Transient Photoinduced Pb2+ Disproportionation for Exciton Self-Trapping and Broadband Emission in Low-Dimensional Lead Halide Perovskites.

Low-dimensional lead halide perovskites with broadband emission hold great promise for single-component white-light-emitting (WLE) devices. The origin of their broadband emission has been commonly attributed to self-trapped excitons (STEs) composed of localized electronic polarization with a distorted lattice. Unfortunately, the exact electronic and structural nature of the STE species in these WLE materials remains elusive, hindering the rational design of high-efficiency WLE materials. In this study, by combining ultrafast transient absorption spectroscopy and ab initio calculations, we uncover surprisingly similar STE features in two prototypical low dimensional WLE perovskite single crystals: 1D (DMEDA)PbBr4 and 2D (EDBE)PbBr4, despite of their different dimensionalities. Photoexcited excitons rapidly localize to intrinsic STEs within ∼250 fs, contributing to the white light emission. Crucially, STEs in both systems exhibit characteristic absorption features akin to those of Pb+ and Pb3+. Further atomic level theoretical simulations confirm photoexcited electrons and holes are localized on the Pb2+ site to form Pb+- and Pb3+-like species, resembling transient photoinduced Pb2+ disproportionation. This study provides conclusive evidence on the key excited state species for exciton self-trapping and broadband emission in low dimensional lead halide WLE perovskites and paves the way for the rational design of high-efficiency WLE materials.

Multispectral transparent electromagnetic-wave-absorbing optical window technology based on a random grid.

A multimode detection system has stringent requirements in terms of electromagnetic characteristic control and electromagnetic compatibility. To meet these requirements, we designed and manufactured a type of transparent electromagnetic-wave-absorbing optical window based on a random grid (EAOWRG) in this study. Owing to the design and regulation of the materials of the random grid and the structures of the metasurface, the optical window has excellent multispectral transparency, electromagnetic wave absorption, and electromagnetic shielding performance. The experimental results showed that the transmissivity of the EAOWRG in the optical spectral ranges of 460-800 nm and 8-12 µm is above 89.77%, the electromagnetic reflectivity in the frequency ranges of 3.6-7.2 GHz and 14.3-17.7 GHz is not more than - 5 dB, the bandwidth at which the electromagnetic reflectivity is not more than -10 dB is 4.4 GHz, the electromagnetic shielding effectiveness in the frequency range of 2-18 GHz is above 31 dB. The average radar cross section of the detection system using the EAOWRG in the ± 60° angle domain at 6 GHz is 8.79 dB lower than that before processing. The detection system has a good imaging effect in the visible and infrared bands, meeting the requirements of the electromagnetic characteristic control and electromagnetic compatibility, and has good application prospects.

Elucidating the Unique Hot Carrier Cooling in Two-Dimensional Inorganic Halide Perovskites: The Role of Out-of-Plane Carrier-Phonon Coupling.

Two-dimensional (2D) halide perovskites represent the natural semiconductor quantum wells (QWs), which hold great promise for optoelectronics. However, due to the hybrid structure of Ruddlesden-Popper 2D perovskites, the intrinsic nature of hot-carrier kinetics remains shielded within. Herein, we adopt CsPbBr3 nanoplates as a model system to reveal the intrinsic carrier dynamics in inorganic perovskite QWs. Interestingly, we revealed an ultrafast and hot-phonon-bottleneck (HPB)-free carrier cooling in monodisperse CsPbBr3 QWs, which is in sharp contrast to the bulk and nanocrystalline perovskites. The absence of HPB was attributed to the efficient out-of-plane triplet-exciton-LO-phonon coupling in 2D perovskites because of the structural anisotropy. Accordingly, the HPB can be activated by shutting down the out-of-plane energy loss route through forming the layer-stacked perovskite superlattice. The controllable on and off of HPB may provide new possibilities in optoelectronic devices and these findings deepen the understanding of a hot-carrier cooling mechanism in 2D perovskites.

Molecular Level Modulation of Anthraquinone-containing Resorcinol-formaldehyde Resin Photocatalysts for H2 O2 Production with Exceeding 1.2 % Efficiency.

