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Developing novel acceptor unit with stronger electron absorption ability and deeper lowest unoccupied molecular orbital (LUMO) is of great significance for designing efficient red thermally activated delayed fluorescence (TADF) emitters. Herein, a new acceptor pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline (PPP) was developed, and through isomer engineering, two red TADF isomers otriTPA-PPP and mtriTPA-PPP were designed, using triphenylamine (TPA) as donors and PPP as acceptor. Two emitters show small ΔEST, which is beneficial for achieving efficient reverse intersystem crossing (RISC) process. OLEDs devices based on otriTPA-PPP and mtriTPA-PPP exhibit red emission at 597 and 603 nm, respectively. Due to the introducing of intramolecular H-bonding, otriTPA-PPP-based device shows excellent performance with a maximum external quantum efficiency (EQEmax) of 24.8%, comparing to only 8.5% for the mtriTPA-PPP-based device. Furthermore, otriTPA-PPP displays a significant improvement in luminance, with a larger luminance of 13170 cd m−2 compared to 3896 cd m−2 for mtriTPA-PPP. [ABSTRACT FROM AUTHOR]

Reducing the lead (Pb) toxicity has remained a huge challenge toward the commercial development of the mainstream Pb-based halide perovskites. Tin (Sn) has been considered as one of the most promising candidates to replace the Pb component, but it still suffers from severe oxidation of Sn2+ and poor perovskite morphology. In this work, bifunctional additive engineering by incorporating nicotinohydrazide (NHD) with two effective groups (–N2H3 and –C＝O) into the perovskite is employed for improving the device performance of Sn-based PeLEDs. The Sn2+ oxidation is greatly suppressed due to the reducibility of –N2H3 and strong interaction between –C＝O and Sn2+. Meanwhile, the perovskite film morphology is apparently optimized owing to the distinctly retarded crystallization process. Consequently, a pure-red PeLED emitting at 628 nm is achieved with elevated electroluminescence efficiency and stability, and the CIE coordinates match the BT.2020 standard. These results provide an effective strategy for the development of lead-free PeLEDs. [ABSTRACT FROM AUTHOR]

Efficient red emitters are crucial to the widespread use of organic light-emitting diodes (OLEDs) in future display and lighting technologies. However, the highly planar configurations of red TADF emitters lead to intermolecular-interaction-induced quenching. To obtain highly efficient doping-insensitive red emitters, a novel asymmetric red thermally activated delayed fluorescence (TADF) emitter, TPA-AQ-DCP, was developed using an asymmetric donor–acceptor–host (D–A–H) structure. The material consists of anthraquinone as the acceptor, triphenylamine as the donor, and 1,4-di(9H-carbazol-9-yl)benzene (DCP) serving as part of the emitter with host material properties. It is demonstrated that incorporating the host group into the TADF emitter with asymmetrical structure improves electron and hole transport within the emitter, effectively reducing concentration quenching. As a consequence, the asymmetric D–A–H structured TPA-AQ-DCP exhibited a relatively high photoluminescent quantum yield (PLQY) of 79.2% and fast radiative rate constant (kr) of 6.60 × 107 s−1, and thus better EL performance. Consequently, an efficient external quantum efficiency (EQE) of 11.76–17.60% with an emission peak of 610–620 nm was achieved for the TPA-AQ-DCP-based devices with doping ratios ranging from 5 wt% to 25 wt%. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters have been rapidly developed over a decade, and they all have efficiencies in excess of 35% in the visible range. Despite their high external quantum efficiency, the broad 70–110 nm emission spectra and short device lifetime have limited their commercialization. The recent rapid development of multiple resonance (MR) TADF materials with narrow emission has made them attractive to industry. Hence, high color purity MR-TADF materials are expected to be fourth-generation emitters for high definition (HD) organic light-emitting diodes (OLEDs). In this review, we systematically document the TADF emitters based on MR frameworks to date, including molecular design, material structure–photophysical properties and device characteristics. In addition, the molecular design guidelines and future commercial prospects of full-color MR-TADF emitters are envisaged. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters with a high horizontal emitting dipole ratio (Θ‖) remain a hot spot in the application of organic light-emitting diodes (OLEDs), which may achieve an extremely high external quantum efficiency (EQE). Herein, two yellow TADF emitters, 3,6,11-triTPA-BPQ and 3,6,12-triTPA-BPQ based on the triphenylamine (TPA) donor, have been synthesized by reasonably introducing nitrogen atoms into different positions of the acceptor core. Both emitters exhibit high horizontal emitting dipole ratios of 92% in doped organic films with photoluminescence quantum yields (ΦPL) of 94.0% and 78.4%, respectively. Because of high ΦPL and high light out-coupling ratio induced by high Θ‖ for 3,6,11-triTPA-BPQ, yellow OLEDs using this emitter achieve a maximum external quantum efficiency of 30.3%, which provides a novel way to boost the efficiency of yellow TADF-OLEDs. [ABSTRACT FROM AUTHOR]

