The Role of Trap-assisted Recombination in Luminescent Properties of Organometal Halide CH3NH3PbBr3 Perovskite Films and Quantum Dots

Thermal Stress Mitigation and Improved Performance in Perovskite Solar Cells via Lattice Matched Alkali Halide Passivation

This study utilizes a method to enhance the structural and thermal stability of perovskite solar cells (PSCs) by incorporating an alkali halide interlayer between the electron transport layer (ETL) and perovskite, which is known to improve device efficiency. This passivation technique significantly reduces residual stress within the perovskite at room temperature (3.68 MPa → 2.56 MPa) and maintains structural integrity under thermal cycling (-40 to 85 °C) as per IEC 61215: 2016 standards. Following 50 cycles, the treated film exhibits a minimal increase in residual stress (≈5.34 MPa), in contrast to the control film (≈29.72 MPa) based on Williamson-Hall 2θ - Sin2Ψ analysis. The incorporation of wide-bandgap alkali halides facilitates a strong lattice registry, thereby enhancing structural reliability. Moreover, fluorescence lifetime imaging microscopy (FLIM) confirms a reduction in defect formation, correlating with macroscopic lifetime studies. This also increases open circuit voltage (VOC) (1.08 V → 1.15 V) and device efficiency (17.9% → 20.6%). Notably, the treated device retains ≈71% of its initial PCE after 50 thermal cycles, whereas control devices ceased operation after 30 cycles due to thermal stress-induced interfacial delamination. This approach effectively prevents interlayer delamination, improving long-term structural reliability and, thereby, enabling efficient and thermally stable PSC deployment.

Elucidating Interfacial Carrier Transfer Dynamics for Circularly Polarized Emission in Self-Assembled Perovskite Heterostructures

By integrating carrier transfer with spin-selectivity in mixed-dimensional perovskites heterostructures (HSs), exceptional chiroptical behaviors can be activated, offering avenues for advanced applications in spintronics and quantum information technologies. However, the critical role of interface effects in this photophysical process remains insufficiently explored. We demonstrate the fabrication of self-assembled chiral 2D/achiral nanocrystal (NC) HSs with different morphologies and chiroptical activities. Using femtosecond transient reflection spectroscopy, the underlying interface-dependent carrier transfer was unraveled. Spin-polarized holes generated in the chiral 2D component can transfer within an ultrafast time scale of ∼362 fs across the coherent heterointerface, inducing circularly polarized luminescence (CPL) in the intrinsically achiral NCs with a high Pc of ∼10.3%. Furthermore, interfacial halide exchange can be utilized to extend the CPL wavelength from green to near-infrared. Our findings reveal the correlation between interfacial properties, charge transfer, and CPL activity, providing insights for the development of high-quality HSs with optimized optical properties.

Improving the performance of floating gate phototransistor memory with perovskite nanocrystals embedded in fluorinated polyamic acids

This study aims to develop a hybrid material using fluorine-containing polyamic acid (PAA) polymers and a perovskite (PVSK) for application in transistor-based photomemory devices to enhance both structural and electrical performance. Adding fluorides to the PAA material creates a structure with Lewis acid-base interactions, improving the interface between PVSK and PAA, reducing defect density in the floating gate dielectric layer, and passivating grain defects. Furthermore, the hydrophobic PAA structure provides an improved crystalline nucleation interface for the semiconductor pentacene, thereby significantly enhancing the hole mobility of the transistor. In electrical performance tests, devices utilizing ODA-6FDA (poly(4,4'-diaminodiphenyl ether-alt-4,4'-(hexafluoroisopropylidene)diphthalic anhydride)) as the floating gate exhibited a superior ON/OFF current ratio, approaching 106, compared to other PAA materials, and demonstrated stable dynamic switching currents. Additionally, incorporating fluorides into the PVSK material resulted in a more stable memory window, enabling the devices to maintain excellent performance during cyclic operation and long-term storage stability tests. These findings highlight the potential of combining fluorinated polymers with PVSK materials, further advancing the development and application of optoelectronic materials.

Effect of ring substituent position on the structural and optoelectronic properties of novel quasi low-dimensional hybrid 2-, 3-, and 4-(bromomethyl)pyridinium lead bromides

In this work, the synthesis, crystal structure, morphology, and optoelectronic properties of novel quasi low-dimensional hybrid (bromomethyl)pyridinium crystalline phases (2-BrCH2PyH)Pb2Br5, (3-BrCH2PyH)PbBr3, and (4-BrCH2PyH)PbBr3 are reported. The first compound formed quasi 2D crystals, while the other two species crystallized as quasi 1D structures. DFT calculations predicted that the bromides are semiconductors featuring electronic VB-to-CB transitions of ∼3.2 eV. (2-BrCH2PyH)Pb2Br5 and (3-BrCH2PyH)PbBr3 exhibited low-temperature (77 K) photoluminescence (PL). However, PL was not observed in (4-BrCH2PyH)PbBr3, which was presumably due to the small distance between the lead cations and bromine atoms of the 4-BrCH2PyH cations, resulting in a radiationless electronic transition. Furthermore, the studied species demonstrated quantum size effects.

