An ultra-thin film photovoltaic cell, which incorporates an Al_xIn_1-xAs/GaAs heterojunction, is simulated using Adept 1D simulation tool, and it is with an energy conversion efficiency of 20.06% (under 1 sun, AM1.5G illumination) for 604 nm cell thickness (excluding the substrate thickness), and optimized layer thickness and doping concentration for each layer of the device. The device has an n-type AlAs window layer (highly doped), an n-type AlxIn1-xAs emitter layer and a p-type GaAs base layer. Germanium (Ge) substrate is used for the structure. The device parameters are optimized separately for each layer. Based on these optimizations, the ultra-thin film solar cell design is proposed after careful consideration of lattice mismatch between two adjacent layers of the device.