Reverse electron transfer at the interface of semiconductor film in dye-sensitized solar cells

The photovoltaic performance of dye-sensitized solar cells (DSCs) based on nanocrystalline TiO2 films with co-absorbed [NBu4]2[cis-Ru(Hdcbiq)2(NCS)2] (BQ; [NBu4]+ = tetrabutyl ammonium cation; H2dcbiq = 4,4′-dicarboxy-2,2′-biquinoline) and [NBu4]2[cis-Ru(Hdcbpy)2(NCS)2] (N719; H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) has been investigated. The excited state (Eox*) of BQ is more positive than that of N719 and is comparable to the conduction band edge (Ecb) of the TiO2. The short circuit photocurrent density (Jsc) and open circuit photovoltage (Voc) decreased with increasing adsorbed BQ on the TiO2 when N719 and BQ were co-adsorbed on the TiO2. The dark current of DSCs with co-absorbed BQ and N719 increased with increasing adsorbed BQ on the TiO2. Therefore, we propose that an electron injected from N719 to TiO2 and then trapped by BQ subsequently reacted with I2. The photovoltaic performance of DSCs with co-adsorbed N719 and BQ can be explained by reverse electron transfer from TiO2 to I2 through BQ.