Large Photoluminescence Research Articles

Multifunctional Molecule Passivated Quasi‐2D Perovskite Film for Efficient and Stable Luminescent Solar Concentrator

AbstractQuasi−2D perovskite films with multiple quantum well structures can provide a large Stokes shift and efficient photoluminescence (PL), which have great potential in the application of luminescent solar concentrators (LSCs). However, its low photoelectric conversion efficiency and poor stability remain obstacles to commercial application. Here, a multifunctional molecule additive octyltriphenylphosphonium bromide is introduced to prepare high‐quality perovskite LSCs. The multifunctional molecule can simultaneously passivate perovskite cations and anions, reducing the defect sites and improving the photoluminescence quantum yield (PLQY) of the perovskite nanocrystals. Triphenylphosphine groups with high steric hindrance effect can hinder ion migration; the hydrophobic properties of the long alkyl chain prevent water erosion, which together improves the stability of the perovskite films. The multifunctional molecule passivated perovskite film has a high PLQY of 100% and retains 96% of the initial PL intensity after 30 days of indoor storage. The perovskite LSC device achieves a maximum optical efficiency of 6.5% at a geometric factor of 3.1. The findings open a new way to boost the performance of perovskite‐based LSCs.

Double-Shell Encapsulation of Lead-Free Tin Halide Perovskite for Self-Powered Smart Windows.

Luminescent solar concentrators (LSC) have the potential application in building integrated photovoltaic (BIPV). 0D tin-based perovskites are a promising embedding phosphor in LSC due to the large Stokes shift and high photoluminescence quantum yield. But the instability and uncontrollable crystal growth are severe limiting their successful utilization in device fabrication. To tackle these issues, double-shell encapsulated configurations are presented, soft ligands of hypophosphorous acid and hard-shell of hollow mesoporous silica are simultaneously suppressing the oxidation of Sn2+ and restricting crystal growth within nano-matrix. The stable phosphor is subsequently embedded into LSC for harvesting solar energy and the resulting output power efficiently drives the combined electrochromic glass under natural light. These fabricated devices also offer the self-adaptable switch on/off functionality to regulate light absorption with variable solar irradiation intensity in real time. This approach is anticipated to open new avenues for utilizing lead-free perovskite nanomaterials in self-powered smart windows for BIPV.

(Invited) Towards Non-Blinking Strongly Confined Perovskite Quantum Dots

Quantum information science has shown its capabilities to enable secure quantum communications. Single photon emitters (SPEs) that emit photons one at a time are fundamental elements of such transformative technologies. Colloidal cesium lead halide (CsPbX3, X=Br, I) perovskite QDs are ideal for next-generation SPEs because of their high room-temperature luminescence efficiency and low-cost, scalable syntheses. Unfortunately, individual perovskite QDs show insufficient photostability and severe photoluminescence (PL) intensity fluctuations (also called blinking). This has greatly limited the spectroscopic studies of perovskite QDs for SPE developments. One major roadblock toward non-blinking, photostable perovskite QDs is their highly ionic crystal structure. When preparing single perovskite QD samples, QD colloids often need to be diluted. During this process ligands can detach from the QD, introducing defects. This is particularly detrimental to strongly confined perovskite QDs since exciton-surface lattice interaction is greatly enhanced. As a result, strongly size confined perovskite QDs are experiencing severe PL blinking with large PL “OFF” occurrences.To suppress perovskite QD blinking and photodegradation, we embedded QDs in an organic crystal matrix consists of Phenethylammonium bromide (PEABr) salts. The bromide rich surface of a QD can be epitaxially anchored onto the PEABr crystals and therefore be stabilized. Individual strongly confined CsPbBr3 QDs in our matrix show nearly non-blinking behavior under non-resonant laser excitations at room temperature. These QDs remain photostable without PL intensity decrease and spectral shift after more than 12 hours of continues excitations. We anticipate that these QDs will lead to more accurate and detailed study of exciton dynamics and structural-optical property relationships in strongly confined perovskite QDs.