Achieving deep blue thermally activated delayed fluorescent (TADF) materials with narrow emission still remains a challenge. Herein, two deep blue TADF emitters DCBTRZ and PPCTRZ were designed and synthesized by directly using host materials, 1,4-di(carbazol-9-yl)benzene (DCB) and 2-[4-(9H-carbazol-9-yl)phenyl]-1-phenyl-1H-phenanthro[9,10-d]imidazole (PPC) as donors, and 2,4,6-triphenyl-1,3,5-triazine (TRZ) as an acceptor. The two emitters have typical TADF properties. Both of them can emit deep blue fluorescence with a very narrow full-width at half-maximum (FWHM) of 42 nm in toluene. The photoluminescence quantum yields (PLQYs) of DCBTRZ and PPCTRZ are 39.6% and 38.2% in doped films, respectively. Devices based on DCBTRZ and PPCTRZ achieve deep blue emission (440 and 442 nm) with maximum external quantum efficiencies (EQEmax) of 6.68% and 6.55%, respectively. Meanwhile, they exhibit very high color purity. The CIEy and FWHM of devices employing DCBTRZ and PPCTRZ are 0.059/0.063 and 56/54 nm, respectively, meeting the standard of NTSC for deep-blue emitters, which indicates excellent application prospects in displays. The results suggest that this strategy for developing deep blue TADF materials with narrow emission and CIEy < 0.06 directly employing host materials as donors is feasible and effective. [ABSTRACT FROM AUTHOR]

To realize high-performance perovskite light-emitting diodes (PeLEDs), the underlying charge transport layer plays a vital role in charge injection and perovskite growth. Herein, a rational interface engineering method has been proposed to enhance the interfacial crystallization of perovskite films and simultaneously suppress the nor-radiative recombination of excitons by modifying the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole transport layer with ammonium thiocyanate (AT). The morphological control of the perovskite film with the improved crystal orientation arises from the strong interfacial chemical force between AT and perovskite, which promotes enhanced radiative recombination. The optimized PeLEDs achieve a peak external quantum efficiency of 14.7% and current efficiency of 45.4 cd A−1, which are approximately 4.6 times and 4.5 times higher than the control device, respectively. [ABSTRACT FROM AUTHOR]

Metal halide perovskite light-emitting diodes (PeLEDs) have been recognized as emerging candidates for next-generation solution-processed electronic display technology, and great progress has been made in recent years. However, their poor operational stability is yet to be solved. Here, we present a reformative all-inorganic red-emission PeLED based on the passivation effect caused by the incorporation of zinc ions into perovskites for enhancing the efficiency and operational stability. A high-quality quasi-two-dimensional perovskite CsPbI3−xBrx thin film with good crystallinity is obtained, benefiting from the combination of large organic ammonium ions (PEA+ ions) and inorganic salt zinc iodide (ZnI2). As a result, ZnI2-passivated CsPbI3−xBrx PeLEDs emitting at 666 nm show a maximum external quantum efficiency of 9.93% and a maximum brightness of 4258 cd m−2. More importantly, the PeLEDs achieve a long half-lifetime of about 2667 h at an initial luminance of 100 cd m−2. This work provides a facile method to obtain stable and efficient red-emission PeLEDs. [ABSTRACT FROM AUTHOR]

