One-Pot Colloidal Synthesis Enables Highly Tunable InSb Short-Wave Infrared Quantum Dots Exhibiting Carrier Multiplication.

Colloidal quantum dots (CQDs) are emerging materials for short-wave infrared (SWIR, ≈1100-3000 nm) photodetectors, which are technologically important for a broad array of applications. Unfortunately, the most developed SWIR CQD systems are Pb and Hg chalcogenides; their toxicity and regulated compositions limit their applications. InSb CQD system is a potential environmentally friendly alternative, whose bandgap in theory, is tunable via quantum confinement across the SWIR spectrum. However, InSb CQDs are difficult to exploit, due to their complex syntheses and uncommon reactive precursors, which greatly hinder their application and study. Here, a one-pot synthesis strategy is reported using commercially available precursors to synthesize-under standard colloidal synthesis conditions-high-quality, size-tunable InSb CQDs. With this strategy, the large Bohr exciton radius of InSb can be exploited for tuning the bandgap of the CQDs over a wide range of wavelengths (≈1250-1860 nm) across the SWIR region. Furthermore, by changing the surface ligands of the CQDs from oleic acid (OA) to 1-dodecanthiol (DDT), a ≈20-fold lengthening in the excited-state lifetime, efficient carrier multiplication, and slower carrier annihilation are observed. The work opens a wide range of SWIR applications to a promising class of Pb- and Hg-free CQDs.

InAs Nanorod Colloidal Quantum Dots with Tunable Bandgaps Deep into the Short-Wave Infrared.

InAs colloidal quantum dots (CQDs) have emerged as candidate lead- and mercury-free solution-processed semiconductors for infrared technology due to their appropriate bulk bandgap, which can be tuned by quantum confinement, and promising charge-carrier transport properties. However, the lack of suitable arsenic precursors and readily accessible synthesis conditions have limited InAs CQDs to smaller sizes (<7 nm), with bandgaps largely restricted to <1400 nm in the near-infrared spectral window. Conventional InAs CQD synthesis requires highly reactive, hazardous arsenic precursors, which are commercially scarce, making the synthesis hard to control and study. Here, we present a controlled synthesis strategy (using only readily available and less reactive precursors) to overcome the practical wavelength limitation of InAs CQDs, achieving monodisperse InAs nanorod CQDs with bandgaps tunable from ∼1200 to ∼1800 nm, thus crossing deep into the short-wave infrared (SWIR) region. By controlling the reactivity through in situ precursor complexation, we isolate the reaction mechanism, producing InAs nanorod CQDs that display narrow excitonic features and efficient carrier multiplication. Our work enables InAs CQDs for a wider range of SWIR applications.

Efficient Spin-Orbit Charge-Transfer Intersystem Crossing and Slow Intramolecular Triplet-Triplet Energy Transfer in Bodipy-Perylenebisimide Compact Dyads and Triads.

Bodipy (BDP)-perylenebisimide (PBI) donor-acceptor dyads/triad were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). For BDP-PBI-3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (ΦΔ =85 %) was observed than the bay-substituted derivative BDP-PBI-1 (ΦΔ =30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP-PBI-3, while for BDP-PBI-1, CS takes 2.8 ps. For triad BDP-PBI-2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long-lived 3 PBI* (179.8 µs) state is populated. Nanosecond transient absorption spectra of BDP-PBI-3 show that the CR gives upper triplet excited state (3 BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 µs), the 3 PBI* state is finally populated, indicating that upper triplet state is involved in SOCT-ISC. Time-resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT-ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S-T+1 /T0 states mixing.

Radioluminescent Cu-Au Metal Nanoclusters: Synthesis and Self-Assembly for Efficient X-ray Scintillation and Imaging.

