Organic light-emitting diode (OLED) has gain numerous attentions since an efficient OLED was firstly demonstrated by Tang and VanSlyke in 1987. Thanks to lots of efforts paid on their progress including materials and device architecture in past three decades, red and green OLEDs have great success in efficiency and lifetime. However, the development of high efficiency deep-blue counterparts with Commission Internationale de L'Eclairage (CIE) coordinate of y<0.1 currently remains in demand in the market of full-color display applications. For example, to realize the BT.2020 color space standard, the standard blue emission must have CIE coordinates of (0.131,0.046), and that is extremely challenging, especially in material development.
Here, a new compound consisting of phenyls groups to connect a benzene core was successfully synthesized. The newly obtained compound exhibited a super wide bandgap of 3.5 eV and a deep-blue emission of approximately 397 nm as well as a photoluminescence quantum yield (PLQY) by 68% in thin film. Consequently, a non-doped OLED using the pristine new compound as emitting layer showed a peak efficiency of 4.9% in external quantum efficiency (EQE) and deep-blue emission with CIE coordinates of (0.16, 0.04). Note that the OLED configuration was bottom emission, which meant the such deep-blue emission resulted from the material itself, rather than microcavity effect. Grazing incidence wide-angle X-ray scattering (GIWAXS) of new compound displayed an order parameter (SGIWAXS) of 0.44, indicating molecules primarily aligned horizontally to the substrate, which contributed to the high efficiency.
Pure 2-iodo-9,9′-spirobifluorene was synthesized by an efficient method without troublesome iodination of 9,9- spirobifluorene (SP) or the Sandmeyer reaction of 2-amino-9,9′-spirobifluorene. A series of main group element-bridged bis-9,9′-spirobifluorene derivatives were synthesized via coupling reactions of 2-iodo-9,9′-spirobifluorene and main group element-containing precursors. These heteroatom-bridged bis-spirobifluorenes show large triplet state energy gaps, high glass transition temperatures, and varied charge-transporting properties advantageous to the host materials for blue phosphorescence organic light-emitting diodes (PhOLEDs).
A new series of platinum complexes containing 4-hydroxy-1,5-naphtyridine derivative with different substitutens such as methyl, dimethyl, phenyl, phenoxy, dimethyl amine, piperidine, morpholine, phenoxazine or carbazole unit as the primary ligand and 2-(2,4-difluorophenyl)pyridine as the secondary ligand were synthesized and characterized. Single crystal X-ray diffraction studies of FPtOPhND, FPtCzND and FPtdmaND showed trans-coordinated in distorted square-planar geometry. Their photophysical properties and electrochemical properties were examined. All platinum complexes in these series exhibited dual emissions not only in solution but also in solid state thin film. Employing CBP or 4P-NPD as host material, high efficiency monochromatic and high quality hybrid white organic light emitting diodes (WOLEDs) were achieved with the single platinum complex dopant device, a relatively simple device configuration.
A new series of mixed ligand platinum(II) complexes with a formula of FPtXND, where XND = 4-hydroxy-1,5-naphthyridine derivates and F = 2-(2,4-difluorophenyl)pyridine, were newly synthesized, and their photophysical properties were examined. Single crystal X-ray diffraction of FPtOPhND were determined to elucidate their variation of solid state phosphorescence and electroluminescence. Organic hole transporting as well as blue light-emitting NPB (1-naphthylphenylbiphenyl diamine) or 4P-NPD (1-naphthylphenylquaterphenyl diamine) was employed in the platinum complex-based hybrid white organic light emitting diodes (WOLEDs) with a simplified device configuration of ITO/4P-NPD or NPB/CBP:FPtXND/TPBI/LiF/Al or ITO/4P-NPD /4P-NPD:FPtXND/TPBI/LiF/Al.
Three new solution processable small molecular host materials based on bis-[3,5-di(9H-carbazol-9-yl)phenyl] structural moiety have been developed for blue phosphorescence (FIrpic dopant) organic light-emitting diodes. 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 potentially being bipolar for charge transport due to the electron deficient unit 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 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 surpassing CzPAMe devices reach current efficiency as high as 9.32 cd/A (or power efficiency of 4.97 lm/W), which is one of the highest of the kind. Corresponding single-layer white phosphorescence OLEDs are also fabricated with the small molecular host material demonstrated herein.
We present an in situ vacuum measurement in the study of electrical characteristics of n-channel OFETs based
on NTCDI-C8F15 semiconductors. Electron mobility of NTCDI-C8F15 OFETs was estimated as a function of
the number of ML using in situ electrical measurement. The electron mobility has been observed for the thin
film transistor of NTCDI-C8F15 as thin as 2 ML (~ 5.4 nm).
Field-effect mobilities rapidly increase with the
increase of the film thickness. Electron mobility of OFETs reaches saturation thickness (d0) about 3.5 ML. Our
experimental results indicated that the molecular layers beyond d0 contribute little to the carrier transport in the
semiconducting channel. Our experimental results have demonstrated that the grown fashion of the first few ML
of NTCDI-C8F15 on the substrate strongly influences the carrier mobility, threshold voltage, and on-off ratio.
