Previously we proposed a highly effective extraction-enhancing OLED display pixel structure based on embedding the top-emitting OLED inside a 3D reflective bank structure covered with a patterned high-index filler. For top-emitting OLED using transparent top electrode (e.g., TCO like ITO), optical simulation indicated that the optical out-coupling efficiency can be enhanced by nearly 2.8 times (e.g., from 24% to 68%). Here in this work, through optical simulation and design, we show that the overall optical out-coupling efficiency of such 3D OLED pixel configuration can be further enhanced by forming the distributed Brag reflector (DBR) between the top-emitting OLED and the high-index filler and strengthening the microcavity effect of the top-emitting OLED.
In optical coherence tomography (OCT) systems, to precisely obtain the scattering properties of samples is an essential issue in diagnostic applications. Especially with a higher density turbid medium, the light interferes among the adjacent scatters. Combining an OCT experiment with the finite-difference time-domain simulation method, the multiple scattering effect is shown to affect the scattering properties of medium depending on the interparticle spacing. The far-field scattering phase function of scatters with various diameters was simulated to further analyze the corresponding anisotropy factors, which can be introduced into the extended Huygens-Fresnel theory to find the scattering coefficient of measured samples.
Quantitative optical analyses were conducted on the mechanisms of impressively high electroluminescence (EL) efficiency (external quantum efficiency of up to 37%) achieved in previously reported blue organic light-emitting devices (OLEDs) using thermally activated delayed fluorescence emitters based on acridine–triazine hybrids. In addition to high photoluminescence quantum yields and preferentially horizontal emitting dipoles, optical simulation shows that the use of both low-index hole-transport layers (HTLs) and electron-transport layers (ETLs) also substantially contribute to enhanced optical outcoupling efficiencies and EL efficiencies of these devices. Further analyses on optical mode distributions and partitions in devices reveal significantly different optical outcoupling enhancement mechanisms for adopting low-index HTLs (i.e., reduced overall waveguided modes and enhanced microcavity effect) or adopting low-index ETLs (i.e., reduced surface plasmon and transverse magnetic waveguided modes), and their effects are combined to give even larger enhancement when reducing refractive indexes of both. Results of this work clearly indicate that optical properties of carrier-transport layers, in addition to their electrical properties, are critical factors and should also be carefully considered for future development of high-efficiency OLEDs.
A series of soluble pentacene precursors are designed, from which pure pentacene can be generated either thermally or
photochemically. Small volatile fragments like CO and CO2 are extruded during the dissociation reactions. Thin films
of pentacene made from these precursors exhibited typical OTFT characteristics.
We have developed an effective approach based on wavelength-selective mirrors to implement three-peak WOLEDs that
have EL spectra matching better with transmission spectra of typical color filters and thus give much enhanced color
gamut for full-color OLED display applications. The wavelength-selective mirror used here is highly compatible with
OLED fabrication.
Thin films of organic semiconductors have been subjected to extensive studies in the last two decades due to
applications in photonics and optoelectronics. In this paper, we investigate wavelength tuning phenomena based on
fluorene-cored oligomers (T3). We studied the optical properties of oligofluorene vacuum-deposited thin films and
found that they show high optical anisotropy but become optical isotropic after annealing at a temperature around the
glass-transition temperature. The results indicate the molecular reorientation in thin films after annealing. Using this
property, we investigated the influence of molecular orientation on stimulated emission properties of organic thin films.
Employing such properties, we have also demonstrated continuous tuning of the stimulated emission wavelength of a
slab waveguide within one sample. Finally, we employed T3 thin film for laser application. We have also demonstrated
the wavelength tuning of the organic laser with DFB structures.
Optical characteristics of microcavity organic light-emitting devices (OLEDs) having two mirrors are examined.
Analyses show that a high-reflection back mirror and a low-loss high-reflection exit mirror are essential for such
microcavity devices to obtain luminance enhancement relative to conventional noncavity devices. The capping layer in
the composite mirror plays the role of enhancing reflection and reducing absorption loss, rather than enhancing
transmission. In addition, by setting the normal-direction resonant wavelength around the peak wavelength of the
intrinsic emission, one obtains the highest luminance enhancement along the normal direction and hardly detectable
color shift with viewing angles, yet accompanied by highly directed emission and lower external quantum efficiency. On
the other hand, the highest enhancement in external quantum efficiencies and the most uniform brightness distribution
are obtained by setting the normal-direction resonant wavelength 20-40 nm longer than the peak wavelength of the
intrinsic emission, yet with noticeable color shift over viewing angles. Due to the tradeoffs between different emission
characteristics in choosing the resonant wavelength, the exact design of microcavity devices would depend on actual applications.
