Highly Efficient Phosphorescent Light-Emitting Diodes by Using New Host Material.

An electron-transporting triazole (Tz) and hole transporting carbazole (Cz) moieties were introduced into the fluorene structure via a Suzuki coupling reaction. The resulting material, 9-(4'-(9-(4'-(4,5-diphenyl-4H-1,2,4-triazol-3-y)-[1,1'-biphenyl]-4-yl)-9H-fluoren-9-yl)-[1,1'-biphenyl]-4-yl)-9H-carbazole (Tz-FL-Cz), was synthesized and can be used as the host layer in phosphorescent OLEDs. The synthesized material exhibits wide energy gap (3.46 eV). The absorption and emission maximum of the Tz-FL-Cz was observed at 300 nm and 389 nm, respectively. Blue phosphorescent devices using Tz-FL-Cz as host material exhibited maximum current efficiency of 3.2 cd/A and power efficiency of 1.3 lm/W, respectively.

Synthesis and characterization of poly(vinylpyrrolidone)-capped silicon-nanoparticles.

Polymer-capped silicon nanoparticles (Si NPs) were prepared using poly(vinylpyrrolidone) (PVP) as a protection layer to minimize the surface oxidation of the surface. The wet synthetic procedure were performed at room temperature via a one-pot synthesis. The PVP capped Si-NPs were characterized by thermo gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), high resolution transmission electron microscopy (HR-TEM) and field emission scanning electron microscopy (FE-SEM). HR-TEM micrograms confirm the presence of crystalline Si-NPs. The pattern by Selected Area Electron Diffraction (SAED) reveals (111), (220) lattice planes which are consistent with the cubic-structured crystalline silicon. The mean size of Si-NPs is estimated to be ca. 10 nm, which is larger than those from conventional wet synthetic methods for silicon quantum dots.

Structural modification of fluorene-thiophene-based copolymers for OPVs.

A series of fluorene-thiophene-based pi-conjugated copolymers, PFT2, PFT4, and PFT2BP, have been synthesized using the Suzuki coupling reaction. Compared to PFT2 and PFT2BP, PFT4 film showed clearly red-shifted UV-visible absorption and photoluminescence (PL) spectra with maxima at 474 and 597 nm, respectively. The optical and electrochemical properties of copolymers could be tuned by changing conjugation lengths of comonomer units from bithiophene (PFT2) to quaterthiophene (PFT4). Moreover, PFT4 containing quaterthiophene showed relatively lower optical band gap of 2.16 eV. The organic photovoltaic cells (OPVs) were fabricated using the copolymers as donor with a sandwich configuration of ITO/PEDOT:PSS/copolymer:PC71BM/LiF/Al. PFT4 showed the best device performance with the optimized power conversion efficiency (PCE) of 2.26% (J(SC) = 7.67 mA/cm2, V(OC) = 0.81 V, FF = 0.36). The higher OPV performance of PFT4 can be explained by its relatively low energy band gap and improved UV absorption property.

Synthesis and photovoltaic properties of a new thiophene-cyclopentadiene-based conjugated polymer.

A new low-band gap polymer containing thiophene and cyclopentadiene, poly(5,2,2'-dioctyldithio-phenylcyclopentadiene) (PDTCP), has been synthesized via the FeCl3 oxidative polymerization. PDTCP showed a broad absorption band and a low energy band gap of 1.82 eV. The photoluminescence (PL) of PDTCP is completely quenched upon addition of PCBM indicative of efficient charge transfer. Bulk heterojunction organic photovoltaic cells (OPVs) fabricated from PDTCP as an electron donor showed an open-circuit voltage (V(OC)) of 0.50 V, a short-circuit current (J(SC)) of 1.24 mA/cm2, and the power conversion efficiency of up to 0.20% under AM 1.5 (100 mW/cm2).

Synthesis and photovoltaic properties of new conjugated polymers based on benzo[1,2-b:4,5-b']dithiophene.

New conjugated polymers based on benzo[1,2-b:4,5-b']dithiophene, poly(benzo[1,2-b:4,5-b'] dithiophene-alt-bithiophene) (PBT-2T) and poly(benzo[1,2-b:4,5-b']dithiophene-alt-terthiophene) (PBT-3T), have been synthesized via the Stille coupling reaction and their optical and electrochemical properties were characterized by UV-vis, photoluminescence (PL) and cyclic voltammetry (CV) measurements. Compared to PBT-2T, PBT-3T film showed clearly red-shifted UV-visible absorption and PL spectra with maxima at 511 and 653 nm, respectively. Especially, PBT-3T containing terthiophene showed very small optical band gap of 1.65 eV. The PBT-2T and PBT-3T showed the power conversion efficiencies of 0.25% and 0.34%, respectively, under AM 1.5 (100 m/cm2).

Improved photovoltaic properties of fluorene-thiophene-based copolymers by an electron-transporting triazole unit in the main chain.

