Elucidating the impact of N-arylanilino substituents of squaraines on their photovoltaic performances

Abstract Squaraines have attracted increasing research attention for organic photovoltaic applications because of their facile and low-cost synthesis, and intense and broad absorption in the visible and near-infrared (Vis-NIR) regions. Herein, three new squaraines, N -phenylanilino substituted SQ-EP , N -2-naphthylanilino substituted SQ-EN , and N -fluorenylanilino substituted SQ-EF , were developed for improving the hole mobility of aniline-based squaraines. The substitution effect of N -arylanilino groups on the optoelectronic properties of squaraines was investigated. These squaraines exhibited similar absorbance spectra, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, and hole mobilities in their neat films. However, compared to their neat films, the hole mobility of the SQ-EP -based BHJ blend film showed the smallest decrease when blending with PC 71 BM ([6,6]-phenyl C 71 butyric acid methyl ester). Besides, morphological studies have shown that a phase separation of 20–30 nm can only be observed in the SQ-EP /PC 71 BM blend film. Consequently, a solution-processed bulk-heterojunction (BHJ) organic photovoltaic (OPV) device fabricated with the as-cast SQ-EP /PC 71 BM blend film exhibited obviously higher power conversion efficiency (PCE: 5.4%) than those of the SQ-EN /PC 71 BM (PCE: 4.3%) or SQ-EF /PC 71 BM systems (PCE: 2.8%). This is one of the highest recorded PCE values in aniline-based squaraines single-junction BHJ-OPV devices. This much-enhanced PCE can be attributed to the significant increases in short-circuit current density ( J sc ) and fill factor (FF) of the SQ-EP /PC 71 BM-based OPV device.