Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition

Direct Observation of Size-Dependent Phase Transition in Methylammonium Lead Bromide Perovskite Microcrystals and Nanocrystals

Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr₃) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr₃ microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.

Optical Properties of a CsMnBr3 Single Crystal

Lead-free perovskite materials with good stability are promising for various applications. In order to explore their application in optoelectronic devices, it is essential to investigate their fundamental optical properties. In this work, we have synthesized a CsMnBr₃ single crystal (SC) with red emission at ∼621 nm and studied their optical properties. Through the measurement of temperature-dependent photoluminescence (PL) spectra, it is found that a phase transition occurs at approximately 100 K in the SC, which is absent in the CsMnBr₃ nanocrystals (NCs). Furthermore, the SC exhibits stronger electron and longitudinal optical phonon coupling strength than that of the NCs at low temperatures. In addition, under the resonant excitation at 600 nm, the SC possesses strong saturable absorption property, with a modulation depth of ∼27%. Interestingly, the SC also exhibits a large two-photon absorption coefficient of ∼0.035 cm GW^-1 at 800 nm and an excellent optical limiting behavior. The experimental results indicate that the CsMnBr₃ SC is a class of excellent environmentally friendly optoelectronic materials.

Time-resolved vibrational-pump visible-probe spectroscopy for thermal conductivity measurement of metal-halide perovskites

Understanding thermal transport at the microscale to the nanoscale is crucially important for a wide range of technologies ranging from device thermal management and protection systems to thermal-energy regulation and harvesting. In the past decades, non-contact optical methods, such as time-domain and frequency-domain thermoreflectance, have emerged as extremely powerful and versatile thermal metrological techniques for the measurement of material thermal conductivities. Here, we report the measurement of thermal conductivity of thin films of CH₃NH₃PbI₃ (MAPbI₃), a prototypical metal-halide perovskite, by developing a time-resolved optical technique called vibrational-pump visible-probe (VPVP) spectroscopy. The VPVP technique relies on the direct thermal excitation of MAPbI₃ by femtosecond mid-infrared optical pump pulses that are wavelength-tuned to a vibrational mode of the material, after which the time dependent optical transmittance across the visible range is probed in the ns to the μs time window using a broadband pulsed laser. Using the VPVP method, we determine the thermal conductivities of MAPbI₃ thin films deposited on different substrates. The transducer-free VPVP method reported here is expected to permit spectrally resolving and spatiotemporally imaging of the dynamic lattice temperature variations in organic, polymeric, and hybrid organic-inorganic semiconductors.

Local Morphology Effects on the Photoluminescence Properties of Thin CsPbBr3 Nanocrystal Films

Lead halide perovskites are emerging as extremely interesting active materials for a wide variety of optoelectronic and photonic devices. A deep understanding of their photophysics is thus fundamental in order to properly understand the origins of the materials active properties and to provide strategies for improving them. In this work, we exploit the local morphological variations in a drop-cast thin CsPbBr3 nanocrystal film to show that the aggregation of NCs has strong effects on the peak wavelengths of PL spectra, the linewidth, and the intensity of dependence on temperature. An analysis based on models that are frequently used in the literature led to completely different conclusions about the intrinsic NC emission properties extracted from spectra measured in points with different contribution of the PL from the aggregates. Our results demonstrate that extreme care has to be used in order to correctly correlate the spectral PL features with the intrinsic emission properties of lead halide perovskite NC films.

Low-Temperature Induced Enhancement of Photoelectric Performance in Semiconducting Nanomaterials

The development of light-electricity conversion in nanomaterials has drawn intensive attention to the topic of achieving high efficiency and environmentally adaptive photoelectric technologies. Besides traditional improving methods, we noted that low-temperature cooling possesses advantages in applicability, stability and nondamaging characteristics. Because of the temperature-related physical properties of nanoscale materials, the working mechanism of cooling originates from intrinsic characteristics, such as crystal structure, carrier motion and carrier or trap density. Here, emerging advances in cooling-enhanced photoelectric performance are reviewed, including aspects of materials, performance and mechanisms. Finally, potential applications and existing issues are also summarized. These investigations on low-temperature cooling unveil it as an innovative strategy to further realize improvement to photoelectric conversion without damaging intrinsic components and foresee high-performance applications in extreme conditions.