Designing polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H2 O2 ) photosynthesis. Here, we introduce electron-deficient 1,4-dihydroxyanthraquinone (DHAQ) into the framework of resorcinol-formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H2 O2 can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H2 O2 production. RF-DHAQ exhibits excellent overall photosynthesis of H2 O2 with a solar-to-chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high-efficiency artificial photosynthesis.

Ultrafast and High-Yield Polaronic Exciton Dissociation in Two-Dimensional Perovskites.

Layered two-dimensional (2D) lead halide perovskites are a class of quantum well (QW) materials, holding dramatic potentials for optical and optoelectronic applications. However, the thermally activated exciton dissociation into free carriers in 2D perovskites, a key property that determines their optoelectronic performance, was predicted to be weak due to large exciton binding energy (Eb, about 100-400 meV). Herein, in contrast to the theoretical prediction, we discover an ultrafast (<1.4 ps) and highly efficient (>80%) internal exciton dissociation in (PEA)2(MA)n-1PbnI3n+1 (PEA = C6H5C2H4NH3+, MA = CH3NH3+, n = 2-4) 2D perovskites despite the large Eb. We demonstrate that the exciton dissociation activity in 2D perovskites is significantly promoted because of the formation of exciton-polarons with considerably reduced exciton binding energy (down to a few tens of millielectronvolts) by the polaronic screening effect. This ultrafast and high-yield exciton dissociation limits the photoluminescence of 2D perovskites but on the other hand well explains their exceptional performance in photovoltaic devices. The finding should represent a common exciton property in the 2D hybrid perovskite family and provide a guideline for their rational applications in light emitting and photovoltaics.

Auger-Assisted Electron Transfer between Adjacent Quantum Wells in Two-Dimensional Layered Perovskites.

Two-dimensional (2D) layered perovskites are naturally formed multiple quantum-well (QW) materials, holding great promise for applications in many optoelectronic devices. However, the further use of 2D layered perovskites in some devices is limited by the lack of QW-to-QW carrier transport/transfer due to the energy barrier formed by the insulating ligands between QWs. Herein, we report an Auger-assisted electron transfer between adjacent QWs in (CmH2m+1NH3)2PbI4 2D perovskites particularly with m = 12 and 18, where the electron energy barrier (Eb) is similar to the QW band gap energy (Eg). This Auger-assisted QW-to-QW electron transfer mechanism is established by the observation of a long-lived and derivative-like transient absorption feature, which is a signature of the quantum confined Stark effect induced by the electron-hole separation (thus an internal electric field) between different QW layers. Our finding provides a new guideline to design 2D perovskites with an optically tunable QW-to-QW charge transport property, advancing their applications in optoelectronics and optical modulations.

Trap-Enabled Long-Distance Carrier Transport in Perovskite Quantum Wells.

Layered two-dimensional (2D) hybrid perovskites are naturally formed multiple quantum well (QW) materials with promising applications in quantum and optoelectronic devices. In principle, the transport of excitons in 2D perovskites is limited by their short lifetime and small mobility to a distance within a few hundred nanometers. Herein, we report an observation of long-distance carrier transport over 2 to 5 µm in 2D perovskites with various well thicknesses. Such a long transport distance is enabled by trap-induced exciton dissociation into long-lived and nonluminescent electron-hole separated state, followed by a trap-mediated charge transport process. This unique property makes 2D perovskites comparable with 3D perovskites and other traditional semiconductor QWs in terms of a carrier transport property and highlights their potential application as an efficient energy/charge-delivery material.

Host-Guest Thin Films by Confining Ultrafine Pt/C QDs into Metal-Organic Frameworks for Highly Efficient Hydrogen Evolution.

