Visible and near-infrared organic photosensitizers comprising isoindigo derivatives as chromophores: synthesis, optoelectronic properties and factors limiting their efficiency in dye solar cells

Abstract

The development of ruthenium-free organic photosensitizers showing panchromatic absorption up to the near-infrared (NIR) region for application in dye-sensitized solar cells (DSSCs) is still scarce. Among the sensitizers with absorption beyond 700 nm and developed for DSSCs, only zinc-phthalocyanine and boron-dibenzopyrromethene-based dyes have been able to reach efficiencies as high as 6%. Here we report metal-free organic dyes based on isoindigo, thieno-isoindigo or benzo-thieno-isoindigo chromophores that absorb in the UV-visible and NIR spectral range up to 900 nm. These molecules, that exhibit purple, blue, or green hues, were used to sensitize TiO₂ mesoporous electrodes in order to fabricate DSSCs with an iodide/triiodide-based electrolyte. Advanced photophysical characterizations, including charge extraction, transient photovoltage, and laser transient absorption spectroscopy experiments, combined with density functional theory modeling and computational investigations allow us to fully unravel the interfacial processes at the origin of the solar cell performances and to identify the limiting factors. A power conversion efficiency as high as 7% associated with a J_sc close to 19 mA cm⁻² was obtained with one of the dyes, which is comparable to those of the best panchromatic organic dyes reported so far. We also demonstrate in this work that the V_oc of the solar cells is linearly correlated to the dipolar moments of the oxidized dyes, the molecules possessing larger dipoles leading to the highest V_oc values.