Copper indium sulfide thi̇n film production with spray pyrolysis and its usage in photoelectrochemical cells

Photoelectrochemical cells are the systems that convert sunlight energy into chemical energy. In principle, this systems are obtained by immersing photoelectrodes, which has a high catalytic activity, and a counter electrode in a proper electrolyte. Under illumination, photons with enough energies are absorbed by semiconductor photoelectrodes and creates electron-hole pairs. These pairs participate in the oxidation and reduction reactions of water and creates hydrogen and oxygen gases. The main aim of this thesis is to investigate the photoelectrochemical performance of spray pyrolyzed CuInS2 photoelectrodes. In the first part of the study, CuInS2 stoichiometry has been changed by controlling the elemental molarities in the spray precursor. Furthermore the performance of CuInS2 electrodes with increasing Cu/In ratio (from 1.33 to 2.37), photosensitivity, photo current densities and photoconversion efficiencies have been increased. Also it has been detected that S/Cu ratio in CuInS2 films has been responsible for the durability of the electrode during measurements. In other words, STO 1 sample, which has the highest S/Cu ratio (1.58) stayed stable at the end of the J-V measurements. However, other CuInS2 photoelectrodes with different stoichiometries have been peeled off during the measurements. At the second part of the study, thickness of the CuInS2 electrodes have been tailored by changing the number of passes (12, 24, 48) in spray pyrolysis process. With increasing thickness, Cu/In and S/Cu ratios have been increased. Also from thin to thick films, current densities under dark and illuminated conditions increased. At the end of the measurement, 48-pass sample had the highest efficiency between all samples. Also in the thesis, CuInS2 thin films have been doped by gallium and zinc in an orderly manner and the effect of these elements on photoelectrochemical performance has been examined. With increasing gallium molarities in precursor solution, Cu/(Ga+In) ratio decreased and crystallinity increased. The band gaps of films increased with gallium content. With all these positive results, photoconversion efficiencies and photo current densities increased. In part five, CuInS2 films have been doped with zinc via spray pyrolysis. It has been seen that photoelectrochemical properties of Zn-doped CuInS2 electrodes were very sensitive to Zn amount in the films. Zinc content with 0,094 mM molarity had the highest current densities under illuminated and dark conditions. However, these current densities decreased when zinc content were increased to 0,47 and 0,94 molarities. With zinc concentration, morphology and band gap changed. To increase the performance of CuInS2 photoelectrodes, In2S3 semiconductor materials were coated over CuInS2 thin films via spray pyrolysis. After this coating, photosensitivity and photocurrent densities increased and also CuInS2 photoelectrodes stayed stable during the current density-voltage (J-V) measurements. In the last part CuInS2 thin films were annealed in N2 atmosphere. It has been seen that crystallinity and photosensitivity have been improved with annealing. CuInS2 films that annealed had 0.5 % photoconversion efficiency, which was also ten times higher efficiency than as-deposited samples.. After coated with In2S3, this value increased up to 0.8%. The conversion efficiency of CuInS2(annealed)/In2S3 photoelectrode has reached to 1.25 % . Keywords: Copper indium sulfide, Spray pyrolysis, Photoelectrochemical cells