Over the past decades, three-dimensional hybrid perovskites (HPs) have gained prominence in optoelectronics, extending beyond traditional photovoltaic applications. Their versatile potential as light-emitting diodes, photocatalysts, and photodetectors positions them as key candidates for efficient, low-cost, and flexible photonic devices. Among HPs, methylammonium lead iodide (MAPI) has continued to attract interest due to its broad application potential, where defect engineering plays a crucial role in tuning its intrinsic properties. Notably, the synthesis method significantly influences the defect landscape and, consequently, the performance of MAPI. 

In this work, we present a solvent-free mechanosynthesis approach to produce MAPI [1] in large quantities for electromagnetic wave absorption (EMWA) studies—a very seldom investigated application for HPs [2]-[4]. Our results reveal that 4 h-ball-milled MAPI (MAPI4h) powders with a powder size lower than 20 µm exhibits a significant improvement of dielectric loss at 11.4 GHz within the X-band frequency range (8–12 GHz) compared to powders ball-milled for only 30 minutes (MAPI30). The enhanced performance of MAPI4h is attributed to improved dipole polarization relaxation and reduced particle sizes.

Structural characterizations of MAPI4h confirm the preservation of the expected I4/mcm crystalline structure of MAPI with no apparent bulk defects but revealed smaller grain and crystallite sizes and increased strains in comparison with MAPI30. A fractal microstructure has been evidenced with aggregated grains constituted of nanograins and these aggregates form agglomerate of aggregates with different sizes. In addition, an orientation of nanograins inside grains of MAPI30 is evidenced when in MAPI4h, this oriented nanograins aggregation could be extended between the adjacent grains due to prolonged milling and localized temperature increases. Detailed defect analysis through static and time-resolved photoluminescence, Urbach energy calculations, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy-energy-dispersive X-ray spectroscopy (HRTEM-EDXS), and positron annihilation lifetime spectroscopy (PALS) highlights significant surface modifications with a raise of surface vacancy-type (MA⁺ and I^- vacancies) defect level in MAPI4h powders (<20µm). PALS, in particular, identifies the presence of specific open defects in MAPI that differ from those earlier observed in MAPI layers or single crystals prepared by solution or dry processes. A density peak in the defect distribution is located at a mean depth of about 55.9 nm from the surface of the first layer of monocrystalline grains. Its existence suggests that the defect population results from competitive reaction of generation and recombination where the grain surface plays a role of sink. The density peak tends to increase with longer grinding time.

These surface vacancies in MAPI4h behave as dielectric polarization centres by stabilizing methylammonium dipoles, which results in enhanced dipole polarization relaxations. The heterogeneous interface between the bulk MAPI and the modified surface could largely enhanced the interfacial polarization of EMWA material, improving dielectric loss [5]. Additionally, the improved dispersion of sub-20 µm powders within polymeric matrices and high specific surface area exhibiting more surface defects enable stronger particle-wave interactions during EMWA testing. These synergistic effects, driven by surface defects and structural optimization, resulted in promising EMWA properties. 

This work emphasizes the potential of mechanosynthesis to tailor defects in hybrid perovskites, providing a pathway to explore defect-driven properties and expand their applications beyond optoelectronics.