Random copolymerization of polythiophene for simultaneous enhancement of in-plane and out-of-plane charge transport for organic transistors and perovskite solar cells

High-performance conjugated polymers for electronic applications can be developed by modulating an appropriate chemical structure that optimizes their crystal characteristics and charge-transport behavior. Herein, we demonstrated the simultaneous enhancement of the in-plane and out-of-plane charge transport of polythiophenes by random polymerization. We synthesized a polythiophene polymer by varying the ratio of two different dialkyl-substituted bi-thiophene and triethylene glycol-substituted mono-thiophene units; this polymer exhibited weakened orientation preferences of polymer crystallite films, a denser packing, and a more homogeneous surface morphology in comparison with its homopolymer analogue. Furthermore, this optimized random polymer afforded an enhanced in-plane mobility of 7.72 cm² V⁻¹ second⁻¹, measured by field-effect transistor, and out-of-plane mobility of 8.86 × 10⁻⁴ cm² V⁻¹ second⁻¹, measured by space-charge-limited-current device. These are respectively 2.4 times and 10 times higher than the mobilities of the homopolymer (field-effect mobility = 3.25 cm² V⁻¹ second⁻¹ and space-charge-limited-current mobility = 8.73 × 10⁻⁵ cm² V⁻¹ second⁻¹). The enhanced charge transport in out-of-plane direction was also confirmed by fabricating perovskite solar cells using optimized polythiophene as a hole-transporting material, which exhibited a higher efficiency of nearly 16.2% than the device with homopolymer analogue (12.0%).