Sub-Second Long Lifetime Triplet Exciton Reservoir for Highly Efficient and Stable Organic Light-Emitting Diode.

In organic light-emitting diode (OLED), achieving high efficiency requires effective triplet exciton confinement by carrier-transporting materials, which typically have higher triplet energy (ET) than the emitter, leading to poor stability. Here, an electron-transporting material (ETM), whose ET is 0.32 eV lower than that of the emitter is reported. In devices, it surprisingly exhibits strong confinement effect and generates excellent efficiency. Additionally, the device operational lifetime is 4.9 times longer than the device with a standard ETM, 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl) phenyl (whose ET 0.36 eV is higher than the emitter). This anomalous finding is ascribed to the exceptionally long triplet state lifetime (≈0.2 s) of the ETM. It is named as long-lifetime triplet exciton reservoir effect. The systematic analysis reveals that the long triplet lifetime of ETM can compensate the requirement for high ET with the help of endothermic energy transfer. Such combination of low ET and long lifetime provides equivalent exciton confinement effect and high molecular stability simultaneously. It offers a novel molecular design paradigm for breaking the dilemma between high efficiency and prolonged operational lifetime in OLEDs.

Passivating A-Site and X-Site Vacancies Simultaneously via N-Heterocyclic Amines for Efficient Cs2AgBiBr6 Solar Cells.

To address the toxicity and stability issues of traditional lead halide perovskite solar cells (PSCs), the development of lead-free PSCs, such as Cs2AgBiBr6 solar cells, is of great significance. However, due to the low defect formation energy of Cs2AgBiBr6, a large number of vacancies, including A-site vacancies and X-site vacancies, form during the fabrication process of the Cs2AgBiBr6 film, which seriously damage the performance of the devices. The traditional phenylethylammonium (PEA) cation, mainly focusing on passivating A-site vacancies, is incapable of reducing X-site vacancies and so results in a limited performance improvement in Cs2AgBiBr6 solar cells. Herein, inspired by the capability of the Lewis base to coordinate with metal cations, a series of N-heterocyclic amines are introduced to serve as a dual-site passivator, reducing A-site and X-site vacancies at the same time. The highest power conversion efficiency of modified Cs2AgBiBr6 solar cells has been increased 36% from 1.10 to 1.50%. Further investigation reveals that the higher electron density of additives would lead to a stronger interaction with metal cations like Ag+ and Bi3+, thus reducing more X-site defects and improving carrier dynamics. Our work provides a strategy for passivating perovskite with various kinds of defects and reveals the connection between the coordination capability of additives and device performance enhancement, which could be instructive in improving the performance of lead-free PSCs.

Superstable CsxSnBrx+2@CsBr Nanocrystals with Over 1200 h of Half-Value PLQY in Air.

Tin-based perovskites comprise one of the preferred nontoxic alternatives to Pb-based perovskites due to their desirable optoelectronic properties. However, there remains a crucial stability problem due to the property of Sn2+ oxidation. In this study, we reported stable tin-based perovskite nanocrystals (NCs) using stannous acetate as the Sn2+ source because of its stronger Sn-O bonding. To prevent the oxidation of Sn2+, a thin layer of CsBr coverage was formed in situ; tin-based perovskite NCs, CsxSnBrx+2@CsBr (1 < x < 4), show a high photoluminescence quantum yield (PLQY) of 78.2% and high stability. The measured lifetime of PLQY decrease to half of the initial value is ∼1287 h under ambient conditions and ∼2200 h under a nitrogen atmosphere, respectively. Furthermore, the as-fabricated light-emitting diodes based on CsxSnBrx+2@CsBr NCs as the emitting layer exhibit a maximum luminescence of 16 cd/m2 and an external quantum efficiency of 0.035% with peaks at 451 and 615 nm, corresponding to the emissions of CsBr and CsxSnBrx+2, respectively. This work provided a new way to obtain stable Sn-based perovskite NCs and exhibited their potential for application in white light-emitting diodes (LEDs).

Separating Crystal Growth from Nucleation Enables the In Situ Controllable Synthesis of Nanocrystals for Efficient Perovskite Light-Emitting Diodes.

Colloidal perovskite nanocrystals (PNCs) display bright luminescence for light-emitting diode (LED) applications; however, they require post-synthesis ligand exchange that may cause surface degradation and defect formation. In situ-formed PNCs achieve improved surface passivation using a straightforward synthetic approach, but their LED performance at the green wavelength is not yet comparable with that of colloidal PNC devices. Here, it is found that the limitations of in situ-formed PNCs stem from uncontrolled formation kinetics: conventional surface ligands confine perovskite nuclei but fail to delay crystal growth. A bifunctional carboxylic-acid-containing ammonium hydrobromide ligand that separates crystal growth from nucleation is introduced, leading to the formation of quantum-confined PNC solids exhibiting a narrow size distribution. Controlled crystallization is further coupled with defect passivation using deprotonated phosphinates, enabling improvements in photoluminescence quantum yield to near unity. Green LEDs are fabricated with a maximum current efficiency of 109 cd A-1 and an average external quantum efficiency of 22.5% across 25 devices, exceeding the performance of their colloidal PNC-based counterparts. A 45.6 h operating half-time is further documented for an unencapsulated device in N2 with an initial brightness of 100 cd m-2 .

Multifunctional π-Conjugated Additives for Halide Perovskite.

