Shun-Wei Liu

High-efficiency blue organic light-emitting diodes using a 3,5-di(9H-carbazol-9-yl)tetraphenylsilane host via a solution-process

We present a solution-processed blue organic light-emitting diode (OLED) with markedly high current efficiency of 41.2 cd A−1 at 100 cd m−2 and 31.1 cd A−1 at 1000 cd m−2. The high efficiency was partly attributed to the use of a molecular host, 3,5-di(9H-carbazol-9-yl)tetraphenylsilane, which possesses a wide triplet band gap, high carrier mobility, ambipolar transport property and high glass transition temperature. Besides the intrinsically good physical properties, the solution-process also played an important role in fabricating the high-efficiency device, since it could make the molecular distribution of host and guest homogeneous in the emissive layer. Moreover, the device efficiency at higher brightness could be markedly enhanced by using an electron-blocking layer. As the microlens was introduced on the glass substrate to enhance the light outcoupling, the resultant device efficiency of the blue OLED further increased to 50.1 cd A−1 at 100 cd m−2 and 37.3 cd A−1 at 1000 cd m−2.

Mixed host organic light-emitting devices with low driving voltage and long lifetime

In this letter, we present and analyze the device performance of the mixed host (MH) organic light-emitting devices (OLEDs). The host of the emitting layer (EML) material in this device consists of a hole transport layer (HTL) and an electron transport layer (ETL) fabricated by coevaporation. The bipolar transport characteristic of the MH layer helps to reduce the driving voltage. Device lifetime is increased due to the elimination of the sharp boundary of the HTL/EML interface. Combining the MH structure with a high mobility electron ETL material, bis(10-hydroxybenzo[h]qinolinato)beryllium, the OLED has shown a brightness of 27600cd∕m2 at a driving voltage of 5 V, and a lifetime four times longer than that of a conventional OLED.

High Photoelectric Conversion Efficiency of Metal Phthalocyanine/Fullerene Heterojunction Photovoltaic Device

This paper introduces the fundamental physical characteristics of organic photovoltaic (OPV) devices. Photoelectric conversion efficiency is crucial to the evaluation of quality in OPV devices, and enhancing efficiency has been spurring on researchers to seek alternatives to this problem. In this paper, we focus on organic photovoltaic (OPV) devices and review several approaches to enhance the energy conversion efficiency of small molecular heterojunction OPV devices based on an optimal metal-phthalocyanine/fullerene (C60) planar heterojunction thin film structure. For the sake of discussion, these mechanisms have been divided into electrical and optical sections: (1) Electrical: Modification on electrodes or active regions to benefit carrier injection, charge transport and exciton dissociation; (2) Optical: Optional architectures or infilling to promote photon confinement and enhance absorption.

Charge carrier mobility of mixed-layer organic light-emitting diodes

The authors report the investigation of the charge transport behaviors in mixed thin films of N,N′-diphenyl-N,N′-bis(1-napthyl)-1,1′-biphenyl-4,4′-diamine and tris(8-hydroxyquinoline) aluminum. The extracted electron and hole drift mobility were found to be sensitive to the compositional fraction and interpreted by energy levels, charge mobilities of neat compounds, and microscopic networks within the mixed systems. The carrier conduction characteristics, therefore, were used to illustrate the electrical and optical properties of the organic light emitting devices with a mixed layer and present direct evidences on the role of the mixed layer in these devices.

Highly efficient red electrophosphorescent device incorporating a bipolar triphenylamine/bisphenylsulfonyl-substituted fluorene hybrid as the host

We have fabricated highly efficient red phosphorescent organic light-emitting diodes (PHOLEDs) incorporating a bipolar host material, 2,7-bis(phenylsulfonyl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (SAF), doped with 7 wt% tris(1-phenylisoquinolinolato-C2,N)iridium(III) [Ir(piq)3]. Attaching the electron-donating (p-type) triphenylamine group onto the electron-accepting (n-type) 2,7-bis(phenylsulfonyl)fluorene segment (through the C9 position of the fluorene unit) imparts SAF with good morphological stability, high triplet energy gap (ET), bipolar transporting ability, and matching energy levels with adjacent carrier-transporting layers. Consequently, the SAF-based red-PHOLED exhibited a very low turn-on voltage (2.4 V) and high electroluminescence efficiencies of 15.8% and 22.0 lm W−1, superior to those of the corresponding device incorporating a conventional host material, 4,4′-N,N′-dicarbazolbiphenyl (CBP; 3.2 V, 8.5%, and 8.4 lm W−1, respectively). At a practical brightness of 1000 cd m−2, the efficiencies of the SAF-based red-PHOLED remained high (13.1%, 14.4 lm W−1).

