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The photo-electrochemical (PEC) cell, which combines the technology of electrolyzers and photovoltaics, is a promising candidate for the electrolysis of hydrogen with reduced external energy supply. Multiple parameters of such cells have to be adjusted to achieve the most efficient assembly for given requirements. These parameters include the choice of photoactive material, type and kinetics of electrolyte and the cell geometry, which finally determine the photocurrent. As the dependency between these parameters and the cell performance is complex, numerical simulations are an indispensable tool for the optimization process. To guarantee a reliable reference model that applies for all possible parameter changes, iterative verification with experiments is necessary. Especially the precise modelling of the interfaces between photoactive substrate and electrolyte poses a significant challenge for the numerical simulations. Nevertheless, precise modelling of this region is crucial, as it largely determines the cell efficiency by controlling the reaction rate, gas saturation, electrode corrosion and thus the photocurrent. In this work we present results of numerical modelling of PEC cells with COMSOL Multiphysics, their benchmarking with experimental data measured with varying 3D-printed geometries, electrode substrates and electrolyte compositions as well as new approaches for optimizing the efficiency of PEC cells.