Highly Efficient n-Doping via Proton Abstraction of an Acceptor1-Acceptor2 Alternating Copolymer toward Thermoelectric Applications

Abstract

Electron transporting (n-type) polymers are the coveted complementary counterpart to more thoroughly studied hole transporting (p-type) semiconducting polymers. Besides intrinsic stability issues of the doped form of n-type polymer toward ubiquitous oxidizing agents (H2O and O2), the choice of suitable n-dopants and underlying mechanism of doping is an open research field. Using a low LUMO, n-type unipolar acceptor1-acceptor2 copolymer poly(DPP-TPD) in conjunction with bulk n-doping using Cs2CO3 these issues can be addressed. A solid-state acid-base interaction between polymer and basic carbonate increases the backbone electron density by deprotonation of the thiophene comonomer while forming bicarbonate, as revealed by NMR and optical spectroscopy. Comparable to N-DMBI hydride/electron transfer, Cs2CO3 proton abstraction doping shifts the poly(DPP-TPD) work function toward the LUMO. Thereby, the anionic doped state is resilient against O2 but is susceptible toward H2O. Based on GIWAXS, Cs2CO3 is mostly incorporated into the amorphous regions of poly(DPP-TPD) with the help of hydrophilic side chains and has minor impact on the short-range order of the polymer. Cs2CO3 proton abstraction doping and the acceptor1-acceptor2 copolymer architecture creates a synergistic n-doped system with promising properties for thermoelectric energy conversion, as evidenced by a remarkable power factor of (5.59 ± 0.39) × µW m−1 K−2.