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

Qi Wang, Xin Dong, Mu He, Miaomiao Li, Hongkun Tian, Jun Liu, Yanhou Geng

Index: 10.1016/j.polymer.2018.02.035

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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 Å) 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 cm2 V−1s−1, much higher than that (0.073 cm2 V−1s−1) of PTT. Polymer solar cells (PSCs) with the two polymers as donor materials and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as acceptor material were fabricated, and both PBT- and PTT-based PSCs showed open circuit voltage (Voc) 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.