Perovskite light-emitting diodes (PeLEDs) are promising for full-color displays due to their high color purity, low-cost and solution processability. However, pure-red PeLEDs still lag behind their green and deep-red counterparts in efficiency and brightness. Here, we employ three structurally similar dual-Lewis-base molecules ethylenediamine, ethanolamine and beta-alanine to modify the interface between the hole transport layer (HTL) and (Cs/K/PEA)PbI3-xBrx perovskite layer for efficient pure-red PeLEDs. The functional groups in these molecules can enhance the hydrophilicity on the surface of the HTL, which improves the perovskite film quality and inhibits the formation of unfavorable low-order (n = 1 and n = 2) phases. Besides, amino groups can coordinate with interfacial defects. In particular, ethanolamine and beta-alanine enable the strong film growth modulation and passivation effect due to the presence of hydroxyl and carboxyl groups. The perovskites based on ethanolamine and beta-alanine interfacial modification give rise to enhanced photoluminescence quantum yields of up to 36.5% and 34.4%, respectively. Consequently, the optimal PeLEDs achieve a pure-red emission at 634 nm with a maximum luminance of 9218 cd m−2 and a peak external quantum efficiency of 5.27%. [ABSTRACT FROM AUTHOR]

A sulfur (S) atom generally undergoes sp3 hybridization when linking two moieties, not only for the sulfane status but also its fully oxidized form, sulfone. In the research reported here an S atom was used as a bridge to construct two thermally activated delayed fluorescence (TADF) emitters, Trz-S-DMAc and Trz-SO2-DMAc. The only difference between these two emitters is the valence of the S atoms in the bridges; however, their electron distributions are greatly affected and the charge-transfer (CT) transitions can be well-modulated between intermolecular and intramolecular modes. In a Trz-S-DMAc molecule, the S atom breaks the conjugation between donor (D) and acceptor (A) moieties, and their CT transition can only then occur with other Trz-S-DMAc molecules. The OLEDs based on Trz-S-DMAc achieve the best external quantum efficiency (EQE) of 18.1% at a high concentration of 55 wt% and maintain a satisfactory EQE of 13.0% for a non-doped device. Whereas for Trz-SO2-DMAc, the sulfone group acts as a second A moiety rather than a simple bridge. The Trz-SO2-DMAc is thus a typical TADF emitter with an effective intramolecular CT transition. The optimized OLED based on Trz-SO2-DMAc achieves a maximum EQE of 18.3%, whereas a significant efficiency drop can be observed with increasing doping concentrations. It is expected that these results will help to increase the understanding of how to link donor and acceptor moieties via sp3 atoms to construct TADF emitters and the roles of the different CT transitions. [ABSTRACT FROM AUTHOR]

High-efficiency orange and red thermally activated delayed fluorescent (TADF) organic light-emitting diodes were fabricated based on a pair of isomers 3,6,11-triAC-BPQ and 3,6,12-triAC-BPQ, containing a rigid dibenzo[f,h]pyrido[2,3-b]quinoxaline (BPQ) core and three 9,9-dimethyl-9,10-dihydroacridine (Ac) donors. Both materials exhibited red emission in neat films and small singlet–triplet energy gaps. Compared with the stable intramolecular charge transfer (ICT) state of 3,6,12-triAC-BPQ, 3,6,11-triAC-BPQ showed hybridized local and charge transfer (HLCT) character due to the H-bond between the 11-position Ac and 10-position N atom. As a result, the doped film of 3,6,11-triAC-BPQ showed a high photoluminescence quantum yield (PLQY) and the related device showed a high external quantum efficiency (EQE) of 22.0% in the orange–red region. [ABSTRACT FROM AUTHOR]

