The photovoltage response of a semiconductor electrode to the periodic excitation by short laser pulses has been described in a quasi-steady-state approximation. Measurements with electrically-biased single- and polycrystalline n-TiO2 electrodes illuminated by a N-2 impulse laser (337 nm, 10 ns) were used to demonstrate the validity of the proposed model for harmonic components in the frequency spectrum of the photovoltage and to illustrate its application to highly doped semiconductors. It has been shown that the higher harmonics of the ac photovoltage can be used to obtain information on the potential and frequency dependence of space-charge capacitance, which is not distorted by the influence of other elements of the equivalent circuit. The approach developed opens the possibility of determining the flatband potential and doping density in the case of polycrystalline semiconductor electrodes showing quite dispersive non-linear Mott-Schottky behaviour.