Unraveling the correlation between reduced thickness and enhanced electrical conductivity in HNO3-treated PEDOT:PSS ultrathin nanofilms

Although the research of organic polymer thermoelectric materials has witnessed significant progress in recent decade, the in-depth mechanism at molecular level still remains unclear for nanoscale films in thickness. Here, we report a strategy to dramatically enhance the electrical conductivity by reducing the thickness for ultrathin nanoscale films of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). First, PEDOT:PSS nanofilms were prepared by spin coating and subsequent HNO3 post-treatment. We found that the reduction of film thickness resulted in a remarkably improved electrical conductivity. When the thickness reduced from 23 nm to 15 nm, the electrical conductivity enhanced significantly from 1772 S cm- 1 to 3059 S cm(-1). Moreover, the underlying mechanism was studied. With the decrease of film thickness, the content of the insulating PSSH greatly reduced, and the oxidation level of PEDOT chains increased. In addition, a fibrous morphology with large conductive micro-domains occurred, and the degree of crystallinity for PEDOT enhanced. The present study helps to prepare high-performance polymer thermoelectric materials and elucidate the carrier transport mechanism for nanoscale films of conducting polymers.