The role of nanoparticle inks in determining the performance of solution processed Cu2ZnSn(S,Se)4thin film solar cells

Cu2ZnSnS4 (CZTS) nanoparticle inks synthesized by the injection of metal precursors into a hot surfactant offer an attractive route to the fabrication of Earth-abundant Cu2ZnSn(S,Se)4 (CZTSSe) thin film photovoltaic absorber layers. In this work it is shown that the chemical reaction conditions used to produce CZTS nanoparticle inks have a fundamental influence on the performance of thin film solar cells made by converting the nanoparticles to large CZTSSe grains in a selenium rich atmosphere and subsequent cell completion. The reaction time, temperature and cooling rate of the nanoparticle fabrication process are found to affect doping level, secondary phases and crystal structure respectively. Specifically, prolonging the reaction offers a new route to increase the concentration of acceptor levels in CZTSSe photovoltaic absorbers and results in higher device efficiency through an increase in the open circuit voltage and a reduction in parasitic resistance. Quenching the reaction by rapid cooling introduces a wurtzite crystal structure in the nanoparticles which significantly degrades the device performance, while elevating the reaction temperature of the nanoparticle synthesis introduces a secondary phase Cu2SnS3 in the nanoparticles and results in the highest cell efficiency of 6.26%. This is correlated with increased doping in the CZTSSe absorber and the results demonstrate a route to controlling this parameter.