Optimization and performance analysis of n-ZnO/p-CdTe thin heterojunction solar cells via two-dimensional numerical simulation

Abstract

The unique properties of cadmium telluride (CdTe) and zinc oxide (ZnO) semiconductors suggest promising photovoltaic performance for n-ZnO/p-CdTe heterojunctions. In this work, two-dimensional numerical simulation was utilized to study and optimize n-ZnO/p-CdTe thin heterojunction solar cells, aiming to demonstrate the highest achievable conversion efficiency for this simple structure. The effects of CdTe acceptor concentration, CdTe thickness, ZnO thickness, ZnO band-gap, ZnO donor concentration, defect density in ZnO layer, and interface defects density on the photovoltaic performance of n-ZnO/p-CdTe heterojunction were investigated under standard illumination conditions (AM1.5, 100 mW/cm²). The results revealed significant sensitivity of the photovoltaic performance to variations in CdTe acceptor and ZnO donor concentrations. Additionally, the optimal thicknesses for CdTe and ZnO were found to be 3 µm and 250 nm, respectively. Consequently, these optimal parameters yielded the following photovoltaic parameter values: J_SC = 22.73 mA/cm², V_OC = 1.056 V, FF = 85.73 %, and η = 20.57 %, for a ZnO donor concentration of 10²¹ cm⁻³ and a CdTe acceptor concentration of 10¹⁷ cm⁻³. The analysis of ZnO bandgap energy, adjusted through Mg doping, shown that a slight increase in efficiency occurs at a band gap of 3.75 eV, corresponding to about 20 % Mg content. However, these performances deteriorate significantly when the defect density in the ZnO layer exceeds 5 $\times$ 10¹⁵ cm⁻³ or when the interface defect density rises above 10¹² cm⁻².