Blue Phosphorescent Bipyridine-Based Iridium(III) Complex for Vacuum-Deposited Organic Light-Emitting Diodes.

We have synthesized and characterized a blue phosphorescent iridium(III) complex (dfpypy)2Ir(tftamp), which contains 2',6'-difluoro-2,3'-bipyridine (dfpypy) as the main ligand and 4-methyl-2-(3'-trifluoromethyl-1'H-1',2',4'-triazol-5'-yl)pyridine (tftamp) as the ancillary ligand. The photophysical, electrochemical, and electroluminescent (EL) properties of (dfpypy)2Ir(tftamp) were investigated. Vacuum-deposited blue and white organic light-emitting diodes (OLEDs) were fabricated using (dfpypy)2Ir(tftamp) in 1,3-bis(carbazol-9-yl)benzene (mCP) as the emitting layer. The EL spectrum of (dfpypy)2Ir(tftamp) exhibited emission maximum at 472 nm with a full-width at half-maximum (FWHM) of 81 nm and Commission Internationale de L'Eclairage (CIE) coordinates of (0.17, 0.27) at 100 cd · m-2. In addition, white-light-emitting devices were fabricated, which exhibited CIE coordinates of (0.42, 0.40) and a correlated color temperature (CCT) of 3,237 K at 1000 cd · m-2, close to the standard warm-white light CIE coordinates of (0.44, 0.40) and CCT of 3,000 K.

Synthesis of a Zr-Based Metal-Organic Framework with Spirobifluorenetetrabenzoic Acid for the Effective Removal of Nerve Agent Simulants.

A new microporous Zr(IV)-based metal-organic framework (MOF) containing 4,4',4″,4‴-(9,9'-spirobi[fluorene]-2,2',7,7'-tetrayl)tetrabenzoic acid (Spirof-MOF) was synthesized, characterized, and size-controlled for the adsorption and decomposition of a nerve agent simulant, dimethyl 4-nitrophenylphosphate (DMNP). Spirof-MOF showed a hydrolysis half-life (t1/2) of 7.5 min to DMNP, which was confirmed by using in situ 31P NMR spectroscopy. Additionally, size-controlled Spirof-MOFb (∼1 µm) exhibited a half-life of 1.8 min and 99% removal within 18 min for DMNP. The results show that Spirof-MOF is a new active material in removing nerve agent simulants by adsorption and hydrolytic decomposition.

Synthesis and Characterization of a Soluble A-D-A Molecule Containing a 2D Conjugated Selenophene-Based Side Group for Organic Solar Cells.

A new acceptor-donor-acceptor (A-D-A) small molecule based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) is synthesized via a Stille cross-coupling reaction. A highly conjugated selenophene-based side group is incorporated into each BDT unit to generate a 2D soluble small molecule (SeBDT-DPP). SeBDT-DPP thin films produce two distinct absorption peaks. The shorter wavelength absorption (400 nm) is attributed to the BDT units containing conjugated selenophene-based side groups, and the longer wavelength band is due to the intramolecular charge transfer between the BDT donor and the DPP acceptor. SeBDT-DPP thin films can harvest a broad solar spectrum covering the range 350-750 nm and have a low bandgap energy of 1.63 eV. Solution-processed field-effect transistors fabricated with this small molecule exhibit p-type organic thin film transistor characteristics, and the field-effect mobility of a SeBDT-DPP device is measured to be 2.3 × 10-3 cm2 V-1 s-1 . A small molecule solar cell device is prepared by using SeBDT-DPP as the active layer is found to exhibit a power conversion efficiency of 5.04% under AM 1.5 G (100 mW cm-2 ) conditions.

Impact of the Crystalline Packing Structures on Charge Transport and Recombination via Alkyl Chain Tunability of DPP-Based Small Molecules in Bulk Heterojunction Solar Cells.

A series of small compound materials based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) with three different alkyl side chains were synthesized and used for organic photovoltaics. These small compounds had different alkyl branches (i.e., 2-ethylhexyl (EH), 2-butyloctyl (BO), and 2-hexyldecyl (HD)) attached to DPP units. Thin films made of these compounds were characterized and their solar cell parameters were measured in order to systematically analyze influences of the different side chains of compounds on the film microstructure, molecular packing, and hence, charge-transport and recombination properties. The relatively shorter side chains in the small molecules enabled more ordered packing structures with higher crystallinities, which resulted in higher carrier mobilities and less recombination factors; the small molecule with the EH branches exhibited the best semiconducting properties with a power conversion efficiency of up to 5.54% in solar cell devices. Our study suggested that tuning the alkyl chain length of semiconducting molecules is a powerful strategy for achieving high performance of organic photovoltaics.

Thermally cross-linkable hole transport polymers for solution-based organic light-emitting diodes.

