The two-level model in the permanent (left panel) and transitory (right panel) regime of illumination. (Left panel) A molecular photocell with one HOMO and one LUMO orbitals attached to the electrodes. The red line represents the electron-hole interaction and recombination inside the molecule and the hopping integrals of electron and hole are denoted by C and J. E is the energy of absorbed photon. (Right panel) Schematic representation of a donor-acceptor system. Charge evacuation leads are considered as semi-infinite chains. Here, J_e and J_h are the coupling energies between two adjacent sites in the electron and hole chains, respectively. Also, C_e and C_h represent the first coupling energies between the interface states and charge evacuation leads.