- Email: [email protected]
Properties of flash evaporated chalcopyrite absorber films and solar cells
Thin Solid Films 387 Ž2001. 47᎐49
Properties of flash evaporated chalcopyrite absorber films and solar cells M. Klenk a,U , O. Schenker a , V. Alberts b , E. Bucher a a
Uni¨ ersitat ¨ Konstanz, Fakultat ¨ fur ¨ Physik, Fach X916, 78457 Konstanz, Germany b Rand Afrikaans Uni¨ ersity, Auckland Park 2006, Johannesburg, South Africa
Abstract Chalcopyrite thin films were produced by flash evaporation. While the focus was on CuGaSe 2 also some CuInSe 2 and CuIn 1yxGa x Se 2 layers were fabricated for structural investigations. Flash evaporated films, obtained at different deposition conditions and after post-deposition annealing, were compared. The influence of the main evaporation parameters, as boat and substrate temperature, on layer composition and structure were studied. CuGaSe 2 solar cells were produced and tested. The effects of annealing in ambient air and vacuum on the cell characteristics are reported. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: GuGaSe 2 ; CuInSe 2 ; Flash evaporation; Thin film; Solar cells
1. Introduction CuInSe 2 related chalcopyrite compounds showed their suitability for obtaining solar cell efficiencies high enough to compete with silicon technology w1x. However, for a future low cost production, simple techniques with high throughput are desirable. Up to now the best results were achieved by evaporation of the elements, either simultaneously or in sequential processes with adjacent treatment. As the chalcopyrite materials used for thin film solar cells are multinary compounds consisting of at least three elements it is obvious that measuring and controlling the respective fluxes during the evaporation process is quite complex, requiring cost intensive control mechanisms and leading to reproducibility problems. A quite simple method to deposit elements and compounds is the flash evaporation technique. Pulverized material is transported to an evaporation boat hot
U
Corresponding author. Tel.: q49-7531-883-732; fax: q49-7531883-895. E-mail address: [email protected] ŽM. Klenk..
enough to provide a quick evaporation of the material upon contact. However, this technique is also said to lead to compositional shifts in the elemental ratio when evaporating compounds. In contrast to what is stated elsewhere w2x no shift in the Cu to In andror Ga ratio could be observed by us when comparing the flash evaporated layers and the pre- reacted material, if only the evaporation boat temperature is high enough. Considering this, the composition control of the metals can therefore be simply adjusted by weighing the elements before pre-reacting them. This might be a way to overcome the costly control mechanisms needed to provide absorber layers of a desired composition. Only one crystal quartz monitor is then sufficient to measure the film thickness during evaporation. 2. Structural properties 2.1. E¨ aporation conditions and base material For deposition a vibrating feeder system was used to transport the pulverized pre-reacted materials Žremains from CVT crystal growth experiments. via a tube to a flat tungsten evaporation boat. The deposition was
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 3 9 - 3
48
M. Klenk et al. r Thin Solid Films 387 (2001) 47᎐49
Fig. 1. Three CuGaSe 2 flash layers with changing grain structure. Flash deposited on unheated substrate Ža., at a substrate temperature of 350⬚C Žb., and after an RTP annealing step at 550⬚C Žc..
done in a high vacuum chamber. Substrate temperatures varied from room temperature to 350⬚C. The chalcopyrite layers were flash evaporated onto approximately 1-m thick molybdenum layers which were in turn RF-sputtered onto soda lime glass. To produce solar cells a CdS buffer layer was deposited, followed by an RF-sputtered ZnOrZnOŽAl. structure. 2.2. Compositional shifts Flash evaporation of compounds is sometimes called a process that is difficult to control, because it leads to compositional shifts of the deposited films compared to the base material. Especially for chalcopyrite compounds a lack of reproducibility with regard to the ratio of the metals is reported to be a main restriction of this technique w2x. We found that in our case this effect is solely due to incomplete evaporation of the powder leading to material left behind in the boat. Analyzing this remaining material we always found a very copper rich composition, corresponding to a copper poor composition of the respective deposited layers. The problem of shifting the Cu to In andror Ga ratio could totally be avoided if the boat temperature is chosen high enough to provide a quick and complete evaporation of the powder, and by also reducing the feeding rate to prevent an accumulation of molten material in
the boat. With the system used it was possible to deposit at a rate of approximately 1 nmrs at a boat temperature of approximately 1400⬚C without having material left in the boat and without a shift in the Cu to Ga andror In ratio. While it was possible to obtain films with Cu to In andror Ga ratios of the pre- reacted material, the selenium concentration of the evaporated layers was always found to be lower than the desired 50%. Raising of the boat temperature in this case proved to decrease the selenium concentration. In all cases the measured selenium concentration of the films was between 42 and 47%. For boat temperatures necessary to prevent a shift of the Cu to Ga andror In ratio the layers showed a selenium concentration of typically 42᎐43 at.%. No significant influence of the substrate temperature on the composition could be observed Žapplied substrate temperatures varied from room temperature to 350⬚C.. By mixing CuInSe 2 and CuGaSe 2 powder it was possible to obtain single phase CuIn 1y xGa x Se 2 layers without any diffraction peaks of either binary phases or the ternary starting materials. The desired composition could easily be adjusted by the amount of the respective powders. No phase separation due to heat treatment could be observed. An example will be shown in Section 2.3. 2.3. Grain size and structure
Fig. 2. Single phase CuIn 0.7 Ga 0.3 Se 2 film, as deposited at 350⬚C, and after RTP.
