Organic thermoelectric device utilizing charge transfer interface as the charge generation by harvesting thermal energy.

We propose an organic thermoelectric device having a new power generation mechanism that extracts small-scale thermal energy, i.e., a few tens of millielectronvolts, at room temperature without a temperature gradient.

Simultaneous control of carrier transport and film polarization of emission layers aimed at high-performance OLEDs.

The orientation of a permanent dipole moment during vacuum deposition results in the occurrence of spontaneous orientation polarization (SOP). Previous studies reported that the presence of SOP in organic light-emitting diodes (OLEDs) lowers electroluminescence efficiency because electrically generated excitons are seriously quenched by SOP-induced accumulated charges. Thus, the SOP in a host:guest-based emission layer (EML) should be finely controlled.

Hexacarbazolylbenzene: An Excellent Host Molecule Causing Strong Guest Molecular Orientation and the High-Performance OLEDs.

Hexacarbazolylbenzene (6CzPh), which is benzene substituted by six carbazole rings, is a simple and attractive compound. Despite the success of a wide variety of carbazole derivatives in organic light-emitting diodes (OLEDs), 6CzPh has not received attention so far. Here, excellent performances of 6CzPh are revealed as a host material in OLEDs regarding conventional host materials.

Impact of Spontaneous Orientational Polarization on Triplet-Triplet Upconversion-Based Blue Organic Light-Emitting Diodes.

The spontaneous orientation polarization (SOP) of a permanent dipole moment of the molecule induces a giant surface potential (GSP) in an organic semiconductor film, and GSP is expected to be a crucial parameter for understanding the operational mechanism of organic light-emitting diodes (OLEDs). This study demonstrates that the voltage-dependent migration of a carrier recombination zone induced by a polar electron transporting layer (ETL) having a positive SOP causes a decline in the overall performance of the OLED in triplet-triplet upconversion (TTU) based on OLEDs. Specifically, the TTU efficiency in an OLED with 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as the ETL decreased by 20% due to the reduction of electrically generated triplet exciton density.

Three-dimensional sensing of surfaces by projection of invisible electroluminescence from organic light-emitting diodes.

Organic light-emitting diodes (OLEDs) that efficiently emit near-infrared (NIR) light and consume little power will create valuable applications for OLEDs beyond just displays. Here, we report such a NIR-OLED with high operational stability that can be used as a light source for three-dimensional sensing of object's surfaces. Using a narrow-energy-gap material as a host for producing NIR hyperfluorescence system, we fabricated a NIR-OLED exhibiting intense emission at 930 nm with a high external electroluminescence quantum efficiency of more than 1% at a current density of 100 milliamperes per square meter without any degradation even after more than 300 hours of operation.

Anti-Stokes Luminescence in Multi-Resonance-Type Thermally-Activated Delayed Fluorescence Molecules.

Photon-upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon-upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light-driving cooling system. However, only a few organic molecular systems have been reported.

Multiple donor-acceptor design for highly luminescent and stable thermally activated delayed fluorescence emitters.

A considerable variety of donor-acceptor (D-A) combinations offers the potential for realizing highly efficient thermally activated delayed fluorescence (TADF) materials. Multiple D-A type compounds are one of the promising families of TADF materials in terms of stability as well as efficiencies. However, those emitters are always composed of carbazole-based donors despite a wide choice of moieties used in linearly linked single D-A molecules.

Enhancing Triplet-Triplet Upconversion Efficiency and Operational Lifetime in Blue Organic Light-Emitting Diodes by Utilizing Thermally Activated Delayed Fluorescence Materials.

In the process of triplet-triplet upconversion (TTU), a bright excited singlet can be generated because of the collision of two dark excited triplets. In particular, the efficiency of TTU is crucial for achieving a high exciton production yield in blue fluorescence organic light-emitting diodes (OLEDs) beyond the theoretical limit. While the theoretical upper limit of TTU contribution yield is expected to be 60%, blue OLEDs with the maximum TTU contribution are still scarce.

Post-Modification of Pyrrolopyrrole Aza-BODIPY toward High Near-Infrared Fluorescence Brightness.

Pyrrolopyrrole aza-BODIPYs (PPABs), dimeric aza-BODIPY analogues, exhibit intense absorption and fluorescence in the visible and near-infrared (NIR) regions. Here, we developed a facile postmodification by palladium-catalyzed coupling reactions to synthesize a series of donor-acceptor-donor (D-A-D) PPABs. Despite the possible fluorescence quenching dictated by the energy-gap low, D-A-D PPABs exhibit high-fluorescence brightness in the NIR region, implying their potential use as a bright NIR emitter.

