Length-scale effects on electrical and thermal transport in polyaniline thin films

Conducting polymers are prevalently used in organic and flexible electronics and energy conversion devices. While their thermo-physical properties are well characterized for different processing and doping conditions, very little is known about the effect of length-scale, especially their thermal conductivity and thermal contact resistance of their interfaces with substrates. In this paper we develop an analytical model to capture the heat transfer in ultra-thin polyaniline thin films and their interfaces with other substrate materials. The model is demonstrated on 20-1000 nm thick 50% camphor-sulphonic acid doped polyaniline films patterned on silicon substrate using photolithography and reactive ion etching. The four-probe based technique allows simultaneous electrical and thermal characterization. Experimental results show a 500% and 200% increase in the in-plane thermal conductivity and electrical conductivity, respectively as the film thickness is increased from 20 nm to 1000 nm. These findings suggest up to 300% improvement in thermoelectric performance for 20 nm thick films when compared to the bulk. Such strong length-scale effects are observed to decay rapidly after 100 nm film thickness and can be attributed to the enhanced phonon scattering at the surface and interface boundaries at length-scales comparable to phonon mean free paths.