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CuInSe2 thin film solar cells
Solar Energy Materials 11 (1985) 409-417 North-Holland, Amsterdam
409
STRUCTURAL CHARACTERISTICS O F CulnSe 2 THIN FILMS AND ITS INFLUENCE ON PV ACTIVITY OF C d S / C u I n S e 2 T H I N FILM SOLAR CELLS RAM JANAM and O.N. SRIVASTAVA Thin Film & Electron Microscope Lab., Department of Physics, Banaras Hindu University, Varanasi221005, India Received 24 April 1984; in revised form 6 August 1984 The photovoltaic (PV) behaviour of CdS/CulnSe2 heterojunction thin film solar cells has been explored in terms of the structural characteristics of the CulnSe 2 thin films. It has been found that depending on the growth conditions, the CulnSe 2 thin films possess a disordered/ordered structural phase. It has been found that whereas the heterojunctions formed from the ordered CulnSe 2 phases show good PV conversion, the one having disordered CulnSe 2 films exhibit very poor response. Evidences and arguments have been put forward to show that the defect disorder states (DDS) arising from the presence of disorder are detrimental to the PV conversion efficiencies. For good PV conversions the growth conditions should be adjusted so that the synthesised films correspond to ordered phases.
1. Introduction
The development of photovoltaic power depends upon the employment of solar photons for production of electricity, i.e. the electrons and holes. This can be most conveniently done by the absorption of solar radiation in a suitable semiconducting material. Since the solar photons have specific energies, they will be able to create electrons-holes efficiently only in certain specific materials. The PV technology is thus material limited. A myriad many materials are being studied for an efficient harnessing of solar energy through solid state solar cells. CuInSe 2 is one such material which is recently being investigated by several workers. This is a direct band gap ( = 1.04 eV) [1] material which is stable and where the electronic behaviour including the type of conductivity (n or p) can be controlled with small variations in stoichiometry of the constituent elements (Se, Cu, In) through simple annealing treatments [2-4]. This ternary semiconductor has been found to be of particular interst in a CdS based all thin film solar cell. The usual absorbing material with CdS window - the copper sulphide - is known to be a problematic partner [5] in the C d S / C u 2 S junction thin film solar cells. The CuInSe 2, on the other hand, which unlike CuzS is quite stable and has a lower lattice mismatch ( = 1%) than Cu2 S ( -- 4%), has led to the fabrication of stable and high efficiency thin film solar cells [6-8]. Although there has been a surfeit of studies aimed at improvement of efficiency of the CdS/CuInSe 2 solar cells, through the variations in the electronic 0165-1633/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
410
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe,
:3
5, 213 0
b:l ~,3il2
5 , 3 23 0 ,332
4 Fig. 1. Guinier X-ray diffraction pattern of the as synthesised bulk CulnSe 2. The as synthesised phase corresponds to the chalcopyrite phase with a = 5.78 and c = 11.62 ,~..
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe :
411
properties of CuInSe 2, there are not many studies available which explore the influence of structural characteristics of CulnSe 2 thin film on PV conversion efficiency. The central theme of the present investigation is to explore the role of structural characteristics of CulnSe 2 on the working efficiency of C d S / C u l n S e 2 thin film solar cells.
2. Experimental details An initial agglomerate of CulnSe 2 was synthesised by a solid state interdiffusion process. The stoichiometric powder mixture of Cu (99.99%), In (99.99%) and Se (99.999%) was homogenised in a silica capsule at a high temperature ( = 800°C) under vacuum of the order of ] 0 - 6 Torr. The bulk flux so formed was characterised by Guinier X-ray diffraction technique. A representative X-ray diffraction pattern bringing out the chalcopyrite type structure of CulnSe 2 with a = 5.78 and c -- 11.62 ,~ is shown in fig. 1. The analysis for proper stoichiometry was also carried out on a Jeol JSM 35 CF scanning electron microscope employing EDAX technique (KEVEX 7000 system). The structural characterisation of the thin films was carried out by the technique of electron microscopy employing both the imaging and diffraction modes. For electron microscopic study, the films of thicknesses in between 30-100 nm were deposited on formvar based copper grids by thermal evaporation under a vacuum of ] 0 - 6 Torr. For monitoring the photovoltaic activity, the films of CulnSe 2 were deposited on a predeposited CdS thin film on a glass substrate at -- 200°C. The thin film solar cell configuration ( A g - C d S - C u l n S e 2 - A g ( G r i d ) ) was formed by the deposition of the various thin films through appropriate masks.
