In this paper, the effects of p-layer hole concentration and p-layer thickness on the performances of InGaN p-i-n homojunction solar cells with different indium components and their intrinsic mechanisms are investigated by numerical simulations. it is found that the conversion efficiency of solar cells first increases and then decreases slightly with the increase of p-layer hole concentration and p-layer thickness. Moreover, the change of p-layer hole concentration and p-layer thickness will cause great changes of the conversion efficiency of the solar cells, especially as the indium composition increases. In order to better clarify and understand the physical mechanism of this phenomenon, the collection efficiency, I-V characteristic, built-in electric field and carrier transport of solar cells are analyzed in this paper. When the hole concentration is insufficient, the build-in electric filed is not strong enough to separate the most of the electric-hole pairs. This will reduce the collection efficiency. In addition, the lower the hole concentration, the higher the series resistance of solar cells will be and the more the power loss. So a conclusion can be drawn that the lower hole concentration of p-layer would be accompanied by the reduction of collection efficiency and the increase of series resistance, thus resulting in a lower conversion efficiency. With the increase of the hole concentration which is below an optimal value, the built-in electric field reaches the threshold, which can improve the collection efficiency. At the same time, although the series resistance is reduced to a certain extent, it still reduces the effective output power and limits the conversion efficiency. When the hole concentration is higher than the optimal value, the carrier mobility becomes the main factor limiting the conversion efficiency. As for the p-layer thickness, the simulation results indicate that the lateral transport of carriers from the p-layer to the anode electrodes becomes more obstructive with the thinning of p-layer thickness. This is because when the p-layer thickness decreases, thus causing the p-layer sectional area to decrease, the lateral series resistance becomes higher. It is clear that when the p-layer is too thin, the lateral series resistance is one of the main limiting factors affecting the conversion efficiency of solar cells.