Tuning the Seebeck effect in C60-based hybrid thermoelectric devices through temperature-dependent surface polarization and thermally-modulated interface dipoles.

Abstract

Fullerene (C60) is an important n-type organic semiconductor with high electron mobility and low thermal conductivity. In this work, we report the experimental results on the tunable Seebeck effect of C60 hybrid thin-film devices by adopting different oxide layers. After inserting n-type high-dielectric constant titanium oxide (TiOx) and zinc oxide (ZnO) layers, we observed a significantly enhanced n-type Seebeck effect in oxide/C60 hybrid devices with Seebeck coefficients of -5.8 mV K-1 for TiOx/C60 and -2.08 mV K-1 for ZnO/C60 devices at 100 °C, compared with the value of -400 μV K-1 for the pristine C60 device. However, when a p-type nickel oxide (NiO) layer is inserted, the C60 hybrid devices show a p-type to n-type Seebeck effect transition when the temperature increases. The remarkable Seebeck effect and change in Seebeck coefficient in different oxide/C60 hybrid devices can be attributed to two reasons: the temperature-dependent surface polarization difference and thermally-dependent interface dipoles. Firstly, the surface polarization difference due to temperature-dependent electron-phonon coupling can be enhanced by inserting an oxide layer and functions as an additional driving force for the Seebeck effect development. Secondly, thermally-dependent interface dipoles formed at the electrode/oxide interface play an important role in modifying the density of interface states and affecting the charge diffusion in hybrid devices. The surface polarization difference and interface dipoles function in the same direction in hybrid devices with TiOx and ZnO dielectric layers, leading to enhanced n-type Seebeck effect, while the surface polarization difference and interface dipoles generate the opposite impact on electron diffusion in ITO/NiO/C60/Al, leading to a p-type to n-type transition in the Seebeck effect. Therefore, inserting different oxide layers could effectively modulate the Seebeck effect of C60-based hybrid devices through the surface polarization difference and thermally-dependent interface dipoles, which represents an effective approach to tune the vertical Seebeck effect in organic functional devices.