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 ionic–electronic 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.