Determination of electron and hole energy levels in mesoporous nanocrystalline TiO2 solid-state dye solar cell

A study of a hybrid heterojunction solar cell based on nanocrystalline mesoporous TiO2 and the hole conductor spiro-OMeTAD (2,2′7,7′-tetrakis(N,N′-di-p-methoxyphenyl-amine)-9,9′-spiro-bifluorene) has been realized. Impedance and cyclic voltammetry techniques were used to measure the interfacial properties of the hybrid heterojunction and establish the energy levels of the solid-state electrolyte. It was observed that the energy levels of the organic hole transport material are changed when it forms a film deposited onto indium-doped tin oxide (ITO). Moreover, the HOMO level of the mono oxidized spiro-OMeTAD is well coupled with the HOMO level of the dye N719 (Ru(4,4′-dicarboxy-2,2′-bipyridyI)2(SCN)2) which implies that it is not convenient to increase the doping of the hole conductor much further than this first oxidized state. This doping level (n ≈ 1019 cm−3) also assures a high enough hole conductivity. The implications of our results to the solid-state dye solar cell performance are discussed.