Hsin-Hua Chang

Anisotropic optical properties and molecular orientation in vacuum-deposited ter(9,9-diarylfluorene)s thin films using spectroscopic ellipsometry

This article reports on the investigation of anisotropic optical properties of vacuum-deposited thin films of high-efficiency blue-emitting ter(9,9-diarylfluorene)s using variable-angle spectroscopic ellipsometry. Under deposition conditions typical for thin-film organic devices, both real and imaginary parts of refractive indices of vacuum-deposited ter(9,9-diarylfluorene) films exhibit rather significant uniaxial anisotropy with the optical axis along the surface normal. In particular, for the absorption associated with the π–π* transition of the terfluorene backbone, they show substantially larger in-plane extinction coefficients than the out-of-plane extinction coefficients. It is thus inferred that the vacuum-deposited ter(9,9-diarylfluorene) molecules tend to align their molecular axes and π–π* transition dipole moments along the substrate surface as observed previously in spin-coated films of alkyl-substituted polyfluorenes or oligofluorenes, even though the present ter(9,9-diarylfluorene)s have ri...

The Tapestry Cellular Automata phase unwrapping algorithm for interferogram analysis

A newly developed phase-unwrapping algorithm, which is termed Tapestry Cellular Automata, are presented. Fundamental restrictions of traditional path-dependent phase unwrapping algorithms such as noise propagation and inconsistent data reconstruction are discussed first. The advantages and drawbacks of a path-independent algorithm, Cellular Automata, are then examined. The parallel and distributed processing nature of Tapestry Cellular Automata is shown to be able to keep the merits of traditional Cellular Automata algorithm while taking advantage of the rapid advancement of personal computers such as distributed computing over internet or intranet and multi-tasks operating environment. Both numerical simulation and experiments used to examine the effectiveness of this newly developed algorithm are presented in detail as well.

Finite-source dye-diffusion thermal transfer for doping and color integration of organic light-emitting devices

An effective process of performing controllable doping of polymer films in organic light-emitting devices is reported. In this approach, a film to be doped is brought into direct contact with a dye-dispersed polymer donor film to permit direct dye-diffusion thermal transfer. Theoretical and experimental studies indicate that this doping process can be modeled by Fick’s diffusion theory and that a desired dopant distribution may be obtained in a single transfer step by adjusting the diffusion conditions. Doped-polymer light-emitting devices made by this process exhibited the same device characteristics as those by the conventional blending process. Along with patterned color donor plates, we also demonstrated multicolor devices of arbitrary patterns over large areas with a single thermal transfer step.

Graded doping profiles for reduction of carrier trapping in organic light-emitting devices incorporating doped polymers

Dispersing emissive dopants into luminescent polymers is an effective approach to enhance luminescence and to tune emission color in organic light-emitting devices incorporating polymer films. However, the carrier trapping effect due to emissive dopants often causes deterioration of electrical characteristics. In this letter, we show that, by introducing a graded doping profile to match the carrier recombination zone in the doped polymer, the carrier trapping, and the deterioration of electrical characteristics can be minimized while the enhancement in efficiency maintains. The finite-source dye-diffusion thermal transfer is used to produce graded doping profiles into a luminescent polymer. The effectiveness of this approach has been demonstrated in both single-layer and heterostructure devices incorporating doped polymers.

Using an embedded nanocomposite scattering film for increasing out-coupling of white phosphorescent organic light-emitting devices

For the lighting purpose, white organic light-emitting devices (OLEDs) need to be operated at a high current density to ensure an ample flux, which will lead the limited lifespan of the device. This situation could be improved through diversified light-extraction methods. In this study, transparent photoresist mixed with titanium oxide (TiO2) nanoparticles of different sizes could be utilized to form an internal extraction structures between the indium-tin-oxide and glass substrate and thereby the out-coupling efficiency of white OLEDs could be significantly improved by this sophisticated device architecture engineering. The high refractive index of TiO2 is essentially operative for increasing the refractive index of nanocomposite film and thus diminishing the total internal reflection between the interfaces. In addition, the nanoparticles served scattering function to multiply the ratio of the substrate and radiation modes. By employing nanocomposite substrate with mixed dual-sized nanoparticles, we obtained external quantum efficiencies of the white phosphorescent OLEDs that were about 1.6 times higher than that of the control device at the high luminescence of 104 cd/m2.

Multicolor organic LEDs processed by integration of screen printing and thermal transfer printing

Color integration in organic LEDs (OLEDs) on a substrate has always been a challenge due to the incompatibility of OLED materials with the conventional photolithography. In this paper, we report a process for the fabrication of large-area multicolor OLEDs of arbitrary patterns by combination of thermal-transfer printing and screen-printing. Thermal transfer printing is used to introduce color-tuning dyes into a thermally stable OLED polymer layer from a dye- dispersed polymer layer on the donor plate. Such a process permits controllable and uniform doping of a polymer layer over large areas. By using a patterned color donor plate, color integration in OLEDs could be accomplished with a single thermal transfer step. In this work, the source plate containing multicolor patterns is fabricated by screen- printing. The RGB color patterns were printed sequentially by using RGB inks prepared by dispersing nile red,C6 and perylene into a commercial screen-printing paste. Based on these printing approaches, we have successfully fabricated multicolor single-layer and heterostructure OLEDs.

