New poly(selenophene-thiophene) bearing π-conjugating spacers for polymer field-effect transistors and photovoltaic cells

Five new poly(selenophene-thiophene) polymers, including PSe4TV, PSe4TT, PSe4T2T, PSe4TTT, and P2Se4TTT, were synthesized via Stille coupling polymerization and used various π-conjugated spacers of vinylene (V), thiophene (T), bithiophene (2T), and thieno[3,2-b]thiophene (TT). Tuneable structural, optical, and electrochemical properties of polymers were observed because different π-conjugated building blocks in the main polymer chain affected the conformation of the polymer backbone. Details of polymer morphologies were systematically investigated using transmission electron microscopy (TEM), atomic force microscopy (AFM), and grazing incidence X-ray diffraction (GIXD). P2Se4TTT possessed the smallest energy band gap, densest molecular packing structure as well as fibrillar-like nanostructures among the studied polymers because the TT moiety could enhance the coplanarity of the polymer backbone and the inserted biselenophene reduced the hindrance of thiophene side chains. Hence, the highest field-effect mobility of this set of polymers was determined to be 0.27 cm2 V-1 s-1 from a P2Se4TTT-based field-effect transistor device with a high on/off ratio over 105. The power conversion efficiencies (PCEs) of the photovoltaic cells based on polymer/PCBM blends were in the range of 0.43%-1.18% for our synthesized polymers. Among them, the P2Se4TTT-based device could achieve the best PCE of 2.29% using a o-dichlorobenzene-1,8-diiodooctane (98:2 v/v) mixture as the processing solvent. The enhancement of active layer uniformity responded to the increment of efficiency. The abovementioned results demonstrated that the newly designed polymers could serve as promising candidates for optoelectronic device applications of polymers. © 2015 The Royal Society of Chemistry.