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

Abstract

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.

Computational microscopy study of the granular structure and pH dependence of PEDOT:PSS

“…12,[28][29][30][31][32][33] Changing the pH-value through acidic and basic treatment modifies the optical properties, film morphology (roughness), thermoelectric properties and electrical conductivity. [22][23][24][25]27,[34][35][36] The suggested mechanism for electrical conductivity enhancement by acidic treatment is the formation of PSS-H. For example, the conductivity enhancement was related to the transformation of non-conductive PSS chains into PSS-H form, 34 phase separation between PEDOT and PSS-H, 37 phase separation between PEDOT and PSS and enhanced p-p stacking between PEDOT chains 36 and removing PSS domains and enhanced inter-chain coupling. 35 X-ray photoelectron spectroscopy also shows the change in the chemical environment of PSS and reducing the PSS weight ratio after acidic treatment.…”

“…[22][23][24][25]27,[34][35][36] The suggested mechanism for electrical conductivity enhancement by acidic treatment is the formation of PSS-H. For example, the conductivity enhancement was related to the transformation of non-conductive PSS chains into PSS-H form, 34 phase separation between PEDOT and PSS-H, 37 phase separation between PEDOT and PSS and enhanced p-p stacking between PEDOT chains 36 and removing PSS domains and enhanced inter-chain coupling. 35 X-ray photoelectron spectroscopy also shows the change in the chemical environment of PSS and reducing the PSS weight ratio after acidic treatment. 22,[38][39][40][41] The morphology of PEDOT:PSS shows a very complex character and depends on various factors as outlined above.…”

“…They include (a) formation of small crystallites consisting of several p-p stacking PEDOT chains, 15,17,[42][43][44][45][46] and (b) granular structure featuring PEDOT-rich and PSS-rich regions (micellar) of characteristic sizes of 20-30 nm. [13][14][15][26][27][28]35,[46][47][48][49] Based on the available experimental observations, there are many morphological pictures for PEDOT:PSS proposed in various studies. 23,25,27,40,48,[50][51][52][53][54][55][56][57][58][59][60][61] Some representative pictures are summarized in Fig.…”

Heme protein-mediated synthesis of PEDOT:PSS: enhancing conductivity by inhibiting heme degradation

“…25 This synthetic route produces a pristine polymer with relatively low conductivity (<1 S cm À1 ), 1,25 that is then doped for higher-conductivity applications. [26][27][28] Alternatively PEDOT:PSS can be synthesized with biomolecules. [29][30][31] Other conductive polymers such as polyaniline 32,33 and polyphenols [34][35][36] have also been polymerized using biomolecular oxidants.…”

“…54,55 Indeed lms of PEDOT:PSS doped by acid can achieve conductivities as high as 4000 S cm À1 . 27,56 While lms treated with organic solvents reach 1100 S cm À1 . 28 For a comprehensive discussion of PEDOT:PSS conductivity enhancements see the review by Shi, et al 26 Importantly, ABTS and EDTA do not alter PEDOT:PSS aer polymerization, rather they optimize the oxidant used for polymerization.…”