Chih-Chien Lee

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.

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.

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.

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.

Effect of deposition rate on device performance and lifetime of planar molecule-based organic light-emitting diodes

Electrical and optical characteristics, surface morphology and device lifetime of planar molecule bis(10-hydroxybenzo[h]qinolinato)beryllium (Bebq2) were studied as a function of the deposition rate. These devices exhibited a significant decrease in the photoluminescence (PL) efficiency due to the formation of large aggregation in the emitting layer during a slower deposition process. Time-of-flight studies showed that the molecule-packing configuration in the ordered aggregates could enhance the probability for site-to-site hopping via intermolecular interactions. The effects of the deposition rate on the device performance of an organic light-emitting diode were exhaustively analysed by examining the electrical property, morphology, PL decay and device simulation. These results provide valuable insights into the effects of varying deposition rates on the electroluminescence efficiency and device reliability.

Numerical Simulation of Electrical Model for Organic Light-Emitting Devices with Fluorescent Dopant in the Emitting Layer

We present a numerical model for the quantitative simulation of electrical characteristics for organic light-emitting devices (OLEDs) with fluorescent dopants in the host. We use drift-diffusion equations in terms of the electron and hole current densities coupled with the Poissons equation. Compared with other models proposed in previous literature, we include charge carrier trapping and direct carrier recombination phenomena on the fluorescent dopants in the simulation. Furthermore, current density, charge distribution, and recombination data in the device are obtained from this numerical study. Results for several multilayer devices with different fluorescent dopant concentrations are presented in this article. On the basis of the experimental data of a typical doped device, we have found good agreement between the simulation results and the experimental results.

In situ vacuum measurement of the thickness dependence of electron mobility in naphthalenetetracarboxylic diimide-based field-effect transistors

We present an in situ electrical measurement for the characteristics of n-channel organic field-effect transistors(OFETs) based on N , N ′ -dipentadecafluorooctyl-1,4,5,8-naphtalene tetracarboxylic diimide (NTCDI-C8F15) organic semiconductors. The field-effect mobilities have been estimated as a function of the number of monolayers (MLs). The electron mobility ( μ e ) of NTCDI-C8F15 OFET as thin as 2 ML has been determined. μ e increases rapidly with increasing film thickness, although it reaches saturation thickness ( d 0 ) around 3.5 ML. Atomic force microscopy confirms island mode growth mechanism of NTCDI-C8F15 with near upright position stacking on SiO 2 substrate and it is attributed to the fluorophobic effect of the material.

Stamped Self-Assembled Monolayers on Electrode for Connecting Organic Light-Emitting Diode and Organic Photovoltaic Device

The stamping process of self-assembled monolayers (SAMs) with polydimethylsiloxane (PDMS) mold on the Ag electrode was demonstrated, for use as the anode of a top-emission organic light-emitting device (OLED) and the cathode of an organic photovoltaic (OPV) device stacked underneath. Such SAMs reduce the barrier for hole injection from the Ag to the hole-transport layer of OLED. The PDMS mold coated with SAMs can be used repeatedly, for more than nine times. In this tandem device, the power conversation efficiency under OPV-mode operation and the current efficiency under OLED-mode operation were nearly identical to those in individual devices without compromise.

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.

Characterization of intrinsic hysteresis of pentacene-based organic thin-film transistor through in-situ real-time electrical measurement

The intrinsic hysteresis of a pentacene-based organic thin-film transistor was characterized through home-designed in-situ real-time electrical measurement. The device exhibited intrinsic hysteresis after the device fabrication without breaking the vacuum, which has not been observed previously. Similar behavior was observed when introducing the nitrogen gas. Compared with the measurement condition of vacuum or nitrogen gas, exposure to the ambient air resulted in a severe hysteresis. It was attributed to both the acceptor-like traps at the organic/dielectric interface and the donor-like traps in the transport channel. When the chamber was vacuumed out again, a significantly reduced hysteresis was obtained almost the same as that measured just after device fabrication, indicating the reversibility of the extrinsic hysteresis. We also related the hysteresis to the morphological change under different deposition rates of pentacene. The smoother surface at higher deposition rate caused reduced hysteresis because of the elimination of vacancies near the pentacene/dielectric interface.