Solution processed CZTS solar cells using amine-thiol systems: understanding the dissolution process and device fabrication

Solar energy is one of the main renewable energy sources currently being researched, with commercial thin film solar cells currently made of CdTe or CuIn(1-x)GaxSe2 (CIGS) absorbers. However, whilst these materials make up the majority of the thin film commercial market, these solar cells have various problems relating to materials cost, and toxicity of constituent elements. Kesterite (Cu2ZnSn(S,Se)4) solar cells are becoming increasingly popular due to their tuneable band gap, relative affordability of the constituent elements, and the ability to produce high efficiency devices from solution processes. However, often expensive and toxic materials are used in production. In this paper we report on a newly developed amine-thiol solvent system based on 10% cysteamine in ethanolamine, which has low toxicity, is user-friendly and is able to readily dissolve all kesterite constituent elements, including metals and their oxides. The dissolution process and the structures of the prevalent metal complexes formed were investigated with the aid of spectroscopic methods, such as electrospray ionization mass spectrometry (ESI-MS) and infrared multiple photon dissociation (IRMPD). In most cases, two molecules of cysteamine were bound to the metals as bidentate ligands. By employing spin coating of the resulting inks, devices of up to 8.1% power conversion efficiency were fabricated.