Numerical analysis of Cu2ZnSnS4 solar cells on Si substrate

The Cu2ZnSnS4 (CZTS) solar cell prepared on Si substrate has an advantage of low lattice mismatch between CZTS and Si substrate, but the conversion efficiency of reported p-CZTS/n-Si solar cells is still low at present. In this work, the CZTS solar cells on Si substrate are calculated numerically by heterojunction solar cell simulation software Afors-het. The calculated results show that the p-CZTS and n-Si act as window layer and absorber respectively in the p-CZTS/n-Si solar cell because the band gap of p-CZTS is larger than that of n-Si. The conversion efficiency of p-CZTS/n-Si solar cell increases as the thickness of p-CZTS window layer decreases. The highest calculated conversion efficiency of p-CZTS/n-Si solar cell is 18.57%. In the best pCZTS/n-Si solar cell, most of the incident light cannot pass through the p-CZTS window layer due to the high absorption coefficient of p-CZTS, which limits the conversion efficiency of solar cell. In order to solve the problems existing in the p-CZTS/n-Si structure, a novel n-ZnO:Al/i-ZnO/n-CdS/p-CZTS/p-Si solar cell structure is proposed, where n-ZnO:Al and i-ZnO are window layers, n-CdS is buffer layer, p-CZTS is absorber, and p-Si is substrate and back electrode. The dark current density-voltage (J-V) characteristic curves of pCZTS/p-Si structure varying with the thickness and doping concentration of p-Si and the doping concentration of p-CZTS are calculated to investigate the feasibility of p-Si as a back electrode of p-CZTS. All the calculated J-V characteristic curves of p-CZTS/p-Si structure are linear, indicating the formation of ohmic contact between p-CZTS and p-Si. The photovoltaic properties of n-ZnO:Al/i-ZnO/n-CdS/p-CZTS/p-Si solar cell are further calculated. The built-in electric field distributed in n-ZnO:Al, i-ZnO, n-CdS, and p-CZTS contribute to the collection of photo-generated carriers. The conversion efficiency of n-ZnO:Al/i-ZnO/n-CdS/p-CZTS/p-Si solar cell is enhanced with the decrease of the thickness of p-Si and the increase of doping concentrations of p-Si and p-CZTS and the thickness of p-CZTS. Without considering the effect of parasitic series resistance and parallel resistance and defect states, the highest conversion efficiency of ideal n-ZnO:Al/i-ZnO/n-CdS/pCZTS/p-Si solar cell is 28.41%. The calculated results in this work show that the n-ZnO:Al/i-ZnO/n-CdS/p-CZTS/p-Si solar cell has an appropriate structure for CZTS solar cell on Si substrate.