Evgueni Polikarpov

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.

Thermal stability of MnBi magnetic materials

MnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The produced powder exhibits 74.6 emu g(-1) saturation magnetization at room temperature with 9 T applied field. After proper alignment, the maximum energy product (BH)max of the powder reached 11.9 MGOe, and that of the sintered bulk magnet reached 7.8 MGOe at room temperature. A comprehensive study of thermal stability shows that MnBi powder is stable up to 473 K in air.

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.

Highly efficient blue organic light emitting device using indium-free transparent anode Ga:ZnO with scalability for large area coating

Organic light emitting devices have been achieved with an indium-free transparent anode, Ga doped ZnO (GZO). A large area coating technique was used (RF magnetron sputtering) to deposit the GZO films onto glass. The respective organic light emitting devices exhibited an operational voltage of 3.7 V, an external quantum efficiency of 17%, and a power efficiency of 39 lm/W at a current density of 1 mA/cm2. These parameters are well within acceptable standards for blue OLEDs to generate a white light with high enough brightness for general lighting applications. It is expected that high-efficiency, long-lifetime, large area, and cost-effective white OLEDs can be made with these indium-free anode materials.

Development of MnBi permanent magnet: Neutron diffraction of MnBi powder

MnBi attracts great attention in recent years for its great potential as permanent magnet materials. MnBi phase is difficult to obtain because of the rather drastic peritectic reaction between Mn and Bi. In this paper, we report our effort on synthesizing high purity MnBi compound using conventional powder metallurgical approaches. Neutron diffraction was carried out to investigate the crystal and nuclear structure of the obtained powder. The result shows that the purity of the obtained powder is about 91 wt. % at 300 K, and the magnetic moment of the Mn atom in MnBi lattice is 4.424 and 4.013 μB at 50 K and 300 K, respectively.

Emission zone control in blue organic electrophosphorescent devices through chemical modification of host materials

We report blue organic light-emitting devices with iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate as an emitter doped into a series of phosphine oxide-based host materials that have significantly different charge transport properties: 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), N-(4-diphenylphosphoryl phenyl) carbazole (PO12), 9-[6-(diphenylphosphoryl)pyridin-3-yl]-9H-carbazole (HM-A5), and 6-(diphenylphosphoryl)-N,N-diphenylpyridin-3-amine (HM-A6). Depending on the nature of the host material, the location of the emission zone can be moved within the emissive layer from the hole transport layer interface to the electron-transport layer interface. The charge transport properties of the materials were evaluated using single carrier devices.

Synthesis and characterization of p-type conductivity dopant 2-(3-(adamantan-1-yl)propyl)-3,5,6-trifluoro-7,7,8,8-tetracyanoquinodimethane

We report the synthesis and characterization of 2-(3-(adamantan-1-yl)propyl)-3,5,6-trifluoro-7,7,8,8-tetracyanoquinodimethane (F3TCNQ-Ad1), a substituted analog of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), designed for p-type conductivity doping. The dopant is designed as a model for substituted alternatives to F4TCNQ that maintain similar electronic properties with the goal of engineering dopants with superior fabrication characteristics over F4TCNQ. We describe the design strategy for F3TCNQ-Ad1 based on molecular modeling predictions that substitution of a single fluorine atom of F4TCNQ has little effect on the electronic properties of the molecule. Photophysical and electrochemical characterization reveal that the adamantyl substituent in F3TCNQ-Ad1 does not significantly alter the electronic properties of the substituted dopant relative to F4TCNQ. Unfortunately, F3TCNQ-Ad1 degrades under standard sublimation conditions, preventing sublimation deposition processing. Instead, hole-only devices were made via solution-processing of the p-doped films with the structure glass/ITO/2.3 × 103 A PVK:(MTDATA:dopant)/2.0 × 102 A Au/1.0 × 103 A Al, where dopant is either F4TCNQ or F3TCNQ-Ad1. We demonstrate that F3TCNQ-Ad1 increased the conductivity of the films by at least 1000 times compared to an undoped device.

Blue phosphorescent organic light-emitting devices utilizing cesium–carbonate-doped 2,4,6-tris(2′,4′-difluoro-[1,1′-biphenyl]-4-yl)-1,3,5-triazine

We report an alternative, high-yielding synthesis for the known compound 2,4,6-tris(2′,4′-difluoro-[1,1′-biphenyl]-4-yl)-1,3,5-triazine (tris-(dFB)Tz). The energy of the lowest unoccupied molecular orbital (ELUMO) for tris-(dFB)Tz is estimated to be −3.5 eV from electrochemical measurements. The deep ELUMO of tris-(dFB)Tz affords a material with excellent electron acceptor characteristics for use in n-doped electron transport layers. Tris-(dFB)Tz shows a four order of magnitude increase in the number of carriers on doping with 8 wt. % Cs2CO3. Enhanced electron injection was also observed on doping with Cs2CO3, which eliminated the necessity for a separate LiF electron injection layer. Blue phosphorescent organic light-emitting devices (OLEDs) were fabricated using n-doped tris-(dFB)Tz electron transport layers. OLEDs with thick (700-A) Cs2CO3-doped tris-(dFB)Tz electron transport layers had lower operating voltages than OLEDS with an undoped electron transport layer of bis(diphenylphosphoryl)dibenzothiophene (PO15), which has previously been used in low-voltage, high-efficiency OLEDs. The tris-(dFB)Tz results indicate that aromatic substituted triazines may be promising materials for use as electron acceptors in n-doped organic electronic systems.