Solution-processed small molecules with ethynylene bridges for highly efficient organic solar cells

Abstract

Two acceptor–donor–acceptor (A–D–A) conjugated molecules DPP-E-BDT and DPP-E-BDT-T, using diketopyrrolopyrrole (DPP) as the A-unit, ethynylene bridge flanked benzo-[1,2-b:4,5-b′]dithiophene (E-BDT) as the central D-unit, and different 4,8-substitutions on the BDT were synthesized by Sonogashira coupling reactions for solution-processed organic solar cells (OSCs). The insertion of electron-withdrawing ethynylene bridges (sp hybridization) in the DPP-E-BDT and DPP-E-BDT-T molecules leads to planar and enlarged aromatic skeletons, larger band gaps, and deeper HOMO levels. 4,8-Dithienyl substitution on BDT in DPP-E-BDT-T results in additional conjugation extension to give a slightly smaller band gap compared to the 4,8-dialkoxy substitution in target molecules. Bulk heterojunction solar cells using DPP-E-BDT and DPP-E-BDT-T as the donor materials and fullerene acceptor showed a high open-circuit voltage of 0.89 V and moderate current densities of 10.9 mA cm⁻². Besides, quite high fill factors (73.6%) could be obtained. Power conversion efficiencies (PCE) of 7.12% were obtained for DPP-E-BDT-T blends, which is the highest efficiency among small molecules based on DPP and BDT units. In active layer fabrication, 1,8-diiodooctane (DIO) was used as a solvent additive and subsequent thermal annealing treatment was also employed. We saw that these combined treatments led to balanced hole and electron transports, with values around 1.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ for the active layers. These results demonstrated that ethynylene bridges in small molecule donors are quite useful, both in tuning the electronic structure and in defining the thin film morphology, thus would be a promising method to enhance photovoltaic performances of the resulting materials.