Structure and morphology of chemical-sprayed CdS films

Abstract

Chemical spray pyrolysis is a technique for film growth which is of interest because of its potential for very large-scale, inexpensive production of semiconductor materials such as CdS for photoconductor and photovoltaic applications. In this study, CdS films were prepared by spraying ∠0.1-M aqueous solutions of CdCl3 and NH2CSNH2 (thiourea) onto SnO2-coated glass substrates heated to 380°C. The films developed very intricate surface morphologies consisting of a variety of nodular and serpentine structures about 5–10 μm in size. Grain sizes were about 200 nm, with their c-axis generally normal to the local surface. The grain structure and morphology produce very large optical scattering and high effective absorption for wavelengths larger than the CdS band-gap wavelength of 500 nm. Films 3–4 μm thick typically have effective absorption constants of about 500 cm−1 at 1000-nm wavelength. The films were relatively impure, with total impurity contents of about 2 wt. % consisting of reaction products (principally chlorine), impurities picked up from the spray process (oxygen, iron, and others), and mobile glass species (such as sodium and potassium). These impurities may be an important factor in limiting grain size. Electrically, the carrier concentrations are on the order of 1016 cm−3, with mobilities of the order of 10 cm3/V s. Some of the effects of these properties on fabricated CdS/Cu2S photovoltaic devices are discussed.