Inverted P3HT:PC61BM organic solar cells incorporating a π-extended squaraine dye with H- and (or) J-aggregation

In this report, we investigated the impact of adding a large π-extended squaraine dye to the classic bulk heterojunction P3HT:PC61BM system. The compound, SQIQ-A, is a bis-indole squaraine dye appended with acetylene-indoloquinoxaline end-capping units that exhibits strong optical absorption in thin film from 650 to 750 nm, beyond the absorption wavelength cut-off for P3HT:PC61BM films (ca. ∼650 nm). The dye SQIQ-A can form H- or J-aggregates (blue or red shifting the optical absorption profiles) via solvent vapour annealing with CHCl3 or THF, respectively, thus providing a simple method to control the morphology and optical properties of the molecule. Ternary blended films composed of P3HT:PC61BM:SQIQ-A (1:1:0.3 mass ratio) were investigated as-cast, after solvent vapour annealing, and after thermal annealing. Films were characterized using optical absorption spectroscopy, X-ray diffraction, atomic force microscopy, and polarized light microscopy. Solar cells were fabricated using an inverted architecture in air and analysed using current–voltage and external quantum efficiency measurements. It was found that solar cells thermally annealed at 130 °C for 10 min gave good power conversion efficiencies of 3%, similar to the control P3HT:PC61BM but with improved fill factors and noticeable photocurrent generation from 650 to 760 nm where the SQIQ-A molecule absorbs. Exposure to solvent vapour (CHCl3 or THF) results in aggregation of all components but specifically leads to the formation of micron-sized domains, lowering the overall photovoltaic performance.