CuInSe₂ (CIS) Thin Film Solar Cells by Direct Coating and Selenization of Solution Precursors

TLDR

Coating and drying reduce Cu²⁺ to Cu⁺, forming precursor films of CuCl crystals and amorphous In embedded in an ethyl‑cellulose matrix. The authors formed CIS absorbers by doctor‑blade coating of Cu(NO₃)₂/InCl₃ methanol solutions (viscosity tuned with EC) followed by Se‑vapor selenization that produced double‑layer films with an upper chalcopyrite CIS layer and a carbon‑rich bottom layer. Selenization caused significant In loss from evaporation of In₂Se, as shown by Cu/In ratio shifts, yet the resulting double‑layered films enabled proof‑of‑concept solar cells with reproducible efficiencies of about 2%.

Abstract

CuInSe2 (CIS) absorber layer was formed by a direct nonvacuum coating and a subsequent selenization of precursor solutions of Cu(NO3)2 and InCl3 dissolved in methanol. The viscosity of precursor solutions was adjusted by adding ethyl-cellulose (EC) to be suitable for the doctor-blade coating. During the coating and drying process Cu2+ ions in the starting solution were reduced to Cu+, resulting in precursor films consisting of CuCl crystals and amorphous In compound embedded in EC matrix. Selenization of the precursor films with Se vapor at elevated temperature generated double-layered films with an upper layer of chalcopyrite CIS and a carbon residue bottom layer. Significant In loss was observed during the selenization, which was attributed to the evaporation of the In2Se binary phase, confirmed by investigating the change in the Cu/In ratio of the selenized film as a function of Se flux and substrate temperature. As a proof-of-concept, thin film solar cells were fabricated with the double-layered absorber film and the devices exhibited reproducible conversion efficiency as high as about 2%.