Photoinduced electrification of solids: II. Photovoltage transients
  In relevance to the photoelectrification mechanisms proposed in Part I, we derive rate equations for the negative-u and STE photoelectrification modes and solve them under conditions close to experimental ones. Exact solutions are obtained for the case of a slow electron-hole recombination at negative-U sites. These solutions are compared with photovoltage versus time transients by "short circuit currents" in photocharging experiments. 
M. Georgiev and O. Ivanov