Polythiophenes with carboxylate side chains and vinylene linkers in main chain for polymer solar cells

Abstract Two polythiophenes with carboxylate side chains and vinylene linkers in conjugated backbones, i.e. , poly[5,5′-( E )-bis(2-hexyldecyl)-2,2′-(ethene-1,2-diyl)bis(thiophene-3-carboxylate)- alt -5,5′- 2,2′-bithiophene] ( PBT) and poly[5,5′-( E )-bis(2-hexyldecyl)-2,2′-(ethene-1,2-diyl) bis(thiophene-3-carboxylate)- alt -5,5′-thieno[3,2- b ]thiophene] ( PTT ), were synthesized and characterized. Compared to poly(3-hexylthiophene) (P3HT), both polymers displayed deeper HOMO energy levels caused by the introduction of the electron-withdrawing carboxylate group and lower band gaps and shorter π-π stacking distances (ca. 3.5 A) due to the incorporation of vinylene linkers in the conjugated backbones. Polymer PBT possesses greater self-organization ability and thereby more ordered intermolecular packing in solid state. Consequently, organic field-effect transistors (OFETs) based on PBT showed a hole mobility of 0.24 cm 2  V −1 s −1 , much higher than that (0.073 cm 2  V −1 s −1 ) of PTT. Polymer solar cells (PSCs) with the two polymers as donor materials and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as acceptor material were fabricated, and both PBT - and PTT -based PSCs showed open circuit voltage ( V oc ) above 0.8 V. Owing to the better hole transport property and more ordered nano-fibrillar film morphology, PBT -based PSCs exhibited a superior photovoltaic performance with power conversion efficiency (PCE) of 6.25%, higher than that of PTT -based PSCs (PCE = 5.53%). This study indicates that simultaneously introducing electron-withdrawing carboxylate in side chains and vinylene linkers in conjugated backbones could efficiently tune the energy levels and enhance the π-π interactions between conjugated backbones.