Wavelength-tunable photoluminescence of CsPbBr3 microcrystal via in situ halogen doping

Lead halide perovskite semiconductors have received significant attention in recent years as promising candidates for optoelectronic applications. Their controllable bandgap in nanostructures (nanocrystals, nanowires) has been widely and successfully investigated. Intentionally manipulating the atomic behavior with dopants in their macroscopic structure with high quality and stability is still challenging. We studied the crystal structure and photoluminescence (PL) behavior of the anion (Cl) doped CsPbBr3 microcrystals. The doped microcrystal with a low doping concentration has shown a large wavelength-tunable PL emission from 535 nm to 457 nm. Two separate emission peaks (blue and green) are carefully studied in the excitation power dependent PL spectra. The doping and excitation endow the perovskite microcrystal adjustable PL emission band and peak intensities, which may serve as a new degree of manipulation freedom in the color-tunable display and functional optoelectronics.

Eu2+ doped cesium alkaline earth chloride nanocrystals in glass for X-ray imaging

High-performance and low-cost metal halide perovskite scintillator materials have important applications in X-ray detection and imaging. However, large-sized perovskite scintillator panel with high transmittance, low toxicity and long-term stability is still lacking. Herein, Eu2+ doped cesium alkaline earth chloride nanocrystals (Eu2+: CsCaCl3, Eu2+: CsSrCl3, and Eu2+: Cs2BaCl4 NCs) embedded in glasses through conventional melt-quenching and subsequent heat-treatment were developed. These NCs doped glasses exhibited highly efficient and narrowband photoluminescence in blue region. Benefitting from the large X-ray absorption coefficient and high photoluminescence quantum yied, these Eu2+: CsCaCl3 NCs doped glass scintillator shows light yield of ∼9300 photons/MeV, detection limit of ∼213.8 μGy/s, and spatial resolution of 30.0 lp/mm. These results demonstrated that glasses containing Eu2+ doped cesium alkaline earth chloride nanocrystals are promising for X-ray detection and imaging.

Harnessing the Synergetic Effects of Ag, Mn Dopants in Eco-Friendly Ultraviolet Selective Quantum Dots for Luminescent Solar Concentrators.

Quantum dots (QDs) are promising building blocks for luminescent solar concentrators (LSCs), yet most QD-based LSCs suffer from toxic metal composition and color tinting. UV-selective harvesting QDs can enable visible transparency, but their development is restricted by large reabsorption losses and low photoluminescence quantum yield (PLQY). The developed here Ag, Mn: ZnInS2/ZnS QDs show a high PLQY of 53% due to the passivating effect of ZnS shell. These QDs selectively absorb UV light and emit orange-red light with a large Stokes shift of 180nm. A LSC of 5×5×0.2cm3, fabricated using a poly(lauryl methacrylate) (PLMA) as a matrix, maintains 87% of integrated PL after 7h of UV exposure. The QD-PLMA achieved 90.7% average visible transparency (AVT) and a color rendering index (CRI) of 95.8, which is close to plain PLMA (AVT=90.8%; CRI=99.5), yielding excellent visible light transparency. Incorporating Si-PVs at LSC edges, the Ag, Mn: ZIS/ZnS QD-LSC achieved an optical efficiency of 1.42%, ranking competitively among high-performing UV-harvesting LSCs.

Ba4Cd2Ge2Te9: A Promising Infrared Functional Crystal with Large Birefringent and Photoluminescence Properties

Herein, a new Cd-based telluride Ba4Cd2Ge2Te9 was successfully synthesized and characterized, which was first identified in the Ba-Cd-Ge-Te system. It crystallizes in the Pbcm space group and its structure features infinite [Cd2Ge2Te9]8- chains, with Ba2+ filling in the interstitial space to balance the charge. The fluorescence spectrum indicates that the title compound generates purple-emission the range of 415-459nm under the 300nm excitation, with a peak lifetime of about 1.18ns at 436nm. In addition, first-principles calculations show that Ba4Cd2Ge2Te9 has a birefringence of 0.298@2090nm, which is determined by the superposition of the polarization of the terminal Te atom and Ge2Te6 unit. Thermoelectricity measurements show that the title compound possesses a large Seebeck coefficient (655 μV K−1 @300K) and a low thermal conductivity (0.87Wm-1 K-1 @300K).