3. Results and discussion The initial structures of all the bulk as well as the deposited films of CulnSe2 were found to be chalcopyrite type tetragonal phase with a = 5.78 and c = 11.62 A [9,10]. However, when the as deposited films were annealed through temperatures ranging between 200-300°C, the chalcopyrite structure transformed to sphalerite type cubic with a = (5.80 + 0.05) ~, [10]. This transformation which was found to be generally present is crucially important, since the C d S / C u l n S e 2 thin film solar cells exhibit a significant PV activity only when the said heterojunction is annealed to temperatures around 200°C [6,7]. The details of the possible influence of this transformation ore the PV activity of the C d S / C u l n S e 2 heterojunctions have already been described elsewhere [10]. An extensive investigation of the annealed films was undertaken with a view to explore the influence of structural characteristics on the PV activity. A curious result ensued out of these investigations. This related to the presence of both the ordered and disordered CulnSe 2 sphalerite structures under nearly similar thin film deposition conditions. Figs. 2 and 3 which embody the diffraction patterns from the
412
Ram Janam, O.N. Srivastat~a / Structural characteristics of CulnSe :
t r a n s f o r m e d C u l n S % films b r i n g out the o r d e r e d a n d d i s o r d e r e d phases. W h e r e a s the o r d e r e d phases c o n t a i n e d sharp Bragg spots (see fig. 2) the d i s o r d e r e d phases h a d streaks i n t e r c o n n e c t i n g the Bragg spots (see fig. 3). A n i n t e r e s t i n g p o i n t in r e l a t i o n to the d i s o r d e r e d phase was the fact that the basic lattice in these cases was the same as for the o r d e r e d phases (see fig. 3). A detailed analysis of the m i c r o s t r u c ture of the d i s o r d e r e d phase revealed that the d i s o r d e r o r i g i n a t e d due to the p r e s e n c e of p l a n a r fauls in the p a r e n t matrix. A r e p r e s e n t a t i v e e x a m p l e is s h o w n in fig. 4. A
Fig. 2. (a) Electron diffraction pattern of the as deposited film (after annealing at = 200°C). The diffraction pattern exhibits the (111) reciprocal lattice section of the sphalerite type cubic phase of CulnSe2. The indices of diffraction spots for 220 and 02.2 have been suggested. This represents the ordered CulnSe2 phase; (b) yet another electron diffraction pattern from the ordered phase in (211) orientation.
Fig. 3. (a) Electron diffraction pattern from the disordered CulnSe2 phase. The streaking interconnecting the diffraction spots along [111] direction is noticeable. The streaking signifies the random stacking of layers along this direction and brings out the disorder: (b) another example of diffraction pattern from the disordered phase. Crossed streakings suggest faulting in two different directions.
413
Fig. 4. Electron micrograph of the stacking fault fringes in the disordered CulnSe 2 phase. The stacking fault fringes are marked by arrow S. The inset diffraction pattern which was taken from a region around (P) reveals the streaking interconnecting the spots along the [111] reciprocal lattice direction.
Fig. 5. Electron micrograph showing the vacancy precipitates. The vacancy clusters are easily discernible near the extinction contours. A region like this has been encircled.
414
Ram Janam, O.N. Srit~asta~a / Structural characteristics of ('ulnSe,
Fig. 6. Another electron micrograph revealing the presence of vacancy precipitates producing loop like features.
Fig. 7. Electron micrograph of the ordered CulnSe 2 phase. The absence of faults can be noticed. The corresponding electron diffraction from region (A) is shown in fig. 2.