While the influence of the substrate temperature was found to be negligible with regard to the composition, it is of major importance for the grain structure. The layers deposited on unheated substrates were very smooth and shiny layers which were amorphous or of very small grain size. With increasing substrate temperature the grain size is improving. In the used system substrate temperatures up to 350⬚C could be applied. In an attempt to overcome the lack of selenium and the restricted grain size, the layers were annealed at temperatures of approximately 550⬚C in selenium containing atmosphere. Rapid thermal processing ŽRTP. and longer annealing in quartz tubes were applied to the samples and compared. The structural changes due to the heat treatment were remarkably independent of
M. Klenk et al. r Thin Solid Films 387 (2001) 47᎐49
49
Fig. 3. XRD scan and electron microscope depiction of a CuInSe 2 layer deposited with additional selenium effusion source.
the type of chalcopyrite material ŽCIS, CIGS, CGS.. The grain structure can be described as a droplet like conglomeration of smaller grains, quite similar to the samples obtained by laser evaporation w3x, or stacked elemental layers followed by RTP treatment w4x. In Fig. 1 electron microscope depictions of three CuGaSe 2 layers are shown. Fig. 1a shows a layer as deposited at room temperature, Fig. 1b as deposited at 350⬚C and Fig. 1c after an RTP step at 550⬚C in an ArrSe atmosphere. As a typical example for the change of the resulting diffraction patterns in Fig. 2 two scans of a single phase CuIn 0.7 Ga 0.3 Se 2 layer Žsee Section 2.2. are shown. The first scan of a sample as deposited at 350⬚C substrate temperature, and the same layer after a rapid thermal annealing step for 6 min in an ArrSe atmosphere. With processing temperatures over 450⬚C the samples changed their shiny appearance to a more dull one. Longer annealing times Žup to 1 h. were also used but resulted in no significant improvement of the material
properties, whereas the MoSe2 formation was increased and lateral inhomogenities started to appear. A very different layer structure could be observed when extra selenium was supplied by an additional effusion cell during flash evaporation. The obtained layers showed a selenium content of the desired 50 at.% after deposition and had an extremely preferred 112 orientation combined with a very small grain size. In Fig. 3 the X-ray diffraction scan and a electron microscope depiction of a CuInSe 2 layer deposited at 350⬚C substrate temperature is shown as an example. Further heat treatment only caused small structural changes for suchlike produced films. 3. Photoelectrical properties of the CuGaSe 2 solar cells The best CuGaSe 2 cell prepared in this study showed an efficiency of 4.0% ŽFig. 4.. It was obtained from an RTP-treated, flash evaporated layer. The completed cell was vacuum annealed and light soaked. Other CuGaSe 2 cells showed the following independent maximum values under AM1.5 illumination conditions: Voc : 610 mV; Isc 12.9 mArcm2 ; FF: 61%. References
Fig. 4. IV-Curve of the best CuGaSe 2 cell.
w1x M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Haason, R. Noufi, Prog. Photov. Res. Appl. 7 Ž1999. 311. w2x J.M. Merino, M. Leon, ´ F. Rueda, R. Diaz, Thin Solid Films 361r362 Ž2000. 22᎐27. w3x K.T. Ramakrishna Reddy, P. Jayarama Reddy, J. Cryst. Growth 108 Ž1991. 765᎐769. w4x M. Klenk, O. Schenker, E. Bucher, Thin Solid Films 361r362 Ž2000. 229᎐233.