Secondary donor-acceptor group enable efficient pure green organic light-emitting devices based on multi-resonance TADF emitters

Multi-resonance thermally activated delayed fluorescence (MR-TADF) presents great potential for applications in high-quality organic light emitting diodes (OLEDs) displays due to their superb properties of high color purity and high quantum efficiencies. Nonetheless, most reported MR-TADF emitters show unsatisfactory devices performances with severe efficiency roll-off resulted from their inherent issues of slow reverse intersystem crossing rate (kRISC) and aggregation-caused quenching (ACQ). Herein, we propose a novel strategy by employing quinazoline derivatives as secondary acceptor (PQ) and secondary donor–acceptor (PQCz) to provide intermediate locally excited triplet (3LE) states and then enhance spin-orbital coupling (SOC). In addition, the secondary donor–acceptor group PQCz in tCzBN-PQCz can suppress intermolecular interaction and provide dual long-range charge transfer (LRCT) channels to further enhance kRISC. Therefore, tCzBN-PQ and tCzBN-PQCz exhibit high kRISC of 5.6 × 105 s−1 and 7.3 × 105 s−1 with large photoluminescence quantum yield (PLQY) of 89 % and 95 % respectively. Consequently, OLED based on tCzBN-PQ and tCzBN-PQCz show excellent external quantum efficiency (EQE) of 30.2 % and 35.1 % with small efficiency roll-off and small full width at half maxima (FWHM) of 37 nm and 34 nm respectively. Especially, tCzBN-PQCz exhibits better performances and less-sensitive to the doping concentration which proves the effectiveness of this secondary donor–acceptor group.

Ultrafast intrinsic excited state localization m 2D layered As2S3 by interlayer bond formation

The family of two-dimensional (2D) layered materials with strong excitonic effect offers fascinating opportunities for studying excited state exciton behavior at 2D limit. While exciton dynamics in conventional 2D semiconductors (e.g. transition metal dichalcogenides) has been extensively studied, little is known about exciton properties and dynamics in 2D layered semiconductors with strong electron/exciton-phonon coupling. Here, by combining experimental and theoretical approaches, we reveal the intrinsic highly localized exciton (i.e. self-trapped exciton) in 2D layered As2S3, driven by strong exciton-phonon interaction. It is shown that photoexcited electron/hole charges in As2S3 localize spontaneously in ~110 fs, giving rise to large stokes-shifted and broad photoluminescence. An interlayer partial bond is formed between chalcogen atoms, triggering lattice distortion and carrier localization. Together with Urbach-Martienssen analysis, this study provides a comprehensive physical picture to understand the complex interplay between exciton and lattice dynamics in 2D semiconductors, which has strong implications to their optoelectronic properties and applications.

Tuning the optoelectronic properties of enargite (Cu3AsS4) solar cells by Ag alloying: A DFT-informed synthesis

The enargite phase of Cu3AsS4 (ENG) is an emerging photovoltaic material with a ∼1.4 eV bandgap and is composed of earth abundant elements with favorable defect properties arising from the differing ionic radii of the constituent elements. Unfortunately, ENG-based photovoltaic devices have experimentally been shown to have low power conversion efficiencies, possibly due to defects in the material. In this joint computational and experimental study, we explore the defect properties of ENG and employ synthesis approaches, such as silver alloying, to reduce the density of harmful defects. We show that shallow copper vacancies (VCu) are expected to be the primary defects in ENG and contribute to its p-type character. However, as shown through photoluminescence (PL) measurements of synthesized ENG, a large mid-bandgap PL peak is present at ∼0.87 eV from a band edge, potentially caused by a copper- or sulfur-related defect. To improve the properties of ENG films and mitigate the mid-bandgap PL, we employed an amine-thiol molecular precursor-based synthesis approach and utilized silver alloying of ENG films. While silver alloying did not affect the mid-bandgap PL peak, it increased grain size and lowered film porosity, improving device performance. In conclusion, we found that incorporating silver such that [Ag]/([Ag] + [Cu]) is 0.05 in the film using an amine-thiol based molecular precursor route with As2S3 as the arsenic source resulted in improved photovoltaic device performance with a champion device of efficiency 0.60%, the highest reported efficiency for an Cu3AsS4 (ENG)-based device to date.

