Theoretical Study of Multi-stacked GaSb/GaAs Quantum Dot Solar Cells

Multi-stacked GaSb/GaAs quantum dot (QD) solar cells are theoretically studied by two-dimensional simulation with the implementation of Poisson's equation, continuity equations, drift-diffusion transport model and thermionic emission model. Such physical models are numerically solved by finite element method. QD layers are inserted into the i-region of the GaAs p-in structure. Each QD layer is separated by a GaAs spacer layer. Effects of spacer layer thickness and number of QD layers on the solar cell characteristics are investigated. Spacer layer thickness also determines on the distribution of hole wave functions which are altered by strain and impact absorption properties. Thinner spacer layer results in higher strain which shifts the absorption edge towards shorter wavelengths and is responsible for open-circuit voltage enhancement, while thicker spacer layer increases the Shockley-Read-Hall (SRH) recombination that degrades both open-circuit voltage and fill factor. Increasing the number of QD layers leads to higher short-circuit current density and lowers both open-circuit voltage and fill factor which are derived from higher GaSb amount and SRH recombination. The simulation results suggest that the highest efficiency of 25.82% is achieved by using the 40-stacked QD layers with 15-nm spacer layer.