Hsiu-Chih Yeh

Blue Light-Emitting Devices Based on Molecular Glass Materials of Tetraphenylsilane Compounds**

Only the full gap, at 2 lm, is shaded along the whole Brillouin zone. We can establish a clear correspondence between all the peaks observed experimentally and the calculated PBGs along C‐L. This comparison indicates that the third reflectance peak (near k = 2.1 lm, as indicated with an asterisk) in the black line spectrum in Figure 4 corresponds to a full PBG in the inverted opal. Good agreement is also found between the theoretical and experimental full photonic gap to mid-gap ratio, being 8.6 % and 6.5 %, respectively. As light impinges on the samples at angles slightly deviated from the C‐L direction, reflectance peak widths and positions are slightly different from those extracted from Figure 5. Although the results shown here indicate the opening of a full PBG in the Ge inverse opal, different kinds of disorder might affect it, and a thorough optical analysis of all crystalline directions should be done to confirm it. This is the focus of future research. In summary, we have made germanium inverse opals by a CVD technique. By varying the synthesis conditions, the degree of germanium infiltration can be controlled, which enables the optical properties of the materials to be engineered. The optical characterization of the materials confirms their PBG behavior in the near infrared region. Good agreement has been found between theory and experiment which indicates that a full PBG has opened up near 2 lm.

Non-doped red organic light-emitting diodes

A convenient and improved procedure has been developed for preparing the red fluorophore N-methyl-bis(4-(N-(1-naphthyl)-N-phenylamino)phenyl)maleimide (NPAMLMe) through the efficiently synthesized bis(4-bromophenyl)fumaronitrile, the necessary precursor in preparing NPAMLMe. This allows NPAMLMe to be an easily accessible material compared with other known red, organic light-emitting diode (OLED) materials. We also report an unusual approach in fabricating red OLEDs, which does not adopt a conventional red dopant but rather NPAMLMe as the host red emitter. The performance of the non-doped red devices has been studied in depth for the first time. Devices with varied layer thickness were fabricated for examining the compatibility of NPAMLMe with commonly known materials, electron-transporting tris(8-hydroxyquinolinolato)aluminium (Alq3) and hole-transporting 4,4′-bis(4-(N-(1-naphthyl)-N-phenylamino)phenyl)biphenyl (NPB). In the presence of a hole-blocking layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), the devices emit pure red electroluminescence (EL), and it is essentially voltage-independent. Red EL with a brightness near 4600 cd m−2 and an external quantum efficiency as high as 1.6% has been achieved. The performance of such non-doped, red OLEDs is comparable with or better than contemporary, dopant-based, red OLEDs, and the simple fabrication is the advantage of the approach.

Readily synthesised arylamino fumaronitrile for non-doped red organic light-emitting diodes

Bright (maximum 10034 cd m(-2), 455 cd m(-2) at 20 mA cm(-2)) and efficient (maximum 2.4% at 4 mA cm(-2)) red (lambda(max)el 634-636 nm) organic light-emitting diodes employ arylamino-substituted fumaronitrile as the novel host emitter, which is readily prepared and easily purified.

The colourful fluorescence from readily-synthesised 3,4-diaryl-substituted maleimide fluorophores

A new synthesis procedure has been developed for a series of maleimide-based fluorophores, exhibiting a large variation of emission spectra spanning the entire visible range.

Optimization of Tetraphenylsilane-Based Blue Organic Light-Emitting Devices with Copper Phthalocyanine

Multilayer organic light-emitting devices emitting with blue elelctroluminescence were fabricated. Devices with and whithout a layer of copper phthalocyanine (CuPc) as hole injection material were studied. / (current density)-V (voltage)-L (luminance) characteristics and electroluminescence efficiency of both devices were compared.

Nondoping red fluorophores for red organic light-emitting diodes

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.

Blue light-emitting devices based on glassy tetraphenylsilane molecular materials

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.

Glassy charge-transporting and electroluminescent tetraphenylmethane-based molecules for organic light-emitting devices

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.

Electroluminescence of bipolar tetraphenylmethane-based molecular materials

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.

Tetraphenylmethane-Based 1,3,4-Oxadiazole as Electron Transporting Materials in Organic Light-Emitting Devices

A series of tetrahedral tetramers of 2,5-diphenyl substituted 1,3,4-oxadiazole compounds were synthesized and characterized for electron-transporting layer (ETL) in organic light-emitting diode (OLED). The multiple-branch design of the oxadiazole tetramers intends to increase the melting temperature and to generate glass phase of the low molar mass derivative such as 2-(4-biphenylyl)-5-(4- tert -butylphenyl)-1,3,4-oxadiazole (PBD). We observed temperatures of the glass phase transition for the oxadiazole tetramer with appropriate peripheral substituents, indicative of amorphous characteristics of the molecule in spite of highly symmetrical molecular framework. The luminescence-current-voltage characteristics of multilayer OLED devices containing the oxadiazole tetramer or PBD as ETL were examined to evaluate the efficiency of our multiple-branch molecular design.