A Rapid and Robust Light-and-Solution-Triggered In Situ Crafting of Organic Passivating Membrane over Metal Halide Perovskites for Markedly Improved Stability and Photocatalysis

Despite intriguing optoelectronic attributes in solar cells, light-emitting diodes, and photocatalysis, the instability of organic-inorganic perovskites poises a grand challenge for long-term applications. Herein, we report a simple yet robust strategy via light-and-solution treatment to create an organic membrane that effectively passivates CH₃NH₃PbI₃ (MAPbI₃). Specifically, the restructuring of MA⁺ is observed on MAPbI₃ in aqueous hydrogen iodide. HIO₃ molecules are generated via the reaction between water and I₂ induced by photocatalysis when MAPbI₃ is illuminated. The hydrogen bonding between HIO₃ molecules at different perovskite particles not only directs the creeplike growth of perovskite particles but also in situ forms a passivating layer firmly anchoring on the perovskite surface with hydrophilic -NH₃⁺ groups tethering to perovskites and hydrophobic -CH₃ moieties exposed to air. Intriguingly, such MA⁺ film greatly improves the stability of perovskites against moisture as well as their crystal quality, considerably enhancing the photocatalytic hydrogen evolution rate.

Disentangling Electron-Phonon Coupling and Thermal Expansion Effects in the Band Gap Renormalization of Perovskite Nanocrystals

The complex electron-phonon interaction occurring in bulk lead halide perovskites gives rise to anomalous temperature dependences, like the widening of the electronic band gap as temperature increases. However, possible confinement effects on the electron-phonon coupling in the nanocrystalline version of these materials remain unexplored. Herein, we study the temperature (ranging from 80 K to ambient) and hydrostatic pressure (from atmospheric to 0.6 GPa) dependence of the photoluminescence of ligand-free methylammonium lead triiodide nanocrystals with controlled sizes embedded in a porous silica matrix. This analysis allowed us to disentangle the effects of thermal expansion and electron-phonon interaction. As the crystallite size decreases, the electron-phonon contribution to the gap renormalization gains in importance. We provide a plausible explanation for this observation in terms of quantum confinement effects, showing that neither thermal expansion nor electron-phonon coupling effects may be disregarded when analyzing the temperature dependence of the optoelectronic properties of perovskite lead halide nanocrystals.

Transmission electron microscopy of organic-inorganic hybrid perovskites: myths and truths

Organic-inorganic hybrid perovskites (OIHPs) have attracted extensive research interest as a promising candidate for efficient and inexpensive solar cells. Transmission electron microscopy (TEM) characterizations that can benefit the fundamental understanding and the degradation mechanism are widely used for these materials. However, their sensitivity to the electron beam illumination and hence structural instabilities usually prevent us from obtaining the intrinsic information or even lead to significant artifacts. Here, we systematically investigate the structural degradation behaviors under different experimental factors to reveal the optimized conditions for TEM characterizations of OIHPs by using low-dose electron diffraction and imaging techniques. We find that a low temperature (-180 °C) does not slow down the beam damage but instead induces a rapid amorphization for OIHPs. Moreover, a less severe damage is observed at a higher accelerating voltage. The beam-sensitivity is found to be facet-dependent that a (1 0 0) exposed CH₃NH₃PbI₃ (MAPbI₃) surface is more stable than a (0 0 1) surface. With these guidance, we successfully acquire the atomic structure of pristine MAPbI₃ and identify the characterization window that is very narrow. These findings are helpful to guide future electron microscopy characterizations of these beam-sensitive materials, which are also useful for finding strategies to improve the stability and performance of the perovskite solar cells.

Hot carriers perspective on the nature of traps in perovskites

Amongst the many spectacular properties of hybrid lead halide perovskites, their defect tolerance is regarded as the key enabler for a spectrum of high-performance optoelectronic devices that propel perovskites to prominence. However, the plateauing efficiency enhancement of perovskite devices calls into question the extent of this defect tolerance in perovskite systems; an opportunity for perovskite nanocrystals to fill. Through optical spectroscopy and phenomenological modeling based on the Marcus theory of charge transfer, we uncover the detrimental effect of hot carriers trapping in methylammonium lead iodide and bromide nanocrystals. Higher excess energies induce faster carrier trapping rates, ascribed to interactions with shallow traps and ligands, turning these into potent defects. Passivating these traps with the introduction of phosphine oxide ligands can help mitigate hot carrier trapping. Importantly, our findings extend beyond photovoltaics and are relevant for low threshold lasers, light-emitting devices and multi-exciton generation devices.

