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The primary factor that affects the lifetime of blue FOLEDs is the bond dissociation energy (BDE) in anionic state. The BDE of host molecules has a greater impact on the lifetime than that of electron blocking layer molecules.

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.

Extremely stable white emission out of a hybrid white quantum dot-organic light-emitting diode (WQD-OLED) was achieved by developing a novel concept of device architecture. The new inverted device structure employs a thermally-evaporated red phosphorescent emitting layer (EML) with an n-type modulation and a multi-stacked hole transporting layer (HTL) on the top of solution-processed ZnO nanoparticles for an electron transporting layer and blue and green QD-mixed EML. The multi-stacked HTL, tris(4-carbazoyl-9-ylphenyl)amine (TCTA)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), and the n-type modulation layer, 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) successfully balanced low-mobility holes with high-mobility electrons, and uniformly distributed the charges across the blue/green QD and red EMLs. In particular, the role of the n-type modulation layer was comprehensively analyzed with impedance spectroscopy and verified to minimize the undesired Auger recombination by excess charges and broaden the recombination zone. Consequently, highly stable white emission (0.32 ± 0.02, 0.31 ± 0.02) in CIExy color coordinates over 4 V operating voltage range (or two decades of current density), 3.92% external quantum efficiency, and 3.83 h device lifetime at 500 cd m−2 up to 50% of the initial luminance (LT50) were simultaneously obtained.

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.

Phosphorescent organic light-emitting diodes (PHOLEDs) have shown a heterojunction system that is a bulk heterojunctioned emitting layer (EML). In our work, we have demonstrated an improvement in efficiency with a partial mixed host system in an EML. A mixed host system, which is mixed with a host material and an electron transporting material, was used on the red PHOLEDs. In general, the recombination zone is located at the electron transporting layer (ETL) side in typical PHOLEDs due to excessive accumulation at the interface. However, this study proposed the removal of interfaces through the advantage of a mixed host system and a mixed layer between EML and ETL sides to increase the electron mobility of the experiments. Detailed optical and electrical analysis of these phenomena, were demonstrated.

The authors have demonstrated a highly efficient and long-lifetime blue phosphorescent organic light emitting diode (PHOLED) that uses two types of mixed layers. The mixed layers play the role of carrier injection control and exciton generation zone extension. One of the layers is applied for mixing the hole transport layer (HTL) and host material at the HTL side for carrier injection control. The other works as a mixed electron transporting layer (ETL) and host material at the ETL side. The optimized blue PHOLED has been shown to achieve high performance owing to the mixed layer effects. It gave a maximum luminous efficiency of 25.55 cd/A, maximum external quantum efficiency of 13.05%, and lifetime of 7.24 h under 500 cd/m2. These results indicate that applying mixed layers is a simple and efficient method that does not require significant structural change.

In this paper, we fabricated tri-metal layered thin film semitransparent electrodes consisting of a thin conductive metal layer, sandwiched between two nickel layers. An equal red phosphorescent organic light-emitting diode (PHOLED) structure was deposited on the anodes of indium tin oxide (ITO) and three types of tri-metal layers (Ni/Al/Ni, Ni/Cu/Ni, and Ni/Ag/Ni, thickness of 3/7/3 nm in common) on a glass substrate. The optical and electrical performances of the device using Ni/Ag/Ni were improved more than the performances of the other devices due to the micro-cavity effect in accordance with the various electrode characteristics. Moreover, we fabricated the same red PHOLED structures on a flexible substrate, as a consequence, showed competitive emission characteristics compared to the devices fabricated on a glass substrate. Therefore, this study could succeed to additional research on flexible display panel and light-emitting devices with ITO-free electrodes.

In this study, we fabricated hybrid white organic light-emitting diodes (WOLEDs) based on triplet harvesting with simple structure. All the hole transporting material and host in emitting layer (EML) of devices were utilized with same material by using N,N'-di-1-naphthalenyl-N,N'-diphenyl-[1,1':4',1":4",1"'-quaterphenyl]-4,4"'-diamine (4P-NPD) which were known to be blue fluorescent material. Simple hybrid WOLEDs were fabricated three color with blue fluorescent and green, red phosphorescent materials. We was investigated the effect of triplet harvesting (TH) by exciton generation zone on simple hybrid WOLEDs. Characteristic of simple hybrid WOLEDs were dominant hole mobility, therefore exciton generation zone was expected in EML. Additionally, we was optimization thickness of hole transporting layer and electron transporting layer was fabricated a simple hybrid WOLEDs. Simple hybrid WOLED exhibits maximum luminous efficiency of 29.3 cd/A and maximum external quantum efficiency of 11.2%. Commission Internationale de l'Eclairage (International Commission on Illumination) coordinates of (0.45, 0.43) at about 10,000 cd/m^2., Comment: 16 page, 5 figures, 1 table

In this work, we proposed that indium-tin-oxide (ITO)-free, which is the most generally used to a transparent electrode for light applications and displays, semitransparent Ni/Ag/Ni electrode could be applied in flexible blue phosphorescent organic light-emitting diodes (OLEDs). Although the highest electrical characteristics values, such as luminance efficiency and external quantum efficiency, using ITO electrode were better than that of devices using Ni/Ag/Ni electrodes, roll-off efficiency and device stability were suitable for blue OLEDs. Therefore, this study could be suggested to additional flexible OLEDs and light applications for ITO-free process.

The hybrid blue organic light-emitting diodes (HB OLEDs) with triplet harvesting (TH) structures within an emitting layer (EML) are fabricated with fluorescent and phosphorescent EMLs. The TH is to transfer triplet excitons from fluorescence to phosphorescence, where they can decay radiatively. Remarkably, the half-decay lifetime of a hybrid blue device with fluorescent and phosphorescent EML thickness of 5 and 25 nm, measured at an initial luminance of 500 cd/m2, has improved twice than that of using a conventional structure. Additionally, the blue device's efficiency improved. We attribute this improvement to the efficient triplet excitons energy transfer and the optimized distribution of the EML which depends on singlet and triplet excitons diffusion length that occurs within each the EML.