Electron Injection Efficiency from Excited N3 into Nanocrystalline ZnO Films:  Effect of (N3−Zn²⁺) Aggregate Formation

Dye-sensitized solar cells based on cis -bis-(4,4‘-dicarboxy-2,2‘-bipyridine) dithiocyanato ruthenium (II) (Ru(dcbpy) 2 (NCS) 2, N3) adsorbed on nanocrystalline ZnO films are a promising system as highly efficient solar cells. However, the performance of the cells decreases with increasing concentration of N3 dye on the surface of ZnO films, suggesting the formation of aggregates. We have studied the efficiency of electron injection at two concentrations through fluorescence and transient absorption measurements. From fluorescence measurements, we confirmed that the aggregates (N3−Zn 2+ ) of N3 dyes with Zn 2+ ions are formed in ZnO films and electron injection from (N3−Zn 2+ ) to ZnO films is inefficient. Fluorescence and scanning electron microscope measurements show that the aggregate forms not only in meso pore of ZnO films but also at the surface as micrometer-sized particles. From transient absorption measurements, it is found that the efficiency of the electron injection is decreased by the presence of (N3−Zn 2+ ) on the surface of ZnO films.