Growth and Characterization of Non Hydrazine Solution Processed Cu2 ZnSn SSe 4 Thin Films for Solar Cells

Abstract

Kesterite Cu2ZnSnS4 (CZTS) has emerged as a very promising absorber layer in thin film solar cells due to its earth abundant environmentally benign constituents and suitable optical properties. So far, the highest conversion efficiency of 12.6% for Se-alloyed CZTS (CZTSSe) films has been obtained by the solution process using hydrazine based slurry. Considering the explosive, hepatotoxic and carcinogenic nature of hydrazine, there have been wider research on facile synthesis of CZTS films from non-hydrazine non-toxic solutions. In this work, the CZTSSe films have been prepared from ethanol based homogeneous solutions using a direct solution coating route via dip coating. Firstly, ethanol, besides being an environmentally friendly solvent, solvates many common inorganic salts and can evaporate quickly (boiling point is ~78.5 °C) which can minimize residual carbon and/or oxygen related impurities in the films. Secondly, contrary to the approaches relying on fabricating thin films from CZTS particle-containing inks, homogeneous solution method offers several advantages such as better stoichiometric control (due to mixing at molecular-scale) and no requirement of pre-fabrication and stabilization of CZTS particles. In the CZTS device technology, the requirement of phase purity, dense microstructure with large grains, conformally deposited film thickness of ~ 1 and#61549;m, appropriate band gap ( 1.0 - 1.5 eV) and electrical properties of the films are very critical for obtaining high efficiency of the devices. In the solution based approaches for semiconductor thin films, the film growth proceeds from the release of cations and anions from the precursor salts, and their subsequent reactions. The reaction pathway depends on various process parameters (such as temperature, precursor composition, deposition duration, complexing agent, etc.) and determines the eventual microstructure and phase of the films.