Optically tunable Seebeck effect from intramolecular proton-transfer materials in organic vertical thin-film thermoelectric device

Abstract This paper reports Seebeck effects from optically-induced intramolecular proton-transfer HPI-Cbz molecules based on vertical electrode/organic film/electrode thin-film devices. We observed large Seebeck coefficients of 428 μV/K and 390 μV/K from HPI-Cbz based thin-film devices at 60°C when proton-transfer was induced by the photoexcitation of a 325 nm laser with an intensity of 12 mW/cm² and 6 mW/cm² respectively. Under dark condition without proton transfer occurring, the Seebeck coefficient was measured to be 342 μV/K at 60°C. The Seebeck coefficient enhancement by the induced intramolecular charge transfer can be attributed to the enhanced polarization difference between high- and low-temperature surface due to the stronger electron-phonon coupling followed with the proton-transfer in HPI-Cbz under photoexcitation, and the strength of electron-phonon coupling is proportional to the photoexcitation intensity. The enhanced temperature-dependent electrical polarization between the high and low-temperature surfaces acts as an additional driving force to diffuse the majority charge carriers for the development of a large Seebeck effect. Therefore, using intramolecular proton-transfer presents an effective approach of enhancing Seebeck effect in organic materials.