Pure exciplex-based white organic light-emitting diodes with imitation daylight emissions

An exciplex could be formed by blending a selected hole-transporting material (HTM)/electron-transporting material (ETM) pair, and the corresponding energy band gap is roughly determined by the energy difference between the lowest unoccupied molecular orbital (LUMO) of the ETM and the highest occupied molecular orbital (HOMO) of the HTM. In this study, three HTM/ETM combinations are adopted to generate blue, green, and red exciplexes, allowing us to design precise device architectures for the fabrication of exciplex-based white OLEDs (WOLEDs) with daylight-like emissions. The CIE coordinates of this WOLED varied close to the Planckian locus as the biases increase, with a high color rendering index of about 96. This high performance suggests this exciplex-based WOLED can provide high-quality white-light illumination. Photoluminance and lifetime measurements of the exciplex behavior of the HTM/ETM combinations indicate that the HTM and ETM selected should possess higher triplet energy bandgaps than those of their corresponding exciplex to avoid energy loss.

Improving the efficiency of white OLEDs based on a gradient refractive index substrate

For use in lighting applications, white organic light-emitting devices (WOLEDs) must operate at higher biases to ensure an ample flux. However, stressed operation voltages often result in poor performance and limited device lifetime. This could be addressed by modifying the inherent optical properties of OLEDs. This study proposes a gradient refractive index (GRIN) substrate to adjust the ratio of the light-waveguided modes as well as the radiation mode. An embedded nanocomposite film consisting of titanium dioxide (TiO2) nanoparticles (NPs) was inserted between ITO and glass to create an internal light-extraction structure (IES). The high refractive index of TiO2 is essential for increasing the refractive index of the photoresist film and thus diminishing the total internal reflection between the interfaces. In addition, the silicon dioxide NPs mixed polydimethylsiloxane was used to form an external light-extraction structure (EES). The refractive indices of the IES and EES were adjusted to form a GRIN substrate. Compared with a control device, this sophisticated substrate produced a 1.6 fold efficiency improvement.

More light from blue organic LEDs using embedded nanoparticles

In sharp contrast to the rapid progress being made on organic LED (OLED) display technologies, the prospects for OLED lighting are much less optimistic. The slower development of this field can be attributed to the immature technologies and organic materials available for architecture design, especially for light extraction. We simplified the device architecture, reducing the number of thin-film layers and incorporating nanosized particles to improve the overall efficiency and light output of blue phosphorescent OLEDs. For high-quality illumination, OLEDs must be operated at a high current density to ensure an ample flux of light. This requires driving the OLEDs at a high bias, which limits a device’s efficiency and shortens its life span. Because the external quantum efficiency of a lamp depends both on the internal quantum efficiency and the light-extraction efficiency, the challenge to designers of OLEDs for lighting is to find both an architecture and materials that are internally efficient under these conditions and to optimize light extraction.1 The internal efficiency of an OLED depends on charge balance, quantum yield, exciton confinement, and other factors, and can generally be optimized by careful structure design. Phosphorescent OLEDs are attractive because they exploit both singlet and triplet excited states, thus raising the limits of internal efficiency from about 25% to theoretical limits of near 100%. Most phosphorescent OLED designs use many thin-film layers to keep the charge carriers balanced. However, the additional layers reduce the manufacturability of the devices. We designed and built a phosphorescent OLED with only three layers between the electrodes: a hole-transport layer Figure 1. Molecules (left) used in two simplified organic LED (OLED) device architectures (right). Device layers from the top down: Lithium fluoride is used as an electron-injection material and aluminum is used as the cathode. TmPyPB is the electron-transport material. The lightemitting layer uses mCP as a wide-energy-gap host material doped with 8wt% of blue emitter FIrpic. TAPC is the hole-transport material. PEDOT:PSS is a conductive transparent polymer that sits between the hole-transport layer and the indium tin oxide (ITO) anode. The extra layer in Device A consists of two sizes of nanoparticles (NPs) embedded in a layer of transparent photoresist.

Multicolor heterostructure organic LEDs based on selectively doped hole-transport polymer layers

In this paper, we report an effective thermal transfer process for performing controllable and selective doping of polymer films for multicolor organic light-emitting devices (OLEDs) or color pixels in OLED displays. In this process, the polymer receiver film is placed in direct contact with the dye-dispersed polymer donor film (with patterns) to permit direct dye-diffusion thermal transfer. It permits controllable lateral and vertical modulations of the dopant distributions of a polymer film for organic devices. Furthermore, it is the intention to combine the selectively doped (patterned) polymer layers with the vacuum deposited small-molecule films, such that both the heterostructure versatility of small-molecule materials and the patterning advantage of polymeric materials could be conserved in the hybrid polymer/small molecule heterostructure OLEDs. The rather thermally stable polymer poly(N-vinylcarbazole) (PVK), which is hole-transport and deep-blue emitting, in combination with several electron-transport molecules has been studied to demonstrate the feasibility of these concepts.