The benign nature of defects in lead halide perovskites is widely regarded as the basis for their outstanding optoelectronic properties. Here Righetto et al. overthrew this perception, revealing the defects’ surprising potency to hot carriers and devised a strategy to suppress them.

Photoluminescence signatures of thermal expansion, electron-phonon coupling and phase transitions in cesium lead bromide perovskite nanosheets

In this article, photoluminescence (PL) behaviors of inorganic cesium lead bromide (CsPbBr3) nanosheets are investigated in a broad temperature range from 5 to 500 K. As can be seen from the temperature evolution of the PL peak position, the bandgap blueshift induced by thermal lattice expansion is found to be gradually compensated by the bandgap redshift caused by electron-phonon coupling for the temperature variation from 5 K to 360 K. As the temperature is further increased, the nearly completely compensated PL peak position turns to exhibit a rapid blueshift, again at ∼360 K. Such turning behavior is consistent with an orthorhombic-tetragonal (γ-β) phase transition at this critical temperature. For the PL linewidth, it shows a continuous broadening at temperatures beyond 40 K, suggesting the dominant role of phonon scattering, especially at high temperatures. These findings of temperature dependent photophysical properties of CsPbBr3 nanosheets may provide useful information of their further applications in the emerging nano photo-electronic devices.

Composition-, Size-, and Surface Functionalization-Dependent Optical Properties of Lead Bromide Perovskite Nanocrystals

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on surface passivation. We present a study on the temperature-dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by surface passivation, and a postsynthesis ligand exchange with a quaternary ammonium bromide (QAB) results in more stable passivation over a larger temperature range. The PL intensity is highest from 50 to 250 K, which indicates that ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), surface passivation still needs to be improved to achieve maximum emission intensity in nanocrystal films.

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

Metal halide perovskites (MHPs) have recently attracted great attention from the scientific community due to their excellent photovoltaic performance as well as their tremendous potential for other optoelectronic applications such as light-emitting diodes, lasers, and photodetectors. Despite the rapid progress in device applications, a solid understanding of the photophysical properties behind the device performance is highly desirable for MHPs. Here, the properties of excitons and photogenerated charge carriers in MHPs are explored. The unique dielectric constant properties, crystal-liquid duality, and fundamental optical processes of MHPs are first discussed. The properties of excitons and related phenomena in MHPs are then detailed, including the exciton binding energy determined by various methods and their influence factors, exciton dynamics, exciton-photon coupling and related applications, and exciton-phonon coupling in MHPs. The properties of photogenerated free charge carriers in MHPs such as the carrier diffusion length, mobility, and recombination are described. Recent progress in various applications is also demonstrated. Finally, a conclusion and perspectives of future studies for MHPs are presented.

Self-Powered Perovskite/CdS Heterostructure Photodetectors

Methylammonium lead halide perovskites have gained a lot of attention because of their remarkable physical properties and potential for numerous (opto)electronic applications. Here, high-performance photodetectors based on CH₃NH₃PbI₃ (MAPbI₃)/CdS heterostructures are demonstrated. The resulting self-powered MAPbI₃/CdS photodetectors show excellent operating characteristics including a maximum detectivity of 2.3 × 10¹¹ Jones with a responsivity of 0.43 A/W measured at 730 nm. A temporal response time of less than 14 ms was achieved. The mechanisms of charge separation and transport at the interface of the MAPbI₃/CdS junction were investigated via conductive atomic force microscopy (AFM) and photoconductive AFM. Obtained results show that grain boundaries exhibit higher photocurrent than flat regions of the top perovskite layer, which indicates that excitons preferentially separate at the grain boundaries of the perovskite thin film, that is, at the edges of the MAPbI₃ crystals. The study of the photoelectric mechanism at the nanoscale suggests the device performance could potentially be fine-tuned through grain boundary engineering, which provides essential insights for the fabrication of the high-performance photodetector. The demonstrated self-powered photodetector is promising for numerous applications in low-energy consumption optoelectronic devices.

