A Dichlorinated Dithienylethene-Diketopyrrolopyrrole-Based Copolymer with Pronounced P-N Crossover: Evidence for Anionic Seebeck Contribution

Nominally p-doped conductive polymers that display n-type Seebeck coefficients are intriguing because of the possibly high n-type power factors and the complex p-to-n switching mechanism. This study reports a diketopyrrolopyrrole (DPP strong acceptor)-based polymer with a dichlorinated dithienylethene (CITVT) weak donor unit sequentially doped with solutions of varied FeCl3 concentrations as a p-n switchable thermoelectric. Polymer films doped with dilute and moderate FeCl3 solution concentrations (up to 40 mmol/L) exhibit positive (ca. 100 mu V/K) and negative (ca. tens of mu/K) Seebeck coefficients and high electrical conductivities of 80 and 130 S respectively. After rinsing, the conductivity increases to 170 S cm(-1) for both p-type and n-type, with Seebeck coefficients slightly more positive/less negative and Seebeck voltage time evolution consistent with their being completely electronic. The power factor of 15 mu W/m K-2 was unusually high for a p-to-n inversion material. However, polymer films doped with a high-concentration FeCl3 solution (50 mmol/L) without rinsing show a high composite Seebeck coefficient of -2400 mu V K-1. We propose that this high Seebeck coefficient arises from a contribution of Cl- ion thermodiffusion in a near-surface layer. Supporting evidence for this is an analogous, even higher negative Seebeck coefficient obtained from the much less conjugated/conductive polymer poly(p-phenylene oxide) (PPO) on which FeCl3 was applied to form a layer that was removed on rinsing.