Solution-processed Organic Light-emitting Devices Research Articles

Red hyperbranched conjugated polymers with thermally activated delayed fluorescence for solution-processed organic light-emitting diodes

Wet-processable red luminescent materials with thermally activated delayed fluorescence (TADF) are essential in the field of printed electronics. Particularly, hyperbranched conjugated polymers with the advantages of large steric hindrance, effective energy transfer and good solubility had exhibited great potential application to prepare the high-performance solution-processed organic light-emitting devices, but it is still a great challenge. In this work, a novel kind of red hyperbranched conjugated polymers PFCz-(BTZ-DMAC)n (n = 1, 2, 10 and 50) were designed and synthesized using D-A-D-typed BTZ-DMAC as the red TADF cores and poly(fluorene-carbazole) as the backbone. The polymers owned more effective energy transfer process and reverse intersystem crossing that could simultaneously and effectively utilize the singlet and triplet excitons. The polymers also exhibited good thermal stabilities and film forming properties. At the same time electroluminescent emission (about 650 nm) prominently enhanced and had an obvious red-shift with the increasing concentration of red cores. In the non-doped devices, PFCz-(BTZ-DMAC)50 with 50 mol‰ content of red TADF cores showed the best electroluminescent performance that a red chromaticity coordinate of (0.59, 0.39) and a maximum external quantum efficiency of 4.88 % were obtained. More importantly, it exhibited the low roll-off (7 % decrease at 500 cd/m−2) owing to the large steric hindrance. All results revealed that red TADF hyperbranched conjugated polymers had potential application for preparing the high-quality solution-processed organic light-emitting devices.

Influence of steric configurations in the conjugated polymers using acridine-triazine derivates as green chromophores

High-quality polymers with the advantages of chemical modifiability, good morphological stability, and excellent solubility have a wide application in the field of solution-processed organic light-emitting devices. However, the influence of steric configurations in the conjugated polymers has not been explored in detail. Herein, we designed and synthesized two kinds of green conjugated polymers with hyperbranched structure (HGx, x = 1, 5, 8, 10) and linear structure (LG10), respectively, using 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)−9,9-dimethyl-9,10-dihydroacridine (Ac-TRZ) as the chromophores, poly(fluorene-carbazole) as the backbone. Particularly, triphenylamine groups were used as the hyperbranched cores. Owing to the large steric hindrance and multi-arm structure, hyperbranched polymers exhibited higher thermal stability (Tg＞110 ℃) and an effective energy transfer process. Regrettably, both kinds of conjugated polymers had low PLQYs and nanosecond-scale excitons lifetime because the 3LE energy levels induced by the backbone were lower than the 3CT energy levels induced by the green units. By calculation, it is found that fluorescent segments connected to the green units had directly changed the electronic structure. Moreover, the films of linear polymers possessed considerable surface roughness. From the above results, it is confirmed that the rational design of steric conformations in the conjugated polymers is very important for perfecting the thermal stability, photophysical properties, and film morphologies.

Origins of the improved device performance in solution-processed TADF organic light-emitting devices with a polymer underneath layer

In solution-processed organic light-emitting devices (OLEDs), a polymer interface layer underneath the emission layer is essential to improving the device's performance. The typical polymer is Poly (9-vinylcarbazole) (PVK), which can dramatically increase the device efficiency whereas the mechanisms are not well understood. In this work, we explored the related mechanisms by smart experimental design and measurements. The results reveal that in addition to reducing the hole current density and balancing the carrier distribution, the PVK underneath layer also improves the film morphology and enhances the horizontal orientation of the transition dipole moment of the thermally activated delayed fluorescence (TADF) emitters. Consequently, the device's efficiency and stability are notably increased by about three times. This work provides insights into the origins of improved device performance in solution-processed TADF OLEDs with a polymer underneath layer and also highlights the effect of the underneath layer on the film formation of the solution-processed emission layer.

