Concerted companion organic dyes with benzothiadiazole–ethynyl units for high-performance indoor dye-sensitized solar cells

Abstract

In our previous work, a concerted companion organic dye, CCOD-2, has been employed to fabricate highly efficient dye-sensitized solar cells (DSSCs) for indoor lamps. However, the absorption profile of CCOD-2 (maximum at 519 nm) exhibits relatively poor spectral overlap with the emission peak of T5 lamps at 610 nm, limiting its photovoltaic performance. Hence, two novel dyes, CCOD-3 and CCOD-4, have been designed and synthesized by inserting benzothiadiazole–ethynyl units into one of the sub-dyes. As a result of the extended π-system, the absorption shifts to longer wavelengths, affording good overlap with the emission of the T5 lamps. Consequently, the solar cells based on CCOD-3 or CCOD-4 afford current density (J_SC) values (0.451–0.478 mA cm⁻²) higher than that of CCOD-2 (0.424 mA cm⁻²). Meanwhile, CCOD-4 exhibits an impressive open-circuit voltage (V_OC) of 732 mV, which may be ascribed to the enhanced anti-aggregation and anti-charge recombination ability, induced by the two bulky phenothiazine and fluorenyl indoline donors within the two subdyes. Finally, a high power conversion efficiency (PCE) of 34.5% has been achieved for CCOD-4 under 2500 lux T5 lamp illumination, which is superior to those of CCOD-3 (31.2%) and reference dye CCOD-2 (28.0%). Besides, CCOD-4 based solar cells achieve the highest PCE of 11.3% under AM 1.5G simulated sunlight among these dyes. These results demonstrate that introducing benzothiadiazole–ethynyl units into CCOD dyes is an effective approach for enhancing the performance of indoor DSSCs, particularly in terms of J_SC.