Tzung-Fang Guo

Degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes

The degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes (PLEDs) is investigated. The active medium of our PLED is a polymer blend comprising poly(vinylcarbazole) (PVK), [2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole] (t-PBD), and platinum(II)-2,8,12,17-tetraethyl-3,7,13,18-tetramethylporphyrin (PtOX). The cyclic voltammetry result shows that the reductive reversibility of PtOX is poor. This result suggests that PLED doped with PtOX is not stable if PtOXs trap electrons and turn into anionic PtOX species. This was indeed verified by fabricating single-layer PLEDs with various amounts of electron-transporting material, t-PBD. A slower degradation rate was observed from the devices with higher concentration of t-PBD, because of the reduction of the electron accumulation at the PtOX sites. The half decay lifetime of our phosphorescent polymer LED has been improved by a factor of ∼40, from 1.2 to 45 h.

Effects of thermal annealing on the performance of polymer light emitting diodes

Thermal annealing plays an important role in controlling morphologies of polymer thin films and consequently the device performance, such as emission spectra, turn-on voltages, quantum efficiency of photoluminescence (PL) and electroluminescence (EL). In thermal annealing there is a tradeoff between hole-injection efficiency and PL efficiency. Annealing at a temperature higher than the glass transition temperature can improve the efficiency of hole injection at the expense of the PL efficiency, and vice versa. Optimizing the annealing conditions can improve the overall EL efficiency. The high efficiency of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene based polymer light-emitting diodes is demonstrated: 2.7 cd/A at a luminescence of 1000 cd/m2.Thermal annealing plays an important role in controlling morphologies of polymer thin films and consequently the device performance, such as emission spectra, turn-on voltages, quantum efficiency of photoluminescence (PL) and electroluminescence (EL). In thermal annealing there is a tradeoff between hole-injection efficiency and PL efficiency. Annealing at a temperature higher than the glass transition temperature can improve the efficiency of hole injection at the expense of the PL efficiency, and vice versa. Optimizing the annealing conditions can improve the overall EL efficiency. The high efficiency of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene based polymer light-emitting diodes is demonstrated: 2.7 cd/A at a luminescence of 1000 cd/m2.

Highly efficient electrophosphorescent polymer light-emitting devices

Abstract We report a high performance polymer electroluminescent device based on a bi-layer structure consisting of a hole transporting layer (poly(vinylcarbazole)) and an electron transporting layer poly(9,9-bis(octyl)-fluorene-2,7-diyl) (BOc-PF) doped with platinum(II)-2,8,12,17-tetraethyl-3,7,13,18-tetramethylporphyrin (PtOX). The devices show red electrophosphorescence with a peak emission at 656 nm and a full width at half maximum of 18 nm, consistent with exclusive emission from the PtOX dopants. BOc-PF emission is not observed at any bias. The required doping levels for these phosphorescence-based polymer light-emitting diodes (PLEDs) are significantly lower than for other reported phosphorescence-based PLEDs or organic light-emitting diodes (OLEDs). A doping level of 1% or more give an LED with exclusive PtOX emission, whereas related PLEDs or OLEDs doped with phosphorescent dopants require doping levels of >5% to achieve exclusive dye dopant emission. The device external efficiency was enhanced from 1% to 2.3% when doped with PtOX. The lower doping level in BOc-PF/PtOX based PLEDs decreases triplet–triplet annihilation in these devices, leading to quantum efficiency that is only weakly dependent on current density. The luminescence transient decay time for this device is ∼500 μs.

High Performance Polymer Light-Emitting Diodes Fabricated by a Low Temperature Lamination Process**

We report on the successful demonstration of high performance polymer light-emitting diodes (PLEDs) using a low temperature, plastic lamination process. Blue- and red-emitting PLEDs were fabricated by laminating different luminescent polymers and organic compounds together to form the active media. This unique approach eliminates the issue of organic solvent compatibility with the organic layers for fabricating multi-layer PLEDs. In addition, a template activated surface process (TAS) has been successfully applied to generate an optimum interface for the low temperature lamination process. Atomic force microscopy analysis reveals a distinct difference in the surfaces created by the TAS and the spin-coating process. This observation coupled with the device data confirms the importance of the activated interface in the lamination process.

