Organic Electroluminescent Devices

π-Conjugated Nanohoops: A New Generation of Curved Materials for Organic Electronics

Nanohoops, cyclic association of π-conjugated systems to form a hoop-shaped molecule, have been widely developed in the last 15 years. Beyond the synthetic challenge, the strong interest towards these molecules arises from their radially oriented π-orbitals, which provide singular properties to these fascinating structures. Thanks to their particular cylindrical arrangement, this new generation of curved molecules have been already used in many applications such as host-guest complexation, biosensing, bioimaging, solid-state emission and catalysis. However, their potential in organic electronics has only started to be explored. From the first incorporation as an emitter in a fluorescent organic light emitting diode (OLED), to the recent first incorporation as a host in phosphorescent OLEDs or as charge transporter in organic field-effect transistors and in organic photovoltaics, this field has shown important breakthroughs in recent years. These findings have revealed that curved materials can play a key role in the future and can even be more efficient than their linear counterparts. This can have important repercussions for the future of electronics. Time has now come to overview the different nanohoops used to date in electronic devices in order to stimulate the future molecular designs of functional materials based on these macrocycles.

Modeling the electroluminescence of atomic wires from quantum dynamics simulations

Static and time-dependent quantum-mechanical approaches have been employed in the literature to characterize the physics of light-emitting molecules and nanostructures. However, the electromagnetic emission induced by an input current has remained beyond the realm of molecular simulations. This is the challenge addressed here with the help of an equation of motion for the density matrix coupled to a photon bath based on a Redfield formulation. This equation is evolved within the framework of the driven-Liouville von Neumann approach, which incorporates open boundaries by introducing an applied bias and a circulating current. The dissipated electromagnetic power can be computed in this context from the time derivative of the energy. This scheme is applied in combination with a self-consistent tight-binding Hamiltonian to investigate the effects of bias and molecular size on the electroluminescence of metallic and semiconducting chains. For the latter, a complex interplay between bias and molecular length is observed: there is an optimal number of atoms that maximizes the emitted power at high voltages but not at low ones. This unanticipated behavior can be understood in terms of the band bending produced along the semiconducting chain, a phenomenon that is captured by the self-consistency of the method. A simple analytical model is proposed that explains the main features revealed by the simulations. The methodology, applied here at a self-consistent tight-binding level but extendable to more sophisticated Hamiltonians such as density functional tight binding and time dependent density functional theory, promises to be helpful for quantifying the power and quantum efficiency of nanoscale electroluminescent devices.

Two-Coordinate Thermally Activated Delayed Fluorescence Coinage Metal Complexes: Molecular Design, Photophysical Characters, and Device Application

Since the emergence of the first green light emission from a fluorescent thin-film organic light emitting diode (OLED) in the mid-1980s, a global consumer market for OLED displays has flourished over the past few decades. This growth can primarily be attributed to the development of noble metal phosphorescent emitters that facilitated remarkable gains in electrical conversion efficiency, a broadened color gamut, and vibrant image quality for OLED displays. Despite these achievements, the limited abundance of noble metals in the Earth's crust has spurred ongoing efforts to discover cost-effective electroluminescent materials. One particularly promising avenue is the exploration of thermally activated delayed fluorescence (TADF), a mechanism with the potential to fully harness excitons in OLEDs. Recently, investigations have unveiled TADF in a series of two-coordinate coinage metal (Cu, Ag, and Au) complexes. These organometallic TADF materials exhibit distinctive behavior in comparison to their organic counterparts. They offer benefits such as tunable emissive colors, short TADF emission lifetimes, high luminescent quantum yields, and reasonable stability. Impressively, both vacuum-deposited and solution-processed OLEDs incorporating these materials have achieved outstanding performance. This review encompasses various facets on two-coordinate TADF coinage metal complexes, including molecular design, photophysical characterizations, elucidation of structure-property relationships, and OLED applications.

Regulating Excited States by Varying Different Acceptors of D-π-A Emitters for Efficient Non-Doped Blue Electroluminescence with High Luminance and Low Efficiency Roll-Off

Chromophores with hybridized local and charge-transfer (HLCT) excited state are promising for the realization of high performance blue organic light-emitting diodes (OLEDs). The rational manipulation of HLCT excited state for efficient emitters remains challenging. Herein, we present three donor-π-acceptor (D-π-A) molecules (mPAN, mPANPH, and mPNAPH) with phenanthro[9,10-d]imidazole (PI) and pyridinyl as donor and π-bridge respectively. Changes in various kinds of polycyclic aromatic derivative acceptors (anthracene, 9-phenylanthracene, and 1-phenylnaphthalene) could manipulate the excited states and optoelectronic properties. Theoretical calculations reveal that the S1 state of mPNAPH exhibits HLCT nature while the other two molecules show local excited (LE) state dominated feature. The photophysical properties also demonstrate this characteristic. Therefore, compared with mPAN and mPANPH, mPNAPH has higher photoluminescence quantum yield (PLQY) whether in solutions or neat films. Ultimately, the non-doped devices based on these emitters show high luminance larger than 35000 cd m-2 , and high maximum external quantum efficiencies (EQEmax s) larger than 5 % with low efficiency roll-off. In particular, the mPNAPH-based device displays an excellent performance of pure blue emission at 456 nm with Commission Internationale de L'Eclairage coordinate of (0.15, 0.16) and EQEmax of 6.13 % that benefited from the HLCT state and high-lying reverse intersystem crossing process.