Superfluorescence and nonlinear optical response of CH3NH3PbBr3-based mesostructures: applications to amplified spontaneous emission and ultrafast lasers

Metal halide perovskites have unique luminescent properties that make them an attractive alternative for high quality light-emitting devices. However, the poor stability of perovskites with many defects and the long cycle time for the preparation of perovskite nanocomposites have hindered their production and application. Here, we prepared the perovskite mesostructures by embedding MAPbBr3 nanocrystals in the mesopores on the surface of silica nanospheres and mixing the nanospheres with silver nanowires and poly(methyl methacrylate) (PMMA), and further explored their optical properties. The perovskite mesostructures exhibited superfluorescence (SF) under 785 nm laser excitation and the decay time was as short as 0.59 ps using a double exponential decay fit. The optical nonlinearity of these perovskite mesostructures under two-photon excitation was investigated by the Z-scanning technique. The samples exhibited both two-photon absorption (TPA) and saturation absorption (SA) with nonlinear absorption coefficients β ranging approximately 0.15 to 0.89 cm/GW corresponding to the TPA, and a saturation intensity of about 0.026 GW/cm2 was determined. In addition, the quality of the perovskite mesostructures was shown to be significantly improved, including an increase in radiative efficiency and an extension of the optical gain lifetime. The samples were excited by a two-photon femtosecond laser pump at room temperature and atmospheric pressure, and the laser peak appeared at 540.1 nm with a low amplified spontaneous emission (ASE) threshold of 0.52 mJ/cm2. Moreover, the perovskite mesostructures have been added to the fiber laser system as a saturable absorber, and the Q-switched pulse is generated at the wavelength of 1.5 µm. Our work effectively demonstrates that perovskite mesostructured materials can be functionalized for photonic devices, paving the way for further exploration of complex, functional and practical perovskite.

Luminescence Patterns on Multimillimeter Single-Crystal MAPbBr3 Microplatelets via Thermal Ablation Effects

The single crystal distinguishes itself as the most outstanding representative of excellent optoelectronic performance among perovskite semiconductors due to its exceptional charge carrier properties, extraordinary optophysics, precise structural geometries, and superior material stabilities. However, it exhibits mediocre performance in light emission, which can be attributed to its excessive carrier diffusion lengths and extremely low recombination rates. To address this challenge, this study puts forward a controllable high-temperature annealing treatment strategy concentrating on multimillimeter-scale MAPbBr3 micrometer-thick platelet single crystals (MPSCs). Through the utilization of thermal ablation effects to convert the surface monocrystalline lattices into polycrystalline perovskite nanocrystals (PNCs) with a remarkably increased specific surface area, the fluorescence intensity arising from the rapid recombination of free carriers on the PNC surface is enhanced by 320 times compared to the pristine MPSC surface. These PNCs produced through surface annealing, with the characteristic of loose adhesion to surfaces, can be mechanically wiped away and regenerated in situ via reannealing the residual MAPbBr3 MPSCs, guaranteeing high intensity, durability, and standard geometric fluorescence. Moreover, by regulating the surface distributions of thermal ablation effects and carrying out embossing annealing treatment or laser direct writing, we can design and fabricate relatively complex, high-contrast (255×), and clearly resolved fluorescence patterns on MPSC surfaces.

Excited-State Dynamics of MAPbBr3: Coexistence of Excitons and Free Charge Carriers at Ultrafast Times

Methylammonium lead tribromide perovskite (MAPbBr3) is an important material, for example, for light-emitting applications and tandem solar cells. The relevant photophysical properties are governed by a plethora of phenomena resulting from the complex and relatively poorly understood interplay of excitons and free charge carriers in the excited state. In this study, we combine transient spectroscopies in the visible and terahertz range to investigate the presence and evolution of excitons and free charge carriers at ultrafast times upon excitation at various photon energies and densities. For above- and resonant band-gap excitation, we find that free charges and excitons coexist and that both are mainly promptly generated within our 50-100 fs experimental time resolution. However, the exciton-to-free charge ratio increases upon decreasing the phonon energy toward resonant band gap excitation. The free charge signatures dominate the transient absorption response for above-band-gap excitation and low excitation densities, masking the excitonic features. With resonant band gap excitation and low excitation densities, we find that although the exciton density increases, free charges remain. We show evidence that the excitons localize into shallow trap and/or Urbach tail states to form localized excitons (within tens of picoseconds) that subsequently get detrapped. Using high excitation densities, we demonstrate that many-body interactions become pronounced and effects such as the Moss-Burstein shift, band gap renormalization, excitonic repulsion, and the formation of Mahan excitons are evident. The coexistence of excitons and free charges that we demonstrate here for photoexcited MAPbBr3 at ultrafast time scales confirms the high potential of the material for both light-emitting diode and tandem solar cell applications.

Ultrafast photo-induced carrier dynamics of perovskite quantum dots during structural degradation

In this study, the ultrafast photo-induced carrier dynamics of red-emitting PQDs during structural degradation was investigated using time-resolved transient absorption spectroscopy. The spectroscopic analysis revealed how the carrier dynamics varied when PQDs were exposed to a polar solvent. Three decay modes (carrier trapping, radiative carrier recombination and trap-assisted non-radiative recombination) were proposed to analyze the carrier dynamics of PQDs. The light-emitting property of PQDs is primarily influenced by radiative carrier recombination. This study demonstrates that structural degradation induced halide migration within PQDs and the formation of defects within the crystal lattice, leading to a proliferation of carrier trapping states. The increased trap states led to a reduction in carriers undergoing radiative carrier recombination. Additionally, PQDs degradation accelerated radiative carrier recombination, indicating a faster escape of carriers from excited states. Consequently, these factors hinder carriers remaining in excited states, leading to a decline in the light-emitting property of PQDs. Nevertheless, increasing an excitation fluence could reduce the carrier trapping mode and increase the radiative carrier recombination mode, suggesting a diminishment of the impact of carrier trapping. These findings offer a more comprehensive understanding of structural degradation of PQDs and can contribute to the development of PQDs with high structural stability.