Properties of flash evaporated chalcopyrite absorber films and solar cells M. Klenk a,U , O. Schenker a , V. Alberts b , E. Bucher a a
Uni¨ ersitat ¨ Konstanz, Fakultat ¨ fur ¨ Physik, Fach X916, 78457 Konstanz, Germany b Rand Afrikaans Uni¨ ersity, Auckland Park 2006, Johannesburg, South Africa
Abstract Chalcopyrite thin films were produced by flash evaporation. While the focus was on CuGaSe 2 also some CuInSe 2 and CuIn 1yxGa x Se 2 layers were fabricated for structural investigations. Flash evaporated films, obtained at different deposition conditions and after post-deposition annealing, were compared. The influence of the main evaporation parameters, as boat and substrate temperature, on layer composition and structure were studied. CuGaSe 2 solar cells were produced and tested. The effects of annealing in ambient air and vacuum on the cell characteristics are reported. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: GuGaSe 2 ; CuInSe 2 ; Flash evaporation; Thin film; Solar cells
1. Introduction CuInSe 2 related chalcopyrite compounds showed their suitability for obtaining solar cell efficiencies high enough to compete with silicon technology w1x. However, for a future low cost production, simple techniques with high throughput are desirable. Up to now the best results were achieved by evaporation of the elements, either simultaneously or in sequential processes with adjacent treatment. As the chalcopyrite materials used for thin film solar cells are multinary compounds consisting of at least three elements it is obvious that measuring and controlling the respective fluxes during the evaporation process is quite complex, requiring cost intensive control mechanisms and leading to reproducibility problems. A quite simple method to deposit elements and compounds is the flash evaporation technique. Pulverized material is transported to an evaporation boat hot
U
Corresponding author. Tel.: q49-7531-883-732; fax: q49-7531883-895. E-mail address: [email protected] ŽM. Klenk..
enough to provide a quick evaporation of the material upon contact. However, this technique is also said to lead to compositional shifts in the elemental ratio when evaporating compounds. In contrast to what is stated elsewhere w2x no shift in the Cu to In andror Ga ratio could be observed by us when comparing the flash evaporated layers and the pre- reacted material, if only the evaporation boat temperature is high enough. Considering this, the composition control of the metals can therefore be simply adjusted by weighing the elements before pre-reacting them. This might be a way to overcome the costly control mechanisms needed to provide absorber layers of a desired composition. Only one crystal quartz monitor is then sufficient to measure the film thickness during evaporation. 2. Structural properties 2.1. E¨ aporation conditions and base material For deposition a vibrating feeder system was used to transport the pulverized pre-reacted materials Žremains from CVT crystal growth experiments. via a tube to a flat tungsten evaporation boat. The deposition was
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 3 9 - 3
48
M. Klenk et al. r Thin Solid Films 387 (2001) 47᎐49
Fig. 1. Three CuGaSe 2 flash layers with changing grain structure. Flash deposited on unheated substrate Ža., at a substrate temperature of 350⬚C Žb., and after an RTP annealing step at 550⬚C Žc..
done in a high vacuum chamber. Substrate temperatures varied from room temperature to 350⬚C. The chalcopyrite layers were flash evaporated onto approximately 1-m thick molybdenum layers which were in turn RF-sputtered onto soda lime glass. To produce solar cells a CdS buffer layer was deposited, followed by an RF-sputtered ZnOrZnOŽAl. structure. 2.2. Compositional shifts Flash evaporation of compounds is sometimes called a process that is difficult to control, because it leads to compositional shifts of the deposited films compared to the base material. Especially for chalcopyrite compounds a lack of reproducibility with regard to the ratio of the metals is reported to be a main restriction of this technique w2x. We found that in our case this effect is solely due to incomplete evaporation of the powder leading to material left behind in the boat. Analyzing this remaining material we always found a very copper rich composition, corresponding to a copper poor composition of the respective deposited layers. The problem of shifting the Cu to In andror Ga ratio could totally be avoided if the boat temperature is chosen high enough to provide a quick and complete evaporation of the powder, and by also reducing the feeding rate to prevent an accumulation of molten material in
the boat. With the system used it was possible to deposit at a rate of approximately 1 nmrs at a boat temperature of approximately 1400⬚C without having material left in the boat and without a shift in the Cu to Ga andror In ratio. While it was possible to obtain films with Cu to In andror Ga ratios of the pre- reacted material, the selenium concentration of the evaporated layers was always found to be lower than the desired 50%. Raising of the boat temperature in this case proved to decrease the selenium concentration. In all cases the measured selenium concentration of the films was between 42 and 47%. For boat temperatures necessary to prevent a shift of the Cu to Ga andror In ratio the layers showed a selenium concentration of typically 42᎐43 at.%. No significant influence of the substrate temperature on the composition could be observed Žapplied substrate temperatures varied from room temperature to 350⬚C.. By mixing CuInSe 2 and CuGaSe 2 powder it was possible to obtain single phase CuIn 1y xGa x Se 2 layers without any diffraction peaks of either binary phases or the ternary starting materials. The desired composition could easily be adjusted by the amount of the respective powders. No phase separation due to heat treatment could be observed. An example will be shown in Section 2.3. 2.3. Grain size and structure
Fig. 2. Single phase CuIn 0.7 Ga 0.3 Se 2 film, as deposited at 350⬚C, and after RTP.