Directly Unveiling the Energy Transfer Dynamics between Alq3 Molecules and Si by Ultrafast Optical Pump-Probe Spectroscopy

The energy transfer (ET) between organic molecules and semiconductors is a crucial mechanism for enhancing the performance of semiconductor-based optoelectronic devices, but it remains undiscovered. Here, ultrafast optical pump-probe spectroscopy was utilized to directly reveal the ET between organic Alq3 molecules and Si semiconductors. Ultrathin SiO2 dielectric layers with a thickness of 3.2-10.8 nm were inserted between Alq3 and Si to prevent charge transfer. By means of the ET from Alq3 to Si, the SiO2 thickness-dependent relaxation dynamics of photoexcited carriers in Si have been unambiguously observed on the transient reflectivity change (ΔR/R) spectra, especially for the relaxation process on a time scale of 200-350 ps. In addition, these findings also agree with the results of our calculation in a model of long-range dipole-dipole interactions, which provides critical information for developing future optoelectronic devices.

Advances in Solution-Processed OLEDs and their Prospects for Use in Displays

This review outlines problems and progress in development of solution-processed organic light-emitting diodes (SOLEDs) in industry and academia. Solution processing has several advantages such as low consumption of materials, low-cost processing, and large-area manufacturing. However, use of a solution process entails complications, such as the need for solvent resistivity and solution-processable materials, and yields SOLEDs that have limited luminous efficiency, severe roll-off characteristics, and short lifetime compared to OLEDs fabricated using thermal evaporation. These demerits impede production of practical SOLED displays. This review outlines the industrial demands for commercial SOLEDs and the current status of SOLED development in industries and academia, and presents research guidelines for the development of SOLEDs that have high efficiency, long lifetime, and good processability to achieve commercialization.

Changing the Microstructural and Chemical Properties of Graphene Oxide Through a Chemical Route

The aim of this work is to investigate the possibility of engineering desired molecular sp² structures in graphene oxide, via controlled oxidation of graphite powder, in order to achieve tunable chemical and microstructural properties useful for optoelectronics or sensing applications. Specifically, GO powder is obtained by a modified Hummers method, by using different concentrations of potassium permanganate (KMnO₄) in order to change the number of oxygen functionalities in the graphitic structure. Then, a successive alkaline treatment is performed by increasing the KOH concentration. The alkaline treatment induces a noticeable variation of the GO microstructural and chemical properties, which is accompanied by a strong enhancement of photoluminecence. PL and PLE measurements reveal that the configuration of electronic energy states changes as a function of the KMnO₄ and KOH concentration, by introducing further electronic n levels available for n→π* transitions. In particular, the number of sp² small domains embedded among oxygen-sp³ domains, increases under the KOH treatment, due to the addition of OH groups. Most of these sp² domains are lifted-off from GO and thrown away in the surnatant giving it high blue photoluminescence excited at λ_exc ∼ 319 nm. The employ of combined spectroscopy techniques allows a deep investigation of the microstructural and chemical changes induced by chemical treatments, opening the way to the fine tuning of GO functional properties.

Pure Hydrocarbon Materials as Highly Efficient Host for White Phosphorescent Organic Light-Emitting Diodes: A New Molecular Design Approach

To date, all efficient host materials reported for phosphorescent OLEDs (PhOLEDs) are constructed with heteroatoms, which have a crucial role in the device performance. However, it has been shown in recent years that the heteroatoms not only increase the design complexity but can also be involved in the instability of the PhOLED, which is nowadays the most important obstacle to overcome. Herein, we design pure aromatic hydrocarbon materials (PHC) as very efficient hosts in high-performance white and blue PhOLEDs. With EQE of 27.7 %, the PHC-based white PhOLEDs display similar efficiency as the best reported with heteroatom-based hosts. Incorporated as a host in a blue PhOLED, which are still the weakest links of the technology, a very high EQE of 25.6 % is reached, surpassing, for the first time, the barrier of 25 % for a PHC and FIrpic blue emitter. This performance shows that the PHC strategy represents an effective alternative for the future development of the OLED industry.

Asymmetrically Enhanced Coplanar-Electrode Electroluminescence for Information Encryption and Ultrahighly Stretchable Displays

Traditional alternating-current-driven electroluminescent (AC-EL) devices adopting a sandwich structure are commonly used in solid-state lighting and displays, while the emerging coplanar-electrode alternating-current-driven light-emitting variants manifest excellent application prospects in intelligent, multifunctional, and full-color displays, and sensing purposes. In this work, an asymmetrically enhanced coplanar-electrode AC-EL device with a universal and straightforward architecture is designed based on the impedance adjustment strategy. This newly devised asymmetric structure extends the functionalities of the coplanar-electrode AC-EL devices by overcoming the bottlenecks of complicated patterning procedures and high driving voltages of symmetric configuration. The developed device design enables a new type of information encryption and ultrahighly stretchable patterned displays. Notably, the novel encryption appliances demonstrate feasible encryption/decryption features, multiple encryptions, and practical applicability; the biaxially stretchable display devices achieve the highest tensile performance in the field of stretchable electroluminescent pattern displays, and outperform the ultrahighly stretchable sandwich devices in terms of simple patterning process, higher brightness, wider color gamut, and long-term stability. The proposed configuration opens up new avenues for AC-EL devices toward a plethora of smart applications in wearable electronics with intelligent displays, dynamic interaction of human-machine interface, and soft robotics.

