High Electroluminescence Efficiency Research Articles

High-efficiency electroluminescence devices containing Si nanocrystals/SiC multilayers via improved carrier injection and recombination process

Realizing high efficiency of all Si-based light-emitting devices is currently one of interesting issues in order to develop monolithic opto-electronic integration on chips. Here, we report an electroluminescence device based on phosphorus (P)-doped silicon nanocrystals (Si NCs)/silicon carbide (SiC) multilayers by modulating carrier injection and recombination process. The p+-Si substrate is used instead of p-Si substrate for facilitating the hole injection into Si NCs. Additionally, the influences of annealing temperature on the device performance have been studied, and the optimized annealing temperature is achieved by balancing the crystallinity, defect state density, and recombination process.

Hybrid White OLEDs with High Electroluminescence Efficiency and High Color Rendering Index Achieved by a Sandwich Interlayer Sensitizing Configuration

AbstractWhite organic light‐emitting devices (WOLEDs) exhibit promising applications as white illumination and background light, but achieving high efficiencies, and high color quality for the WOLEDs still remains difficult. Herein, to enhance exciton utilization and improve the color quality of WOLEDs, a new sandwich configuration of an ultrathin orange phosphorescent layer located between two blue thermally activated delayed fluorescence (TADF) layers is proposed, and the working mechanism is investigated. In this configuration, the interlayer sensitization from blue TADF layers to orange phosphorescent layer via efficient Förster energy transfer occurs, which effectively alleviates exciton quenching in emitting layers and thus results in excellent electroluminescence (EL) efficiencies and improved color quality. The obtained two‐color hybrid WOLED exhibits an external quantum efficiency (ηext) exceeding 27%, with Commission Internationale del'Eclairage (CIEx,y) coordinates of (0.335, 0.396) and a color rendering index (CRI) of 63. By further incorporating an ultrathin red phosphorescent layer between the orange phosphorescent layer and the blue TADF layer, the ηext maintains at 21.96% and the CRI increases to 91 (CIEx,y = 0.380, 0.366). These results demonstrate the proposed sandwich interlayer sensitizing configuration is an effective and feasible strategy for realizing high EL efficiencies and high color quality simultaneously for hybrid WOLEDs.

Linker aggregation engineering of TADF materials to tune carrier balance for highly efficient organic LEDs with long operational lifetime

AbstractThermally activated delayed fluorescence (TADF) molecules are regarded as promising materials for realizing high‐performance organic light‐emitting diodes (OLEDs). The connecting groups between donor (D) and acceptor (A) units in D–A type TADF molecules could affect the charge transfer and luminescence performance of TADF materials in aggregated states. In this work, we design and synthesize four TADF molecules using planar and twisted linkers to connect the aza‐azulene donor (D) and triazine acceptor (A). Compared with planar linkers, the twisted ones (Az‐NP‐T and Az‐NN‐T) can enhance A–A aggregation interaction between adjacent molecules to balance hole and electron density. As a result, highly efficient and stable deep‐red top‐emission OLEDs with a high electroluminescence efficiency of 57.3% and an impressive long operational lifetime (LT95 ∼ 30,000 h, initial luminance of 1000 cd m−2) are obtained. This study provides a new strategy for designing more efficient and stable electroluminescent devices through linker aggregation engineering in donor–acceptor molecules.

Stepwise Toward Pure Blue Organic Light-Emitting Diodes by Synergetically Locking and Shielding Carbonyl/Nitrogen-Based MR-TADF Emitters.

