Sunlight passes through the transparent electrode into the dye layer where it can excite electrons that then flow into the titanium dioxide.

In the absence of any mechanism to remove electrons from the bulk of the solid irradiation continues to excite electrons to the conduction band producing holes in the valence band.

As these phonons strike other layers in the lattice, they excite electrons, which produce further phonons, which go on to excite more electrons, and so on.