Temperature-Dependent Intraband Relaxation of Hybrid Perovskites

Intraband relaxation in polycrystalline films of hybrid perovskites methylammonium lead tribromide and methylammonium lead triiodide are studied by transient absorption spectroscopy from 80 K to >350 K. This temperature range spans the transitions of these materials from the high-temperature cubic phases, intermediate tetragonal phases, and low-temperature orthorhombic phases. The organic cation undergoes a distinct transition from an ordered lattice in the orthorhombic phase to a plastic crystal in cubic and tetragonal phases, which reportedly influences many optoelectronic properties. The much larger exciton binding energy of orthorhombic MAPbI₃ (compared to cubic or tetragonal phases) or MAPbBr₃ substantially changes the transient spectral responses of the materials by reducing the number of free carriers. However, for these measurements at low fluences, both MAPbBr₃ and MAPbI₃ exhibit subpicosecond intraband relaxation over the entire temperature range studied. Intraband relaxation becomes somewhat faster at higher temperatures, but freezing of organic cations are not accompanied by a discontinuity of the intraband relaxation time. These results suggest that configurational freedom of organic cations does not screen carriers from electron-phonon coupling.

Optical and Physical Probing of Thermal Processes in Semiconductor and Plasmonic Nanocrystals

This article reviews thermal properties of semiconductor and emergent plasmonic nanomaterials, focusing on mechanisms through which hot carriers and phonons are produced and dissipated as well as the related impacts on optoelectronic properties. Elevated equilibrium temperatures, of particular relevance for implementation of nanomaterials in devices, affect absorptive and radiative transitions as well as emission efficiency that can present reversible and irreversible changes with temperature. In noble metal or doped semiconductor/insulator nanomaterials, hot carriers and lattice heating can substantially influence localized surface plasmon resonances and yield large ultrafast changes in transmission or strongly oscillatory coherences. Transient optical and diffraction characterizations enable nonequilibrium investigations of phonon dynamics and cooling such as lattice expansion and crystal phase stability. Timescales of nanoparticle thermalization with surroundings and transport of heat within films of such materials are also discussed.

Influence of mixed organic cations on the structural and optical properties of lead tri-iodide perovskites

Perovskites with mixed organic cations possess better properties in some aspects as compared to their pure counterparts. However, the structural and optical properties of these mixed-type perovskites have been rarely investigated. In this study, probable structures of mixed organic cation perovskites, MAxFA1-xPbI3 (x = 1, 0.8, 0.6, 0.4 and 0.2), were comparatively discussed. Saturation of excitonic emission at 1.66 eV indicates limited orthorhombic phase at 30 K in MAPbI3, which confirms the co-existence of orthorhombic and tetragonal phases at low temperatures. Based on the comprehensive temperature- and power-dependent measurements, it is found that defects are activated in mixed organic cation perovskites under low excitation power at room temperature, whereas this process is not observed in pure MAPbI3. At high excitation power, free exciton recombination is suppressed due to exciton-exciton interaction for all samples. Analysis of stability against temperature based on these photoemission processes shows that the structure with comparable organic cation proportion is more stable; this can be explained by uniformly distributed strains existing at the boundaries between MAPbI3 and FAPbI3 molecules. These analyses of structural and optical properties of mixed organic cation perovskites are meaningful in dictating the future optoelectronic devices.

Stabilization of Lead-Tin-Alloyed Inorganic-Organic Halide Perovskite Quantum Dots

Recently, lead-tin-based alloyed halide perovskite quantum dots (QDs) with improved stability and less toxicity have been introduced. However, the perovskite QDs containing tin are still unstable and exhibit low photoluminescence quantum yields (PLQYs), owing to the presence of defects in the alloyed system. Here, we have attempted to introduce sulfur anions (S^2-) into the host lattice (MAPb_0.75Sn_0.25Br₃) as a promising route to stable alloyed perovskite QDs with improved stability and PLQY. In this study, we used elemental sulfur as a sulfur precursor. The successful incorporation of sulfur anions into the host lattice resulted in a highly improved PLQY (>75% at room temperature), which is believed to be due to a reduction in the defect-related non-radiative recombination centers present in the host lattice. Furthermore, we found that the emission property could be tuned between the bright green and cyan-bluish regions without compromising on color quality. This work invigorates the perovskite research community to prepare stable, bright, and color-tunable alloyed inorganic-organic perovskite QDs without compromising on their phases and color quality, which can lead to considerable advances in display technology.