Deep-blue multi-resonance (MR) emitters with stable and narrow full-width-at-half-maximum (FWHM) are of great importance for widening the color gamut of organic light-emitting diodes (OLEDs). However, most planar MR emitters are vulnerable to intermolecular interactions from both the host and guest, causing spectral broadening and exciton quenching in thin films. Their emission in the solid state is environmentally sensitive, and the color purity is often inferior to that in solutions. Herein, a molecular design strategy is presented that simultaneously narrows the FWHM and suppresses intermolecular interactions by combining intramolecular locking and peripheral shielding within a carbonyl/nitrogen-based MR core. Intramolecularly locking carbonyl/nitrogen-based bears narrower emission of 2,10-dimethyl-12,12-diphenyl-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-4,8(12H)-dione in solution and further with peripheral-shielding groups, deep-blue emitter (12,12-diphenyl-2,10-bis(9-phenyl-9H-fluoren-9-yl)-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-4,8(12H)-dione, DPQAO-F) exhibits ultra-pure emission with narrow FWHM (c.a., 24nm) with minimal variations (∆FWHM ≤ 3nm) from solution to thin films over a wide doping range. An OLED based onDPQAO-F presents a maximum external quantum efficiency (EQEmax) of 19.9% and color index of (0.134, 0.118). Furthermore, the hyper-device of DPQAO-F exhibits a record-high EQEmax of 32.7% in the deep-blue region, representing the first example of carbonyl/nitrogen-based OLED that can concurrently achieve narrow bandwidth in the deep-blue region and a high electroluminescent efficiency surpassing 30%.

Energy Transfer-Dominated Quasi-2D Blue Perovskite Light-Emitting Diodes.

The bromide-chloride mixed quasi-two-dimensional (2D) perovskite, with a natural quantum well structure and tunable exciton binding energy, has gained significant attention for high-performance blue perovskite light-emitting diodes (PeLEDs). However, the relative importance of having a low trap state density or efficient exciton transfer for high-efficiency electroluminescence (EL) performance remains elusive. Here, two molecules with the benzoic acid group, sodium 4-fluorobenzoate (SFB) and 3,5-dibromobenzoic acid (DBA), are used to modulate the phase distribution and trap state to explore the effect between energy transfer and defect passivation. As a result, when the n = 1 phase is inhibited in both films, the DBA@SFB-modified perovskite films achieve a higher photoluminescence quantum yield (PLQY) than the SFB-modified perovskite films due to effective defect passivation. However, DBA@SFB-modified PeLEDs exhibit lower external quantum efficiency (EQE) compared to SFB-modified PeLEDs due to the poor exciton transfer between the low-dimensional phase. This demonstrates that passivation strategies may enhance photoluminescence through reducing nonradiative recombination, but the effect of phase distribution is pivotal for EL performance by efficient energy transfer in quasi-2D perovskites. Femtosecond time-resolved transient absorption measurements confirm the fastest carrier dynamics in SFB-modified perovskite films, further corroborating the above result. This work provides useful information about phase modulation and defect passivation for high-efficiency blue quasi-2D PeLEDs.

Precursor Engineering Induced High-Efficiency Electroluminescence of Quasi-Two-Dimensional Perovskites: A Synergistic Defect Inhibition and Passivation Approach.

Despite light-emitting diodes (LEDs) based on quasi-two-dimensional (Q-2D) perovskites being inexpensive and exhibiting high performance, defects still limit the improvement of electroluminescence efficiency and stability by causing nonradiative recombination. Here, an organic molecule, 1-(o-tolyl) biguanide, is used to simultaneously inhibit and passivate defects of Q-2D perovskites via in situ synchronous crystallization. This molecule not only prevents surface bromine vacancies from forming through hydrogen bonding with the bromine of intermediaries but also passivates surface defects through its interaction with uncoordinated Pb. Via combination of defect inhibition and passivation, the trap density of Q-2D perovskite films can be significantly reduced, and the emission efficiency of the film can be improved. Consequently, the corresponding LED shows an external quantum efficiency of 24.3%, and its operational stability has been increased nearly 15 times.

Prediction of Operational Lifetime of Perovskite Light Emitting Diodes by Machine Learning

Perovskite light‐emitting diodes (LEDs) with advantages of high electroluminescence efficiency at high brightness, good color purity, and tunable bandgap, are believed to have potential applications in the next generation display and lighting technologies. Due to the complex degradation process, mathematic models to describe the degradation process of perovskite LEDs are absent. In this work, it is found that the mathematical fitting methods which have been widely used to describe the decay trend of organic LEDs and quantum‐dot LEDs, are unable to accurately predict the lifetime of perovskite LEDs. Then an ensemble machine learning model is developed, which utilizes data augmentation technique to predict T50 of perovskite LEDs based on features before T80, achieving an accuracy of 0.995. Furthermore, the model can also accurately predict the T90 lifetime of quantum‐dot LEDs (QLEDs) using features before T98, suggesting it is a useful tool to efficiently evaluate LED lifetimes.

