A new concept for organic light-emitting diodes (OLEDs) is presented, which is called exciplex-sensitized triplet-triplet annihilation (ESTTA). The exciplex formed at the organic heterojunction interface of 4,4',4″-tris(N-3-methyphenyl-N-phenyl-amino) triphenylamine and 9,10-bis(2'-naphthyl) anthracene (ADN) is used to sensitize the triplet-triplet annihilation (TTA) process on the ADN molecules. This results in a turn-on voltage (2.2 V) of the blue emission from the OLED below the bandgap (2.9 eV). From the transient electroluminescence measurement, blue emission totally came from the TTA process without direct recombination on the ADN molecules. The blue singlet exciton from the TTA process can be quenched by energy transfer to the exciplex, as revealed by transient photoluminescence measurements. This can be prevented by blocking the energy transfer path and improving the radiative recombination rate of blue emission. With the insertion of the "triplet diffusion and singlet blocking (TDSB)" layer and the incorporation of the dopant material, an ESTTA-OLED with external quantum efficiency of 5.1% was achieved, which consists of yellow and blue emission coming from the exciplex and ESTTA process, respectively.

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The novel ambipolar hosts of o-CbzBz and o-DiCbzBz contain carbazole and benzimidazole through an ortho-connection. The orthogonal conformations cause the triplet state to be confined at the carbazole units to secure efficient energy transfer. The phosphorescent organic light-emitting diodes (PhOLEDs) show a high current efficiency, power efficiency, and low efficiency roll-off. o-DiCbzBz can be used as a host for sky-blue, green, and orange-red PhOLEDs, giving 57.5, 78.4, and 60.3 cd/A, respectively.

This paper introduces the fundamental physical characteristics of organic photovoltaic (OPV) devices. Photoelectric conversion efficiency is crucial to the evaluation of quality in OPV devices, and enhancing efficiency has been spurring on researchers to seek alternatives to this problem. In this paper, we focus on organic photovoltaic (OPV) devices and review several approaches to enhance the energy conversion efficiency of small molecular heterojunction OPV devices based on an optimal metal-phthalocyanine/fullerene (C60) planar heterojunction thin film structure. For the sake of discussion, these mechanisms have been divided into electrical and optical sections: (1) Electrical: Modification on electrodes or active regions to benefit carrier injection, charge transport and exciton dissociation; (2) Optical: Optional architectures or infilling to promote photon confinement and enhance absorption.

Four new donor–acceptor–acceptor (D–A–A) type molecules (DTCPB, DTCTB, DTCPBO, and DTCTBO), wherein benzothiadiazole or benzoxadiazole serves as the central A bridging triarylamine (D) and cyano group (terminal A), have been synthesized and characterized. The intramolecular charge-transfer character renders these molecules with strong visible light absorption and forms antiparallel dimeric crystal packing with evident π–π intermolecular interactions. The characteristics of the vacuum-processed photovoltaic device with a bulk heterojunction active layer employing these molecules as electronic donors combining C70 as electronic acceptor were examined and a clear structure–property–performance relationship was concluded. Among them, the DTCPB-based device delivers the best power conversion efficiency (PCE) up to 6.55% under AM 1.5 G irradiation. The study of PCE dependence on the light intensity indicates the DTCPB-based device exhibits superior exciton dissociation and less propensity of geminated recombination, which was further verified by a steady photoluminescence study. The DTCPB-based device was further optimized to give an improved PCE up to 6.96% with relatively high stability under AM 1.5 G continuous light-soaking for 150 h. This device can also perform a PCE close to 16% under a TLD-840 fluorescent lamp (800 lux), indicating its promising prospect for indoor photovoltaic application.

energy transfer (TET) populates the triplet state of the TTA emitter. When two triplet excitons encounter each other, the TTA process puts the emitter into its singlet excited state to emit fluorescence. One criterion for the sensitizer is efficient ISC to convert the singlet to the triplet, and sensitizers like phosphors, thermally activated delay fluorescence emitters, inorganic quantum dots, and perovskites have been used. [5][6][7] Charge transfer states between electron donor and acceptor materials can also act as sensitizers, resulting in efficient sub-bandgap emission (i.e., the driving voltage is ≈1/2 of the emitted photon energy) for organic light-emitting diode (OLED) applications. [8] For electrical excitation, 75% of the electron-hole pairs recombine as triplets without any ISC. By harnessing these triplet excitons to generate luminescence, TTAUC provides a path to higher efficiency devices.In this Communication, we show that a new device architecture can significantly improve TTAUC blue emission efficiencies, which is one of the important issues for OLED development. [9] The basic idea is to separate the triplet sensitization and upconversion layers while still allowing triplet energy migration. To demonstrate this concept, tris-(8-hydroxyquinoline)aluminum (Alq 3 ) was used as the sensitizer. This molecule has been used as an electron-transporting layer (ETL) and emitting layer material in one of the first OLED 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 (Alq 3 ), 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 Alq 3 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 Alq 3 sensitizer with lower bandgap, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) is inserted between the Alq 3 and ADN as a triplet-diffusion-and-singlet-blocking layer. DMPPP exhibits triplet energy close to Alq 3 and higher than ADN, as well as higher singlet energy than both Alq 3 and ADN. It allows triplet diffusion from Alq 3 to ADN, but blocks the SQ of the blue TTAUC signal by Alq 3 . 86.1% intrinsic efficiency of TTAUC is demonstrated in this trilayer (Alq 3 /DMPPP/ADN) OLED. Organic Light-Emitting DiodesThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Ambipolar triplet hosts comprising 1,2,4-triazole and carbazole in ortho-positions have been developed. The blue PHOLED has a high current efficiency of 47.1 cd A(-1), power efficiency of 41.2 lm W(-1), and low efficiency roll-off. The high efficiency was attributed to the successful control of π-conjugation through orthogonal arrangement of the substituents so that a wide T1-S0 gap could be maintained.

Singlet fission of organic molecules has attracted recent attention owing to its potential advantages in organic photovoltaic and electroluminescence applications. Its microscopic mechanism however remains stymied. Large couplings from charge-transfer (CT) state mediation were invoked to explain the ultrafast singlet fission rate observed in crystalline polyacene, but its experimental confirmation is still lacking. The singlet fission and triplet fusion of amorphous rubrene were investigated with time-resolved photoluminescence spectroscopy at different temperatures to extract the rates of singlet fission, triplet fusion, and triplet hopping. On the basis of the Marcus electron-transfer model, the deduced electronic coupling constant of the singlet fission process was found to be larger than that of the triplet fusion process, indicating that the singlet fission process undertakes a CT-state-mediated channel while the triplet fission process assumes a direct channel. This study thus confers supporting evidence of the existence of the CT-state-mediated channel for singlet fission of rubrene and offers an experimental approach to study singlet fission dynamics.