M.Phil Thesis, Department of Physics, University of Kelaniya (2001)

Supervisors: W. P. Siripala (wps@kln.ac.lk), K. D. Jayasuriya (kdj@kln.ac.lk) and K. T. L. De Silva

Study of Electrodeposited Cu2O, CuInS2 and ZnSe for Applications in Thin Film Solar Cells
L. B. D. R. P. Wijesundera (palitha@kln.ac.lk)

In this study, preparation of Cu2O, CuInS2 and ZnSe thin films for fabrication of Cu2O/CuxS and CuInS2/ZnSe heterojunction solar cells are presented. A general discussion of the history and theoretical background of solar cells are also made. The experimental study deals with growth and characterisation of electrodeposited Cu2O, growth and characterisation of CuInS2, potentiostatic electrodeposition of ZnSe, characterisation of ZnSe and fabrication and characterisation of thin film ITO/Cu2O/CuxS/Metal and Ti/CuInS2/ZnSe/Metal heterostructure solar cells. Cu2O films of ?0.8 ?m thickness were potentiostatically electrodeposited on ITO/glass substrates using previously determined parameters. X-ray diffraction, X-ray fluorescence, scanning electron microscopy, optical absorption and reflectance, C-V measurements were used to study the material. The deposited Cu2O is of high purity having a polycrystalline structure with a grain size of ?(1-2) ?m. It has a direct band gap of 2 eV. As deposited Cu2O was n-type in conductivity and the doping density of the material was of the order of 1018 cm-3. Sulphurisation of Cu-In alloy prepared by one step electrodeposition and sequential electrodeposition was investigated to obtain CuInS2 films. Both methods were successful in producing CuInS2 thin films. Current-potential scan was carried out to establish the deposition parameters for potentiostatic electrodepostion of Cu-In alloy. X-ray diffraction, X-ray fluorescence, scanning electron microscopy, optical reflectance, C-V measurements and photoelectrochemical characterisation were used to study the material. The Cu2+ to In3+ ionic ratios in the Cu-In alloy deposition bath or the Cu/In atomic ratios of the initial Cu and In layers played a major role in determining the composition of the CuInS2 films. Both methods produced single phase polycrystalline chalcopyrite structure CuInS2 having a direct band gap of 1.5 eV. As deposited CuInS2 was found to be n-type in conductivity and the doping density of the material was of the order of 1017 cm-3. The best CuInS2 films produced Voc of ?300 mV and ? Jsc of 6 mA/cm2 in a PEC containing polysulphide as the electrolyte. Current-potential scans were determined in an aqueous solution containing ZnSO4 and SeO2 in order to establish the deposition parameters for potentiostatic electrodepostion of ZnSe. ZnSe films of ?0.4 ?m thickness were potentiostatically electrodeposited on glass/ITO substrate. X-ray diffraction, X-ray fluorescence, optical absorption and reflectance, C-V measurements and photoelectrochemical characterisation were used to study the material. The deposited material was amorphous. As deposited ZnSe was found to be p-type in conductivity and the doping density of the material was of the order of 1016 cm-3. Antimony was a good p-type dopant for ZnSe and doped ZnSe films exhibited a doping density of the order of 1017 cm-3. A glass/ITO/n-Cu2O/p-CuxS/Al heterostructure solar cell was fabricated by partial sulphidation of Cu2O. Current-voltage characteristics and spectral response of the devices were studied. The spectral response of the cell was observed to be limited to shorter wavelengths. The best Cu2O/CuxS cell exhibited Voc of 255 mV, Jsc of 1.62 mA/cm2 and FF of 0.34 under AM1 artificial illumination. A Ti/CuInS2/ZnSe/ITO heterostructure solar cell was fabricated by electrordepositing very thin (?0.2 ?m) p-doped ZnSe film on CuInS2/Ti. Current-voltage and capacitance-voltage characteristics and spectral response of the devices were studied. The best CuInS2/ZnSe cell exhibited Voc of 335 mV, Jsc of 2 mA/cm2 and FF of 0.263 under AM1 artificial illumination. The study reveals the possibility of utilising ITO/Cu2O/CuxS/Metal and Ti/CuInS2/ZnSe/Metal heterostructures in developing low-cost thin film solar cells. Further enhancements of the efficiencies of Cu2O/CuxS and CuInS2/ZnSe solar cells are suggested.