Correlating the structural evolution and charge transport in lightly doped semiconducting polymers of controlled regioregularity

Abstract

As an element of organic thin-film transistors, lightly doped conjugated polymers without high off-state electrical conductivities have received considerable attention. In this study, the impact of dopant loading in a benchmark semiconducting polymer, poly(3-hexyl thiophene) (P3HT), with controlled regioregularity was systemically investigated in view of structural evolution and change in optoelectronic properties, especially in relation to transistor performance. P3HTs doped with the p-type molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) exhibited distinct nanostructures as the molar dopant ratio (MDR, [F₄TCNQ]/[hexylthiophene]) increased. The nucleation and crystal growth of 97 % regioregular P3HT were greatly enhanced by blending with F₄TCNQ at low-level MDRs (≤ 0.015). At MDR = 0.015, the doped P3HT film exhibiting highly ordered conjugated nanodomains yielded a superior field-effect mobility of up to 0.10 cm² V⁻¹ s⁻¹, considerably higher than the undoped P3HT (∼10⁻³ cm² V⁻¹ s⁻¹). At relatively higher doping levels (MDR ≥ 0.05), the doped P3HT films showed large size aggregates with less long range order and resulted in poor transistor performances, while their electrical conductivities gradually increased as the MDR increased. Similarly, a regiorandom P3HT with 55 % regioregularity showed aggregated nanostructures as F₄TCNQ was loaded, but their electrical properties were far inferior to regioregular P3HTs in the overall MDR ranges (0.005 – 0.5). Also, a diketopyrrolopyrrole-based p-type donor-acceptor copolymer was found to exhibit the µ_FET improvement upon light doping, like the µ_FET trends in rr-P3HT and rra-P3HT. These findings would provide valuable insights into the development of various electronic applications with doped conjugated polymers.