Exploiting effective orange-red thermally activated delayed fluorescent (TADF) emitters is still challenging. In this work, we proposed a method by using a crooked donor (D) moiety instead of its conventional planar form to balance the opposite influences of frontier molecular orbital overlap on the reverse intersystem crossing and the singlet radiative decay rate. A novel orange-red TADF emitter, named 2-(4-(9,9-dimethyl-4-phenylacridin-10(9H)-yl)phenyl)anthracene-9,10-dione (AQ-PhDMAC), was accordingly designed and synthesized. Owing to the steric effect of the α-phenyl ring in 9,9-dimethyl-4-phenyl-9,10-dihydroacridine (α-PhDMAC), a relatively small singlet–triplet energy splitting of 0.22 eV and a high fluorescence quantum yield of 0.89 can be realized simultaneously. The orange-red organic light-emitting diode based on AQ-PhDMAC exhibited a maximum external quantum efficiency of 18.1%, much improved compared with the 13.7% efficiency of the α-phenyl ring-free control emitter (AQ-DMAC). These results propose a new viewpoint that enforcing crooked D segments by large steric hindrance actually has a great potential for applications in orange-red TADF molecular design, to improve their electroluminescence efficiency. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters have attracted great attention for their use in organic light-emitting diodes since they can potentially realize 100% internal quantum efficiency by utilizing both singlet and triplet excitons due to an efficient reverse intersystem crossing process. However, forming a rational molecular design strategy is still challenging for high-performance TADF emitters. It is critical to understand the key factors that affect the performance of TADF materials. In this regard, this review focuses on the recent progress made in the design and synthesis of TADF materials controlled by the position and number of donor and acceptor units in different molecular structures. Reasonable molecular design principles have been summarized with the photophysical and electroluminescence characteristics of TADF materials. Furthermore, design guidelines and future prospects for high-performance TADF emitters are proposed for their future development. [ABSTRACT FROM AUTHOR]

For the development of solution-processed organic light-emitting diodes (OLEDs), it is highly desirable yet challenging to realize solution-processable non-doped thermally activated delayed fluorescence (TADF) emitters due to their high efficiency and excellent compatibility to the wet methods. Herein, two pairs of blue TADF isomers are designed and synthesized with a hetero-donor configuration for the realization of high photoluminescent quantum yield. The incorporation of two tert-butyl groups in the molecules can effectively increase the molecular solubility and reduce the aggregation-caused self-quenching of excitons in neat films by inhibiting the intramolecular vibrational relaxation and the intermolecular π–π stacking. Solution-processed non-doped OLEDs are achieved with these blue TADF emitters, exhibiting the record-high external quantum efficiencies (EQE) of 25.8%. Furthermore, an all-TADF white OLED with an EQE of 27.3% is also achieved by employing a single emitting layer with the blue TADF emitter as a host for an orange-red TADF dopant. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters normally suffer from serious concentration sensitivity, realizing peak electroluminescence efficiencies with precise control of the doping concentrations, which may hinder their practical applications in organic light emitting devices (OLEDs). In this work, we developed TADF emitter 2′,7′-di(10H-phenoxazin-10-yl)spiro[fluorene-9,9′-thioxanthene]10′,10′-dioxide (SPFS-PXZ) based on conventional TADF emitter 10,10′-(sulfonylbis(4,1-phenylene))bis(10H-phenoxazine) (DPS-PXZ). By introducing a fluorene group, the molecular rigidity and steric hindrance of SPFS-PXZ are improved greatly, which contributes to suppressing the strong π–π stacking tendency between phenoxazine segments and reduces intermolecular interactions. In turn, SPFS-PXZ successfully exhibits low concentration sensitivity in OLEDs, and keeps high maximum external quantum efficiencies (EQEs) over 20.8% with a small EQE fluctuation of 2.1% in a wide doping concentration range from 10 wt% to 50 wt%. These results prove that SPFS-PXZ is an ideal model to develop efficient TADF emitters with low concentration sensitivity. [ABSTRACT FROM AUTHOR]