New luminescent tetracoordinate boron complexes: an in-depth experimental and theoretical characterisation and their application in OLEDs

New luminescent 2-iminopyrrolyl boron complexes with different BX2 moieties are extensively studied via complementary experimental and theoretical methodologies, including application in OLEDs.

Spatial Anisotropy of Charge Transfer at Perfluoropentacene-Pentacene (001) Single-Crystal Interfaces and its Relevance for Thin Film Devices

Archetypal donor-acceptor (D-A) interfaces composed of perfluoropentacene (PFP) and pentacene (PEN) are examined for charge transfer (CT) state formation and energetics as a function of their respective molecular configuration. To exclude morphological interference, our structural as well as highly sensitive differential reflectance spectroscopy studies were carried out on PFP thin films epitaxially grown on PEN(001) single-crystal facets. Whereas the experimental data supported by complementary theoretical calculations confirm the formation of a strong CT state in the case of a cofacial PFP-PEN stacking, CT formation is energetically less favorable and thus absent for the corresponding head-to-tail configuration as disclosed for the first time. In view of technological implementations, the knowledge gained on the single-crystal references is transferred to thin-film diodes composed of either stacked PFP/PEN bilayers or mixed PFP:PEN heterojunction interfaces. As demonstrated, their electronic and electroluminescent behavior can be consistently described by the absence or presence of interfacial CT states. Thus, our results hint at the thorough design of D-A interfaces to achieve the highest device performances.

Precise Spin Manipulation of Single Molecule Positioning on Graphene by Coordination Chemistry

Precise spin manipulation of single molecules is crucial for future molecular spintronics. However, it has been a formidable challenge due to the complexities of the strong molecule-substrate coupling as well as the response of the molecule to external stimulus. Here we demonstrate by density functional theory calculations that precise spin manipulation can be achieved by extra CO and NO molecules coordination to transition metal phthalocyanine (TMPc) (TM = Co, Fe, Mn) molecules deposited on metal-supported graphene; the spins of TMPc molecules are switched from S to S - ¹/₂ (|S - 1|) after NO (CO) coordination. With the aid of a combination of molecular orbitals (MO) theory and recently developed principal interacting spin-orbital (PISO) analysis, the impacts of NO and CO coordinations on both adsorption configuration and spin polarization of TMPc are well elucidated. We reveal the different coordination geometries that CO always coordinates axially to the TM center with a linear geometry, while NO prefers a bent geometry, which can be attributed to the competition between the σ- and π-type interactions according to the PISO analysis. Particularly, the NO-MnPc complex adopts a bent geometry deviating from the prediction by the existing Enemark-Feltham formalism. In addition, MO analysis suggests that during the CO coordination, the simultaneous existence of σ-donation and π-back-donation promotes electrons flowing from the d_z² to partially occupied d_π (d_xz and d_xz) orbitals with subsequent reordering of the TM d-orbitals, resulting in the spin transition of S → |S - 1|. In comparison, given that NO is regarded as NO^- when it adopts a bent geometry coordinating to the TM center, the complete (CoPc) or partial (FePc and MnPc) quenching of the molecular spins caused by NO coordination is attributed to the electron transfer from TM to NO. These theoretical findings provide important insights into relevant experiments and offer an effective design strategy to realize underlying single-molecular spintronics devices integrated with two-dimensional materials.

Recent Advances in Thermally Activated Delayed Fluorescent Polymer-Molecular Designing Strategies

Thermally activated delayed fluorescent (TADF) materials, as the third generation of organic electroluminescent materials, have many advantages over other organic light-emitting diodes (OLEDs) materials, such as 100% internal quantum efficiency, no doping of heavy metals, and avoiding the shortages of ordinary fluorescent materials and phosphorescent materials. So it is considered to be the most competitive organic light-emitting materials, and has great application prospects in the field of OLEDs. So far, small-molecule TADF materials have achieved high quantum yield and full-color range of red, green, and blue. However, TADF polymers suitable for low-cost and easily scalable solution processing are less developed, which are confined by the preparation methods and polymers designing, and there are still challenges of increasing quantum efficiency and strengthening device performance. This review mainly summarizes different synthesis strategies of TADF polymers and the latest development in the field. Special attention is focused on illustrating the designing and structure-property relationship of TADF polymers, and finally, an outlook is given for the design and application prospect of TADF polymers in the future.

