Photovoltaic performances of type-II dye-sensitized solar cells based on catechol dye sensitizers: retardation of back-electron transfer by PET (photo-induced electron transfer)
Catechol dyes (CAT-PET), possessing PET (photo-induced electron transfer) characteristics as well as dye-to-TiO2 charge-transfer (DTCT) characteristics, which are composed of a catechol unit linked to a diethylamino group as an electron donor moiety via a methylene spacer, have been designed and developed as dye sensitizers for type-II dye-sensitized solar cells (DSSCs). The PET takes place from the nitrogen atom of the amino moiety to the photoexcited catechol unit, that is, the CAT-PET dyes make it possible to retard the back-electron transfer rate from the electrons injected into the TiO2 electrode to the oxidized catechol unit by the PET characteristic in type-II DSSCs. The CAT-PET dyes exhibited a broad absorption band corresponding to DTCT upon binding to TiO2 films, and the DTCT band for the CAT-PET dyes broadened and showed a red-shift compared to that for CAT dyes without PET characteristics. In addition, our results demonstrate that the introduction of moderately electron-withdrawing substituents on the catechol unit for the CAT dyes, but strongly electron-withdrawing substituents as well as moderately electron-withdrawing substituents on the catechol unit for the CAT-PET dyes leads to enhancement of the DTCT characteristics. It was found that the photovoltaic performances of DSSCs based on the CAT-PET dyes are higher than those based on CAT dyes. This indicates that the PET from the diethylamino group to the oxidized catechol dye adsorbed on the TiO2 electrode can efficiently retard the back-electron-transfer, leading to favorable conditions for the type-II electron-injection pathway from the ground state of the catechol unit to the conduction band (CB) of the TiO2 electrode by the photoexcitation of DTCT bands. Consequently, this work proposes that introducing PET characteristics to the catechol dye with a moderately electron-withdrawing substituent is an effective molecular design for type-II dye sensitizers to lead to not only an increase in the DTCT efficiency, but also the retardation of back electron transfer.