Lead Halide Perovskite Single Crystals Research Articles

Photodiodes Made of Cascade Perovskite Single Crystals with Passivation Layers for High-Energy-Resolution γ-ray Spectroscopy.

Lead halide perovskite single crystals (LHPSC) are promising for room-temperature γ-ray spectroscopy in radiation detection. While MA(CH3NH3)-based LHPSCs are the most straightforward and cost-effective to synthesize from solution, their performance in γ-ray spectroscopy is hindered by significant noise and ion migration at high bias, which degrades their energy resolution (ER). The work introduced an n-type/intrinsic/n-type photodiode incorporating passivation layers of MA-based LHPSCs, grown through solution-process epitaxial growth. This single-polarity device demonstrated an outstanding ER of 3.6% for 662 keV γ-ray photons. Overall, this work provides useful information for developing room-temperature γ-ray detectors based on solution-processed lead halide perovskites.

Using N-I-N Photodiodes Made of Perovskite Single Crystals for Low Noise Gamma-Ray Spectroscopy.

Solution-processed lead halide perovskite single crystals (LHPSCs) are believed to have great potential in gamma-ray spectroscopy. However, obtaining low-defect LHPSCs from a solution at low temperatures is difficult compared to obtaining Bridgman single crystals such as CdTe and Si. Herein, noise from the intrinsic defects of LHPSCs is considered as the main problem hindering their gamma-ray detection performance. By isolating the defect-induced holes in LHPSCs via energy barriers, we show that NIN photodiodes based on three types of LHPSCs, i.e., MAPbBr3 (MA = CH3NH3), MAPbBr2.5Cl0.5, and cascade LHPSCs, have demonstrated good energy resolution in the range of 6.7-10.3% for 662 keV 137Cs gamma-ray photons. The noise for >10 mm3 devices is low, in the order of 340-860 electrons, and the electron collection efficiency reaches 23-43%. These results pave the way for obtaining low-cost, large, high energy-resolution gamma-ray detectors at room temperature (300 K).

Electrical and optical investigations lead-free Cesium Bismuth iodide single crystal

Extensive research efforts are currently focused on lead-free all-inorganic perovskite single crystals (SCs) due to their lower biological toxicity and improved material stability. These advancements address two significant obstacles in the implementation of practical device applications. Positive photoconductivity (PPC) under light illumination has been observed in lead-halide perovskite SCs. Given the optical activity of lead-free perovskites, it is anticipated that they will exhibit comparable characteristics. Nevertheless, we present in this study the discovery of negative photoconductivity (NPC) induced by light, coupled with a slow recovery method, in lead-free Cs3Bi2I9 perovskite material, which was successfully synthesized through the novel top-seed solution (TSS) process. In-depth investigations using femtosecond transient reflectance (fs-TR) spectroscopy demonstrate that these unique electronic transport characteristics originate from the generation of light-triggered trap states that are metastable within the perovskite crystal lattice. We computed the performance metrics of Cs3Bi2I9 SC detectors, encompassing detectivity (1.68 × 1012 Jones), responsivity (32 mA/W), and the current ratio (∼10257). The distinctiveness of these findings lies in the unique behavior of Cs3Bi2I9 SCs, which, despite being optically active, exhibit retroactive photocurrent upon irradiation. This behavior has rarely been observed in other lead-free perovskite materials.

Interface Energy Alignment between Lead Halide Perovskite Single Crystals and TIPS-Pentacene.

At present, there is a huge development in optoelectronic applications using lead halide perovskites. Considering that device performance is largely governed by the transport of charges across interfaces and, therefore, the interfacial electronic structure, fundamental investigations of perovskite interfaces are highly necessary. In this study, we use high-resolution soft X-ray photoelectron spectroscopy based on synchrotron radiation to explore the interfacial energetics for the molecular layer of TIPS-pentacene and lead halide perovskite single crystals. We perform ultrahigh vacuum studies on multiple thicknesses of an in situ formed interface of TIPS-pentacene with four different in situ cleaved perovskite single crystals (MAPbI3, MAPbBr3, FAPbBr3, and CsxFA1-xPbBryI3-y). Our findings reveal a substantial shift of the TIPS-pentacene energy levels toward higher binding energies with increasing thickness, while the perovskite energy levels remain largely unaffected regardless of their composition. These shifts can be interpreted as band bending in the TIPS-pentacene, and such effects should be considered when assessing the energy alignment at perovskite/organic transport material interfaces. Furthermore, we were able to follow a reorganization on the MAPbI3 surface with the transformation of the surface C 1s into bulk C 1s.

