Engineering the microstructure and defect landscape of MAPbI3 perovskite solar cells using nitrogen-rich supramolecular Metal–Organic cages

Organic–inorganic lead halide perovskites such as methylammonium lead iodide (MAPbI 3 ) have excellent optoelectronic properties, but their performance and lifetime are still limited by defects, grain-boundary recombination, and poor stability. In this work, we use supramolecular metal–organic cages (MOCs) as a new class of multifunctional additives to control film formation and defect passivation in MAPbI 3 perovskite solar cells (PSCs). Three nitrogen-rich cages with different metal centers and ligands, namely Ni 8 L 12 –H 3 BTC (H 3 BTC = 1,3,5-benzenetricarboxylic acid), Pd 2 L 4 –4,4′-bipyridine, and Rh 6 L 9 –TPT (TPT = 2,4,6-tris(2-pyridyl)-1,3,5-triazine), are added at low concentration to the MAPbI 3 precursor and processed into mesoscopic TiO 2 -based n–i–p devices by a standard spin-coating route. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) show that all MOC-modified films retain the tetragonal MAPbI 3 phase but exhibit narrower diffraction peaks, reduced microstrain, larger grains, and smoother, nearly pinhole-free surfaces compared with the pristine films. Energy-dispersive X-ray spectroscopy (EDX) mapping confirms a uniform Pb/I distribution close to the ideal stoichiometry, while Fourier-transform infrared (FTIR) and Raman spectra indicate that the characteristic MAPbI 3 vibrational modes are preserved, with additional ligand-related bands consistent with the presence of the cages in the film. Ultraviolet–visible (UV–Vis) absorption, photoluminescence (PL), and external quantum efficiency (EQE) measurements reveal stronger optical absorption, and reduced non-radiative recombination, consistent with effective passivation of bulk and interfacial trap states. These microstructural and optical improvements translate into clear device-level gains: relative to the control device (power conversion efficiency, PCE = 16.18%), all MOC-modified PSCs show higher open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF), together with lower series resistance and higher shunt resistance. The best performance is obtained with the Rh 6 L 9 –TPT cage, yielding a PCE of 20.86% with Voc = 1.119 V, Jsc = 23.98 mA cm −2 , and FF = 77.74%. To the best of our knowledge, the use of discrete, soluble supramolecular cages as bulk additives to simultaneously modulate crystallization, relieve microstrain, and passivate grain-boundary and interfacial defects in MAPbI 3 has not previously been reported. Our results therefore identify nitrogen-rich MOCs as a distinct additive platform for engineering high-quality MAPbI 3 films and achieving more efficient and operationally stable perovskite solar cells.