Structure and photoluminescence properties of PbO–Bi2O3–Ga2O3 glass-ceramics containing silver nanoparticles, free lead and bismuth ions

We report the synthesis, structural characterization, and optical properties of PbO–Bi2O3–Ga2O3 glass-ceramics (GCs) containing PbO and Bi2O3 crystallites, free lead (Pb2+) and bismuth (Bi3+) ions, and silver nanoparticles (Ag-NPs). The samples were prepared by applying the melt-quenching method followed by suitable heat-treatment of the samples. Transmission Electron Microscopy, X-ray diffraction, linear optical absorption, and photoluminescence (PL) were the techniques applied to characterize the samples. When the samples were excited at 330 nm a large PL intensity, due to the Pb2+ and Bi3+, is observed. Moreover, large PL intensity enhancement of about 1000% for the samples containing Ag-NPs is observed in comparison with the PL intensity from the samples without Ag-NPs. This is the first time that this behavior is reported for a GCs based on heavy-metal oxides. The potential of this class of materials for photonic applications is discussed.

Giant Out-of-Plane Exciton Emission Enhancement in Two-Dimensional Indium Selenide via a Plasmonic Nanocavity

Out-of-plane (OP) exciton-based emitters in two-dimensional semiconductor materials are attractive candidates for novel photonic applications, such as radially polarized sources, integrated photonic chips, and quantum communications. However, their low quantum efficiency resulting from forbidden transitions limits their practicality. In this work, we achieve a giant enhancement of up to 34000 for OP exciton emission in indium selenide (InSe) via a designed Ag nanocube-over-Au film plasmonic nanocavity. The large photoluminescence enhancement factor (PLEF) is attributed to the induced OP local electric field (Ez) within the nanocavity, which facilitates effective OP exciton-plasmon interaction and subsequent tremendous enhancement. Moreover, the nanoantenna effect resulting from the effective interaction improves the directivity of spontaneous radiation. Our results not only reveal an effective photoluminescence enhancement approach for OP excitons but also present an avenue for designing on-chip photonic devices with an OP dipole orientation.

Large Cation Engineering in Organic Antimony Halides for Low‐Loss Active Waveguide

AbstractActive waveguides have attracted growing attention as a basic component of miniaturized and integrated photonic devices. However, most optical waveguide materials suffer from severe self‐absorption, which greatly increases the optical loss. Therefore, developing highly efficient red‐emitting materials with large Stokes shift and negligible self‐absorption is of great interest. Herein, this work proposes a strategy to achieve both large Stokes shifts and high photoluminescence quantum yield (PLQY) in organic metal halide via large cation engineering. Two zero‐dimensional antimony(III) chlorides, (TMA)2SbCl5 and (TMAA)2SbCl5, are designed and synthesized by screening the ligands with different cationic volumes. Experimental and theoretical results reveal that the large cation can promote the self‐trapped excitons emission and improve stability. Benefiting from the large Stokes shift (291 nm) and high PLQY (98.3%), the as‐prepared (TMAA)2SbCl5 microplate exhibits efficient active waveguide with low loss coefficient of 6.07 × 10−3 dB µm−1. This work not only develops a novel active waveguide material, but also provides insights into the role of organic cation, which will provide new inspiration for the exploitation of excellent luminescent metal halides and photonic devices.

Dual‐State Emission and Two‐Photon Absorption Tuned by Benzonitrile Substitution in 2,3,4,5‐Tetraphenyl‐1H‐Pyrrole

AbstractA series of nine 2,3,4,5‐tetraphenyl pyrrole derivatives presenting benzonitrile substitution at different positions was designed and synthesized for this work. Their structure–emission property relationships were investigated in depth through a joint advanced experimental and computational effort. Our ultrafast and non‐linear spectroscopic results showed more efficient intramolecular charge transfer and enhanced two‐photon absorption cross sections when the benzonitrile substitution is at position 3 of the central pyrrole. On the other hand, an intriguing dual‐state emission behaviour was found to be activated by substitution at position 2, with exceptionally large photoluminescence quantum yields exhibited both in solution and in the solid state. Indeed, the quantum chemical simulations revealed for the derivatives functionalized at position 2 that the distorted geometry of the benzonitrile substituent with respect to the phenyl ring prevents emission quenching in the aggregate species by π‐π stacking interactions. Our findings shed new light on optimized design strategies of highly emitting materials for efficient optoelectronic devices.

