Formamidinium Lead Bromide (FAPbBr3) Perovskite Microcrystals for Sensitive and Fast Photodetectors

Exciton-Phonon-Plasmon Interplay in Hot Carrier Relaxation Dynamics in Perovskite Crystals

Enhancing light-matter interaction in nanostructures using metallic surface plasmons is a dependable route for improving efficiencies in optoelectronic applications. Plasmonic interfaces of organic cation-based halide perovskites that show high quantum efficiency and enhanced carrier lifetimes are seen as a technologically important avenue for new-age photovoltaics and quantum emitters. Here, several interesting multi-particle interplays in hybrid structures of Ga nanodroplets and FAPbBr3 crystals (Ga-NCs) having novel practical applications are reported. In addition to the conventional emission enhancement, a dominant blueshift in the perovskite photoluminescence (PL) is seen in the presence of Ga nanoparticles, which are persistent down to low temperatures. The integrated PL intensity ratio has a non-monotonic temperature dependence indicating a non-trivial exciton-phonon-plasmon interplay. The time-resolved photoluminescence measurements at different excitation wavelengths and transient absorption measurements reveal the strong influence of the Ga nanoparticles on the intrinsic phonon bottleneck typically observed in FAPbBr3 crystals (NCs). Detailed calculations explain the observed results throwing light on the complex interplay of plasmons, excitons, and the phonons in these simple heterojunctions. Ga-supported perovskite nanocrystals with higher quantum yield and ultrafast carrier relaxation pathways are seen to be an exciting system for quantum light emission with facile synthesis techniques.

Challenges and Opportunities of Upconversion Nanoparticles for Emerging NIR Optoelectronic Devices

Upconversion nanoparticles (UCNPs), incorporating lanthanide (Ln) dopants, can convert low-energy near-infrared photons into higher-energy visible or ultraviolet light through nonlinear energy transfer processes. This distinctive feature has attracted considerable attention in both fundamental research and advanced optoelectronics. Challenges such as low energy-conversion efficiency and nonradiative losses limit the performance of UCNP-based optoelectronic devices. Recent advancements including optimized core-shell structures, tailed Ln-doping concentration, and surface modifications show significant promise for improving the efficiency and stability. In addition, combining UCNPs with functional materials can broaden their applications and improve device performance, paving the way for the innovation of next-generation optoelectronics. This paper first categorizes and elaborates on various upconversion mechanisms in UCNPs, focusing on strategies to boost energy transfer efficiency and prolong luminescence. Subsequently, an in-depth discussion of the various materials that can enhance the efficiency of UCNPs and expand their functionality is provided. Furthermore, a wide range of UCNP-based optoelectronic devices is explored, and multiple emerging applications in UCNP-based neuromorphic computing are highlighted. Finally, the existing challenges and potential solutions involved in developing practical UCNPs optoelectronic devices are considered, as well as an outlook on the future of UCNPs in advanced technologies is provided.

Comparative Study of the Electrorheological Properties of Various Halide Perovskites

Although perovskite-structured materials have primarily been widely employed in solar cell applications, limited studies have been conducted in the field of electrorheology (ER). In this study, various halide perovskite materials, including FAPbBr3, FAPbI3, MAPbBr3, MAPbI3, CsPbBr3, and CsPbI3 were synthesized for the first time to evaluate their applicability in ER for the first time. Initially, the morphological and chemical properties of these materials were characterized to confirm the successful formation of the perovskite structures. In addition, the as-synthesized halide perovskite materials were dispersed in silicone oil (3.0 wt %) to evaluate their suitability as dispersants in ER fluids. Among these, the CsPbI3-based ER fluid exhibited the optimal dielectric properties and the greatest dispersion stability of the various systems examined. In ER applications, the CsPbI3-based ER fluid demonstrated the highest ER performance, achieving a shear stress of 99.4 Pa, owing to the synergistic effects of its intrinsic rod-like structure and dielectric properties, which promoted polarization. The aspect ratios of the CsPbI3 rods were further controlled by modifying the synthetic process, resulting in the generation of both shorter and longer rods. Notably, ER fluids based on CsPbI3 synthesized via a hydrothermal method yielded rod-like structures with a high aspect ratio of 20, leading to an enhanced ER activity of 128.0 Pa. These results highlight the potential of halide perovskite materials for use in ER applications.