By introducing two novel electron-donating (D) moieties dibenzothiophene (DBT) and dibenzofuran (DBF) as the secondary D groups, we designed and synthesized four novel blue thermally activated delayed fluorescence (TADF) emitters DBTCz-Trz, DBFCz-Trz, BDBTCz-Trz and BDBFCz-Trz. The secondary DBF and DBT segments all significantly participated in the highest occupied molecular orbital (HOMO) delocalization, resulting in effective TADF characteristics and high photoluminescence quantum yields simultaneously for all four emitters. In the devices, DBTCz-Trz and DBFCz-Trz successfully exhibit identical blue emission with a peak at 472 nm and a CIE coordinate of (0.17, 0.28) in the devices, while BDBTCz-Trz and BDBFCz-Trz exhibit cyan emissions. Moreover, the maximum forward-viewing external quantum efficiencies are determined to be as high as 21.7% for DBTCz-Trz, 21.6% for DBFCz-Trz, 23.4% for BDBTCz-Trz, and 25.1% for BDBFCz-Trz, respectively. These results not only prove the great potential of amine-free electron-donating components such as DBF and DBT in constructing TADF emitters for electroluminescence, but they also reveal the importance of extending the frontier orbital molecular distributions for TADF molecular optimization. [ABSTRACT FROM AUTHOR]

Despite the rapid development of organic solar cells (OSCs), light harvesting efficiency is limited by a compromise between light absorption and charge recombination loss due to the low carrier mobility of organic materials. To circumvent this block, a facile route is to implement the photonic structures into an ultrathin photoactive layer of OSCs to fully use the incident light. Herein, a simple yet cost-effective paradigm for light-trapping enhancement of OSCs is proposed by introducing the dual-grating patterns via a two-step soft nanoimprinting technology. The original grating templates are simply duplicated from a commercial CD or DVD disk, which can avoid the complicated mold fabrication process and allow the easy transfer of various two-dimensional grating patterns into OSCs. With an optimized dual-grating, a substantial increase in the photo-generated current of 11.6% is realized, yielding a power conversion efficiency of 9.92% for PTB7:PC71BM-based OSCs. The experimental and theoretical studies demonstrate that the light-trapping enhancement is attributed to broadband light scattering and the grating period longer than the absorption region can minimize the destructive interference. We anticipate that the proposed light manipulation scheme could help to achieve highly efficient OSCs of various material systems due to the ease of use and drastic improvement in efficiency. [ABSTRACT FROM AUTHOR]

Slow healing of gastric mucosa defects caused by endoscopic surgery is a common but severe clinical problem for lack of an effective treatment. Small intestinal submucosa (SIS) is a bio-derived extracellular matrix scaffold with remarkable repairing ability for soft tissue, but its rapid degradation and poor mechanical properties in the stomach environment limit its application for gastric mucosa regeneration. Herein, we modified SIS by genipin, a natural crosslinking agent, to improve its resistance against degradation in gastric juice and to promote the healing of gastric mucosa defects. The crosslinking characteristics of genipin-crosslinked SIS (GP-CR SIS) were evaluated by crosslinking degree, swelling ratio and FITR, respectively. GP-CR SIS was highly resistant to gastric juice digestion and had a great improvement in mechanical properties. Additionally, GP-CR SIS maintained excellent biocompatibility according to a cytotoxicity test, hemolysis test, and rat subcutaneous implant assay. In an in vivo study, we treated defected gastric mucosa with GP-CR SIS in a rabbit endoscopic submucosal dissection (ESD)-related ulcer model. After two weeks of surgical treatment, GP-CR SIS significantly expedited wound closure and ameliorated newly constructed tissue by providing a protective microenvironment for rapid granulation tissue formation and accelerating angiogenesis/re-epithelialization. In conclusion, this study demonstrates the huge therapeutic potential of GP-CR SIS scaffolds for accelerating defected gastric mucosa regeneration. [ABSTRACT FROM AUTHOR]