Two thermally cross-linkable hole transport polymers that contain phenoxazine and triphenylamine moieties, X-P1 and X-P2, are developed for use in solution-processed multi-stack organic light-emitting diodes (OLEDs). Both X-P1 and X-P2 exhibit satisfactory cross-linking and optoelectronic properties. The highest occupied molecular orbital (HOMO) levels of X-P1 and X-P2 are -5.24 and -5.16 eV, respectively. Solution-processed super yellow polymer devices (ITO/X-P1 or X-P2/PDY-132/LiF/Al) with X-P1 or X-P2 hole transport layers of various thicknesses are fabricated with the aim of optimizing the device characteristics. The fabricated multi-stack yellow devices containing the newly synthesized hole transport polymers exhibit satisfactory currents and power efficiencies. The optimized X-P2 device exhibits a device efficiency that is dramatically improved by more than 66% over that of a reference device without an HTL.

High-crystalline medium-band-gap polymers consisting of benzodithiophene and benzotriazole derivatives for organic photovoltaic cells.

Two semiconducting conjugated polymers were synthesized via Stille polymerization. The structures combined unsubstituted or (triisopropylsilyl)ethynyl (TIPS)-substituted 2,6-bis(trimethylstannyl)benzo[1,2-b:4.5-b']dithiophene (BDT) as a donor unit and benzotriazole with a symmetrically branched alkyl side chain (DTBTz) as an acceptor unit. We investigated the effects of the different BDT moieties on the optical, electrochemical, and photovoltaic properties of the polymers and the film crystallinities and carrier mobilities. The optical-band-gap energies were measured to be 1.97 and 1.95 eV for PBDT-DTBTz and PTIPSBDT-DTBTz, respectively. Bulk heterojunction photovoltaic devices were fabricated and power conversion efficiencies of 5.5% and 2.9% were found for the PTIPSBDT-DTBTz- and PBDT-DTBTz-based devices, respectively. This difference was explained by the more optimal morphology and higher carrier mobility in the PTIPSBDT-DTBTz-based devices. This work demonstrates that, under the appropriate processing conditions, TIPS groups can change the molecular ordering and lower the highest occupied molecular orbital level, providing the potential for improved solar cell performance.

A highly crystalline low band-gap polymer consisting of perylene and diketopyrrolopyrrole for organic photovoltaic cells.

A new, low band-gap donor-acceptor-type conjugated polymer, PDPP-PER, comprising 3,9-perylene (PER) and diketopyrrolo[3,4-c]pyrrole (DPP) was synthesized. The crystallinity of the synthesized polymer film significantly improved with thermal annealing to 150 °C. The PCE of the PDPP-PER device reached 6.35% with a high open-circuit voltage of 0.79 V.

An easily accessible donor-π-acceptor-conjugated small molecule from a 4,8-dialkoxybenzo[1,2-b:4,5-b']dithiophene unit for efficient solution-processed organic solar cells.

A new donor-acceptor-conjugated organic small molecule, BDT(TBT)(2), comprised of benzo[1,2-b:4,5-b']dithiophene and 2,1,3-benzothiadiazole units was designed and synthesized. The small molecule BDT(TBT)(2) in its thin film showed an absorption band in the range of 300-700 nm with an absorption edge at 650 nm and an optical band gap of 1.90 eV. As estimated from the cyclic voltammetry measurements, the HOMO and LUMO energy levels of BDT(TBT)(2) were -5.44 and -3.37 eV, respectively. The spin-coated thin film of BDT(TBT)(2) exhibited p-channel output characteristics with a hole mobility of 2.7 × 10(-6). BDT(TBT)(2), when explored as an electron-donor material in solution-processed bulk-heterojunction organic solar cells in conjunction with a PC(71)BM acceptor with an active layer thickness of 50-55 nm, generated a power conversion efficiency (PCE) of 1.18%. A more impressive PCE of ~2.9% with a short-circuit current density (J(sc)) of 7.94 mA cm(-2) and an open-circuit voltage (V(oc)) of 0.89 V was achieved when the active layer of the cell was annealed at higher temperature (~180 °C).

Synthesis of fluorene-based semiconducting copolymers for organic solar cells.

Semiconducting polymers composed of 2,2'-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophenes (F8T2s) and (2E,2'E)-3,3'-(2,5-bis(octyloxy)-1,4-phenylene) bis(2-(5-bromothiophene-2-yl)acrylonitrile)s (OPTANs) have been synthesized through Pd(O)-catalyzed Suzuki coupling polymerization by controlling the monomer ratio. The synthesized polymers were confirmed to exhibit good solubility in common solvents, simple processability, and thermal stability up to 350 degrees C. The highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), and optical band-gap energies were determined using cyclic voltammetry (CV) and UV-visible spectrometry. The synthesized polymers showed their maximum absorption and edge at around 520 and 650 nm, respectively. The optical band-gap energies of the polymers were determined to be 1.89 eV. Bulk heterojunction organic solar cells were fabricated using the conjugated polymer as the electron donor, and 6,6-phenyl C61-butyric acid methylester (PC61BM) or 6,6-phenyl C71-butyric acid methylester (PC71BM) as the electron acceptor. The power conversion efficiencies (PCEs) of the solar cells based on polymer:PC71BM (1:1) and polymer:PC71BM (1:2) were 0.68% and 1.22%, respectively, under air mass 1.5 global (AM 1.5 G) illumination at 100 mW/cm2.

