We have developed novel formulations through a combination of new materials and a co-dopant/co-host systems approach. These novel formulations offer significant improvements in efficiency, lifetime, and color, which are suitable for fabrication of full-color OLED displays. In this report, we will review the progress made at Kodak and compare it with currently available systems.

Laser Induced Thermal Imaging (LITI) is a high resolution, digital patterning technique developed at 3M for use in a number of applications including the patterning of LCD color filters and OLED emitters. The LITI process is suited for the manufacture of flat panel displays, where both high resolution and absolute placement accuracy are required. In this paper, we present the capabilities of LITI, the basic design of a LITI laser imager, the construction of a LITI donor sheet, and the process by which OLED emitters may be patterned. An OLED device fabricated with the LITI process is described.

Efficient electroluminescence is difficult to achieve using purely organic materials as only S₀ <-> S* transitions (25 % quantum efficiency according to spin statistics) will be allowed when the spin selection rule is obeyed. In most strictly organic systems the triplet state energy will decay non-radiatively. In order to improve the electroluminescent quantum efficiency of light emitting polymers, triplet state emitting complexes have been introduced into the polymer either by blending or copolymerization. The strong spin-orbit coupling from the heavy metal atoms allows effective mixing of singlet and triplet states to provide strong radiative decay. Two approaches to the synthesis of solution-processible phosphorescent polymers have been developed and the degree of energy transfer from the polymer host to the triplet emitter has also been investigated. The phosphorescent core, [Ir(btp)₂(acac)], where btp is 2-(2'-benzo[b]thienyl)pyridinato and acac is acetylacetonate, can either be incorporated into the polymer chain through the extension of the ligands or tethered to the polymer backbone through an alkyl pendant. In addition, the synthesis of high band gap polymer hosts will also be discussed.

Two blue-shifted iridium phenyl-pyridine dopants are compared in identical device structures. While the dopants have very similar optical behavior, it is found that the device efficiencies are very different and dependent on the host material. Upon comparison of molecular energy levels it is proposed that the electronic properties of the dopant influence the device efficiency through an electron trapping mechanism. It is believed that the relative energetics between the host and dopant play an integral role in the operation of the device.

We present highly efficient, low-voltage multilayer organic light emitting diodes based on the phosphorescent emitter tris(2-phenylpyridine) iridium (Ir(ppy)3). The phosphor is doped into various wide gap electron-transport or hole-transport host materials embedded in between doped transport layers. We use 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as p-type hole-injection and transport layer, while cesium (Cs) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are co-evaporated for the n-type doped electron transport layer. Sandwiched between these two transport layers, we insert one or two emission layers. This p-i-n structure results in efficient carrier injection from both contacts into the doped transport layers and low ohmic losses. Thus, lower operating voltages are obtained compared to conventional undoped OLEDs. By doping Ir(ppy)₃ into a double layer structure of predominantly electron and hole transporting hosts, a power efficiency of 70 lm/W and an external quantum efficiency of 19.5% is achieved at 100 cd/m² (2.95V). Besides this, the efficiency decays only weakly with increasing current density (or brightness). A quantum efficiency of 13.5% is still obtained at a current density of 100 mA/m² with a luminance around 50,000 cd/m². This improvement can be attributed mainly to the fact that we prevent any charge accumulation at hole or electron blocking layers and spread the generation region to both sides of the interface between the two parts of the emission layer. Moreover, losses due to non-radiative decay of triplet excitons diffusing into regions without the phosphorescent dye are avoided.

A light-emitting electrochemical cell is a type of organic electroluminescent device of particular interest for large-area lighting. We have assessed the potential applicability of different kinds of light-emitting electrochemical cells. For devices having a blend of an electroluminescent polymer and a polymer electrolyte as active layer, the obtainable efficiency and lifetime were found to be insufficient for practical applications. Light-emitting electrochemical cells with charged transition metal complexes as conducting and electroluminescent material sandwiched between ITO and Ag electrodes resulted in considerable improvement. For a yellow-emitting charged Ir complex, an efficacy of about 4 cd/A over a wide luminance range was obtained. Furthermore, we have studied the dependence of the performance on the active layer thickness, and we demonstrate that thick-layer light-emitting electrochemical cells can be operated at much lower voltage than organic light-emitting diodes.