Photoluminescence and photo-induced conductivity in 2D siloxene nanosheet for optoelectronic applications

Semiconducting 2D siloxene nanosheets of thickness 1.7 nm and band gap of 2.54 eV are synthesized using simple chemical route. Strong photoluminescence is observed in the as-synthesized nanosheets due to presence of oxygen atoms. The photoluminescence behaviour of siloxene nanosheets is investigated by controlling temperature, excitation and pH of the solution to understand the optical response and stability of the material. The as-synthesized sample heated with temperature 200 °C shows a blue shift of 90 nm compared to the sample synthesized at room temperature. The low temperature luminescence measurements of as-synthesized samples dried at different temperatures viz. 27, 100 and 200 °C. It is seen that the luminescence intensity is increasing with decreasing temperature for the sample dried at room temperature. However, after heating the sample at 100 °C, the luminescence intensity is not only increased but also red-shifted up to 52 nm. The photocurrent has been measured for the device structure of ITO/PEDOT: PSS/Siloxene/Al with different film thicknesses to optimize the photocurrent and the maximum percentage change in photo power gain. The maximum photopower gain of 2693% is observed for the film thickness of 600 nm.

Efficient fluorescent OLEDS based on assistant acceptor modulated HLCT emissive state for enhancing singlet exciton utilization

Phenylamine phenanthroimidazole based bipolar compounds with donor-acceptor (D-A) architecture namely, 4-(1-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-6,9-di(pyren-4-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenylaniline (DDPPPA) and 4'-(1-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-6,9-di(pyren-4-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (DDPBA) have been synthesized with highly fluorescent pyrene moieties at C6- and C9-positions. The C6 and C9 modification enhanced the thermal, photochemical and electroluminescent properties. Both molecules were employed as blue emitters in non doped organic light emitting devices (OLEDs) and show high performances due to hybridized local and charge-transfer properties. An OLED with DDPPPA/DDPBA emissive layer shows deep-blue emission with maximum external quantum efficiency (η ex), current efficiency (η c) and power efficiency (η p) of 5.7/6.0%, 10.5/12.0 cd A-1 and 8.3/9.2 lm W-1, respectively. Both devices show high singlet exciton utilizing efficiency (η s) of DDPPPA-31.33% and DDPBA-35.29%. The doped device m-MTDATA:DDPPPA/m-MTDATA:DDPBA shows maximum efficiencies of η c -7.4/8.23 cd A-1; η p -5.8/6.13 lm W-1; η ex -4.72/5.63% (5 wt%):η c -8.36/9.15 cd A-1; η p -6.32/6.65 lm W-1; η ex -4.86/5.45% (10 wt%):η c -9.58/10.02 cd A-1; η p -7.8/8.25 lm W-1; η ex -5.96/6.25% (20 wt%). The doped device based on TAPC host TAPC:DDPPPA/TAPC:DDPBA exhibits maximum efficiencies of η c -9.60/10.03 cd A-1; η p -7.81/8.26 lm W-1; η ex -5.96/6.25% (20 wt%).

Efficient and Stable Organic Light-Emitting Diodes Employing Indolo[2,3-b]indole-Based Thermally Activated Delayed Fluorescence Emitters

Triplet excitons can be effectively harvested in organic light-emitting diodes employing thermally activated delayed fluorescence (TADF) molecules as the emitter and host. A design strategy for blue and green emitters with small S₁-T₁ splitting (ΔE_ST) is to construct a donor-acceptor (D-A) type molecule with moieties combining a high T₁ level with a strong electron-donating/withdrawing character. Here, we report a new kind of TADF emitter with an indolo[2,3-b]indole (IDID) donor. In comparison to other reported indolocarbazole and indoloindole donors, IDID has a higher T₁ level, which is comparable to that of the classical donor 9,9-dimethyl-9,10-dihydroacridine (DMAC) for blue TADF emitters. The sky-blue and green TADF emitters based on the IDID donor and a phenyltriazine acceptor exhibit high photoluminescence quantum yields (0.78-0.92) and short TADF lifetimes (1.1-1.7 μs) in doped films. Devices employing these IDID-based emitters offer an external quantum efficiency of 19.2%, which is comparable to that obtained for a device employing an analogous compound with a DMAC donor, while the stability of the former is higher than that of the latter owing to the just-right D-A twisting angles (∼59°) in the IDID-based emitters leading to a balance between ΔE_ST and the fluorescence rate. The utilization of host materials with a similar polarity to the emitter is found to be an effective strategy to improve device stability.

Printed nanofilms mechanically conforming to living bodies

It is anticipated that flexible wearable/implantable devices for biomedical applications will be established for the development of medical diagnostics and therapeutics. However, these devices need to be compatible with the physical and mechanical properties of the living body. In this minireview, we introduce free-standing polymer ultra-thin films (referred to as "polymer nanosheets"), for which a variety of polymers can be selected as building blocks (e.g., biodegradable polymers, conductive polymers, and elastomers), as a platform for flexible biomedical devices that are mechanically compatible with the living body, and then we demonstrate the use of "printed nanofilms" by combining nanosheets and printing technologies with a variety of inks represented by drugs, conductive nanomaterials, chemical dyes, bio-mimetic polymers, and cells. Owing to the low flexural rigidity (<10-2 nN m) of the polymer nanosheets, which is within the range of living brain slices (per unit width), the flexible printed nanofilms realize bio-integrated structure and display various functions with unique inks that continually monitor or detect biological activities, such as performing surface electromyography, measuring epidermal strain, imaging tissue temperature, organizing cells, and treating lesions in wounds and tumors.