To fully unlock the potential of metallic electrodes in flexible polymer solar cells (PSCs), tuning their optical properties is urgently required. Here we report an efficient light harvesting scheme involving the combination of a silver mesowire grid-based front transparent electrode and a plasmonic meta-mirror-based back reflector electrode. As an alternative to the indium-tin-oxide transparent conductor, the silver mesowire grid on a plastic substrate enables the reduced ohmic loss with competitive mechanical, electrical and optical properties, verifying its superiority in large-area flexible devices. Further implementation of the plasmonic meta-mirror back reflector allows broadband enhancement of light harvesting efficiency over the entire visible wavelength range, yielding a power conversion efficiency of 9.50% due to a 23.2% increase in photocurrent compared to the reference flat device. Experimental and theoretical analysis of light propagation in PSCs elucidates that the optical harvesting enhancement benefits from the synergistic effects of wide-area redistribution of an optical field induced by the silver mesowire grid electrode and broadband recovery of photon loss via surface plasmon and scattering enabled by the meta-mirror back reflector. This method provides a convenient and scalable way for developing high-performance flexible PSCs towards the future photovoltaic applications. [ABSTRACT FROM AUTHOR]

The emergence of smartphones and televisions with curved displays indicates that flexible organic light-emitting diodes (OLEDs) are marching steadily toward commercialization. Regardless of the significant step forward from laboratory to industry, flexible OLEDs are still facing enormous obstacles and challenges in realizing truly wearable, deformable, printable and low-cost electronic products for the future. How to develop highly flexible electrodes, optimize the device efficiency and greatly extend the operation lifetime are among the most important issues to be solved. In this regard, this review summarizes the recent achievements in flexible transparent conductive electrodes, device fabrication as well as light extraction technologies. Furthermore, the device operation stability is discussed with a focus on device degradation mechanisms and encapsulation methods. Some future prospects on new opportunities and research challenges in flexible OLEDs are also covered. [ABSTRACT FROM AUTHOR]

Mechanisms of charge transport between the interconnector and its neighboring layers in tandem organic photovoltaic cells have been systematically investigated by studying electronic properties of the involving interfaces with photoelectron spectroscopies and performance of the corresponding devices. The results show that charge recombination occurs at HATCN and its neighboring hole transport layers which can be deposited at low temperature. The hole transport layer plays an equal role to the interconnector itself. These insights provide guidance for the identification of new materials and the device architecture for high performance devices. [ABSTRACT FROM AUTHOR]

The influence of constituent materials and the operational stability of charge generation layers (CGLs) on the device characteristics of tandem organic light-emitting diodes (OLEDs) is reported upon. The CGLs between two vertically stacked emission units consist of an abrupt heterointerface between a MoO3 film and an n-type doped organic electron-transporting layer, where the n-type dopants vary from reactive metal (Mg) to metallic compounds (i.e., Cs2CO3 and CsN3). Contrary to the almost identical electron injection barriers modulated by the n-type dopants, the device characteristics and operational stability of tandem OLEDs are found to be sensitive to the doping constituent materials used in the CGLs. Using data on the electrical and spectral emission properties, electronic structures and the lifetime characteristics, it is identified that the degradation of the n-type doped layers in CGLs provides the main contribution to the device lifetime, due to the different doping mechanisms. [ABSTRACT FROM AUTHOR]

We have demonstrated a new approach to realize the light extraction enhancement in organic light-emitting diodes (OLEDs) by using platinum–cobalt (Pt3Co) alloy nanoparticles (ANPs). The current efficiencies of 19.2 cd A−1 and 29.3 cd A−1 at a current density of 20 mA cm−2 are obtained for the device with Pt3Co ANPs unannealed and annealed, respectively, which correspond to a ∼46% and ∼123% enhancement compared to the control device without Pt3Co ANPs. A systematic study on the devices with and without unannealed Pt3Co ANPs shows that the significantly enhanced efficiency is mainly due to the resonance of localized surface plasmon (LSP). The analysis of surface topography, angular-dependent EL spectra and theoretical calculations of the devices with and without annealed Pt3Co ANPs reveals that the annealing treatment of Pt3Co ANPs could result in a further enhancement in light extraction due to the increased light scattering effect, thereby achieving a double-enhancement in light extraction by simultaneously realizing the resonance of LSP and the light scattering effect. [ABSTRACT FROM AUTHOR]

