Optoelectronic optimization of graded-bandgap CdTeSe thin-film solar cells

Recent research on CdTeSe thin-film solar cells has indicated that front-junction grading suppresses nonradiative recombination and improves carrier collection, thereby improving the solar-cell efficiency. However, a comprehensive study encompassing different bandgap-grading profiles—such as linear, nonlinear, and piecewise homogeneous—is required to fully understand their impact on device performance. Hence, detailed optoelectronic simulations of these solar cells were performed to determine the effects of compositional grading and absorbing-layer thickness on power-conversion efficiency. The transfer-matrix method was used to calculate the electron-hole-pair (EHP) generation rate, and a one-dimensional drift-diffusion model was used to determine the EHP recombination rate, open-circuit voltage, short-circuit current density, power-conversion efficiency, and fill factor. Optimization using the differential evolution algorithm indicates that linearly and nonlinearly graded CdTeSe photon-absorbing layers of 3000 nm thickness can deliver an efficiency of 21.79% and 21.22%, respectively, when the selenium-to-tellurium ratio is not allowed to exceed 2/3. Also, the simulations indicate that a two-layered piecewise-homogeneous CdTeSe photon-absorbing layer, with a thickness of 3000 nm, can deliver 22.13% efficiency. If higher selenium content is permitted, the maximum efficiency attainable rises to 24.68%.