Electron Lifetime of Over One Month in Disordered Organic Solid-State Films.

Understanding intrinsic carrier lifetime in disordered organic solid-state semiconductors is essential for improving device performance in not only molecule-based optoelectronic devices such as organic solar cells (OSC) but also photocatalysts used for producing solar fuel cells. Carriers in disordered films are generally thought to have short lifetimes on a scale ranging from nanoseconds to milliseconds. These short carrier lifetimes cause loss of charges in OSCs and low quantum yields in photocatalysts and impede the future application of organic semiconductors to, for example, charge-storage-based memory devices.

Multiple resonance type thermally activated delayed fluorescence by dibenzo [1,4] azaborine derivatives.

We studied the photophysical and electroluminescent (EL) characteristics of a series of azaborine derivatives having a pair of boron and nitrogen aimed at the multi-resonance (MR) effect. The computational study with the STEOM-DLPNO-CCSD method clarified that the combination of a BN ring-fusion and a terminal carbazole enhanced the MR effect and spin-orbit coupling matrix element (SOCME), simultaneously. Also, we clarified that the second triplet excited state (T) plays an important role in efficient MR-based thermally activated delayed fluorescence (TADF).

Carbazole-2-carbonitrile as an acceptor in deep-blue thermally activated delayed fluorescence emitters for narrowing charge-transfer emissions.

This work reports a new acceptor for constructing donor-acceptor type (D-A type) blue thermally activated delayed fluorescence (TADF) emitters with narrowed charge-transfer (CT) emissions. A new acceptor core, carbazole-2-carbonitrile (CCN), is formed by the fusion of carbazole and benzonitrile. Three D-A type TADF emitters based on the CCN acceptor, namely 3CzCCN, 3MeCzCCN, and 3PhCzCCN, have been successfully synthesized and characterized.

Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films.

The performance of organic optoelectronic and energy-harvesting devices is largely determined by the molecular orientation and resultant permanent dipole moment, yet this property is difficult to control during film preparation. Here, we demonstrate the active control of dipole direction-that is, vector direction and magnitude-in organic glassy films by physical vapour deposition. An organic glassy film with metastable permanent dipole moment orientation can be obtained by utilizing the small surface free energy of a trifluoromethyl unit and intramolecular permanent dipole moment induced by functional groups.

Significant role of spin-triplet state for exciton dissociation in organic solids.

Clarification of the role of the spin state that initiates exciton dissociation is critical to attaining a fundamental understanding of the mechanism of organic photovoltaics. Although an excited spin-triplet state with an energy lower than that of excited spin-singlet state is disadvantageous in exciton dissociation, a small electron exchange integral results in small singlet-triplet energy splitting in some material systems. This energy splitting leads to a nearly isoenergetic alignment of both excited states, raising a question about the role of excited spin states in exciton dissociation.

Efficiency of Thermally Activated Delayed Fluorescence Sensitized Triplet Upconversion Doubled in Three-Component System.

As in many fields, the most exciting endeavors in photon upconversion research focus on increasing the efficiency (upconversion quantum yield) and performance (anti-Stokes shift) while diminishing the cost of production. In this vein, studies employing metal-free thermally activated delayed fluorescence (TADF) sensitizers have garnered increased interest. Here, for the first time, the strategy of ternary photon upconversion is utilized with the TADF sensitizer 2,4,5,6-tetrakis(carbazol-9-yl)isophthalonitrile (4CzIPN), resulting in a doubling of the upconversion quantum yield in comparison to the binary system employing p-terphenyl as the emitter.

Tetrabenzo[a,c]phenazine Backbone for Highly Efficient Orange-Red Thermally Activated Delayed Fluorescence with Completely Horizontal Molecular Orientation.

Three thermally activated delayed fluorescence (TADF) molecules, namely PQ1, PQ2, and PQ3, are composed of electron-accepting (A) tetrabenzo[a,c]phenazine (TBPZ) and electron-donating (D) phenoxazine (PXZ) units are designed and characterized. The combined effects of planar acceptor manipulation and high steric hindrance between D and A units endow high molecular rigidity that suppresses nonradiative decay of the excitons with improved photoluminescence quantum yields (PLQYs). Particularly, the well-aligned excited states involving a singlet and a triplet charge-transfer excited states and a localized excited triplet state in PQ3 enhances the reverse intersystem crossing rate constant (k ) with a short delay lifetime (τ ).

Highly Efficient Near-Infrared Electrofluorescence from a Thermally Activated Delayed Fluorescence Molecule.

