Fabrication of chalcopyrite thin film solar cells on flexible and rigid substrates by ultrasonic spray pyrolysis technique

In recent years, copper based chalcopyrite thin film solar cells have recorded considerable development in laboratory scale. However, fabrication of these solar cells in industrial scale still has some unresolved problems. Lowering the manufacturing cost and employing the environmentally benign process steps are the primary issues for the fabrication of chalcopyrite photovoltaics. The main theme of this thesis is the fabrication of copper indium sulfide (CuInS2) based thin film solar cells by spray pyrolysis (SP) instead of high-cost vacuum-based deposition techniques. SP is superior to other techniques due to the diversity of starting materials, lower materials consumption and applicability at large scales. To improve the adhesion of CuInS2 thin films, post annealing studies have been performed. The post-treatment studies have revealed that rapid thermal annealing process, which has ramp rate greater than 5 ºC/s, increased the oxidation rate of the films as well as the crack formation. On the other hand, conventional annealing processes increased the mobility of thin films to 30 from 1.6 cm2/Vs. The observed hillocks on the CuInS2 films, which contains more copper than flat zones, have been removed by increasing the concentration of precursor solution. Thus, the efficiency of the photovoltaic devices increased from 0.69 to 1.65 %. Alternatively, spray pyrolyzed indium sulfide (In2S3) buffer layers have been used instead of cadmium sulfide (CdS) which is the traditional material used for commercial solar cells. It has been revealed that 75-cycle processed In2S3 buffer layers with ~1 µm thickness have shown better photovoltaic performance. Moreover, the photosensitivity of the buffer layers has been increased 20 folds and reached to 1780 by doping with 1 % silver. AZO/Ag/AZO sandwich structured transparent conductive oxide (TCO) layers have been deposited via radio frequency (RF) sputtering at low power densities on the CuInS2/In2S3 heterojunctions in order to prevent the plasma deformation of junctions. These sandwich structured TCO layers have provided 80 folds increase in electron lifetime compared to single layer AZO. Effects of the device configuration on the photovoltaic performance has also been investigated. While substrate configuration solar cells presented 4.30 % photo-conversion efficiency, 3.09 % has been obtained from inverted superstrate architecture. The obtained know-how from the fabrication of CuInS2 solar cell on glass substrates has been transferred to processing on flexible substrates. 1.43 % efficiency is obtained on polyimide foils by optimization of back contact properties and post annealing of CuInS2 films. In order to prevent the diffusion of undesired atoms into the device structure, alumina (Al2O3) barrier layers have been deposited on stainless steel (SS) foils. These photovoltaic devices showed 2.74 % efficiency value for record cell. It is noteworthy to mention here that all photovoltaic devices have been fabricated without any toxic processing steps. Moreover, we have fabricated chalcopyrite based thin film solar cells on flexible foils by spray pyrolysis technique for the first time in literature. To conclude, this reported thesis has been revealed the tremendous potential of spray pyrolysis technique for large fabrication of chalcopyrite solar cells in a cost-efficient manner.