Naga Prathibha Jasti

Quasi-2D halide perovskites are chemically synthesized realizations of quantum well stacks with giant exciton oscillator strengths, tunable emission spectra, and very large exciton binding energies. While these features render quasi-2D halide perovskites a promising platform for room-temperature polaritonics, bosonic condensation and polariton lasing in quasi-2D perovskites have so far remained elusive at ambient conditions. Here, we demonstrate room-temperature cavity exciton-polariton condensation in mechanically exfoliated crystals of the quasi-2D Ruddlesden-Popper iodide perovskite (BA) ₂ (MA) ₂ Pb ₃ I ₁₀ in an open optical microcavity. We observe a polariton condensation threshold of 0.41 µJ cm ⁻² per pulse and detect a strong non-linear response. Interferometric measurements confirm the spontaneous emergence of spatial coherence across the condensate with an associated first-order autocorrelation reaching 0.6 with 1 ps coherence time and an effective de Broglie wavelength of 13 µm. Our results lay the foundation for a new class of room-temperature polariton lasers based on quasi-2D halide perovskites with great potential for hetero-integration with other van-der-Waals materials and combination with photonic crystals or waveguides.

We report continuous wave laser-assisted evaporation (CLE), a thin film deposition technique that yields phase-pure and stoichiometric thin films of halide perovskites (HaPs) from stoichiometric HaP targets. We use methylammonium lead bromide (MAPbBr3) to demonstrate the ability to grow with CLE well-oriented and smooth thin films on various substrates. Further, we show the broader applicability of CLE by preparing films of several other 3D HaP compounds, viz., methylammonium lead iodide, formamidinium lead bromide, and a 2D one, butylammonium lead iodide. CLE is a single-source, solvent-free, room-temperature process that needs only roughing pump vacuum; it allows the deposition of hybrid organic-inorganic compound films without needing post-thermal treatment or an additional organic precursor source to yield the intended product. The resulting films are polycrystalline and highly oriented. All these features, and the fact that one stoichiometric source serves as the target, make for an attractive, potentially scalable dry deposition approach.

The term defect tolerance (DT) is used often to rationalize the exceptional optoelectronic properties of halide perovskites (HaPs) and their devices. Even though DT lacked direct experimental evidence, it became a “fact” in the field. DT in semiconductors implies that structural defects do not translate to electrical and optical effects (e.g., due to charge trapping), associated with such defects. We present pioneering direct experimental evidence for DT in Pb-HaPs by comparing the structural quality of 2-dimensional (2D), 2D-3D, and 3D Pb-iodide HaP crystals with their optoelectronic characteristics using high-sensitivity methods. Importantly, we get information from the materials’ bulk because we sample at least a few hundred nanometers, up to several micrometers, from the sample’s surface, which allows for assessing intrinsic bulk (and not only surface-) properties of HaPs. The results point to DT in 3D, 2D-3D, and 2D Pb-HaPs. Overall, our data provide an experimental basis to rationalize DT in Pb-HaPs. These experiments and findings will help the search for and design of materials with real DT.

Humidity is often reported to compromise the stability of lead halide perovskites or of devices based on them. Here we measure the humidity dependence of the elastic modulus and hardness for two series of lead halide perovskite single crystals, varying either by cation or by anion type. The results reveal a dependence on bond length between, hydrogen bonding with, and polarizability/polarization of these ions. The results show an intriguing inverse relation between modulus and hardness, in contrast to their positive correlation for most other materials. This anomaly persists and is strengthened by the effect of humidity. This, and our overall findings are ascribed to the materials’ unique atomic-scale structure and properties, viz nano-polar domains and strong dynamic disorder, yet high-quality average order. Our conclusions are based on comparing results obtained from several different nano-indentation techniques, which separate surface from bulk elastic modulus, and probe different manifestations of the hardness.

The environment humidity effects on performance of halide perovskites (HaPs), especially MAPbI₃, are known. Nevertheless, it is hard to find direct experimental evidence of H₂O in the bulk materials at the levels lower than that of Monohydrate (MAPbI₃.H₂O). Here, for the first time, direct experimental evidence of water being released from bulk (µm‐s deep) of MAPbI₃ single crystal is reported. The thermogravimetric analysis coupled with mass spectrometry (TGA‐MS) of evolved gases is used to detect the MS signal of H₂O from the penetrable depth and correlate it with the TGA mass loss due to H₂O leaving the material. These measurements yield an estimate of the average H₂O content of 1 H₂O molecule per three MAPbI₃ formula units (MAPbI₃.0.33H₂O). Under the relatively low temperature conditions no other evolved gases that can correspond to MAPbI₃ decomposition products, are observed in the MS. In addition to being direct evidence that there is H₂O inside MAPbI₃, the data show that H₂O diffuses into it. With this article, a solid basis is proved for further studies on the mechanisms through which water modifies the properties of MAPbI₃ and all the other halide perovskites.

Adding a 2D character to halide perovskite (HaP) active layers in ambient‐protected cells can improve their stability drastically, which is not obvious from the hydrophobicity of the large cations that force the HaP into a 2D structure. Results of two‐photon confocal microscopy are reported to study inherent photo‐stability of 2D Pb iodide HaPs in the interior of single crystals. Compared to 3D HaP crystals, 2D ones have higher photo‐stability and, under a few sun‐equivalent conditions, self‐heal efficiently after photo‐damage. Using both photoluminescence (PL) intensities (as function of time after photo‐damage) and spectra, self‐healing dynamics of 2D HaP (C₄H₉NH₃)₂PbI₄, 2D/3D (C₄H₉NH₃)₂(CH₃NH₃)₂Pb₃I₁₀ and 3D MAPbI₃ are compared. Differences in response to photo‐damage and self‐healing ability from different degrees of photo‐damage are found between these HaPs. Based on the findings, a possible chemical mechanism for photo‐damage and self‐healing of the 2D HaPs is suggested: the layered lattice arrangement limits out‐diffusion of degradation products, facilitating damage reversal, leading to better 2D HaP photo‐stability and self‐healing uniformity than for 2D/3D HaPs. One implication of the layered structures’ resilience to photo‐damage is transfer of their increased stability to devices made with them, such as photovoltaic solar cells and light‐emitting diodes.