Interface properties of CoPc and CoPcF16 on graphene/nickel: influence of germanium intercalation

Photoelectron spectroscopy was used to investigate electronic interface properties and interactions of the organic semiconductors CoPc and CoPcF₁₆ on graphene/nickel based substrates. Additional focus was put on the influence of germanium intercalation of graphene/nickel. The presented results demonstrate that germanium can decouple graphene from nickel and in this manner restore its buffer layer properties. No charge transfer from the substrate to the organic layer is observed in the germanium intercalated case, while interface related peaks in the Co 2p core level spectra indicate such charge transfer on graphene/nickel. Strong interface dipoles are found for CoPcF₁₆ on graphene/nickel and on germanium intercalated graphene/nickel. Fluorine Auger parameters have been measured, and the results provide evidence for polarization and charge transfer screening effects of different amounts at the unlike film-substrate interfaces. The various contributions to the observed shifts are discussed.

Derivatives of Cyanonaphthyl-Substituted Phenanthroimidazole as Blue Emitters for Nondoped Organic Light-Emitting Diodes

New multifunctional blue-emissive materials with superior thermal properties, viz., 4,4'-bis(1-(4-naphthyl)-1H-phenanthro[9,10-d]imidazol-2-yl)binaphthyl (NPIBN), 4,4'-bis(1-(4-cyanonaphthyl)-1H-phenanthro[9,10-d]imidazol-2-yl)biphenyl (CNPIBP), and 4,4'-bis(1-(4-cyanonaphthyl)-1H-phenanthro[9,10-d]imidazol-2-yl)binaphthyl (CNPIBN) have been synthesized. The said molecules show high photoluminescence quantum yield (Φs/f: NPIBN-0.75/0.68, CNPIBP-0.85/0.76, and CNPIBN-0.90/0.88). The fabricated nondoped/doped device with CNPIBN/4,4'-bis(carbazol-9-yl)biphenyl: CNPIBN display maximum efficiencies (ηex 4.96/5.4%; ηc 7.46/7.56 cd A-1; ηp 6.85/6.91 lm W-1) at low turn-on voltage (3.5/3.8 V). Nondoped devices based on D-π-A architecture, 4-(2-(5-(9H-carbazol-9-yl)thiophen-2-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile exhibit maximum efficiencies (ηex 2.32%; ηc 4.00 cd A-1; ηp 3.42 lm W-1) compared to 4-(2-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (ηex 2.01%; ηc 3.89 cd A-1; ηp 3.15 lm W-1).

Synthesis of OLED Materials of Several Triarylamines by Palladium Catalysts and Their Light Emitting Property

Primary aryl amines reacted with aryl halides to give the secondary aryl amines by the catalysis of [Pd(dba)2/P(t-Bu)3] at 80 °C in toluene. Secondary aryl amines reacted with 1-bromo-pyrene and 9-bromo-phenanthrene and 2,7-dibromo-9H-fluorene to afford the OLED material of aminopyrene derivatives and aminophenanthrene derivatives and diamino-9H-fluorene type by the catalysis of [Pd(OAc)2/P(t-Bu)3] at 120 °C in o-xylene. The product structures were established by 1H NMR, 13C NMR, 13C(DEPT), HRMS spectra. Physical properties were examined by UV-vis, PL and DSC spectra.

Efficient electroluminescent hybridized local and charge-transfer host materials with small singlet–triplet splitting to enhance exciton utilization efficiency: excited state transition configuration

A series of efficient electroluminescent materials with dual carrier transport properties shows enhanced singlet exciton utilization (η_s) due to small singlet–triplet splitting (ΔE_ST). The strong orbital-coupling transitions of N-(4-(1-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-4,5-diphenyl-1H-imidazol-2-yl)naphthalen-4-yl)phenyl)-N-phenyl benzenamine (DDPB) exhibit deep blue emission at 435 nm (CIEy, 0.07) with an external quantum efficiency of 2.01%. The electroluminescent efficiencies of 2-(1-(9H-carbazol-9-yl)naphthalen-4-yl)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazole (CDDPI) (L – 3992 cd m⁻²; η_ex – 3.01%; η_c – 2.56 cd A⁻¹; η_p – 2.12 lm W⁻¹) are higher than those of the N-(4-(1-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-H-phenanthro[9,10-d]imidazole-2-yl)naphthalen-4-yl)phenyl)-N-phenylbenzenamine (DBDPA) based device (L – 3015 cd m⁻²; η_ex – 2.85%; η_c – 2.01 cd A⁻¹; η_p – 1.92 lm W⁻¹). The blue emissive materials CDDPI and DBDPA are used as a host to construct green and red phosphorescent OLEDs: the green device based on CDDPI:Ir(ppy)₃ exhibits higher efficiencies (L – 8812 cd m⁻²; η_ex – 19.0%; η_c – 27.5 cd A⁻¹; η_p – 33.0 lm W⁻¹) at 2.7 V and the red device based on CDDPI:Ir(MQ)₂(acac) exhibits a maximum luminance of 39 661 cd m⁻² with excellent EL efficiencies [η_ex – 19.2%; η_c – 27.9 cd A⁻¹; η_p – 29.2 lm W⁻¹; CIE (0.64, 0.34)] compared with those of the DBDPA:Ir(MQ)₂(acac) based device [L – 37 621 cd m⁻²; η_ex – 18.5%; η_c – 25.2 cd A⁻¹; η_p – 25.8 lm W⁻¹; CIE (0.64, 0.34)].

