this material simultaneously with a mono-crystalline non-dislocation ingot of a large diameter where its bottom polycrystalline part is of a result of an intentional move of the process into a mode of pulling a polycrystal. It is evident that this type of poly-Si meets one of the main requirements of helioengineering, namely the low cost of initial material for the production of solar cells. The purpose of this paper is the comparison of the efficiency of hydrogen passivation of defect in poly-Si produced by the Czochralski method and the cast one (Abdurakhmanov et al., 1992; A b d u r a k h m a n o v et al., 1993). 303
304
B . M . ABDURAKHMANOV and R. R. BILYALOV
EXPERIMENTAL
TECHNIQUES
The i n i t i a l m a t e r i a l was that of p - t y p e w i t h a specific resistance of 1-2 O h m and a m e a n g r a i n size of i-i0 mm, an area of the samples was 4x2 cm 2. The p / n - j u n c t i o n was f o r m e d at a d e p t h of 1 m k m by p h o s p h o r diffusion. The c o n t a c t s w e r e c o a t e d by v a c u u m s p u t t e r i n g Ti-Ni. The a n t i r e f l e c t i o n c o a t i n g was not coated. H y d r o g e n a t i o n was m a d e by l o w - e n e r g y ion i m p l a n t a t i o n (1.2 keY; 0.25 mA/cm2; 20 min) u s i n g the K a u f f m a n source at a t e m p e r a t u r e of 300°C (Bilyalov et al., 1990). B e f o r e and a f t e r h y d r o g e n passivation the current-voltage characteristics were measured under a solar s i m u l a t o r in the c o n d i t i o n s of AM 1.5 at i00 M W t / c m 2. W h i l e m e a s u r i n g the t e m p e r a t u r e was m a i n t a i n e d at a level of 25°C. The k i n e t i c s of g r a i n size and d i s l o c a t i o n p a s s i v a t i o n was d e f i n e d by the L B I C - m e t h o d . The m o n o c h r o m a t i c b e a m w i t h a d i a m e t e r of i0 m k m and a wave l e n g t h of 0.9 m k m was u s e d as a l i g h t probe. The d i f f u s i o n l e n g t h and the rate of s u r f a c e r e c o m b i n a t i o n w e r e d e t e r m i n e d by m e a s u r i n g n o r m a l i z e d photocurrent. RESULTS As a r e s u l t of h y d r o g e n p a s s i v a t i o n the m i n o r i t y c a r r i e r d i f f u s i o n length (Ln)iS i n c r e a s e d by 1.5-2.5 f r o m 20-40 m k m to 50-60 m k m for the C z o c h r a l s k i p o l y - S i and by 1.5-2 for the cast one. T h e i n c r e a s e in L n for the material being s t u d i e d e s s e n t i a l l y d e p e n d s on w h a t p a r t of the ingot the samples were cut. The b o t t o m p a r t of the p o l y - S i ingot g r o w n by the C z o c h r a l s k i method is m o r e d e f e c t i v e and there are a g r e a t n u m b e r of i m p u r i t i e s of t r a n s i t i v e m e t a l s in it. T h e y d e c r e a s e l i f e t i m e of the c h a r g e c a r r i e r s as a r e s u l t of w h i c h the i n i t i a l v a l u e of L n is less than that for the top p a r t of the ingot. The i n c r e a s e in L n c o r r e l a t e s w i t h that in spectral r e s p o n c e w i t h i n the long w a v e r e g i o n of 0.6-1.2 mkm. The d i f f e r e n c e in the m a x i m a l v a l u e s of spectral responce should be s p e c i a l l y noted. It is 0.85 m k m for the C z o c h r a l s k i p o l y - S i and 0.8 m k m for the cast one. The r e s u l t s of m e a s u r i n g the p h o t o v o l t a i c p a r a m e t e r s b e f o r e and after h y d r o g e n p a s s i v a t i o n are g i v e n in Table. It is seen that a f t e r h y d r o g e n a t i o n the p a r a m e t e r s of s o l a r cells on the base of the m a t e r i a l b e i n g s t u d i e d become c o m p a r a b l e w i t h those on the base of the cast poly-Si. Table.
