Aggregation-Enhanced Emission and Thermally Activated Delayed Fluorescence of Derivatives of 9-Phenyl-9H -Carbazole: Effects of Methoxy and tert -Butyl Substituents

Ornamenting of Blue Thermally Activated Delayed Fluorescence Emitters by Anchor Groups for the Minimization of Solid-State Solvation and Conformation Disorder Corollaries in Non-Doped and Doped Organic Light-Emitting Diodes

Motivated to minimize the effects of solid-state solvation and conformation disorder on emission properties of donor-acceptor-type emitters, we developed five new asymmetric multiple donor-acceptor type derivatives of tert-butyl carbazole and trifluoromethyl benzene exploiting different electron-accepting anchoring groups. Using this design strategy, for a compound containing four di-tert-butyl carbazole units as donors as well as 5-methyl pyrimidine and trifluoromethyl acceptor moieties, small singlet-triplet splitting of ca. 0.03 eV, reverse intersystem crossing rate of 1 × 10⁶ s^-1, and high photoluminescence quantum yield of neat film of ca. 75% were achieved. This compound was also characterized by the high value of hole and electron mobilities of 8.9 × 10^-4 and 5.8 × 10^-4 cm² V^-1 s^-1 at an electric field of 4.7 × 10⁵ V/cm, showing relatively good hole/electron balance, respectively. Due to the lowest conformational disorder and solid-state solvation effects, this compound demonstrated very similar emission properties (emission colors) in non-doped and differently doped organic light-emitting diodes (OLEDs). The lowest conformational disorder was observed for the compound with the additional accepting moiety inducing steric hindrance, limiting donor-acceptor dihedral rotational freedom. It can be exploited in the multi-donor-acceptor approach, increasing the efficiency. Using an emitter exhibiting the minimized solid-state solvation and conformation disorder effects, the sky blue OLED with the emitting layer of this compound dispersed in host 1,3-bis(N-carbazolyl)benzene displayed an emission peak at 477 nm, high brightness over 39 000 cd/m², and external quantum efficiency up to 15.9% along with a maximum current efficiency of 42.6 cd/A and a maximum power efficiency of 24.1 lm/W.

Diabatic Decomposition Perspective on the Role of Charge Transfer and Local Excitations in Thermally Activated Delayed Fluorescence

Thermally activated delayed fluorescence (TADF) is a phenomenon that relies on the upconversion of triplet excitons to singlet excitons by means of reverse intersystem crossing (rISC). It has been shown both experimentally and theoretically that the TADF mechanism depends on the interplay between charge transfer and local excitations. However, the difference between the diabatic and adiabatic character of the involved excited states is rarely discussed in the literature. Here we develop a diabatization procedure to implement a four-state model Hamiltonian to a set of TADF molecules. We provide physical interpretations of the Hamiltonian elements and show their dependence on the electronic state of the equilibrium geometry. We also demonstrate how vibrations affect the TADF efficiency by modifying the diabatic decomposition of the molecule. Finally, we provide a simple model that connects the diabatic Hamiltonian to the electronic properties relevant to TADF and show how this relationship translates into different optimization strategies for rISC, fluorescence, and overall TADF performance.

2,7(3,6)-Diaryl(arylamino)-substituted Carbazoles as Components of OLEDs: A Review of the Last Decade

Organic light emitting diode (OLED) is a new, promising technology in the field of lighting and display applications due to the advantages offered by its organic electroactive derivatives over inorganic materials. OLEDs have prompted a great deal of investigations within academia as well as in industry because of their potential applications. The electroactive layers of OLEDs can be fabricated from low molecular weight derivatives by vapor deposition or from polymers by spin coating from their solution. Among the low-molar-mass compounds under investigation in this field, carbazole-based materials have been studied at length for their useful chemical and electronic characteristics. The carbazole is an electron-rich heterocyclic compound, whose structure can be easily modified by rather simple reactions in order to obtain 2,7(3,6)-diaryl(arylamino)-substituted carbazoles. The substituted derivatives are widely used for the formation of OLEDs due to their good charge carrier injection and transfer characteristics, electroluminescence, thermally activated delayed fluorescence, improved thermal and morphological stability as well as their thin film forming characteristics. On the other hand, relatively high triplet energies of some substituted carbazole-based compounds make them useful components as host materials even for wide bandgap triplet emitters. The present review focuses on 2,7(3,6)-diaryl(arylamino)-substituted carbazoles, which were described in the last decade and were applied as charge-transporting layers, fluorescent and phosphorescent emitters as well as host materials for OLED devices.

