Cyano-substitution on the end-capping group: facile access toward asymmetrical squaraine showing strong dipole–dipole interactions as a high performance small molecular organic solar cells material

Abstract

A novel asymmetrical squaraine derivative bearing a cyano-substituted indoline end-capping group, namely ASQ-5-CN, was designed and synthesized. In comparison with the noncyano-substituted ASQ-5, ASQ-5-CN showed an analogous absorption band-gap in the thin solid film state, but a 0.11 eV lowered HOMO energy level, which led to a higher V_oc. Density functional theory calculation results revealed that the dipole moment of ASQ-5-CN was over double that of ASQ-5. Hence the stronger dipole–dipole interactions of ASQ-5-CN might trigger more intense intermolecular packing in ASQ-5-CN, which should account for the higher hole mobility of ASQ-5-CN than that of ASQ-5 (4.00 × 10⁻⁵vs. 1.67 × 10⁻⁵ cm² V⁻¹ s⁻¹). Accordingly, solution-processed bulk-heterojunction small molecular organic solar cells using ASQ-5-CN as the electron donor exhibited a much higher PCE (5.24%) than that of the reference compound ASQ-5-based device (4.22%) due to its simultaneously enhanced V_oc (0.92 vs. 0.82 V), J_sc (11.38 vs. 10.94 mA cm⁻²) and FF (0.50 vs. 0.47). Additionally, the PCE of the ASQ-5-CN-based device could be improved to be as high as 6.11% when measured at 80 °C, which is the record PCE among the hitherto reported squaraine-based solution-processed bulk-heterojunction organic solar cells.