Crystal growth, structural phase transitions and optical gap evolution of FAPb(Br1-xClx)3 hybrid perovskites (FA: formamidinium ion, CH(NH2)2+)

Chemically tuned organic-inorganic hybrid halide perovskites based on bromide and chloride anions CH(NH2)2Pb(Br1-xClx)3 (CH(NH2)2+: formamidinium ion, FA) have been crystallized and investigated by neutron powder diffraction (NPD), single crystal X-ray diffraction (SCXRD), scanning electron microscopy (SEM) and UV-vis spectroscopy. FAPbBr3 and FAPbCl3 experience successive phase transitions upon cooling, lowering the symmetry from cubic to orthorhombic phases; however, these transitions are not observed for the mixed halide phases, probably due to compositional disorder. The band-gap engineering brought about by the chemical doping of FAPb (Br1-xClx)3 perovskites (x = 0.0, 0.33, 0.5, 0.66 and 1.0) can be controllably tuned: the gap progressively increases with the concentration of Cl- ions from 2.17 to 2.91 eV at room temperature, presenting a nonlinear behavior. This study provides an improved understanding of the structural and optical properties of these appealing hybrid perovskites.

Ab initio exploration of A2AlAgCl6 (A = Rb, Cs): unveiling potentials for UV optoelectronic applications

CONTEXT AND RESULTS

In this study, we have explored the electronic and optical properties of A2AlAgCl6 (A = Rb, Cs), revealing their potential applications in UV devices. Our investigation demonstrates that Rb2AlAgCl6 and Cs2AlAgCl6 possess remarkable mechanical and thermodynamic stability, alongside direct band gaps of 4.25 eV and 4.20 eV, respectively. The optical properties, including the dielectric function, absorption coefficient, and reflectivity, underscore the suitability of these materials for UV device applications. This work serves as a foundational reference for future experimental endeavors aiming to leverage these characteristics for practical uses in scientific research.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The study utilizes first-principles calculations based on the Wien2k code, employing GGA-PBE and mBJ exchange-correlation functional to analyze the cubic structure of the space group Fm-3m. Detailed computational analyses were conducted to investigate the band structure, density of states, and optical properties, particularly focusing on Cs2AlAgCl6. This methodological approach not only confirms the materials' impressive stability and optical characteristics but also provides a robust framework for assessing their potential in UV technology applications. Our computational strategy offers insights into the effectiveness of these methodologies for future experimental validation and practical deployment in the research domain.

Enhanced Photostability of Lead Halide Perovskite Nanocrystals with Mn3+ Incorporation

Recently, lead halide perovskite nanocrystals (NCs) have shown great potential and have been widely studied in lighting and optoelectronic fields. However, the long-term stability of perovskite NCs under irradiation is an important challenge for their application in practice. Mn2+ dopants are mostly proposed as substitutes for the Pb site in perovskite NCs synthesized through the hot-injection method, with the aim of improving both photo- and thermal stability. In this work, we employed a facile ligand-assisted reprecipitate strategy to introduce Mn ions into perovskite lattice, and the results showed that Mn3+ instead of Mn2+, even with a very low level of incorporation of 0.18 mol % as interstitial dopant, can enhance the photostability of perovskite binder film under the ambient conditions without emission change, and the photoluminescent efficiency can retain 70% and be stable under intensive irradiation for 12 h. Besides, Mn3+ incorporation could prolong the photoluminescent decay time by passivating trap defects and modifying the distortion of the lattice, which underscores the significant potential for application as light emitters.

