Yung-Ting Chang

High efficiency non-dopant blue organic light-emitting diodes based on anthracene-based fluorophores with molecular design of charge transport and red-shifted emission proof

A new series of 9,10-diphenylanthracene (DPA)-based blue fluorophores have been synthesized and characterized for organic light-emitting diode (OLED) applications. These fluorophores have a bulky substituent, such as triphenylsilane in TPSDPA and mesitylene in TMPDPA, on the C-2 position. The C-2 substituent also includes electron transporting diphenylphosphine oxide in PPODPA and dimesitylene borane in BMTDPA, or hole transporting N-phenylnaphthalen-1-amine in NPADPA. For TMPDPA blue fluorophores, 9,10-diphenyl substituents of the anthracene core are further attached to hole-transporting 9H-carbazole in CBZDPA and electron-transporting 2-phenyl-1,3,4-oxadiazole in OXDDPA. Absorption and emission spectroscopic properties of all DPA-derived fluorophores, either in solution or in the condensed phase, were fully characterized and the HOMO/LUMO energy levels of these fluorophores were determined. The frontier molecular orbitals of the DPA derivatives were analysed by theoretical methods to determine the possible intramolecular charge transfer (ICT) characteristics. Whereas the blue emission is best preserved in TMPDPA, in which the non-conjugated bulky mesitylene group suppresses red-shifted emissions, the ICT is attributed to the deterioration in the emissions of NPADPA and BMTDPA. In the solid state, PPODPA suffered from red-shifted and weakened emissions because of adverse crystallization, which is promoted by the dipolar nature of the diphenylphosphine oxide substituent. Non-dopant OLEDs were fabricated with DPA, TPSDPA, TMPDPA, PPODPA, CBZDPA, and OXDDPA. Except for PPODPA, the electroluminescence efficiency of these DPA derivatives was significantly improved compared with that of the DPA OLEDs. In particular, CBZDPA and OXDDPA OLEDs exhibited the best external quantum efficiency of 4.5% and 4.0% with a true blue colour, with CIEx,y coordinates of (0.17, 0.17) and (0.16, 0.18), respectively. The improved electroluminescence efficiency can be attributed to the molecular charge transport design of CBZDPA and OXDDPA.

Solution-processed bipolar small molecular host materials for single-layer blue phosphorescent organic light-emitting diodes

Three new solution processable small molecular host materials based on the bis-[3,5-di(9H-carbazol-9-yl)phenyl] structural moiety have been developed for blue phosphorescent (FIrpic dopant) organic light-emitting diodes. All three host materials have been characterized as having high glass transition temperatures (Tgs), 155–175 °C, indicative of good morphological stability of their amorphous thin films prepared from the solution process. Whereas N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]methylamine (CzPAMe) has the highest solid state triplet energy gap (ET) of 2.73 eV, tetrakis-[3,3′,5,5′-(9H-carbazol-9-yl)]triphenylphosphine oxide (CzPPO) and N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]pyrimidin-2-amine (CzPAPm) are two host materials which are potentially bipolar for charge transport due to the electron deficient units of phenylphosphine oxide and pyrimidine, respectively. Due to the insufficient ET (2.56 eV) of CzPAPm, CzPPO or CzPAMe devices are significantly better than CzPAPm devices with or without a 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) co-host. Particularly, having no OXD-7 co-host and no vacuum thermal-deposited extra electron transporting layer, single-layer devices of CzPPO surpass CzPAMe devices and reach current efficiencies as high as 9.32 cd A−1 (or power efficiency of 4.97 lm W−1), one of the highest efficiencies among small molecular devices with the same fabrication process and same device configuration.

High-efficiency small-molecule-based organic light emitting devices with solution processes and oxadiazole-based electron transport materials.

We demonstrate high-efficiency small-molecule-based white phosphorescent organic light emitting diodes (PHOLEDs) by single-active-layer solution-based processes with the current efficiency of 17.3 cdA(-1) and maximum luminous efficiency of 8.86 lmW(-1) at a current density of 1 mA cm(-2). The small-molecule based emitting layers are codoped with blue and orange phosphorescent dyes. We show that the presence of CsF/Al at cathodes not only improves electron transport in oxadiazole-containing electron transport layers (ETLs), but also facilitates electron injection through the reacted oxadiazole moiety to reduce interface resistance, which results in the enhancement of current efficiency. By selecting oxadiazole-based materials as ETLs with proper electron injection layer (EIL)/cathode structures, the brightness and efficiency of white PHOLEDs are significantly improved.

