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Reply to the comment on: Preparation and characterization of chemically deposited CuInS2 thin films
Solar Energy Materials 15 (1987) 227-228 North-Holland, Amsterdam
227
Letter to the Editor REPLY T O T H E C O M M E N T ON: P R E P A R A T I O N A N D CHARACTERIZATION O F CHEMICALLY D E P O S I T E D CulnS 2 THIN FILMS G.K. PADAM and S.U.M. RAO National Physical Laboratory, Hillside Road, New Delhi 12, India Received 3 September 1986; in final form 28 November 1986
The Letter to the Editor of Cahen [1] which comments on our paper accepts the importance and the need of low cost methods (like wet chemical technique used by the present authors) for the preparation of photovoltaic quality CulnX 2 compounds, but brings out the doubts about the confirmation of single phase CulnS 2 films with chalcopyrite structure. In this Letter an attempt is made to settle the dispute. As correctly pointed out by Cahen, CulnS 2 can have two different crystal structures. One of them is ordered chalcopyrite which is tetragonal with space group I42d and the other is disordered zinc blende which is cubic with space group F43m. Though it is difficult, diffraction data can resolve these two phases. There Cahen is sceptical of our interpretation of diffraction spectra. According to him in the tetragonal indexing system only (hkl) reflections with l = odd are expected for the chalcopyrite phase, and the present authors [2] and others [3,4] have wrongly concluded the presence of the chalcopyrite phase on the basis of reflections having l = even only. In fact, not only the chalcopyrite but also the sphalerite phase shows both 1= odd and l = even (hkl) reflections (International Table for X-ray Crystallography, Vol. I), and further the strongest reflection (112) in chalcopyrite has 1 = even while the strongest reflection (111) in sphalerite has 1 = odd (ASTM Card No. 27-159). This suggests very strongly that the contention of Cahen needs modifications. Apart from this there are certain reflections with l = odd like (011), (013), (121), (123), (017) etc. which are only present in chalcopyrite, and their presence can be decisive. To confirm the chalcopyrite structure not only X R D (X-ray diffraction) studies were done, but to strengthen the analysis T E D (transmission electron diffraction) was also carried out. As mentioned in fig. 1 and table 1 of ref. [2], nearly all reflections can be attributed to the chalcopyrite phase. Moreover, the presence of reflection (121) with l = odd strongly suggests the formation of the chalcopyrite phase. Not only this, but a careful, critical and thorough T E D study of CulnS 2 thin films revealed the presence of (011) and (013) reflections along with (121). In our opinion the presence of the above reflection with l = odd which exclusively belongs to the chalcopyrite phase, confirms beyond any reasonable doubt the presence of this phase. One such T E D pattern can be seen in fig. 1. 0165-1633/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
228
G.K. Padam, S.U.M. Rao / Chemically deposited CulnS 2
Fig. 1. T h e second o b j e c t i o n is a b o u t the presence of single p h a s e d e s p i t e of the fact that the films are n o n s t o i c h i o m e t r i c in the Cu-rich direction. O u r c o m p o s i t i o n a l d a t a o b t a i n e d f r o m E D S a n d s p e c t r o c h e m i c a l analyses definitely show that films are Cu-rich a n d the i n d i v i d u a l c o m p o n e n t s Cu, In a n d S are in the ratio of 1.08 : 1.0 : 1.5 i n s t e a d of 1 : 1 : 2. A s p o i n t e d o u t b y C a h e n for C u - I n - X systems, C u l n X 2 can b e c o n s i d e r e d as one p h a s e in the ( C u 2 X ) x ( I n 2 X 3 ) 1 x system, with x = 0.5. Moreover, recently he a n d others have r e p o r t e d [5] that C u l n S e 2 exists in single p h a s e in the ( C u 2 S e ) x ( I n 2 S e 3 ) l _ ~ system when 0.45 ~< x ~< 0.6. It is very evident f r o m our d a t a that x for c h e m i c a l l y d e p o s i t e d C u l n S 2 films lies b e t w e e n these values. It seems quite r e a s o n a b l e in the light of the a b o v e discussion t h a t our films are single phase. This c o u l d b e the r e a s o n that, despite the v a r i a t i o n in stoichiometry, we are n o t observing a n y e x t r a line o t h e r t h a n C u l n S 2 p h a s e in b o t h the p o w d e r X R D a n d the T E D patterns. However, a slight v a r i a t i o n in the d-values was observed, which can be a t t r i b u t e d to the d e p a r t u r e f r o m s t o i c h i o m e t r y [2]. T o epitomise o n e can say that the results o b t a i n e d are b e t t e r e x p l a i n e d if we c o n s i d e r that the f o r m a t i o n of C u l n S 2 p h a s e is d o m i n a n t .
References [1] [2] [3] [4]
D. Cahen, this issue, Solar Energy Mater. 15 (1987) 225. G.K. Padam and S.U.M. Rao, Solar Energy Mater. 13 (1986) 287. H.L. Hwang, C.Y. Sun, C.Y. Leu, C.L. Cheng and C.C. Tu, Rev. de Phys. Appl. 13 (1978) 745. P. Rajaram, R. Thangaraj, A.K. Sharma, A. Raza and O.P. Agnihotri, Thin Solid Films 100 (1983) 111. [5] J.C.W. Folmer, J.A. Turner, R. Noufi and D. Cahen, J. Electrochem. Soc. 132 (1985) 1319.
