Exploring Optoelectronic Properties of Heterostructures of Thin Films and Magic-Sized Clusters of Metal Halide Perovskites

The broad spectrum of potential applications presented by metal halide perovskites (MHPs), coupled with their straightforward manufacturing procedures, has captured considerable interest in recent years. Considerable effort has been invested in enhancing the charge transport and extraction efficiencies of MHP thin films for applications in photovoltaics and in refining the film surface quality for usage in sensing. One approach is leveraging heterostructure engineering of MHPs, and prior studies have demonstrated improved photoluminescence (PL) and emission tunability via the adaptation of this method. In our previous work, we have documented how heterostructures comprising thin films of CH₃NH₃PbI₃ (methylammonium lead iodide, or MAPI) and metal halide perovskite quantum dots (PQDs) result in improved stability, defect passivation, increased charge carrier lifetimes, and increased charge extraction efficiency in the MAPI films. This work focuses on charge and energy transfer in heterostructures of MAPI thin films and perovskite magic-sized clusters (PMSCs). Magic-sized clusters refer to small, well-defined stable clusters of atoms or molecules that have unique physical and optoelectronic properties, and our PMSCs are 1 – 3 nm in diameter, smaller than the typical PQDs of equivalent composition. Owing to this higher surface-to-volume ratio, PMSCs have greater potential of higher charge and energy transfer efficiencies and higher PL quantum yield. Furthermore, one of the causes affecting the quality of the MHPs films are point defects at the film surface and at grain boundaries. These defects are in the sub-nanometer spatial scale, which could be addressed better by adding the PMSCs to the MHPs film rather than PQDs.<br/>We have conducted a comparative study of heterostructures comprising MAPI films with colloidally synthesized PMSCs and PQDs. The deposition was carried out two ways: the first, by spin coating MAPbBr₃ (methylammonium lead bromide) MSCs or MAPbBr₃ PQDs on annealed MAPI films; the second, by adding the PMSCs or PQDs to the precursor of MAPI and annealing them together. Scanning electron microscopy and X-ray diffraction were employed to get insights into the morphological and structural characteristics of the films with PMSCs and PQDs. Confocal microscopy, PL spectroscopy and time-resolved PL measurements while varying sample temperature and excitation wavelengths were utilized to understand the energy and charge transfer mechanisms between the coated MAPI films and the PMSCs or PQDs.