Synthesis of Cu2ZnSnS4 nanoparticles and analysis of secondary phases in powder pellets

Sulfur-based kesterite Cu2ZnSnS4 (CZTS) nanoparticles (NPs) have been produced in a wet-chemical synthesis route which provides high yields of CZTS-NPs per synthesis cycle. These NPs can be used as raw material for electronic inks, which can be processed to light-harvesting absorber thin films in photovoltaic devices. In the corresponding phase diagram, the desired kesterite phase is found only in a very narrow, off-stoichiometric composition region. Accordingly, kesterite materials are likely to contain unwanted secondary phases such as ZnS, Cu2S, Cu2SnS3, which may limit the photovoltaic device performance in terms of current-blocking domains and formation of low-energy recombination channels. These phases can form already during the synthesis but may also evolve at high temperatures, for example, during annealing of the thin film. To limit the formation of Cu2S and Cu2SnS3 secondary phases Zn-rich and Cu-poor compositions were chosen corresponding to device-grade CZTS. In this paper we report on the phase and composition analysis of CZTS using X-ray diffraction, Raman spectroscopy with different excitation wavelengths, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analysis was performed on annealed CZTS pellets prepared from condensed CZTS NP powders. To extract the phase constitution and the composition from the surfaces and from the sample interior the CZTS pellets were horizontally cleaved after the annealing step. The bulk of the pellet consists of the required CZTS phase with very small quantities of Cu2-xS and ZnS. The utmost amount of these secondary phases was found on the top of the sample whereby copper sulfide forms preferentially isolated domains. As these surface-located secondary phases can be removed chemically, CZTS absorber films in good crystalline quality are to be expected.