Understanding and harnessing biomimetic molecular machines for NEMS actuation materials

This paper describes the design, assembly, fabrication, and evaluation of artificial molecular machines with the goal of implementing their internal nanoscale movements within nanoelectromechanical systems in an efficient manner. These machines, a unique class of switchable molecular compounds in the shape of bistable [2]rotaxanes, exhibit internal relative mechanical motions of their ring and dumbbell components as a result of optical, chemical, or electrical signals. As such, they hold promise as nanoactuation materials. Although micromechanical devices that utilize the force produced by switchable [3]rotaxane molecules have been demonstrated, the current prototypical devices require a mechanism that minimizes the degradation associated with the molecules in order for bistable rotaxanes to become practical actuators. We propose a modified design in which electricity, instead of chemicals, is employed to stimulate the relative movements of the components in bistable [3]rotaxanes. As an initial step toward the assembly of a wholly electrically powered actuator based on molecular motion, closely packed Langmuir-Blodgett films of an amphiphilic, bistable [2]rotaxane have been characterized and an in situ Fourier transform infrared spectroscopic technique has been developed to monitor molecular signatures in device settings. Note to Practitioners-Biological molecular components, such as myosin and actin in skeletal muscle, organize to perform complex mechanical tasks. These components execute nanometer-scale interactions, but produce macroscopic effects. Inspired by this concept, we are developing a new class of mechanical nanodevices that employ a group of artificial molecular machines called bistable rotaxanes. In this paper, a series of experiments has been conducted to study the molecular properties of bistable rotaxanes in thin films and on solid-state nanodevices. Our results have shed light on the optimization of future molecular machine-based systems particularly with respect to their implementation and manufacture.

In situ study on the reorientation of polymer chains in operating polymer diodes

A reflection-absorption Fourier-transform infrared spectroscopy experiment has been designed to in situ monitor poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based polymer light-emitting diodes under stress test. This method enables the in situ study of the co-relation between device performance and the conformational transformation of a conjugated polymer. The experimental results indicate that the plane of the conjugated π-electron cloud in MEH-PPV tends to align parallel to the substrate. This rearrangement enhances the π–π electron coupling and lowers the device operating voltage under high current densities.

In situ infrared spectroscopic studies of molecular behavior in nanoelectronic devices

An in situ Fourier-transform infrared (FTIR) spectroscopic technique has been developed to monitor molecular behavior in single-molecule thick nanoelectronic devices. This approach is applicable to a range of molecular-based devices and has the potential to provide researchers in the field with a tool to understand the molecular behavior that contributes to device performance.

Investigation of the interfacial properties of laminated polymer diodes

Plastic lamination has become a promising approach to the fabrication of high-performance polymeric electronic devices. [T.-F. Guo, S. Pyo, S.-C. Chang, and Y. Yang, Adv. Funct. Mater. 11, 339 (2001)]. The major challenge for achieving high-performance laminated devices is the control of interface quality. In this letter, the various interfaces between two organic films have been investigated using atomic force microscope and impedance spectroscopy. Our results indicate the device performance is directly related to the formation of the interface. We attribute the better charge injection and higher electroluminescence efficiency of a laminated polymer light-emitting diode is due to the nanoscale surface roughness at the laminating interface.

High-performance flexible polymer light-emitting diodes fabricated via a low-temperature plastic laminated process

In this manuscript, we report on the successful fabrication of high performance polymer light emitting diodes (PLEDs) using a low temperature, plastic lamination process. Blue- and red-emitting PLEDs were fabricated by laminating different luminescent polymers and organic compounds together to form the active media. This unique approach eliminates the issue of organic solvent compatibility with the organic layers for fabricating multi-layer PLEDs. In addition, a template activated surface process (TAS) has been successfully applied to generate an optimum interface for the low temperature lamination process. The atomic force microscopy analysis reveals a distinct difference in the surfaces created by the TAS and the spin-coating process. This observation coupled with the device data confirms the importance of the activated interface in the lamination process.