Combining the features of host templates and guest species is an efficient strategy to optimize the photo/electrocatalytic performance. Herein, novel host-guest thin-film electrocatalysts are designed and developed with Pt doped carbon (Pt/C) confined into porphyrin-based metal-organic frameworks (MOFs). Porous MOF PCN-222 and PCN-221 thin films are used as the host templates and fabricated using vapor-assisted deposition method, and then the guest Pt/C quantum dots are encapsulated into the MOFs by loading the glucose mixed H2 PtCl6 and heating at 200 °C. Thanks to the confinement effect of MOF pores, the homogenous and ultrafine Pt/C nanowires (Pt/CNWs) and nanodots (Pt/CNDs) are confined in nanochannels of PCN-222 and nanocages of PCN-221 (Pt/CNW@PCN-222 and Pt/CND@PCN-221), respectively. The electrocatalytic study shows that the host-guest thin films have highly-efficient electrocatalytic hydrogen evolution performance under light irradiation. Furthermore, the time-resolved photoluminescent results reveal that Pt/CNW@PCN-222 has a faster charge transfer (441 ps) from PCN-222 to Pt/CNWs comparing to that (557 ps) of Pt/CND@PCN-221, indicating the guests with different shapes play an important role in the electrocatalytic performance. This work serves to present both the outstanding level of control in the precise synthesis and high potential for nanocomposite thin films in photo-electrocatalytic application.

Excitation-Dependent Emission Color Tuning from an Individual Mn-Doped Perovskite Microcrystal.

Divalent manganese cation (Mn2+) doped perovskite materials are of great interest for their unique optical, magnetic, and electric properties. Herein, we report an excitation-dependent emission color tuning from an individual Mn-doped CsPbCl3 microcrystal (MC) with a wide color tuning range, reversible and continuous color change, and high photostability. We demonstrate that the Mn-doped CsPbCl3 MCs exhibit dual-color emission from both host excitons (blue) and Mn-dopants (orange) through an internal energy transfer (IET) process. By simple change of the laser excitation repetition rate or pulse intensity, the relative emission intensity between exciton (Iexciton) and Mn-dopant (IMn) can be continuously and reversibly tuned from IMn/Itotal (Itotal = IMn + Iexciton) = 0.9-0.8 to 0.1-0.2, corresponding to a color change from orange to blue. Such emission color tuning is enabled by the saturation of Mn-dopant emission at high excitation intensity and a linear dependence of exciton emission with excitation intensity. Transient spectroscopy and temperature-dependent photoluminescence (PL) measurements confirm that the exciton-to-dopant IET in Mn-doped CsPbCl3 MCs is mediated by some shallow trap states, rather than through a direct transfer pathway. Therefore, the saturation of Mn-dopant emission is caused by a bottlenecked energy transfer effect by saturating the mediating trap states at high excitation intensities. The Mn-doped MCs also exhibit a high photostability on the reversible switch of emission color between orange and blue for more than 300 cycles within a continuous operation time of 14 h. In view of the stable and color-switchable emission properties, Mn-doped perovskite MCs may find application in nanophotonic devices using a single MC.

Long-Distance Charge Carrier Funneling in Perovskite Nanowires Enabled by Built-in Halide Gradient.

The excellent charge carrier transportation in organolead halide perovskites is one major contributor to the high performance of many perovskite-based devices. There still exists a possibility for further enhancement of carrier transportation through nanoscale engineering, owing to the versatile wet-chemistry synthesis and processing of perovskites. Here we report the successful synthesis of bromide-gradient CH3NH3PbBrxI3-x single-crystalline nanowires (NWs) by a solid-to-solid ion exchange reaction starting from one end of pure CH3NH3PbI3 NWs, which was confirmed by local photoluminescence (PL) and energy dispersive X-ray spectroscopy (EDS) measurements. Due to the built-in halide gradient, the long-distance carrier transportation was driven by the energy funnel, rather than the spontaneous carrier diffusion. Indeed, local PL kinetics demonstrated effective charge carrier transportation only from the high-bandgap bromide-rich region to the low-bandgap iodine-rich region over a few micrometers. Therefore, these halide gradient NWs might find applications in various optoelectronic devices requiring long-distance and directional delivery of excitation energy.

An irradiation density dependent energy relaxation in plant photosystem II antenna assembly.

