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In phosphor-sensitized-fluorescent organic light-emitting diodes (PSF-OLEDs), triplet excitons generated in the emitting layer (EML) are harvested by the sensitizer and the excitonic energy is efficiently transferred to the fluorescent dopant molecules. Since a phosphorescent sensitizer is typically doped at around 10 wt.%, the sensitizer not only affects cascade energy transfer but also electrical transporting in the PSF-OLEDs. However, the electrical role of phosphorescent metal complex molecules in the EML is unclear. In this study, we report the impact of the phosphorescent sensitizer on the electrical properties of PSF-OLEDs. Through a comprehensive analysis using impedance spectroscopy in conjunction with current density (J)-voltage (V)-luminance (L) properties of the PSF-OLED, it was revealed that 10 wt.% phosphorescent sensitizer forms an electron transporting pathway inside the EML. Thus, the sensitizer molecules transform a unipolar single host into a bipolar mixed host. In other words, the phosphorescent sensitizer is an n-type host in the PSF-OLEDs. As a result, the charge balance and the operating voltage were simultaneously and significantly improved in the PSF-OLEDs. [ABSTRACT FROM AUTHOR]

A series of metal-quinolate (MQ 2) materials (M = Be2+, Mg2+, Ca2+, and Sr2+) were theoretically investigated to understand their electronic and charge transport properties as an application of electron injection layer (EIL) and additive material to electron transporting layer (ETL). It was found that the Be2+, Mg2+, and Ca2+ are tetragonally coordinated to the quinolate ligands whereas the Sr2+ is planarly coordinated. The analyses of electronic properties indicated that the stable lowest unoccupied molecular orbital (LUMO) energy level of these materials relative to LiQ is expected to improve the electron injection from cathode and enhance the electron transport in ETL. In perspective of electrochemical reactivity, the chemical hardness values of MQ 2 materials showed that hole and electron carriers can be easily facilitated in order of BeQ 2 < MgQ 2 < CaQ 2 < SrQ 2. In particular, the results of charge carrier transport properties clearly showed that k e of SrQ 2 is 64% higher than that of LiQ, which is an advantage for both EIL and ETL additive. Comprehensive analyses showed that SrQ2 is superior to LiQ as a replacement in perspective of electron injection and transport properties. Image 1 • The effect of various metal ions on electronic properties of multiligand metal quinolates was investigated by DFT simulation. • The electronic and electrical properties of SrQ 2 were found to be superior to those of LiQ. • SrQ 2 can be potentially utilized as electron injection layer and/or additive material to electron transport layer. [ABSTRACT FROM AUTHOR]

High triplet energy exciplex hosts are developed using new electron transport type hosts with triphenylsilyl blocking groups to obtain high solid state triplet energy and exciplex triplet energy. Two triphenyltriazine derived hosts, 2‐phenyl‐4,6‐bis(4‐(triphenylsilyl)phenyl)‐1,3,5‐triazine (pSiTrz) and 2‐phenyl‐4,6‐bis(3‐(triphenylsilyl)phenyl)‐1,3,5‐triazine (mSiTrz), are synthesized as the high triplet energy hosts with electron transport properties. The two hosts show high triplet energy over 2.90 eV both in solution and solid film, and afford high triplet energy over 2.90 eV in the mixed hosts with a hole transport type 2,2′‐di(9H‐carbazol‐9‐yl)‐1,1′‐biphenyl (oCBP) host. The doping of a deep blue phosphorescent emitter in the mixed host oCBP:mSiTrz achieves high external quantum efficiency over 20% with a color coordinate of (0.14, 0.16) and device lifetime of longer than 1000 h at 100 cd m‐2. This is the first demonstration of simultaneous achievement over 20% external quantum efficiency (EQE) and 1000 h lifetime in the deep blue organic light‐emitting diodes with a x+y color coordinate value below 0.30. [ABSTRACT FROM AUTHOR]

