The key materials for photo-electrochemical (PEC) processes are the photoelectrocatalysts that are required to absorb sunlight, form electron-hole pairs, and transport the charge carriers efficiently towards the electrolyte-semiconductor interface. Metal-oxides are thought to be the most stable materials under intense interfacial reactive conditions. However, the overall efficiencies of these metal-oxide photoelectrocatalysts are not as high as expected, even for those with near-suitable bandgap energies required for efficient solar energy absorptions. One of the main reasons is attributed to the poor transport properties in metal-oxides with strongly correlated electrons. Typically, strongly correlated systems exhibit polaron conduction via electron-phonon interactions. Polarons act like a bottleneck for photoconductions in such metal-oxides. Recently, there have been significant efforts to characterize polaronic states and their effects on transport properties in photoactive materials. First-principles theoretical and computational studies can reveal the electronic origin of the polaron formation and, therefore, can suggest how to improve overall transport properties in such photoactive materials. In this presentation, representative metal-oxides photo-catalysts will be considered to demonstrate the small polaron formation mechanism. The polaronic states in metal-oxides will be depicted from detailed electronic structure calculations by density functional theory (DFT) based methods. We will show how these polaronic states affect the PEC cell's transport properties and photo-voltages. We will also discuss how these computational results compare with the recent experimental outcome. Some part of the work was funded by National Science Foundation, USA. Computations were performed at the Texas Advanced Computing Center (TACC).