Luminescent Bimetallic Two-Coordinate Gold(I) Complexes Utilizing Janus Carbenes.

A series of bimetallic carbene-metal-amide (cMa) complexes have been prepared with bridging biscarbene ligands to serve as a model for the design of luminescent materials with large oscillator strengths and small energy differences between the singlet and triplet states (ΔEST). The complexes have a general structure (R2N)Au(:carbene─carbene:)Au(NR2). The bimetallic complexes show solvation-dependent absorption and emission that is analyzed in detail. It is found that the molar absorptivity of the bimetallic complexes is correlated with the energy barrier to rotation of the metal-ligand bond. The bimetallic cMa complexes also exhibit short emission lifetimes (τ = 200-300 ns) with high photoluminescence efficiencies (ΦPL > 95%). The radiative rates of bimetallic cMa complexes are 3-4 times faster than that of the corresponding monometallic complexes. Analysis of temperature-dependent luminescence data indicates that the lifetime for the singlet state (τS1) of bimetallic cMa complexes is near 12 ns with a ΔEST of 40-50 meV. The presented compounds provide a general design for cMa complexes to achieve small values for ΔEST while retaining high radiative rates. Solution-processed organic light-emitting devices (OLEDs) made using two of the complexes as luminescent dopants show high efficiency and low roll-off at high luminance.

Spiro[fluorene-9,9ʹ-xanthene] based deep-blue emitter and the hidden electroplex for solution-processed non-doped white organic light-emitting devices

A deep-blue fluorescent emitter with an A−D−A-skeleton, named pTRASX, constructed from two diphenyltriazine acceptors and one spiro[fluorene-9,9ʹ-xanthene] (SFX) donor, was deliberately designed and synthesized. The blocking effect of O-atom in xanthene moiety of SFX and the rigid A−D−A-skeleton endow a deep-blue emission at 410 nm with a photoluminescence quantum yield of 43% in solid film. The solution-processed non-doped white organic light-emitting diode (OLED) was demonstrated by merging the emission of pTRASX and its electroplex. Based on the intermolecular through-space charge-transfer, the electroplex mechanism resulted in the non-doped white OLED with the wide spectral range from 400 to 800 nm, the color coordinate of (0.33, 0.27), the high color rendering index of 85, and the external quantum efficiency of 0.49%. The electro- and photo-luminance measurements clarify that the electroplex emission originates from the intermolecular interaction between pTRASX and the adjacent electron transporting layer, ranging from ca. 540 to 800 nm. This study illustrates that the rational molecular design cooperated with the innovative electroluminescent mechanism blazes a trail for simple solution-processed and metal-free blue and white OLED.

Tetradentate C∧C∧N∧N Ligand-Containing Gold(III) Complexes with Orange to Deep-Red Thermally Activated Delayed Fluorescence (TADF) and Their Application in Organic Light-Emitting Devices

A new class of thermally activated delayed fluorescence (TADF) tetradentate C∧C∧N∧N ligand-containing gold(III) complexes containing acridinyl moieties has been designed and synthesized. These complexes exhibit orange-red to deep-red emission with photoluminescence quantum yields (PLQYs) of up to 0.76 in solid-state thin films. Short excited-state lifetimes of ≤2.0 μs and large radiative decay rate constants (kr) in the order of 105 s-1 have also been found in the complexes. High-performance solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) based on these complexes have been fabricated, demonstrating high maximum external quantum efficiencies (EQEs) of 12.2 and 12.7%, respectively, which are among the best values ever reported for red-emitting gold(III)-based OLEDs. In addition, satisfactory operational half-lifetime (LT50) values of up to 34,058 h have been attained in these red-emitting devices. It is found that the operational stability is strongly dependent on the choice of functional groups on the acridinyl moieties, of which the incorporation of -O- and -S- linkers can effectively prolong the LT50 value by an order of magnitude. The TADF properties of the complexes are substantiated by the hypsochromic shift in emission energies and the remarkable enhancement in the emission intensity upon increasing temperature. The TADF properties have also been supported by temperature-dependent ultrafast transient absorption studies, with the direct observation of reverse intersystem crossing (RISC) and the determination of the activation parameters for the very first time, together with their excited-state dynamics.

