Enhancing the photovoltaic performance of Cd-free Cu2ZnSnS4 heterojunction solar cells using SnS HTL and TiO2 ETL

•Cd-free CZTS solar cells with improved photo-conversion efficiency.•Incorporation of SnS HTL into CZTS-based solar cells reduces the VOC deficit.•TiO2 ETL could be a good alternative to CdS in CZTS-based solar cells.•Metals like Au, Ir, Ni & W can be used as back metal contact materials. Cu2ZnSnS4 chalcogenide semiconductor is a very promising absorber material for solar cells and no longer fulfilled its realistic goals due to the possible challenge of defect-free fabrication, non-optimized buffer layer alignment and device configuration. In this article, we proposed a new planner structure defined as (SnS/CZTS/TiO2/ITO) using Ni and Al as a back and front contact materials, respectively, and simulated its photovoltaic (PV) performance using SCAPS-1D. In the simulation, defect densities were added to each layer and the interface between the buffer layer and the absorber layer. This work demonstrated the promising role of SnS HTL and TiO2 ETL in reducing the Voc deficit of CZTS-based solar cells by forming more favourable band alignment with the absorber layer, reducing non-radiative recombination at the interfaces. In response to changes in the material properties of different layers (such as thickness, carrier concentration, defect density), working temperature, and back contact metal work function, the PV performance of the new architecture for CZTS solar cells was investigated and optimized. The study of PV performance optimization revealed that a relatively thicker absorber layer with low carrier concentration showed better PV performances, while ETL and HTL with a thinner and higher carrier concentration are required to enhance PV performances. Power conversion efficiency of >30%, Voc of >1.09 V, Jsc of >32 mA/cm2 and FF of >87% was predicted for CZTS solar cells with the new architecture. The findings of this study suggest that SnS and TiO2 are expected to become HTL and ETL, respectively, for fabricating low cost, high efficiency, and Cd-free CZTS-based heterojunction solar cell.