Oligothiophene dendron-decorated squaraine dyes: Synthesis, thin film formation, and performance in organic solar cells

Squaraine (SQ) dye-based organic semiconductor hybrids 6T-SQ and 18T-SQ functionalized with oligothiophene dendrons were synthesized via Suzuki-Miyaura coupling. The electronic coupling between the oligothiophene dendrons and the squaraine core was rather weak, as suggested from UV-vis spectra, cyclic voltammetry measurements and molecular modeling. Thin films of pure SQ were characterized by a pronounced solvent-and heat-induced crystallization tendency. The dendrons substantially hindered the squaraine core crystallization, and 18T-SQ films remained amorphous after annealing or storage for several weeks. PCBM disrupted dye crystallization in blends, and smooth and stable films could be coated. Heat treatment of blended films induced dewetting for SQ:PCBM and 6T-SQ:PCBM, but 18T-SQ:PCBM remained again stable. These morphological film features could consistently explain the performance of dye-fullerene solar cells. The best performance (eta similar to 1.5%) was obtained for simple bilayer 6T-SQ:C-60 or 18T-SQ:C-60 cells without annealing. Our results demonstrate that the attachment of decorating moieties to a central light-absorbing core unit in molecular push-pull systems can be used to adjust the optoelectronic and morphological film properties of small molecular semiconductors with a strong tendency towards crystallization. (C) 2012 Elsevier B. V. All rights reserved.