Highly Luminescent Aluminum Complex with β-Diketone Ligands Exhibiting Near-Unity Photoluminescence Quantum Yield, Thermally Activated Delayed Fluorescence, and Rapid Radiative Decay Rate Properties in Solution-Processed Organic Light-Emitting Devices

Abstract A highly luminescent mononuclear Al complex with β-diketone ligands exhibiting superior thermally activated delayed fluorescence (TADF) properties was developed in this work. The complex showed yellow emission with promising photofunctions, including a near-unity photoluminescence quantum yield (PLQY), a rapid radiative decay rate, and a short delayed fluorescence lifetime in the solid state. When applied in a solution-processed organic light-emitting device, an external quantum efficiency (EQE) exceeding 18% and a low turn-on voltage of 2.9 V at 1 cd/m2 were obtained, surpassing those of the corresponding β-diketone ligand. Metal complexation with Al generated unique electronic structures that significantly strengthened the photofunctions of the original β-diketone ligand in the solid state.

Methoxy-substituted carbazole-based polymers obtained by RAFT polymerization for solution-processable organic light-emitting devices

New electroactive vinyl monomers, i.e., 3,6-dimethoxy-9-(4-vinylphenyl)-9H-carbazole and 3,6-dimethoxy-9-(3-vinylphenyl)-9H-carbazole were prepared by palladium catalyzed reactions of dimethoxy-9H-carbazole with 1-bromo-4-vinylbenzene or 1-bromo-3-vinylbenzene, respectively. Living radical polymerization of these monomers was carried out using 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid as a RAFT agent affording polymers with controlled molecular weight (up to Mn = 20,000) and low polydispersity (Đ<1.5). The obtained polymers exhibited delayed fluorescence caused by relaxation of excimer states. The lifetimes of long-lived components up to 2.6 μs were obtained for the excimer emission by multi-exponential fitting of photoluminescence decay curves. The polymers were tested in solution-processed OLEDs. Green thermally-activated-delayed-fluorescence OLEDs with polymers as hole transporting materials demonstrated lower turn-on voltage and enhanced charge balance in emission layer compared to device fabricated using the commercial hole-transporting material. Additionally, the phosphorescent OLEDs based on red phosphor as dopant and the synthesized polymer as a host materials were fabricated and characterized. The best device showed external quantum efficiency of 10.6%.

Orange-red and saturated red thermally activated delayed fluorescent dendrimers for non-doped solution-processed OLEDs

Two thermally activated delayed fluorescence (TADF) dendritic emitters with orange-red and saturated red emission, respectively, were obtained by coupled diphenylamine-carbazole dendron groups as donor with di(pyridin-3-yl)methanone as accepter. The two TADF dendrimers of tBuDPACz-DPyM and MeODPACz-DPyM exhibited small energy gaps between the singlet and triplet excited states (0.04 vs 0.03 eV). As a consequence, the tBuDPACz-DPyM non-doped solution-processed organic light-emitting devices (OLED) exhibited high current efficiency of 2.36 cd A−1 and external quantum efficiency of 1.82% with low turn-on voltage 3.4 V as well as a lower efficiency roll-off. Moreover, Commission Internationale de L'Éclairage (CIE) coordinates of two OLED devices were (0.56, 0.43) and (0.62, 0.37) for tBuDPACz-DPyM and MeODPACz-DPyM respectively.