Zero-dimensional (0D) scintillation materials have drawn tremendous attention due to their inherent advantages in the fabrication of flexible high-energy radiation scintillation screens by solution processes. Although considerable progress has been made in the development of 0D scintillators, such as the current leading lead-halide perovskite nanocrystals and quantum dots, challenges still persist, including potential issues with self-absorption, air stability, and eco-friendliness. Here, we present a strategy to overcome those limitations by synthesis and self-assembly of a new class of scintillators based on metal nanoclusters. We demonstrate the gram-scale synthesis of an atomically precise nanocluster with a Cu-Au alloy core exhibiting high phosphorescence quantum yield, aggregation-induced emission enhancement (AIEE) behavior, and intense radioluminescence. By controlling solvent interactions, the AIEE-active nanoclusters were self-assembled into submicron spherical superparticles in solution, which we exploited as a novel building block for flexible particle-deposited scintillation films with high-resolution X-ray imaging performance. This work reveals metal nanoclusters and their self-assembled superstructures as a promising class of scintillators for practical applications in high-energy radiation detection and imaging.

A long-lived charge-separated state of spiro compact electron donor-acceptor dyads based on rhodamine and naphthalenediimide chromophores.

Spiro rhodamine (Rho)-naphthalenediimide (NDI) electron donor-acceptor orthogonal dyads were prepared to generate a long-lived charge separation (CS) state based on the electron spin control approach, i.e. to form the 3CS state, not the 1CS state, to prolong the CS state lifetime by the electron spin forbidden feature of the charge recombination process of 3CS → S0. The electron donor Rho (lactam form) is attached via three σ bonds, including two C-C and one N-N bonds (Rho-NDI), or an intervening phenylene, to the electron acceptor NDI (Rho-Ph-NDI and Rho-PhMe-NDI). Transient absorption (TA) spectra show that fast intersystem crossing (ISC) (<120 fs) occurred to generate an upper triplet state localized on the NDI moiety (3NDI*), and then to form the CS state. For Rho-NDI in both non-polar and polar solvents, a long-lived 3CS state (lifetime τ = 0.13 µs) and charge separation quantum yield (Φ CS) up to 25% were observed, whereas for Rho-Ph-NDI (τ T = 1.1 µs) and Rho-PhMe-NDI (τ T = 2.0 µs), a low-lying 3NDI* state was formed by charge recombination (CR) in n-hexane (HEX). In toluene (TOL), however, CS states were observed for Rho-Ph-NDI (0.37 µs) and Rho-PhMe-NDI (0.63 µs). With electron paramagnetic resonance (EPR) spectra, weak electronic coupling between the Rho and NDI moieties for Rho-NDI was proved. Time-resolved EPR (TREPR) spectra detected two transient species including NDI-localized triplets (formed via SOC-ISC) and a 3CS state. The CS state of Rho-NDI features the largest dipolar interaction (|D| = 184 MHz) compared to Rho-Ph-NDI (|D| = 39 MHz) and Rho-PhMe-NDI (|D| = 41 MHz) due to the smallest distance between Rho and NDI moieties. For Rho-NDI, the time-dependent e,a → a,e phase change of the CS state TREPR spectrum indicates that the long-lived CS state is based on the electron spin control effect.

Quantum Tunneling Effect in CsPbBr3 Multiple Quantum Wells.

Two-dimensional (2D) lead halide perovskites (LHPs) have garnered incredible attention thanks to their exciting optoelectronic properties and intrinsic strong quantum confinement effect. Herein, we carefully investigate and decipher the charge carrier dynamics at the interface between CsPbBr3 multiple quantum wells (MQWs) as the photoactive layer and TiO2 and Spiro-OMeTAD as electron and hole transporting materials, respectively. The fabricated MQWs comprise three monolayers of CsPbBr3 separated by 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as barriers. By varying the BCP thickness, we show that charge carrier extraction from MQWs to the corresponding extracting layer occurs through a quantum tunneling effect, as elaborated by steady-state and time-resolved photoluminescence measurements and further verified by femtosecond transient absorption experiments. Ultimately, we have investigated the impact of the barrier-thickness-dependent quantum tunneling effect on the photoelectric behavior of the synthesized QW photodetector devices. Our findings shed light on one of the most promising approaches for efficient carrier extraction in quantum-confined systems.

Lecithin Capping Ligands Enable Ultrastable Perovskite-Phase CsPbI3 Quantum Dots for Rec. 2020 Bright-Red Light-Emitting Diodes.

