Abstract
Two unsymmetrical squaraines (USQs) with different donor (D) subunits as photovoltaic materials, namely USQ-11 and USQ-12, were designed and synthesized to investigate the effect of different D subunits on the optoelectronic properties of USQs for the first time. The two USQs compounds were characterized for optical, electrochemical, quantum chemical and optoelectronic properties. By changing the two different D subunits attached to the squaric acid core from 2,3,3-trimethylindolenine to 2-methylbenzothiazole, the HOMO energy levels could be tuned with a stepping of 0.07 eV, and quite different solid state aggregations (H- or J-aggregation) were observed in the thin film by UV-Vis absorption spectra, which were attributed to their distinct steric effects and dipole moments. Solution-processed bulk-heterojunction small molecule organic solar cells fabricated with the USQ-11/PC71BM (1:5, wt%) exhibited extremely higher PCE (4.27%) than that of the USQ-12/PC71BM (2.78%). The much enhanced PCE should be attributed to the simultaneously improved Voc, Jsc and FF.
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