A Universal Fabrication Strategy for High-Resolution Perovskite-Based Photodetector Arrays

Metal halide perovskite photodetector arrays have demonstrated great potential applications in the field of integrated systems, optical communications, and health monitoring. However, the fabrication of large-scale and high-resolution device is still challenging due to their incompatibility with the polar solvents. Here, a universal fabrication strategy that utilizes ultrathin encapsulation-assisted photolithography and etching to create high-resolution photodetectors array with vertical crossbar structure is reported. This approach yields a 48 × 48 photodetector array with a resolution of 317 ppi. The device shows good imaging capability with a high on/off ratio of 3.3 × 105 and long-term working stability over 12 h. Furthermore, this strategy can be applied to five different material systems, and is fully compatible with the existing photolithography and etching techniques, which are expected to have potential applications in the other high-density and solvent-sensitive devices array, including perovskite- or organic semiconductor-based memristor, light emitting diode displays, and transistors.

Unveiling the Valence State of Interstitial Bromine on Charge Carrier Lifetime in CH3NH3PbBr3 by Quantum Dynamics Simulation

Interstitial halogens are detrimental to the optoelectronic properties of metal halide perovskites. Using nonadiabatic (NA) molecular dynamics, we demonstrate that the valence state of interstitial bromine strongly changes the carrier lifetimes of MAPbBr₃ (MA = CH₃NH₃⁺). Both neutral and negatively charged interstitial bromine create no midgap states, and they decrease the bandgap, weaken the NA coupling, and accelerate decoherence in a different extent with respect to pristine MAPbBr₃, making free charge recombination either slow down about a 3-fold or remain largely unchanged. In contrast, a positively charged interstitial bromine forms a Br trimer and introduces a deep electron trap state, causing a 1.4-fold increase of charge recombination followed by a rapid electron trapping or across the bandgap because of an enhanced NA coupling. The simulations uncover the influence of different charged interstitial bromine defects on MAPbBr₃ carrier lifetimes and provide rational guidelines for optimizing perovskite solar cells.

Precise Tuning of Multiple Perovskite Photoluminescence by Volume-Controlled Printing of Perovskite Precursor Solution on Cellulose Paper

Metal halide perovskite nanocrystals (PeNCs) with a controlled quantum size effect have received intense interest for potential applications in optoelectronics and photonics. Here, we present a simple and innovative strategy to precisely tune the photoluminescence color of PeNCs by simply printing perovskite precursor solutions on cellulose papers. Depending on the volume of the printed precursor solutions, the PeNCs are autonomously grown into three discrete sizes, and their relative size population is controlled; accordingly, not only the number of multiple PL peaks but also their relative intensities can be precisely tuned. This autonomous size control is obtained through the efflorescence, which is advection of salt ions toward the surface of a porous medium during solvent evaporation and also through the confined crystal growth in the hierarchical structure of cellulose fibers. The infiltrated PeNCs are environmentally stable against moisture (for 3 months in air at 70% relative humidity) and strong light exposure by hydrophobic surface treatment. This study also demonstrates invisible encryption and highly secured unclonable anticounterfeiting patterns on deformable cellulose substrates and banknotes.

Dynamics of Strong Coupling Between Free Charge Carriers in Organometal Halide Perovskites and Aluminum Plasmonic States

We have investigated a strong coupled system composed of a MAPbI_xCl_3-x perovskite film and aluminum conical nanopits array. The hybrid states formed by surface plasmons and free carriers, rather than the traditional excitons, is observed in both steady-state reflection measurements and transient absorption spectra. In particular, under near upper band resonant excitation, the bleaching signal from the band edge of uncoupled perovskite was completely separated into two distinctive bleaching signals of the hybrid system, which is clear evidence for the formation of strong coupling states between the free carrier–plasmon state. Besides this, a Rabi splitting up to 260 meV is achieved. The appearance of the lower bands can compensate for the poor absorption of the perovskite in the NIR region. Finally, we found that the lifetime of the free carrier–SP hybrid states is slightly shorter than that of uncoupled perovskite film, which can be caused by the ultrafast damping of the SPs modes. These peculiar features on the strong coupled hybrid states based on free charge carriers can open new perspectives for novel plasmonic perovskite solar cells.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Hybrid Perovskite/Polymer Materials: Preparation and Physicochemical Properties

The aim of this work is to investigate the preparation, the optical properties, and the stability over time of a colloidal organic–inorganic hybrid perovskite (CH3NH3PbBr3)/random copolymer P(MMA-co-DMAEMA) system. Different ratios of perovskite to copolymer were used to study its effect on stability and properties. The optical properties were investigated by UV-Vis and fluorescence spectroscopy. Dynamic light scattering was used to determine the size, and the size polydispersity of the colloidal hybrid particles; while morphology was investigated by transmission electron microscopy. Photoluminescence decay studies revealed the interaction of the random copolymer with the perovskite. Finally, thin-films were prepared, to investigate the optical properties of the samples in the absence of the solvent. High temporal stability of the optical properties of thin hybrid films was observed under certain conditions.

