PEDOT:PSS Films with Greatly Enhanced Conductivity via Nitric Acid Treatment at Room Temperature and Their Application as Pt/TCO‐Free Counter Electrodes in Dye‐Sensitized Solar Cells

A room‐temperature HNO3 treatment method is developed that significantly enhances the conductivity of a poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) film, which is one of the most promising transparent conducting materials. The PEDOT:PSS film treated with HNO3 exhibits a conductivity as high as 4100 S cm−1, which is the highest value among reported conductivities achieved by treatments at room temperature. The mechanism of this conductivity enhancement is elucidated by means of electrical, optical, structural, and compositional characterizations of the PEDOT:PSS films. The HNO3 treatment induces phase separation and crystallization in the (100) and (200) directions of PEDOT:PSS through selective removal of the PSS domains. This rearrangement significantly increases the conductivity through an enhancement of the inter‐chain interaction between the conducting polymers. Dye‐sensitized solar cells (DSSCs) that adopt these films as counter electrodes are developed as an example of practical applications. Using the HNO3‐treated PEDOT:PSS films, a power‐conversion efficiency of 8.59% is achieved for Pt/fluorine‐doped tin oxide (FTO)‐free DSSCs. This study demonstrates that a facile HNO3 treatment at room temperature is one of the most promising methods for obtaining highly conductive and transparent PEDOT:PSS films, and it facilitates the commercial use of PEDOT:PSS in next‐generation transparent electrodes.