It becomes clear by drawing a band diagramm for the surface depletion region of p-type material with the downward bending of the the conduction and valence bands. Photoexcited electron-hole pairs are separated by the surface electric field meaning that holes move towards the bulk while electrons are attracted by by the surface. But these additional electrons are needed for the formation of molecular hydrogen from the protons. In other worde, the built-in electric surface field can be used to force the electrons to the surface where the photo-electrochemical reaction takes place.

Thanks sir for your answer

Hi dear M. Ali, Actually the band bending originates from the lattice mismatching of materials,(i.e.,  materials with different band gaps and different electron affinities) which creates built-in potential. In fact, built-in potential is a barrier for carriers to migrate across the depletion region. Downward bending of p-semiconductor would be useful if the VBM of n-semiconductor is lower to that of p-type. In this way valence band offset reduces and rate of carrier transportation is enhanced. In this way, conduction mechanism is improved with low recombination rate.