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe e
415
careful analysis of the microstructural details in conjunction with the diffraction patterns revealed that the planar faults were bound by stacking faults [11]. Typical examples of the microstructures exhibiting the stacking faults are shown in fig. 4. Attempts were made to explore the origin of the planar faults. It was found that the occurrence of faults was often accompanied by the presence of structures which correspond to vacancy loops. The detailed contrast experiments (e.g. inside-outside contrast), however, could not be carried out. Representative examples of the presence of loops are brought out by figs. 5 and 6. This seemed to suggest the fact that material loss leading to precipitation of vacancies formed the prelude to the formation of planar faults. As is well known vacancy precipitation leads to the occurrence of planar faults [11]. In the case of CulnSe 2, annealing treatment of the stoichiometric phases is known to produce loss of selenium and hence it is thought that the vacancy loops are nucleated from selenium vacancy clusters. In order to verify this, films were also prepared from CulnSe 2 phases containing 20% excess selenium. In these cases the resulting films contained only a very low density of vacancy loops. Fig. 7 shows a representative example of the microstructure of the excess selenium films. Selected area diffraction showed that the excess selenium films corresponded to ordered phases. It thus seems evident that the disorder gets created through the occurrence of planar faults which in turn is produced by the nucleation of selenium vacancies. In order to monitor the PV activity of the ordered and disordered CulnSe 2 phases, thin film structures with A g - C d S - C u l n S e 2 - A g configurations of 1 cm 2 area were prepared. The CulnSe 2 films were deposited and then annealed ( = 200°C) on cadmium sulphide film base as well as on plane slides simultaneously. The PV activity was investigated with the C d S / C u l n S e 2 junction structure and the structural behaviour was explored by employing the film on the plane slide which would have presumably the same structure as the film on CdS base. Investigations spread over
Table 1 Comparative PV activity of CdS/CulnSe 2 thin film solar cells Ordered CulnSe2 phase
Disordered CulnSe2 phase
Cell no.
Fo¢ (mY)
Remarks
Cell no.
Voc (mY)
24
20
before annealing after annealing before annealing after annealing before annealing after annealing
27
2
no change on annealing
33
20
no change on annealing
41
24
no change on annealing
350 30
20 365
34
42 370
Remarks
416
Ram Janarn, O.N. Srivastava / Structural characteristics of CulnSe,
twenty different runs revealed that the C d S / C u I n S e 2 junctions with the disordered CuInSe 2 phase invariably exhibited poor PV activity. All the junctions with disordered CuInSe2 exhibited low PV activity. In sharp contrast to this all the C d S / C u l n S e 2 junctions having ordered CuInSe 2 thin film phases exhibited significant PV activity. Comparison of the PV response for the ordered and disordered cases was done by monitoring the Voc in the two cases. Table 1 brings out the PV response of some representative C d S / C u I n S e 2 junctions. In these cases the structural characteristics of the CuInSe 2 film could be unambiguously characterised through transmission electron microscopic (imaging and diffraction techniques) investigations. Representative examples of diffraction patterns and microstructures manifesting the ordered and disordered phases, as described earlier, are already shown in figs. 2 to 7. As is well known the presence of disorder would give rise to additional energy states outside the domain of regular band structure. For the type of present disorder the planar faults embodying stacking faults and antiphase boundaries would result in the creation of interface states. It is by now fairly well established that the interface states affect the efficiency of the thin film heterojunction devices [12,13]. It may be pointed out that one set of interface defect states are the ones that are located at the C d S / C u I n S e 2 boundary, the other typically present in the present case with the disordered phase would be those present in the CuInSe 2 phase and may be termed as defect disorder states (DDS). The exact role of the DDS based on the present investigation are difficult to gauge. However, it is expected that these would distort the band profile of the absorber material and they may act as recombination sites. Another possible effect is Fermi-level pinning. The most prominent detrimental effect of the DDS is likely to be in the vicinity of the heterojunction and within the carrier diffusion length. The carrier recombination may form the major influence of the DDS. Thus the DDS would result in lowering of efficiencies. Further studies seeking to explore the growth conditions which are prone to disorder nucleation and possible passivation of disorder are under progress and results will be forthcoming.
4. Conclusion It is known that PV behaviour of C d S / C u I n S e 2 junctions is rather subtle, the devices fabricated under nearly similar growth conditions show varied characteristics - high and low PV efficiencies are found for nearly similar junctions. The present investigations reveal that this subtlety may be associated with the state of order of CuInSe 2 films - the ordered films exhibit high response but the disordered ones show very poor response.