Energy Resonance Transfer between Quantum Defects in Metal Halide Perovskites

Quantum defects have been shown to play an essential role in nonradiative recombination in metal halide perovskites (MHPs). Nonetheless, the processes of charge transfer assisted by defects are still ambiguous. Herein, we theoretically study the nonradiative multiphonon processes among different types of quantum defects in MHPs using Markvart's model for the induced mechanisms of electron-electron and electron-phonon interactions. We find that the charge carrier can transfer between the neighboring levels of the same type of shallow defects by multiphonon processes, but it will be distinctly suppressed with an increase in the defect depth. For the nonradiation multiphonon transitions between donor- and acceptor-like defects, the processes are very fast and not sensitive to the defect depth, which provides a possible explanation for the phenomenon of blinking of photoluminescence spectra. We also discuss the temperature dependence of these multiphonon processes and find that their variational trends depend on the comparison of the Huang-Rhys factor with the emitted phonon number. These theoretical results not only fill some of the gaps in defect-assisted nonradiative processes in the perovskite materials but also provide deeper physical insights into producing higher-performance perovskite-based devices.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

In-situ observation of trapped carriers in organic metal halide perovskite films with ultra-fast temporal and ultra-high energetic resolutions

We in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy with a sub-25 picosecond time resolution. Upon ultrafast laser excitation, trapped carriers follow a phonon assisted tunneling mechanism and a hopping transport mechanism along ultra-shallow to shallow trap states ranging from 1.72-11.51 millielectronvolts and is demonstrated by time-dependent and independent activation energies. Using temperature as an energetic ruler, we map trap states with ultra-high energy resolution down to < 0.01 millielectronvolt. In addition to carrier mobility of ~4 cm²V^-1s^-1 and lifetime of ~1 nanosecond, we validate the above transport mechanisms by highlighting trap state dynamics, including trapping rates, de-trapping rates and trap properties, such as trap density, trap levels, and capture-cross sections. In this work we establish a foundation for trap dynamics in high defect-tolerant perovskites with ultra-fast temporal and ultra-high energetic resolution.

Laurionite Competes with 2D Ruddlesden-Popper Perovskites During the Saturation Recrystallization Process

The room-temperature saturation recrystallization (RTSR) method has been extensively used to prepare all-inorganic lead halide perovskite (e.g., CsPbBr₃) nanocrystals. Here, we revealed that the composition of the products prepared by the seemingly simple RTSR method could be extremely complex under different experimental parameters. The pH value of the solution and the protonation tendency of the amines influenced by the amounts and types of introduced amines, oleic acid, and water from the environment determined the composition of the final products. PbBr₂, 2D Ruddlesden-Popper perovskites (RPPs) formed by perovskite layers separated by intercalating cations, and laurionite Pb(OH)Br would form under acidic, mildly acidic, and alkaline conditions, respectively. Based on the understanding of the formation mechanism, Pb(OH)Br microparticles with well-defined morphologies were prepared, which could be transformed into highly luminescent CH₃NH₃PbBr₃ with the morphology unchanged. The protonated amine behaves as an intercalating layer during the formation of 2D RPPs. Phenylethylamine (PEA) was proven to be an appropriate amine to prepare pure RPP microplates because of its weaker alkalinity compared to aliphatic amines. The prepared (PEA)₂PbBr₄ RPP microplates showed strong deep-blue light emission with a PL peak at 415 nm, which could be fine-tuned by changing amines. This study proved the complex reaction pathways of the seemingly simple RTSR method and extended the RTSR method into the fabrication of 2D RPPs and laurionite with promising applications in optoelectronic devices.

Enhanced Stabilities and Production Yields of MAPbBr3 Quantum Dots and Their Applications as Stretchable and Self-Healable Color Filters

Organic-inorganic hybrid CH₃NH₃PbBr₃ (MAPbBr₃) perovskite quantum dots (PQDs) are considered as promising and cost-effective building blocks for various optoelectronic devices. However, during centrifugation for the purification of these PQDs, commonly used polar protic and aprotic non-solvents (e.g., methanol and acetone) can destroy the nanocrystal structure of MAPbBr₃ perovskites, which will significantly reduce the production yields and degrade the optical properties of the PQDs. This study demonstrates the use of methyl acetate (MeOAc) as an effective non-solvent for purifying as-synthesized MAPbBr₃ PQDs without causing severe damage, which facilitates attainment of stable PQD solutions with high production yields. The MeOAc-washed MAPbBr₃ PQDs maintain their high photoluminescence (PL) quantum yields and crystalline structures for long periods in solution states. MeOAc undergoes a hydrolysis reaction in the presence of the PQDs, and the resulting acetate anions partially replace the original surface ligands without damaging the PQD cores. Time-resolved PL analysis reveals that the MeOAc-washed PQDs show suppressed non-radiative recombination and a longer PL lifetime than acetone-washed and methanol-washed PQDs. Finally, it is demonstrated that a composite of the MAPbBr₃ PQDs and a thermoplastic elastomer (polystyrene-block-polyisoprene-block-polystyrene) is feasible as a stretchable and self-healable green color filter for a white light-emitting diode device.

Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping

The outstanding optoelectronic performance of lead halide perovskites lies in their exceptional carrier diffusion properties. As the perovskite material dimensionality is reduced to exploit the quantum confinement effects, the disruption to the perovskite lattice, often with insulating organic ligands, raises new questions on the charge diffusion properties. Herein, we report direct imaging of >1 μm exciton diffusion lengths in CH₃NH₃PbBr₃ perovskite nanocrystal (PNC) films. Surprisingly, the resulting exciton mobilities in these PNC films can reach 10 ± 2 cm² V^-1 s^-1, which is counterintuitively several times higher than the carrier mobility in 3D perovskite films. We show that this ultralong exciton diffusion originates from both efficient inter-NC exciton hopping (via Förster energy transfer) and the photon recycling process with a smaller yet significant contribution. Importantly, our study not only sheds new light on the highly debated origins of the excellent exciton diffusion in PNC films but also highlights the potential of PNCs for optoelectronic applications.