Recent advances in solution-processed organic and perovskite nanocrystal light-emitting devices

Recent advances in solution-processed organic and perovskite nanocrystal light-emitting devices

Preface

The 2022 International Conference on Energy Utilization and Automation (ICEUA 2022) was held on February 25th to 27th, 2022 as a virtual conference due to the growing concerns over the coronavirus outbreak (COVID-19), and in order to protect the well-being of our attendees, partners, and staff as our number one priority. The meeting arounds the network communication and recent research achievements in the field of information security, from institutions of higher learning, scientific research institutes, enterprises and institutions at home and abroad experts, professors, scholars, engineers and so on to provide a share professional experience, expand the professional network, face to face to new ideas and show the research results of the international platform, Exploring key challenges and research directions in this field, with a view to promoting the development and application of theories and technologies in this field in universities and enterprises, as well as establishing business or research connections for attendees and seeking global partners for future ventures.This scientific event brings together more than 100 national and international researchers in Energy Utilization and Automation. On top of the local participants coming from different national universities, international participants are also registered from different countries, namely Malaysia, India, Ethiopia, Australia. During the conference, the conference model was divided into three sessions, including oral presentations, keynote speeches, and online Q&A discussion. In the first part, some scholars, whose submissions were selected as the excellent papers, were given about 5-10 minutes to perform their oral presentations one by one. Then in the second part, keynote speakers were each allocated 30-45 minutes to hold their speeches.In the second part, we invited four professors as our keynote speakers. Assoc.Prof. Guohua Xie, Wuhan University, performed a speech: Solution-Processed Organic Light-Emitting Devices. And then we had Assoc.Prof. Muhyiddine Jradi, Center for Energy Informatics, University of Southern Denmark. He presented an insightful speech: Auditing and Smartness Evaluation of Building Automation and Control Systems. Prof. Rajan Jose, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, delivered a speech: Energy for 21st Century and Beyond. Prof. Thanikaivelan Palanisamy, our finale keynote speaker, from CSIR-Central Leather Research Institute. He presented an insightful speech: Advanced Sustainable Materials from Leather Wastes for Energy and Environmental Applications. Their insightful speeches had triggered heated discussion in the third session of the conference. Every participant praised this conference for disseminating useful and insightful knowledge.List of Committee member are available in this pdf.

Electroluminescent Aggregation-Induced Emission-Active Discotic Liquid Crystals Based on Alkoxy Cyanostilbene-Functionalized Benzenetricarboxamide with Ambipolar Charge Transport

There is significant demand for molecular functional materials with tailored light-emissive and charge transport properties for their utilization in organic optoelectronic devices. Motivated by such promising properties, we present the synthetic design and emissive and semiconducting properties of aggregation-induced emission (AIE)-active columnar discotic liquid crystals based on cyanostilbene-modified benzenetricarboxamide derivatives (1a, 1b, and 1c). Enantiotropic mesomorphic behavior over a wide temperature range, including room temperature, with columnar hexagonal self-assembly was observed for all of the compounds. The space-charge limited current (SCLC) technique revealed the ambipolar charge transport for reported materials with balanced electron and hole transport of the order of 10–3 cm2/(V s). On the other hand, all of the compounds 1a–c were tested as emitter materials in solution-processed organic light-emitting devices at different concentrations with several hosts, viz., poly(vinylcarbazole) (PVK), 1,3-bis(N-carbazolyl)benzene (mCP), bis[3,5-di(9H-carbazol-9-yl)phenyl]diphenylsilane (SimCP2), and (carbazolyl)-1,10-biphenyl (CBP). The maximum luminance of 1255 cd/m2 corresponding to sky-blue emission was observed for compound 1a at 3.0 wt % with the CBP host.

Incorporation of Fluorene and Its Heterocyclic Spiro Derivatives To Realize High-Performance and Stable Sky-Blue-Emitting Arylgold(III) Complexes.

