Tsung-Hao Su

Transparent Organic Upconversion Devices for Near‐Infrared Sensing

Transparent organic upconversion devices are shown in a night-vision demonstration of a real object under near-infrared (NIR) illumination in the dark. An extraordinarily high current gain - reflecting the on-off switching effect - greater than 15 000 at a driving voltage of 3 V is demonstrated, indicating the high sensitivity to NIR light and potential of using the proposed upconverter in practical applications. A maximum luminance exceeding 1500 cd m(-2) at 7 V is achieved. Unlike previous studies, where 2D aperture projection is reported, the current study shows 3D images of real objects under NIR illumination in the dark.

Decoupling the optical and electrical properties of subphthalocyanine/C70 bi-layer organic photovoltaic devices: improved photocurrent while maintaining a high open-circuit voltage and fill factor

We demonstrate a simple method for achieving high-performance subphthalocyanine (SubPc)/C70 bi-layer organic photovoltaic (OPV) devices through the changing of the C70 thickness. The optical and electrical properties of the OPV devices were decoupled and could be individually manipulated to obtain a significantly increased short-circuit current density (JSC) without reducing the open-circuit voltage and the fill factor. The thickness-independent electrical property of the C70 layer was systematically studied in terms of the dark currents of the OPV devices and the carrier mobilities of the organic layers; the results indicate that the considerable difference in mobility between SubPc and C70 is not detrimental, while the optical-field distribution can be optimized by tuning the C70 thickness. The power conversion efficiency was improved from 2.7 to 4.2% by optimizing the C70 thickness. The optical effect upon the change in the C70 thickness was thoroughly investigated by calculating the optical-field profile and the power dissipation inside the OPV devices on the basis of the transfer matrix method. The calculated results suggest that the optical-field intensity is insufficient in predicting the trend in JSC. Instead, the power dissipation involving the absorption properties of materials and the optical-field distribution of OPV devices can provide deeper insight into the optical condition and indicates the importance of optimizing the film thickness in bi-layer OPV devices.

Cathodic-controlled and near-infrared organic upconverter for local blood vessels mapping.

Organic materials are used in novel optoelectronic devices because of the ease and high compatibility of their fabrication processes. Here, we demonstrate a low-driving-voltage cathodic-controlled organic upconverter with a mapping application that converts near-infrared images to produce images of visible blood vessels. The proposed upconverter has a multilayer structure consisting of a photosensitive charge-generation layer (CGL) and a phosphorescent organic light-emitting diode (OLED) for producing clear images with a high resolution of 600 dots per inch. In this study, temperature-dependent electrical characterization was performed to analyze the interfacial modification of the cathodic-controlled upconverter. The result shows that the upconverter demonstrated a high conversion efficiency of 3.46% because of reduction in the injection barrier height at the interface between the CGL and the OLED.

Downscaling the Sample Thickness to Sub-Micrometers by Employing Organic Photovoltaic Materials as a Charge-Generation Layer in the Time-of-Flight Measurement

Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N’-diphenyl-N,N’-bis(1,1’-biphenyl)-4,4’-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-μm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

Low resistance and high work-function WO3/Ag/MoO2 multilayer as transparent anode for bright organic light-emitting diodes

The authors report efficient organic light-emitting diodes (OLEDs) using a high conductive transparent WAM multilayer as anode electrode [WAM=WO3 (30nm)/Ag (10 nm)/MoO2 (5 nm)], which was prepared by thermal evaporation under room temperature condition to form the smooth morphology on anode surface, leading to reduce the injection barrier between metal/organic interface. The WAM anode shows a low sheet resistance of 8.27 Ω/sq, suitable injection workfunction of ~5.68 e V, and high optical transmittance of ~80% at range of visible light from 400 to 550 nm. In addition, a hole only WAM-device by atomic-force microscopy and electrical characterizations has demonstrates that the efficient hole injection property is existing at WAM/NPB interface as compared with the standard transparent conductive oxide of indium tin oxide. From the device characterization, the device performance of green fluorescent OLED based on WAM anode exhibiting a high brightness of 150 000 cd/m2 and current efficiency of 20 cd/A at driving voltage of 8 V has been achieved.

A wide absorption donor-acceptor active layer for vacuum-deposited organic photovoltaic devices with a 6.8 % power conversion efficiency

In this letter, the authors present a highly efficient small-molecule organic photovoltaic (OPV) with a donor-acceptor bulk-heterojunction of a novel electron donor material and C70. As a result, the OPV exhibited a highest power conversion efficiency of 6.8%, an open circuit voltage of 0.79 V, a short-circuit current density of 16.13 mA/cm2 and a fill factor (FF) of 53.9 % under 1 sun solar illumination (AM 1.5G).

Low resistance and high work-function WO 3 /Ag/MoO 2 multilayer as transparent anode for bright organic light-emitting diodes

The authors report efficient organic light-emitting diodes (OLEDs) using a high conductive transparent WAM multilayer as anode electrode [WAM=WO3 (30nm)/Ag (10 nm)/MoO2 (5 nm)], which was prepared by thermal evaporation under room temperature condition to form the smooth morphology on anode surface, leading to reduce the injection barrier between metal/organic interface. The WAM anode shows a low sheet resistance of 8.27 Ω/sq, suitable injection workfunction of ~5.68 e V, and high optical transmittance of ~80% at range of visible light from 400 to 550 nm. In addition, a hole only WAM-device by atomic-force microscopy and electrical characterizations has demonstrates that the efficient hole injection property is existing at WAM/NPB interface as compared with the standard transparent conductive oxide of indium tin oxide. From the device characterization, the device performance of green fluorescent OLED based on WAM anode exhibiting a high brightness of 150 000 cd/m 2 and current efficiency of 20 cd/A at driving voltage of 8 V has been achieved.