Before I get into my question, it is helpful to note the paper I am referring to. "Photovoltage in nanocrystalline porous TiO2" by V. Duzhko, et al. DOI: 10.1103/PhysRevB.64.075204 In Section B. "Spectral photovoltage in well-passivated porous TiO2 layers", the authors mention that "The [photovoltage] amplitude increases strongly in the region of the forbidden gap." From the data, it is clear that this is true, but I am struggling trying to determine exactly why. I understand that photovoltages can be measured because an incident beam of light causes excess carriers in space (as per the introduction of the paper). From what I understand, the light can move some of the electrons around, leading to an anisotropic charge distribution in the material. This altered charge distribution leads to a (fairly weak) induced electric field which creates a voltage across the sample which can then be measured. (Please correct me if I am thinking about this wrong). It is stated slightly further along in the paper (same section as above) that the photovoltage signal arises "due to the concentration gradient of excess carriers in the porous layer." From my description of a photovoltage above, it makes sense to me that a carrier concentration gradient would lead to a photovoltage signal, simply because of the anisotropy of the charge density, thus inducing an electric field and a voltage. However, would this describe such a strong increase in measured photovoltage in middle of the band gap? Surely charges cannot reside in the forbidden gap. I know what matters is spatial charge separation, not necessarily energy disparities between carriers, but I still cannot reason why such a sharp increase in photovoltage is measured in the gap. Any suggestions or ideas would be greatly appreciated. Thanks in advance.