Mixed-halide lead perovskite (MHP) materials are rapidly advancing as next-generation high-efficiency perovskite solar cells due to enhanced stability and bandgap tunability. In this work, we demonstrate the ability to readily and stoichiometrically tune the halide composition in methylammoniumbased MHPs using a mechanochemical synthesis approach. Using this solvent-free protocol we are able to prepare domain-free MHP solid solutions with randomly distributed halide ions about the Pb center. Up to seven distinct [PbCl x Br 6−x ] 4− environments are identified, based on the 207 Pb NMR chemical shifts, which are also sensitive to the changes in the unit cell dimensions resulting from the substitution of Br by Cl, obeying Vegard's law. We demonstrate a straightforward and rapid synthetic approach to forming highly tunable stoichiometric MHP solid solutions while avoiding the traditional solution synthesis method by redirecting the thermodynamically driven compositions. Moreover, we illustrate the importance of complementary characterization methods, obtaining atomic-scale structural information from multinuclear, multifield, and multidimensional solid-state magnetic resonance spectroscopy, as well as from quantum chemical calculations and long-range structural details using powder X-ray diffraction. The solvent-free mechanochemical synthesis approach is also compared to traditional solvent synthesis, revealing identical solid-solution behavior; however, the mechanochemical approach offers superior control over the stoichiometry of the final mixed-halide composition, which is essential for device engineering.

Mixed-halide lead perovskites are becoming of paramount interest in the optoelectronic and photovoltaic research fields, offering band gap tunability, improved efficiency, and enhanced stability compared to their single halide counterparts. Formamidinium-based mixed halide perovskites (FA-MHPs) are critical to obtaining optimum solar cell performance. Here, we report a solvent-free mechanochemical synthesis (MCS) method to prepare FA-MHPs, starting with their parent compounds (FAPbX; X = Cl, Br, I), achieving compositions not previously accessible through the solvent synthesis (SS) technique. By probing local Pb environments in MCS FA-MHPs using solid-state nuclear magnetic resonance spectroscopy, along with powder X-ray diffraction for long-range crystallinity and reflectance measurements to determine the optical band gap, we show that MCS FA-MHPs form atomic-level solid solutions between Cl/Br and Br/I MHPs. Our results pave the way for advanced methods in atomic-level structural understanding while offering a one-pot synthetic approach to prepare MHPs with superior control of stoichiometry.

An NMR investigation of methylammonium lead iodide, the leading member of the hybrid organic− inorganic perovskite class of materials, and of its putative decomposition products as a result of exposure to heat and humidity, has been undertaken. We show that the 207 Pb NMR spectra of the compound of interest and of the proposed leadcontaining decomposition products, CH 3 NH 3 PbI 3 •H 2 O, (CH 3 NH 3) 4 PbI 6 •2H 2 O, and PbI 2 , have distinctive chemical shifts spanning over 1400 ppm, making 207 Pb NMR an ideal tool for investigating this material; further information may be gained from 13 C and 1 H NMR spectra. As reported in many investigations of CH 3 NH 3 PbI 3 on films, the bulk material hydrates in the presence of high relative humidity (approximately 80%), yielding the monohydrated perovskite CH 3 NH 3 PbI 3 •H 2 O. This reaction is reversible by heating the sample to 341 K. We show that neither (CH 3 NH 3) 4 PbI 6 •2H 2 O nor PbI 2 is observed as a decomposition product and that, in contrast to many studies on CH 3 NH 3 PbI 3 films, the bulk material does not decompose or degrade beyond CH 3 NH 3 PbI 3 •H 2 O upon prolonged exposure to humidity at ambient temperature. However, exposing CH 3 NH 3 PbI 3 concurrently to heat and humidity, or directly exposing it to liquid water, leads to the irreversible formation of PbI 2. In spite of its absence among the decomposition products, the response of (CH 3 NH 3) 4 PbI 6 •2H 2 O to heat was also investigated. It is stable at temperatures below 336 K but then rapidly dehydrates, first to CH 3 NH 3 PbI 3 •H 2 O and then to CH 3 NH 3 PbI 3. The higher stability of the bulk material as reported here is a promising advance, since stability is a major concern in the development of commercial applications for this material.

We demonstrate that the often observed double peak emission in lead halide perovskites is caused by an extensive self-absorption effect.

Here, the phase‐transition from tetragonal to orthorhombic crystal structure of the halide perovskite methylammonium lead iodide single crystal is investigated. Temperature dependent photoluminescence (PL) measurements in the temperature range between 165 and 100 K show complex PL spectra where in total five different PL peaks can be identified. All observed PL features can be assigned to different optical effects from the two crystal phases using detailed PL analyses. This allows to quantify the fraction of tetragonal phase that still occurs below the phase transition temperature. It is found that at 150 K, 0.015% tetragonal phase remain, and PL signatures are observed from quantum confined tetragonal domains, suggesting their size to be about 7–15 nm down to 120 K. The tetragonal inclusions also exhibit an increased Urbach Energy, implying a high degree of structural disorder. The results first illustrate how a careful analysis of the PL can serve to deduce structural information, and second, how structural deviations in halide perovskites have a significant impact on the optoelectronic properties of this promising class of semiconductors.