Chih-Hsien Yuan

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.

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.

High-efficiency green electrophosphorescent organic light-emitting diodes with a simple device structure

In this study, we demonstrate electrophosphorescent organic light-emitting diodes (PHOLEDs) with a simple structure and high efficiency, utilizing fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) as a green phosphorescent emitter. The device structure (ITO/TAPC/TAPC:Ir(ppy)3/TPBi:Ir(ppy)3/TPBi /LiF/Al) was fabricated by the hole transporting layer of 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC; T1 = 2.87 eV) and the electron transporting layer of 2,2,2-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] (TPBI; T1 = 2.74 eV) to assemble a double emitting zone without common host materials. The emitting region with a TAPC/TPBI bilayer interface was able to provide an effective energy transference of Ir(ppy)3 (T1 = 2.4 eV). Thus, the green PHOLED exhibited high current efficiency (70 cd/A at 100 cd/m2) and low roll-off in these devices.

A new model for optimization of organic light-emitting device by concurrent incorporation of electrical and optical simulations

To optimize the performance of organic light-emission devices (OLEDs), optical simulation or electrical simulation is often used to help designing the device structures. However, employing electrical or optical simulation separately to optimize the device might lead to incorrect conclusions. A few researches have combined optical and electrical simulations to design OLED structures by merely inserting the maximum carrier recombination rate calculated from electrical simulation into optical simulation programs, which is still insufficient for optimization of OLEDs due to lack of considering the influence of optical interference positions. In this paper, we investigate the OLED performance by using three simulation methods, pure optical, pure electrical, or combination of both, to design the devices. Using the models incorporating both electrical and optical simulations, we found that the optimal emission position occurs neither at the place with the best optical interference nor at the point where carrier recombination rate is the maximum. In order to verify the simulation results, we design the testing devices, red fluorescence OLEDs of bi-layer structures, with various positions of recombination emission. It is found that the position of recombination emission has major impact on the device performance of OLEDs, which lead to some important design rules. With integration of electrical and optical simulations, the real emission position could be predicted with excellent agreements to the experimental results. Applying this method to design the red fluorescent bi-layer OLEDs, the device with very high efficiency of 8.44 cd/A was achieved.

Comparison of light out-coupling enhancements in single-layer blue-phosphorescent organic light emitting diodes using small-molecule or polymer hosts

Single-layer blue phosphorescence organic light emitting diodes (OLEDs) with either small-molecule or polymer hosts are fabricated using solution process and the performances of devices with different hosts are investigated. The small-molecule device exhibits luminous efficiency of 14.7 cd/A and maximum power efficiency of 8.39 lm/W, which is the highest among blue phosphorescence OLEDs with single-layer solution process and small molecular hosts. Using the same solution process for all devices, comparison of light out-coupling enhancement, with brightness enhancement film (BEF), between small-molecule and polymer based OLEDs is realized. Due to different dipole orientation and anisotropic refractive index, polymer-based OLEDs would trap less light than small molecule-based OLEDs internally, about 37% better based simulation results. In spite of better electrical and spectroscopic characteristics, including ambipolar characteristics, higher carrier mobility, higher photoluminescence quantum yield, and lar...

Solution-Processed Electrophosphorescent Devices Based on Small-Molecule Mixed Hosts

Highly efficient electrophosphorescent organic light-emitting diodes (PHOLEDs) containing mixed hosts of bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2) and 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) and a soluble derivative of the green emitter fac-tris(2-phenylpyridine)iridium (III) [(Ir(ppy)3 ] have been demonstrated. All organic layers were mixed in a single layer for solution processing during the fabrication of the PHOLEDs. The amorphous mixed host of SimCP2:OXD-7:Ir(ppy)3 exhibited ambipolar charge transport as well as high hole and electron mobilities on the order of 10-7 cm2 /Vs from space-charge-limited current measurement. The values of both the hole and electron mobilities were 100 times greater than that of the widely used poly(9-vinylcarbazole) (PVK):OXD-7:Ir(ppy)3 host, resulting in efficient charge balance in the SimCP2 host. Based on a simple fabrication process of single-layer PHOLED, a green device with the maximum luminous and power efficiencies of 42 cd/A and 20 lm/W, respectively, was obtained at 600 cd/m2.

Organic Photovoltaic Cells Employing an Ultrathin Electron Donor of Arylamino-Substituted Fumaronitrile Material

In this paper, a novel electron donor, bis4-[N-(1-naphthyl)phenylamino]phenylfumaronitrile (NPAFN), was demonstrated as a potential for application in high open-circuit voltage (VOC) organic photovoltaic (OPV) cells. Devices based on NPAFN/C60 heterojunction were firstly manipulated to their optimum optical property by tuning the thickness of C60 without compromising the transport property. With the appropriate thickness of C60, the electrical configuration of the devices was improved when the thickness of NPAFN was less than 6.3 nm. The optimum efficiency was 2.1% with a VOC of 0.87 V when the thickness of NPAFN was 5 nm. In addition, the increase in VOC by increasing the thickness of NPAFN was observed and discussed.