Plant photosystem II (PSII) is a multicomponent pigment-protein complex that harvests sunlight via pigments photoexcitation, and converts light energy into chemical energy. Against high light induced photodamage, excess light absorption of antenna pigments triggers the operation of photoprotection mechanism in plant PSII. Non-photochemical energy relaxation as a major photoprotection way is essentially correlated to the excess light absorption. Here we investigate the energy relaxation of plant PSII complexes with varying incident light density, by performing steady-state and transient chlorophyll fluorescence measurements of the grana membranes (called as BBY), functional moiety PSII reaction center and isolated light-harvesting complex LHCII under excess light irradiation. Based on the chlorophyll fluorescence decays of these samples, it is found that an irradiation density dependent energy relaxation occurs in the LHCII assemblies, especially in the antenna assembly of PSII supercomplexes in grana membrane, when irradiation increases to somewhat higher density levels. Correspondingly, the average chlorophyll fluorescence lifetime of the highly isolated BBY fragments gradually decreases from ~1680 to ~1360 ps with increasing the irradiation density from 6.1×10(9) to 5.5×10(10) photon cm(-2) pulse(-1). Analysis of the relation of fluorescence decay change to the aggregation extent of LHCIIs suggests that a dense arrangement of trimeric LHCIIs is likely the structural base for the occurrence of this irradiation density dependent energy relaxation. Once altering the irradiation density, this energy relaxation is quickly reversible, implying that it may play an important role in photoprotection of plant PSII.

Aziridinyl Fluorophores Demonstrate Bright Fluorescence and Superior Photostability by Effectively Inhibiting Twisted Intramolecular Charge Transfer.

Replacing conventional dialkylamino substituents with a three-membered aziridine ring in naphthalimide leads to significantly enhanced brightness and photostability by effectively suppressing twisted intramolecular charge transfer formation. This replacement is generalizable in other chemical families of fluorophores, such as coumarin, phthalimide, and nitrobenzoxadiazole dyes. In highly polar fluorophores, we show that aziridinyl dyes even outperform their azetidinyl analogues in aqueous solution. We also proposed one simple mechanism that can explain the vulnerability of quantum yield to hydrogen bond interactions in protonic solvents in various fluorophore families. Such knowledge is a critical step toward developing high-performance fluorophores for advanced fluorescence imaging.

Hierarchical NiMoO4 nanowire arrays supported on macroporous graphene foam as binder-free 3D anodes for high-performance lithium storage.

Novel three-dimensional (3D) NiMoO4 nanowire arrays (NWAs) grown directly onto the surface of macroporous graphene foams (GF) with robust adhesion were synthesized via a facile chemical vapor deposition (CVD) and subsequent hydrothermal route. The as-prepared NiMoO4 nanowires are composed of ultra-small nanoparticles (∼5 nm) with a diameter of 70-150 nm and are several micrometers in length. Such as-grown NiMoO4 NWA/3DGF composites are then evaluated as monolithic electrodes for lithium-ion batteries (LIBs) without the need of binders or metal-based current collectors. Benefitting from the unique three-dimensional arrayed architecture and characteristics with a high specific surface area and more active sites which facilitate fast electron and ionic transport within the electrode, the NiMoO4 NWA/GF composites deliver a high reversible specific capacity of 1088.02 mA h g(-1) at a current density of 200 mA g(-1) and 867.86 mA h g(-1) after 150 cycles (79.77% retention of the second cycle), and excellent rate capability. With the advantages of excellent electrochemical performance and a facile synthesis method, the NiMoO4 nanowire arrays supported on 3DGF exhibit great potential as anode materials for LIBs.

Limiting Perovskite Solar Cell Performance by Heterogeneous Carrier Extraction.

Although the power conversion efficiency of perovskite solar cells has improved rapidly, a rational path for further improvement remains unclear. The effect of large morphological heterogeneity of polycrystalline perovskite films on their device performance by photoluminescence (PL) microscopy has now been studied. Contrary to the common belief on the deleterious effect of morphological heterogeneity on carrier lifetimes and diffusivities, in neat CH3 NH3 PbI3 (Cl) polycrystalline perovskite films, the local (intra-grain) carrier diffusivities in different grains are all surprisingly high (1.5 to 3.3 cm2 s-1 ; comparable to bulk single-crystals), and the local carrier lifetimes are long (ca. 200 ns) and surprisingly homogenous among grains, and uniform across grain boundary and interior. However, there is a large heterogeneity of carrier extraction efficiency at the perovskite grain-electrode interface. Improving homogeneity at perovskite grain-electrode contacts is thus a promising direction for improving the performance of perovskite thin-film solar cells.

