Electron trapping in electrochemical solar cells based on porous nanocrystalline TiO2 electrodes has been investigated. As light-absorbing dye, zinc 5,10,15,20-tetracarboxyphenylporphyrin is used. The spectral photocurrent response under front- and back-side irradiation reveals the existence of an active region, located near the transparent conducting substrate, where charge separation takes place. Due to electron trapping, this region increases when an additional white light bias is applied. In an indifferent electrolyte, the shape of the photocurrent action spectra depends on the applied potential. The contribution of the weaker absorbing Q bands is enhanced when negative potentials are applied. In this case the electron traps in the porous TiO2 film are filled. Their location in the bandgap is 0.5-0.6 eV below the conduction band edge. Consequently, electron trap filling enlarges the active region where charge separation takes place, which leads to an enhanced collection efficiency.