CDDPI:Ir(ppy)₃ exhibits higher efficiencies: L = 8812 cd m⁻²; η_ex = 19.0%; η_c = 27.5 cd A⁻¹; η_p = 33.0 lm W⁻¹ at 2.7 V.

Strategic tuning of excited-state properties of electroluminescent materials with enhanced hot exciton mixing

Deep blue emitters with excellent stability, high quantum yield and multifunctionality are the major issues for full-color displays. In line with this, new multifunctional, thermally stable blue emitters viz., N-(4-(10-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)anthracen-9-yl)phenyl)-N-phenylbenzenamine (DPIAPPB) and 2-(10-(9H-carbazol-9-yl)anthracen-9-yl)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazole (CADPPI) with hybridized local charge transfer state (HLCT) and hot exciton properties have been synthesized. These molecules show high photoluminescence quantum yield (Φ_s/f): (DPIAPPB – 0.82/0.70 and CADPPI – 0.91/0.83). The CADPPI based device (EL – 467 nm) shows high efficiencies [η_c – 9.85 cd A⁻¹; η_p – 10.84 lm W⁻¹; η_ex – 4.78% at 2.8 V; CIE (0.15, 0.10)] compared to the DPIAPPB device (EL − 472 nm) [η_c – 6.56 cd A⁻¹; η_p – 6.16 lm W⁻¹; η_ex – 4.15% at 2.8 V with CIE (0.15, 0.12)]. The green device with CADPPI:Ir(ppy)₃ exhibits a maximum L – 59 012 cd m⁻²; η_ex – 16.8%; η_c – 37.3 cd A⁻¹; η_p – 39.8 lm W⁻¹ with CIE (0.30, 0.60) and the red device with CADPPI:Ir(MDQ)₂(acac) shows a maximum L – 43 456 cd m⁻²; η_ex – 21.9%; η_c – 36.0 cd A⁻¹; η_p – 39.6 lm W⁻¹ with CIE (0.64, 0.35).

The CADPPI:Ir(ppy)₃ device exhibits L – 90 12 cd m⁻²; η_ex – 18.8%; η_c − 27.3 cd A⁻¹; η_p – 29.8 lm W⁻¹; CIE (0.30, 0.60).

Efficient full-colour organic light-emitting diodes based on donor–acceptor electroluminescent materials with a reduced singlet–triplet splitting energy gap

A series of efficient blue-emitting materials, namely, Cz-DPVI, Cz-DMPVI, Cz-DEPVI and TPA-DEPVI, possessing a donor–acceptor architecture with dual carrier transport properties and small singlet–triplet splitting is reported. These compounds exhibit excellent thermal properties with a very high glass-transition temperature (T_g), and thus, a stable uniform thin film was formed during device fabrication. Among the weak donor compounds, specifically, Cz-DPVI, Cz-DMPVI and Cz-DEPVI, the Cz-DEPVI-based device showed the maximum efficiencies (L: 13 955 cd m⁻², η_ex: 4.90%, η_c: 6.0 cd A⁻¹, and η_p: 5.4 lm W⁻¹) with CIE coordinates of (0.15, 0.06) at 2.8 V. The electroluminescent efficiencies of Cz-DEPVI were higher than that of the strong donor TPA-DEPVI-based device (L: 13 856 cd m⁻², η_ex: 4.70%, η_c: 5.7 cd A⁻¹, and η_p: 5.2 lm W⁻¹). Furthermore, these blue emissive materials were used as hosts to construct efficient green and red phosphorescent OLEDs. The green device based on Cz-DEPVI:Ir(ppy)₃ exhibited the maximum L of 8891 cd m⁻², η_ex of 19.3%, η_c of 27.9 cd A⁻¹ and η_p of 33.4 lm W⁻¹ with CIE coordinates of (0.31, 0.60) and the red device based on Cz-DEPVI:Ir(MQ)₂(acac) exhibited the maximum L of 40 565 cd m⁻², η_ex of 19.9%, η_c of 26.0 cd A⁻¹ and η_p of 27.0 lm W⁻¹ with CIE coordinates of (0.64, 0.37).

The Cz-DEPVI device showed high efficiencies of L: 13955 cd m⁻², η_ex: 4.90%, η_c: 6.0 cd A⁻¹, η_p: 5.4 lm W⁻¹ and CIE coordinates of (0.15, 0.06) at 2.8 V.