A series of arylgold(III) complexes of tridentate diphenylpyridine ligand incorporated with fluorene and its heterocyclic spiro derivatives, spiro[fluorene-9,9'-xanthene] and spiro[acridine-9,9'-fluorene], as auxiliary ligands has been prepared. This class of complexes exhibits high decomposition temperatures of up to 387 °C, excellent film morphologies in solid-state thin films with a root-mean-square roughness smaller than 0.20 nm, as well as high photoluminescence quantum yields of up to 0.72 in solid-state thin films. Solution-processed organic light-emitting devices (OLEDs) fabricated from this series of complexes as dopants show intense electroluminescence in the sky-blue region with maximum external quantum efficiencies of 10.0%. Taking advantage of their high thermal stability, vacuum-deposited OLEDs have also been fabricated and satisfactory operational lifetimes of ∼300 h have been recorded.

Synthesis of luminescent phosphine-containing rigid-rod dinuclear alkynylgold(I) complexes and their X-Ray structural, photophysical, self-assembly and electroluminescence studies

A series of rigid-rod dinuclear alkynylgold(I) complexes bearing triethylene glycol monomethyl ether or dibutoxy-substituted oligo(p-phenylene ethynylene)s moieties as the backbone, and phosphine ligands that are charge-neutral or bear a positive or negative charge, has been designed and synthesized. The alkynylgold(I) complexes with charge-neutral phosphines are found to show polymeric zig-zag chain structure with short gold‧‧‧gold distances in their X-ray crystal structures, while those with negatively charged phosphine ligands demonstrate spherical aggregates in dimethyl sulfoxide (DMSO)-water dispersions. The aggregation behaviour of the complex with positively charged phosphine ligand is found to follow an isodesmic mechanism upon increasing the water content in DMSO. Representative complexes have been applied as the emitting material in solution-processed organic light-emitting devices, exhibiting sky-blue emission with emission maxima at 432 nm. This work has provided further insights into the aggregation behaviour of this class of alkynylgold(I) complexes, as well as the understanding of their molecular packing and alignment for the future design of structurally-related rigid-rod organometallic materials.

Triphenylamine-based trinuclear Pt(II) complexes for solution-processed OLEDs displaying efficient pure yellow and red emissions

Highly efficient phosphors are critical in solution-processed organic light-emitting devices (OLEDs). Multinuclear Ir(III) complexes containing more than one metal center have showed great potential in fabricating high performance OLEDs, yet the electroluminescent (EL) properties of multinuclear Pt(II) complexes are rarely studied. In this work, two neutral trinuclear Pt(II) complexes are synthesized based on the triphenylamine core bearing three bidentate ligand arms. Both the yellow emitter (PyTPt) and deep-red emitter (IqTPt) exhibit improved photoluminescent quantum yields (PLQYs) compared with their corresponding mononuclear Pt(II) complexes. Furthermore, the PLQYs of PyTPt and IqTPt doped films are increased to 0.63 and 0.47, respectively. The solution-processed pure yellow-emitting device based on PyTPt achieves impressively high external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 16.92%, 56.74 cd/A and 29.09 lm W−1, respectively, which are among the best performance reported for the OLEDs employing multinuclear Pt(II) complexes. The solution-processed device based on IqTPt shows pure red emission with the peak EQE approaching 9.0%. Both PyTPt and IqTPt display much higher EL efficiencies than their corresponding mononuclear Pt(II) complexes. This work demonstrates that it is an attritive strategy to develop multinuclear Pt(II) complexes for high-performance OLEDs.

Novel Series of Mononuclear Aluminum Complexes for High-Performance Solution-Processed Organic Light-Emitting Devices.

Light metal complexes, such as lithium (Li), sodium (Na), magnesium (Mg), and aluminum (Al) complexes, are attractive candidates for the fabrication of thermally activated delayed fluorescent (TADF) materials. Nevertheless, mononuclear Al complexes with delayed fluorescence have not been developed so far. In this study, we successfully developed a novel series of highly luminescent Al complexes with two phenylacridine-modified asymmetric acetylacetonate-type ligands. These complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 79 % in the solid state with a short delayed fluorescence lifetime of approximately 4 μs. Solution-processed organic light-emitting devices (OLEDs) using these Al complexes exhibit excellent performance with an external quantum efficiency of 17.5 % at 100 cd m-2 . This is the best performance in light metal-based TADF OLEDs reported so far. The results are expected to guide the advancement of the next-generation solid-state lighting technology.

