Session F11 - Focus Session: Organic Electronic Materials and Devices III.
FOCUS session, Tuesday morning, March 19
104, Indiana Convention Center

[F11.001] Displays, Memories, and Sensors Based on Organic Transistors

For many applications of organic transistor-based electronics, properties other than mobility may be determining factors. In this work, we consider four "higher order" applications: an electrophoretic pixel switch, a complementary voltage inverter, a simple nonvolatile memory element, and a chemical sensor array, and discuss the particular device and material properties that need to be optimized for each one. The pixel switch requires minimal off current, the inverter depends on an ambipolar (hole carrying and electron carrying) transistor pair, the memory depends on a dielectric-semiconductor pairing in which both charge storage and threshold voltage adjustment occur, and the sensor is based on reversible and selective chemical interaction between an analyte and the semiconductor. Achievements reported here include a dynamic range >10 for the inverter and memory element, and sensory discrimination between organic functional groups such as alcohols and ketones. Solution-based processes will also be emphasized.

[F11.002] Printed Organic Transistors

We use scalable printing technology in the fabrication of the organic transistors. We formulated printable conducting layers that can be imaged onto a transistor structures forming source and drains lines with 10 micron wide channels. The performance of printed transisors compare compare favorable with those prepared using metal contacts evaporated via shadow mask

[F11.003] Direct writing of 3D features in self-assembled photonic crystals

Colloidal self-assembly is a less tedious and expensive alternative to layer-by layer fabrication techniques for the generation of large-area photonic crystals. Many researchers have addressed the problems of inherent defects in the self-assembly process as well as the infiltration of colloidal crystals with high refractive index materials. While they have made substantial progress in these areas, a perfect high index replica of a perfect colloidal crystal would still not be appropriate for most photonic band gap applications. Their operation typically relies on structures within the photonic that are specifically designed to guide and otherwise interact with light so as to function as desired. This necessitates an appropriate method for the controllable incorporation of pre-defined structures in the interior of colloidal crystals. We have recently demonstrated controlled fabrication within photonic crystals via multi-photon polymerization and laser scanning confocal microscopy. Further progress in the multi-photon polymerization of embedded structures within photonic crystals as well as their infiltration will be presented.

[F11.004] Soft, Conformable Electrical Contacts for Organic Transistors: High Resolution Circuits by Lamination

Soft, conformable electrical contacts for organic transistors were fabricated over large areas via microcontact printing on elastomeric polydimethylsiloxane (PDMS). To complete the circuit, the gold contacts were brought into contact and directly laminated onto the organic semiconductor. This versatile lamination approach allows us to non-invasively establish electrical contact with what is frequently the most fragile component of the circuit, making it an attractive alternative to conventional top- and bottom-contact device geometries. Laminated transistors containing evaporated films of copper hexadecafluorophthalocyanine (FCuPC; n-type organic semiconductor) and pentacene (p-type) fabricated in this fashion exhibited I-V characteristics comparable to those completed with shadow-mask gold electrodes. Additionally, this lamination technique enables us to completely encapsulate the active circuit and places it near the midplane of the encapsulated device. Thus, this process yields transistors that are more robust to both the environment and to deformation than those fabricated by conventional approaches.

[F11.005] BRIGHT WHITE SMALL MOLECULAR ORGANIC LIGHT-EMITTING DEVICES (OLED'S) BASED ON RED-EMITTING DCM2-DOPED \alpha-NPD AND BLUE-EMITTING DPVBi.

Bright multilayer white OLED's were fabricated by depositing CuPc on indium tin oxide, followed by N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (\alpha-NPD), red-emitting DCM2-doped \alpha-NPD, blue-emitting 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi), Alq_3, CsF, and Al. The devices were optimized with respect to the DCM2-doped layer thickness and DCM2 concentration. The blue emission band increased relative to the red emission band with increasing bias, and the CIE coordinates of the emission were well within the white region at V > 10 V. The highest luminous power and external quantum efficiencies of \eta_lum = 3.3 lum/W and \eta_ext ~ 1most lightly-doped devices, which contained a 5 nm-thick layer of 0.5the efficiency of the undoped blue-emitting OLED, with \eta_ext ~ 3.5the doped devices, the maximum brightness of > 50,000 cd/m^2 was also achieved with the most lightly doped devices mentioned above, at ~ 2 A/cm^2. It was also observed that doping decreased the injected current at a given bias, probably due to trapping of holes by the dopant.

