Asanga B. Padmaperuma

Electron and hole transport in a wide bandgap organic phosphine oxide for blue electrophosphorescence

We report blue phosphorescent organic light-emitting devices (OLEDs) using an ambipolar host, N-(4-diphenylphosphoryl phenyl) carbazole (MPO12), doped with iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). The external quantum efficiency and operating voltage is 9.1(±0.1)% and 4.8V, respectively, measured at a brightness of 800cd∕m2 with no outcoupling enhancement. By varying the layer structure of the OLEDs, we show that MPO12 is capable of transporting both electrons and holes, in contrast to previous demonstrations using diphosphine oxides, which only transported electrons. The improved hole transport results in improved device efficiency.

Ultraviolet electroluminescence and blue-green phosphorescence using an organic diphosphine oxide charge transporting layer

We report electroluminescence at 338nm from a simple bilayer organic light-emitting device (OLED) made using 4,4′-bis(diphenylphosphine oxide) biphenyl (PO1). In an OLED geometry, the material is preferentially electron transporting. Doping the PO1 layer with iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C2′)picolinate (FIrpic) gives rise to electrophosphorescence with a peak external quantum efficiency of 7.8% at 0.09mA∕cm2 and 5.9% at 13mA∕cm2. The latter current density is obtained at 6.3V applied forward bias.

High efficiency and low roll-off blue phosphorescent organic light-emitting devices using mixed host architecture

We report high efficiency and low roll-off for blue electrophosphorescent organic light emitting devices based on a mixed host layer architecture. The devices were fabricated using a mixed layer of di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, a hole transport material, and 2,8-bis(diphenylphosphoryl)dibenzothiophene, an electron transport material, as the host layer doped with the blue phosphor iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate. Using a mixed layer as the host allowed us to achieve high power efficiency (59 lm/W at 100 cd/m2), low turn-on voltage (2.7 V for >10 cd/m2), and low roll-off in these devices.

An ambipolar phosphine oxide-based host for high power efficiency blue phosphorescent organic light emitting devices

We report blue electrophosphorescent organic light emitting devices with an ambipolar host material, 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), doped with FIrpic (iridium (III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate). The ambipolar nature of the host was verified using single carrier devices. The power efficiency of devices with PO15 (2,8-bis(diphenylphosphoryl)dibenzothiophene) electron transport layer (ETL) showed optimized performance when the ETL thickness was 500 A, giving a peak power efficiency of 46 lm/W (corresponding external quantum efficiency (EQE) of 17.1%). The EQE and power efficiency at the brightness of 800 cd/m2 were measured with no light outcoupling enhancement and found to be 15.4% and 26 lm/W, respectively.

Process Design and Economics for the Conversion of Lignocellulosic Biomass to Hydrocarbon Fuels: Fast Pyrolysis and Hydrotreating Bio-oil Pathway

This report describes a proposed thermochemical process for converting biomass into liquid transportation fuels via fast pyrolysis followed by hydroprocessing of the condensed pyrolysis oil. As such, the analysis does not reflect the current state of commercially-available technology but includes advancements that are likely, and targeted to be achieved by 2017. The purpose of this study is to quantify the economic impact of individual conversion targets to allow a focused effort towards achieving cost reductions.

Synthesis and application of pyridine-based ambipolar hosts: control of charge balance in organic light-emitting devices by chemical structure modification.

We studied the influence of a pyridine moiety versus a phenyl moiety when introduced in the molecular design of an ambipolar host. These pyridine-based host materials for organic light-emitting diodes (OLEDs) were synthesized in three to five steps from commercially available starting materials. The isomeric hosts have similar HOMO/LUMO energies; however, data from OLEDs fabricated using the above host materials demonstrate that small structural modification of the host results in significant changes in its carrier-transporting characteristics.

