Size Dependence of Electrical Conductivity and Thermoelectric Enhancements in Spin-Coated PEDOT:PSS Single and Multiple Layers

This work reveals that the electrical conductivity s of a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) film can be significantly increased by spin-coating multiple thin layers onto a substrate. Generally, s can be improved by more than fourfold for multiple layers, as compared to a single thicker one. A gradual enhancement is observed for pristine PEDOT: PSS films (up to 2.10 +/- 0.26 S cm(-1) for five-layered films), while a plateau in s at around 200 S cm(-1) is reached after only three layers, when using a PEDOT: PSS solution with 5 vol% dimethyl sulfoxide. By contrast, only a small change in s is observed for single layers of varying thickness. Accordingly, the thermoelectric power factor is also increased by up to 3.4 times for the multiple layers. Based on atomic force microscopy, X-ray photoelectron spectroscopy, UV-vis, and Raman spectroscopy measurements, two mechanisms are also proposed, involving an increase in percolation by inclusion of smaller grains within the existing ones, respectively, a reorganization of the PEDOT: PSS chains. These findings represent a direct strategy for enhancing the thermoelectric performance of conductive polymer films without additional reagents, while the mechanistic insights explain existing literature results.