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)

Abstract

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.