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The exciplex forming co-host with phosphorescent dopant system has potential to realize highly efficient phosphorescent organic light emitting didoes (PhOLEDs). However, the exciplex forming co-host for blue phosphorescent OLEDs has been rarely introduced because of higher triplet level of the blue dopant than green and red dopants. In this work, a novel exciplex forming co-host with high triplet energy level is developed by mixing a phosphine oxide based electron transporting material, PO-T2T, and a hole transporting material, N,N′-dicarbazolyl-3,5-benzene (mCP). Photo-physical analysis shows that the exciplexes are formed efficiently in the host and the energy transfer from the exciplex to blue phosphorescent dopant (iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate; FIrpic) is also efficient, enabling the triplet harvest without energy loss. As a result, an unprecedented high performance blue PhOLED with the exciplex forming co-host is demonstrated, showing a maximum external quantum efficiency (EQE) of 30.3%, a maximum power efficiency of 66 lm W−1, and low driving voltage of 2.75 at 100 cd m−2, 3.29 V at 1000 cd m−2, and 4.65 V at 10 000 cd m−2, respectively. The importance of the exciton confinement in the exciplex forming co-host is further investigated which is directly related to the performance of PhOLEDs.

The use of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) thin layers, particularly the solution-processed type, as an efficient hole-injection layer (HIL) for organic optoelectronic devices is demonstrated herein. Among the solvents commonly used for solution processing, 2-propanone was found to selectively dissolve HAT-CN, allowing the fabrication of a rigid film. The alignment of the electronic energy levels of the solution-processed HAT-CN and thermally polymerized 2,7-disubstituted fluorene-based triaryldiamine (VB-FNPD) species was evaluated using ultraviolet photoelectron spectroscopy. The results revealed that the lowest unoccupied molecular orbital of HAT-CN and the highest occupied molecular orbital of VB-FNPD were very close to the Fermi level, which facilitated charge transfer at the interface and improved hole injection. The utilization of HAT-CN as HIL resulted in a dramatic enhancement of the performance of solution-processed red, green, and blue organic light-emitting diodes. The external quantum efficiency, current efficiency, and power efficiency of the HAT-CN-based devices were higher than or almost similar to those of optimized poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based devices. Because of the efficient carrier-injection capability and the capacity to prevent interfacial mixing and erosion during fabrication, solution-processed HAT-CN is promising as a novel alternative to conventional PEDOT:PSS HILs.

The correlation of accumulation charges at the interfaces of organic layers and carrier mobility in organic light emitting devices (OLEDs) were investigated through the impedance versus voltage (Z–V) characteristics of devices. The properties of devices with various combinations of cathode structures, HTLs and ETLs were investigated to understand the impedance transition in Z–V characteristics of OLEDs. It was observed that there is an extra impedance transition before devices turn on when the hole mobility in the HTL is much greater than the electron mobility in the ETL in the devices, which makes the Z–V characteristics a potential tool to compare the electron mobility in ETL and hole mobility in HTL.

The improvement of current injection efficiency in organic light emitting diodes is demonstrated with incorporation of SubPc-doped NPB as hole injection layers. The photoemission spectra illustrate the presence of gap states and the occurrence of axial reaction on SubPc with NPB. Quantum chemistry calculation provides simulated valence-band spectra indicating that the formation of gap states is due to the HOMOs of new complexes created from the reaction of SubPc and NPB, which facilitate the hole injection at the anode interfaces.

Graphene thin films have great potential to function as transparent electrodes in organic electronic devices, due to their excellent conductivity and high transparency. Recently, organic light-emitting diodes (OLEDs)have been successfully demonstrated to possess high luminous efficiencies with p-doped graphene anodes. However, reliable methods to fabricate n-doped graphene cathodes have been lacking, which would limit the application of graphene in flexible electronics. In this paper, we demonstrate fully solution-processed OLEDs with n-type doped multilayer graphene as the top electrode. The work function and sheet resistance of graphene are modified by an aqueous process which can also transfer graphene on organic devices as the top electrodes. With n-doped graphene layers used as the top cathode, all-solution processed transparent OLEDs can be fabricated without any vacuum process.

We demonstrate the fabrication of solution-processed MoOx-treated (s-MoOx) silver nanowire (AgNW) transparent conductive electrodes (TCEs) utilizing low-temperature (sub-100 °C) processes. The s-MoOx aggregates around the AgNW and forms gauze-like MoOx thin films between the mesh, which can effectively lower the sheet resistance by more than two orders of magnitude. Notably, these s-MoOx-treated AgNW TCEs exhibit a combination of several promising characteristics, such as a high and broad transmittance across a wavelength range of 400 to 1000 nm, transmission of up to 96.8%, a low sheet resistance of 29.8 ohm sq(-1), a low haze value of 0.90%, better mechanical properties against bending and adhesion tests, and preferable gap states for efficient hole injection in optoelectronic applications. By utilizing these s-MoOx-treated AgNW TCEs as the anode in ITO-free organic light emitting diodes, promising performance of 29.2 lm W(-1) and 10.3% external quantum efficiency are demonstrated. The versatile, multi-functional s-MoOx treatment presented here paves the way for the use of low-temperature, solution-processed MoOx as both a nanowire linker and a hole injection interfacial layer for future flexible optoelectronic devices.

We demonstrate an alternating current (AC)-driven organic light emitting diodes (OLED) with lithium fluoride (LiF) insulating layers fabricated using simple thermal evaporation. Thermal evaporated LiF provides high stability and excellent capacitance for insulating layers in AC devices. The device requires a relatively low turn-on voltage of 7.1 V with maximum luminance of 87 cd/m(2) obtained at 10 kHz and 15 Vrms. Ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy are employed simultaneously to examine the electronic band structure of the materials in AC-driven OLED and to elucidate the operating mechanism, optical properties and electrical characteristics. The time-resolved luminance is also used to verify the device performance when driven by AC voltage.

Pretreatment of the indium tin oxide (ITO) surface is generally adopted to improve the charge injection and device performance in the fabrication of organic light-emitting diodes (OLEDs). For the common approaches of surface treatment, such as oxygen plasma treatment, self-assembled monolayer (SAM) adsorption, and the PEDOT:PSS coating, different effects on the device lifetime were observed. A distinctly different driving voltage change with device operation time was obtained and was correlated with the device lifetime. The fast increase in driving voltage for devices based on oxygen-plasma-treated ITO is attributed to the work function change as a result of the change in the composition of the interface with device operation, whereas a rather stable work function for SAM-modified ITO is suggested due to the permanent dipoles associated with the monolayer and the protecting effect of the covalently bound monolayer on the surface composition.

The degradation of organic light-emitting diodes (OLEDs) is a very complex issue, which might include interfacial charge accumulation, material diffusion, and electrical-induced chemical reaction during the operation. In this study, the origins of improvement in device stability from inserting a hole injection layer (HIL) at the indium tin oxide (ITO) anode are investigated. The results from aging single-layer devices show that leakage current increases in the case of ITO/hole transport layer contact, but this phenomenon can be prevented by inserting molybdenum oxide (MoO3) or 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN6) as an HIL. Moreover, X-ray photoemission spectroscopy suggests that the diffusion of indium atoms and active oxygen species can be impeded by introducing MoO3 or HAT-CN6 as an HIL. These results reveal that the degradation of OLEDs is related to indium and oxygen out-diffusion from the ITO substrates, and that the stability of OLEDs can be improved by impeding this diffusion with HILs.