Acknowledgements One of the authors (R.J.) wishes to thank C.S.I.R. New Delhi, India for the award of a Senior Research Fellowship. The authors are also thankful to Prof. A.R. Verma
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe_,
417
a n d Prof. V . G . B h i d e for discussions. T h a n k s are also d u e to Dr. S.K. S h a r m a a n d Mr. S . U . M . R a o for c a r r y i n g o u t the E D A X a n a l y s i s o n J E O L s c a n n i n g m i c r o s c o p e .
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
W. Horig, H. Neumann and H. Sobotta, Thin Solid Films 48 (1978) 67. F.R. Fray and P. Alloyd, Thin Solid Films 58 (1979) 29. J.L. Shay, S. Wagner and H.M. Kasper, Appl. Phys. Lett. 27 (1975) 89. M. Gorska, R. Baulieu, J.J. Loferski and B. Roessler, Solar Energy Mater. 2 (1980) 343. M. Savelli and J. Bougnot, Solar Energy Conversion, ed. B.O. Seraphin (New York, 1979) p. 215. J.J. Loferski, Physics of Semiconductor Devices, eds. S.C. Jain and Radhakrishna (Wiley Eastern, New Delhi, 1981) p. 408. K.W. Mitchell, Physics of Semiconductor Devices, eds. S.C. Jain and Radhakrishna (Wiley Eastern, New Delhi, 1981) p. 420. R.A. Mickelsen and W.S. Chen, Proc. 16th IEEE Photovoltaic Spec. Conf. (New York, 1982) p. 781. L.L. Kazmerski, M.S. Ayyagari, F.R. White and G.A. Sanborn, J. Vac. Sci. Technol. 13 (1976) 139. Ram Janam and O.N. Srivastava, Crystal Res. & Technol. 18 (1983) 1475. J.P. Hirth and J. Lothe, Theory of Dislocations (McGraw-Hill, New York, 1968). K.L. Chopra and S.R. Das, Thin Film Solar Cells (Plenum Press, New York, 1983). A.L. Fahrenbruch and R.H. Bube, Fundamentals of Solar Cells (Academic Press, New York, 1983).
409
STRUCTURAL CHARACTERISTICS O F CulnSe 2 THIN FILMS AND ITS INFLUENCE ON PV ACTIVITY OF C d S / C u I n S e 2 T H I N FILM SOLAR CELLS RAM JANAM and O.N. SRIVASTAVA Thin Film & Electron Microscope Lab., Department of Physics, Banaras Hindu University, Varanasi221005, India Received 24 April 1984; in revised form 6 August 1984 The photovoltaic (PV) behaviour of CdS/CulnSe2 heterojunction thin film solar cells has been explored in terms of the structural characteristics of the CulnSe 2 thin films. It has been found that depending on the growth conditions, the CulnSe 2 thin films possess a disordered/ordered structural phase. It has been found that whereas the heterojunctions formed from the ordered CulnSe 2 phases show good PV conversion, the one having disordered CulnSe 2 films exhibit very poor response. Evidences and arguments have been put forward to show that the defect disorder states (DDS) arising from the presence of disorder are detrimental to the PV conversion efficiencies. For good PV conversions the growth conditions should be adjusted so that the synthesised films correspond to ordered phases.
1. Introduction
The development of photovoltaic power depends upon the employment of solar photons for production of electricity, i.e. the electrons and holes. This can be most conveniently done by the absorption of solar radiation in a suitable semiconducting material. Since the solar photons have specific energies, they will be able to create electrons-holes efficiently only in certain specific materials. The PV technology is thus material limited. A myriad many materials are being studied for an efficient harnessing of solar energy through solid state solar cells. CuInSe 2 is one such material which is recently being investigated by several workers. This is a direct band gap ( = 1.04 eV) [1] material which is stable and where the electronic behaviour including the type of conductivity (n or p) can be controlled with small variations in stoichiometry of the constituent elements (Se, Cu, In) through simple annealing treatments [2-4]. This ternary semiconductor has been found to be of particular interst in a CdS based all thin film solar cell. The usual absorbing material with CdS window - the copper sulphide - is known to be a problematic partner [5] in the C d S / C u 2 S junction thin film solar cells. The CuInSe 2, on the other hand, which unlike CuzS is quite stable and has a lower lattice mismatch ( = 1%) than Cu2 S ( -- 4%), has led to the fabrication of stable and high efficiency thin film solar cells [6-8]. Although there has been a surfeit of studies aimed at improvement of efficiency of the CdS/CuInSe 2 solar cells, through the variations in the electronic 0165-1633/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
410
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe,
:3
5, 213 0
b:l ~,3il2
5 , 3 23 0 ,332
4 Fig. 1. Guinier X-ray diffraction pattern of the as synthesised bulk CulnSe 2. The as synthesised phase corresponds to the chalcopyrite phase with a = 5.78 and c = 11.62 ,~..