We have studied the temperature-dependent photoluminescence (PL) characteristics of oxidised PFO thin films at temperatures above 298K. We find the relative strength of the green emission band (g-band) to increase greatly at temperatures corresponding to the onset of crystallisation. Based on the proposal that the g-band arises from a fluorenone-based excimer, this finding would seem to indicate that a close approach of neighbouring fluorenone-containing segments may be energetically favourable. Finally, by successfully identifying diffusion-limited and dynamic equilibrium regimes within the temperature dependence of the ratio R=I_D/I_M, we extract an activation energy for excimer formation of ~ 0.05 eV and an excimer binding energy of ~ 0.51 eV. This is a relatively high binding energy for an excimer and lends credibility to the notion of an energetically favourable fluorenone:fluorenone coupling configuration, perhaps as a result of the considerable ground state dipole moments associated with the ketone group.

An attractive approach to full color OLED displays is based on white emitting copolymers and color filters. In this paper the special impact of broadband emitting copolymers based on polyspirobifluorene structures is discussed. The EL spectra of broadband emitters in PLEDs are strongly influenced by interference effects as well as by the driving conditions. Experimental results could be confirmed by modelling. Adjustment of emission spectra to the color filter characteristics lead to improved efficiency.

White emission from polymer light-emitting diodes (PLEDs) is demonstrated by spin-casting polymer blends from solution containing poly(9,9-dioctylfluorenyl-2.7-diyl) (PFO) and tris (2,5-bis-2'-(9',9'-dihexylfluorene) pyridine) iridium (III), Ir(HFP)₃. The white electrophosphorescence PLEDs exhibit luminance of 1.2 x 104 cd/m² at 17 volts and luminous efficiency of 4.3 cd/A at current density of 5.2 mA/cm². Because a single semiconducting polymer, PFO, was used as the common host for red, green and blue emission, the color coordinates, the color temperatures and the color rendering indices of the white emission are insensitive to the brightness, applied voltage and applied current density.

Semiconducting polymers can be used in light-emitting-diodes (LEDs), photovoltaics (PVs), and field-effect-transistors (FETs). In all of these devices charge carrier transport is a major issue, the mobility being directly related to device performance. In LEDs and PVs, charge transport occurs vertically through a bulk semiconducting polymer film. This bulk mobility is determined by the average interchain hopping distance a, the polaron relaxation energy λ, the level of energetic and spatial disorder σ and Σ, the presence of charge traps and different structural phases. In FETs, charge transport occurs horizontally along the interface between the semiconducting polymer film and the insulating material. The FET mobility is also determined by the above parameters but these may be different from the bulk. Also, there are additional factors such as surface features which have to be circumnavigated, specific interface trap states, and the high charge carrier densities effectively filling all the deep sites. Here we present results looking at the difference between the bulk mobility, as measured by time-of-flight (TOF) photocurrent, versus the FET mobility, as measured by the FET transfer characteristics. Three different polyfluorene copolymers are investigated. In all three materials, the room temperature hole TOF bulk mobility was found to be greater than the FET mobility. This indicates that models based on deep site filling due to the high FET carrier densities cannot be correct. Temperature measurements also show that the level of energetic disorder σ in the FETs is the same or less than that in the bulk, as is the polaron relaxation energy λ or thermal activation energy of any deep traps. The results instead indicate that it is the average interchain hopping distance which is greater at the insulator-semiconductor interface in FETs than in the bulk films, and it is this which is responsible for the difference in mobility.

Electromodulation (EM) spectroscopy has been used to probe the electric field distribution in polymer light-emitting diodes. Below the turn-on bias, the EM spectrum is dominated by electroabsorption of the emissive layer. The electroabsorption signal vanishes at the turn-on bias. Under operation, the EM spectrum is due to by excited state absorption from injected charge and bleaching of the ground state absorption of the emissive layer. We conclude that the internal electric field is effectively screened by accumulation of trapped electrons at the anode.

Experimental studies of charge injection and transport of holes and electrons in LUMATION^TM Green 1300 Series Light-Emitting Polymer (LEP) by dark injection space-charge-limited technique are performed. It is found that hole mobility is lower than electron mobility and the former exhibits steeper electric field dependence thus reducing the disbalance between charge mobilities at higher device operating voltages. Electron current is affected by trapping, mainly due to deep traps prevailing at low electric fields and with an estimated concentration of 10¹⁶ cm^-3. Hole current is affected both by trapping and injection limitation, with the trapping being approximately independent of electric field and injection efficiency increasing with increasing electric field. Electron trapping is found to be significantly reduced in dual carrier devices, which is believed to be the effect of faster exciton formation and recombination rates, compared to electron trapping processes.