P a r a m e t e r s of solar cells f r o m p o l y - S i g r o w n by the C z o c h r a l s k i m e t h o d (A) and the cast one (B) before and a f t e r passivation
Isc
Uoc
ff
eff
N
before a~ter mA/cm k
before
after
before
after
before
after
V
A
i0.0
18.0
0.5
0.53
0.55
0.55
3.25
5.57
B
18.2
20.1
0.55
0.57
0.7
0.7
7.0
8.0
The d i f f e r e n c e in the e f f i c i e n c y of t r a n s f o r m a t i o n is a p p a r e n t l y due d i f f e r i n g the d e f e c t s t r u c t u r e in the C z o c h r a l s k i and cast poly-Si.
to
Hydrogen passivation of defects
305
In the cast one the efficiency of passivation is due to passivation of defects inside the grains. The passivation of vacancy defects is especially effective (Saidov et al., 1989a). In addition, the electroactive complexes of carbon and transitive metals are well passivated (Saidov et al., 1989b). The efficiency of the grain boundary passivation is less than that in the Czochralski poly-Si. In the Czochralski poly-Si the concentration of undesired impurities little exceeds their concentration in mono-Si. Therefore the main defects in this material are the grain boundaries and the dislocation pile-up. In spite of the considerable concentration of dislocations they are well passivated by hydrogen because the most part of them is l i n e a r dislocations. The recombination rate is decreased from 3.6"106 to 4.4"105 cm/s. As shown by the LBIC-analysis the passivation of the grain boundaries occurs during the first i0 minutes of the hydrogenation after that the linear grain boundaries are passivated. Only the boundaries between the grains oriented in different directions remain unpassivated. Thus, the hydrogenation results in the considerable decrease of the recombination activity of the Czochralski poly-Si. Therefore this material is more suitable for manufacturing cheap and effective solar cells. REFERENCES Abdurakhmanov, B.M., T.H.Achilov, A.L.Kadirov, Sh.T.Kasimov, M.S.Saidov, V.Y.Tadjiev, M.Halikov, S.Hoshimov and O.I.Chechetka (1992). Production technology of secondary cast polycrystalline silicon and solar cells on its base. G~liotehnika, 28, No.4, 8-14. Abdurakhmanov, B.M., N.I.Abrosimova, R.R.Bilyalov and M.S.Saidov (1993). Photovoltaic properties of polycrystalline cast silicon. Geliotehnika, 29, No.l, 9-12. Bilyalov, R.R., M.S.Saidov and V.P.Chirva (1990). Hydrogen passivation of polycrystalline silicon solar cells. Geliotehnika, 2__6_, No.4, 36-39. Hanoka, J.I., C.H.Seager, D.J.Sharp and J.K.G.Panitz (1983). Hydrogen passivation of defects in silicon ribbon grown by the edge-defined filmfed growth process. Appl.Phys.Lett., 42, No.7, 618-620. Lewalski, N., R.Schindler and B.Voss (1987). Characterization of hydrogen passivated polycrystalline silicon solar cells. In: Proc.19th IEEE Pho~QvQltaic Spec. Conf, New Orlean, 1059-1062. Muller, J., Y.Ababou, A.Barhdadi, E.Courcelle, S.Unamuno, D.Salles and P.Siffert (1986). Passivation of polycrystalline silicon solar cells by low-energy hydrogen ion implantation. Solar Cells, 17, No.2-3, 201231. Martinuzzi, S., H.EI.Chitani, L.Ammor, M.Pasquinelli and H.Poitevin (1987). Improvement of polycfystalline silicon cells by backside hydrogenations. In: Proc.19th IEEE Photovoltaic Spec. Conf, New Orlean, 1069-1074. Saidov, H.S., B.M.Abdurakhmanov, R.R.Bilyalov and V.P.Chirva (1989). The effect of radiation by hydrogen plasma on IR-absorbtion of polycrystalline silicon. Dokladi Academii Nauk UzSSR, 8, 24-26. Saidov, M.S., B.M.Abdurakhmanov, R.R.Bilyalov and V.P.Chirva (1989). The EPR-study of polycrystalline silicon films passivated by hydrogen plasma. Dokladi Academii Nauk UzSSR, !, 25-27.