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe :
411
properties of CuInSe 2, there are not many studies available which explore the influence of structural characteristics of CulnSe 2 thin film on PV conversion efficiency. The central theme of the present investigation is to explore the role of structural characteristics of CulnSe 2 on the working efficiency of C d S / C u l n S e 2 thin film solar cells.
2. Experimental details An initial agglomerate of CulnSe 2 was synthesised by a solid state interdiffusion process. The stoichiometric powder mixture of Cu (99.99%), In (99.99%) and Se (99.999%) was homogenised in a silica capsule at a high temperature ( = 800°C) under vacuum of the order of ] 0 - 6 Torr. The bulk flux so formed was characterised by Guinier X-ray diffraction technique. A representative X-ray diffraction pattern bringing out the chalcopyrite type structure of CulnSe 2 with a = 5.78 and c -- 11.62 ,~ is shown in fig. 1. The analysis for proper stoichiometry was also carried out on a Jeol JSM 35 CF scanning electron microscope employing EDAX technique (KEVEX 7000 system). The structural characterisation of the thin films was carried out by the technique of electron microscopy employing both the imaging and diffraction modes. For electron microscopic study, the films of thicknesses in between 30-100 nm were deposited on formvar based copper grids by thermal evaporation under a vacuum of ] 0 - 6 Torr. For monitoring the photovoltaic activity, the films of CulnSe 2 were deposited on a predeposited CdS thin film on a glass substrate at -- 200°C. The thin film solar cell configuration ( A g - C d S - C u l n S e 2 - A g ( G r i d ) ) was formed by the deposition of the various thin films through appropriate masks.
3. Results and discussion The initial structures of all the bulk as well as the deposited films of CulnSe2 were found to be chalcopyrite type tetragonal phase with a = 5.78 and c = 11.62 A [9,10]. However, when the as deposited films were annealed through temperatures ranging between 200-300°C, the chalcopyrite structure transformed to sphalerite type cubic with a = (5.80 + 0.05) ~, [10]. This transformation which was found to be generally present is crucially important, since the C d S / C u l n S e 2 thin film solar cells exhibit a significant PV activity only when the said heterojunction is annealed to temperatures around 200°C [6,7]. The details of the possible influence of this transformation ore the PV activity of the C d S / C u l n S e 2 heterojunctions have already been described elsewhere [10]. An extensive investigation of the annealed films was undertaken with a view to explore the influence of structural characteristics on the PV activity. A curious result ensued out of these investigations. This related to the presence of both the ordered and disordered CulnSe 2 sphalerite structures under nearly similar thin film deposition conditions. Figs. 2 and 3 which embody the diffraction patterns from the
412
Ram Janam, O.N. Srivastat~a / Structural characteristics of CulnSe :
t r a n s f o r m e d C u l n S % films b r i n g out the o r d e r e d a n d d i s o r d e r e d phases. W h e r e a s the o r d e r e d phases c o n t a i n e d sharp Bragg spots (see fig. 2) the d i s o r d e r e d phases h a d streaks i n t e r c o n n e c t i n g the Bragg spots (see fig. 3). A n i n t e r e s t i n g p o i n t in r e l a t i o n to the d i s o r d e r e d phase was the fact that the basic lattice in these cases was the same as for the o r d e r e d phases (see fig. 3). A detailed analysis of the m i c r o s t r u c ture of the d i s o r d e r e d phase revealed that the d i s o r d e r o r i g i n a t e d due to the p r e s e n c e of p l a n a r fauls in the p a r e n t matrix. A r e p r e s e n t a t i v e e x a m p l e is s h o w n in fig. 4. A
Fig. 2. (a) Electron diffraction pattern of the as deposited film (after annealing at = 200°C). The diffraction pattern exhibits the (111) reciprocal lattice section of the sphalerite type cubic phase of CulnSe2. The indices of diffraction spots for 220 and 02.2 have been suggested. This represents the ordered CulnSe2 phase; (b) yet another electron diffraction pattern from the ordered phase in (211) orientation.
