Yue-Feng Liu

Solving Efficiency–Stability Tradeoff in Top‐Emitting Organic Light‐Emitting Devices by Employing Periodically Corrugated Metallic Cathode

The introduction of a periodic corrugation into TOLEDs is demonstrated to be effective in relieving the tradeoff between device stability and efficiency, through the cross coupling of the SPPs associated with the Ag cathode and the microcavity modes. The thickness of the Ag cathode for the corrugated TOLEDs was increased from 20 to 45 nm, and both the device lifetime and efficiency are significantly improved. The figure shows a schematic cross section of a red TOLED with periodic microstructure and an operating TOLED with both corrugated and planar area.

Enhanced efficiency of organic light-emitting devices with metallic electrodes by integrating periodically corrugated structure

Photons trapped in form of surface-plasmon polariton (SPP) modes associated with the metallic electrode/organic interface results in a large energy loss in organic light-emitting devices (OLEDs). We demonstrate efficient outcoupling of SPP modes from one of two metal electrodes by integrating a periodic wavelength-scale corrugation into the device structure. 30% enhancement in efficiency has been obtained from the corrugated OLEDs with appropriate grating period. The efficient outcoupling of the SPPs has been verified by numerical simulations of both absorption spectra and field distribution.

Broadband Light Extraction from White Organic Light‐Emitting Devices by Employing Corrugated Metallic Electrodes with Dual Periodicity

A dual-periodic corrugation consisting of two sets of gratings with different periods to realize a broadband light extraction in white organic light-emitting diodes (WOLEDs) is shown. A 37% enhancement in current efficiency and 48% enhancement in the external quantum efficiency compared to those of the conventional planar devices have been obtained. Besides the much improved efficiency, the dual-periodic corrugated WOLEDs exhibit satisfying viewing characteristics.

Enhanced hole injection in organic light-emitting devices by using Fe3O4 as an anodic buffer layer

Hole injection improvement in organic light-emitting devices with Fe3O4 as a buffer layer on indium tin oxide (ITO) has been demonstrated. The luminance and the current density are significantly enhanced by using the Fe3O4/ITO anode, as well as the turn-on voltage is reduced by 1.5 V compared to the devices without the buffer. Results of atom force microscopy, x ray, and UV photoelectron spectroscopy studies reveal that the enhanced hole injection is attributed to the modification of the ITO surface and the reduced hole-injection barrier by the insertion of the Fe3O4 thin film between the ITO and hole-transporting layer.

Efficient and mechanically robust stretchable organic light-emitting devices by a laser-programmable buckling process

Stretchable organic light-emitting devices are becoming increasingly important in the fast-growing fields of wearable displays, biomedical devices and health-monitoring technology. Although highly stretchable devices have been demonstrated, their luminous efficiency and mechanical stability remain impractical for the purposes of real-life applications. This is due to significant challenges arising from the high strain-induced limitations on the structure design of the device, the materials used and the difficulty of controlling the stretch-release process. Here we have developed a laser-programmable buckling process to overcome these obstacles and realize a highly stretchable organic light-emitting diode with unprecedented efficiency and mechanical robustness. The strained device luminous efficiency −70 cd A−1 under 70% strain - is the largest to date and the device can accommodate 100% strain while exhibiting only small fluctuations in performance over 15,000 stretch-release cycles. This work paves the way towards fully stretchable organic light-emitting diodes that can be used in wearable electronic devices.

Omnidirectional emission from top-emitting organic light-emitting devices with microstructured cavity

We demonstrate optimized viewing-angle characteristics from top-emitting organic light-emitting devices by integrating a periodic microstructure into the cavity. A holographic lithography technique combined with filling process of the groove by spin coating of a polymer film has been employed to enable its periodically and gradually changed cavity length and suppress the viewing-angle dependence of the peak emission wavelength and intensity. The theoretical and experimental results support that the proposed microstructured cavity can resolve the angular-dependence effect in a very simple and effective way, and a desired omnidirectional emission has been obtained.

Highly flexible inverted organic solar cells with improved performance by using an ultrasmooth Ag cathode

Inverted organic solar cells (OSCs) with high efficiency and flexibility have been demonstrated. A thick Ag film with ultrasmooth morphology fabricated on a photopolymer substrate by template-stripping process and a semitransparent Ag film has been employed as cathode and anode of the top-illuminated OSCs, respectively. An improved performance has been obtained compared with that of the OSCs deposited on Si substrate due to the enhanced charge extraction and reduced charge loss resulted from the employment of the ultrasmooth cathode. Moreover, the flexible OSCs obtained by this method keep good performance under a small bending radius and after repeated bending.

Simultaneous efficiency enhancement and self-cleaning effect of white organic light-emitting devices by flexible antireflective films

In this Letter, we report the improved light outcoupling efficiency of conventional white organic light-emitting devices (OLEDs) by a kind of multifunctional film with both antireflective and superhydrophobic ability. This film consisted of regular polydimethylsiloxane (PDMS) nanopillar arrays, which were readily batch produced by low-cost imprint lithography. The nanopillar arrays could effectively eliminate the light total reflection and enhance the device efficiency of OLEDs by producing the gradual refractive index due to the decreasing material density from glass to air. Moreover, owing to its superhydrophobicity (contact angle ∼151°), the antireflective film exhibited self-cleaning ability, which was beneficial for keeping the OLEDs substrate clean and ensure the high efficiency of OLEDs. This method is simple, cost-effective, and reproducible. The OLEDs showed an efficiency enhancement of 25% with the multifunctional film.

Hybrid Tamm plasmon-polariton/microcavity modes for white top-emitting organic light-emitting devices

White top-emitting organic light-emitting devices (WTOLEDs), emitting white light through a transparent top metallic electrode, have emerged as promising candidates as energy-efficient solid-state lighting sources and full-color flat-panel displays. The microcavity effect due to usage of metallic electrodes results in emission enhancement solely at a particular color, and therefore sets an obstacle for WTOLEDs, where at least two colors with balanced intensity should be emitted. Current efforts solving the problem basically rely on the relaxation of the microcavity effect, resulting in sacrificed light outcoupling efficiency in the original resonance region. Here, we demonstrate that by integrating a photonic crystal structure upon the top metallic electrode, an additional emission enhancement peak other than the one determined by the microcavity resonance could be provided by the Tamm plasmon-polariton mode. Mode hybridization induced dual hybrid modes with comparable light outcoupling efficiency can then be excited, from which two colors with balanced intensity could be emitted. Both experimental and theoretical results demonstrate that the proposed mode hybridization strategy may pave the way for the realization of WTOELDs towards high white color quality, improved viewing characteristics, and electroluminescence efficiency.

Efficiency Enhancement in Organic Light-Emitting Devices With a Magnetic Doped Hole-Transport Layer

Magnetic field effects on tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices (OLEDs) by employing Fe3O4 as a magnetic dopant in the hole-transport layer (HTL) have been studied. The magnetic doped OLEDs exhibit efficient injection and transport of holes, and its performances are further enhanced after a magnetic field is applied. The enhancement of luminance and current efficiency of 20% and 24% has been obtained from the magnetic doped devices, while they are only 8% and 9%, respectively, for the nondoped devices under an applied magnetic field of 500 mT. Organic magnetoresistance induced by the magnetic doped HTL is the main origin of increased electroluminescence for the magnetic doped OLEDs.