A mechanistic study of exciplex formation and efficient red light-emitting devices based on rare earth complexes

The mechanisms of exciplex formation between hole-transporting material N,N^′-diphenyl-N,N^′-bis(3-methylphenyl)-1,1^′-diphenyl-4,4^′-diamine (TPD) and electron-transporting materials tris(dibenzoylmethanato)-mono(bathophenanthroline)-rare earth (RE(DBM)₃bath) in TPD/RE(DBM)₃bath bilayer electroluminescence (EL) devices were studied. The formation process was identified by using fluorescent dye as dopant. It was found that interaction between the excited states of RE(DBM)₃bath and the ground state of TPD molecules resulted in the exciplex. The recombination zone of the TPD/RE(DBM)₃bath device was proved to be mainly in the RE(DBM)₃bath layer near the organic interface. On the other hand, by using dopant as efficient energy acceptor in RE-complex hosts, we found that exciplex emission was quenched thoroughly and efficient red light emission was observed, proving that RE(DBM)₃bath may act as an efficient energy donor in EL devices. In the case of Eu³⁺ as the central ion, maximum EL efficiency and highest brightness of red light emission reached 2.6% and 2000 cd/m², respectively.     

Efficient exciplex emission from intramolecular charge transfer material

New exciplexes formed between a typical intramolecular charge transfer (ICT) material (bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS)) and a series of electron donor and acceptors in donor:acceptor system have been systematically demonstrated. It is found that such ICT materials could form exciplex with both standalone electron donor and acceptor materials with itself as acceptor and donor components, which is based on the presence of both donor and acceptor species in the ICT material. The emission spectra of exciplex OLEDs based on ICT materials could be regularly tuned ranging from blue to yellow color by changing energy level alignment between ICT and standalone donor/acceptor materials. Among these exciplexes, DMAC-DPS:PO-T2T combination offered the highest exciplex EL performance, with its peak external quantum efficiency, luminance and current efficiency of 9.08%, 35,000 cd/m² and 30 cd/A, respectively. On the other hand, we also found that the exciplex efficiency was insensitive with the weight ratio between ICT material and acceptor, which means ‘doping’ of ICT material into the acceptor. Our finding extend the usage and selection scope of the TADF material.     

Novel Strategy to Develop Exciplex Emitters for High‐Performance OLEDs by Employing Thermally Activated Delayed Fluorescence Materials

To develop high‐performance thermally activated delayed fluorescence (TADF) exciplex emitters, a novel strategy of introducing a single‐molecule TADF emitter as one of the constituting materials has been presented. Such a new type of exciplex TADF emitter will have two reverse intersystem crossing (RISC) routes on both the pristine TADF molecules and the exciplex emitters, benefiting the utilization of triplet excitons. Based on a newly designed and synthesized single‐molecule TADF emitter MAC, a highly efficient exciplex emitter MAC:PO‐T2T has been obtained. The device based on MAC:PO‐T2T with a weight ratio of 7:3 exhibits a low turn‐on voltage of 2.4 V, high maximum efficiency of 52.1 cd A^?1 (current efficiency), 45.5 lm W^?1 (power efficiency), and 17.8% (external quantum efficiency, EQE), as well as a high EQE of 12.3% at a luminance of 1000 cd m^?2. The device shows the best performance among reported organic light‐emitting devices based on exciplex emitters. Such high‐efficiency and low‐efficiency roll‐off should be ascribed to the additional reverse intersystem crossing process on the MAC molecules, showing the advantages of the strategy described in this study.     

A host material with a small singlet–triplet exchange energy for phosphorescent organic light-emitting diodes: Guest,host, and exciplex emission

A host material containing a triazine core and three phenylcarbazole arms, called 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was developed for phosphorescent organic light-emitting diodes (OLEDs). Ultra-low driving voltages were achieved by utilizing TCPZ as the host due to its decreased singlet–triplet exchange energy (ΔE_ST) and low-lying lowest unoccupied molecular orbital (LUMO) energy level. Interaction between the RGB triplet emitters and TCPZ were studied in both photoluminescent and electroluminescent processes. Transient photoluminescence (PL) measurement of the co-deposited film of fac-tris(2-phenylpyridine) iridium (Ir(PPy)₃):TCPZ exhibits a shoulder at 565 nm whose lifetime is about two times longer than that of the Ir(PPy)₃ triplet excitons and can be attributed to the triplet exciplex formed between Ir(PPy)₃ and TCPZ. Such exciplex was also found for the green phosphorescent OLED, giving the most efficient phosphorescent OLED with triplet exciplex emission hitherto. Different from the PL process, a broad featureless band with a maximum at 535 nm was found for the OLED based on an EML of iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C²′)picolinate (FIrpic):TCPZ, which can be attributed to the emission from the singlet excited state of TCPZ formed by direct hole-electron recombination. A multi-emitting-layer white OLED was also fabricated by utilizing FIrpic and tris(1-phenylisoquinolinolato-C²,N)iridium(III) (Ir(piq)₃) as the complementary triplet emitters and TCPZ as the host. Different from most of ever reported white OLEDs fabricated with blue/red complementary triplet emitters that exhibit color rendering index (CRI) lower than 70, a high CRI of 82 is achieved due to the combination of blue and red phosphorescence emissions from FIrpic and Ir(piq)₃, and the emerging green fluorescence emission from TCPZ.     