Comprehensive study to understand the degradation mechanism of high efficiency blue organic light-emitting diodes by charge transport layers was carried out by managing the device parameters and charge transport materials. Although it has been generally accepted that the charge transport layers are important for long lifetime, the degradation mechanism was not clear. From this work, it was revealed that exciton quenching of emitters by the degradation products of charge transport layers is the main degradation pathway of blue phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes. It was described that the degradation of the blue devices by the charge transport layers would be largely avoided by separating the exciton generating emitters from the charge transport layers by shifting the exciton formation zone to the center of the emitting layer. Eventually, the lifetime of the blue devices with the emission zone fall apart from the charge transport layers was not affected by the charge transport layers because exciton quenching by the degradation products of the charge transport layers was suppressed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Density functional theory calculations were performed to understand the charge transport property of Liq-mixed ETL matrix. • The stabilization energy calculations revealed that forming the ETL-Liq complex is favorable. • The co-deposition of Liq in ETL matrix generally weakens the charge transport property via forming an ETL-Liq complex. The effect of coordinated structures of electron transporting materials (ETM s) with lithium quinolate (Liq) on charge transport characteristics was theoretically investigated. The reorganization energies of holes (λ h) and electrons (λ e) of ETM - Liq simultaneously increased compared with those of ETMs, indicating that the charge transporting is retarded. Interestingly, λ e of ETM -(Liq) n increased, while λ h decreased. Regarding transfer integrals for holes (t h) and electrons (t e), t h 's of T2T - Liq and BPyTP2 - Liq were significantly reduced with t e of T2T - Liq decreasing tremendously. Consequently, the hopping rates of holes of Liq- complexes were reduced, while that of electrons of BPyTP2 - Liq increased and that of T2T - Liq / TSPO1 - Liq was reduced. [ABSTRACT FROM AUTHOR]

In general, electron transport layer (ETL) in organic light-emtting diodes (OLEDs) consists of single component of electron transporting material (ETM) or a mixture with n-dopant such as 8-hydroxyquinolinolato-lithium (Liq). However, there exists a limit to controlling a wide range of carrier density in OLEDs according to the required characteristics of the devices due to electrically insulating property of Liq. Here, we suggest a universal strategy to construct an efficient ETL. We synthesized two ETMs, diphenyl-[4-(10-phenyl-anthracene-9-yl)-phenyl]-amine (An-Ph) and phneyl-[4-(10-phenyl-anthracene-9-yl)-phenyl]-pyridin-3-yl-amine (An-Py) that have the same core structures with different polarities in functional groups. The electrical characteristics of electron-only-devices (EODs) were investigated by space charge limited current (SCLC) modeling and impedance spectroscopy analysis. Interestingly, the homostructure type ETL composed of An-Ph and An-Py showed not only superior electron transporting capability, but also the possibility of controlling electron injection and transporting in a wide range compared to the heterostructure type ETL of An-Ph and Liq. Compared to the An-Ph-only EOD, the electron mobility in 75% An-Py-mixed homostructure EOD increased by almost 4 orders of magnitude. Such dramatic variation of electron mobility was achieved thanks to the molecular design strategy to separate charge injection and charge transport regions within a molecule, which consequently induced the giant surface potential (GSP) effect between the ETL/cathode interface. As a result, the external quantum efficiency (EQE) of blue fluorescent and phosphorescent OLEDs with the homostructure ETLs was enhanced by 28.6% and 34%, respectively, compared to that of each control device without manipulating outcoupling effects. [Display omitted] • The anthracene-based electron transporting materials, An-Py and An-Ph with different polarities were designed and synthesized for universal electron transporting layers. • The superior charge injection property was comprehensively investigated with impedance spectroscopy and attributed to the giant surface potential in the ETL. • Compared to Liq-based ETL, the EQE of fluorescent and phosphorescent OLEDs with An-Py and An-Ph ETL was enhanced by 28.6% and 34%, respectively. [ABSTRACT FROM AUTHOR]