Multi-functional ion-pairing additive for efficient quasi-2D perovskite light-emitting diodes and solar cells

Due to their unique quantum confinement, dielectric coupling, and tunable high-efficiency electroluminescence, quasi-two-dimensional (quasi-2D) perovskite light-emitting diodes (PeLEDs) have attracted considerable attention. However, imperfections in perovskite thin films introduced during the solution-based fabrication process lead to significant non-radiative recombination, hindering device performance. Addressing this challenge is crucial for realizing high-performance quasi-2D PeLEDs. In this study, we introduce an ion-pairing-type additive, 6-maleimidohexanehydrazide trifluoroacetate (EMCH-TFA), designed to cooperatively passivate defects in quasi-2D perovskites. The diverse functional groups (–COOH, –NH2, -CF3) on EMCH-TFA could provide disparate modification to perovskite, among which the CO group tends to coordinate with the uncoordinated Pb2+ ions, thereby passivating the defects in the perovskite crystal. Additionally, the –NH2 and -CF3 groups establish hydrogen bonds with Br- ions and BA+ spacer cations, respectively. This multifaceted approach suppresses the formation of the small n phase and induces alterations in the crystalline state by interacting with numerous sites in perovskites, including A’, B, and X sites. Consequently, the quasi-2D PeLED based on BA2Csn-1PbnBr3n+1 exhibits enhanced brightness, achieving a current efficiency of 66.7 cd/A and an impressive external quantum efficiency of 19.77 %.

Efficient deep-red Aggregate-Induced emission material and “Hot-exciton” electroluminescence

Organic deep-red fluorescent materials have a wide range of applications in organic light-emitting diodes (OLEDs), biological imaging, night-vision equipment, and other fields. Here, we designed and synthesized a new deep-red emissive donor–acceptor (D-A) compound TPANZPPICN, whose non-doped OLED achieves stable pure deep-red electroluminescence (EL) at 692 nm with a maximum external quantum efficiency (EQE) of 2.67 % in non-doped OLED and a maximum EQE of 7.96 % at 660 nm in doped OLED. Magneto-electroluminescence (MEL) measurements reveal a negative response in the doped OLED, which is a significant character of reversed intersystem crossing (RISC) on high-lying excited states. Both theoretical calculations and experimental results confirm that the hot-exciton mechanism is responsible for the high efficiency EL of doped OLED, which proves to be a novel approach to realize the efficient deep-red organic fluorescent OLEDs.

Achieving High External Quantum Efficiency for ITIC‐Based Organic Solar Cells with Negligible Homo Energy Offsets

AbstractMinimizing energy loss in organic solar cells (OSCS) is critical for attaining high photovoltaic performance. Among the parameters that correlated to photovoltaic performance, the energy offsets between donor–acceptor pairs play a vital role in photoelectric conversion processes. For so far reported a large number of non‐fullerene acceptors (NFAs), only Y6 and its derivatives can achieve external quantum efficiencies (EQEs) over 80% with negligible energy offsets when combined with polymeric donors. Thus, understanding the relationship between energy offsets and energy losses in representative NFAs is the key to further enhancing the efficiency of OSCs. In this study, a series of wide‐bandgap polymer donors based on pyrrolo[3,4‐f]benzotriazole‐5,7(6H)‐dione (TzBI) and benzo[1,2‐c:4,5‐c′] dithiophene‐4,8‐dione building blocks are combined with representative NFAs, including ITIC and Y6, to gain deep insights into their photovoltaic performances and related energy losses. Outstanding EQEs (≈70%) and suppressed non‐radiative recombination are achieved at negligible energy offsets. Moreover, it is noted that a prolonged exciton lifetime of acceptor is not essential to obtain high EQEs in OSCs with negligible energy offsets. Eventually, ITIC derivatives with high electroluminescence efficiencies and near‐infrared absorptions have the potential to be assembled to obtain high‐efficiency OSCs.

