Solar cells have attracted much attention since they can convert solar energy directly into electricity, 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 restricted by the Shockley-Queisser (SQ) limit of single-junction solar cells. To exceed the SQ limit and further obtain high efficiency solar cells, researchers have proposed the concept of tandem solar cells. In this work, a systematic theoretical study of chalcopyrite CuGaSe<sub>2</sub>/CuInSe<sub>2</sub> tandem solar cells was carried out by combining first-principle calculations and SCAPS-1D device simulations. Firstly, the electronic structure, defect properties and corresponding macroscopic performance parameters of CuGaSe<sub>2</sub> (CGS) were obtained by first-principles calculations, which were adopted as input parameters for subsequent device simulations of CGS solar cells. Subsequently, the simulations of single-junction CGS and CuInSe<sub>2</sub> (CIS) solar cells were carried out 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 revealed that CGS single-junction solar cells had the highest short-circuit current (J<sub>sc</sub>) and PCE at the growth condition of Cu-rich, Ga-rich and Se-poor chemical growth condition. Further optimization found that the open-circuit voltage (V<sub>oc</sub>) and PCE of CGS solar cells can be improved by replacing the electron transport layer (ETL) with ZnSe after the devices with the highest short-circuit current (J<sub>sc</sub>). Finally, after the optimized CGS and CIS solar cells were 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 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 recorded efficiency of the current single-junction solar cells, and shows a good application prospect.