Lead Vacancy Can Explain the Suppressed Nonradiative Electron-Hole Recombination in FAPbI3 Perovskite under Iodine-Rich Conditions: A Time-Domain Ab Initio Study

Experiments show that the excited-state lifetime of hybrid organic-inorganic perovskites, synthesized under iodine-rich conditions, can be greatly enhanced. Ab initio nonadiabatic (NA) molecular dynamics demonstrates that a single lead vacancy, which constitutes a major defect within the iodine-rich environment, significantly suppresses the nonradiative electron-hole recombination in HC(NH₂)₂PbI₃ (FAPbI₃) perovskite. The simulations show that electron-hole recombination in pristine FAPbI₃ occurs within one nanosecond. Introduction of a single lead vacancy weakens the NA electron-phonon coupling and prolongs the coherence time simultaneously. The weaker NA coupling competes successfully with the longer coherence time, extending the lifetime over tens of nanoseconds. The calculated recombination time scales show excellent agreement with experiment. Our study rationalizes the microscopic mechanism responsible for experimental observations, suggesting a rational choice of defect can modulate perovskite excited-state lifetimes and improve solar cell performance.

Direct Observation of Perovskite Photodetector Performance Enhancement by Atomically Thin Interface Engineering

Lead trihalide perovskites have been integrated with atomically thin WS₂ and served as absorption layers to improve photoresponsivity in photodetectors. The combination of perovskites and two-dimensional (2D) transition-metal dichalcogenide (TMDC) materials provides the platform to study light-matter interactions and charge transfer mechanisms in optoelectronic devices. Herein, conductive and photoconductive atomic force microscopy were used to image the dark current and photocurrent generated in WS₂/CH₃NH₃PbI₃ (MAPbI₃) heterostructures. Dark current measurement in the applied voltage range displays characteristic diode behavior, which can be well described by thermionic emission theory. Under laser illumination at 532 nm, the spatially resolved photocurrent images exhibit location-dependent photoresponse, where the photocurrent increases remarkably for the WS₂/MAPbI₃ heterostructures compared with the bare MAPbI₃ regions. Furthermore, comparative surface roughness and 2D Fourier analysis of the topographic and current maps reveal that the interfacial conditions of the WS₂/MAPbI₃ heterojunctions play an important role in the charge separation process. In addition, WS₂/MAPbI₃-based photodetectors have been fabricated. Our study provides direct evidence that atomically thin TMDC monolayers can effectively assist the charge separation process and improve the light-to-electric energy conversion, which aids in the design principles and understanding of 2D heterostructured optoelectronic devices.

Phonon Interaction and Phase Transition in Single Formamidinium Lead Bromide Quantum Dots

Formamidinium lead bromide (FAPbBr₃) quantum dots (QDs) are promising materials for light emitting applications in the visible spectral region because of their high photoluminescence (PL) quantum yield (QY) and the enhanced chemical stability as compared to, for instance, methylammonium based analogues. Toward practical harnessing of their compelling optical characteristics, the exciton recombination process, and in particular the exciton-phonon interaction and the impact of crystal phase transition, has to be understood in detail. This is addressed in this contribution by PL studies on single colloidal FAPbBr₃ QDs. Polarization-resolved PL measurements reveal a fine structure splitting of excitonic transitions due to the Rashba effect. Distinct phonon replica have been observed within energetic distances of 4.3 ± 0.5, 8.6 ± 0.9, and 13.2 ± 1.1 meV from the zero phonon line, which we attribute to vibrational modes of the lead bromide lattice. Additional vibrational modes of 18.6 ± 0.3 and 38.8 ± 1.1 meV are found and related to liberation modes of the formamidinium (FA) cation. Temperature-dependent PL spectra reveal a line broadening of the emission caused by exciton phonon interaction as well an unusual energy shift which is attributed to a crystal phase transition within the single QD.

Light-Emitting Halide Perovskite Nanoantennas

Nanoantennas made of high-index dielectrics with low losses in visible and infrared frequency ranges have emerged as a novel platform for advanced nanophotonic devices. On the other hand, halide perovskites are known to possess high refractive index, and they support excitons at room temperature with high binding energies and quantum yield of luminescence that makes them very attractive for all-dielectric resonant nanophotonics. Here we employ halide perovskites to create light-emitting nanoantennas with enhanced photoluminescence due to the coupling of their excitons to dipolar and multipolar Mie resonances. We demonstrate that the halide perovskite nanoantennas can emit light in the range of 530-770 nm depending on their composition. We employ a simple technique based on laser ablation of thin films prepared by wet-chemistry methods as a novel cost-effective approach for the fabrication of resonant perovskite nanostructures.