Interpreting time-resolved photoluminescence of perovskite materials

Time-resolved photoluminescence (TRPL) spectroscopy is a powerful technique to investigate excited charge carrier recombinations in semiconductors and molecular systems. The analysis of the TRPL decays of many molecular systems (e.g. molecules and organic materials) is usually fairly straightfoward and can be fitted with an exponential function allowing extraction of the rate constants. Due to the non-excitonic nature of charge carriers in lead halide perovskite materials coupled with the presence of localised trap states in their band-gap, the TRPL of these materials is much more complicated to interpret. Here we discuss two models used in the literature to simulate charge carrier recombinations and TRPL in perovskites. These models consider the bimolecular nature of direct electron-hole recombination but differ in their treatment of trap-mediated recombination with one model describing trapping as a monomolecular process whereas the other as a bimolecular process between free carriers and the available trap states. In comparison, the classical analysis of perovskite TRPL decay curves (using a sum of exponentials) can lead to misinterpretation. Here we offer some recommendations for meaningful measurements of lead halide perovskite thin-films. The fluence dependence as well as charge carrier accumulation due to incomplete depopulation of all photoexcited carriers between consecutive excitation pulses are discussed for both models.

Synthesis conditions influencing formation of MAPbBr3 perovskite nanoparticles prepared by the ligand-assisted precipitation method

This work reports on an optimized procedure to synthesize methylammonium bromide perovskite nanoparticles. The ligand-assisted precipitation synthetic pathway for preparing nanoparticles is a cost-effective and promising method due to its ease of scalability, affordable equipment requirements and convenient operational temperatures. Nevertheless, there are several parameters that influence the resulting optical properties of the final nanomaterials. Here, the influence of the choice of solvent system, capping agents, temperature during precipitation and ratios of precursor chemicals is described, among other factors. Moreover, the colloidal stability and stability of the precursor solution is studied. All of the above-mentioned parameters were observed to strongly affect the resulting optical properties of the colloidal solutions. Various solvents, dispersion media, and selection of capping agents affected the formation of the perovskite structure, and thus qualitative and quantitative optimization of the synthetic procedure conditions resulted in nanoparticles of different dimensions and optical properties. The emission maxima of the nanoparticles were in the 508–519 nm range due to quantum confinement, as confirmed by transmission electron microscopy. This detailed study allows the selection of the best optimal conditions when using the ligand-assisted precipitation method as a powerful tool to fine-tune nanostructured perovskite features targeted for specific applications.

Anti-Stokes photoluminescence study on a methylammonium lead bromide nanoparticle film

Photon cooling via anti-Stokes photoluminescence (ASPL) is a promising approach to realize all-solid-state cryo-refrigeration by photoexcitation. Photoluminescence quantum yields close to 100% and a strong coupling between phonons and excited states are required to achieve net cooling. We have studied the anti-Stokes photoluminescence of thin films of methylammonium lead bromide nanoparticles. We found that the anti-Stokes photoluminescence is thermally activated with an activation energy of ∼80 meV. At room temperature the ASPL up-conversion efficiency is ∼60% and it depends linearly on the excitation intensity. Our results suggest that upon further optimization of their optical properties, the investigated particles could be promising candidates for the demonstration of photon cooling in thin solid films.

Ligand-Length Modification in CsPbBr3 Perovskite Nanocrystals and Bilayers with PbS Quantum Dots for Improved Photodetection Performance

Nanocrystals surface chemistry engineering offers a direct approach to tune charge carrier dynamics in nanocrystals-based photodetectors. For this purpose, we have investigated the effects of altering the surface chemistry of thin films of CsPbBr₃ perovskite nanocrystals produced by the doctor blading technique, via solid state ligand-exchange using 3-mercaptopropionic acid (MPA). The electrical and electro-optical properties of photovoltaic and photoconductor devices were improved after the MPA ligand exchange, mainly because of a mobility increase up to 5 × 10⁻³cm2/Vs. The same technology was developed to build a tandem photovoltaic device based on a bilayer of PbS quantum dots (QDs) and CsPbBr₃ perovskite nanocrystals. Here, the ligand exchange was successfully carried out in a single step after the deposition of these two layers. The photodetector device showed responsivities around 40 and 20 mA/W at visible and near infrared wavelengths, respectively. This strategy can be of interest for future visible-NIR cameras, optical sensors, or receivers in photonic devices for future Internet-of-Things technology.

Modulating the optical and electrical properties of MAPbBr3 single crystals via voltage regulation engineering and application in memristors

Defect density is one of the most significant characteristics of perovskite single crystals (PSCs) that determines their optical and electrical properties, but few strategies are available to tune this property. Here, we demonstrate that voltage regulation is an efficient method to tune defect density, as well as the optical and electrical properties of PSCs. A three-step carrier transport model of MAPbBr3 PSCs is proposed to explore the defect regulation mechanism and carrier transport dynamics via an applied bias. Dynamic and steady-state photoluminescence measurements subsequently show that the surface defect density, average carrier lifetime, and photoluminescence intensity can be efficiently tuned by the applied bias. In particular, when the regulation voltage is 20 V (electrical poling intensity is 0.167 V μm-1), the surface defect density of MAPbBr3 PSCs is reduced by 24.27%, the carrier lifetime is prolonged by 32.04%, and the PL intensity is increased by 112.96%. Furthermore, a voltage-regulated MAPbBr3 PSC memristor device shows an adjustable multiresistance, weak ion migration effect and greatly enhanced device stability. Voltage regulation is a promising engineering technique for developing advanced perovskite optoelectronic devices.