An Exciplex Forming Host for Highly Efficient Blue Organic Light Emitting Diodes with Low Driving Voltage

The exciplex forming co‐host with phosphorescent dopant system has potential to realize highly efficient phosphorescent organic light emitting didoes (PhOLEDs). However, the exciplex forming co‐host for blue phosphorescent OLEDs has been rarely introduced because of higher triplet level of the blue dopant than green and red dopants. In this work, a novel exciplex forming co‐host with high triplet energy level is developed by mixing a phosphine oxide based electron transporting material, PO‐T2T, and a hole transporting material, N,N′‐dicarbazolyl‐3,5‐benzene (mCP). Photo‐physical analysis shows that the exciplexes are formed efficiently in the host and the energy transfer from the exciplex to blue phosphorescent dopant (iridium(III)bis[(4,6‐difluorophenyl)‐pyridinato‐N,C2′]picolinate; FIrpic) is also efficient, enabling the triplet harvest without energy loss. As a result, an unprecedented high performance blue PhOLED with the exciplex forming co‐host is demonstrated, showing a maximum external quantum efficiency (EQE) of 30.3%, a maximum power efficiency of 66 lm W^?1, and low driving voltage of 2.75 at 100 cd m^?2, 3.29 V at 1000 cd m^?2, and 4.65 V at 10 000 cd m^?2, respectively. The importance of the exciton confinement in the exciplex forming co‐host is further investigated which is directly related to the performance of PhOLEDs.     

Simplified and high-efficiency warm/cold phosphorescent white organic light-emitting diodes based on interfacial exciplex co-host

Possessing the reverse intersystem crossing (RISC) process, exciplex system has vast potential to enhance the efficiency of the white organic light-emitting diodes (WOLEDs). Nevertheless, general structures of the emitting layer always employ triple-doping in a long range (20–30 nm) which is complicated on fabrication progress. In this paper, based on the interfacial exciplex co-host, a flexible and simplified structure design is proposed to realize both warm and cold phosphorescent WOLEDs. In the two devices, with strategically locating the ultrathin orange phosphorescent emitting layers at two sides of the blue phosphorescent emitting layer (2 nm), respectively, multiple energy transfer channels are created to carry out highly efficient exciton utilization. Owing to the different energy transfer mechanisms, different organic emission ratios are obtained in two WOLEDs. The cold WOLEDs exhibited superior maximum external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) of 28.37%, 72.17 cd A⁻¹ and 87.17 lm W⁻¹, respectively. Also, the warm WOLEDs showed high values as EQE of 23.80%, CE of 67.70 cd A⁻¹ and PE of 81.10 lm W⁻¹. Furthermore, both the devices presented rather stable color output in the luminance range from 2000 cd m⁻² to 10000 cd m.⁻²     

Thermally activated delayed-fluorescence organic light-emitting diodes based on exciplex emitter with high efficiency and low roll-off

Highly efficient thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) based on exciplex are demonstrated in a blended system with commercially available 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC) and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T). By well adjusting the ratio between these two materials, the optimized device shows a low turn-on voltage of 2.4 V and a high external quantum efficiency (EQE) of 11.6%. More importantly, the device retains an EQE of 9.4% even at a high luminescence of 1000 cd/m². The low efficiency roll-off is attributed to the small singlet-triplet splitting and the short of the delayed fluorescence lifetime. Both EQE and efficiency roll-off are ones of the best performance among the reported TADF OLEDs based on exciplex.     