Ultraviolet-Visible-Short-Wavelength Infrared Broadband and Fast-Response Photodetectors Enabled by Individual Monocrystalline Perovskite Nanoplate

Perovskite-based photodetectors exhibit potential applications in communication, neuromorphic chips, and biomedical imaging due to their outstanding photoelectric properties and facile manufacturability. However, few of perovskite-based photodetectors focus on ultraviolet-visible-short-wavelength infrared (UV-Vis-SWIR) broadband photodetection because of the relatively large bandgap. Moreover, such broadband photodetectors with individual nanocrystal channel featuring monolithic integration with functional electronic/optical components have hardly been explored. Herein, an individual monocrystalline MAPbBr3 nanoplate-based photodetector is demonstrated that simultaneously achieves efficient UV-Vis-SWIR detection and fast-response. Nanoplate photodetectors (NPDs) are prepared by assembling single nanoplate on adjacent gold electrodes. NPDs exhibit high external quantum efficiency (EQE) and detectivity of 1200% and 5.37 × 1012 Jones, as well as fast response with rise time of 80 µs. Notably, NPDs simultaneously achieve high EQE and fast response, exceeding most perovskite devices with multi-nanocrystal channel. Benefiting from the high specific surface area of nanoplate with surface-trap-assisted absorption, NPDs achieve high performance in the near-infrared and SWIR spectral region of 850-1450 nm. Unencapsulated devices show outstanding UV-laser-irradiation endurance and decent periodicity and repeatability after 29-day-storage in atmospheric environment. Finally, imaging applications are demonstrated. This work verifies the potential of perovskite-based broadband photodetection, and stimulates the monolithic integration of various perovskite-based devices.

In Search of a Double Perovskite in the Phase Triangle of Bromides CsBr-CuBr-InBr3

New bromide compounds A₂B^IB^IIIBr₆ with a double perovskite structure provide variety and flexibility of optoelectronic properties, and some of them are of poor toxicity in comparison with such popular lead halides. The promising compound with a double perovskite structure was proposed recently for the ternary system of CsBr-CuBr-InBr₃. Analysis of phase equilibria in the CsBr-CuBr-InBr₃ ternary system showed stability of the quasi-binary section of CsCu₂Br₃-Cs₃In₂Br₉. Formation of the estimated phase Cs₂CuInBr₆ by melt crystallization or solid-state sintering was not observed, most likely, as a result of higher thermodynamic stability of binary bromides CsCu₂Br₃ and Cs₃In₂Br₉. The existence of three quasi-binary sections was observed, while no ternary bromide compounds were found.

Achieving a Highly Stable Perovskite Photodetector with a Long Lifetime Fabricated via an All-Vacuum Deposition Process

Hybrid organic-inorganic metal halide perovskites (HOIP) have become a promising visible light sensing material due to their excellent optoelectronic characteristics. Despite the superiority, overcoming the stability issue for commercialization remains a challenge. Herein, an extremely stable photodetector was demonstrated and fabricated with Cs_0.06FA_0.94Pb(I_0.68Br_0.32)₃ perovskite by an all-vacuum process. The photodetector achieves a current density up to 1.793 × 10^-2 A cm^-2 under standard one sun solar illumination while maintaining a current density as low as 8.627 × 10^-10 A cm^-2 at zero bias voltage. The linear dynamic range (LDR) and transient voltage response were found to be comparable to the silicon-based photodetector (Newport 818-SL). Most importantly, the device maintains 95% of the initial performance after 960 h of incessant exposure under one sun solar illumination. The achievements of these outstanding results contributed to the all-vacuum deposition process delivering a film with high stability and good uniformity, which in turn delays the degradation process. The degradation mechanism is further investigated by impedance spectroscopy to reveal the charge dynamics in the photodetector under different exposure times.