227
Letter to the Editor REPLY T O T H E C O M M E N T ON: P R E P A R A T I O N A N D CHARACTERIZATION O F CHEMICALLY D E P O S I T E D CulnS 2 THIN FILMS G.K. PADAM and S.U.M. RAO National Physical Laboratory, Hillside Road, New Delhi 12, India Received 3 September 1986; in final form 28 November 1986
The Letter to the Editor of Cahen [1] which comments on our paper accepts the importance and the need of low cost methods (like wet chemical technique used by the present authors) for the preparation of photovoltaic quality CulnX 2 compounds, but brings out the doubts about the confirmation of single phase CulnS 2 films with chalcopyrite structure. In this Letter an attempt is made to settle the dispute. As correctly pointed out by Cahen, CulnS 2 can have two different crystal structures. One of them is ordered chalcopyrite which is tetragonal with space group I42d and the other is disordered zinc blende which is cubic with space group F43m. Though it is difficult, diffraction data can resolve these two phases. There Cahen is sceptical of our interpretation of diffraction spectra. According to him in the tetragonal indexing system only (hkl) reflections with l = odd are expected for the chalcopyrite phase, and the present authors [2] and others [3,4] have wrongly concluded the presence of the chalcopyrite phase on the basis of reflections having l = even only. In fact, not only the chalcopyrite but also the sphalerite phase shows both 1= odd and l = even (hkl) reflections (International Table for X-ray Crystallography, Vol. I), and further the strongest reflection (112) in chalcopyrite has 1 = even while the strongest reflection (111) in sphalerite has 1 = odd (ASTM Card No. 27-159). This suggests very strongly that the contention of Cahen needs modifications. Apart from this there are certain reflections with l = odd like (011), (013), (121), (123), (017) etc. which are only present in chalcopyrite, and their presence can be decisive. To confirm the chalcopyrite structure not only X R D (X-ray diffraction) studies were done, but to strengthen the analysis T E D (transmission electron diffraction) was also carried out. As mentioned in fig. 1 and table 1 of ref. [2], nearly all reflections can be attributed to the chalcopyrite phase. Moreover, the presence of reflection (121) with l = odd strongly suggests the formation of the chalcopyrite phase. Not only this, but a careful, critical and thorough T E D study of CulnS 2 thin films revealed the presence of (011) and (013) reflections along with (121). In our opinion the presence of the above reflection with l = odd which exclusively belongs to the chalcopyrite phase, confirms beyond any reasonable doubt the presence of this phase. One such T E D pattern can be seen in fig. 1. 0165-1633/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
228
G.K. Padam, S.U.M. Rao / Chemically deposited CulnS 2
Fig. 1. T h e second o b j e c t i o n is a b o u t the presence of single p h a s e d e s p i t e of the fact that the films are n o n s t o i c h i o m e t r i c in the Cu-rich direction. O u r c o m p o s i t i o n a l d a t a o b t a i n e d f r o m E D S a n d s p e c t r o c h e m i c a l analyses definitely show that films are Cu-rich a n d the i n d i v i d u a l c o m p o n e n t s Cu, In a n d S are in the ratio of 1.08 : 1.0 : 1.5 i n s t e a d of 1 : 1 : 2. A s p o i n t e d o u t b y C a h e n for C u - I n - X systems, C u l n X 2 can b e c o n s i d e r e d as one p h a s e in the ( C u 2 X ) x ( I n 2 X 3 ) 1 x system, with x = 0.5. Moreover, recently he a n d others have r e p o r t e d [5] that C u l n S e 2 exists in single p h a s e in the ( C u 2 S e ) x ( I n 2 S e 3 ) l _ ~ system when 0.45 ~< x ~< 0.6. It is very evident f r o m our d a t a that x for c h e m i c a l l y d e p o s i t e d C u l n S 2 films lies b e t w e e n these values. It seems quite r e a s o n a b l e in the light of the a b o v e discussion t h a t our films are single phase. This c o u l d b e the r e a s o n that, despite the v a r i a t i o n in stoichiometry, we are n o t observing a n y e x t r a line o t h e r t h a n C u l n S 2 p h a s e in b o t h the p o w d e r X R D a n d the T E D patterns. However, a slight v a r i a t i o n in the d-values was observed, which can be a t t r i b u t e d to the d e p a r t u r e f r o m s t o i c h i o m e t r y [2]. T o epitomise o n e can say that the results o b t a i n e d are b e t t e r e x p l a i n e d if we c o n s i d e r that the f o r m a t i o n of C u l n S 2 p h a s e is d o m i n a n t .
References [1] [2] [3] [4]
D. Cahen, this issue, Solar Energy Mater. 15 (1987) 225. G.K. Padam and S.U.M. Rao, Solar Energy Mater. 13 (1986) 287. H.L. Hwang, C.Y. Sun, C.Y. Leu, C.L. Cheng and C.C. Tu, Rev. de Phys. Appl. 13 (1978) 745. P. Rajaram, R. Thangaraj, A.K. Sharma, A. Raza and O.P. Agnihotri, Thin Solid Films 100 (1983) 111. [5] J.C.W. Folmer, J.A. Turner, R. Noufi and D. Cahen, J. Electrochem. Soc. 132 (1985) 1319.