Fig. 3. (a) Electron diffraction pattern from the disordered CulnSe2 phase. The streaking interconnecting the diffraction spots along [111] direction is noticeable. The streaking signifies the random stacking of layers along this direction and brings out the disorder: (b) another example of diffraction pattern from the disordered phase. Crossed streakings suggest faulting in two different directions.
413
Fig. 4. Electron micrograph of the stacking fault fringes in the disordered CulnSe 2 phase. The stacking fault fringes are marked by arrow S. The inset diffraction pattern which was taken from a region around (P) reveals the streaking interconnecting the spots along the [111] reciprocal lattice direction.
Fig. 5. Electron micrograph showing the vacancy precipitates. The vacancy clusters are easily discernible near the extinction contours. A region like this has been encircled.
414
Ram Janam, O.N. Srit~asta~a / Structural characteristics of ('ulnSe,
Fig. 6. Another electron micrograph revealing the presence of vacancy precipitates producing loop like features.
Fig. 7. Electron micrograph of the ordered CulnSe 2 phase. The absence of faults can be noticed. The corresponding electron diffraction from region (A) is shown in fig. 2.
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe e
415
careful analysis of the microstructural details in conjunction with the diffraction patterns revealed that the planar faults were bound by stacking faults [11]. Typical examples of the microstructures exhibiting the stacking faults are shown in fig. 4. Attempts were made to explore the origin of the planar faults. It was found that the occurrence of faults was often accompanied by the presence of structures which correspond to vacancy loops. The detailed contrast experiments (e.g. inside-outside contrast), however, could not be carried out. Representative examples of the presence of loops are brought out by figs. 5 and 6. This seemed to suggest the fact that material loss leading to precipitation of vacancies formed the prelude to the formation of planar faults. As is well known vacancy precipitation leads to the occurrence of planar faults [11]. In the case of CulnSe 2, annealing treatment of the stoichiometric phases is known to produce loss of selenium and hence it is thought that the vacancy loops are nucleated from selenium vacancy clusters. In order to verify this, films were also prepared from CulnSe 2 phases containing 20% excess selenium. In these cases the resulting films contained only a very low density of vacancy loops. Fig. 7 shows a representative example of the microstructure of the excess selenium films. Selected area diffraction showed that the excess selenium films corresponded to ordered phases. It thus seems evident that the disorder gets created through the occurrence of planar faults which in turn is produced by the nucleation of selenium vacancies. In order to monitor the PV activity of the ordered and disordered CulnSe 2 phases, thin film structures with A g - C d S - C u l n S e 2 - A g configurations of 1 cm 2 area were prepared. The CulnSe 2 films were deposited and then annealed ( = 200°C) on cadmium sulphide film base as well as on plane slides simultaneously. The PV activity was investigated with the C d S / C u l n S e 2 junction structure and the structural behaviour was explored by employing the film on the plane slide which would have presumably the same structure as the film on CdS base. Investigations spread over
Table 1 Comparative PV activity of CdS/CulnSe 2 thin film solar cells Ordered CulnSe2 phase
Disordered CulnSe2 phase
Cell no.
Fo¢ (mY)
Remarks
Cell no.