Improving the Performance and High-Field Stability of FAPbBr3 Single Crystals in X-Ray Detection with Chenodeoxycholic Acid Additive

Organometal halide perovskite single crystals are one of the most promising radiation detection materials due to their unique advantages of high absorption coefficient, long carrier diffusion length, and low defect density. However, the severe ion migration in perovskites deteriorates the X-ray detection performance under longtime and high-field operating conditions. This work reports an effective additive of chenodeoxycholic acid (CDCA), which can suppress the ion migration and improve the performance and the operational stability of FAPbBr₃ single crystals (SCs) in X-ray detection significantl. The CDCA molecules in precursors effectively suppress the decomposition of FA ions, resulting in a better crystal orientation and stoichiometry. The trace amounts of CDCA residues in FAPbBr₃ SCs improve the thermal stability and effectively suppress the ion migration. The resulting detector shows an impressive X-ray sensitivity up to 21 386.88 µC Gy_air ^-1 cm^-2 under -500 V and a detection limit of 15.23 nGy_air s^-1 . The response current of the detector at 225 V cm^-1 field is barely changed under the 7200 s irradiation with a dose rate of 1.949 mGy_air s^-1 . This work provides insights for the additive selection and improving the operational stability of perovskite single crystals for commercial applications.

Phase Equilibria in Ternary System CsBr-AgBr-InBr3

The double perovskite halides A₂B^IB^IIIX₆ provide flexibility for various formulation adjustments and are of less toxicity in comparison with well-discussed complex lead halide derivatives. Such type of structure can be formed by replacing two Pb²⁺ ions in the cubic lattice with a pair of non-toxic heterovalent (monovalent and trivalent) metal cations, such as silver and indium. The aim of this work is to briefly characterize the phase equilibria in the ternary system CsBr-AgBr-InBr₃ and investigate the thermodynamic availability of synthesis of Cs₂AgInBr₆ double perovskite phase by solid-state sintering or melt crystallization. The results demonstrate the unfeasibility of the Cs₂AgInBr₆ phase but high stability of the corresponding binary bromides perspective for optoelectronics.

Synthesis, Photoluminescence and Vibrational Properties of Aziridinium Lead Halide Perovskites

Three-dimensional lead halide perovskites are known for their excellent optoelectronic properties, making them suitable for photovoltaic and light-emitting applications. Here, we report for the first time the Raman spectra and photoluminescent (PL) properties of recently discovered three-dimensional aziridinium lead halide perovskites (AZPbX₃, X = Cl, Br, I), as well as assignment of vibrational modes. We also report diffuse reflection data, which revealed an extended absorption of light of AZPbX₃ compared to the MA and FA counterparts and are beneficial for solar cell application. We demonstrated that this behavior is correlated with the size of the organic cation, i.e., the energy band gap of the cubic lead halide perovskites decreases with the increasing size of the organic cation. All compounds show intense PL, which weakens on heating and shifts toward higher energies. This PL is red shifted compared to the FA and MA counterparts. An analysis of the PL data revealed the small exciton binding energy of AZPbX₃ compounds (29-56 meV). Overall, the properties of AZPbX₃ are very similar to those of the well-known MAPbX₃ and FAPbX₃ perovskites, indicating that the aziridinium analogues are also attractive materials for light-emitting and solar cell applications.

A-Site Substitute for Fabricating All-Inorganic Perovskite CsPbCl3 with Application in Self-Powered Ultraviolet Photodetectors

Because of its stable chemical properties and wide band gap, CsPbCl₃ perovskite has shown great application prospects in ultraviolet photodetectors (UPDs). However, the poor solubility of CsCl in organic solvents impedes the fabrication of high-quality CsPbCl₃ films. Herein, we introduced an A-site substitute route for fabricating a high-quality CsPbCl₃ microcrystalline (MC) film by spin-coating cesium acetate on a MAPbCl₃ MC film followed by a high-temperature annealing process. To enhance the device performance of the FTO/SnO₂/CsPbCl₃ MCs/carbon structure UPD, a pressure-assisted annealing strategy was carried out, which reduced the void density and surface roughness of the microcrystal film. Finally, our optimized PDs showed high device performances with an on/off ratio of 6 × 10⁴, a responsivity of 0.13 A W^-1, a detectivity of as high as 1.07 × 10¹² Jones, and a rise/fall time of 10/24 μs. Moreover, our unpacked PDs showed good storage and light stability. Our results lay a foundation for the application of all inorganic perovskite in the ultraviolet region.