Development of naphtho[1,2-b:5,6-b']dithiophene based novel small molecules for efficient bulk-heterojunction organic solar cells.

Two new small molecules with a rigid planar naphtho[1,2-b:5,6-b']dithiophene (NDT) unit were designed and synthesized. Solution processed bulk-hetereojunction organic solar cells based on blends of the small molecules and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) exhibited promising photovoltaic device performance with a maximum power conversion efficiency up to 2.20% under the illumination of AM 1.5G, 100 mW cm(-2).

Synthesis and properties of copolymers composed of arylenevinylene and phenothiazine for organic solar cells.

A series of new organic semiconducting copolymers composed of {(2E,2'E)-3,3'-[2,5-bis(octyloxy)-1,4-phenylene]-bis[2-(thiophen-2-yl)acrylonitrile]}(OPTAN) and 10(2'-ethylhexylphenothiazine) (PTZ) monomers, (the copolymers are hereafter referred to as poly(OPTAN-co-PTZ)s), were synthesized by using Suzuki coupling polymerization in which the monomer ratios were controlled. An increase in the OPTAN content shifted the peak and onset absorption of the copolymers to the longer wavelength regions, which resulted in a decrease in the band gap energy. The maximum UV absorption of the polymer films was in the range 523-540 nm and the optical band gap energies were in the range 1.90-1.87 eV. Energy levels of the highest occupied molecular orbital (HOMO) of the polymers were determined by cyclic voltammetry (CV). The HOMO energy level of the copolymers was between -5.07 and -5.12 eV. Photovoltaic devices were fabricated by using the copolymers as the p-type donor and C60-PCBM or C70-PCBM as the electron acceptors. The device with poly(50OPTAN-alt-50PTZ) and C70-PCBM showed the best performance among the fabricated devices; the open circuit voltage, short circuit current, fill factor, and maximum power conversion efficiency of this device were 0.79 V, 5.25 mA/cm2, 0.30, and 1.25%, respectively.

Synthesis and characterization of a thiazolo[5,4-d]thiazole-based copolymer for high performance polymer solar cells.

A highly processable, new semiconducting polymer, PCDTTz, based on alternating thiazolothiazole and carbazole units was synthesized. The new polymer exhibited a field-effect carrier mobility of up to 3.8 × 10(-3) cm(2) V(-1) s(-1) and bulk heterojunction solar cells made from PCDTTz produced a power conversion efficiency of 4.88% under AM 1.5 G (100 mW cm(-2)) conditions.

Effects of post thermal annealing on the electrical properties of vertical type organic thin film transistors using poly(3-hexylthiophene) and its application in organic light emitting transistor.

We have fabricated the vertical type organic thin film transistor (OTFT) using electrically conductive poly(3-hexylthiophene) (P3HT) as a p-type organic material. Effects of post thermal annealing and thickness of active layer on the performance of vertical type transistors were investigated. Especially, the correlation between carrier mobility of P3HT after post thermal annealing and static characteristics of the transistor was studied. Carrier mobility was calculated by space charge limited current (SCLC) model from the I-V curves of the prepared device. The vertical type OTFT after post thermal annealing at 120 degrees C (Tg) showed high current of 0.383 mA and on-off ratio of 22.5 at a low gate voltage of +2.0 V. Additionally, we report on emission characteristics from the vertical type transistor using P3HT.

Effects of morphological control on the characteristics of vertical-type OTFTs using Alq3.

We have fabricated vertical-type organic thin-film transistors (OTFTs) using tris-(8-hydroxyquinoline) aluminum (Alq(3)) as an n-type active material. Vertical-type OTFT using Alq(3) has a layered structure of Al(source electrode)/Alq(3)(active layer)/Al(gate electrode)/Alq(3)(active layer)/ITO glass(drain electrode). Alq(3) thin films containing various surface morphologies could be obtained by the control of evaporation rate and substrate temperature. The effects of the morphological control of Alq(3) thin layer on the grain size and the flatness of film surface were investigated. The characteristics of vertical-type OTFT significantly influenced the growth condition of Alq(3) layer.

Synthesis and characterization of highly twisted and bulky tetraoctyloxybiphenyl-containing polyfluorene copolymers: toward efficient blue polymer light emitting diodes.

We have synthesized new blue light emitting random copolymers, poly(9,9'-n-dioctylfluorene-co-2,2',6,6'-tetraoctyloxybiphenyl-3,3'-diyl)s (PFTOBPs), via Ni(0)-mediated coupling reactions. The PL emission peaks of the resulting copolymers closely resembled those of the polyfluorene (PF) homopolymer. The EL devices fabricated using these copolymers exhibited highly pure blue emission with approximate 1931 CIE coordinates of (0.15, 0.15) at 1000 cd/m2. The maximum brightnesses ranged from 2000 to 12000 cd/m2 with maximum efficiencies from 0.53 to 0.97 cd/A. The efficiencies were found to increase as the fraction of TOBP in the copolymers was increased, which may result the inhibition of exciton quenching that is produced by the introduction of the highly twisted and bulky TOBP moieties into the copolymers.