Efficient Flexible Inorganic Perovskite Light-Emitting Diodes Fabricated with CsPbBr3 Emitters Prepared via Low-Temperature in Situ Dynamic Thermal Crystallization

The present study systematically investigates the morphology and crystallization process of inorganic CsPbBr₃ perovskite layer films fabricated by thermal coevaporation in conjunction with continuous low-temperature thermal annealing to promote in situ dynamic thermal crystallization. The results confirm for the first time that both the crystal grain size and the compactness of the CsPbBr₃ films can be tuned during the thermal coevaporation fabrication process via in situ dynamic thermal crystallization. The performance of the PeLEDs employing the CsPbBr₃ films as the emitter layer is investigated in detail with respect to the substrate temperature and deposition rate employed during deposition of the CsPbBr₃ film. This study provides guidelines for developing suitable film production processes and highlights future challenges that must be addressed to facilitate the commercial development of large-area, uniform, and flexible perovskite-based optoelectronic devices.

Optical Properties and First-Principles Study of CH3NH3PbBr3 Perovskite Structures

Solution-processed organic-inorganic hybrid perovskites have attracted attention as light-harvesting materials for solar cells and photonic applications. The present study focuses on cubic single crystals and microstructures of CH₃NH₃PbBr₃ perovskite fabricated by a one-step solution-based self-assembly method. It is seen that, in addition to the nucleation from the precursor solution, crystallization occurs when the solution is supersaturated, followed by the formation of a small nucleus of CH₃NH₃PbBr₃ that self-assembles into bigger hollow cubes. A three-dimensional (3D) fluorescence microscopy investigation of hollow cubes confirmed the formation of hollow plates on the bottom; then, the growth starts from the perimeter and propagates to the center of the cube. Furthermore, the growth in the (001) direction follows a layer-by-layer growth model to form a complete cube, confirmed by scanning electronic microscopy (SEM) observations. Two-dimensional (2D)-3D fluorescence microscopy and photoluminescence (PL) measurements confirm a peak emission at 535 nm. To get more insights into the structural and optical properties, density functional theory (DFT) simulations were conducted. The electronic and optical properties calculated by DFT are in agreement with the obtained experimental values. The density-of-state (DOS) calculations revealed that the valence band maximum (VBM) consists of states contributed by Br and Pb, which agrees with the X-ray photoelectron spectroscopy valence band (XPS VB) measurements.

Surface Pb-Dimer Passivated by Molecule Oxygen Notably Suppresses Charge Recombination in CsPbBr3 Perovskites: Time-Domain Ab Initio Analysis

Experiments show that excess lead atoms accelerate charge recombination while oxygen passivation can heal the defects and enhance solar cell efficiency. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that an excess lead atom forms a Pb-dimer with a single surface lead atom of CsPbBr₃(001) surface and creates a deep hole trap. The electron-hole recombination is accelerated to over 10 ps via fast hole trapping or bypassing the hole trap compared to the pristine CsPbBr₃, occurring on tens of picoseconds. Pb-dimer passivated with oxygen molecules forms Pb-O bonds, breaks the Pb-dimer, and removes the trap state, leading to a decrease in the recombination and extending excited-state lifetime to over 100 ps. The deceleration arises mainly due to the reduced NA coupling and short decoherence time. The study advances our understanding of excited-state dynamics of all-inorganic perovskites in the presence of excess lead and oxygen atmosphere.

Electrospun Fibers Containing Emissive Hybrid Perovskite Quantum Dots

We demonstrate a single-step fabrication process of highly stable and luminescent polymer fibers embedded with quantum dots (QDs) of the organic-inorganic hybrid perovskite (OIP) (CH₃NH₃PbBr₃) using the electrospinning process. The fiber (∼2 μm diameter) primarily consists of poly(methyl methacrylate) dispersed with clusters of OIP quantum dots. The OIP clusters are radially distributed, normal to the fiber axis. The photoluminescence quantum yield (PLQY) is high (∼80%) and comparable to that of conventional QDs. The emission maxima are tunable by varying the concentration of OIP precursor in the electrospinning solution. Submicron emission maps show an isotropic and continuous emission along the fiber, suggesting uniform distribution of QD clusters. Temperature-dependent PL response indicates features which are a function of the particle size. For small QDs, the PLQY(T) maxima are close to the ambient temperature, whereas the PLQY(T) maxima shift sizably to T < 50 K for larger QDs. Significant waveguiding of QDs emission and amplified spontaneous emission, a prerequisite for lasing, were observed in the fiber confined OIP system at room temperature.

Influence of Defects on Excited-State Dynamics in Lead Halide Perovskites: Time-Domain ab Initio Studies

This Perspective summarizes recent research into the excited-state dynamics in lead halide perovskites that are of paramount importance for photovoltaic and photocatalytic applications. Nonadiabatic molecular dynamics combined with time-domain ab initio density functional theory allows one to mimic time-resolved spectroscopy experiments at the atomistic level of detail. The focus is placed on realistic aspects of perovskite materials, including point defects, surfaces, grain boundaries, mixed stoichiometries, dopants, and interfaces. The atomistic description of the quantum dynamics of electron and hole trapping and recombination, provided by the time-domain ab initio simulations, generates important insights into the mechanisms of charge and energy losses and guides the development of high-performance perovskite solar cell devices.

