Enhancing the open-circuit voltage and efficiency of CZTS thin-film solar cells via band-offset engineering

Abstract

Limits on the conversion efficiency and open-circuit voltage lead to low kesterite thin-film solar cell performance. The conduction band offset (CBO) between the absorber and buffer layers (p-CZTS and n-CdS) is considered to be one of the effective factors for the performance of these devices. The simulation results demonstrated that the optimal thickness and doping concentration for obtaining a high performance in the solar cell which is based on CZTS/CdS absorber/buffer layers would respectively be 50 nm and 1.5 × 1017 cm−3. The CBO at absorber/buffer interface plays a significant role in solar cell performance and the highest CZTS-based solar cell efficiency is obtained within the range of − 0.4 eV and + 0.4 eV of the CBO. Solar cell performance decreases from 5.12 to about 3% when the positive CBO increases from 0.4 to 0.7 eV. Moreover, the impact of CBO on device quantum efficiency was investigated. The optical response range changed with an increase in + ΔEC and the highest value (about 80%) was observed in visible and near ultraviolet light within the range of 400–520 nm. In − ΔEC, electrons can be transferred from the buffer layer to the window layer and the optical photons proportional to the buffer layer’s band gap help generate free electrons and holes, which cause a slower reduction of the quantum efficiency within the negative offsets and the above-mentioned range, through electric field bending. In addition, a large number of n-type materials such as CdS, SnS2, ZnO, ZnTe, ZnS, Zn(O, S), (Mg, Zn)O, In2S3, TiO2, and CdTe were analyzed as buffer layers in CZTS-based thin-film solar cell. The CZTS-based thin-film solar cells with SnS2, ZnO0.55S0.45, ZnO0.80S0.20, and Mg0.2Zn0.8O led to a conversion efficiency of about 13%. Furthermore, an open-circuit voltage of above 700 mV was obtained by using ZnO0.40S0.60 and Mg0.1Zn0.9O buffer layers. By introducing the CZTSe/CZTS absorber layer and SnS2 buffer layer in the structure of the proposed solar cell, the best efficiency and open-circuit voltage is deduced as 16.28% and 780 mV, respectively.