Charge transport comparison of FA, MA and Cs lead halide perovskite single crystals for radiation detection

We report a systematic comparison of the charge transport and radiation detection properties of inorganic and organic metal bromide single crystal perovskites. We studied the performance of Bridgman-grown CsPbBr3 single crystals, together with solution-grown FAPbBr3 and MAPbBr3 single crystals. Laser time of flight is used to measure the drift mobilities for all samples, and we report a maxium mobility value of 121 ± 10 cm V−1 s−1 for CsPbBr3. Alpha particle measurements were used to assess the mobility-lifetime products, with values recorded in the range of 2 × 10−4 cm2 V−1 to 1 × 10−3 cm2 V−1. Low temperature measurements showed an increase in bulk resistivity at temperatures down to 260 K, but no significant change to the drift mobilities. The overall performance of the Cs, FA and MA samples is compared and their potential for use in gamma spectroscopy measurements is discussed.

Additive-Enhanced Crystallization of Inorganic Perovskite Single Crystals for High-Sensitivity X-Ray Detection.

Inorganic cesium lead halide perovskite single crystals are particularly intriguing to ionizing radiation detection by virtue of their material stability and high attenuation coefficients. However, the growth of high-quality inorganic perovskite single crystals remains challenging, mainly due to the limited solubility. In this work, an additive-enhanced crystallization method is proposed for cesium lead perovskites. The additive can remarkably increase the solubility of cesium bromide in dimethyl sulfoxide (DMSO) forming a balanced stoichiometric precursor solution, which prevents the formation of impurity phases. In addition, the additives would react with DMSO generating glyoxylic acid (GLA) via nucleophilic substitution and Kornblum oxidation reactions. The GLA can form stable PbBr2 -DMSO-GLA complexes, which enables better crystallinity, uniformity and much longer carrier lifetimes for the grown single crystals. The X-ray detectors using the additive-enhanced crystals exhibit an ultra-high sensitivity of 3.0 × 104 µC Gyair -1 cm-2 which is more than two orders of magnitude higher than that for the control devices.

Universal approach for diffusion quantification applied to lead halide perovskite single crystals

A universal approach to calculating diffusion coefficients in lead halide perovskite single crystals, which have ionic and mixed ionic–electronic conductivity, is proposed. Using impedance spectroscopy, it is demonstrated how to model a non-ideal Warburg element and transmission line equivalent circuit to identify ionic diffusion in the material. The proposed method is applicable to samples of any thickness and electrical properties. Additionally, it is shown how to overcome the challenges of low-frequency impedance measurement and the non-ideal behavior of the elements through extrapolative modeling and approximation.

Perovskite-like Silver Halide Single-Crystal Microbelt Enables Ultrasensitive Flexible X-ray Detectors.

Lead halide perovskite single crystals have attracted wide interest in the field of X-ray detection due to their excellent photophysical properties. However, their inherent toxicity and high thickness restrict their applications in flexible devices. In this paper, designing a micronanometer-scale X-ray detector based on all-inorganic lead-free CsAg2I3 (CAI) single crystal microbelts (MBs) has addressed the above issues. These CAI single crystal MBs can be synthesized on various substrates with high crystal quality and excellent stability. Based on their excellent characteristics of the CAI MBs, we fabricate single CAI MB devices with an Au/CAI/Au structure, which shows not only good ultraviolet photoresponse characteristics, but also excellent X-ray detection performance. The optimized CAI photodetectors exhibit a responsivity of 23.59 mA/W, a high detectivity of 1010 Jones, and a fast response speed. For X-ray detection performance, a sensitivity of up to 515.49 μC Gyair-1 cm-2 and a detection limit of as low as 14.65 μGyair s-1 are achieved with outstanding operation stability and excellent long-term stability. Furthermore, our devices also showed excellent applicability for X-ray imaging, which is promising for their use in X-ray detection and imaging. Finally, flexible X-ray detectors are fabricated by using thin CAI single-crystal MBs and demonstrate good flexibility under different bending radii and bending cycles. Our work shows the potential for developing highly sensitive flexible integrated micro/nano optoelectronic devices by using lead-free perovskite analogue single crystals.