Electrical and optical simulation of organic light-emitting devices with fluorescent dopant in the emitting layer

A complete model for the quantitative simulation of electrical and optical characteristics for organic light-emitting devices with fluorescent dopant in the host is presented. This simulation model consists of three parts: charged carrier transport model, exciton model, and emission and optical model. In the first part, we include not only charge carrier trapping but also direct carrier recombination phenomena on the fluorescent dopant. In the second part, Forster [Discuss. Faraday Soc.27, 7 (1959)]energy transfer from the host molecule to the dopant molecule is included in exciton model. In addition, the quenching phenomena related to dopant concentration and electrode are also considered in this study. In the optical model, the thin-film optics is applied to calculate the interference effect of the device. Results for several multilayer devices with different fluorescent dopant concentrations are presented. On the basis of the experimental data of a typical doped device, we have found good agreement between the simulation results and the experimental data.

Fabrication and characterization of heteropolyanion Langmuir-Blodgett films

The possibility of the formation of Langmuir-Blodgett (LB) films with dimethyldioctadecylammonium (DODA) after the addition of cobalt(II)-substituted Dawson-type tungstodiphosphate anion (briefed as (H2O)(CoP2W17O618-)-P-11) in the subphase has been explored. Marked modifications of the compression isotherms are observed when this anion is dissolved in the subphase, which demonstrates that the polyanions interact with the monolayers. LB films have been readily obtained from this system. The adsorption Fourier transform IR (FT IR) spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD) and cyclic voltammetry (CV) have been used to investigate the morphological and molecular structure of the deposited film. The FT IR results showed the presence of the polyanion within the LB films, and the shift for its characteristic bands may be related to the presence of positively charged DODA. AFM measurement reveals that the LB films of DODA/(H2O)(CoP2W17O618)-P-II are regularly and uniformly deposited on the substrate. XRD experiments prove that the lamellar structure of the LB films of DODA/(H2O)(CoP2W17O618-)-P-II is well-defined. The LB films of DODA/(H2O)(CoP2W17O618-)-P-II immobilized onto an indium-oxide (ITO) glass, in aqueous solutions of pH 2.0-5.0, show quite facile redox reactions even for multilayers. All the experiments carried out in the present study suggest that the new materials of heteropolyanions can be formed by LB techniques and beneficial physicochemical properties of heteropolyanions can be maintained/enhanced through molecular-level design

4-Hydroxy-8-methyl-1,5-naphthyridine aluminium chelate: a morphologically stable and efficient exciton-blocking material for organic photovoltaics with prolonged lifetime

A stable exciton-blocking layer (EBL) with rigid ball-like 4-hydroxy-8-methyl-1,5-naphthyridine aluminium chelate (AlmND3) has been newly developed for organic photovoltaics (OPVs) based on a double-heterostructure of pentacene/C60/EBL. Unlike the common EBL materials, such as tris-(8-hydroxyquinoline)aluminium (Alq3) and bathocuproine (BCP), AlmND3 exhibits a relatively high electron mobility (∼10−4 cm2 V−1 s−1 at the electric field of 6.4 × 105 V cm−1) and a wide band gap energy (∼3.3 eV) in addition to a high glass transition temperature (Tg ∼ 194 °C). Having such EBL between active region (pentacene/C60) and the metal cathode, pristine device performances were comparable with those of BCP-based OPV due to its high mobility and wide band gap energy. Moreover, due to its significantly higher Tg than that of BCP, an extended lifetime performance was observed for AlmND3-based OPV, compared with the BCP-based OPV aged at the elevated temperature of 75 °C.

Modification of silver anode and cathode for a top-illuminated organic photovoltaic device

We have demonstrated a top-illuminated organic photovoltaic device with a thick Ag anode and a thin Ag cathode capped with an α-naphthylphenylbiphenyl diamine (NPB) thin film. The surface of the Ag anode was oxidized by UV–ozone which improved the carrier collection and reduced the exciton quenching. Compared with the control device with an indium tin oxide anode, a 15.59 times reduction in the serial resistance and a 1.72 times increase in the shunt resistance were observed with a fill factor of 0.61 in such a device. The NPB capping layer not only improved the light transmission from the semitransparent cathode, but also hindered the formation of Ag island growth and thereby improved the device stability.

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.