Zinc Oxysulfide Thin Films as Window Layer for Solar Cells
Ratheesh R. Thankalekshmi and Alok Rastogi
Thin films of ZnO doped with sulfur show electronic and optical properties which are attractive for optoelectronic device applications. Recently several investigations have been reported on the growth of homogeneous ZnO1-xSx thin films by oxidation of ZnS, reactive sputtering and atmospheric vapor transport. Nanostructure ZnO1-xSx thin films have however received limited attention. Here we report the development of ZnO1-xSx prepared with chemical spray pyrolysis technique and solution growth method. In the chemical spray pyrolysis technique, the atomized precursor solution is directed downward at the heated substrate, which upon decomposition and oxidation reaction forms a thin film. The ZnO1-xSx films prepared by the chemical spray process at 300°C over quartz substrates are highly crystalline with the ZnO1-xSx films having a preferred c-axis orientation. Optical plot shows that band edge shifts towards the lower energies by addition sulfur and a minimum of 2.78 eV is obtained for ZnO0.45S0.55 and further addition of sulfur, the band gap energy retracts initially and approaches towards that of pure ZnS. In solution growth process ZnO1-xSx are deposited over ZnO nanorods in a core-shell like nanostructure for application to thin film solar cells. ZnO films with a nanostructure dominated by arrays were deposited by hydrothermal synthesis over surface activated quartz substrates. ZnO1-xSx is deposited over ZnO nanorods by chemiplating process. Microstructure studies confirm conformal surface conversion with accumulated ZnO1-xSx growth over (0001) facets of ZnO rods. X-ray diffraction studies show c-axis orientated growth of ZnO rods. The optical absorption analysis shows concurrent band gaps corresponding to the ZnO1-xSx shell and ZnO core. The reduced band gap of ZnO1-xSx offers unique advantage in application to solar cells. It will replace Cd from conventional CdS heterojunction layer in non-toxic earth abundant materials based thin film solar cells.