An electron-transporting triazole (Tz) unit was introduced into the fluorene-thiophene-based copolymer backbone via a Suzuki coupling reaction. The resulting copolymer, poly[9,9'-dioctyl-fluorene-co-5, 5-(4', 7'-di-2-thienyl-2', 1',3'-benzothiadiazole)-co-(4-(4-butyl-phenyl)-3, 5-diphenyl-4H-1,2,4]triazole)] (PF3TBTz), was soluble in common organic solvents and can be easily used as the active layer in organic photovoltaic cells (OPVs). By the introduction of the triazole unit, the OPV performance was nearly doubled from 0.62% (PF3TB) to 1.25% (PF3TBTz) under the same conditions. The higher performance can be explained by the improved surface morphology, resulting in better charge photogeneration and higher short circuit current (J(sc)) value in PF3TBTz in comparison with PF3TB. The possibility of the use of triazole units in OPV applications are described herein.

Polymeric white organic light-emitting diodes with two emission layers.

A white light-emitting diode was fabricated by preparing multilayer emitting films with an inserted buffer layer. The device structures are ITO/PEDOT:PSS/Emissive layer/LiF/Al. The emissive layer comprises a yellow-emitting layer of Poly[9,9-dioctylfluorenyl-2,7-diyl]-co-1,4-benzo-(2,1,3)-thiadiazole (F8BT), a blue-emitting layer of Poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl] (BEHF) and PEDOT:PSS as a buffer layer between the emission layers. The solution processed multi-layer polymer light-emitting diodes (PLED) were prepared by introduction of a water-soluble buffer layer between organic solvent soluble layers. We present white organic light-emitting diodes (WOLEDs) that has bilayer emission zones. This device exhibits a brightness of 280 cd/m2 and emission efficiency of 1.18 cd/A at 12.6 V. The device with a doped PEDOT:PSS layer and a thicker blue-emission layer exhibits CIE color coordinates of (0.30, 0.34), which is close to the white coordinates of (0.33, 0.33) used by the standard CIE color coordinates.

Synthesis and characterization of regioregular alkyl-substituted poly(3,4-ethylenedioxythiophenes).

In order to increase the solubility of PEDOT, we introduced an alkyl chain into polymer backbone. Depending on the position of the alkyl bond, the steric hindrance between the alkyl chains can also leads to regioregularity in the polymer. The regioregular polymers can easily occur self-assembly, both in solution and in the solid state, resulting in highly ordered two- and three-dimensional polymer architectures. A series of soluble regioregular alkyl-substituted PEDOTs were synthesized using the Grignard Metathesis (GRIM) for the purpose of study on regiorandom and regioregular PEDOT. The ethylene proton adjacent to the oxygen atom of regioregular methoxydodecyl-PEDOT appears at 3.51 ppm, whereas the corresponding peak of regiorandom methoxydodecyl-PEDOT appears at 3.42 ppm.

Synthesis, characterization, and electrical properties of regioregular alkyl-substituted PEDOTt.

The regioregular hexyl-substituted poly(3,4-ethylenedioxy-2-thiophenylthiophene) (PEDOTt), structurally alternating copolymer of EDOT and thiophene, was synthesized via the Grignard Metathesis (GRIM) using catalytic amounts of Ni(dppp)Cl2. The products were characterized by 1H NMR spectroscopy, UV-vis spectroscopy and XRD. The alkyl-substituted conjugated polymer showed an enhancement of solubility in common organic solvents. Based on XRD data, the regioregular hexyl-substituted poly(3,4-ethylenedioxy-2-thiophenylthiophene) (PEDOTt) shows better crystallinity than the regiorandom hexyl-substituted PEDOTt owing to self-ordering. In addition, regioregular hexyl-PEDOTt has a conductivity of 0.788 S/cm by iodine doping.

Quantitative analysis of X-ray absorption spectra using a 2D map representation.

A promising possibility for the quantitative analysis of X-ray absorption near edge structure (XANES) spectra of nanosized electrode materials is demonstrated. We used a 2D map representation technique, which utilizes the values of the first derivatives of the absorbance with respect to the inserted Li(+) content plotted over the two-dimensional space defined by the inserted Li(+) content (mole) versus photon energy (eV) as a single map. The technique was applied to XANES spectra of the Li(y)CoO system in the first Li(+) insertion reaction for determining the structural and electronic variations associated with the change in Li(+) content. The obtained show that the intensities of two peaks at 7725 and 7711 eV increased with the Li(+) content and the difference of intensity change of these two peaks carried out for successive couples of spectra yielded the largest changes at 1.05 and 1.98 mol of Li content. This approach for quantitative analysis of XANES without using conventional simulation techniques enable us to interpret X-ray absorption spectroscopy (XAS) as a quantitative analytical technique with greater confidence.

Improved photovoltaic performance of P3HT:PCBM cells by addition of a low band-gap oligomer.

The introduction of a low band-gap oligomer, oligo(benzo[1,2,5]thiadiazole-alt-3,3'''-dihexylquaterthiophene) (BT4T) improved the performance of P3HT:PCBM bulk heterojunction organic photovoltaic cells due to improved UV-vis absorption and increased P3HT crystallinity.