Additive is a conventional way to enhance halide perovskite active layer performance in multiaspects. Among them, π-conjugated molecules have significantly special influence on halide perovskite due to the superior electrical conductivity, rigidity property, and good planarity of π-electrons. In particular, π-conjugated additives usually have stronger interaction with halide perovskites. Therefore, they help with higher charge mobility and longer device lifetime compared with alkyl-based molecules. In this review, the detailed effect of conjugated molecules is discussed in the following parts: defect passivation, lattice orientation guidance, crystallization assistance, energy level rearrangement, and stability improvement. Meanwhile, the roles of conjugated ligands played in low-dimensional perovskite devices are summarized. This review gives an in-depth discussion about how conjugated molecules interact with halide perovskites, which may help understand the improved performance mechanism of perovskite device with π-conjugated additives. It is expected that π-conjugated organic additives for halide perovskites can provide unprecedented opportunities for the future improvement of perovskite devices.

High stability and luminescence efficiency in donor-acceptor neutral radicals not following the Aufbau principle.

With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor-acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl or tris(2,4,6-trichlorophenyl)methyl radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor-acceptor radicals have a strong emission yield (up to 54%) and high photostability, with estimated half-lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light-emitting diodes based on such a radical emitter show deep-red/near-infrared emission with a maximal external quantum efficiency of 5.3%. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.

Circularly polarized luminescence of achiral open-shell π-radicals.

Stable luminescent π-radicals with doublet emission have attracted growing attention for functional molecular materials. However, their chiroptical properties, particularly their doublet emission-based circularly polarized luminescence, have never been investigated. Here, we investigate the circularly polarized luminescence (CPL) properties of a series of achiral luminescent open-shell π-radicals through various chirality regulation approaches, including induction by a magnetic field, supramolecular coassembly and chiral liquid crystal encapsulation.

Radical-Based Organic Light-Emitting Diodes with Maximum External Quantum Efficiency of 10.6.

A new luminescent radical, tris-2,4,6-trichlorophenylmethyl-1,5-diazarcarbazole (TTM-DACz), was synthesized and characterized. The photoluminescence quantum yield of TTM-DACz in solid 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene matrix film (5 wt %) is as high as 57.0%. Organic light-emitting diodes (OLEDs) employing TTM-DACz as the emitter were fabricated. By rational design of the device structure and host-guest doping system, external quantum efficiency (EQE) of up to 10.6% of the optimized device with a red CIE chromaticity of (0.62, 0.36) was obtained, which is among the highest values for red OLEDs using nonphosphorescent materials as the emitters. This work will accelerate the development of luminescent radical materials for high-performance OLEDs.

Efficient radical-based light-emitting diodes with doublet emission.

Organic light-emitting diodes (OLEDs)1-5, quantum-dot-based LEDs6-10, perovskite-based LEDs11-13 and micro-LEDs14,15 have been championed to fabricate lightweight and flexible units for next-generation displays and active lighting. Although there are already some high-end commercial products based on OLEDs, costs must decrease whilst maintaining high operational efficiencies for the technology to realise wider impact.  Here we demonstrate efficient action of radical-based OLEDs16, whose emission originates from a spin doublet, rather than a singlet or triplet exciton. While the emission process is still spin-allowed in these OLEDs, the efficiency limitations imposed by triplet excitons are circumvented for doublets. Using a luminescent radical emitter, we demonstrate an OLED with maximum external quantum efficiency of 27 per cent at a wavelength of 710 nanometres-the highest reported value for deep-red and infrared LEDs. For a standard closed-shell organic semiconductor, holes and electrons occupy the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs), respectively, and recombine to form singlet or triplet excitons. Radical emitters have a singly occupied molecular orbital (SOMO) in the ground state, giving an overall spin-1/2 doublet. If-as expected on energetic grounds-both electrons and holes occupy this SOMO level, recombination returns the system to the ground state, giving no light emission. However, in our very efficient OLEDs, we achieve selective hole injection into the HOMO and electron injection to the SOMO to form the fluorescent doublet excited state with near-unity internal quantum efficiency.

Effects of substituents on luminescent efficiency of stable triaryl methyl radicals.

A series of perchlorotriphenyl methyl (PTM) and tris(2,4,6-trichlorophenyl)methyl (TTM) radical derivatives were synthesized. The factors affecting the photoluminescence quantum yields (PLQYs) of π-radicals were studied systematically for the first time through comparing the photophysical properties of the synthesized PTM and TTM radicals. The room-temperature PLQY of a PTM radical derivative achieves to be 56.6%, which is the highest value among the organic near-infrared materials with peak wavelength over 650 nm. The photostabilities of the radicals was significantly enhanced via incorporation of substituent groups. The molecular rigidity, electron donating ability of the donor and dihedral angle between D-A system were found to be the potential factors to affect the luminescent efficiency of the open-shell molecules.

Stable Organic Radicals as Hole Injection Dopants for Efficient Optoelectronics.

Precursors of reactive organic radicals have been widely used as n-dopants in electron-transporting materials to improve electron conductivity and enhance electron injection. However, the utilization of organic radicals in hole counterparts has been ignored. In this work, stable organic radicals have been proved for the first time to be efficient dopants to enhance hole injection. From the absorbance spectra and the ultraviolet photoelectron spectra, we could observe an efficient electron transfer between the organic radical, (4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl (TTM-1Cz), and the widely used hole injection material, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN). When the unpaired electron of TTM-1Cz is transferred to HAT-CN, it would be oxidized to a TTM-1Cz cation with a newly formed lowest unoccupied molecular orbital which is quite close to the highest occupied molecular orbital (HOMO) of the hole-transporting material (HTM). In this way, the TTM-1Cz cation would promote the electron extraction from the HOMO of the HTM and improve hole injection. Using TTM-1Cz-doped HAT-CN as the hole injection layer, efficient organic light-emitting diodes with extremely low voltages can be attained.