Carbonyl- and Nitrogen-Embedded Multi-Resonance Emitter with Ultra-Pure Green Emission and High Electroluminescence Efficiencies.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with narrow emission spectra have garnered significant attention in future organic light-emitting diode (OLED) displays. However, current C=O/N-embedded MR-TADF systems still lack satisfactory performance in terms of electroluminescence bandwidths and external quantum efficiencies (EQEs). In this study, a C=O/N-embedded green MR-TADF emitter, featuring two acridone units incorporated in a sterically protected 11-ring fused core skeleton, is successfully synthesized through finely controlling the reaction selectivity. The superior combination of multiple intramolecular fusion and steric wrapping strategies in the design of the emitter not only imparts an extremely narrow emission spectrum and a high fluorescence quantum yield to the emitter but also mitigates aggregation-induced spectral broadening and fluorescence quenching. Therefore, the emitter exhibits leading green OLED performance among C=O/N-based MR-TADF systems, achieving an EQE of up to 37.2 %, a full width at half maximum of merely 0.11 eV (24 nm), and a Commission Internationale de l'Éclairage coordinate of (0.20, 0.73). This study marks a significant advance in the realization of ideal C=O/N-based MR-TADF emitters and holds profound implications for the design and synthesis of other MR-TADF systems.

Constructing Multiresonance Thermally Activated Delayed Fluorescence Emitters for Organic LEDs: A Computational Investigation.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials have acquired substantial attention due to their high electroluminescence efficiency with narrow emission spectra. However, the existing MR-TADF emitters suffer from substantial efficiency roll-off due to insufficient rate constants of the reverse intersystem crossing (kRISC) process compared to the traditional TADF materials. Herein, we employ the DLPNO-STEOM-CCSD method, which is computationally less expensive than the wave function-based EOM-CCSD method, to evaluate the electronic and photophysical properties of MR-TADF materials accurately. The predicted singlet-triplet energy gap (ΔEST), one of the critical parameters governing the TADF efficiency, exhibits remarkable agreement with the experimental measurement, with a standard deviation value of 0.026 eV (obtained based on five experimentally synthesized MR-TADF systems). The proposed technique was utilized to determine the suitability of 15 newly designed MR-TADF emitters via their computed radiative and nonradiative rates, luminescence efficiencies, and exciton characteristics. Moreover, most conceived molecules exhibit blue emission with decent to strong oscillator strengths, making them potential candidates for practical light-emitting applications. The proposed computational route will undoubtedly accelerate the designing and prescreening of potential MR-TADF emitters before their expensive laboratory synthesis and characterization.

Thienoacene-fused fluorophores for highly efficient deep red/near-infrared OLEDs

Highly efficient deep red/near-infrared (DR/NIR) thermally activated delayed fluorescence (TADF) emitters were rarely reported by now, which was partially due to the lack of appropriate donor (D) and acceptor (A) units. Therefore, the exploration of novel building blocks is highly demanded for the development of efficient TADFs. Thienoacene (TA) with a ladder-type structure containing thiophene units demonstrated intriguing electronic properties. Importantly, the ring-fused system of TA lowered the lowest singlet excited state (S1) significantly and thus pushed the emission into long wavelength region. In this work, the polycyclic π-extended TA was applied to construct TADF materials (TA-TPA, TA-tBu-TPA and TA-CN-TPA), where the robust configuration resulted in high photoluminescent quantum yield (PLQY). Consequently, TADF OLED based on TA-CN-TPA demonstrated a high electroluminescence (EL) efficiency with external quantum efficiency (EQE) of 15.2% at 710 nm and 12.7% at 724 nm. This work indicated that TA-fused TADFs could exhibit attractive OLED properties, complementary to the N/O-heteroacene based emitters, and the result will pave a new path for the development of efficient heteroacene-based NIR TADFs.

