Influence of ethynyl position on benzothiadiazole based D–A–π–A dye-sensitized solar cells: spectral response and photovoltage performance

Abstract

The ethynyl group has been widely employed in dye-sensitized solar cells (DSSCs) as an efficient π-spacer to prolong conjugation and promote electronic coupling at the interface of TiO₂ films. However, systematic studies of the ethynyl position on metal-free organic sensitizers remain relatively rare. Herein we report indoline-based organic dyes bearing an ethynyl group at different positions of D–A–π–A organic dyes. Based on the reference dye D1, we inserted an ethynyl unit in either the left or right side of benzothiadiazole to construct two novel dyes D2 and D3. It was found that inserting an ethynyl unit to the side of the anchoring group obtained a higher molar extinction coefficient with a red shift in the absorption band. Interestingly, D2 and D3 displayed a better photovoltaic performance with respect to D1. In particular, D3 exhibits an over 90 mV enhanced open-circuit voltage (V_OC) than D2 owing to a longer electron lifetime and slower charge recombination. With this incredible increase of V_OC, D3 bestows a high efficiency of 7.13% with respect to D1 and D2. Coadsorption strategies are exploited for further improving the cell behaviour. As a result, cosensitization with a long-wavelength-responsive dye WS-2 was demonstrated to efficiently compensate the light-harvesting, achieving an excellent efficiency of 9.83% in the iodine electrolyte. This work has paved a useful and practical way for molecular engineering in D–A–π–A metal-free organic dyes.