Visualizing Carrier Diffusion in Individual Single-Crystal Organolead Halide Perovskite Nanowires and Nanoplates.

Single-crystal CH3NH3PbX3 (X = I(-), Cl(-), Br(-)) perovskite nanowires (NWs) and nanoplates (NPs), which demonstrate ultracompact sizes and exceptional photophysical properties, offer promises for applications in nanoscale photonics and optoelectronics. However, traditional electronic and transient techniques are limited by the dimensions of the samples, and characterizations of the carrier behavior (diffusion coefficient, charge mobility and diffusion length) in these NWs and NPs are extremely difficult. Herein, we report the direct visualization of the carrier diffusion process in individual single-crystal CH3NH3PbI3 and CH3NH3PbBr3 NWs and NPs using time-resolved and photoluminescence-scanned imaging microscopy. We report the diffusion coefficient (charge motility), which varies significantly between different NWs and NPs, ranging from 1.59 to 2.41 cm(2) s(-1) (56.4 to 93.9 cm(2) V(-1) s(-1)) for CH3NH3PbI3 and 0.50 to 1.44 cm(2) s(-1) (19.4 to 56.1 cm(2) V(-1) s(-1)) for CH3NH3PbBr3 and find this variation is independent of the shape and size of the sample. The average diffusion length is 14.0 ± 5.1 µm for CH3NH3PbI3 and 6.0 ± 1.6 µm for CH3NH3PbBr3. These results provide information that is essential for the practical applications of the single-crystal perovskite NWs and NPs, and the imaging microscopy may also be applicable to other optoelectronic materials.

Singlet oxygen phosphorescence lifetime imaging based on a fluorescence lifetime imaging microscope.

The feasibility of singlet oxygen phosphorescence (SOP) lifetime imaging microscope was studied on a modified fluorescence lifetime imaging microscope (FLIM). SOP results from the infrared radiative transition of O2(a(1)Δg → X(3)Σg(-)) and O2(a(1)Δg) was produced in a C60 powder sample via photosensitization process. To capture the very weak SOP signal, a dichroic mirror was placed between the objective and tube lens of the FLIM and used to divide the luminescence returning from the sample into two beams: the reflected SOP beam and the transmitted photoluminescence of C60 (C60-PL) beam. The C60-PL beam entered the scanner of the FLIM and followed the normal optical path of the FLIM, while the SOP steered clear of the scanner and directly entered a finely designed SOP detection channel. Confocal C60-PL images and nonconfocal SOP images were then simultaneously obtained by using laser-scanning mode. Experimental results show that (1) under laser-scanning mode, the obstacle to confocal SOP imaging is the infrared-incompatible scanner, which can be solved by using an infrared-compatible scanner. Confocal SOP imaging is also expected to be realized under stage-scanning mode when the laser beam is parked and meanwhile a pinhole is added into the SOP detection channel. (2) A great challenge to SOP imaging is its extraordinarily long imaging time, and selecting only a few interesting points from fluorescence images to measure their SOP time-dependent traces may be a correct compromise.

Tuning the Optoelectronic Property of All-Inorganic Lead-Free Perovskite via Finely Microstructural Modulation for Photovoltaics.

