High thermoelectric power factor from multilayer solution-processed organic films

Abstract

We investigate the suitability of the “sequential doping” method of organic semiconductors for thermoelectric applications. The method consists of depositing a dopant (F₄TCNQ) containing solution on a previously cast semiconductor (P3HT) thin film to achieve high conductivity, while preserving the morphology. For very thin films (∼25 nm), we achieve a high power factor around 8 μW/mK⁻² with a conductivity over 500 S/m. For the increasing film thickness, conductivity and power factor show a decreasing trend, which we attribute to the inability to dope the deeper parts of the film. Since thick films are required to extract significant power from thermoelectric generators, we developed a simple additive technique that allows the deposition of an arbitrary number of layers without significant loss in conductivity or power factor that, for 5 subsequent layers, remain at ∼300 S/m and ∼5 μW/mK⁻², respectively, whereas the power output increases almost one order of magnitude as compared to a single layer. The efficient doping in multilayers is further confirmed by an increased intensity of (bi)polaronic features in the UV-Vis spectra.