An ion migration induced self-powered photoelectrical detector based on FAPbBr3 single crystals

Irreversible ion migration was utilized to design a built-in electric field and energy band bending in a symmetrically structured Au/FAPbBr3/Au device, which successfully leads to a self-powered photoelectric device based on FAPbBr3 crystals.

Influence of Different Rotations of Organic Formamidinium Molecule on Electronic and Optical Properties of FAPbBr3 Perovskite

Hybrid organic–inorganic halide perovskites (HOIPs) have recently represented a material breakthrough for optoelectronic applications. Obviously, studying the interactions between the central organic cation and the Pb-X inorganic octahedral could provide a better understanding of HOIPs. In this work, we used a first-principles theoretical study to investigate the effect of different orientations of central formamidinium cation (FA+) on the electronic and optical properties of FAPbBr3 hybrid perovskite. In order to do this, the band structure (with and without spin–orbit coupling (SOC)), density of states (DOS), partial density of states (PDOS), electron density, distortion index, bond angle variance, dielectric function, and absorption spectra were computed. The findings revealed that a change in the orientation of FA+ caused some disorders in the distribution of interactions, resulting in the formation of some specific energy levels in the structure. The interactions between the inorganic and organic parts in different directions create a distortion index in the bonds of the inorganic octahedral, thus leading to a change in the volume of PbBr6. This is the main reason for the variations observed in the electronic and optical properties of FAPbBr3. The obtained results can be helpful in solar-cell applications.

Quasi-Zero Dimensional Halide Perovskite Derivates: Synthesis, Status, and Opportunity

In recent decades, many technological advances have been enabled by nanoscale phenomena, giving rise to the field of nanotechnology. In particular, unique optical and electronic phenomena occur on length scales less than 10 nanometres, which enable novel applications. Halide perovskites have been the focus of intense research on their optoelectronic properties and have demonstrated impressive performance in photovoltaic devices and later in other optoelectronic technologies, such as lasers and light-emitting diodes. The most studied crystalline form is the three-dimensional one, but, recently, the exploration of the low-dimensional derivatives has enabled new sub-classes of halide perovskite materials to emerge with distinct properties. In these materials, low-dimensional metal halide structures responsible for the electronic properties are separated and partially insulated from one another by the (typically organic) cations. Confinement occurs on a crystal lattice level, enabling bulk or thin-film materials that retain a degree of low-dimensional character. In particular, quasi-zero dimensional perovskite derivatives are proving to have distinct electronic, absorption, and photoluminescence properties. They are being explored for various technologies beyond photovoltaics (e.g. thermoelectrics, lasing, photodetectors, memristors, capacitors, LEDs). This review brings together the recent literature on these zero-dimensional materials in an interdisciplinary way that can spur applications for these compounds. The synthesis methods, the electrical, optical, and chemical properties, the advances in applications, and the challenges that need to be overcome as candidates for future electronic devices have been covered.

Advances in Halide Perovskite Memristor from Lead-Based to Lead-Free Materials

Memristors have attracted considerable attention as one of the four basic circuit elements besides resistors, capacitors, and inductors. Especially, the nonvolatile memory devices have become a promising candidate for the new-generation information storage, due to their excellent write, read, and erase rates, in addition to the low-energy consumption, multistate storage, and high scalability. Among them, halide perovskite (HP) memristors have great potential to achieve low-cost practical information storage and computing. However, the usual lead-based HP memristors face serious problems of high toxicity and low stability. To alleviate the above issues, great effort has been devoted to develop lead-free HP memristors. Here, we have summarized and discussed the advances in HP memristors from lead-based to lead-free materials including memristive properties, stability, neural network applications, and memristive mechanism. Finally, the challenges and prospects of lead-free HP memristors have been discussed.

Growth of two-dimensional formamidine lead halide perovskite single-crystalline sheets and their optoelectronic properties

Formamidine-based hybrid perovskite is an excellent optoelectronic material; however, its intrinsic non-layered crystalline structure makes it hard to isolate the corresponding 2D counterparts. In this work, a unique liquid-epitaxy technique was introduced to grow micro-sized two-dimensional FAPbX3 perovskite sheets. Such ultrathin sheets exhibited excellent photo-induced carrier properties with high crystalline quality, as well as provided new opportunities for next-generation optoelectronic devices.