Bright-red light-emitting diodes (LEDs) with a narrow emission line width that emit between 620 and 635 nm are needed to meet the latest industry color standard for wide color gamut displays, Rec. 2020. CsPbI3 perovskite quantum dots (QDs) are one of the few known materials that are ideally suited to meet these criteria. Unfortunately, CsPbI3 perovskite QDs are prone to transform into a non-red-emitting phase and are subject to further degradation mechanisms when their luminescence wavelength is tuned to match that of the Rec. 2020 standard. Here, we show that zwitterionic lecithin ligands can stabilize the perovskite phase of CsPbI3 QDs for long periods in air for at least 6 months compared to a few days for control samples. LEDs fabricated with our ultrastable lecithin-capped CsPbI3 QDs exhibit an external quantum efficiency (EQE) of 7.1% for electroluminescence centered at 634 nm─a record for all-inorganic perovskite nanocrystals in Rec. 2020 red. Our devices achieve a maximum luminance of 1391 cd/m2 at 7.5 V, and their operational half-life is 33 min (T50) at 200 cd/m2─a 10-fold enhancement compared to control samples. Density functional theory results suggest that the surface strain in CsPbI3 QDs capped with the conventional ligands, oleic acid and oleylamine, contributes to the instability of the perovskite structural phase. On the other hand, lecithin binding induces virtually no surface strain and shows a stronger binding tendency for the CsPbI3 surface. Our study highlights the tremendous potential of zwitterionic ligands in stabilizing the perovskite phase and particle size of CsPbI3 QDs for various optoelectronic applications.

Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free Cs3Bi2I9 Perovskite Single Crystal.

In this report, we study the hot carrier cooling mechanism of the Cs3Bi2I9 single crystal by using femtosecond transient reflectance (fs-TR) spectroscopy. We find an unusual slow hot carrier cooling associated with longitudinal optical (LO) and coherent longitudinal acoustic phonons (CLAPs) emission during the deexcitation of the hot carriers. We posit the interplay between the hot-carriers and the LO and CLA phonons in subpicosecond to subnanosecond time scales, respectively, by analyzing the TR kinetics upon perturbation with excess energy. Furthermore, we measured the CLAPs propagation velocity in Cs3Bi2I9, the crystal, ranging from 1820 to 2000 ms-1. The elastic constants and frequency of Brillouin oscillations were estimated as 20.08 GPa and 14.66 GHz, respectively. Our discovery delivers new physical insights into how the hot carriers in Cs3Bi2I9 single crystal are coupled with a crystal lattice that controls the hot carrier dynamics.

Visible-Light Copper Nanocluster Catalysis for the C-N Coupling of Aryl Chlorides at Room Temperature.

Activation of aryl chlorides in cross-coupling reactions is a long-standing challenge in organic synthesis that is of great interest to industry. Ultrasmall (<3 nm), atomically precise nanoclusters (NCs) are considered one of the most promising catalysts due to their high surface area and unsaturated active sites. Herein, we introduce a copper nanocluster-based catalyst, [Cu61(StBu)26S6Cl6H14] (Cu61NC) that enables C-N bond-forming reactions of aryl chlorides under visible-light irradiation at room temperature. A range of N-heterocyclic nucleophiles and electronically and sterically diverse aryl/hetero chlorides react in this new Cu61NC-catalyzed process to afford the C-N coupling products in good yields. Mechanistic studies indicate that a single-electron-transfer (SET) process between the photoexcited Cu61NC complex and aryl halide enables the C-N-arylation reaction.

Insight into the role of reduced graphene oxide in enhancing photocatalytic hydrogen evolution in disordered carbon nitride.

