Effect of Cd1−xZnxS Window Layer Incorporation in CdTe Solar Cell by Numerical Simulation

CdTe is a very potential binary semiconductor material for solar photovoltaic application due to its superior optoelectronic properties. The overall performances of incorporating $\mathbf{Cd}_{\pmb{1}-\mathbf{X}}\mathbf{Zn}_{\mathbf{x}}\mathbf{S}$ window layer in lieu of CdS layer in CdTe solar cell were investigated by SCAPS-1D simulator. The $\mathbf{Cd}_{\pmb{1}-\mathbf{X}}\mathbf{Zn}_{\mathbf{x}}\mathbf{S}$ is an alloy of CdS and ZnS which increase band gap of window layer from 2.42 e V to 3.7 e V as a function of x (from $\pmb{x=0}$ to 1). The spectral response of the design $\mathbf{Cd}_{\pmb{1}-\mathbf{x}}\mathbf{Zn}_{\mathbf{x}}\mathbf{S}/\mathbf{CdTe}$ cell improves in blue region which implies the big improvement of short-circuit current density $\pmb{J_{sc}}$. In addition, in the traditional back contact of CdTe cell a small positive conduction band $(\pmb{\Delta Ec} < \pmb{0.3}\ \ \mathbf{eV})$ offset is necessary to reduce the forward current $\pmb{J_{0}}$ as well as the recombination losses in the back contact interface. To achieve this goal a highly doped $\mathbf{ZnTe}$: Cu extra layer was used as an electron reflector (ER) above back contact. Furthermore, this ER interface allows electron tunnelling by reducing the barrier height of the valence band which in turn leads to an improvement of open-circuit voltage and fill factor. The performance of the proposed cell was examined by varying thickness and doping concentration of Transparent Conducting Oxide (TCO) layer, window layer, absorber layer and finally ER layer. The simulated results of the proposed cell had shown that the open-circuit voltage $\pmb{(V_{oc})}$ overcame the 1-volt barrier of CdTe cell with energy conversion efficiencies of 19.93 %.