Hot exciton transition for organic light-emitting diodes: tailoring excited-state properties and electroluminescence performances of donor-spacer-acceptor molecules

The photophysical, electrochemical and electroluminescent properties of newly synthesized blue emitters with donor-π-acceptor geometry, namely, 4'-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1'-biphenyl]vinyl)-4-amine (NSPI-TPA), 4'-(1-(2-methylnaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1'-biphenyl]vinyl)-4-amine (MNSPI-TPA), 4-(2-(4'-(diphenylamino)-(2-[1,1'-biphenyl]vinyl)-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-1-naphthalene-1-carbonitrile (SPNCN-TPA) and 4-(2-(4-(9H-carbazol-9-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (SPNCN-Cz) were analyzed. The conjugation length in the emitters is not conducive to pure emission and hence, a molecular twisting strategy was adopted in NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz to enhance pure emission. The emissive state (HLCT) of twisted D-π-A molecules containing both LE and CT (HLCT) states was tuned for high PL (η PL) (LE) and high exciton utilization (η s) (CT) efficiencies by replacing triphenylamine (strong donor) with carbazole (weak donor). Among strong donor compounds, namely, NSPI-TPA, MNSPI-TPA and SPNCN-TPA, the SPNCN-TPA-based device exhibited blue emission (451 nm) with CIE coordinates (0.15, 0.08), maximum current efficiency (η c) of 2.32 cd A-1, power efficiency (η p) of 2.01 lm W-1 and external quantum efficiency (η ex) of 3.02%. The device with SPNCN-Cz emitter exhibited higher electroluminescence efficiencies than the SPNCN-TPA-based device, with pure blue emission (443 nm, CIE: 0.15,0.07), η ex of 3.15%, η c of 2.56 cd A-1 and η p of 2.45 lm W-1.

Efficient donor-acceptor host materials for green organic light-emitting devices: non-doped blue-emissive materials with dual charge transport properties

Comparative optical, electroluminescence and theoretical studies were performed for (E)-4'-(1-(4-(2-(1-(4-morpholinophenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (SMPI-TPA) and (E)-4-(4-(2-(4-(2-(4-(9H-carbazol-9-yl)phenyl)-1H-phenanthro[9,10-d]imidazol-1-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)phenyl)morpholine (SMPI-Cz). These compounds show excellent thermal properties, dual charge transport properties and form thin films under thermal evaporation. Blue OLEDs (CIE: 0.16, 0.08) based on SMPI-TPA show efficient device performance (η ex 6.1%; η c 5.3 cd A-1; η p 5.2 lm W-1) at low turn-on voltages. Both SMPI-TPA and SMPI-Cz were utilised as hosts for green OLEDs. The devices with SMPI-Cz (30 nm):5 wt% Ir(ppy)3 exhibit maximum luminance of 20 725 cd m-2, and η c and η p values of 61.4 cd A-1 and 63.8 lm W-1, respectively. In comparison, devices with SMPI-TPA (30 nm):5 wt% Ir(ppy)3 exhibit high η c and η p values of 65.2 cd A-1 and 67.1 lm W-1, respectively. Maximum η ex values of 19.6% and 23.4% were obtained from SMPI-TPA:Ir(ppy)3 and SMPI-Cz:Ir(ppy)3, respectively. These device performances indicate that the phenanthroimidazole unit is a tunable building unit for efficient carrier injection and it may also be employed as a host for green OLEDs.

Aggregation-induced emission and the working mechanism of 1-benzoyl and 1-benzyl pyrene derivatives

Over the past decade, research studies on solid-state luminescent materials featuring aggregation-induced emission (AIE) have achieved great success. It has been proved that lots of planar ACQ (aggregation-caused quenching) chromophores can be converted to AIE luminogens (AIEgens) by combining with an AIE-active unit such as tetraphenylethene (TPE). In this work, we present a new method to create AIE luminogens just by introducing benzoyl or benzyl to a planar chromophore, pyrene. The generated 1-benzoyl and 1-benzyl pyrene derivatives exhibit weak emission when molecularly dissolved in good solvents but strong emission from pyrene dimers when aggregated in poor solvent or the solid state. Their crystal structure analysis and theoretical calculations are performed to depict the working mechanism of these new AIEgens. The results show that the structural rigidification of these 1-benzoyl pyrene derivatives is the major cause for their AIE effect. This new AIE system along with a clear working mechanism will contribute to the development of AIE-related functional materials and theories.

Salen-indium/triarylborane triads: synthesis and ratiometric emission-colour changes by fluoride ion binding

Salen-based indium triads, [{(3-tBu)2-(5-Mes2B)2-salen}In-Me] (1) and [{(3-tBu)2-(5-Mes2Bphenyl)2-salen}In-Me] (2), bearing triarylborane (TAB) units were prepared and fully characterised by NMR spectroscopy and elemental analysis. The major absorption bands of 1 and 2 appeared in the region centred at 347 nm and 374 nm, respectively, and the intense emission spectra were observed in the sky blue (λem = 491 nm for 1) and bluish-green (λem = 498 nm for 2) regions, respectively. The solvatochromism effects in various organic solvents and computational calculation results strongly suggested that these absorption and emission features are mainly attributed to intramolecular charge transfer (ICT) transitions between the salen ligand moieties and the TAB units. Furthermore, UV-vis and photoluminescence (PL) titration experiments by the addition of fluoride anions demonstrated ratiometric quenching patterns in both the absorption and emission spectra, indicating that binding of the fluoride anion to the boron centres interrupts these ICT transitions in each compound. Interestingly, both triads exhibited a gradual red-shifted response in each emission spectrum upon the addition of the fluoride anions, resulting in a dramatic colour-change to yellow. The computational calculation results of the S1 states revealed that these emission-colour change properties arise from the elevation of HOMO levels, which are mainly localised on the TAB moieties, resulting from the fluoride anion binding to the borane centres.