Compared to crystalline carbon nitride, the performance of disordered carbon nitride (d-CN) as a hydrogen production photocatalyst is extremely poor. Owing to its disordered atomic orientation, it is prone to numerous defect states. These energy states are potential sites for trapping and recombination of photogenerated charge carriers. As a result, rapid recombination of photogenerated charge carriers places a fundamental photophysical challenge in charge separation and transport, which inhibits its photocatalytic activity. In the presence of reduced graphene oxide (rGO), d-CN shows enhanced photocatalytic production of hydrogen. However, photophysical insight into the tacit role of rGO is not well understood which limits the rational design of d-CN as a photocatalyst. Particularly, understanding of the early time-scale (in fs to ps) recombination mechanism and the charge transport kinetics has not yet been achieved. With the help of ultrafast transient absorption spectroscopy, femtosecond time-resolved photoluminescence spectroscopy and transient photocurrent measurements, this article deciphers the ultrafast dynamics of the separation and transport of photogenerated charge carriers in d-CN facilitated by rGO. It is found that rGO substantially suppresses the bimolecular and trap-assisted recombination and enables a faster separation of charge carriers. As a result, it increases the lifetime of the charge carriers to be transported to the surface catalytic sites, and therefore, augments the rate of hydrogen production almost by an order of magnitude. Our findings therefore offer a proof-of-concept for overcoming the trap-mediated recombination problems in disordered carbon nitride.

Ultrafast transient infrared spectroscopy for probing trapping states in hybrid perovskite films.

Studying the charge dynamics of perovskite materials is a crucial step to understand the outstanding performance of these materials in various fields. Herein, we utilize transient absorption in the mid-infrared region, where solely electron signatures in the conduction bands are monitored without external contributions from other dynamical species. Within the measured range of 4000 nm to 6000 nm (2500-1666 cm-1), the recombination and the trapping processes of the excited carriers could be easily monitored. Moreover, we reveal that within this spectral region the trapping process could be distinguished from recombination process, in which the iodide-based films show more tendencies to trap the excited electrons in comparison to the bromide-based derivatives. The trapping process was assigned due to the emission released in the mid-infrared region, while the traditional band-gap recombination process did not show such process. Various parameters have been tested such as film composition, excitation dependence and the probing wavelength. This study opens new frontiers for the transient mid-infrared absorption to assign the trapping process in perovskite films both qualitatively and quantitatively, along with the potential applications of perovskite films in the mid-IR region.

Tunable Selectivity in CO2 Photo-Thermal Reduction by Perovskite-Supported Pd Nanoparticles.

Photo-thermal catalysis has recently emerged as a promising alternative to overcome the limitations of traditional photocatalysis. Despite its potential, most of the photo-thermal systems still lack adequate selectivity patterns and appropriate analysis of the underlying reaction pathways, thus hampering a wide implementation. Herein, a novel photocatalyst based on Pd nanoparticles (NPs) supported on barium titanate (BTO) was prepared for the selective photo-thermal reduction of CO2 and displayed catalytic rates of up to 8.2 molCO gPd -1 h-1 . The photocatalyst allowed for a tailored selectivity towards CO or CH4 as a function of the metal loading or the light intensity. Mechanistic studies indicated that both thermal and non-thermal contributions of light played a role in the overall reaction pathway, each of them being dominant upon changing reaction conditions.

There is plenty of room at the top: generation of hot charge carriers and their applications in perovskite and other semiconductor-based optoelectronic devices.

Hot charge carriers (HC) are photoexcited electrons and holes that exist in nonequilibrium high-energy states of photoactive materials. Prolonged cooling time and rapid extraction are the current challenges for the development of future innovative HC-based optoelectronic devices, such as HC solar cells (HCSCs), hot energy transistors (HETs), HC photocatalytic reactors, and lasing devices. Based on a thorough analysis of the basic mechanisms of HC generation, thermalization, and cooling dynamics, this review outlines the various possible strategies to delay the HC cooling as well as to speed up their extraction. Various materials with slow cooling behavior, including perovskites and other semiconductors, are thoroughly presented. In addition, the opportunities for the generation of plasmon-induced HC through surface plasmon resonance and their technological applications in hybrid nanostructures are discussed in detail. By judiciously designing the plasmonic nanostructures, the light coupling into the photoactive layer and its optical absorption can be greatly enhanced as well as the successful conversion of incident photons to HC with tunable energies can also be realized. Finally, the future outlook of HC in optoelectronics is highlighted which will provide great insight to the research community.

Chromophore Orientation-Dependent Photophysical Properties of Pyrene-Naphthalimide Compact Electron Donor-Acceptor Dyads: Electron Transfer and Intersystem Crossing.