Understanding of the charge transport properties is of great importance for the operation and the efficiency of polymer based light-emitting diodes (LEDs). We investigate the charge transport in hole-only diodes based on poly(p-phenylene vinylene) (PPV) as function of temperature T, charge carrier density p and electric field E. At low voltages the hole mobility is independent on the electric field and charge carrier density. At high voltages both the charge-carrier density and electric-field dependence of the mobility have to be taken into account to describe the hole transport in polymer LEDs.

Energy level alignment at interfaces between tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) films on representative anode materials Au, ITO and PEDOT-PSS are investigated by ultraviolet photoelectron spectroscopy (UPS). Both materials have broadly uniform behavior independent of the chemical composition of the substrate. The position of the vacuum level of the deposited films is fixed with respect to the substrate Fermi energy, and the hole injection barrier is likewise constant. The magnitude of the vacuum level shift is a simple linear function of the substrate work function. A 4 nm thick interlayer in TTF/TCNQ/Au and TCNQ/TTF/Au compound interfaces does not alter the alignment of the outer material with the substrate. Charge transfer in conjunction with subsequent structural relaxation of the ionic donor or acceptor molecule is proposed to explain the results. Lastly, we discuss how donor-acceptor molecules might be used to mediate charge injection into organic light emitting diodes (OLEDs) and similar organic electronic devices.

Bilayers of aluminum (Al) and alkali fluoride (such as sodium fluoride) are well-known top cathode contacts for organic light-emitting devices (OLEDs) in which the alkali fluoride is inserted in between the Al and organic materials. However, the configuration, to date, has never been successfully applied as bottom cathode contacts. In this article, we describe a novel bilayer bottom cathode contact for OLEDs utilizing the same materials but with a reversed structure, i.e. the Al rather than the alkali fluoride contacts the organic material. Electron-only devices were fabricated showing enhanced electron injection from this bottom contact with respect to an Al-only contact. Kelvin probe, X-ray photoelectron spectroscopy, Auger electron spectroscopy experiments and thermodynamic calculations suggest that the enhancement results from n-doping of the organic material by dissociated alkali metals.

Inverted organic light-emitting diodes showing light emission from the top are discussed. Top-emitting organic light-emitting diodes are required for next-generation active-matrix organic light-emitting displays , as Si-driving circuitry has to be incorporated into the display itself. We focus on hybrid anodes, thereby giving a simple model for spin-coating of PEDOT:PSS on top of an organic layer-stack, LiF-based cathodes and phosphorescent emitters, allowing for highly efficient inverted organic light emitting diodes. A maximum current efficiency of 55.4 cd/A at 140 cd/m² and a maximum luminous efficiency of 17.2 lm/W at 50 cd/m² has been obtained.

Molecular design concepts, synthesis, and properties of a few novel classes of emitting amorphous molecular materials with desired bipolar character are described. They include α,ω-bis{4-[bis(4-methylphenyl)amino]- phenyl}oligothiophene (BMA-nT), π-electron systems end-capped with 4-[bis(9,9-dimethylfluoren-2-yl)amino]-phenyl group (BFA), and α-{4-[bis(9,9-dimethyl-fluoren-2-yl)amino]phenyl}-ω-(dimesitylboryl)oligothiophene (FlAMB-nT) families. Fabrication and performance of organic light-emitting diodes using these emitting materials are discussed.

We have been researching blue-emitting polymers and developed some novel polymers with blue fluorescence, which are not fluorene copolymers. Our previous works showed the dependence of EL efficiency on molecular weight of poly(p-phenylene vinylene) derivatives. We concluded that polymers must be synthesized by the method which gave larger molecular weight in order to achieve higher performance. Firstly, we modified the synthetic method of polyarylenes and confirm that Mw of ca.1,000 to over 1,000,000 is obtainable depending on the condition. Polymers having moderately large Mw showed good solubility in solvents and film-forming properties. PLEDs using these polymers had longer lifetime as well as higher EL efficiency. Secondly, we understood the importance of charge balance between electrons and holes, by using polyfluorene derivatives as model polymers. We measured electron, hole and total currents by electron-only, hole-only and usual bipolar devices respectively. We found out that the hole mobility could be changed from 10^-7cm²/Vs to 10^-3cm²/Vs depending on the content of hole transporting units. In addition, electron injection from a cathode seemed to be assisted by trapped holes. We utilized these results in order to design our blue polymers. Finally, we obtained a new blue-emitting polymer, which showed longer device lifetime among non-fluorene polymers.