While the influence of the substrate temperature was found to be negligible with regard to the composition, it is of major importance for the grain structure. The layers deposited on unheated substrates were very smooth and shiny layers which were amorphous or of very small grain size. With increasing substrate temperature the grain size is improving. In the used system substrate temperatures up to 350⬚C could be applied. In an attempt to overcome the lack of selenium and the restricted grain size, the layers were annealed at temperatures of approximately 550⬚C in selenium containing atmosphere. Rapid thermal processing ŽRTP. and longer annealing in quartz tubes were applied to the samples and compared. The structural changes due to the heat treatment were remarkably independent of
M. Klenk et al. r Thin Solid Films 387 (2001) 47᎐49
49
Fig. 3. XRD scan and electron microscope depiction of a CuInSe 2 layer deposited with additional selenium effusion source.
the type of chalcopyrite material ŽCIS, CIGS, CGS.. The grain structure can be described as a droplet like conglomeration of smaller grains, quite similar to the samples obtained by laser evaporation w3x, or stacked elemental layers followed by RTP treatment w4x. In Fig. 1 electron microscope depictions of three CuGaSe 2 layers are shown. Fig. 1a shows a layer as deposited at room temperature, Fig. 1b as deposited at 350⬚C and Fig. 1c after an RTP step at 550⬚C in an ArrSe atmosphere. As a typical example for the change of the resulting diffraction patterns in Fig. 2 two scans of a single phase CuIn 0.7 Ga 0.3 Se 2 layer Žsee Section 2.2. are shown. The first scan of a sample as deposited at 350⬚C substrate temperature, and the same layer after a rapid thermal annealing step for 6 min in an ArrSe atmosphere. With processing temperatures over 450⬚C the samples changed their shiny appearance to a more dull one. Longer annealing times Žup to 1 h. were also used but resulted in no significant improvement of the material
properties, whereas the MoSe2 formation was increased and lateral inhomogenities started to appear. A very different layer structure could be observed when extra selenium was supplied by an additional effusion cell during flash evaporation. The obtained layers showed a selenium content of the desired 50 at.% after deposition and had an extremely preferred 112 orientation combined with a very small grain size. In Fig. 3 the X-ray diffraction scan and a electron microscope depiction of a CuInSe 2 layer deposited at 350⬚C substrate temperature is shown as an example. Further heat treatment only caused small structural changes for suchlike produced films. 3. Photoelectrical properties of the CuGaSe 2 solar cells The best CuGaSe 2 cell prepared in this study showed an efficiency of 4.0% ŽFig. 4.. It was obtained from an RTP-treated, flash evaporated layer. The completed cell was vacuum annealed and light soaked. Other CuGaSe 2 cells showed the following independent maximum values under AM1.5 illumination conditions: Voc : 610 mV; Isc 12.9 mArcm2 ; FF: 61%. References
Fig. 4. IV-Curve of the best CuGaSe 2 cell.
w1x M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Haason, R. Noufi, Prog. Photov. Res. Appl. 7 Ž1999. 311. w2x J.M. Merino, M. Leon, ´ F. Rueda, R. Diaz, Thin Solid Films 361r362 Ž2000. 22᎐27. w3x K.T. Ramakrishna Reddy, P. Jayarama Reddy, J. Cryst. Growth 108 Ž1991. 765᎐769. w4x M. Klenk, O. Schenker, E. Bucher, Thin Solid Films 361r362 Ž2000. 229᎐233.