Prospects of ZnxCd1−xS window layer in CdTe thin film solar cells from numerical analysis

A high conversion efficiency has been achieved at reduced CdTe absorber thickness and Zn_xCd_1-xS window layer incorporation in conventional CdS/CdTe solar cells. The conventional CdTe baseline case was the starting point of this investigation to analyze ultra thin and high efficiency CdTe solar cell. The initial step of the analysis was to decrease the CdTe absorber layer to the extreme limit of 1 μm and at this thickness the proposed cell has shown satisfactory level of efficiencies. The front contact of the best cell was adapted to the proposed cell and the Zn_xCd_1-xS window layer thickness was reduced with a buffer layer to improve the quantum efficiency at the blue region. The ultimate step was to insert a suitable back surface field (BSF) with Sb₂Te₃ to reduce the back contact barrier height and back surface recombination of the ultra thin cell. All the analysis was done using the widely used simulator Analysis of Microelectronic and Photonic Structures (AMPS 1D). It was observed that the proposed cell provided conversion efficiency of 17.82% (Voc = 0.894 V, Jsc = 25.29 mA/cm², FF = 0.778) without BSF and an efficiency of 20.1% (Voc = 0.927 V, Jsc = 25.89 mA/cm², FF = 0.825) with Sb₂Te₃ BSF from 1 μm and 0.6 μm CdTe absorber layer, respectively. Moreover, the normalized efficiency of the proposed ultra thin cells with BSF linearly decreased with the increasing operating temperature at the gradient of -0.25%/°C, which also indicates better stability of the ultra thin cells.