^*Ames Laboratory is operated by Iowa State University for the USDOE under Contract W-7405-Eng-82.

[F11.006] SINGLET EXCITON (SE) QUENCHING MECHANISMS IN SMALL MOLECULAR ORGANIC LIGHT-EMITTING DEVICES (OLED'S).

Several significant SE quenching mechanisms in small molecular OLED's are revealed by electroluminescense (EL)-detected magnetic resonance (ELDMR):

(i) Quenching by polarons. The OLED's exhibit a positive spin 1/2 polaron resonance, whose dependence on the temperature and current is similar to the dependence of the positive spin ½ photoluminescence (PL)-detected magnetic resonance (PLDMR), observed in p-conjugated polymers, on T and the excitation power. As the latter has been shown to result from reduced quenching of SE's by polarons,

(ii) Quenching by triplet excitons (TE's). The ELDMR signature of this mechanism is also similar to that of the TE PLDM, which has recently been shown to result from reduced quenching of SE's by TE's.

(iii) Quenching by charges, including negative bipolarons, at the organic/cathode interface. The ELDMR also includes a negative spin 1/2 resonance which is attributed to the spin-dependent formation of negative bipolarons at the organic/cathode interface. The results indicate that the charge density at this interface depends strongly on its nature, but is generally very high, and may account for the differences in the behavior of OLED's with different interfaces.

^*Ames Laboratory is operated by Iowa State University for the USDOE under Contract W-7405-Eng-82.

[F11.007] Spin-polarized injection in polymeric light-emitting diode structures

We discuss initial results on spin-polarized injection in polymeric light-emitting diode structures. We use half-metallic LaCaMnO and LaSrMnO as the hole injector, PEDOT/PSS as an interfacial layer, MEH-PPV as the light emitting ogranic, and Ca/Al as electron-injector. The magnetic oxides La(Ca,Sr)MnO are highly spin-polarized at the Fermi energy, resulting in changes in the excitonic spin configurations in the polymer layer. These structures will allow us to address issues such as spin depolarization at the injection barrier, the spin diffusion length in MEH-PPV, and the effect on light-emitting recombination due to the enhanced hole spin-polarization. We also characterize the MEH-PPV on- and off-device using photoabsorption and photoluminescence.

[F11.008] Ito modification for more efficient hole injection in organic light emitting diodes

Indium Tin Oxide is the most commonly used anode electrode in organic light emitting diodes (OLEDs). A critical parameter for charge injection is its workfunction, varies between 4.5 and 5.1 eV, depending on the sample preparation and cleaning procedure. These large variations in the workfunction translate to even larger variations in the injected current, which is a major issue for the fabrication of efficient OLEDs. We demonstrate a way to treat ITO and get a contact with good injection characteristics, regardless of the ITO preparation procedure. We have carried out direct measurements of the injection efficiency at the ITO/TPD contact (TPD is N-N’-diphenyl-N-N’-bis(3-methylphenyl)-1-1-biphenyl-4,4’-diamine, a commonly used hole transport layer). The contact is found to be current-limiting, supplying TPD only with 1 per cent of the space charge limited current. By introducing a thin layer of polyaniline, the injection efficiency approaches 100 per cent, i.e. the contact becomes Ohmic. The performance of the contact shows little sensitivity to the details of the ITO preparation. A mechanism for this improvement is proposed.

[F11.009] Microcavity Effects in MEH-DOO-PPV Red Emitting PLEDS

We explore the optical properties and device performance of microcavity PLEDs. Devices are constructed on glass substrates with a patterned ITO/PEDOT-PSS anode and Ca/Al cathode, and polymer films are spin cast at varying speeds to achieve a range of film thicknesses. Devices are made with Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-ethenylene-2,5-dioctyloxy-1,4-phenylene-ethenylene] (MEH-DOO-PPV) with luminosity up to 10000 cd/m^2 at 5 V and quantum efficiencies up to 1%. We construct devices with a thin (9.3 nm, 1.1nm, and 1.5nm) layer of silver between the transparent anode and the polymer layers to explore microcavity effects. Equivalent non-microcavity PLEDs are constructed for comparison. Data is compared to simulation based on a transfer matrix formalism.

