Formation of wide-bandgap, highly transparent and compact Cd1−xZnxS films with dynamically controlled pH in chemical bath deposition

Abstract

Cadmium sulfide has long been used as the buffer layer in thin-film solar cells. Despite its strengths, the problems associated with CdS – a low bandgap, absorption loss, and toxicity of Cd – have resulted in a search for alternate materials. Incorporating Zn results in a higher bandgap but obtaining complete coverage with a flake-free, compact morphology of Cd_1−xZn_xS films using chemical bath deposition remains an unsolved problem. In this paper, we address the coverage issue and engineer the morphology of Cd_1−xZn_xS films (x = 0.2, 0.5, and 0.8), focusing on controlling the formation mechanism by dynamically maintaining the pH of the precursor solution throughout the deposition. Heterogeneously grown Cd_1−xZn_xS films show high uniformity and improved structural properties. With a successive increment of the Zn amount, the Cd_1−xZn_xS optical bandgap enhances considerably from 2.55 eV to 3.77 eV. By depositing at the right pH, we report – for both low and high Zn – films that are compact and pin-hole free with 100% coverage which is rarely found in the literature. The significant improvement in average visible transmittance (AVT) from 51% (pristine CdS) to 75% (for Cd_0.8Zn_0.2S) and then to 85% (for Cd_0.2Zn_0.8S) indicates the potential to improve the efficiency of thin-film solar cells including for some novel applications such as semi-transparent photovoltaics.