In order to study the effect of mutual orientation of the chromophores in compact electron donor-acceptor dyads on the spin-orbit charge transfer intersystem crossing (SOCT-ISC), we prepared naphthalimide (NI)-pyrene (Py) compact electron donor-acceptor dyads, in which pyrene acts as an electron donor and NI is an electron acceptor. The connection of the two units is at the 4-C and 3-C positions of the NI unit and the 1-position of the pyrene moiety for dyads NI-Py-1 and NI-Py-2, respectively. A charge transfer absorption band was observed for both dyads in the UV-vis absorption spectra. Upon nanosecond pulsed laser excitation, long-lived triplet states (lifetime is 220 µs) were observed and the triplet state was confined to the pyrene moiety. The ISC efficiency is moderate to high in nonpolar to polar solvents (singlet oxygen quantum yield: ΦΔ = 14-52%). Ultrafast charge separation (ca. 0.81 ps) and charge recombination-induced ISC (∼3.0 ns) were observed by femtosecond transient absorption spectroscopy. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism; interestingly, the observed electron spin polarization pattern of the triplet state is chromophore orientation-dependent; and the population rates of the triplet sublevels of NI-Py-1 (Px:Py:Pz = 0.2:0.8:0) are drastically different from those of NI-Py-2 (Px:Py:Pz = 0:0:1).

Manipulation of hot carrier cooling dynamics in two-dimensional Dion-Jacobson hybrid perovskites via Rashba band splitting.

Hot-carrier cooling processes of perovskite materials are typically described by a single parabolic band model that includes the effects of carrier-phonon scattering, hot phonon bottleneck, and Auger heating. However, little is known (if anything) about the cooling processes in which the spin-degenerate parabolic band splits into two spin-polarized bands, i.e., the Rashba band splitting effect. Here, we investigated the hot-carrier cooling processes for two slightly different compositions of two-dimensional Dion-Jacobson hybrid perovskites, namely, (3AMP)PbI4 and (4AMP)PbI4 (3AMP = 3-(aminomethyl)piperidinium; 4AMP = 4-(aminomethyl)piperidinium), using a combination of ultrafast transient absorption spectroscopy and first-principles calculations. In (4AMP)PbI4, upon Rashba band splitting, the spin-dependent scattering of hot electrons is responsible for accelerating hot-carrier cooling at longer delays. Importantly, the hot-carrier cooling of (4AMP)PbI4 can be extended by manipulating the spin state of the hot carriers. Our findings suggest a new approach for prolonging hot-carrier cooling in hybrid perovskites, which is conducive to further improving the performance of hot-carrier-based optoelectronic and spintronic devices.

Twisted BODIPY derivative: intersystem crossing, electron spin polarization and application as a novel photodynamic therapy reagent.

The photophysical properties of a heavy atom-free BODIPY derivative with a twisted π-conjugated framework were studied. Efficient intersystem crossing (ISC quantum yield: 56%) and an exceptionally long-lived triplet state were observed (4.5 ms in solid polymer film matrix and 197.5 µs in solution). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations confirmed the delocalization of the triplet state on the whole twisted π-conjugated framework and the zero-field-splitting (ZFS) D parameter of D = -69.5 mT, which is smaller than that of 2,6-diiodoBODIPY (D = -104.6 mT). The electron spin polarization (ESP) phase pattern of the triplet state TREPR spectrum of the twisted BODIPY is (a, a, e, a, e, e), which is different from that of 2,6-diiodo BODIPY (e, e, e, a, a, a), indicating that the electron spin selectivity of the ISC of the twisted structure is different from that of the spin orbital coupling effect. According to the computed spin-orbit coupling matrix elements (0.154-1.964 cm-1), together with the matched energy of the S1/Tn states, ISC was proposed to occur via S1→T2/T3. The computational results were consistent with TREPR results on the electron spin selectivity (the overpopulation of the TY sublevel of the T1 state). The advantage of the long-lived triplet state of the twisted BODIPY was demonstrated by its efficient singlet oxygen (1O2) photosensitizing (ΦΔ = 50.0%) even under a severe hypoxia atmosphere (pO2 = 0.2%, v/v). A high light toxicity (EC50 = 1.0 µM) and low dark toxicity (EC50 = 78.5 µM) were observed for the twisted BODIPY, and thus the cellular studies demonstrate its potential as a novel potent heavy atom-free photodynamic therapy (PDT) agent.