High-efficiency turquoise-blue electrophosphorescence from a Pt(II)-pyridyltriazolate complex in a phosphine oxide host

We demonstrate high-efficiency turquoise-blue electrophosphorescence from bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II) [Pt(ptp)2] doped in 4-(diphenylphosphoryl)-N,N-diphenylaniline(HM-A1). Organic light-emitting diodes (OLEDs) with 5% Pt(ptp)2:HM-A1 attain peak power efficiency of 61.2 lm/W, versus 40.8 lm/W for analogous devices employing the standard turquoise-blue phosphor bis[(4,6-difluorophenyl)-pyridinato-N,C2′](picolinato)iridium(III) (FIrpic). Devices with x% Pt(ptp)2:HM-A1 exhibit blue emission maxima (λmax∼480 nm) with monotonic increase in excimer/monomer intensity ratio at higher doping levels within 1%–10%, causing color shift toward green and less charge balance. This work represents a significant step toward optimizing future white OLEDs from the same phosphor via combination of low-doped and higher-doped or neat films.

Designing organic phosphine oxide host materials using heteroarmatic building blocks: inductive effects on electroluminescence

Phosphine oxide substitution of small molecules with high triplet exciton energies allows development of vacuum sublimable, electron transporting host materials for blue OLEDs. Heteroaromatic building blocks (carbazole, dibenzofuran and dibenzothiophene) with ET ~ 3 eV were incorporated into phosphine oxide (PO) structures. External quantum efficiencies (EQEs) at lighting brightness (i.e., 800 cd/m2) reached as high as 9.8% at 5.2V for OLEDs using the heteroaromatic PO hosts doped with the sky blue phosphor, iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C2,) picolinate (FIrpic). Comparing device properties at a similar current density (i.e., J = 13 mA/cm2) showed the dibenzothiophene-bridged PO compound exhibits the highest EQEs and lowest operating voltages at all phosphor dopant levels. These results are explained with respect to the effects of the inductive phosphine oxide substituents on electrochemical, photophysical and electroluminescence properties of the substituted heteroaromatic building blocks.

Design strategies for achieving high triplet energy electron transporting host materials for blue electrophosphorescence

High efficiency small molecule organic light emitting devices (OLEDs) based on light emission from an electrophosphorescent dopant dispersed in an organic host matrix are well known. Achieving blue phosphorescent OLEDs is particularly challenging because the host triplet energy should ideally be > 2.8 eV to prevent back-transfer of energy from the dopant to the host matrix resulting in loss of efficiency. A design strategy for developing new host materials with high triplet energies by using phosphine oxide (P=O) moieties as points of saturation in order to build sublimable, electron transporting host materials starting from small, wide bandgap molecular building blocks (i.e., biphenyl, phenyl, naphthalene, octafluorobiphenyl, and N-ethylcarbazole) is described. Electrophosphorescent OLEDs using the organic phosphine oxide compounds as host materials for the sky blue organometallic phosphor, iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C2,) picolinate (FIrpic) give maximum external quantum efficiencies of ~ 8% and maximum luminance power efficiencies up to 25 lm/W.

Tuning the optical properties of mesoporous TiO2 films by nanoscale engineering.

The optical properties of spin-coated titanium dioxide films have been tuned by introducing mesoscale pores into the inorganic matrix. Differently sized pores were templated using Pluronic triblock copolymers as surfactants in the sol-gel precursor solutions and adjusted by varying the process parameters, such as the polymer concentration, annealing temperature, and time. The change in refractive index observed for different mesoporous anatase films annealed at 350, 400, or 450 °C directly correlates with changes in the pore size. Additionally, the index of refraction is influenced by the film thickness and the density of pores within the films. The band gap of these films is blue-shifted, presumably due to stress the introduction of pores exerts on the inorganic matrix. This study focused on elucidating the effect different templating materials (Pluronic F127 and P123) have on the pore size of the final mesoporous titania film and on understanding the relation of varying the polymer concentration (taking P123 as an example) in the sol-gel solution to the pore density and size in the resultant titania film. Titania thin film samples or corresponding titanium dioxide powders were characterized by X-ray diffraction, cross-section transmission electron microscopy, nitrogen adsorption, ellipsometery, UV/vis spectrometry, and other techniques to understand the interplay between mesoporosity and optical properties.