The first aggregation-induced emission fluorophore as a solution processed host material in hybrid white organic light-emitting diodes

A judicious structure design of bis(3,5-di(9H-carbazol-9-yl)phenyl)amine (CzPA) and 1,1,2,2-tetraphenylethene (TPE) hybrid generates the first aggregation-induced emission (AIE) fluorophore, CzPATPE, as a solution processed host material in a hybrid white organic light-emitting diode (OLED). Small molecule CzPATPE has been verified for the amorphous characteristics by a featureless halo in powder X-ray diffraction spectra and a glass transition temperature as high as 180 °C. Fluorescence images indicate that a homogenous thin film of CzPATPE is obtained from a solution drop-casting process. Solution (THF/water) fluorescence spectroscopy study clearly demonstrates typical AIE properties of CzPATPE. Whereas CzPATPE is virtually non-emissive in common organic solvents, thin film photoluminescence (PL) spectroscopy shows that CzPATPE has a strong sky blue emission around 495 nm with a relatively wide full width at half maximum (FWHM) about 95 nm and a reasonable PL quantum yield of 40%. Low temperature time-gated PL spectroscopy identifies CzPATPEs triplet optical gap energy ET ∼ 2.05–2.16 eV, which is barely enough for the phosphorescent dopant, tris(2-phenylquinoline) iridium(III) (Ir(2-phq)3) (ET ∼ 2.13 eV). CzPATPE is solution processable to be a sky blue emissive host material for an orange dopant in the hybrid white OLED, ITO/PEDOT:PSS (65 nm)/CzPATPE:Ir(2-phq)3(99.9 : 0.1, ∼40 nm)/TmPyPB (50 nm)/CsF (2 nm)/Al (100 nm). This AIE hybrid white OLED displays white electroluminescence (EL) with a colour chromaticity of CIEx,y(0.36, 0.43). Although the maximum EL efficiency is modest 3.4 cd A−1 or 1.8 lm W−1, the white OLED has a colour rendering index (CRI) of 72, which is unusually high considering the orange dopant and a two-colour white feature. We attribute such CRI to the relatively large FWHM of CzPATPE.

Emitter apodization dependent angular luminance enhancement of microlens-array film attached organic light-emitting devices

Taking organic emitter apodization calculated from electromagnetic theory as input, the angular luminance enhancement of a microlens-array-film (MAF) attached OLED (organic light-emitting device) can be further evaluated by ray-tracing approach. First, we assumed artificial emitters and revealed that not every OLED with MAF has luminance enhancement. Then, the OLEDs of different Alq(3) thickness were fabricated and their angular luminance measurement validated simulation results. Mode analyses for different layers were performed to estimate the enhancement potential of the MAF attached devices. In conclusion, the organic emitters with higher off-axis-angle luminous intensity cause lower out-coupling efficiency but gain higher enhancement after the MAF attached.

Improve efficiency of white organic light-emitting diodes by using nanosphere arrays in color conversion layers

The authors demonstrated an efficient color conversion layer (CCL) by using nanosphere arrays in down-converted white organic light-emitting diodes (WOLEDs). The introduced periodical nanospheres not only helped extract the confined light in devices, but also increased the effective light path to achieve high-efficiency color conversion. By applying a CCL with red phosphor on a 400-nm-period nanosphere array, we achieved 137% color conversion ratio for blue OLEDs, which was 2.68 times higher than conventional flat CCL. The resulting luminous efficiency of WOLEDs with patterned CCLs (20.97 cd/A, 1000 cd/m2) was two times higher than the efficiency of the flat device (10.26 cd/A, 1000 cd/m2).

Pump-Probe Study of E(TO) Polaritons in Zn-Doped LiNbO3

Results of femtosecond pump-probe measurement of E(TO)-like polaritons in Zn-doped lithium niobate crystals are presented. The impulsive-stimulated-Raman-induced three delay-time oscillations, ω1, ω2 and ω3, are found and assigned to the linear combinations of three lowest E(TO)-like polaritons, ν1, ν2, and ν3, which are ω1=ν2-ν1, ω2=ν3-ν1 and ω3=2ν1. The obtained frequencies of ω1, ω2 and ω3 are well explained by dielectric dispersion calculation. The lifetimes of these oscillations induced by the electro optical effect are also deduced, and they are strongly correlated with the Raman lineshape of E(TO) phonons. Our results also imply that the vibrations of E(TO1) and E(TO2) are mainly attributed to the Nb and O atomic motions, while the vibration of E(TO3) is dominated by the defect structure.

