Thieno[3,2-b]indole (TI) bridged A-π−D-π−A small molecules: Synthesis, characterizations and organic solar cell applications

Two novel A-π-D-π-A small molecules, BDT-TITRh and BDT-TI2TRh, containing alkylthienyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT) as a core building block, together with 2-(thiophen-2-yl)-N-alkyl-thieno [3,2-b]indole (TIT) or 2,6-di(thiophen-2-yl)-N-alkyl-thieno [3,2-b]indole (TI2T) as π-bridge unit, and 3-ethylrodanine as the electron-withdrawing unit were synthesized, characterized, and employed as the donor materials for BHJ SMOSCs. The impacts of TI π-bridge on the absorption spectra, energy levels, hole mobility, film morphology, and the photovoltaic performance were investigated. The two small molecules both exhibited a high extinction coefficients (∼10⁵ M⁻¹ cm⁻¹) and hole mobility (∼10⁻⁵ cm⁻² V⁻¹ s⁻¹) due to the introduction of the extensively conjugated TI as π-bridge unit. A promising PCE of 4.19% with a Jsc of 13.23 mA cm⁻² was achieved for the device based on BDT-TI2TRh/PC₇₁BM (1:2, w/w) as the active layer. It is worth mentioning that no additive or thermal annealing was exploited for the fabrication of the current BHJ SMOSCs, and this is an advantage for simplifying the device fabrication and improving the repeatability of the photovoltaic performance. Our preliminary results demonstrate that tricyclic TI as the π-bridge unit can be used for building A-π-D-π-A type small molecules, providing the guidance for the molecular design of high hole mobility SM-materials for efficient BHJ SMOSCs in the future.