Ultra-fast co-sensitization and tri-sensitization of dye-sensitized solar cells with N719, SQ1 and triarylamine dyes

Abstract

This paper describes the synthesis of a new, yellow triphenylamine dye, 4-[2-(4-diphenylaminophenyl)vinyl]benzoic acid] (6), with a sorption maximum at 380 nm in solution for which EQE data show shifts to 420 nm on sorption to TiO₂. The performance of this dye has been measured in dye-sensitized solar cell (DSC) devices, showing η = 2.6% for 1 cm² devices. Light soaking of (6) shows excellent long-term stability with <10% variation in device performance over 1800 h. Full characterization data are reported for (6) and the intermediates used in its synthesis including single-crystal X-ray structural analysis of all compounds. The paper also describes the ultra-fast dye sensitization and co-sensitization of TiO₂ photo-electrodes in 5 minutes using one or two dyes and the first example of ultra-fast tri-sensitization. The dyes tested include the ruthenium dye N719, the squaraine dye SQ1, the red triphenylamine dye 2-cyano-3-{4-[2-(4-diphenylaminophenyl)vinyl]phenyl}acrylic acid (5) and (6). DSC efficiencies of 7.5% have been achieved for 1 cm² devices co-sensitized using (6) and N719. These efficiencies exceed those recorded for single dye devices and EQE measurements confirm efficient photon capture from two or more dyes in a single photo-electrode. Photo-acoustic calorimetry (PAC) has also been used to measure the energy of the charge separation states formed for (6) and N719, showing a larger value (1.47 eV) for (6) compared to N719 (1.08 eV), whilst a TiO₂ film co-sensitized with both (6) and N719 gave an intermediate value (1.28 eV). These data have been used to calculate dye HOMO, LUMO and λ_max levels for (6) and N719 leading to important insights for future successful co-sensitization.