In order to improve photovoltaic performance of solar cells based on ZnTe thin films two device structures have been proposed and its photovoltaic parameters have been numerically simulated using Solar Cell Capacitance Simulator software. The first one is the ZnO/CdS/ZnTe conventional structure and the second one is the ZnO/CdS/ZnTe/P +-ZnTe structure with a P +-ZnTe layer inserted at the back surface of ZnTe active layer to produce a back surface field effect which could reduce back carrier recombination and thus increase the photovoltaic conversion efficiency of cells. The effect of ZnO, CdS and ZnTe layer thicknesses and the P +-ZnTe added layer and its thickness have been optimized for producing maximum working parameters such as: open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, photovoltaic conversion efficiency η. The solar cell with ZnTe/P +-ZnTe junction showed remarkably higher conversion efficiency over the conventional solar cell based on ZnTe layer and the conversion efficiency of the ZnO/CdS/ZnTe/P +-ZnTe solar cell was found to be dependent on ZnTe and P +-ZnTe layer thicknesses. The optimization of ZnTe, CdS and ZnTe layers and the inserting of P +-ZnTe back surface layer results in an enhancement of the energy conversion efficiency since its maximum has increased from 10% for ZnO, CdS and ZnTe layer thicknesses of 0.05, 0.08 and 2 µm, respectively to 13.37% when ZnO, CdS, ZnTe and P +-ZnTe layer thicknesses are closed to 0.03, 0.03, 0.5 and 0.1 µm, respectively. Furthermore, the highest calculated output parameters have been Jsc = 9.35 mA/cm 2 , Voc = 1.81 V, η=13.37% and FF= 79.05% achieved with ZnO, CdS, ZnTe, and P +-ZnTe layer thicknesses about 0.03, 0.03, 0.5 and 0.1 µm, respectively. Finally, the spectral response in the long-wavelength region for ZnO/CdS/ZnTe solar cells has decreased at the increase of back surface recombination velocity. However, it has exhibited a red shift and showed no dependence of back surface recombination velocity for ZnO/CdS/ZnTe/P+-ZnTe solar cells.

This work reports on modeling IB (intermediate band) solar cells based on ZnTe:O semiconductor and determination of their photovoltaic parameters using SCAPS (solar cell capacitance simulator) software. A comparative study between photovoltaic performance of ZnTe and ZnTe:O based solar cells has been carried out. It has been found that the energy conversion efficiency η, short-circuit current density J sc , EQE (external quantum efficiency) and FF (fill factor) increased with increasing oxygen doping concentration N t up to the shallow acceptor density N A and decreased when N t was higher than N A . The open circuit-voltage V oc remained constant for N t lower than the acceptor doping concentration N A and decreased for N t higher than N A . The increase of η, J sc and FF is due to the fact that IB is fully empted, so sub-bandgap photons can be absorbed by hole photoemission process from the VB (valence band) to the IB. The decrease of η, J sc , EQE and FF is attributed to overcompensation for the base doping N A making electron photoemission process from IB to the CB (conduction band) maximized. This indicates that there is a competition between oxygen doping and intrinsic acceptor defects. The optimal concentrations of oxygen and shallow acceptor carriers were found to be N t ≈ 10 15 cm -3 and N A ≈ 10 14 cm -3 . The corresponding photovoltaic parameters were η = 41.5%, J sc = 31.2 mA/cm 2 , V oc = 1.80 V and FF = 75.1%. Finally, the EQE spectra showed a blue shift of absorption edge indicating that the absorption process is extended to the sub-bandgap photons through IB.