Engineering Larger Grains in MAPbI₃ Solar Cells via Covalent Organic Framework (COF)-Assisted Crystallization:...

Methylammonium lead iodide (MAPbI₃) is a benchmark absorber for low-cost, high-efficiency perovskite solar cells (PSCs), but its practical deployment is constrained by small grain sizes, high defect densities, and moisture-sensitive surfaces that limit both efficiency and stability. Here, we demonstrate that covalent organic frameworks (COFs) built from commercially available building blocks provide a powerful, multifunctional route to overcome these limitations by simultaneously regulating crystallization, passivating defects, and tuning surface properties in MAPbI₃ thin films. Three hydrazone-linked and β-ketoenamine frameworks, denoted DMTH-COF, Boc-PDA-COF and Tp-COF, were prepared by condensing 2,5-dimethoxyterephthalohydrazide (DMTH), Boc-PDA-phenylenediamine (Boc-PDA), and 1,3,5-triformylphloroglucinol (Tp), respectively, and were incorporated at low loading into the MAPbI₃ precursor solution. X-ray diffraction and FE-SEM reveal that all three additives preserve the tetragonal perovskite phase while enlarging the grain size from a few hundred nanometers in pristine films to about 1 μm for DMTH-modified layers, accompanied by narrowed diffraction peaks and reduced microstrain. Raman and FTIR spectroscopy show COF-induced modifications of Pb-I and CH₃NH₃⁺ vibrational modes, indicating specific coordination and hydrogen-bonding interactions that stabilize the lattice and reduce trap density. UV-Vis spectra confirm strong visible-light absorption at an unchanged optical bandgap of 1.56 eV across all films, while steady-state PL and EQE measurements evidence enhanced radiative efficiency and improved photon-to-current conversion, particularly at longer wavelengths. X-ray photoelectron spectroscopy confirms that the framework is retained within the film after annealing, the modified absorber showing a N 1s component at 399.7 eV that is absent from the pristine reference, while the Pb 4f and I 3d core levels shift in parallel with their spin-orbit splittings preserved and no metallic lead detected. These material-level improvements translate into substantial device gains in n-i-p PSCs (FTO/c-TiO₂/m-TiO₂/COF:MAPbI₃/Spiro-OMeTAD/Au). Relative to pristine MAPbI₃ cells (PCE ≈ 14.7%), COF-modified devices exhibit higher Voc, Jsc, and FF, with DMTH-COF-modified cells achieving Voc = 1.183 V, Jsc = 22.98 mA cm^-2, FF = 76.46%, and PCE = 20.78%, alongside reduced series resistance, increased shunt resistance, and improved device-to-device reproducibility. Water-contact-angle and EDX analyses confirm that COF addition increases surface hydrophobicity and preserves near-ideal Pb:I stoichiometry, supporting enhanced environmental stability. This study introduces bulk COF doping as a simple, scalable strategy to engineer MAPbI₃ microstructure, energetics, and surface chemistry in a single processing step, and identifies DMTH-COF as the most effective of the three frameworks. More broadly, it positions COF-perovskite hybrids as a tunable platform for high-efficiency, durable perovskite optoelectronics.