Spatially Resolved Local Electronic Properties of 2D Lead Halide Perovskite Single Crystals Studied by X‐Ray Photoemission Electron Microscopy

Recently, research on the edge states of 2D lead halide perovskites (LHPs) has been attracting much attention. The lower‐energy edge state (LES) is believed to provide an efficient pathway for the dissociation of photoexcited excitons. However, the mechanism of the LES formation remains controversial, and studies that establish precisely the local electronic properties are lacking. Herein, the first study of spatially resolved electronic structures in 2D LHP single‐crystal flakes by X‐ray photoemission electron microscopy is presented, specifically identifying the contribution from the edge area. The results show that blueshifts occur in the Pb 5d core‐level peaks at the edge area compared to the interior area with much less difference in I 4d core‐level peaks. The shift becomes more pronounced as n varies from 1 to 3 (≈0.2–1.0 eV). This phenomenon is attributed to the surface restructuring of the edge area induced by the release of mechanical strain through lattice expansion. This work provides an important reference on the origin of the LES of 2D LHPs and is beneficial for future optoelectronic device applications.

Nanomechanical signatures of degradation-free influence of water on halide perovskite mechanics

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

Searching for High-Quality Halide Perovskite Single Crystals toward X-ray Detection.

Metal halide perovskite materials, which combine outstanding physical properties, large absorption coefficient, tailored composition, and low-cost solution-processing, have aroused wide attention for use in various optoelectronic devices. Recently, perovskite single crystals have been rapidly outpacing traditional semiconductor materials in the field of radiation detection. As a prerequisite, achieving high-quality single crystals under controllable solution-phase growth must be tackled to fulfill their full potential as a new paradigm in this stagnated field. This Perspective summarizes the advances in X-ray detectors based on lead halide perovskite single crystals, presenting a comprehensive picture of the relationship among composition engineering, synthesis, and device properties. Additionally, we share our thoughts on several outstanding challenges of perovskite single crystals as promising X-ray detectors and propose possible approaches to the unresolved issues. We anticipate that this Perspective can open up new opportunities to improve their optoelectronic properties, which confers fascinating photonics applications with above and beyond state-of-the-art performance.

Long carrier diffusion length in two-dimensional lead halide perovskite single crystals

<h2>Summary</h2> Ruddlesden-Popper (RP) perovskites are two-dimensional semiconductors for high-performance optoelectronic devices. In this work, we report a long in-plane carrier diffusion length in 2D RP perovskite single crystals probed by scanning photocurrent microscopy. Carrier diffusion lengths of 7–14 μm are observed when the number of PbI₆⁻² octahedra between organic spacers increases from 1 to 3. Using detailed light intensity and electric-field-dependent photocurrent measurements, we attribute the observed long diffusion length to the dominating dissociated free carrier transport. This is further validated by time-resolved photoluminescence measurements, where the decay lifetime increases in the presence of an electric field. From our experiments, we conclude that the in-plane transport in RP perovskites is efficient because of the partial free carrier generation, which overcomes strong excitonic effects. Our results suggest that semiconducting devices fabricated from RP perovskite single crystals can be as efficient as their 3D counterparts.

Spin-orbit-coupled exciton-polariton condensates in lead halide perovskites.

