The fabrication of electronic devices based on organic materials, known as’ printed electronics’, is an emerging technology due to its unprecedented advantages involving flexibility, light weight, and portability, which will ultimately lead to future ubiquitous applications.[1] The solution processability of semiconducting and metallic polymers enables the cost-effective fabrication of optoelectronic devices via high-throughput printing techniques.[2] These techniques require high-performance flexible and transparent electrodes (FTEs) fabricated on plastic substrates, but currently, they depend on indium tin oxide (ITO) coated on plastic substrates. However, its intrinsic mechanical brittleness and inferior physical properties arising from lowtemperature (T) processing below the melting T of the plastic substrates (ie, typically below 150 C) have increased the demand for alternative FTE materials.[3] Conducting polymers (CPs) have been considered a promising candidate for FTEs due to their mechanical flexibility and solution processability. The high transparency of CPs originates from the charge carrier density (n) of approximately 10 21 cm− 3 because both the reflectance and absorption are confined in the IR region below the plasma frequency (ωP, ωP 2= 4 πe2n/m* where m* is the effective mass of the charge carrier) at approximately h–ωP≈ 1 eV.[2] A complex of poly (3, 4-ethylenedioxythiophene)(PEDOT) and poly (4-styrenesulfonate)(PSS), in which PSS acts as both a counter-ion and a soluble template for PEDOT, is a successful CP due to its high electrical conductivity (σdc) and excellent transparency in the visible range.[4] The conducting films, which were coated from PEDOT: PSS solution in an aqueous dispersion, consist of hydrophobic and conducting PEDOT-rich grains encapsulated by hydrophilic and insulating PSS-rich shells.[5] These morphological features involve an excess amount of PSS as well as low chain alignment, resulting in a low σdc of approximately 1 S cm− 1. Over the past decade, pre-and/or post-treatment with various organic solvents, surfactants, salts, and acids have been found to enhance the σdc of PEDOT: PSS by two to three orders of magnitude.[6–8] Recently, the high σdc (≈ 3065 S cm− 1) was achieved using a treatment of dropping a 1.0 MH 2SO 4 solution onto the PEDOT: PSS films.[8] Although numerous studies suggested that the σdc enhancement could be attributed to morphological changes in the PEDOT: PSS complex, such as grain growth, polymer chain expansion, and phase separation, a clear understanding of the mechanism of the σdc enhancement is still required for both the basic material studies on CPs and developing high-performance FTEs.[6–8]

Herein, we report the solution-processed crystalline formation in PEDOT: PSS via H2so 4 post-treatment. By rigorously controlling the post-treatment conditions (ie, the H2so 4 concentration, treatment T, and processing details), we obtained insight into how the H2so 4 solution processing induces crystallinity in the PEDOT: PSS complex. The concentrated H2so 4 treatment induced a significant structural rearrangement in the PEDOT: PSS with the removal of PSS and led to the formation of crystallized nanofibrils via a charge-separated transition mechanism. The σdc values increased with the improvement in the crystallinity, resulting in σdc≈ 4380 S cm− 1, which is the highest value for this system and nearly comparable to that of ITO.