White Organic LED with a Luminous Efficacy Exceeding 100 lm W−1 without Light Out‐Coupling Enhancement Techniques

Luminous efficacy (LE), which is given by the ratio of luminous flux to power, is commonly used to measure the power consumption of a light source. Unfortunately, the LE of white organic light‐emitting diodes (OLEDs) still lags behind those of inorganic LED for practically used (>100 lm W^?1). In this paper, an ultraefficient white OLED is discussed based on a newly designed thermally activated delayed fluorescent exciplex host. The resulting white OLED delivers an unusually high forward‐viewing LE of 105.0 lm W^?1 and external quantum efficiency (EQE) η_ext of ≈30% (without using any optical out‐coupling techniques). As far as it is known, specifically, these efficiencies are the highest values among the published white OLEDs to date. Two‐color warm white emission is realized with Commission International de I'Eclairage coordinates of (0.40, 0.48) at a brightness of 1000 cd m^?2. Furthermore, the well‐matched energy alignment endows the device with an extremely low turn‐on voltage (≈2.5 V). Such high efficiencies and excellent device performance should benefit from the advantages of exciplex material solely used as the host. Therefore, this study anticipates that the findings have great potential to boost the LE of OLEDs, and more importantly, fulfill the power efficacy requirement for lighting applications.     

Unraveling Temperature‐Dependent Contact Electrification between Sliding‐Mode Triboelectric Pairs

The underlying mechanism on contact electrification (CE) has remained a topic of debate over centuries, and it is argued to be due to electron transfer, ion transfer, and/or even material species transfer. Recently, a previous study shows that CE is dominated by electrons, at least for solid–solid cases. Herein, by using a model detailing the charge transfer between triboelectric surfaces and thermionic emission of electrons via employing a sliding mode Ti–SiO₂ triboelectric nanogenerator (TENG), surface charge decay behavior is scrutinized in lateral‐sliding mode during operation at high temperature. The temperature dependence of TENG electric output contributes to characteristic metal–dielectric and dielectric–dielectric CEs, thereby providing further evidence that electrons are the dominating transferred charges in CE. The total surface charge output of the TENG is rationalized as a direct consequence of the coupling of the rate of electron thermionic emission, the charge transfer rate of CE, and the changing rate of the contacted area between the two materials. When the contacting area is larger than the displaced area, the CE between the two materials is the major contributor to measured surface charge. Conversely, the thermionic emission of the exposed surfaces dictates when the contacting area is smaller.     

Measurement of Charge‐Density Dependence of Carrier Mobility in an Organic Semiconductor Blend

Here, a new methodology for analyzing the charge‐density dependence of carrier mobility in organic semiconductors, applicable to the low‐charge‐density regime (10¹⁴–10¹⁷ cm^?3) corresponding to the operation conditions of many organic optoelectronic devices, is reported. For the P3HT/PCBM blend photovoltaic devices studied herein, the hole mobility µ is found to depend on charge density n according to a power law µ(n) ∝ n^δ, where δ = 0.35. This dependence is shown to be consistent with an energetic disorder model based upon an exponential tail of localized intra‐band states.     

Many Exciplex Systems Exhibit Organic Long‐Persistent Luminescence

Organic long‐persistent luminescence (OLPL) is a long‐lasting luminescence from a photogenerated intermediated state, such as a charge separated state. Here, it is shown that many exciplex systems exhibit OLPL and that emission pathways of OLPL can be controlled by the relationship among local excited states and charge‐transfer excited states of materials.     

Color Tuning of Avobenzone Boron Difluoride as an Emitter to Achieve Full‐Color Emission

Organic light‐emitting diodes (OLEDs) displaying a wide range of emission colors with emission peaks from 450 to 665 nm using a single emitting material, avobenzone boron difluoride (AVB‐BF₂), are reported. Color tuning is achieved by controlling the aggregation of AVB‐BF₂ and the formation of a “triadic” exciplex of an AVB‐BF₂ dimer and a host molecule. Various electroluminescent devices containing AVB‐BF₂ cover the whole visible light spectrum and a white‐emitting device with CIE coordinates of (0.35, 0.37) is obtained with a single emitting material in a single emissive layer. Furthermore, an exceptionally high external quantum efficiency of nearly 13% is achieved for a green‐emitting OLED because AVB‐BF₂ exhibits thermally activated delayed fluorescence by forming the exciplex.     