Axially chiral biphenoxazine-based thermally activated delayed fluorescence materials for solution-processed circularly polarized organic light-emitting diodes

Circularly polarized organic light-emitting diodes (CP-OLEDs) from pure organic thermally activated delayed fluorescence (TADF) materials are a research hotspot due to their low price and high electroluminescence efficiency for antiglare filters and 3D displays. Herein, one pair of intrinsically chiral octahydro-binaphthalenes-based biphenoxazine dyes were designed, prepared, and characterized. Bearing one axially chiral electron-donor (D) biphenoxazine core and two electron-acceptor (A) 2,4,6-triphenyl-s-triazineine fragments, they are tri-propeller-type, which would be in favour of both charge separation and A-D-D-A charge transfer to achieve aggregation-induced yellow-green TADF with a quantum yield of 25.2% and 57.6% for neat and doped film, respectively. Most importantly, they can be acted as emitters in solution processed CP-OLEDs to achieve luminance, current efficiency, external quantum efficiency, and dissymmetry factor up to 1.66 × 104 cd/m−2, 35.2 cd/A, 11.3% and 0.88 × 10−3, respectively. Moreover, the dissymmetry factor can be further enhanced to 1.69 × 10−3, if semitransparent electrodes are adopted.

B/N/O-participated multi-resonance TADF emitters by a simple peripheral decoration strategy enable high-efficiency electroluminescence with EQEs up to 36.5%

This work reports highly efficient B/N/O-participated multi-resonance thermally activated delayed fluorescence emitters with a simple peripheral decoration strategy, enabling electroluminescence device with external quantum efficiency up to 36.5%.

Efficient emission of quasi-two-dimensional perovskite films cast by inkjet printing for pixel-defined matrix light-emitting diodes

Quasi-two-dimensional (quasi-2D) perovskites are promising materials for potential application in light-emitting diodes (LEDs) due to their high exciton binding energy and efficient emission. However, their luminescent performance is limited by the low-n phases that act as quenching luminescence centers. Here, a novel strategy for eliminating low-n phases is proposed based on the doping of strontium bromide (SrBr2) in perovskites, in which SrBr2 is able to manipulate the growth of quasi-2D perovskites during their formation. It was reasonably inferred that SrBr2 readily dissociated strontium ions (Sr2+) in dimethyl sulfoxide solvent, and Sr2+ was preferentially adsorbed around [PbBr6]4− through strong electrostatic interaction between them, leading to a controllable growth of quasi-2D perovskites by appropriately increasing the formation energy of perovskites. It has been experimentally proved that the growth can almost completely eliminate low-n phases of quasi-2D perovskite films, which exhibited remarkably enhanced photoluminescence. A high electroluminescent efficiency matrix green quasi-2D perovskite-LED (PeLED) with a pixel density of 120 pixels per inch fabricated by inkjet printing technique was achieved, exhibiting a peak external quantum efficiency of 13.9%, which is the most efficient matrix green quasi-2D PeLED so far to our knowledge.

Organic Photovoltaic Cells Based on Nonhalogenated Polymer Donors and Nonhalogenated A-DA'D-A-Type Nonfullerene Acceptors with High VOC and Low Nonradiative Voltage Loss.

Compared with other all-inorganic/organic-inorganic hybrid solar cells, the large voltage loss (Vloss) of organic photovoltaic (OPV) cells, especially the nonradiative voltage loss (ΔVnonrad), limited the further improvement of performance. Although A-DA'D-A-type Y-series nonfullerene acceptors (NFAs) largely improve the power conversion efficiencies (PCEs) to 18%, the open-circuit voltage (VOC) of this kind of material was still restricted to below 1.0 V. Herein, we designed and synthesized a narrow bandgap (Eg = 1.41 eV) acceptor BTA77 with an A-DA'D-A-type backbone containing a nonhalogenated terminal group to achieve high electroluminescence efficiency and high VOC. Combined with the nonhalogenated polymer PBDB-T with a conjugated thiophene side chain, BTA77 realized a VOC of 0.944 V, a Vloss of 0.552 V, and a PCE of 13.75%, which is one of the highest PCEs based on nonhalogenated A-DA'D-A-type acceptors with VOC > 0.9 V. After further blending with the nonhalogenated donor polymer PBT1-C with a conjugated phenyl side chain, the VOC increases to 1.021 V with a super low ΔVnonrad of 0.14 V owing to the greatly improved electroluminescence external quantum efficiency (EQEEL) of 4.42 × 10-3. Our results indicate that there is still a large room to decrease the ΔVnonrad and increase VOC by synergistic molecular engineering of p-type polymers and n-type small molecules.

