Nanoscale Characterization and Unexpected Photovoltaic Behavior of Low Band Gap Sulfur-Overrich-Thiophene/Benzothiadiazole Decamers and Polymers

Highly soluble air-stable conjugated decamers and polymers (compounds 1-6) with alternating "sulfur-overrich" bis(3,4?-S-alkyl)-2,2?-bithiophenes as the donor units and 2,1,3-benzothiadiazoles as the acceptor units were designed and expediently synthesized with the aid of microwave and ultrasound enabling technologies. Solid-state cyclovoltammetry showed that 1-6 had oxidation and reduction potentials in the range 0.6-0.9 V and -1/-1.2 V (vs SCE), respectively, with energy gaps below 2 eV. The electronic properties of spin-coated films of pure 1-6 were investigated with nanoscale resolution by Kelvin probe force microscopy (KPFM). KPFM measurements showed that charge generation and separation were obtained for all films under illumination. Consequently, 1-6 were tested on single-material organic solar cells (SMOCs). In agreement with KPFM results, photovoltaic behavior was observed for all compounds with power conversion efficiencies in line with the best results obtained so far for the few donor-acceptor molecules already shown to perform in single-component solar cells. To our knowledge, this is the first time in which thiophene-benzothiadiazole co-oligomers and copolymers are shown to be photoactive materials in SMOCs.