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.

Yousuke Ooyama,Yohei Adachi,Joji Ohshita,Masahiro Kanda,Toshiaki Enoki,Kensuke Furue

doi:10.1039/c6cp06513a

Abstract

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 TiO2 electrode through both the coordination bond at Lewis acid sites and the bidentate binuclear bridging linkage at Brønsted acid sites on the TiO2 surface, which makes it possible to inject an electron into the conduction band of the TiO2 electrode by the intramolecular charge-transfer (ICT) excitation (type-I pathway) and by the photoexcitation of the dye-to-TiO2 charge transfer (DTCT) band (type-II pathway). It was found that the type-I/type-II hybrid dye sensitizer adsorbed on TiO2 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 TiO2 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 TiO2 film, photocurrent-voltage (I-V) curves, incident photon-to-current conversion efficiency (IPCE) spectra, and electrochemical impedance spectroscopy (EIS) for DSSC.

Highlights

Working toward the creation of a type-I/type-II hybrid dye sensitizer for dye-sensitized solar cells (DSSCs) that makes it possible to inject electron into the conduction band (CB) of the TiO2 electrode by intramolecular charge-transfer (ICT) excitation and by the photoexcitation of the dye-toTiO2 charge transfer (DTCT) band, we have designed and developed the dye sensitizer OF-Py-Cat with a pyridyl group and a catechol unit as the anchoring group possessing the ability to adsorb on the TiO2 electrode through both the coordination bond at the Lewis acid sites and the bidentate binuclear bridging linkage at the Brønsted acid sites on the TiO2 surface
To gain insight into the influence of the molecular structure of the type-I/type-II hybrid dye sensitizer on the appearance of the ICT and dyeto-TiO2 charge transfer (DTCT) bands and the electron-injection mechanism, and to investigate the impacts of the ICT and DTCT characteristics of type-I/ type-II hybrid dye sensitizer on the photovoltaic performances of DSSCs, type-I dye sensitizer OF-Car-Car with two carboxyl groups, type-I dye sensitizer OF-Py-Py with two pyridyl groups, type-I dye sensitizer OF-Py-Car with a pyridyl group and a carboxyl group, and type-II dye sensitizer OF-Cat-Cat with two catechol units have been synthesized
The photovoltaic performance of the type-I/type-II DSSC based on OF-Py-Cat is lower than in the type-I DSSC based on the dye sensitizers (OF-Car-Car, OF-Py-Py, and OF-Py-Car) with a carboxyl group or a pyridyl group, but is equivalent to that of type-II DSSC based on the dye sensitizer (OF-Cat-Cat) with a catechol unit

Summary

Introduction

TiO2 electrode, (iii) high dye loading and high surface coverage of the TiO2 electrode, (iv) efficient re-reduction of the oxidized dyes by electron transfer from IÀ ions in the electrolyte containing the I3À/IÀ redox couple, (v) reduction of charge recombination between the injected electrons in the CB of the TiO2 electrode and the oxidized dyes or I3À ions in the electrolyte, and (vi) reduction of dye aggregation on the TiO2 surface.[3,4]. To provide a direction in molecular design of Cat dye sensitizers for type-II DSSCs, in our previous work, we designed and synthesized a D–p–Cat dye, which shows a broad absorption band corresponding to the DTCT upon binding to TiO2 film.29a It was found that the photocurrent for the DSSC based on the D–p–Cat dye is mainly generated by a direct electron-injection pathway from the D–p–Cat dye to the CB of the TiO2 electrode, that is, type-II DSSCs. we have demonstrated that the stabilization of the LUMO level and the expansion of the p-conjugated system by the introduction of a long p-bridge such as terthiophene on the Cat moiety can lead to an increase in the ICT excitation based on the p–p* transition with a decrease in the DTCT characteristics, resulting in enhancement of an indirect electron injection pathway from the excited dye to the CB of the TiO2 electrode by photoexcitation of the local band of the adsorbed dye on the TiO2 surface. To gain insight into the influence of the molecular structure of type-I/type-II hybrid dye sensitizer on the appearance of ICT and DTCT bands and the electron-injection mechanism, and to investigate the impacts of the ICT and DTCT characteristics of the type-I/type-II hybrid dye sensitizer on the photovoltaic performances of DSSCs, we synthesized a type-I dye sensitizer OF-Car-Car with two carboxyl groups, a type-I dye sensitizer OF-Py-Car with a pyridyl group and a carboxyl group, a type-II dye sensitizer OF-Cat-Cat with two catechol units, as well as a type-I dye sensitizer OF-Py-Py (YNI-2) with two pyridyl groups.18d It was found that the type-I/type-II hybrid dye sensitizer adsorbed on TiO2 film exhibits a broad photoabsorption curves, incident photon-to-current conversion efficiency (IPCE) spectra, and electrochemical impedance spectroscopy (EIS) for DSSC

Findings

Results and discussion

Conclusions