Yan-Gang Bi

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.

Surface-plasmon enhanced absorption in organic solar cells by employing a periodically corrugated metallic electrode

We demonstrate improved efficiency of organic solar cells (OSCs) by employing a periodically corrugated metallic electrode in the OSCs. The improved efficiency can be attributed to the absorption enhancement resulted from the excitation of propagating surface-plasmon polariton (SPP) modes at the corrugated metal/organic interface. Through tuning the SPP resonance to the intrinsic absorption region, the short circuit current of the corrugated device with appropriate period has been increased from 4.1 mA/cm2 for planar device to 5.5 mA/cm2. The power conversion efficiency exhibits an enhancement of 35%.

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.

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 and efficient top-emitting organic light-emitting devices with ultrasmooth Ag anode.

We demonstrate highly flexible and efficient top-emitting organic light-emitting devices (TOLEDs) by using an ultrasmooth Ag anode. A template-stripping process has been employed to create the ultrasmooth Ag anode on a photopolymer substrate. The flexible TOLEDs obtained by this method keep good electroluminescence properties under a small bending radius and after repeated bending. The efficiency of the flexible TOLEDs is improved by 60% compared with the conventional TOLEDs deposited on Si substrate due to the enhanced hole injection from the ultrasmooth anode.

Surface Plasmon-Polariton Mediated Red Emission from Organic Light-Emitting Devices Based on Metallic Electrodes Integrated with Dual-Periodic Corrugation

We demonstrate an effective approach to realize excitation and outcoupling of the SPP modes associated with both cathode/organic and anode/organic interfaces in OLEDs by integrating dual-periodic corrugation. The dual-periodic corrugation consists of two set gratings with different periods. The light trapped in the SPP modes associated with both top and bottom electrode/organic interfaces are efficiently extracted from the OLEDs by adjusting appropriate periods of two set corrugations, and a 29% enhancement in the current efficiency has been obtained.

Enhanced efficiency of organic light-emitting devices with corrugated nanostructures based on soft nano-imprinting lithography

An enhanced efficiency organic light-emitting device (OLED) with corrugated nanostructures on a small-molecule organic film has been demonstrated. By patterning the hole transport layer via soft nano-imprinting lithography and coating with Ag, a nanostructured cathode is introduced to enhance the light extraction of the OLED without affecting the flatness and conductivity of the indium-tin-oxide film. Both luminance and current efficiency are improved compared with those of conventional planar devices. The observable improvement in luminance and current efficiency can be ascribed to the surface plasmonic and scattering effects caused by the Ag nanostructures. Moreover, theoretical simulations also demonstrate that the power loss to surface plasmon-polariton modes has been recovered.

Improved Performance of ITO-Free Organic Solar Cells Using a Low-Workfunction and Periodically Corrugated Metallic Cathode

The indium–tin-oxide-free organic solar cells (OSCs) have been fabricated using a semitransparent Au film as anode. A comparative study between the OSCs with different metal cathodes shows that formation of an ideal ohmic contact at low-workfunction Ca and the organic interface reduces series resistance and increases the carrier collection. Moreover, a periodic corrugation was introduced into the device structure, which leads to an enhanced light absorption in the OSCs by elongating the optical path of the incident light inside the absorber material. The highest power conversion efficiency of an optimal OSC with a Au anode and a corrugated Ca/Ag cathode is 1.4%, which is significantly improved in contrast to 0.61% for the OSC with a planar Al cathode.