The surface hole concentrations shown in Figure are around 10 17 h + cm -3 for both models and do not change much with changes in light intensity. Beneath the surface, bulk hole concentrations decrease with increasing depth because of light attenuation at each light intensity, and increase overall with increasing light intensities. We can calculate the absorbance of surface and bulk holes in ΔmO.D. (SI Section 5, Figure , Table ) using the absorption coefficient reported by Durrant et al. for comparison to their measurements. As shown in Figure (a) we find that the calculated hole concentrations are about 1000 times lower for a given light intensity than the measured ones. Figure (b) shows the calculated absorbance in ΔmO.D. is 1 to 2 orders of magnitude smaller and increases more rapidly than the experimental results. The experimental absorbance starts to flatten with intensity at 28 mW•cm -2 while the simulated absorbance continues to increase. This suggests that there are many more holes present in the experimental sample than is predicted by the simulations for a perfect crystal, and that their population reaches a maximum as a function of light intensity. This difference is worth understanding more fully through detailed sample characterizations: it may be that the very large internal surface area in the nanocrystalline film plays a role. In addition, our calculated electron current (0.27 mA•cm -2 ) is very close to experimental values (0.5 mA•cm -2 ) at the light intensity of 57.9 mW•cm -2 , which indicates that the electron and O2 production currents are not necessarily equivalent because of the multiple factors influencing them (see SI Section 5 and Discussion for more details).