Chapter 102 - Stepwise electron injection in the dye-sensitized nanocrystalline films of ZnO and TiO2 with novel coumarin dye

Abstract

Photoexcitation of dye molecules adsorbed on semiconductor surfaces such as TiO2 is known to induce ultrafast electron injection when the energy level of the excited state of the dye is higher than that of the conduction band bottom of the semiconductor. This phenomenon is used in dye-sensitized solar cells (so called Gratzel cell), in which visible light excites the dye, and electron injection generates carriers in the semiconductor. Ultrafast spectroscopy is applied to this system to study the mechanism of interfacial electron transfer for the last several years. Recently, the importance of localized surface states in the electron injection process from a photo-excited ruthenium complex dye (Ru(dcbpy)2(NCS)2, so-called N3) into the conduction band of a ZnO nanocrystalline film has been reported. An intermediate state in the course of the electron injection is found that indicates the reaction proceeds in stepwise manner. The intermediate is assigned to a charge transfer (CT) complex, or an exciplex, formed at the ZnO surface due to strong interaction with the excited dye and the surface states of ZnO. This chapter discusses another example of such a stepwise electron injection process on ZnO. The dye investigated is a novel coumarin derivative called NKX-2311which is reported to work as an efficient sensitizer for dye-sensitized TiO2 solar cells. The observed stepwise injection from NKX-2311 to ZnO is compared with that from NKX-2311 to TiO2. These injection processes are discussed in terms of exciplex formation with localized surface states and subsequent electron injection into the conduction bands.