A multi-physics model of a planar photoelectrochemical water-splitting device was developed, validated and used to identify and quantify the most significant materials-related bottlenecks in photoelectrochemical device performance. The model accounted for electromagnetic wave propagation within electrolyte and semiconductor, and charge carriers transport within the semiconductors and at the semiconductor-electrolyte interface. Interface states at the semiconductor-electrolyte interface were considered using an extended Schottky contact model. The device model was validated with current-voltage measurements using an n-type GaN photoanode as a model device. Numerical design of experiments and parametric analysis were conducted using the validated model in order to identify and optimize key factors for photoelectrochemical water splitting devices. The methodology developed using an experimentally-validated numerical model coupled to statistical analysis provides a general approach to identify and quantify main material challenges and design considerations in working PEC devices. In the case of n-type GaN, the surface recombination, hole transfer kinetics, and doping concentration were identified as the most significant parameters for the photocurrent density.