via endothermally assisted reverse intersystem crossing (RISC), thereby achieving a theoretical 100% exciton utilization. [1b,3,4] The strategy for a sufficiently small ΔE ST is to reduce the electron exchange of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) by separating them into different segments. [5][6][7][8] As a classic example, a donor-π-acceptor (D-π-A) molecular architecture can form a twisted conformation at excitation and induce effective charge transfer (CT) along with the HOMO (@ donor) and LUMO (@ acceptor) separation. [6,9] The structure relaxation between the ground and twist induced excited states, however, inevitably results in large Stokes shift and broad CT emission, which is extremely detrimental to the color purity of the emitters for high-resolution display applications.In an exceptional solution for this problem proposed by Hatakeyama et al., a series of N/B rigid heterocycles were used as multiple resonances induced TADF (MR-TADF) emitters based on a DABNA-1 core to show good color purity. [10] Different from the D-π-A type TADF emitters, these MR-TADF emitters possess a rigid configuration and realize the HOMO and LUMO separation by opposite resonance effect of the nitrogen and boron atoms. Very recently, by modifying the original DABNA-1 with a peripheral carbazole group, Zheng and co-workers obtained a more efficient MR-TADF emitter TBN-TPA, and the corresponding device realized a maximum external quantum efficiency (EQE max ) of 32.1% and a sharp blue emission peak with full-width at half-maximum (FWHM) of 27 nm, showing the potential of MR-TADF. [11] However, the attempts with oxygen atoms replacing nitrogen atoms as the electron-donating group in a polycyclic framework did not activate the TADF phenomenon, [12] indicating new MR-TADF systems need more investigation and the versatility of the molecular design should be further explored.In this work, we design a novel system bearing MR-TADF activity aside from the reported N/B system, which consists of rigidified aryl ketones and amine. In the previous study, carbonyl group was widely used as a functional motif for room temperature phosphorescent materials and TADF emitters, owing to the electron-withdrawing nature and the small ΔE ST caused by its electronic transition from the n orbital to Multiple resonances induced thermally activated delayed fluorescence (MR-TADF) has great advantages in high color purity display. Up to now, current MR-TADF emitters are only based on the boron-nitrogen-containing fragment. Reported herein is a novel class of MR-TADF emitter, quinolino[3,2,1-de]acridine-5,9-dione (QAO), realized by the opposite resonance effect of the carbonyl and the nitrogen atoms, which is also the smallest TADF emitter reported so far. The QAO-based pure blue organic light-emitting diode achieves a maximum external quantum efficiency (EQE max ) of 19.4% with a small full width at half maximum of 39 nm. Moreover, tert-butyl modified QAO can be employed as an efficient electr...

This work describes a strategy to produce circularly polarized thermally activated delayed fluorescence (CP-TADF). A set of two structurally similar organic emitters SFST and SFOT are constructed, whose spiro architectures containing asymmetric donors result in chirality. Upon grafting within the spiro frameworks, the donor and acceptor are fixed proximally in a face-to-face manner. This orientation allows intramolecular through-space charge transfer (TSCT) to occur in both emitters, leading to TADF properties. The donor units in SFST and SFOT have a sulfur and oxygen atom, respectively; such a subtle difference has great impacts on their photophysical, chiroptical, and electroluminescence (EL) properties. SFOT exhibits greatly enhanced EL performance in doped organic light-emitting diodes, with external quantum efficiency (EQE) up to 23.1%, owing to the concurrent manipulation of highly photoluminescent quantum efficiency (PLQY, ∼90%) and high exciton utilization. As a comparison, the relatively larger sulfur atom in SFST introduces heavy atom effects and leads to distortion of the molecular backbone that lengthens the donor–acceptor distance. SFST thus has lower PLQY and faster nonradiative decay rate. The collective consequence is that the EQE value of SFST, i.e., 12.5%, is much lower than that of SFOT. The chirality of these two spiro emitters results in circularly polarized luminescence. Because SFST has a more distorted molecular architecture than SFOT, the luminescence dissymmetry factor (|g lum|) of circularly polarized luminescence of one enantiomer of the former, namely, either (S)-SFST or (R)-SFST, is almost twice that of (S)-SFOT/(R)-SFOT. Moreover, the CP organic light-emitting diodes (CP-OLEDs) show obvious circularly polarized electroluminescence (CPEL) signals with g EL of 1.30 × 10–3 and 1.0 × 10–3 for (S)-SFST and (S)-SFOT, respectively.