Low-Dimensional Metal Halide Perovskite Photodetectors

Metal halide perovskites (MHPs) have been a hot research topic due to their facile synthesis, excellent optical and optoelectronic properties, and record-breaking efficiency of corresponding optoelectronic devices. Nowadays, the development of miniaturized high-performance photodetectors (PDs) has been fueling the demand for novel photoactive materials, among which low-dimensional MHPs have attracted burgeoning research interest. In this report, the synthesis, properties, photodetection performance, and stability of low-dimensional MHPs, including 0D, 1D, 2D layered and nonlayered nanostructures, as well as their heterostructures are reviewed. Recent advances in the synthesis approaches of low-dimensional MHPs are summarized and the key concepts for understanding the optical and optoelectronic properties related to the PD applications of low-dimensional MHPs are introduced. More importantly, recent progress in novel PDs based on low-dimensional MHPs is presented, and strategies for improving the performance and stability of perovskite PDs are highlighted. By discussing recent advances, strategies, and existing challenges, this progress report provides perspectives on low-dimensional MHP-based PDs in the future.

Constructing Caesium-Based Lead-Free Perovskite Photodetector Enabling Self-Powered Operation with Extended Spectral Response

Since the discovery of the state-of-the-art hybrid halide perovskites, their application in optoelectronic systems has drawn considerable attention. However, the toxicity from lead (Pb) and the volatility induced by organic constituents hinder their future large-scale market development. Herein, a fully inorganic Pb-free halide perovskite based on robust Cs₃Bi₂I₉ is synthesized and realized its potential in photodetector application. The material property investigation suggests the good crystalline quality with strong absorption coefficient suitable for photodetection. An interesting feature based on the extended absorption is obtained, which is the characteristic of a weak phonon-assisted transition. Additionally, the morphological features display the beautifully grown micrometer-sized crystals of Cs₃Bi₂I₉. The fabricated photodetector demonstrated the self-powered operation (zero-bias state) with a very low dark current of 0.46 pA. Profiting from this, an improved photosensitivity of 1.4 × 10⁴ is achieved. Moreover, along with self-powered photodetection, the photodetector exhibits a broad spectral response (450-950 nm), high detectivity (1.2 × 10¹⁰/1.6 × 10¹² Jones), high responsivity (0.59 μA W^-1/3.8 mA W^-1), and fast response speed (ms) under a weak optical signal of 0.1 mW cm^-2 with a larger active area of 0.25 cm². The photodetector shows high photostability which was well retained for almost 2000 repetitive cycles without degradation. More strikingly, based on the core stability of the perovskite film, an excellent long-term stability of 3 months (90 days) is achieved for the photodetector even after exposure to moist air (75% relative humidity). This study thus highlights one of the few Pb-free all-inorganic perovskite photodetectors employing a simple device architecture with a larger active area that outshines by showing efficient and comparable performance under the self-powered mode under low light conditions.

A General Wet Transferring Approach for Diffusion-Facilitated Space-Confined Grown Perovskite Single-Crystalline Optoelectronic Thin Films

Hybrid perovskite single-crystalline thin films are promising for making high-performance perovskite optoelectronic devices due to their superior physical properties. However, it is still challenging to incorporate them into multilayer devices because of their on-substrate growth. Here, a wet transfer method is used in transferring perovskite single-crystalline films perfectly onto various target substrates. More importantly, large millimeter-scaled single-crystalline films can be obtained via a diffusion-facilitated space-confined growth method as thin as a few hundred nanometers, which are capable of sustaining excellent crystalline quality and morphology after the transferring process. The availability of these crystalline films offers us a convenient route to further investigate their intrinsic properties of hybrid perovskites. We also demonstrate that the wet transfer method can be used for scalable fabrication of perovskite single-crystalline film-based photodetectors exhibiting a remarkable photoresponsivity. It is expected that this transferring strategy would promise broad applications of perovskite single-crystalline films for more complex perovskite devices.