Keywords: NTCDI, OFET, carrier mobility, in-situ, n-type semiconductor, monolayer
Long life-time molecular-based organic electronics, such as organic light-emitting diodes (OLEDs), organic solar
cells, or organic transistors etc, inevitably demand their constituent molecules to be highly thermal-stable. Coupling with
special needs in molecular design, the resultant increasing molecular weight (MW) will eventually make the molecules
difficult to deposit if via dry-process, while using wet-process would frequently result in undesired relatively poorer
efficiency. Surprisingly, two high-molecule composing OLEDs with relatively high-efficiency were obtained by using
solution-process. A blue OLED with a blue dye doped in a novel
high-MW, wide band-gap host,
3,5-di(9H-carbazol-9-yl) tetraphenylsilane (SimCP2), yielded 24 lm/W (38 cd/A) at 100 nits, and a green OLED using a novel
high-MW green dye, bis[5-methyl-7-trifluoromethyl-5H-benzo (c)(1,5) naphthyridin-6-one] iridium (picolinate)
(CF3BNO), yielded 70 lm/W (89 cd/A), while their dry-processed blue and green counterparts yield 1.7 and 21 lm/W,
respectively. Importantly, although the comparatively high MW has made the resulting molecules extremely difficult to
vacuum-evaporate and has resulted in poor device performance, the wet-process has been proven effective in fabricating
two high molecule-containing OLEDs with relatively high efficiency. The successful demonstration suggests that the
same approach may as well be extended to other organic devices that compose or should compose high molecules.
In this report, we have investigated electroluminsecence (EL) characteristics and field-induced recovery of organic light-emitting devices (OLEDs) with a mixed emitting layer (EML). The mixed EML which is composed of a mixture of a hole transport layer (HTL), N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB), and an electron transport layer (ETL), bis(10-hydroxybenzo[h]quinolinato) beryllium (Bebq2), was fabricated by co-evaporation. Evident recovery of luminance-voltage characteristic was observed in the mixed device. It is explained by a dipole rearrangement model. The lifetime of this mixed layer OLED can reach 348 min. with initial luminance of 11,000 cd/m2 which is two times better that of the comparable heterojunction device.
Crystallinic red fluorophores based on donor-acceptor substituted spirofluorene, i.e., PhSPDCV show strong fluorescence in solution (Φf ~ 70 %) as well as in solid state (Φf > 30 %). Non-doped red OLEDs fabricated with PhSPDCV exhibit authentic red (CIE, x = 0.65, y = 0.35) electroluminescence with brightness over 12,000 cd m-2 (or > 600 cd m-2 at 20 mA cm-2) and remarkable external quantum efficiency as high as 3.6%. On the other hand, the bis-substituted derivatives of spirofluorene BisPhSPDCV show relatively weak fluorescence both in solution (&PHgr;f < 20 %) and in solid state (Φf < 10%). Although saturated red electroluminescence (CIE, x = 0.65, y = 0.34) is also observed, non-doped red OLED containing BisPhSPDCV performs much worse than PhSPDCV OLEDs. Both PhSPDCV and BisPhSPDCV are not amorphous forming loosely packed crystallinic materials in solid state with no intimate π-π interaction.
One of the first bright (~8,000 cd/m2 of maximum electroluminance) and efficient (2.4% of maximum external quantum efficiency) saturated red (coordinates x = 0.66, y = 0.32 of 1931 CIE chromaticity) nondopign OLEDs has been achieved based on novel N-Methyl-bis(4-(N-(1-naphyl)-N-phenylamino)phenyl)maleimide red fluorophore. The developing process in achieving the high performance of the device is presented.
Bipolar p-TPAOXD and m-TPAOXD are both tetraphenylmethane- based isomeric compounds containing triphenylamine- substituted 1,3,4-oxadizaole moieties. Two are amorphous materials showing no melting or crystallization transition but only glass transition at 187 and 149 degree(s)C, respectively. Single-layer organic light-emitting devices based on p-TPAOXD or m-TPAOXD as active layer were fabricated via spin-casting method. With varied fabrication conditions (solvent, cathode material, and film thickness) devices with turn-on voltage as low as 4V, blue electroluminance with intensity of 1,700 cd/m2, and photometric efficiency larger than 0.7 cd/A can be achieved for p-TPAOXD. The luminance and photometric efficiency of the device containing m-TPAOXD are low (always less than 100 cd/m2 and 0.2 cd/A) as compared to those containing p- TPAOXD.
Tetraphenylsilane derivatives attached with one to four of N,N-diphenylaninooxadiazole(TPAOXD) blue fluorescent moieties, namely Si(PhTPAOXD)4, PhSi(PhTPAOXD)3, PH2Si(PhTPAOXD)2, and Ph3Si(PhTPAOXD), were synthesized and characterized. Tetraphenylsilane core structure provides an effective molecular skeleton for inhibiting the crystallization of the material. Very bright blue electroluminescence (>20,000 cd/m2) was observed for a trilayer device with configuration of ITO/NPB/PH3Si)PhTPAOXD)/Alq3/Mg:Ag. The electroluminescence of the device, in terms of emission wavelength and chromaticity, is highly dependent on the thickness of emitting layer. The current-voltage-electroluminescence characteristics of the devices are described.
Bipolar tetraphenylmethane derivatives carrying both hole transporting and electron transporting units were synthesized and characterized. Tetraphenylmethane core structure of the molecule provides an effective molecular skeleton for inhibiting the crystallization of the attached functional moieties and greatly enhances the glass-forming ability of the molecules. A single-layer light-emitting device was fabricated based on the highly luminescent bipolar tetraphenylmethane-based derivatives containing conjugated triphenylamino and 2,5 diaryl substituted 1,3,4- oxadiazole moieties. The current-voltage-electroluminescence characteristics of the devices are described.
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