Development of type-I/type-II hybrid dye sensitizer with both pyridyl group and catechol unit as anchoring group for type-I/type-II dye-sensitized solar cell

A type-I/type-II hybrid dye sensitizer with a pyridyl group and a catechol unit as the anchoring group has been developed and its photovoltaic performance in dye-sensitized solar cells (DSSCs) is investigated. The sensitizer has the ability to adsorb on a TiO electrode through both the coordination bond at Lewis acid sites and the bidentate binuclear bridging linkage at Brønsted acid sites on the TiO surface, which makes it possible to inject an electron into the conduction band of the TiO electrode by the intramolecular charge-transfer (ICT) excitation (type-I pathway) and by the photoexcitation of the dye-to-TiO charge transfer (DTCT) band (type-II pathway). It was found that the type-I/type-II hybrid dye sensitizer adsorbed on TiO film exhibits a broad photoabsorption band originating from ICT and DTCT characteristics. Here we reveal the photophysical and electrochemical properties of the type-I/type-II hybrid dye sensitizer bearing a pyridyl group and a catechol unit, along with its adsorption modes onto TiO film, and its photovoltaic performance in type-I/type-II DSSC, based on optical (photoabsorption and fluorescence spectroscopy) and electrochemical measurements (cyclic voltammetry), density functional theory (DFT) calculation, FT-IR spectroscopy of the dyes adsorbed on TiO film, photocurrent-voltage (I-V) curves, incident photon-to-current conversion efficiency (IPCE) spectra, and electrochemical impedance spectroscopy (EIS) for DSSC.