Strategy to improve the efficiency of solution-processed phosphorescent organic light-emitting devices by modified TADF host with tert-butyl carbazole

A new bipolar host material t3Cz-SO was designed and synthesized by introducing tert-butyl carbazole unit to the peripheral of parent TADF units. The calculated triplet energy and ΔEST of t3Cz-SO are 2.85 and 0.11 eV, respectively. Moreover, t3Cz-SO exhibited excellent film-forming ability and morphological stability, which are suitable for the fabrication of solution-processed OLEDs. The FIrpic-based device host with t3Cz-SO achieved low turn-on voltage of 3.5 V and high power efficiency of 16.5 lm W−1, which is almost ten times higher than the unmodified parent TADF molecule. This study shows that the introduction of tert-butyl carbazole unit to the periphery of small molecular TADF emitter is an effective method for the development of efficient electroluminescent materials.

Highly efficient carbazolylgold(iii) dendrimers based on thermally activated delayed fluorescence and their application in solution-processed organic light-emitting devices.

A new class of C^C^N ligand-containing carbazolylgold(iii) dendrimers has been designed and synthesized. High photoluminescence quantum yields of up to 82% in solid-state thin films and large radiative decay rate constants in the order of 105 s−1 are observed. These gold(iii) dendrimers are found to exhibit thermally activated delayed fluorescence (TADF), as supported by variable-temperature emission spectroscopy, time-resolved photoluminescence decay and computational studies. Solution-processed organic light-emitting diodes (OLEDs) based on these gold(iii) dendrimers have been fabricated, which exhibit a maximum current efficiency of 52.6 cd A−1, maximum external quantum efficiency of 15.8% and high power efficiency of 41.3 lm W−1. The operational stability of these OLEDs has also been recorded, with the devices based on zero- and second-generation dendrimers showing maximum half-lifetimes of 1305 and 322 h at 100 cd m−2, respectively, representing the first demonstration of operationally stable solution-processed OLEDs based on gold(iii) dendrimers.

Molecular design of efficient yellow- to red-emissive alkynylgold(iii) complexes for the realization of thermally activated delayed fluorescence (TADF) and their applications in solution-processed organic light-emitting devices.

Here, we report the design and synthesis of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which show tunable emission colors spanning from the yellow to red region in the solid state and exhibit thermally activated delayed fluorescence (TADF) properties. These complexes display high photoluminescence quantum yields of up to 0.87 and short excited-state lifetimes in sub-microsecond timescales, yielding high radiative decay rate constants on the order of up to 106 s−1. The observation of the drastic enhancement in the emission intensity of the complexes with insignificant change in the excited-state lifetime upon increasing the temperature from 200 to 360 K indicates an increasing radiative decay rate. The experimentally estimated energy splitting between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔES1–T1, is found to be as small as ∼0.03 eV (250 cm−1), comparable to the value of ∼0.05 eV (435 cm−1) obtained from computational studies. The delicate choice of the cyclometalating ligand and the fused heterocyclic ligand is deemed the key to induce TADF through the control of the energy levels of the intraligand and the ligand-to-ligand charge transfer excited states. This work represents the realization of highly emissive yellow- to red-emitting gold(iii) TADF complexes incorporated with fused heterocyclic alkynyl ligands and their applications in organic light-emitting devices.

Mono-, di- and tri-nuclear PtII(C^N)(N-donor ligand)Cl complexes showing aggregation-induced phosphorescent emission (AIPE) behavior for efficient solution-processed organic light-emitting devices

Mono-, di- and tri-nuclear PtII(C^N)(N-donor ligand)Cl complexesPh-PAYPt1,Ph-PAYPt2andPh-PAYPt3are designed and prepared.