Enhanced triplet-triplet fusion for high efficiency and long lifetime of multiresonant pure blue organic light emitting diodes

Multiresonant (MR) narrowband emitters became extensive attention as a blue fluorescent dopant for organic light-emitting diodes (OLEDs) due to their high color purity and large photoluminescence quantum yields (PLQY), but they show poor operational lifetime. Herein, we present a double emissive layer (DEML) architecture to enhance the efficiency and lifetime of pure blue fluorescent OLEDs for MR type emitters. Mainly the DEML are divided into recombination zone and triplet–triplet fusion (TTF) zone based on different triplet energy hosts. Notably, we achieved a tremendous TTF ratio of 32.4% for MR-type dopant, which is close to the theoretical maximum of 37.5%. Furthermore, fabricated fluorescent devices based on DEML structure manifested tremendous maximum external quantum efficiency (EQEmax) of nearly 11% and revealed a longer lifetime (LT95) of 79 h at 1000 cd/m2 due to efficient triplet harvesting, which is almost double compared to the single host device. We believe that our findings can pave the way for enhancement in EQE and lifetime for blue OLEDs.

High-Efficiency and Stable Long-Persistent Luminescence from Undoped Cesium Cadmium Chlorine Crystals Induced by Intrinsic Point Defects.

Application of long-persistent luminescence (LPL) materials in many technological fields is in the spotlight. However, the exploration of undoped persistent luminescent materials with high emission efficiency, robust stability, and long persistent duration remains challenging. Here, inorganic cesium cadmium chlorine (CsCdCl3 ) is developed, featuring remarkable LPL characteristics at room temperature, which is synthesized by a facile hydrothermal method. Excited by ultraviolet light, the CsCdCl3 crystals exhibit an intense yellow emission with a large photoluminescence quantum yield of ≈90%. Different from the reported systems with lanthanides or transition metals doping, the CsCdCl3 crystals without dopants perform yellow LPL with a long duration of 6000 s. Joint experiment-theory characterizations reveal the intrinsic point defects of CsCdCl3 act as the trap centers of excited electrons and the carrier de-trapping process from such trap sites to localized emission centers contributes to the LPL. Encouraged by the attractive fluorescence and persistent luminescence as well as good stability of CsCdCl3 against environment oxygen/moisture (75%), heat (100°C for 10h), and ultraviolet light irradiation, an effective dual-mode information storage-reading application is demonstrated. The results open up a new frontier for exploring LPL materials without dopants and provide an opportunity for advanced information storage compatible for practical applications.

Scale-up synthesis of high-quality solid-state-processed CsCuX (X = Cl, Br, I) perovskite nanocrystal materials toward near-ultraviolet flexible electronic properties.

High-quality CsCu2X3 and Cs3Cu2X5 (X = Cl, Br, I) nanocrystals (NCs) exhibit excellent optoelectronic, physical, and chemical properties for detection of UV radiation due to large carrier mobility and lifetime, and heavy atoms. The nanocrystal materials can be prepared via a low-cost and simple solid-state synthesis. However, poor reproducibility and complex synthesis methods of obtaining perovskite NC thin films represent a drawback for the fabrication of the commercial photoelectric device. To address these issues, we develop highly stable CsCu2X3 and Cs3Cu2X5 NC materials using a facile solid-state reaction method for the scale-up production of halogen lead-free perovskites. We suggest a distinctive way to design a series of nanocrystalline perovskites using short-term synthesis and study the mechanism of perovskite formation using thermal solid-state synthesis. These all-inorganic and lead-free CsCu2X3 and Cs3Cu2X5 exhibit large photoluminescence quantum yields (PLQYs) up to 95.2%. Moreover, flexible paper photodetectors based on this series of lead-free perovskites show strong photoselectivity and bending stability at 254 nm, 365 nm, and 405 nm wavelengths. High-quality responses with a responsivity of 1.1 × 10-3 A W-1 and detectivity of 2.71 × 109 jones under UV illumination (10 μW cm-2) at a bias voltage of 5 mV are demonstrated. These results open prospects for designing photodetectors, LEDs, and other photosensitive devices.