Vivid and Fully Saturated Blue Light-Emitting Diodes Based on Ligand-Modified Halide Perovskite Nanocrystals

CsPbX₃ (X = I, Br, Cl) perovskite nanocrystals (NCs) have recently emerged as emitting materials for optoelectronic and display applications owing to their easily tunable emissions, high photoluminescence quantum yield (PLQY), and vivid color purity (full width at half maximum of approximately 20 nm). However, the lagging quantum yields of blue-emitting perovskite NCs have resulted in low efficiency compared to green or red perovskite light-emitting diodes (PeLEDs); moreover, the long insulating ligands (such as oleylamine and oleic acid) inhibit charge carrier injection. In this study, we demonstrated a facile ligand-mediated post-treatment (LMPT) method for high-quality perovskite NCs with changing optical properties to allow fine-tuning of the target emission wavelength. This method involves the use of a mixed halide ion-pair ligand, di-dodecyl dimethyl ammonium bromide, and chloride, which can induce a reconstruction through a self-anion exchange. Using the LMPT method, the PLQY of the surface-passivated blue-emitting NCs was dramatically enhanced to over 70% within the 485 nm blue emission region and 50% within the 467 nm deep-blue emission region. Through this treatment, we achieved highly efficient blue-PeLED maximum external quantum efficiencies of 0.44 and 0.86% within the 470 and 480 ± 2 nm electroluminescence emission regions, respectively.

Unravelling the Effects of A-Site Cations on Nonradiative Electron-Hole Recombination in Lead Bromide Perovskites: Time-Domain ab Initio Analysis

Lead bromide perovskites APbBr₃ (A = Cs, MA, FA) hold great promise in optoelectronics and photovoltaics. Because the band gaps of the three materials are similar, and also because the A-site cation does not contribute to band edges, one would expect a minor influence of A-site cation on the excited-state lifetime of the perovskites. Experiments defy that expectation. By performing ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we demonstrate that the nonradiative electron-hole recombination times are in the order FAPbBr₃ > MAPbBr₃ > CsPbBr₃, which are determined by the NA electron-phonon coupling because decoherence times are similar. The simulations show that the larger A-site cation and the smaller NA coupling because larger A-site cation suppresses the Pb-Br cages' motion. The electron-hole recombination is slow, ranging from subnanosecond to nanoseconds, because the NA coupling is small, less than 3 meV, and because decoherence time is slow, less than 7 fs. Both the trend of recombination and the time scales show excellent agreement with experiments. The time-domain atomistic simulations rationalize the experimental observations and advance our understanding of the cations' influence on perovskite excited-state lifetimes.

Study of the Partial Substitution of Pb by Sn in Cs-Pb-Sn-Br Nanocrystals Owing to Obtaining Stable Nanoparticles with Excellent Optical Properties

Halide perovskites are revolutionizing the photovoltaic and optoelectronic fields with outstanding performances obtained in a remarkably short time. However, two major challenges remain: the long-term stability and the Pb content, due to its toxicity. Despite the great effort carried out to substitute the Pb by a less hazardous element, lead-free perovskite still remains more unstable than lead-containing perovskites and presents lower performance as well. In this work, we demonstrate the colloidal preparation of Cs-Pb-Sn-Br nanoparticles (NPs) where Sn is incorporated up to 18.8%. Significantly, we have demonstrated that the partial substitution of Pb by Sn does not produce a deleterious effect in their optical performance in terms of photoluminescence quantum yield (PLQY). We observed for the first time a positive effect in terms of enhancement of PLQY when Sn partially substitutes Pb in a considerable amount (i.e., higher than 5%). PLQYs as high as 73.4% have been obtained with a partial Pb replacement of 7% by Sn. We present a systematic study of the synthesis process in terms of different growth parameters (i.e., precursor concentration, time, and temperature of reaction) and how they influence the Sn incorporation and the PLQY. This high performance and long-term stability is based on a significant stabilization of Sn²⁺ in the NPs for several months, as determined by XPS analysis, and opens an interesting way to obtain less Pb-containing perovskite NPs with excellent optoelectronic properties.

Insulated Interlayer for Efficient and Photostable Electron-Transport-Layer-Free Perovskite Solar Cells

Currently, the most efficient perovskite solar cells (PSCs) mainly use planar and mesoporous titanium dioxide (TiO₂) as an electron-transport layer (ETL). However, because of its intrinsic photocatalytic properties, TiO₂ can decompose perovskite absorber and lead to poor stability under solar illumination (ultraviolet light). Herein, a simplified architectural ETL-free PSC with enhanced efficiency and outstanding photostability is produced by the facile deposition of a bathocuproine (BCP) interlayer. Power conversion efficiency of the ETL-free PSC improves from 15.56 to 19.07% after inserting the BCP layer, which is the highest efficiency reported for PSCs involving an ETL-free architecture, versus 19.03% for the n-i-p full device using TiO₂ as an ETL. The BCP interlayer has been demonstrated to have several positive effects on the photovoltaic performances of devices, such as "modulation doping" of the perovskite layer, modification of FTO surface work function, and enhancing the charge-transfer efficiency between FTO and perovskite. Moreover, the BCP-based ETL-free devices exhibit outstanding photostability: the unencapsulated BCP-based ETL-free PSCs retain over 90% of their initial efficiencies after 1000 h of storage in air and maintain 92.2% after 450 h of exposure to full solar irradiation (without a UV filter), compared to only 14.1% in the n-i-p full cells under the same condition.