Controllable Modulation of Efficient Phosphorescence Through Dynamic Metal-Ligand Coordination for Reversible Anti-Counterfeiting Printing of Thermal Development

Dynamically tunable room-temperature phosphorescence (RTP) organic materials have attracted considerable attention in recent years due to their great potential over a wide variety of advanced applications. However, the precise regulation of the intersystem crossing (ISC) process for efficient RTP materials with dynamically modulated properties in a rigid environment is challenging. Herein, an effective strategy for RTP material preparation with controllably regulated properties is developed via the construction of dynamic metal-ligand coordination in a host-guest doped system. The coordination interaction promotes ISC and phosphorescence emission of the guest, thus allowing the modulation of the photophysical properties of doped materials by changing the doping ratio and Zn²⁺ counterions. By taking advantage of the reversible metal-ligand coordination interaction, the coordination-activated, and dissociation-deactivated RTP is dynamically manipulated. With the unique Zn²⁺-responsible RTP enhancement materials, the anti-counterfeiting applications of thermal development, and color inversion have been constructed for inkjet printing of high-resolution patterns with high reversibility for many write/erase cycles. The results show that dynamic metal-ligand coordination strategy is a promising approach for achieving efficient RTP materials with controllably modulated properties.     

Aggregation induced intermolecular charge transfer in simple nonconjugated donor–acceptor system

The exploration of exciplex for organic light-emitting diodes (OLEDs) has been fleetly developed. However, many of them confront with the problems like phase separation and poor solubility, hampering their utilization in solution process. Hence, a series of soluble exciplex luminophores with the simple architecture of D-spacer-A (mCP-6C-TRZ, phCz-6C-TRZ and 2phCz-6C-TRZ) are synthesized and characterized, in which, the alkyl chain as ample spacer breaks the molecular backbone conjugation, induces intermolecular charge transfer process instead of intramolecular charge transfer in solid state. These materials are endowed with narrowed singlet−triplet splitting energy (ΔE_ST), efficient reverse intersystem crossing (RISC) process, and distinct thermally activated delayed fluorescence (TADF) characteristics. In view of their high triplet energy level (E_T) and bipolar carrier transport ability, where efficient exciplexes are applied as the host, the solution-processed phosphorescence devices realize a low efficiency roll-off of 7.0% at 1000 cd m⁻², high luminance, current efficiency (CE) and external quantum efficiency (EQE) of 25,990 cd m⁻², 20.0 cd A⁻¹ and 6.7%, respectively. These results offer a promising tactic to the establishment of exciplex with TADF feature as host for fabricating efficient solution processed OLEDs.     

Color-Stable,Efficient, and Bright Blue Light-Emitting Electrochemical Cell Using Ionic Exciplex Host

The lack of high-performance blue light-emitting electrochemical cells (LECs) has remained a formidable challenge for fabricating white LECs for lighting applications. Here, a ionic exciplex host is used for color-stable, efficient, and bright blue LECs by taking advantage of its facilitated carrier injection, bipolar charge-transport, and efficient energy transfer to the guest dopant. A cationic donor molecule, 1-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-3-methyl-1H-imidazol-3-ium hexafluorophosphate (tbuCAZ-ImMePF₆), and a cationic acceptor molecule, 1-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (TRZ-ImEtPF₆), are developed to form the ionic exciplex host. The mixed film of tbuCAZ-ImMePF₆ and TRZ-ImEtPF₆ affords blue exciplex with fast reverse intersystem crossing and thermally activated delayed fluorescence. For the film doped with a blue-emitting iridium complex, energy is efficiently transferred from the exciplex to the complex. Host-guest LECs using the doped film as the active layer show stable blue emission color and high current efficiencies of up to 25.8 cd A⁻¹. More importantly, they attain simultaneously high efficiency and high brightness (14.1/17.4/16.8 cd A⁻¹ at 705/872/1680 cd m⁻²), which are the most efficient and bright host-guest blue LECs reported so far. The primary host-guest LEC also exhibits promising operational stability. The work reveals that the use of an ionic exciplex host is a promising avenue toward high-performance blue LECs.     

Highly Efficient TADF OLEDs: How the Emitter–Host Interaction Controls Both the Excited State Species and Electrical Properties of the Devices to Achieve Near 100% Triplet Harvesting and High Efficiency

New emitters that can harvest both singlet and triplet excited states to give 100% internal conversion of charge into light, are required to replace Ir based phosphors in organic light emitting diodes (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence (TADF). Here, it is shown that a D–A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on a previously studied D–A–D molecule in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices, it is unambiguously established that the triplet states are harvested via TADF, but more interestingly, these results are found to be independent of whether the emitter is the ICT state or the D–A–D/host exciplex.     

