Top cell design and optimization of all-chalcopyrite CuGaSe<sub>2</sub>/CuInSe<sub>2</sub> two-terminal tandem solar cells

Solar cells have attracted much attention, for they can convert solar energy directly into electric energy, and have been widely utilized in manufacturing industry and people’s daily life. Although the power conversion efficiency (PCE) of single-junction solar cells has gradually improved in recent years, its maximum efficiency is still limited by the Shockley-Queisser (SQ) limit of single-junction solar cells. To exceed the SQ limit and further obtain high-efficiency solar cells, the concept of tandem solar cells has been proposed. In this work, the chalcopyrite CuGaSe₂/CuInSe₂ tandem solar cells are studied systematically in theory by combining first-principle calculations and SCAPS-1D device simulations. Firstly, the electronic structure, defect properties and corresponding macroscopic performance parameters of CuGaSe₂ (CGS) are obtained by first-principles calculations, and are used as input parameters for subsequent device simulations of CGS solar cells. Then, the single-junction CGS and CuInSe₂ (CIS) solar cells are simulated by using SCAPS-1D software, respectively. The simulation results for the single junction CIS solar cells are in good agreement with the experimental values. For single-junction CGS cells, the device simulations reveal that the CGS single-junction solar cells have the highest short-circuit current (<i>J</i>_sc) and PCE under the Cu-rich, Ga-rich and Se-poor chemical growth condition. Further optimization in the growth environment with the highest short circuit current (<i>J</i>_sc) shows that the open-circuit voltage (<i>V</i>_oc) and PCE of CGS solar cells can be improved by replacing the electron transport layer (ETL) with ZnSe. Finally, after the optimized CGS and CIS solar cells are connected in series with two-terminal (2T) monolithic tandem solar cell, the device simulation results show that under the growth temperature of 700 K and the growth environment of Cu-rich, Ga-rich, and Se-poor, with ZnSe serving as the ETL, the CGS thickness of 2000 nm and the CIS thickness of 1336 nm, the PCE of 2T monolithic CGS/CIS tandem solar cell can reach 28.91%, which is higher than the ever-recorded efficiency of the current single-junction solar cells, and shows that this solar cell has a good application prospect.