Tandem solar cells with copper tin sulfide quantum dot sensitization: A SCAPS-1D approach

Tandem solar cells are effective in overcoming thermalization and transmission losses in a solar cell. In this work, the tandem structure of copper tin sulfide (CTS) quantum dot sensitized solar cell has been simulated and then optimized using solar cell capacitance simulator software. A CTS sensitizer at 2 different bandgaps of 1.5 eV and 2.5 eV has been used for the tandem structure. Individual solar cells of structure ITO/TiO₂/CTS/CuSbS₂/Au, where CTS has bandgaps of 1.5 eV and 2.5 eV, yielded efficiencies of 17.86% and 18.14%, respectively. The tandem structure after optimization yielded a cell efficiency of 20.20%. The effect of layer defects in hole-transporting layer and sensitizer layers was investigated. The interface defects in the HTL/CTS, CTS(Eg-1.5 eV)/CTS(Eg-2.5 eV), and CTS/ETL interfaces were also investigated. Permissible levels of defect density are found to be 10¹⁴ cm⁻² in all cases, provided the defect levels are shallow, allowing only Shockley-Read-Hall recombination. On increasing the number of sensitizer layers with bandgap lower than 1.5 eV and higher than 0.9 eV, the bulk bandgap, the cell efficiency can be further increased. Introduction of a third layer of CTS sensitizer with a bandgap of 1.2 eV further improved the cell efficiency by 2%.