Prospects of back surface field effect in ultra-thin high-efficiency CdS/CdTe solar cells from numerical modeling

Polycrystalline CdTe shows greater promises for the development of cost-effective, efficient, and reliable thin film solar cells. Results of numerical analysis using AMPS-1D simulator in exploring the possibility of ultrathin, high efficiency, and stable CdS/CdTe cells are presented. The conventional baseline case structure of CdS/CdTe cell has been explored with reduced CdTe absorber and CdS window layer thickness, where 1m thin CdTe and 50 nm CdS layers showed reasonable efficiencies over 15%. The viability of 1μm CdTe absorber layer together with possible back surface field (BSF) layers to reduce minority carrier recombination loss at the back contact in ultra thin CdS/CdTe cells was investigated. Higher bandgap material like ZnTe and low bandgap materials like Sb2Te3 and As2Te3 as BSF were inserted to reduce the holes barrier height in the proposed ultra thin CdS/CdTe cells. The proposed structure of SnO₂/Zn₂SnO₄/CdS/CdTe/As ₂Te₃/Cu showed the highest conversion efficiency of 18.6% (Voc = 0.92V, Jsc = 24.97mA/cm², and FF = 0.81). However, other proposed structures such as SnO₂/Zn₂SnO ₄/CdS/CdTe/Sb₂Te₃/Mo and SnO ₂/Zn₂SnO₄/CdS/CdTe/ZnTe/Al have also shown better stability at higher operating temperatures with acceptable efficiencies. Moreover, it was found that the cells normalized efficiency linearly decreased with the increased operating temperature with relatively lower gradient, which eventually indicates better stability of the proposed ultra thin CdS/CdTe cells.