Towards the Bisbenzothienocarbazole Core: A Route of Sulfurated Carbazole Derivatives with Assorted Optoelectronic Properties and Applications

Ladder-type molecules, which possess an extended aromatic backbone, are particularly sought within the optoelectronic field. In view of the potential of the 14H-bis[1]benzothieno[3,2-b:2',3'-h]carbazole core as a p-type semiconductor, herein we studied a set of two derivatives featuring a different alkylation patterning. The followed synthetic route, involving various sulfurated carbazole-based molecules, also resulted in a source of fluorophores with different emitting behaviors. Surprisingly, the sulfoxide-containing fluorophores substantially increased their blue fluorescence with respect to the nearly non-emitting sulfur counterparts. On this basis, we could shed light on the relationship between their chemical structure and their emission as an approach for future applications. Considering the performance in organic thin-film transistors, both bisbenzothienocarbazole derivatives displayed p-type characteristics, with hole mobility values up to 1.1 × 10-3 cm2 V-1 s-1 and considerable air stability. Moreover, the role of the structural design has been correlated with the device performance by means of X-ray analysis and the elucidation of the corresponding single crystal structures.

Branched Electron-Donor Core Effect in D-π-A Star-Shaped Small Molecules on Their Properties and Performance in Single-Component and Bulk-Heterojunction Organic Solar Cells †

Star-shaped donor-acceptor molecules are full of promise for organic photovoltaics and electronics. However, the effect of the branching core on physicochemical properties, charge transport and photovoltaic performance of such donor-acceptor materials in single-component (SC) and bulk heterojunction (BHJ) organic solar cells has not been thoroughly addressed. This work shows the comprehensive investigation of six star-shaped donor-acceptor molecules with terminal hexyldicyanovinyl blocks linked through 2,2′-bithiophene π-conjugated bridge to different electron-donating cores such as the pristine and fused triphenylamine, tris(2-methoxyphenyl)amine, carbazole- and benzotriindole-based units. Variation of the branching core strongly impacts on such important properties as the solubility, highest occupied molecular orbital energy, optical absorption, phase behavior, molecular packing and also on the charge-carrier mobility. The performance of SC or BHJ organic solar cells are comprehensively studied and compared. The results obtained provide insight on how to predict and fine-tune photovoltaic performance as well as properties of donor-acceptor star-shaped molecules for organic solar cells.

Spherical Hole-Transporting Interfacial Layer Passivated Defect for Inverted NiOx-Based Planar Perovskite Solar Cells with High Efficiency of over 20

In this study, we achieved a facile and low-cost (18-22 USD/g) synthesis of spiro[fluorene-9,9-phenanthren-10-one]-based interfacial layer materials (MSs; designated MS-PC, MS-PA, MS-OC, and MS-OA). Carbazoles and dimethylacridine substituents with an extended π-conjugation achieved through ortho- or para-orientations were used as donors at the spiro[fluorene-9,9'-phenanthren-10'-one] moiety. Highly efficient and stable inverted perovskite solar cells (PSCs) with the device architecture of ITO/NiO_x/MSs/perovskite/PC₆₁BM/BCP/Ag can be achieved to improve the surface morphology of NiO_x when MSs are adopted as the interfacial layer. During a morphological study, the ortho-orientated donor of MS-OC and MS-OA has spherical structures indicated that the films were smooth and that the films of perovskite deposited on them had large grain size and uniformity. The photoluminescence properties of the perovskite layers on the NiO_x/MSs were showed better hole-transporting capabilities than the bare NiO_x. The dual-functional interfacial layer has shown defect passivation effect, it not only improved the surface morphology of NiO_x but also enlarged the perovskite layer grain size. The best PSC device performance of the NiO_x/MS-OC was characterized by 22.34 mA cm^-2 short-circuit current density (J_sc), 1.128 V open-circuit voltage (V_oc), and 80.8% fill factor (FF), resulting in 20.34% power conversion efficiency (PCE). The NiO_x/MS-OC PSCs showed good long-term device stability, even retained the original PCE of 93.16% after 370 days under argon (25 °C). Owing to the superior perovskite morphologies of the NiO_x/MSs, the resulting devices outperformed the bare NiO_x-based PSCs.