[F11.010] VIOLET POLYMER LIGHT EMITTING DEVICES (PLED'S) BASED ON PYRROLOPYRIDAZINE TRIESTER (PPZ)-DOPED POLY(N-VINYL CARBAZOLE) (PVK).

Violet spun-coated PLED's based on PPZ-doped PVK (PPZ:PVK) are described. The photoluminescence (PL) of the PPZ-oil derivative and the electroluminescence (EL) of the devices, with either an indium tin oxide (ITO) anode or an ITO/poly(3,4-ethylene dioxy-2,4-thiophene)-polystyrene sulfonate (PEDOT-PSS) anode, peak at 428 nm. Although absolute PL quantum yield \eta_PL measurements on the PPZ-oil yielded \eta_PL \approx 40fabricate devices by vacuum sublimation of PPZ failed. However, the ITO/PEDOT-PSS/PPZ:PVK/Ca devices yielded a luminance of 34 Cd/m^2 at a bias of ~16 V, which corresponds to ~3000 Cd/m^2 at 555 nm. In comparison, the luminance of similar devices without the PEDOT-PSS layer was weaker by a factor of ~6. As the unencapsulated devices degraded, the 430 nm emission band weakened and a red band, peaking at ~610 nm, grew. However, the growth of this red emission band was also observed in undoped ITO/PEDOT-PSS/PVK/Ca devices, and it is therefore not attributable to the PPZ dopant. ^*Ames Laboratory is operated by Iowa State University for the USDOE under Contract W-7405-Eng-82.

[F11.011] ELECTROLUMINESCENCE- AND ELECTRICALLY-DETECTED MAGNETIC RESONANCE STUDY OF ELECTROPHOSPHORESCENT ORGANIC LIGHT EMITTING DEVICES (OLED'S).

The electroluminescence- and electrically-detected magnetic resonance (ELDMR and EDMR, respectively) of 2,3,7,8,12,13,17,18-octaethylporphine Pt (PtOEP)-based electrophosphorescent OLED's is described. At room temperature the measurements yield a negative (EL-quenching) spin 1/2 resonance similar to those exhibited by fluorescence-based OLED's. This resonance was concluded to result from magnetic resonance enhancement of the formation of negative bipolarons at the organic-cathode interface, which enhances the nonradiative quenching of singlet excitons (SE's). It is therefore suspected that similar quenching of SE's by charges at the organic/cathode interface may compete significantly with the transfer of the SE energy to TE's in the electrophosphorescent devices as well.

[F11.012] Giant Nerst effect and Thermoelectric Power at Magic Angles in (TMTSF)_2PF_6

Here we report the results of the a-axis thermoelectric power and Nernst effect in (TMTSF)_2PF_6 at 10 Kbar. The electrical transport along a-axis was measured in situ. The experiment has been carried out in a temperature range of 2-0.2 K with a rotational magnetic field in the bc-plane. Giant Nernst signals were found when magnetic field is aligned near the c-axis (perpendicular to the conducting ab-plane) and the first Lebed magic angles. Strikingly, the Nernst coefficient steeply changes its polarity from negative to positive when it passes through both the magic angles and the c-axis. However, the field induced spin density wave suppresses the sign change at the c-axis while it dramatically enhances the feature at the magic angles.

[F11.013] Efficient Electroluminescent Devices Based on a Chelated Osmium(II) Complex

We report on the fabrication and characterization of efficient, solid state electroluminescent devices from the Os complexes. Devices were fabricated by spin coating on Indium Tin Oxide (ITO) covered glass slides, using evaporated Au counter electrodes. Upon the application of a small bias voltage the devices emit red-orange light with an efficiency of the order of 1 per cent. Their brightness exceeds 6000cd/m2 at just 6V, making osmium complexes a promising new class of solid state emitters. The mechanism of device operation will be discussed.

Part F of program listing