Cubic Halide Double Perovskite Nanocrystals with Anisotropic Free Excitons and Self-Trapped Exciton Photoluminescence

In this work, we first developed Cs2KBiCl6 cubic double perovskite nanocrystals and a series of morphologically isotropic double perovskite nanocrystals. Different contributions of different elements to self-trapped states were revealed by density functional theory. Meanwhile, these double perovskite nanocrystals exhibit the coexistence of free and self-trapped exciton dual-color photoluminescence. Femtosecond transient absorption spectroscopy can confirm that the double perovskite nanocrystals produce a relatively obvious structural deformation in the excited state. We infer that this can lead to a large deviation of the excitation and emission transition dipoles, thus causing large photoluminescence anisotropy. Most importantly, we observe for the first time that both free exciton emission and self-trapped exciton emission are highly anisotropic, which are comparable to or even better than that of lead halide perovskites. This research paves the way for exploring more possibilities and practical applications.

Large photoluminescence enhancement in the chlorophyll from the energy transfer in perovskite–chlorophyll hybrid derivatives

Determining the photophysical processes for biomaterial–semiconductor systems has been beneficial for developing optoelectronic devices that exhibit biodegradability and biocompatibility. Here, we systematically investigated the optical properties and photophysical mechanisms of CsPbBr3 nanoparticles (NPs)-incorporated chlorophyll material. Steady-state photoluminescence (PL) studies reveal a large fluorescence enhancement in the chlorophyll once the perovskite was incorporated in the pristine chlorophyll with an associated PL quenching of the CsPbBr3 emission. A spectral overlap was measured from the PL and absorption spectra of CsPbBr3 NPs and chlorophyll indicative of a Förster-type resonance energy transfer (FRET). Using time-resolved PL, faster PL decay curves were observed from the CsPbBr3 NPs in the mixture suggesting that most of its energy was transferred to the chlorophyll. The corresponding Jablonski diagram was built and the energy transfer parameters, such as FRET efficiencies and transfer rates, were calculated to fully explain the FRET process. Slow PL degradation for the mixtures was also observed, highlighting the advantage of the FRET proposed. The demonstration of the photophysical mechanism in biomaterial–semiconductor systems is influential in improving the performance of emerging bio-inspired optoelectronic devices.

Single‐Component White Circularly Polarized Luminescence in Chiral 1D Double‐Chain Perovskites

AbstractCircularly polarized luminescence (CPL) with polychromatic colors has been widely studied due to its great applications in chiroptical, optoelectronics, and spintronics. However, the realization of white CPL in single‐component solid‐state materials remains a great challenge and suffers from the incompatibility between high efficient luminescence and large asymmetric discrimination. Here, by exploiting self‐trapped exciton mechanism and chirality induction strategy, a pair of 1D chiral perovskites, (RR/SS‐DMPZ)PbBr4 (where DMPZ = cis‐2,5‐dimethylpiperazine divalent cation), is reported to achieve white CPL with both high quantum yield of 28.4% and large photoluminescence asymmetry factor |glum| of 2.32 × 10−2. The crystal structures are featured by 1D double‐chain structure composed of distorted octahedra with short PbPb distances as the structural origin of the high photoluminescence yield. The high |glum| value is caused by efficient chiral induction due to the multiple hydrogen bonds between the chiral host composed of the enantiopure organic cations containing two stereocenters and the inorganic emitting guest. The self‐trapped exciton emission mechanism is demonstrated by density functional theory calculations and variable‐temperature photoluminescence and femtosecond‐transient absorption spectroscopy studies. The photo‐luminescent white light‐emitting diodes exhibit good stability and can be used as single‐component white light emitters. This work provides applicable strategies to explore single‐component white CPL emitters.