Carbazole-modified thiazolo[3,2-c][1,3,5,2]oxadiazaborinines exhibiting aggregation-induced emission and mechanofluorochromism

Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and synthesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability and comparatively high photoluminescence quantum yields (up to 34%) in the solid state. The presence of a carbazole donor unit, linked with the oxadiazaborinine acceptor via a phenyl linker, restricted intramolecular rotation, leading to enhanced aggregation-induced emission properties of the compounds: in THF/water mixtures with a large water percentage, they demonstrated the formation of emissive nanoaggregates with an average size of 79 and 89 nm for complexes 2 and 3, respectively. The introduction of bulky tert-butyl groups attached to the carbazole moiety induced significant mechanofluorochromic properties of the compounds.

In-Depth Ab Initio Study of Thermally Activated Delayed Fluorescence in 4,5-Di(9H-carbazol-9-yl)-phthalonitrile

Molecules capable of thermally activated delayed fluorescence (TADF) are promising as emitters in organic light-emitting devices. Processes leading to and competing with TADF in 4,5-di(9H-carbazol-9-yl)-phthalonitrile are analyzed in detail. It is demonstrated that the key features of an efficient TADF emitter include the presence of two triplet states of different natures with potential energy surfaces crossing between the T₁ and S₁ minima and a noticeable dependence of the S₁ → S₀ oscillator strength on molecular deformations from low-frequency antisymmetric vibrational modes. These conclusions can be useful in the targeted design of efficient TADF emitters.

Clustering-Triggered Ultralong Room-Temperature Phosphorescence of Organic Crystals through Halogen-Mediated Molecular Assembly

To achieve efficient room-temperature phosphorescence of organic materials with ultralong lifetime, it is imperative to resolve the dilemma that the introduction of heavy atoms simultaneously improves emission efficiencies and shortens the emission lifetimes. Herein, we report a new molecular design approach for halogenated luminogens with a methylene bridge to avoid the lifetime shortening induced by heavy halogens and propose a general molecular engineering strategy to realize efficient and ultralong room-temperature phosphorescence via halogen-mediated molecular clustering. The halogenated N-benzylcarbazole derivatives show distinct photophysical behaviors depending on different physical states, including single-molecule state and cluster state. Their crystals demonstrate the halogen-dependent emission duration of room-temperature phosphorescence upon excitation. Experimental data and theoretical analysis indicate that halogen-regulated molecular clustering in the crystal is responsible for the generation of efficient ultralong room-temperature phosphorescence, and halogen-dominated molecular engineering favors the promotion of the intersystem crossing process and the following triplet emissions.

Facile Generation of Thermally Activated Delayed Fluorescence and Fabrication of Highly Efficient Non-Doped OLEDs Based on Triazine Derivatives

A series of donor-acceptor-donor triazine-based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain highly efficient blue-emitting OLEDs with non-doped emitting layers (EMLs). The targeted molecules use a triazine core as the electron acceptor, and a benzene ring as the conjugated linker with different electron donors to alternate the energy level of the HOMO to further tune the emission color. The introduction of long alkyl chains on the triazine core inhibits the unwanted intermolecular D-D/A-A-type π-π interactions, resulting in the intermolecular D-A charge transfer. The weak aggregation-caused quenching (ACQ) effect caused by the suppressed intermolecular D-D/A-A-type π-π interaction further enhances the emission. The crowded molecular structure allows the electron donor and acceptor to be nearly orthogonal, thereby reducing the energy gap between triplet and singlet excited states (ΔE_ST ). As a result, blue-emitting devices with TH-2DMAC and TH-2DPAC non-doped EMLs showed satisfactory efficiencies of 12.8 % and 15.8 %, respectively, which is one of the highest external quantum efficiency (EQEs) reported for blue TADF emitters (λ_peak <475 nm), demonstrating that our tailored molecular designs are promising strategies to endow OLEDs with excellent electroluminescent performances.

