Abstract

The integration of two-dimensional covalent organic frameworks (2D COFs) as thin films on transparent conductive substrates remains a major challenge due to limited control over crystallization, thickness, and film continuity. Herein, we report a surface-initiated solvothermal polymerization strategy for the controlled synthesis of ultrathin TpBpy covalent organic framework (COF) films on indium tin oxide (ITO) substrates. The approach combines self-assembled monolayer (SAM) functionalization with a grafting step to confine nucleation and enable directional growth at the solid–liquid interface. The resulting films exhibit homogeneous coverage, controlled thickness (?20 nm). A comprehensive characterization by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), atomic force microscopy (AFM), X-ray reflectivity (XRR), wide-angle X-ray scattering (WAXS) and X-ray photoelectron spectroscopy (XPS) confirms the structural integrity and compositional uniformity of the films. Nanomechanical AFM analysis revealed that the TpBpy COF films possess good mechanical robustness (?1 GPa). Conductive-AFM and electrochemical impedance spectroscopy (EIS) demonstrated the semiconducting nature of the films, as well as capacitive behavior. Time-resolved studies elucidate a fiber-to-film morphological transition, consistent with a self-assembly mechanism driven by ?–? stacking and hydrogen bonding. These findings elucidate the growth pathway and highlight the importance of surface engineering and reaction time in achieving uniform, defect-free COF films. This work provides a versatile route for the fabrication of high-quality COF thin films on transparent conductive substrates, advancing their integration into optoelectronic and electrochemical devices. © 2025 Elsevier Ltd