Bismuth-based halide perovskite materials have attracted extensive attention for optoelectronic applications due to nontoxicity and ambient stability. However, limited by low-dimensional structure and isolate octahedron arrangement, the undesirable photophysical properties of bismuth-based perovskites are still not well modulated. Here, the rational design and synthesis of Cs3 SbBiI9 with improved optoelectronic performance via premeditatedly incorporating antimony atoms with a similar electronic structure to bismuth into the host lattice of Cs3 Bi2 I9 is reported. Compared with Cs3 Bi2 I9 , the absorption spectrum of Cs3 SbBiI9 is broadened from ≈640 to ≈700 nm, the photoluminescence intensity enhances by two orders of magnitude indicating the extremely suppressed carrier nonradiative recombination, and the charge carrier lifetime is further increased from 1.3 to 207.6 ns. Taking representative applications in perovskite solar cells, the Cs3 SbBiI9 exhibits a higher photovoltaic performance benefiting from the improved intrinsic optoelectronic properties. Further structure analysis reveals that the introduced Sb atoms regulate the interlayer spacing between dimers in c-axis direction and the micro-octahedral configuration, which correlate well with the improvement of optoelectronic properties of Cs3 SbBiI9 . It is anticipated that this work will benefit the design and fabrication of lead-free perovskite semiconductors for optoelectronic applications.

Excited State Dynamics and Transport of Self-Trapped Excitons in Bi-Doped Cs2Na0.4Ag0.6In(1-y)BiyCl6 Double Perovskites.

Lead-free double perovskites (DPs) have become notable in white light emission applications due to the self-trapped exciton (STE) formation in the excited state. However, the mechanism understanding of the excited state dynamics and transport of STE remains ambiguous. Here, we demonstrate a new STE (Bi-STE) forming in tiny Bi-doped Cs2Na0.4Ag0.6InCl6, alongside its intrinsic STE (i-STE), resulting in the DPs photoluminescence quantum yield (PLQY) increasing to as high as >90%. The i-STE exhibits faster formation (60 fs) and slower relaxation dynamics (2.8 µs) compared to the Bi-STE. Moreover, we unveil that the Bi doping can augment the i-STE diffusion properties to attain a diffusion coefficient (diffusion length) of 0.012 cm2 s-1 (1.7 µm) at room temperature, indicating their promise in photovoltaic applications. Our results shed light on significant STE dynamics and transport mechanisms in DPs, providing a new roadmap for advancing existing and crafting new DPs in light emission applications.

Trap-Induced Long-Lived Internal Charge Separation in Sn-Doped MAPbBr3 Perovskite Films.

Sn-doped lead halide perovskites (LHPs) have attracted considerable attention for their lower bandgap and lower toxicity. While it is well-established that Sn doping easily introduces a lot of structural defects into LHP films, the extent to which these defects impact carrier dynamics has yet to be fully elucidated. Herein, we take Sn-doped MAPbBr3 films as an example to explore the influence of Sn doping on their carrier dynamics. The results show that Sn doping can simultaneously introduce many fillable electron traps and unfillable hole traps, consequently instigating an ultrafast carrier capture process. This further elicits long-lived internal charge separation between band edge and trap states or between two kinds of trap states, thereby enabling these carriers to persist for up to ∼2.6 µs. Our findings suggest that Sn doping potentially serves as an effective strategy to prolong the carrier lifetime in LHPs, which could pave the way for potential applications within Sn-based perovskites.

Printable Block Molecular Assemblies with Controlled Exciton Dynamics.

Creating hierarchical molecular block heterostructures, with the control over size, shape, optical, and electronic properties of each nanostructured building block can help develop functional applications, such as information storage, nanowire spectrometry, and photonic computing. However, achieving precise control over the position of molecular assemblies, and the dynamics of excitons in each block, remains a challenge. In the present work, the first fabrication of molecular heterostructures with the control of exciton dynamics in each block, is demonstrated. Additionally, these heterostructures are printable and can be precisely positioned using Direct Ink Writing-based (DIW) 3D printing technique, resulting in programable patterns. Singlet excitons with emission lifetimes on nanosecond or microsecond timescales and triplet excitons with emission lifetimes on millisecond timescales appear simultaneously in different building blocks, with an efficient energy transfer process in the heterojunction. These organic materials also exhibit stimuli-responsive emission by changing the power or wavelength of the excitation laser. Potential applications of these organic heterostructures in integrated photonics, where the versatility of fluorescence, phosphorescence, efficient energy transfer, printability, and stimulus sensitivity co-exist in a single nanowire, are foreseen.