Novel Dicyano-Phenylenevinylene Fluorophores for Low-Doped Layers: A Highly Emissive Material for Red OLEDs

Two efficient deep red (DR)-emitting organic dicyano-phenylenevinylene derivatives with terminal withdrawing or donor groups were synthesized. The spectroscopic properties of the neat solids and the low-doped layers in polystyrene or polyvinylcarbazole host matrixes were analyzed, and the luminescence performance was explained using density functional theory (DFT) analysis. A noteworthy 89% fluorescence quantum yield was observed for the brightest red-emissive polyvinylcarbazole (PVK) blend. This result pushed us to successfully produce an emissive red organic light-emitting device (OLED) as a preliminary feasibility test.

Rational Molecular Design of Dibenzo[a,c]phenazine-Based Thermally Activated Delayed Fluorescence Emitters for Orange-Red OLEDs with EQE up to 22.0

The design and synthesis of highly efficient thermally activated delayed fluorescence (TADF) emitters with an electroluminescence wavelength beyond 600 nm remains a great challenge for organic light-emitting diodes (OLEDs). To solve this issue, three TADF molecules, xDMAC-BP (x = 1, 2, 3), are developed in combination with the rigid planar dibenzo[a,c]phenazine (BP) acceptor core and different numbers of 9,9-dimethylacridan (DMAC) donors. All these emitters possess stable internal charge transfer and a large dihedral angle between the donors and planar BP core. The emission wavelength can be regulated from 541 to 605 nm by increasing the number of the donor DMAC units because of the controllable tuning of the intramolecular charge transfer effect and the molecular geometrical structure. The photoluminescence quantum yields of these emitters are improved from 42 to 89% with the increase in the number of DMAC units. The orange-red OLEDs employing the xDMAC-BP emitters exhibit maximum external quantum efficiency (EQE) of 22.0% at 606 nm, which is the highest EQE of the previously reported TADF OLEDs exceeding 600 nm.

Triggering Thermally Activated Delayed Fluorescence by Managing the Heteroatom in Donor Scaffolds: Intriguing Photophysical and Electroluminescence Properties

Establishment of the structure-property relationships of thermally activated delayed fluorescence (TADF) materials has become a significant quest for the scientific community. Herein, two new donors, 10H-benzofuro[3,2-b]indole (BFI) and 10H-benzo[4,5]thieno[3,2-b]indole (BTI), have been developed and integrated with a aryltriazine acceptor to design the green TADF emitters benzofuro[3,2-b]indol-10-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (BFICNTrz) and 2-(10H-benzo[4,5]thieno[3,2-b]indol-10-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (BTICNTrz), respectively. The physicochemical and electroluminescence properties of the compounds were tuned by exchanging the heteroatom in the donor scaffold. Intriguingly, the electronegativity of the heteroatom and the ionization potential of the donor unit played vital roles in control of the singlet-triplet energy splitting and TADF mechanism of the compounds. Both compounds showed similar singlet excited states that originated from the charge transfer (CT) states (¹ CT), whereas the triplet excited states were tuned by the heteroatom in the donor unit. The origin of phosphorescence in the BTICNTrz emitter was CT emission from the triplet state (³ CT), whereas that in the BFICNTrz emitter stemmed from the local triplet excited state (³ LE). Consequently, BTICNTrz showed a small singlet-triplet energy splitting of 0.08 eV, compared with 0.26 eV for BFICNTrz. Thus, BTICNTrz showed efficient delayed fluorescence with a high quantum yield and a short delayed exciton lifetime, whereas BFICNTrz displayed weak delayed fluorescence with a relatively long lifetime. Furthermore, a BTICNTrz-based device exhibited a maximum external quantum efficiency (EQE) of 15.2 % and reduced efficiency roll-off (12 %) compared with its BFICNTrz-based counterpart, which showed a maximum EQE of 6.4 % and severe efficiency roll-off (55 %) at a practical brightness range of 1000 cd m^-2 . These results demonstrate that the choice of subunit plays a vital role in the design of efficient TADF emitters.