Anthracene-based blue fluorescence materials utilized in non-doped OLEDs with high luminance and a low efficiency roll-off

In this work, we designed and synthesized two novel blue-emitting compounds (m-TAC and p-TAC) with a twisted D-π-A configuration by incorporation of 2,4,6-triphenyl-1,3,5-triazine as an acceptor and carbazole as a donor through an anthracene π-spacer. Therein, p-TAC exhibits a hybridized local and charge-transfer (HLCT) characteristic with a sky-blue emission at 462 nm in toluene while m-TAC shows emission maximum at 431 nm. The two compounds exhibit superb thermal stability in the company of a thermal decomposition temperature (Td) as high as 507 and 516 °C. It is noticed that the non-doped devices with m-TAC and p-TAC as blue emitters display a turn-on voltage (Von) of 3.2 V and achieve sky-blue emission at 460 and 476 nm. In particular, the optimized device based on p-TAC realizes a maximum luminance above 10 000 cd m−2 and the maximum external quantum efficiency (EQEmax) reaching 5.03%. As a sky-blue HLCT emitter, p-TAC reveals high-efficiency electroluminescence (EL) performance, providing a new insight for further industrial applications.

Electroluminescence enhancement of ZnO nanorod array/GaN heterojunction with MgZnO barrier layer

ZnO nanorod arrays were fabricated by solution method, and highly vertical-oriented ZnO NRA were obtained with MgxZn1-xO seed layer. The temperature-dependent photoluminescence showed a typical transformation process between bound exciton to free exciton emission, causing a domination of near-band edge photoluminescence at room temperature. For ZnO NRA/GaN heterojunction device, an obvious blue light could be observed after current injection, and electroluminescence profiles originated from ZnO exciton, ZnO/GaN interface and GaN accepter related recombination could be enhanced due to the increasing current injection. By introducing MgZnO barrier layer, the interface state on ZnO side could be effectively passivated, and tunneling effect through the barrier could significantly enhance the recombination probability, achieving high efficiency electroluminescence comparing with heterojunction without MgZnO barrier layer.

High Steric‐Hindrance Windmill‐Type Molecules for Efficient Ultraviolet to Pure‐Blue Organic Light‐Emitting Diodes via Hybridized Local and Charge‐Transfer Excited‐State

AbstractHybridized local and charge‐transfer (HLCT) excited‐state compounds that enable full exciton utilization through a reverse intersystem conversion (RISC) from a high‐lying triplet to a singlet state have attracted attention. Developing high‐performance ultraviolet (UV) and blue organic light‐emitting diodes (OLEDs) is challenging due to difficulties acquiring HLCT molecules with a large energy bandgap and a high photoluminescence. Herein, a new strategy for excellent‐performance UV to blue emitters based on high steric‐hindrance windmill‐type structure is proposed. These emitters exhibit good thermal, morphological, and electrochemical stabilities, as well as HLCT excited‐state characteristics. Results suggest that OLED using CTPPI efficiently emits UV light (396 nm, CIEx,y = 0.16, 0.04) with a maximum external quantum efficiency (EQE) of 7.9% and currently ranks third in UV OLEDs. The lights of these devices are well modulated from UV/deep‐blue to pure‐blue with EQEs greater than 5% by regulating the locally excited (LE) and charge‐transfer (CT) components. Experimental and theoretical investigations indicate that harvesting triplet excitons afford high electroluminescence efficiencies via HLCT excited states in the devices. This study provides an efficient strategy to achieve high‐performance UV and blue OLEDs and offers great flexibility for material design.