Cation exchange improves the efficiency and stability of the n-doping of π-conjugated polymers

Abstract

Currently, the development of n-type organic thermoelectric materials lags far behind that of their p-type counterparts due to the low efficiency and poor stability of n-doping. Herein, we report a cation exchange strategy to improve the efficiency and stability of n-doping of π-conjugated polymers, and enhance their thermoelectric performance. By treating the tetrakis(dimethylamino)ethylene (TDAE) doped poly(benzimidazobenzophenanthroline) (BBL) film with ionic liquid 1-benzyl-3-methylimidazolium tetrafluoroborate (BzMIM-BF₄), BzMIM⁺ cations spontaneously exchanged with TDAE·⁺ radical cations. This cation exchange facilitated the redox doping reaction between BBL and TDAE, and inhibited the dedoping reaction related with diffusion of TDAE species, thus significantly improving the efficacy and stability of n-doping. The conductivity was elevated by 58% to 12.3 S cm⁻¹ and could be maintained at 120 °C for 2 days under a nitrogen atmosphere. The cation exchange also enhanced charge transport in the film, improving the Seebeck coefficient by 80% and the power factor by 3.6 fold.