Investigations of carrier transport mechanism and junction formation in Si/CZTS dual absorber solar cell technology

Abstract

We report on dual absorber n-silicon/p-Cu2ZnSnS4 (Si/CZTS) solar cell model with CZTS as top absorber, silicon as bottom absorber and ITO acting as transparent conducting oxide layer on top of CZTS material in order to exploit the advantages of well-established silicon technology and low-cost earth abundant CZTS materials. The valence band edge develops a triangular potential at the Si/CZTS interface, and hence, the drift–diffusion and thermionic carrier transport mechanisms are investigated using Silvaco ATLAS 2D simulator. The thermionic emission current component is suggested to improve the fill factor at the cost of short circuit current and open circuit voltage. The band discontinuities, built-in potential developed across the heterojunction, generation–recombination profile, electron and hole current densities are carefully examined to understand the transport phenomena of Si/CZTS structure for varying thickness, acceptor concentration, electron affinity and optical band gap of top CZTS absorber materials. Synthesis and deposition of CZTS thin films were carried out via sol-gel spin coating process with different preheating temperature range between 250 and 350 °C for 10 min, and absorption data obtained from experiment is used to explore the benefits of tunable band gap properties of CZTS absorbers in the Si/CZTS heterojunction solar cell device. The increased band gap for 350 °C preheated sample has minimized the electrical recombination losses and improved the infrared light absorption resulting in efficiency enhancement. Then the CZTS physical parameters are optimized for the Si/CZTS solar cell model and the conversion efficiency is improved from 8.97 to 14.74% by minimizing the electrical and optical losses of the materials.