Reported efficiency records of >70 lm/W and the community's performance roadmaps indicate the potential of OLEDs (Organic Light Emitting Diodes) for use in general lighting applications. Within a shorter timeframe, OLED technology may be exploited for signage applications. Key differences of OLED signage devices to display and lighting devices are discussed. Recent results are presented on large area device design, polymer deposition technology, device and material performance, and encapsulation technology. Finally we discuss performance and cost targets for potential applications indicating the main challenges for future developments.

We focused on the light out coupling efficiency to be improved. To improve out coupling efficiency, we considered air/substrate interface is the key place to be modified. We simulated various shaped array at that interface for high out coupling efficiency. As a result of simulation, we found that the pyramid array was best. We successfully fabricated a pyramid array film and attached to a white emission OLED panel for evaluation. The white emission OLED was optimized to be suitable for the pyramid array. The luminance and flux of OLED with pyramid array were increased by a factor of 2.0, 1.7 relatively. For further improvement in luminance, we found that additional use of Brightness Enhancement Film (BEF) with optimized OLED device is able to achieve much higher luminous efficiency of 3.0 times comparing to the normal OLED.

Subwavelength silica particle layers have been applied between glass and thin film luminescent layers of Alq3 and a polymer MEH-PPV layer, respectively. The layers acted as a randomised two-dimensional diffraction lattice, which increased the fraction of emitted power from thin film organic layers into air. In contrast to perfectly ordered structures strong interference emission patterns did not occur. Still, an optical feedback of the particle layer on the emission spectrum could be observed, which can be used to improve colour saturation for blue and green emission and to increase the lumen efficiency for red emission, without changing the colour point of the red emitter. In photoluminescence experiments a gain factor of 3 and 2.5 in light outcoupling was realized for Alq₃ and MEH-PPV layers, respectively. With a MEH-PPV polymer OLED device an efficiency gain of 30 to 40 % has been realized in electroluminescence.

Following a brief review of OLED structures and emission models, we have proposed a phase holographic method to modify both sides of the substrate of an OLED to increase its output light emission efficiency. The holograms are designed with special masks simulating the emission layer of the OLED. A single source macroscopic phase holographic experiment is used to illustrate the idea. Limitations on diffraction efficiency due to the dynamic density and position variations of the excitons and near-field subwavelength wave propagation phenomena in a real OLED device have been discussed.

Efficient and highly saturated red organic, polymer and white organic/inorganic hybrid light-emitting devices (OLEDs, PLEDs and OIHLEDs) employing platinum(II) meso-tetrakis(pentafluorophenyl) porphyrin (PtF₂₀TPP) are reported. The presence of fluorine atoms on the periphery of porphyrin ring suppresses the self-quenching behavior and improves the electron mobility, solubility, and stability of PtF₂₀TPP. Both the PLEDs and OLEDs give a red emission with a peak maximum at 650 nm (full width at half maximum = 26 nm) and a shoulder at 705 nm. The maximum external quantum efficiency (hext), luminous efficiency (hL), and brightness of the 2.1 wt% PtF20TPP-doped PLED are 3.2%, 1.1 cd/A, 350 cd/m² respectively; while those for the 4.0 wt% PtF20TPP-doped OLEDs are 5.1%, 1.5 cd/A, 890 cd/m². The efficiency and luminous power efficiency (hP) of luminescence conversion of white OIHLEDs based on near-ultraviolet (n-UV) light GaN-LEDs as light source, tris-(8-hydroxyquinolato) aluminum (Alq₃) as green emitter, and PtF₂₀TPP as red emitter are 3.3% and 10 lm/W, respectively.

We present a structural study of poly(9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl) (PF2/6) in aligned thin films on a rubbed polyimide substrate. PF2/6 forms 5/2-helical hairy-rodlike molecular structure which is self-organized in the hexagonal structure. In thin films, the aligned rigid polymer chains are parallel to the substrate in the direction corresponding to the molecular backbone, c axis. The cells are flattened in the direction of the surface normal and -in particular- reveal a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface but a small proportion are aligned with the crystal axis a parallel to surface. We find further that by reducing the polymer chain length to approximately twenty repeat units the degree of axial alignment of PF2/6 is considerably improved while the local order changes from hexagonal packing towards nematic phase with no sign of multiple orientation.