High‐Performance Dibenzoheteraborin‐Based Thermally Activated Delayed Fluorescence Emitters: Molecular Architectonics for Concurrently Achieving Narrowband Emission and Efficient Triplet–Singlet Spin Conversion

Thermally activated delayed fluorescence (TADF) materials, which enable the full harvesting of singlet and triplet excited states for light emission, are expected as the third‐generation emitters for organic light‐emitting diodes (OLEDs), superseding the conventional fluorescence and phosphorescence materials. High photoluminescence quantum yield (Φ_PL), narrow‐band emission (or high color purity), and short delayed fluorescence lifetime are all strongly desired for practical applications. However, to date, no rational design strategy of TADF emitters is established to fulfill these requirements. Here, an epoch‐making design strategy is proposed for producing high‐performance TADF emitters that concurrently exhibiting high Φ_PL values close to 100%, narrow emission bandwidths, and short emission lifetimes of ≈1 µs, with a fast reverse intersystem crossing rate of over 10⁶ s^?1. A new family of TADF emitters based on dibenzoheteraborins is introduced, which enable both doped and non‐doped TADF‐OLEDs to achieve markedly high external electroluminescence quantum efficiencies, exceeding 20%, and negligible efficiency roll‐offs at a practical high luminance. Systematic photophysical and theoretical investigations and device evaluations for these dibenzoheteraborin‐based TADF emitters are reported here.     

Unique P?Co?N Surface Bonding States Constructed on g‐C3N4 Nanosheets for Drastically Enhanced Photocatalytic Activity of H2 Evolution

Developing high‐efficiency and low‐cost photocatalysts by avoiding expensive noble metals, yet remarkably improving H₂ evolution performance, is a great challenge. Noble‐metal‐free catalysts containing Co(Fe)? N? C moieties have been widely reported in recent years for electrochemical oxygen reduction reaction and have also gained noticeable interest for organic transformation. However, to date, no prior studies are available in the literature about the activity of N‐coordinated metal centers for photocatalytic H₂ evolution. Herein, a new photocatalyst containing g‐C₃N₄ decorated with CoP nanodots constructed from low‐cost precursors is reported. It is for the first time revealed that the unique P(δ^?)? Co(δ⁺)? N(δ^?) surface bonding states lead to much superior H₂ evolution activity (96.2 µmol h^?1) compared to noble metal (Pt)‐decorated g‐C₃N₄ photocatalyst (32.3 µmol h^?1). The quantum efficiency of 12.4% at 420 nm is also much higher than the record values (≈2%) of other transition metal cocatalysts‐loaded g‐C₃N₄. It is believed that this work marks an important step toward developing high‐performance and low‐cost photocatalytic materials for H₂ evolution.     

Unique PCoN Surface Bonding States Constructed on g‐C3N4 Nanosheets for Drastically Enhanced Photocatalytic Activity of H2 Evolution

Developing high‐efficiency and low‐cost photocatalysts by avoiding expensive noble metals, yet remarkably improving H₂ evolution performance, is a great challenge. Noble‐metal‐free catalysts containing Co(Fe)?N?C moieties have been widely reported in recent years for electrochemical oxygen reduction reaction and have also gained noticeable interest for organic transformation. However, to date, no prior studies are available in the literature about the activity of N‐coordinated metal centers for photocatalytic H₂ evolution. Herein, a new photocatalyst containing g‐C₃N₄ decorated with CoP nanodots constructed from low‐cost precursors is reported. It is for the first time revealed that the unique P(δ^?)?Co(δ⁺)?N(δ^?) surface bonding states lead to much superior H₂ evolution activity (96.2 µmol h^?1) compared to noble metal (Pt)‐decorated g‐C₃N₄ photocatalyst (32.3 µmol h^?1). The quantum efficiency of 12.4% at 420 nm is also much higher than the record values (≈2%) of other transition metal cocatalysts‐loaded g‐C₃N₄. It is believed that this work marks an important step toward developing high‐performance and low‐cost photocatalytic materials for H₂ evolution.     

Organic charge-transfer interface enhanced graphene hybrid phototransistors

Benefited from the advantages of light sensitivity of organic materials and high carrier mobility of graphene, organic semiconductor/graphene hybrid phototransistors exhibit ultrahigh photoconductive gain and photoresponsivity, attributed to the carrier multiplication effect in the organic layer. However, with single absorbing layer, the built-in electrical field between organic layer and graphene is insufficient, and spectral response range is limited. In this work, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) organic tandem structure with high crystal quality fabricated via van der Waals vapor phase growth is adopted as the light absorbing layer, where suppressed carrier trap states density along with the additional charge-transfer interface between the organic layers result in overall improvement of device performance. Prominent photo-responses are observed with photoresponsivity up to 5.76 × 10⁵A/W and photoconductive gain up to 1.38 × 10⁹. Further more, complementary light absorption spectrum of these two organic materials fulfills effective photodetecting at ultraviolet and visible range.