Temperature-Dependent Optical Band Gap in CsPbBr3, MAPbBr3, and FAPbBr3 Single Crystals

Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr3, CsPbBr3 and for the first time FAPbBr3) by spectroscopic ellipsometry in the range of 1-5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic-tetragonal and tetragonal-cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.

Temperature-Dependent Optical Band Gap in CsPbBr3, MAPbBr3, and FAPbBr3 Single Crystals

Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr₃, CsPbBr₃ and for the first time FAPbBr₃) by spectroscopic ellipsometry in the range of 1-5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic-tetragonal and tetragonal-cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.

Polarization-enhanced photoelectric performance in a molecular ferroelectric hexane-1,6-diammonium pentaiodobismuth (HDA-BiI5)-based solar device

Molecular ferroelectric HDA-BiI₅ has been utilized as the light-absorbing layer for organic-inorganic hybrid solar cells.

Ultrastable Carbon Quantum Dots-Doped MAPbBr3 Perovskite with Silica Encapsulation

Having suffered from intrinsic structural lability, perovskite quantum dots (PQDs) are extremely unstable under high-temperature and moisture conditions, which have greatly limited their applications. In this work, we propose a novel method to synthesize ultrastable carbon quantum dots (CQDs)-doped methylamine (MA) lead bromide PQDs with SiO₂ encapsulation (CQDs-MAPbBr₃@SiO₂). The kernel CQDs-MAPbBr₃ is formed by the interaction of carboxyl-rich CQDs with MAPbBr₃ via H-bond, which greatly improves the thermal stability of CQDs-MAPbBr₃. Furthermore, highly compact SiO₂ encapsulates the proposed CQDs-MAPbBr₃ via a facile in situ growth strategy, which effectively enhances the water resistance and air stability of CQDs-MAPbBr₃@SiO₂. As a result, the proposed nanomaterial shows extremely high water stability in aqueous solution for over 9 months and ideal thermal stability with strong fluorescence (FL) emission after 150 °C annealing. Based on the superior stability and ultrahigh FL efficiency of this proposed nanomaterial, a primary sensing method for ion (Ag⁺ and Zn²⁺) FL detection has been developed and the mechanism of PQDs-based ion determination has also been discussed, thus exhibiting the potential applications of CQDs-MAPbBr₃@SiO₂ in the area of FL assay and environment monitoring.

Ultrastable, Deformable, and Stretchable Luminescent Organic-Inorganic Perovskite Nanocrystal-Polymer Composites for 3D Printing and White Light-Emitting Diodes

Organic-inorganic perovskite nanocrystals with excellent optoelectronic properties have been utilized in various applications, despite their stability issues. The perovskite materials are sensitive to environments such as polar solvents, moisture, and heat. Thus, they are not used for extrusion three-dimensional (3D) printing, as it is usually conducted in the ambient environment and requires heating to liquefy the printed materials. In this work, 11 thermoplastic polymers conventionally used for extrusion 3D printing were investigated to test their capability as protective encapsulation materials for perovskite nanocrystals. Three of them exhibited good protective properties, and one (polycaprolactone, PCL) of these three could be blended with perovskite nanocrystals to form perovskite nanocrystal-PCL composites, which were deformable and stretchable once heated. Because of the low melting point of PCL, the perovskite nanocrystals maintained their optical properties after 3D printing, and the printed objects were still having fluorescent behavior. Moreover, fluorescent micrometer-sized fibers based on the perovskite nanocrystal-PCL composites could also be simply prepared using cotton candy makers. Perovskite nanocrystal-PCL composite films with different emission wavelengths were incorporated with blue light-emitting diodes (LEDs) to realize white LEDs with Commission Internationale de l'Éclairage chromaticity coordinates of (0.33, 0.33).