Perovskite Light-Emitting Diodes via Laser Crystallization: Systematic Investigation on Grain Size Effects for Device Performance

Owing to unique potential for high color purity luminance based on low-cost solution processes, organic/inorganic hybrid perovskite light-emitting diodes (PeLEDs) are attracting a great deal of research attention. For high performance PeLEDs, optimum control of the perovskite film morphology is a critical parameter. Here, we introduce a reliable and well-controllable PeLED crystallization process based on beam-damage-free near-infrared laser (λ = 808 nm) irradiation. Morphology of the CH₃NH₃PbBr₃ films can be precisely controlled by laser irradiation condition parameters: power density and beam scan rate. We systematically investigate the perovskite film morphology and device performance of the PeLEDs under different processing conditions. In the optimum power density and high beam scan rate (30 W cm^-2, 0.1 mm s^-1), a dense and smooth perovskite film is attained with a small crystal grain size. The critical relationship between the crystal grain size and LED efficiency is established while attaining the device performance of 0.95 cd A^-1 efficiency and 1784 cd m^-2.

Aggregation-induced emission in lamellar solids of colloidal perovskite quantum wells

The outstanding excitonic properties, including photoluminescence quantum yield (η_PL), of individual, quantum-confined semiconductor nanoparticles are often significantly quenched upon aggregation, representing the main obstacle toward scalable photonic devices. We report aggregation-induced emission phenomena in lamellar solids containing layer-controlled colloidal quantum wells (QWs) of hybrid organic-inorganic lead bromide perovskites, resulting in anomalously high solid-state η_PL of up to 94%. Upon forming the QW solids, we observe an inverse correlation between exciton lifetime and η_PL, distinct from that in typical quantum dot solid systems. Our multiscale theoretical analysis reveals that, in a lamellar solid, the collective motion of the surface organic cations is more restricted to orient along the [100] direction, thereby inducing a more direct bandgap that facilitates radiative recombination. Using the QW solids, we demonstrate ultrapure green emission by completely downconverting a blue gallium nitride light-emitting diode at room temperature, with a luminous efficacy higher than 90 lumen W^-1 at 5000 cd m^-2, which has never been reached in any nanomaterial assemblies by far.

Fabrication-Method-Dependent Excited State Dynamics in CH3NH3PbI3 Perovskite Films

Understanding the excited-state dynamics in perovskite photovoltaics is necessary for progress in these materials, but changes in dynamics depending on the fabrication processes used for perovskite photoactive layers remain poorly characterised. Here we report a comparative study on femtosecond transient absorption (TA) in CH₃NH₃PbI₃ perovskite films fabricated by various solution-processing methods. The grain sizes and the number of voids between grains on each film varied according to the film synthesis method. At the low excitation fluence of 0.37 μJ cm⁻², fast signal drops in TA dyanmics within 1.5 ps were observed in all perovskite films, but the signal drop magnitudes differed becuase of the variations in charge migration to trap states and band gap renormalisation. For high excitation fluences, the buil-up time of the TA signal was increased by the activated hot-phonon bottleneck, while the signal decay rate was accelerated by fluence-dependent high-order charge recombination. These fluence-dependent dynamics changed for different perovskite fabrication methords, indicating that the dynamics were affected by morphological features such as grain sizes and defects.

Amino-Acid-Induced Preferential Orientation of Perovskite Crystals for Enhancing Interfacial Charge Transfer and Photovoltaic Performance

Interfacial engineering of perovskite solar cells (PSCs) is attracting intensive attention owing to the charge transfer efficiency at an interface, which greatly influences the photovoltaic performance. This study demonstrates the modification of a TiO₂ electron-transporting layer with various amino acids, which affects charge transfer efficiency at the TiO₂ /CH₃ NH₃ PbI₃ interface in PSC, among which the l-alanine-modified cell exhibits the best power conversion efficiency with 30% enhancement. This study also shows that the (110) plane of perovskite crystallites tends to align in the direction perpendicular to the amino-acid-modified TiO₂ as observed in grazing-incidence wide-angle X-ray scattering of thin CH₃ NH₃ PbI₃ perovskite film. Electrochemical impedance spectroscopy reveals less charge transfer resistance at the TiO₂ /CH₃ NH₃ PbI₃ interface after being modified with amino acids, which is also supported by the lower intensity of steady-state photoluminescence (PL) and the reduced PL lifetime of perovskite. In addition, based on the PL measurement with excitation from different side of the sample, amino-acid-modified samples show less surface trapping effect compared to the sample without modification, which may also facilitate charge transfer efficiency at the interface. The results suggest that appropriate orientation of perovskite crystallites at the interface and trap-passivation are the niche for better photovoltaic performance.

In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly Efficient Light-Emitting Diode

Excellent color purity with a tunable band gap renders organic-inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr₃ (MA = CH₃NH₃) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr₂ in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr₃, thereby forming nanocrystals and creating type I band alignment with core MAPbBr₃ by embedding MAPbBr₃ nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr₃ thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated.

Ultrafast carrier dynamics in bimetallic nanostructure-enhanced methylammonium lead bromide perovskites

In this work, we examine the impact of hybrid bimetallic Au/Ag core/shell nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr₃) mesoporous perovskite solar cells (PSCs). Plasmon-enhanced PSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in the photobleach formation time, which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr₃ perovskites. TAS also showed a decrease in carrier decay lifetimes, indicating that nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO₂ prior to bulk recombination. Furthermore, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.

Size-dependent one-photon- and two-photon-pumped amplified spontaneous emission from organometal halide CH3NH3PbBr3 perovskite cubic microcrystals

In the past few years, organometal halide light-emitting perovskite thin films and colloidal nanocrystals (NCs) have attracted significant research interest in the field of highly purified illuminating applications.