unite solution processing with desirable optoelectronic properties such as tunable light emission and long carrier lifetimes and diffusion lengths. [9][10][11] The rapid development of mixed-halide CsPbBr x /I 3−x nanocrystals via compositional tuning has enabled an EQE of 20.3% in the red with a full-width at half maximum (FWHM) of 40 nm. [12] Unfortunately, they have yet to rise to match the operating stability of organic [13,14] and inorganic quantum dot [15,16] LEDs: device operating stability (T 50 ) to date, at an initial luminance of 140 cd m -2 , has been limited to hours. [12] The organic ligands used in synthesis, and the subsequent exchange, provide colloidal stability in solution. However, these surface ligands are labile, exhibit limited surface binding affinity, and provide high surface coverage only when present in excess in solution. [18,19] They are readily desorbed upon dilution and washing, inducing incomplete passivation of surface sites.Introducing inorganic ligands in order to better passivate surfaces has shown promising progress in single halide composition in our previous work in both blue and red LEDs. [10,27] However, these strategies rely on a highly polar solvent (DMF) to carry out the inorganic salts. The introduction of the highly polar solvent compromises perovskite nanocrystal structural stability and harms the operating stability of LEDs.Here, we report a strategy wherein we introduce inorganic ligands in the antisolvent used in nanocrystal purification. We show that working with a mildly polar antisolvent, such as ethyl acetate used in this work, allows a gentler processing of the vulnerable perovskite quantum dots. Only by introducing ultrasonication during the introduction of the antisolvent were we able to develop an exchange that was successful, and substantially complete. The inorganic ligands replace, in situ, the organic ligands that are detached from the dot surface (Figure 1a): the small inorganic cations provide a rich surface coverage superior to that offered by long-chain organic ligands, and thus prevent trap formation. The inorganic ligands fill surface defects and improve material conductivity and charge-carrier injection in LEDs. The strategy improves bandgap stability, resulting in MHP solids with a storage stability of 1 year in ambient conditions (25 °C and 40% humidity), in comparison to controls that show phase changes after 7 days under the same conditions.Instability in mixed-halide perovskites (MHPs) is a key issue limiting perovskite solar cells and light-emitting diodes (LEDs). One form of instability arises during the processing of MHP quantum dots using an antisolvent to precipitate and purify the dots forming surface traps that lead to decreased luminescence, compromised colloidal stability, and emission broadening. Here, the introduction of inorganic ligands in the antisolvents used in dot purification is reported in order to overcome this problem. MHPs that are colloidally stable for over 1 year at 25 °C and 40% humidity are demonstrated and films ...

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Metal halide perovskite semiconductors have demonstrated remarkable potentials in solution‐processed blue light‐emitting diodes (LEDs). However, the unsatisfied efficiency and spectral stability responsible for trap‐mediated non‐radiative losses and halide phase segregation remain the primary unsolved challenges for blue perovskite LEDs. In this study, it is reported that a fluorene‐based π‐conjugated cationic polymer can be blended with the perovskite semiconductor to control film formation and optoelectronic properties. As a result, sky‐blue and true‐blue perovskite LEDs with Commission Internationale de l'Eclairage coordinates of (0.08, 0.22) and (0.12, 0.13) at the record external quantum efficiencies of 11.2% and 8.0% were achieved. In addition, the mixed halide perovskites with the conjugated cationic polymer exhibit excellent spectral stability under external bias. This result illustrates that π‐conjugated cationic polymers have a great potential to realize efficient blue mixed‐halide perovskite LEDs with stable electroluminescence.