Thermally Induced Formation of HF₄TCNQ^– in F₄TCNQ-Doped Regioregular P3HT

Abstract

The prototypical system for understanding doping in solution-processed organic electronics has been poly(3-hexylthiophene) (P3HT) p-doped with 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ). Multiple charge transfer states, defined by the fraction of electron transfer to F4TCNQ, are known to coexist and are dependent on polymer molecular weight, crystallinity, and processing. Less well understood is the loss of conductivity after thermal annealing of these materials. Specifically, in thermoelectrics, F4TCNQ-doped regioregular (rr) P3HT exhibits significant conductivity losses at temperatures lower than other thiophene-based polymers. Through detailed spectroscopic investigation of progressively heated P3HT films coprocessed with F4TCNQ, we demonstrate that this diminished conductivity is due to formation of the non-chromophoric, weak dopant HF4TCNQ -. This species is likely formed through hydrogen abstraction from the alpha aliphatic carbon of the hexyl chain at the 3-position of thiophene rings of rr-P3HT. This reaction is eliminated for polymers with ethylene glycol-containing side chains, which retain conductivity at higher operating temperatures. In total, these results provide a critical materials design guideline for organic electronics.

p-Type molecular doping by charge transfer in halide perovskite

“…To achieve this goal, our understanding of the chemical and physical processes involved in the solution, during crystallization and in the final film must be improved. Additionally, we can expect to face impediments already encountered in organic semiconductor doping, including the formation of aggregates, [43][44][45][46][47][48] instability, 36,[49][50][51][52] or limited doping efficiency. [53][54][55] The latter issue is usually overcome with large concentrations of dopant (few %), which however can negatively impact the film morphology and could be particularly detrimental for crystalline halide perovskites, as observed in the present study for 5% doped MAPb 0.5 Sn 0.5 I 3 .…”

Effect of polar side chains on neutral and p-doped polythiophene

“…The reduced thermal stability with increased polar group content may be explained by recent work by Watts et al who observe thermally induced formation of HF4TCNQ anions in F4TCNQ doped P3HT via proton abstraction of the alpha proton on the side chain. 51 Within our system, the carbon spacer with an oxygen adjacent could promote this proton abstraction, leading to a larger decrease in conductivity with increasing polar content.…”

Microstructure and heteroatom dictate the doping mechanism and thermoelectric properties of poly(alkyl-chalcogenophenes)

“…P3RTe (Figure 1c) shows the highest degree of F4TCNQ doping, and the optical features at 410 nm, 760 nm and 875 nm are indicative of ICT being the predominant doping mechanism at play. 19,[27][28][29] While P3RSe also demonstrates these ICT features, peaks at 364 nm and 685 nm suggest the presence of CTC doping as well (Figure 1b). 19,[27][28][29] In contrast, P3RT demonstrates predominantly CTC doping peaks (Figure 1a).…”

“…19,[27][28][29] While P3RSe also demonstrates these ICT features, peaks at 364 nm and 685 nm suggest the presence of CTC doping as well (Figure 1b). 19,[27][28][29] In contrast, P3RT demonstrates predominantly CTC doping peaks (Figure 1a). To facilitate direct comparisons with prior literature, measurements were also made on P3HT films (Figure 1d), which demonstrate strong ICT features at 410 nm, 760 nm and 875 nm and a fair degree of F4TCNQ doping susceptibility, like P3RTe.…”