Scaling behaviors of ZnO transparent thin-film transistors (TTFTs) have been studied by fabricating series of
miniaturized ZnO TTFTs having various channel widths and lengths. Mobility of >8 cm2/V.s and on/off ratio of up to
107 are achieved with these TTFTs. Results show that these ZnO TTFTs retain rather well-behaved transistor
characteristics down to the channel length of ~5 &mgr;m, rendering possible high-resolution applications. More apparent
short-channel effects (e.g., lowering of threshold voltages, degradation of the subthreshold slope with the decrease of the
channel length and the increase of the drain voltage, and loss of hard saturation, etc.) are observed when the channel
length is reduced below 5 &mgr;m. Influences of parasitic effects on TFT characteristics are also studied by extracting
parasitic resistance and channel resistance using devices of various dimensions. The ratio of parasitic resistance to
channel resistance at VG = 10 V was increased from 0.04 to 0.36, when the channel length decreased from 20 &mgr;m to 2 &mgr;m.
This indicates that parasitic resistance has substantial influences on device performances (e.g., output drain current,
apparent field effect mobility, etc.) when the channel length is reduced, and better contact techniques may be required.
The synthesis and properties of 3,6-bis(triphenylsilyl)carbazole as an effective host material for blue electrophosphorescence are reported. The electrochemically active C3 and C6 sites of carbazole are non-conjugated blocked with the steric, bulky and large-gap triphenylsilyls, the resulting new compound retains the large triplet energy of carbazole yet exhibits much enhanced morphological stability and electrochemical stability in comparison with previous carbazole-based host materials. Using this new host material, blue phosphorescent OLEDs having high efficiencies up to 16%, 30.6 cd/A and 26.7 lm/W are demonstrated.
The concept of tandem organic light-emitting devices (OLEDs) provides a pathway for developing highly stable and efficient OLEDs. The connecting structure that bridges adjacent light-emitting units, substantially affects the device performance of tandem OLEDs. In this letter, we introduce an effective connecting structure in which an ultrathin middle metal layer is sandwiched between efficient electron- and hole-injection layers for the tandem OLEDs, which in essence, avoids the use of reactive metals during fabrication. Two-unit tandem OLEDs with such connecting structure exhibit less than double the driving voltage, yet more than double the efficiency, more saturated emission color, and longer operational lifetime compared to those of single-unit devices. A model based on a hypothesis of energy level pinning effect has been proposed as the mechanism of the connecting structure in the tandem devices. This model is also consistent with the results obtained from the photovoltaic effect measurements in tandem OLEDs.
Fluorene-based materials, such as polyfluorenes or oligofluorenes, are conventionally well known as efficient blue emitting materials. Intriguingly, it is recently unveiled that ter(9,9-diarylfluorene)s also exhibit efficient unusual nondispersive bipolar carrier-transport characteristics and high carrier mobilities of >10-3 cm2/Vs for both holes and electrons in the amorphous state. Making use of convenient substitution on C9 of fluorenes and corresponding variety in molecular structures and physical properties, in this work we systematically investigate influences of molecular structures, such as dialkyl-substitution vs. diaryl-substitution and oligomer length etc., on charge transport of oligofluorenes. Furthermore, utilizing liquid-crystalline properties of oligofluorenes with appropriate substitutions, comparative studies of carrier transport of oligofluornes in both vacuum-deposited amorphous states and aligned liquid-crystal glass states are also performed. Insights on intermolecular charge transport derived from the experimental observations and theoretical examinations are then discussed accordingly.
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.
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.
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 from shallow to flat profiles may be obtained in a single transfer step by adjusting the diffusion conditions. Doped-polymer light-emitting devices made by this process and the conventional blending process exhibited same device characteristics. Along with patterned color donor plates, we demonstrated multicolor OLEDs of arbitrary patterns over large areas with a single thermal transfer step.
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