Voc (mY)
24
20
before annealing after annealing before annealing after annealing before annealing after annealing
27
2
no change on annealing
33
20
no change on annealing
41
24
no change on annealing
350 30
20 365
34
42 370
Remarks
416
Ram Janarn, O.N. Srivastava / Structural characteristics of CulnSe,
twenty different runs revealed that the C d S / C u I n S e 2 junctions with the disordered CuInSe 2 phase invariably exhibited poor PV activity. All the junctions with disordered CuInSe2 exhibited low PV activity. In sharp contrast to this all the C d S / C u l n S e 2 junctions having ordered CuInSe 2 thin film phases exhibited significant PV activity. Comparison of the PV response for the ordered and disordered cases was done by monitoring the Voc in the two cases. Table 1 brings out the PV response of some representative C d S / C u I n S e 2 junctions. In these cases the structural characteristics of the CuInSe 2 film could be unambiguously characterised through transmission electron microscopic (imaging and diffraction techniques) investigations. Representative examples of diffraction patterns and microstructures manifesting the ordered and disordered phases, as described earlier, are already shown in figs. 2 to 7. As is well known the presence of disorder would give rise to additional energy states outside the domain of regular band structure. For the type of present disorder the planar faults embodying stacking faults and antiphase boundaries would result in the creation of interface states. It is by now fairly well established that the interface states affect the efficiency of the thin film heterojunction devices [12,13]. It may be pointed out that one set of interface defect states are the ones that are located at the C d S / C u I n S e 2 boundary, the other typically present in the present case with the disordered phase would be those present in the CuInSe 2 phase and may be termed as defect disorder states (DDS). The exact role of the DDS based on the present investigation are difficult to gauge. However, it is expected that these would distort the band profile of the absorber material and they may act as recombination sites. Another possible effect is Fermi-level pinning. The most prominent detrimental effect of the DDS is likely to be in the vicinity of the heterojunction and within the carrier diffusion length. The carrier recombination may form the major influence of the DDS. Thus the DDS would result in lowering of efficiencies. Further studies seeking to explore the growth conditions which are prone to disorder nucleation and possible passivation of disorder are under progress and results will be forthcoming.
4. Conclusion It is known that PV behaviour of C d S / C u I n S e 2 junctions is rather subtle, the devices fabricated under nearly similar growth conditions show varied characteristics - high and low PV efficiencies are found for nearly similar junctions. The present investigations reveal that this subtlety may be associated with the state of order of CuInSe 2 films - the ordered films exhibit high response but the disordered ones show very poor response.
Acknowledgements One of the authors (R.J.) wishes to thank C.S.I.R. New Delhi, India for the award of a Senior Research Fellowship. The authors are also thankful to Prof. A.R. Verma
Ram Janam, O.N. Srivastava / Structural characteristics of CulnSe_,
417
a n d Prof. V . G . B h i d e for discussions. T h a n k s are also d u e to Dr. S.K. S h a r m a a n d Mr. S . U . M . R a o for c a r r y i n g o u t the E D A X a n a l y s i s o n J E O L s c a n n i n g m i c r o s c o p e .
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
W. Horig, H. Neumann and H. Sobotta, Thin Solid Films 48 (1978) 67. F.R. Fray and P. Alloyd, Thin Solid Films 58 (1979) 29. J.L. Shay, S. Wagner and H.M. Kasper, Appl. Phys. Lett. 27 (1975) 89. M. Gorska, R. Baulieu, J.J. Loferski and B. Roessler, Solar Energy Mater. 2 (1980) 343. M. Savelli and J. Bougnot, Solar Energy Conversion, ed. B.O. Seraphin (New York, 1979) p. 215. J.J. Loferski, Physics of Semiconductor Devices, eds. S.C. Jain and Radhakrishna (Wiley Eastern, New Delhi, 1981) p. 408. K.W. Mitchell, Physics of Semiconductor Devices, eds. S.C. Jain and Radhakrishna (Wiley Eastern, New Delhi, 1981) p. 420. R.A. Mickelsen and W.S. Chen, Proc. 16th IEEE Photovoltaic Spec. Conf. (New York, 1982) p. 781. L.L. Kazmerski, M.S. Ayyagari, F.R. White and G.A. Sanborn, J. Vac. Sci. Technol. 13 (1976) 139. Ram Janam and O.N. Srivastava, Crystal Res. & Technol. 18 (1983) 1475. J.P. Hirth and J. Lothe, Theory of Dislocations (McGraw-Hill, New York, 1968). K.L. Chopra and S.R. Das, Thin Film Solar Cells (Plenum Press, New York, 1983). A.L. Fahrenbruch and R.H. Bube, Fundamentals of Solar Cells (Academic Press, New York, 1983).