Spin-orbit coupling (SOC) is responsible for a range of spintronic and topological processes in condensed matter. Here, we show photonic analogs of SOCs in exciton-polaritons and their condensates in microcavities composed of birefringent lead halide perovskite single crystals. The presence of crystalline anisotropy coupled with splitting in the optical cavity of the transverse electric and transverse magnetic modes gives rise to a non-Abelian gauge field, which can be described by the Rashba-Dresselhaus Hamiltonian near the degenerate points of the two polarization modes. With increasing density, the exciton-polaritons with pseudospin textures undergo phase transitions to competing condensates with orthogonal polarizations. Unlike their pure photonic counterparts, these exciton-polaritons and condensates inherit nonlinearity from their excitonic components and may serve as quantum simulators of many-body SOC processes.

Template directed perovskite X-ray detectors towards low ionic migration and low interpixel cross talking

The metal halide perovskites exhibit excellent performance as the direct X-ray detectors owing to their strong absorption capability, long carrier lifetime and diffusion length, radiation ruggedness, etc. For imaging applications, the ionic migration of perovskites and charge sharing effect between the adjacent pixels have a significantly negative impact on the spatial resolution. Herein, for the first time, the porous anodic aluminum oxides (AAO) have been used as a template to grow the CsPbBr2I thick film for the direct X-ray detection. Benefiting from the oxygen passivation effect, the activation energy for ionic migration has been observed to increase to 0.701 eV, whereas the dark current drift (1.01 × 10−5 nA cm−1s−1V−1) is one to two orders of magnitude lower than the other lead halide perovskite single crystals and films. Moreover, the AAO insulating wall effectively blocks the charge diffusion effect across a pixel pitch of 10 μm. Overall, the findings reported in this study open a new route for reducing the ionic migration and pixel crosstalk, thus, bringing the perovskite X-ray detectors close to the practical applications.

Giant Bulk Photostriction and Accurate Photomechanical Actuation in Hybrid Perovskites

AbstractIt is well known that a material may be strained by mechanical, thermal, electric, magnetic, and light stimuli, and this effect has been extensively utilized in industries. However, the observed photostrictive effect usually occurs in the very thin surface layer and the photostriction of most bulk materials has been too small to be applied in a device until now. Here, a giant bulk photostriction is achieved, evidenced by the measured linear strain ε ≈ 0.72–0.43% for MAPbI3 single crystal plates of 0.05–0.5 mm in thickness under the 532 nm illumination. More importantly, the MAPbI3 single crystals also exhibit accurate photomechanical actuation functionality and the actuator can precisely adjust the displacement from hundreds of pm to tens of μm or the angle of a hard mirror from ≈10−6 to 0.2 degree. The present work not only unveils the huge bulk photostriction in lead halide perovskite single crystals but also demonstrates a wireless photomechanical actuator which is much simpler and smaller than conventional piezoelectric actuators.

Free Carriers versus Self-Trapped Excitons at Different Facets of Ruddlesden-Popper Two-Dimensional Lead Halide Perovskite Single Crystals.

The physical origin of sub-band gap photoluminescence in Ruddlesden–Poppers two-dimensional (2D) lead halide perovskites (LHPs) is still under debate. In this paper, we studied the photoluminescence features from two different facets of 2D LHP single crystals: the in-plane facet (IF) containing the 2D inorganic layers and the facet perpendicular to the 2D layers (PF). At the IF, the free carriers (FCs) dominate due to the weak electron–phonon coupling in a symmetric lattice. At the PF, the strain accumulation along the 2D layers enhances the electron–phonon coupling and facilitates self-trapped exciton (STE) formation. The time-resolved PL studies indicate that free carriers (FCs) at the IF can move freely and display the trapping by the intrinsic defects. The STEs at the PF are not likely trapped by the defects due to the reduced mobility. However, with increasing STE density, the STE transport is promoted, enabling the trapping of STE by the intrinsic defects.