Free‐carrier absorption of Hg1−xCdxTe epitaxial films

Free‐carrier absorption (FCA) of Hg1−xCdxTe epitaxial films is analyzed by considering the composition‐in‐depth nonuniformity of epilayers. The results show that epilayers exhibit different FCA behavior from bulk materials. Based on the analyses, the carrier concentration, the density and size distribution of Te precipitates, as well as the inclusion in Hg1−xCdxTe epilayers are derived from fitting the measured FCA spectra.

New platinum complexes exhibiting host dependent photoluminescence as single dopants in double emitting layer, voltage independent hybrid white electroluminescence devices

Four heteroleptic platinum complexes (FPtXNDs) bearing 4-hydroxy-1,5-naphthyridine (XND) derivatives functionalized with phenyl (X = Ph), dimethylamino (X = dma), morpholino (X = mor), or phenoxazino (X = pxz) units as one ligand and 2-(2,4-difluorophenyl)pyridine (F) as the other common ligand were newly synthesized and characterized. Each platinum complex has absorption and photoluminescence (PL) spectroscopic characteristics. As demonstrated by X-ray crystallography (of FPtdmaND and FPtpxzND), electrochemical and theoretical studies, the unusual spectroscopic features of FPtpxzND (such as its weak absorption band around 450–600 nm, its significant concentration dependent solution PL wavelength but low host dependent thin film PL wavelength (which is relatively long) and its particularly low solution PL quantum yield (ΦPL)) the relatively strong absorption of FPtdmaND and FPtmorND, and the relatively long PL wavelength of FPtPhND have been reasonably explained. After each platinum complex had been examined for monochromatic organic light-emitting diodes (OLEDs), having a relatively high ΦPL as well as an appropriate greenish yellow PL in the monomeric form and an orange to red PL in the aggregate/excimeric form, FPtdmaND and FPtmorND have been chosen as dopant materials in the construction of single or double emitting layer (EML) hybrid white organic light-emitting diodes (OLEDs). A high performance warm white electroluminescence (EL) (CIEx,y (0.42–0.43, 0.43–0.44)) was obtained where the same platinum complex dopant FPtdmaND was doped (5 or 8 wt%) in two EMLs, 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP) harvesting monomeric emission and the highly fluorescent blue N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1′′:4′′,1′′′-quaterphenyl]-4,4′′′-diamine (4P-NPD) harvesting excimer/aggregate emission. This warm white EL showed a high colour rendering index (CRI) of 86–88, and a reasonably good EL efficiency of 9.7%, 22.5 cd A−1, or 10.7 lm W−1. EL with improved white chromaticity (CIEx,y (0.37–0.39, 0.39–0.41)) can be obtained using FPtmorND as the single dopant, although it has a somewhat inferior CRI (84–86) and EL efficiency (8.3%, 20.1 cd A−1, or 9.5 lm W−1). Nevertheless, both double EML hybrid white OLEDs exhibit a rather constant CRI and CIEx,y under a driving voltage between 7 and 10 V.

Device‐dependent angular luminance enhancement and optical responses of organic light‐emitting devices with a microlens‐array film

— By taking the organic emitter apodization calculated from electromagnetic theory as input, the angular luminance enhancement of organic light-emitting devices (OLEDs) with a microlens-array film (MAF) can be further evaluated by the ray-tracing approach. First, the OLEDs of different Alq3 thickness are fabricated and their angular luminance measurements are compared to simulation results. Second, mode analyses for different layers are performed to estimate the enhancement potential of the MAF-attached devices. Finally, by decreasing the Alq3 thickness, increasing the viewing angle, and attaching the MAF, the EL spectral peak shifts of the OLEDs seem irregular, but the spectral blue shifts induced by the optical structures are all explained by the optical responses (EL spectra divided by the intrinsic PL spectrum). In conclusion, the organic emitters with higher off-axis-angle luminous intensity cause lower out-coupling efficiency but gain higher enhancement after the MAF is attached. With the choices of apodizations and microstructures, the tailored or customized angular radiation patterns can be also made possible.