Near-IR organic light-emitting diodes (NIR-OLEDs) are potential light-sources for various sensing applications as OLEDs have unique features such as ultra-flexibility and low-cost fabrication. However, the low external electroluminescence (EL) quantum efficiency (EQE) of NIR-OLEDs is a critical obstacle for potential applications. Here, we demonstrate a highly efficient NIR emitter with thermally activated delayed fluorescence (TADF) and its application to NIR-OLEDs.

Precise Exciton Management of Quaternary Emission Layers for Highly Stable Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence.

Simultaneous achievement of both high electroluminescence efficiency and high operational stability in organic light-emitting diodes (OLEDs) is required for their use in various practical applications. Although OLEDs based on thermally activated delayed fluorescence-assisted fluorescence (TAF) are considered to possess a promising device architecture to exploit the full potential of OLEDs, the operational stability of such systems still requires further improvement. In this study, a quaternary emission layer consisting of a combination of TAF and mixed-host systems is developed.

Molecular Design Based on Donor-Weak Donor Scaffold for Blue Thermally-Activated Delayed Fluorescence Designed by Combinatorial DFT Calculations.

Quantum chemical calculations are necessary to develop advanced emitter materials showing thermally-activated delayed fluorescence (TADF) for organic light-emitting diodes (OLEDs). However, calculation costs become problematic when more accurate functionals were used, therefore it is judicious to use a multimethod approach for efficiency. Here we employed combinatorial chemistry to develop the deep blue TADF materials with a new concept of homo-junction design.

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff.

Aromatic organic deep-blue emitters that exhibit thermally activated delayed fluorescence (TADF) can harvest all excitons in electrically generated singlets and triplets as light emission. However, blue TADF emitters generally have long exciton lifetimes, leading to severe efficiency decrease, i.e.

Utilization of Multi-Heterodonors in Thermally Activated Delayed Fluorescence Molecules and Their High Performance Bluish-Green Organic Light-Emitting Diodes.

We report a series of pentacarbazolyl-benzonitrile derivatives such as 2,4,6-tri(9-carbazol-9-yl)-3,5-bis(3,6-di(pyridin-3-yl)-9-carbazol-9-yl)benzonitrile (), 3,5-bis(3,6-bis(4-(trifluoromethyl)phenyl)-9-carbazol-9-yl)-2,4,6-tri(9-carbazol-9-yl)benzonitrile (), 2,4,6-tri(9-carbazol-9-yl)-3-(3,6-di(pyridin-3-yl)-9-carbazol-9-yl)-5-(3,6-diphenyl-9-carbazol-9-yl)benzonitrile (), 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9-carbazol-9-yl)-2,4,6-tri(9-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9-carbazol-9-yl)benzonitrile (), and 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9-carbazol-9-yl)-2,6-di(9-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9-carbazol-9-yl)-4-(9-pyrido[3,4-]indol-9-yl)benzonitrile () in which some of the carbazoles are substituted with modified 3,5-diphenyl carbazoles, exhibiting thermally activated delayed fluorescence (TADF) properties. These emitters comprised two, three, and four different types of donors, capable of bluish-green emission of around 480 nm with relatively high photoluminescence quantum yields over 90% in solution. Emitters, namely, , , and , composed of three and four different types of donors endowed a rather short delayed lifetime (τ) of 4.

The Role of Reverse Intersystem Crossing Using a TADF-Type Acceptor Molecule on the Device Stability of Exciplex-Based Organic Light-Emitting Diodes.

Exciplex system exhibiting thermally activated delayed fluorescence (TADF) holds a considerable potential to improve organic light-emitting diode (OLED) performances. However, the operational lifetime of current exciplex-based devices, unfortunately, falls far behind the requirement for commercialization. Herein, rationally choosing a TADF-type electron acceptor molecule is reported as a new strategy to enhance OLEDs' operating lifetime.

Observation of Nonradiative Deactivation Behavior from Singlet and Triplet States of Thermally Activated Delayed Fluorescence Emitters in Solution.

Excited states of emissive organic molecules undergo various kinds of quenching phenomena such as vibration-coupled quenching depending on their environmental conditions. Because bright singlet excitons in purely organic molecules exhibiting thermally activated delayed fluorescence (TADF) can access dark triplet excited states, photogenerated singlet excitons can decay nonradiatively through both singlet and triplet excited states. Here, we investigated nonradiative decay behavior, including internal and external exciton quenching processes, of various types of TADF materials in solution.

Slow recombination of spontaneously dissociated organic fluorophore excitons.

The harvesting of excitons as luminescence by organic fluorophores forms the basis of light-emitting applications. Although high photoluminescence quantum yield is essential for efficient light emission, concentration-dependent quenching of the emissive exciton is generally observed. Here we demonstrate generation and accumulation of concentration-dependent "long-lived" (i.