A polymer controlled nucleation route towards the generalized growth of organic-inorganic perovskite single crystals

Recently, there are significant progresses in the growth of organic-inorganic lead halide perovskite single crystals, however, due to their susceptible nucleation and growth mechanisms and solvent requirements, the efficient and generalized growth for these single crystals is still challenging. Here we report the work towards this target with a polymer-controlled nucleation process for the highly efficient growth of large-size high-quality simple ternary, mixed-cations and mixed-halide perovskite single crystals. Among them, the carrier lifetime of FAPbBr3 single crystals is largely improved to 10199 ns. Mixed MA/FAPbBr3 single crystals are synthesized. The crucial point in this process is suggested to be an appropriate coordinative interaction between polymer oxygen groups and Pb2+, greatly decreasing the nuclei concentrations by as much as 4 orders of magnitudes. This polymer-controlled route would help optimizing the solution-based OIHPs crystal growth and promoting applications of perovskite single crystals.

Dark Self-Healing-Mediated Negative Photoconductivity of a Lead-Free Cs3Bi2Cl9 Perovskite Single Crystal.

Recently, halide perovskites have emerged as a promising material for device applications. Lead-based perovskites have been widely explored, while investigation of the optical properties of lead-free perovskites remains limited. Lead-halide perovskite single crystals have shown light-induced positive photoconductivity, and as lead-free perovskites are optically active, they are expected to demonstrate similar properties. However, we report here light-induced negative photoconductivity with slow recovery in lead-free Cs3Bi2Cl9 perovskite. Femtosecond transient reflectance (fs-TR) spectroscopy studies further reveal that these electronic transport properties are due to the formation of light-activated metastable trap states within the perovskite crystal. The figure of merits were calculated for Cs3Bi2Cl9 single-crystal detectors, including responsivity (17 mA/W), detectivity (6.23 × 1011 Jones), and the ratio of current in dark to light (∼7160). These observations for Cs3Bi2Cl9 single crystals, which were optically active but showed retroactive photocurrent on irradiation, remain unique for such materials.

Highly Sensitive and Stable X-ray Detector Based on a 0D Structural Cs4PbI6 Single Crystal.

Lead halide perovskite single crystals with a high X-ray stopping power and large μτ product have been successfully used for X-ray detection. However, poor air stability and ionic migration lead to the degradation of the devices for long-term operations. Here, we report a solution-processed lead 0D bulk Cs4PbI6 single crystal, which have I atoms in isolated [PbI6]4-. This 0D Cs4PbI6 single crystal has a μτ product of 9.7 × 10-4 cm2 V-1 and an activation energy of 321.28 meV. We have fabricated a photoconductor device with a symmetric Au/Cs4PbI6/Au sandwich structure, which exhibits a high sensitivity of 451.49 μC Gy-1 cm-2 under 30 keV X-ray irradiation at an applied voltage of 30 V. After storing the device in air at room temperature for three months, the device retains nearly the same sensitivity as the original. These results demonstrate that this 0D Cs4PbI6 perovskite single crystal has great potential for practical X-ray detection applications.

Halide Perovskite Single Crystals: Optoelectronic Applications and Strategical Approaches

Halide perovskite is one of the most promising semiconducting materials in a variety of fields such as solar cells, photodetectors, and light-emitting diodes. Lead halide perovskite single crystals featuring long diffusion length, high carrier mobility, large light absorption coefficient and low defect density, have been attracting increasing attention. Fundamental study of the intrinsic nature keeps revealing the superior optoelectrical properties of perovskite single crystals over their polycrystalline thin film counterparts, but to date, the device performance lags behind. The best power conversion efficiency (PCE) of single crystal-based solar cells is 21.9%, falling behind that of polycrystalline thin film solar cells (25.2%). The oversized thickness, defective surfaces, and difficulties in depositing functional layers, hinder the application of halide perovskite single crystals in optoelectronic devices. Efforts have been made to synthesize large-area single crystalline thin films directly on conductive substrates and apply defect engineering approaches to improve the surface properties. This review starts from a comprehensive introduction of the optoelectrical properties of perovskite single crystals. Then, the synthesis methods for high-quality bulk crystals and single-crystalline thin films are introduced and compared, followed by a systematic review of their optoelectronic applications including solar cells, photodetectors, and X-ray detectors. The challenges and strategical approaches for high-performance applications are summarized at the end with a brief outlook on future work.