The impact of electronic structures on the functionality of transition metal oxide-based intermediate connectors for tandem organic light-emitting devices is investigated by studying the interfaces and the corresponding devices. For a typical transition metal oxide-based intermediate connector, consisting of a heterointerface between MoO3and Mg-doped tris(8-quinolinolato)aluminum (Mg:Alq3), it is identified that MoO3is essential to the charge generation and separation process, which occurs at the interface between MoO3and the adjacent hole-transporting layer (HTL) viaelectron transfer from the highest occupied molecular orbital of the HTL into the conduction band of MoO3. In addition, the incorporation of a Mg:Alq3layer is indispensable to the functionality of the intermediate connector, which not only facilitates the electron injection from MoO3into the electron-transporting layer of the adjacent electroluminescent (EL) unit, but also blocks the leakage of holes across the intermediate connector into the HTL of the other adjacent EL unit. [ABSTRACT FROM AUTHOR]

Intramolecular exciplex systems featuring thermally activated delayed fluorescence (TADF) have garnered significant attention in the realm of organic light-emitting diodes (OLEDs). Nonetheless, the occurrence of organic sandwich intramolecular exciplexes remains rare due to structural limitations and synthetic challenges. Herein, we present a novel rigid acceptor-donor-acceptor (A-D-A) sandwich complex, dSFQP, characterized by two sp 3 C-locking moieties. This compound exhibits TADF characteristics facilitated by a multiple through-space charge-transfer process. X-ray crystallographic analysis confirms the distinctive sandwich configuration. The parallel spatial arrangement and minimized A-D-A configuration enhance electronic interactions, resulting in a high photoluminescence quantum yield, rapid reverse intersystem crossing rate, and sluggish nonradiative decay rate. OLEDs employing dSFQP as the dopant achieve a maximum external quantum efficiency (EQE) of 28.5% with a low efficiency roll-off of merely 2.8% at 1000 cd m -2 . Even at a high brightness of 10 000 cd m -2 , the EQE remains notably high at 17.5%. Our current results provide an effective way to further innovate the design of new organic charge-transfer complexes.

Thermally activated delayed fluorescence (TADF) emitters with aggregation-induced emission (AIE) features are hot candidates for non-doped organic light-emitting diodes (OLEDs), as they are highly emissive in solid states upon photoexcitation. Nevertheless, not every AIE-TADF emitter in the past had guaranteed decent efficiencies in non-doped devices, indicating that the AIE character alone does not necessarily afford ideal non-doped TADF emitters. As intermolecular electron-exchange interaction that involves long-lived triplet excitons plays a dominant role in the whole quenching process of TADF, we anticipate that it is the main reason for the different electroluminescence performances of AIE-TADF emitters. Therefore, in this work, we designed two TADF emitters SPBP-DPAC and SPBP-SPAC by modifying a reported less successful emitter BP-DPAC with extra fluorenes to increase intermolecular distances and attenuate this electron-exchange interaction. With the fluorene lock as steric hindrance, SPBP-DPAC and SPBP-SPAC exhibit significantly higher exciton utilization in non-doped films due to the suppressed concentration quenching. The non-doped OLEDs based on SPBP-DPAC and SPBP-SPAC show an excellent maximum external quantum efficiency (EQE) of 22.8% and 21.3% respectively, and what's even more promising is that ignorable roll-offs at practical brightness ( e.g. , 1000 and 5000 cd m -2 ) were realized. These results reveal that locking the phenyl rings as steric hindrance can not only enhance the molecular rigidity, but also cause immediate relief of concentration quenching, and result in significant performance improvement under non-doped conditions. Our approach proposes a feasible molecular modification strategy for AIE-TADF emitters, potentially increasing their applicability in OLEDs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)