III-nitrides material systems have attracting growing interests in photovoltaic (PV) applications after huge success in optoelectronics. In this work, a semi-analytical model is used to analyze the PV performance of single junction InGaN solar cells. Through clarifying four basic types of loss mechanisms, including transmission loss, thermalization loss, spatial relaxation loss and recombination loss, we discover that transmission loss accounts for the primary part of efficiency loss due to the large bandgaps of III-nitride materials. As for all recombination-related losses, Shockley-Reed-Hall (SRH) recombination loss is dominant over others. By incorporating non-step-like absorptance and emittance with below-bandgap absorption, we discover that reducing SRH recombination current by improving the material quality of InGaN layers proves an efficient approach to optimize the cell performance. Furthermore, the energy conversion efficiency increases with higher material quality and larger solar concentration. Our calculations show that energy conversion efficiency of 7.35% can be achieved under one sun and maximum efficiency of 8.43% under 1000 suns. This theoretical study offers detailed guidance for the future design of high-performance thin film InGaN solar cells.