Chloride transport in conductive polymer films for an n-type thermoelectric platform

Cl- transport in a conductive polymer (CP) film was demonstrated for n-type thermoelectric (TE) harvesting. CPs have been considered as an important group of p-type TE materials due to their high TE functionalities plus simple processing steps for a device. In particular, recently emerging p-type ionic CPs can be unique candidates due to their high Seebeck coefficients (S). However, n-type materials based on CPs suffer from very poor TE functionalities, and n-type ionic TE CP materials have not been realized so far. Here, we report the first example of n-type mixed ionic-electronic CP composite (NPC) films. The p-type TE properties of the PEDOT:PSS films was drastically converted into the n-type TE properties in the presence of CuCl2 through metal binding with polymers, thus resulting in the formation of Cl- channels. Fluorescence imaging using Cl- as an indicator and time-of-flight secondary ion mass spectrometry mapping confirmed that Cl- is transported in the film from the hot to the cold electrode. In addition, electron spin resonance spectroscopy indicated the major spin density transition from a polaron of PEDOT:PSS to the polymer-bound unpaired electron spin of Cu ions by increasing the CuCl2 content to prove the binding of metal ions with the PSS unit of the polymer chain. These mixed ionicelectronic NPC films recorded a surprisingly high negative S value of over -18.2 mV K-1 and a power factor of 1.7 mW m(-1) K-2 at 80% RH with 40 wt% of CuCl2. Taking advantage of this high performance, the CP films were integrated with a p-type CP film as a flexible module-type TE harvester with 10 pairs of p-n legs on CNT electrodes. This TE harvester showed a thermovoltage of 1.55 V for a low temperature gradient of 4.5 K. This high anion transport in a TE CP hydrogel film might be a useful solution for environmentally benign and body-worn electronics. Broader context Thermoelectrics harvest electrical energy directly from waste heat through electron and ion transport between the temperature gradient. While TE materials have been explored to provide increased TE output by optimizing molecular and nanostructures, the output from the currently available TE materials is still low because of the difficulties in controlling the carrier transport. In particular, a high TE performance for flexible and low-cost polymeric TE materials is a challenging issue in wearable thermoelectrics. In this work, we tackled this challenge by optimizing the transport of chloride ions as n-type carriers in PEDOT:PSS films, which further benefited from the formation of copper ion-complexed polymer channels. With such a new n-type TE mechanism, the p-type TE property of the PEDOT:PSS film was drastically converted into an n-type TE property. The Seebeck coefficient and power factor of the n-type film were over 18 mV K1 and 1.7 mW m1 K-2, respectively, which provided flexible module-type thermoelectrics, generating 1.55 V for a small DT value of 4.5 K. This high anion transport in a polymer film can be useful to power wearable electronics and inspire new strategies for carrier control for mixed ionic-electronic devices.