Abstract In this paper, a simulation and analysis on the short-circuit current density ( J sc ) of the P-GaSb window/P-Ga x In 1 −x As 1 −y Sb y emitter/N-Ga x In 1 −x As 1− y Sb y base/N-GaSb substrate structure is performed. The simulations are carried out with a fixed spectral control filter at a radiator temperature ( T rad ) of 950 °C, diode temperature ( T dio ) of 27 °C and diode bandgap ( E g ) of 0.5 eV. The radiation photons are injected from the front P-side. Expressions for minority carrier mobility and absorption coefficient of Ga x In 1− x As 1− y Sb y semiconductors are derived from Caughey–Thomas and Adachi’s model, respectively. The P-Ga x In 1− x As 1− y Sb y emitter with a much longer diffusion length is adopted as the main optical absorption region and the N-Ga x In 1− x As 1− y Sb y base region contribute little to J sc . The effect of P-GaSb window and P-Ga x In 1− x As 1− y Sb y emitter region parameters on J sc is mainly analyzed. Dependence of J sc on thickness and carrier concentration of the window are analyzed; these two parameters need to be properly selected to improve J sc . Contributions from the main carrier recombination mechanisms in the emitter region are considered; J sc can be improved by suppressing the carrier recombination rate. Dependence of J sc on the carrier concentration and layer thickness of the emitter P-region are also analyzed; these two parameters have strong effect on J sc . Moreover, adding a back surface reflector (BSR) to the diode can improve J sc . The simulated results are compared with the available experimental data and are found to be in good agreement. These theoretical simulations help us to better understand the electro-optical behavior of Ga x In 1− x As 1− y Sb y TPV diode and can be utilized for performance enhancement through optimization of the device structure.