[Cu15 (PPh3 )6 (PET)13 ]2+ : a Copper Nanocluster with Crystallization Enhanced Photoluminescence.

Due to their atomically precise structure, photoluminescent copper nanoclusters (Cu NCs) have emerged as promising materials in both fundamental studies and technological applications, such as bio-imaging, cell labeling, phototherapy, and photo-activated catalysis. In this work, a facile strategy is reported for the synthesis of a novel Cu NCs coprotected by thiolate and phosphine ligands, formulated as [Cu15 (PPh3 )6 (PET)13 ]2+ , which exhibits bright emission in the near-infrared (NIR) region (≈720 nm) and crystallization-induced emission enhancement (CIEE) phenomenon. Single crystal X-ray crystallography shows that the NC possesses an extraordinary distorted trigonal antiprismatic Cu6 core and a, unique among metal clusters, "tri-blade fan"-like structure. An in-depth structural investigation of the ligand shell combined with density functional theory calculations reveal that the extended CH···π and π-π intermolecular ligand interactions significantly restrict the intramolecular rotations and vibrations and, thus, are a major reason for the CIEE phenomena. This study provides a strategy for the controllable synthesis of structurally defined Cu NCs with NIR luminescence, which enables essential insights into the origins of their optical properties.

[Ag9(1,2-BDT)6]3-: How Square-Pyramidal Building Blocks Self-Assemble into the Smallest Silver Nanocluster.

The emerging promise of few-atom metal catalysts has driven the need for developing metal nanoclusters (NCs) with ultrasmall core size. However, the preparation of metal NCs with single-digit metallic atoms and atomic precision is a major challenge for materials chemists, particularly for Ag, where the structure of such NCs remains unknown. In this study, we developed a shape-controlled synthesis strategy based on an isomeric dithiol ligand to yield the smallest crystallized Ag NC to date: [Ag9(1,2-BDT)6]3- (1,2-BDT = 1,2-benzenedithiolate). The NC's crystal structure reveals the self-assembly of two Ag square pyramids through preferential pyramidal vertex sharing of a single metallic Ag atom, while all other Ag atoms are incorporated in a motif with thiolate ligands, resulting in an elongated body-centered Ag9 skeleton. Steric hindrance and arrangement of the dithiolated ligands on the surface favor the formation of an anisotropic shape. Time-dependent density functional theory based calculations reproduce the experimental optical absorption features and identify the molecular orbitals responsible for the electronic transitions. Our findings will open new avenues for the design of novel single-digit metal NCs with directional self-assembled building blocks.

Engineering Band-Type Alignment in CsPbBr3 Perovskite-Based Artificial Multiple Quantum Wells.

Semiconductor heterostructures of multiple quantum wells (MQWs) have major applications in optoelectronics. However, for halide perovskites-the leading class of emerging semiconductors-building a variety of bandgap alignments (i.e., band-types) in MQWs is not yet realized owing to the limitations of the current set of used barrier materials. Here, artificial perovskite-based MQWs using 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), tris-(8-hydroxyquinoline)aluminum, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as quantum barrier materials are introduced. The structures of three different five-stacked perovskite-based MQWs each exhibiting a different band offset with CsPbBr3 in the conduction and valence bands, resulting in a variety of MQW band alignments, i.e., type-I or type-II structures, are shown. Transient absorption spectroscopy reveals the disparity in charge carrier dynamics between type-I and type-II MQWs. Photodiodes of each type of perovskite artificial MQWs show entirely different carrier behaviors and photoresponse characteristics. Compared with bulk perovskite devices, type-II MQW photodiodes demonstrate a more than tenfold increase in the rectification ratio. The findings open new opportunities for producing halide-perovskite-based quantum devices by bandgap engineering using simple quantum barrier considerations.

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.