Lead-Free Halide Double Perovskite Materials: A New Superstar Toward Green and Stable Optoelectronic Applications

Lead-based halide perovskites have emerged as excellent semiconductors for a broad range of optoelectronic applications, such as photovoltaics, lighting, lasing and photon detection. However, toxicity of lead and poor stability still represent significant challenges. Fortunately, halide double perovskite materials with formula of A₂M(I)M(III)X₆ or A₂M(IV)X₆ could be potentially regarded as stable and green alternatives for optoelectronic applications, where two divalent lead ions are substituted by combining one monovalent and one trivalent ions, or one tetravalent ion. Here, the article provides an up-to-date review on the developments of halide double perovskite materials and their related optoelectronic applications including photodetectors, X-ray detectors, photocatalyst, light-emitting diodes and solar cells. The synthesized halide double perovskite materials exhibit exceptional stability, and a few possess superior optoelectronic properties. However, the number of synthesized halide double perovskites is limited, and more limited materials have been developed for optoelectronic applications to date. In addition, the band structures and carrier transport properties of the materials are still not desired, and the films still manifest low quality for photovoltaic applications. Therefore, we propose that continuing efforts are needed to develop more halide double perovskites, modulate the properties and grow high-quality films, with the aim of opening the wild practical applications.

Solvent-Assisted Tuning of the Size and Shape of CsPbBr3 Nanocrystals via Redispersion Process at Ambient Condition

All-inorganic CsPbBr₃ perovskite nanocrystals are emerging as a new class of semiconductors with outstanding optoelectronic properties and great potential for various applications, such as, lasing, photon detection, photovoltaics, etc. This article provides the effect of solvents on the reprecipitation of CsPbBr₃ perovskite at room temperature. The results observed for CsPbBr₃ perovskite in various antisolvents showed various cubes (nano- to microsized), self-assembly of nanocubes and nanorods. In addition, all of the various sizes (nano to micro) of cubes and self-assembly of nanocubes and shape-controlled nanorods exhibited band gap tuning at the green light region. The corresponding microscopy (field emission scanning electron microscopy and high-resolution transmission electron microscopy) images and photoluminescence quantum yield as well as lifetime decay are presented. To the best of our literature knowledge, this is the first report on various solvent-assisted studies on CsPbBr₃ perovskite nanocrystals.

Pressure-Assisted Annealing Strategy for High-Performance Self-Powered All-Inorganic Perovskite Microcrystal Photodetectors

Owing to their low trap-state density, high carrier mobility, and high thermal stability, CsPbBr₃ perovskite microcrystals (MCs) have attracted significant attention for applications as photodetectors (PDs). However, solution synthesis processes lead to MC films with high void density, seriously limiting the performance of the PDs. Here, a pressure-assisted annealing strategy is introduced to significantly reduce the void density and decrease the surface roughness. The resulting self-powered all-inorganic CsPbBr₃ perovskite MC thick-film PDs show improved performance characteristics, with responsivities and detectivities of up to 0.206 A W^-1 and 7.23 × 10¹² Jones, respectively. Moreover, the on/off ratios of the devices are up to 10⁶, and the highest linear dynamic range reaches 123.5 dB. These improved results indicate that the pressure-assisted annealing method is an effective strategy to enhance the performance of solution-synthesized perovskite MC PDs.

Constructing Sensitive and Fast Lead-Free Single-Crystalline Perovskite Photodetectors

We developed a high-performance photodetector based on (CH₃NH₃)₃Sb₂I₉ (MA₃Sb₂I₉) microsingle crystals (MSCs). The MA₃Sb₂I₉ single crystals exhibit a low-trap state density of ∼10¹⁰ cm^-3 and a long carrier diffusion length reaching 3.0 μm, suggesting its great potential for optoelectronic applications. However, the centimeter single crystal (CSC)-based photodetector exhibits low responsivity (10^-6 A/W under 1 sun illumination) due to low charge-carrier collection efficiency. By constructing the MSC photodetector with efficient charge-carrier collection, the responsivity can be improved by three orders of magnitude (under 1 sun illumination) and reach 40 A/W with monochromatic light (460 nm). Furthermore, the MSC photodetectors exhibit fast response speed of <1 ms, resulting in a high gain of 108 and a gain-bandwidth product of 10⁵ Hz. These numbers are comparable to the lead-perovskite single-crystal-based photodetectors.