A sensitized thermal cell is a new thermal energy device based on a dye-sensitized solar cell; thermally excited carriers in a semiconductor are used instead of photon-excited carriers in a dye. In semiconductor-sensitized batteries, open-circuit voltage in thermal excitation is lower than in optical excitation, but the reason has been unclear. By changing the doping amount of the electron transport layer, we experimentally show that the voltage in thermal excitation is dependent on the Fermi level of the "entire" semiconductor electrode.