2, Fraunhofer IWM, Freiburg, , Germany
Cu2ZnSnS4 is a potential p-type absorber layer for photovoltaic and PEC water splitting applications. It has optimum band gap energy (1.4-1.5 eV), high absorption coefficient and involves earth abundant compositions. CZTS can be used as photocathode for hydrogen evolution, where its conduction band minima (CBM) edge is more negative than the hydrogen evolution potential. The quaternary semiconductors Cu2Zn – IV-VI4 (IV- Si, Ge, Sn and VI- S, Se) are widely considered as fourth generation solar cell absorbing materials with tunable band gap of around 1.0–3.0 eV. Two photon absorber layer devices having different band gaps (1.0 eV and 1.7 eV) show better PEC response, however, different properties (e.g. crystal structure) of constituent semiconductor materials and band alignment issue can create hindrance in efficiency enhancement. Polycrystalline Si layer over CIGS is found to improve net efficiency as compared to the bottom cell (CIGS) alone. In present work, Si is doped on CZTS, which can make its surface stable and increase the band gap, which is favorable for PEC application due to reduction in recombination loss of charge carriers. The first principle calculations were performed using DFT simulations for Cu2Zn(Sn0.5Si0.5)S4 structure. The optical band gap of this kesterite structure was calculated to be ~2.0 eV, which is higher than purely Sn based structure (CZTS). The experimental synthesis of Cu2Zn(SnxSi1-x)S4 structure is performed by magnetron co-sputtering method. The metallic precursor Cu, Zn and Sn were deposited on Mo-coated glass and bare glass substrates by co-sputtering for 60 minutes at 10-12 mTorr Ar working pressure. The Si has been deposited over Cu-Zn-Sn films for 5 and 7 minutes of duration followed by sulphurization at 520οC for 20 minutes giving samples, S0, S1 and S2. The composition ratios Cu/(Zn+Sn), Zn/(Sn+Si), Si/Sn for samples S0 to S2 are observed as ~ 0.97, 1.4, 0; 0.84, 1.04, 0.35; 0.96, 0.84, 0.57, respectively. The XRD analysis for samples shows kesterite single phase, which is also confirmed by micro-Raman analysis carried out using 532 nm laser excitation source. The Raman peaks observed at ~337 and 286 cm-1 belong to kesterite CZTS phase. The band gap increases from samples S0 to S2 as 1.43, 1.45 and 1.47. Hence, the doping of Si in CZTS results in a slight increase in band gap which can enhance the photocurrent density, because thermodynamic reaction loss decreases due to water reduction potential (~1.23 eV) straddle well with photo-electrode. The photocurrent density for pure CZTS thin film sample (S0) on FTO protected by i-ZnO is found to be ~0.5 mA/cm2. In pure CZTS, the crystallites of sizes ~1-3 µm are observed to be lying on the top surface, but the density of these crystallites increases on going from samples S0 to S2. Hence, this new material can be potential photo –electrode material for future solar water splitting applications. The PEC measurements for Cu2Zn(SnxSi1-x)S4 samples are in progress.