Synthesis of Linear and V-Shaped Carbazolyl-Substituted Pyridine-3,5-dicarbonitriles Exhibiting Efficient Bipolar Charge Transport and E-Type Fluorescence

With the aim of developing all-organic bipolar semiconductors with high charge mobility and efficient E-type fluorescence (so-called TADF) as environmentally friendly light-emitting materials for optoelectronic applications, four noble metals-free dyes with linear and V-shapes were designed using accepting pyridine-3,5-dicarbonitrile and donating carbazole units. By exploiting a donor-acceptor design strategy and using moieties with different donating and accepting abilities, TADF emitters with a wide variety of molecular weights were synthesized to achieve the optimum combination of charge-transporting and fluorescent properties in one TADF molecule. Depending on molecule structures, different TADF emitters capable of emitting in the range from 453 to 550 nm with photoluminescence quantum yields up to 98 % for the solutions in oxygen-free toluene were obtained. All compounds showed bipolar charge-transport. Hole mobility of 2.8×10^-3  cm² /Vs at 7×10⁵  V cm^-1 was observed for the compound containing two di-tert-butyl-substituted carbazole moieties. The compounds were tested in both non-doped and doped organic light-emitting diodes using different hosts. It was shown that the developed TADF emitters are suitable for different color devices with electroluminescence ranging from blue to yellow and with brightness, maximum current and external quantum efficiencies exceeding 10 000 cd m^-2 , 15 cd/A, and 7 %, respectively.

Efficient Triplet-Triplet Annihilation Upconversion in an Electroluminescence Device with a Fluorescent Sensitizer and a Triplet-Diffusion Singlet-Blocking Layer

Solid-state triplet-triplet annihilation upconversion (TTAUC) blue emission in an electroluminescence device (i.e., an organic light-emitting diode (OLED)) is demonstrated. A conventional green fluorophore, tris-(8-hydroxyquinoline)aluminum (Alq₃ ), is employed as the sensitizer that generates 75% triplet under electrical pumping for the blue triplet-triplet annihilation emitter, 9,10-bis(2'-naphthyl) anthracene (ADN), with the heterojunction bilayer structure. The operation lifetime is elongated both for ADN blue (4.1x) and Alq₃ green (34.8%) emission due to efficient use of excitons and separation of recombination and emission zone. To reduce the singlet quenching (SQ) of blue TTAUC signal by the Alq₃ sensitizer with lower bandgap, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) is inserted between the Alq₃ and ADN as a triplet-diffusion-and-singlet-blocking layer. DMPPP exhibits triplet energy close to Alq₃ and higher than ADN, as well as higher singlet energy than both Alq₃ and ADN. It allows triplet diffusion from Alq₃ to ADN, but blocks the SQ of the blue TTAUC signal by Alq₃ . 86.1% intrinsic efficiency of TTAUC is demonstrated in this trilayer (Alq₃ /DMPPP/ADN) OLED.

In Situ Generation of Red-Emissive AIEgens from Commercial Sources for Nondoped OLEDs

Luminescent materials with red emission are promising materials for optoelectronic devices and biological sciences. However, their synthesis is often complicated. In this work, we developed a facile approach to generating red-emissive luminogens with aggregation-induced emission (AIE) characteristics from commercially available reactants. The new compounds, abbreviated PN-BTZ-Cz and DP-BTZ-Cz, were prepared by coupling 4,7-dibromobenzo[c][1,2,5]thiadiazole with (1-naphehyl)phenylamine or diphenylamine followed by the reaction of the monobromo-substituted compounds with carbazole. Although all of the reactants were non-AIE-active, PN-BTZ-Cz and DP-BTZ-Cz showed AIE characteristics. This suggested that the AIE chromophores were generated in situ through the reactions. PN-BTZ-Cz and DP-BTZ-Cz were soluble in common organic solvents and showed red emission with high fluorescence quantum yields of 42.2 and 38.3% in the film, respectively. They were thermally and morphologically stable, as revealed by their high decomposition temperature (up to 327 °C) and glass-transition temperature (up to 120 °C). Nondoped organic light-emitting diodes with a configuration of ITO/HATCN/TAPC/PN-BTZ-Cz or DP-BTZ-Cz/Bphen/Liq/Al were fabricated using these compounds as emitting layers, which emitted red electroluminescence at a low turn-on voltage (down to 2.8 V) with a maximum external quantum yield of up to 2.7% and a small efficiency roll-off.