Fig. 12 shows the mechanism of current generation. When the
light falls on donor P3HT, the excitons are generated. The excitons
are dissociated at the interface between P3HT and TiO2. The electrons
move from LUMO of TiO2 to the graphene and then ITO.
Similarly, the holes move from HUMO of PCBM to the aluminium
via PEDOT layer.