Conductivity Boost by the Loading-Soaking Doping (LSD) Procedure: A Crystalline Structure-Preserving Strategy

Molecular doping plays a central role in tuning the electrical properties of conjugated polymers in organic optoelectronic devices. However, conventional solution doping methods often disrupt the crystalline order of polymers, limiting the charge carrier mobility and overall device performance. In this work, a crystalline structure-preserving doping strategy called loading-soaking doping (LSD) is reported, where dopants are selectively localized in amorphous regions, and the crystalline domains of the polymer remain intact. In this method, since the complex formed by F4TCNQ and cyclopentanone (CPO) cannot undergo charge transfer with the conjugated main chain of P3HT and does not have a π-π interaction with the thiophene ring, it is distributed in the amorphous region of P3HT to form a thin film, thereby effectively suppressing premature charge transfer and maintaining crystalline integrity. Subsequently, it was immersed in an F4TCNQ-acetonitrile (ACN) solution. Due to the solubility of cyclopentanone and acetonitrile and the high content of F4TCNQ in the immersion solution, only CPO in the amorphous region of the film diffused out. Then, the exposed F4TCNQ in the amorphous region of the thin film is preferentially doped into the crystal region with a higher HOMO energy level by driving F4TCNQ toward the crystalline interfaces. Unlike conventional solution-mixed doping (SMD), which significantly disrupts polymer ordering and reduces crystalline domain size from 104.3 to 52.4 Å, the LSD method enhances the domain size to 121.7 Å while preserving the relative degree of crystallinity and molecular orientation. As a result, LSD-doped P3HT films achieve a conductivity of 66.8 S cm^-1, which represents an 11-fold improvement over SMD without sacrificing structural integrity. This work demonstrates a promising route for optimizing the dopant distribution and charge transport in conjugated polymers for high-performance organic electronics.