Solution-processed thermally activated delayed fluorescence organic light-emitting diodes using a new polymeric emitter containing non-conjugated cyclohexane units

A new solution-processable polymeric emitter containing non-conjugated cyclohexane units was developed for high-performing TADF-OLEDs.

Chain Length Dependence of the Dielectric Constant and Polarizability in Conjugated Organic Thin Films

Dielectric materials are ubiquitous in optics, electronics, and materials science. Recently, there have been new efforts to characterize the dielectric performance of thin films composed of molecular assemblies. In this context, we investigate here the relationship between the polarizability of the constituent molecules and the film dielectric constant, using periodic density functional theory (DFT) calculations, for polyyne and saturated alkane chains. In particular, we explore the implication of the superlinear chain length dependence of the polarizability, a specific feature of conjugated molecules. We show and explain from DFT calculations and a simple depolarization model that this superlinearity is attenuated by the collective polarization. However, it is not completely suppressed. This confers a very high sensitivity of the dielectric constant to the thin film thickness. This latter can increase by a factor of 3-4 at reasonable coverages, by extending the molecular length. This significantly limits the decline of the thin film capacitance with the film thickness. Therefore, the conventional fit of the capacitance versus thickness is not appropriate to determine the dielectric constant of the film. Finally, we show that the failures of semilocal approximations of the exchange-correlation functional lead to a very significant overestimation of this effect.

Effect of Injection Layer Sub-Bandgap States on Electron Injection in Organic Light-Emitting Diodes

It is generally considered that the injection of charges into an active layer of an organic light-emitting diode (OLED) is solely determined by the energetic injection barrier formed at the device interfaces. Here, we demonstrate that the density of surface states of the electron-injecting ZnO layer has a profound effect on both the charge injection and the overall performance of the OLED device. Introducing a dopant into ZnO reduces both the energy depth and density of surface states without altering the position of the energy levels-thus, the magnitude of the injection barrier formed at the organic/ZnO interface remains unchanged. Changes observed in the density of surface states result in an improved electron injection and enhanced luminescence of the device. We implemented a numerical simulation, modeling the effects of energetics and the density of surface states on the electron injection, demonstrating that both contributions should be considered when choosing the appropriate injection layer.

Highly efficient non-doped blue organic light emitting diodes based on a D–π–A chromophore with different donor moieties

The non-doped OLED based on MPPIS-Cz exhibits blue emission with CIE of (0.16, 0.08), maximum current and external quantum efficiency of 1.52 cd A⁻¹ and of 1.42%, respectively.

Carbazole functionalized new bipolar ligand for monochromatic red light-emitting europium(iii) complex: combined experimental and theoretical study

Complete energy transfer observed from carbazole functionalized bipolar ligand as well as β-diketonate to Eu(iii) ion and Eu(iii) complex shows efficient PLQE (41.4%) with appropriate CIE color coordinates (x = 0.64, y = 0.36).

Achieving Low-Energy Driven Viologens-Based Electrochromic Devices Utilizing Polymeric Ionic Liquids

Herein, three kinds of viologens-based electrochromic devices (ECDs) (heptyl viologen (HV(BF₄)₂), octyl viologen (OV(BF₄)₂), and nonyl viologen (NV(BF₄)₂)) were fabricated utilizing ferrocene (Fc) as a redox mediator. Among them, the NV(BF₄)₂-based ECD exhibits the highest coloration efficiency (36.2 cm²/C) owing to the lowest driving energy. Besides, switching between 0 and 1.2 V, the NV(BF₄)₂-based ECD shows a desirable initial transmittance change (ΔT = 56.7% at 605 nm), and long-term stability (ΔT = 45.4% after 4000 cycles). Furthermore, a UV-cured polymer electrolyte containing polymeric ionic liquid (PIL, 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and ethoxylated trimethylolpropane triacrylate (ETPTA) was introduced to the NV(BF₄)₂-based ECD. By controlling the weight percentage of the PIL, different curing degrees of the polymer electrolytes were obtained and led to an improved stability of the NV(BF₄)₂-based ECD because of the immobilization of NV(BF₄)₂. This observation was explained by calculating the apparent diffusivity (D_app) of the redox species in the NV(BF₄)₂-based ECD under various curing degrees. In addition, increasing the amount of PIL leads to a lower driven energy needed for the NV(BF₄)₂-based ECD, following the same trend as the value of D_app. Among all NV(BF₄)₂-based ECDs, 20 wt % of PIL addition (20-PIL ECD) exhibits large transmittance change (ΔT = 55.2% at 605 nm), short switching times (2.13 s in coloring and 2.10 s in bleaching), high coloration efficiency (60.4 and 273.5 cm²/C at 605 nm, after excluding the current density at the steady state), and exceptional cycling stability (ΔT = 53.8% after 10,000 cycles, or retained 97.5% of its initial ΔT).