Aligned and unaligned thin polyfluorene (PF) films doped with varying degrees of the dyes tetraphenyl porphyrin (TPP) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) have been investigated via polarised steady-state and time-resolved fluorescence anisotropy measurements. In the aligned films, the PF and DCM emission have dichroic ratios of approximately 10 and 4 respectively, while the TPP emission is completely depolarised. The polarised dopant emission in the case of the DCM is likely to arise from its linear shape and possible orientation parallel to the polymer chains, whereas the TPP is planar and exhibits no preferential orientation. These experiments show the possibility of polarised, red-shifted luminescence from polymer films.

The crosstalk, equivalent circuit, voltage drop and power consumption of passive matrix organic light-emitting diodes (PMOLEDs) are quantitatively analyzed, and a mathematical model to calculate the power of PMOLEDs is built. In particular, the advantages of dual-panel PMOLEDs are discussed. The model demonstrates that the power of dual-panel PMOLEDs can be significantly reduced comparing with that of single-panel PMOLEDs. Two 2.5-in. 128×64-pixel green small molecule PMOLEDs have been fabricated. One is single-panel, and the other dual-panel. The power consumption of dual-panel PMOLEDs is 25% less than that of single-panel when both are operated at an average luminance of 100cd/m².

A novel transparent organic light emitting diode (TOLED) has been developed. This TOLED features Lanthanum hexaborides (LaB₆) as transparent cathode and has the device structures of ITO/TPD/Alq₃/LaB₆. LaB₆ film was prepared by the electrons evaporation. This device has the transparency of ~70% in visible spectra, and reach a luminance of 100cd/m² at 7.2v corresponding to injected current density of 4.8mA/cm².

The paper explores the behaviors of thin-film formation through the microfluidic deposition method, in which the spherical liquid droplet on solid surface may turn into a thin film. Small liquid drops with diameter of less than 100 micro-meters are virtually to be generated with some certain velocity by current inkjet-like mechanism. Fundamental physical modeling for the dynamic situation is first built up. Thus the time evolutions of fluid behavior are solved by numerical method. Drop-on-demand droplets are secondly applied to demonstrate the thin film on the glass that the solid surface is patterned with rectangular well. Experimental results show that uneven surface of thin film with curvature may always be formed mainly due to the patterned well. It suggests that the topographic pattern can make significant effect on the formation of thin film from the liquid droplets.

In this paper, we demonstrated an organic light emitting device (OLED) of low driving voltage, high current efficiency and long lifetime by using an n-type organic material as the electron transport layer (ETL). Such a layer is composed of a large alkali metal, i.e. Cesium (Cs), doped into the organic material. Cs atom is heavy and hard to diffuse into the emitting layer (EML) material that decreases the metal quenching and increase the operation lifetime. 2,9-dimethyl-4,7-diphenyl 1,10- phenanthrolin (BCP) and 2,5-diaryl-1,3,4-oxadiazoles (OXD) were used as the host organic material. OXD is thermally stable and exhibits a high glass transition temperature (Tg) of 147°C that can further increase the operation lifetime. The resistivity of those doped materials is about 3x10⁵ Ω-cm that is two orders of magnitude lower than the resistivity of the pure organic materials. Compared with the conventional LiF/Al device, about two-volt reduction in driving voltage was observed. Current efficiency is also increased due to better carrier balance. Operation lifetimes of metal dopant devices are longer than that of the conventional device especially by using OXD as the host of the ETL.

The interface between the hole transport layer (HTL) and emissive layer (EML) in polymer light-emitting diodes (PLEDs) has attracted intense research attentioin since the initial discovery of PLEDs in 1989. In this contribution, we analyze the electron-blocking properties of various HTL at this interface and their effect on PLED device performance. We find that poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) - a conventional PLED HTL - does not possess optimum electron-blocking properties and that PLED device performance can be significantly enhanced by inserting a new type of electron-blocking layer (EBL) between the PEDOT-PSS HTL and EML. The new EBLs developed in this study consist of two major components: a siloxane-derivatized, crosslinkable, TPD-like triarylamine hole-transporting material, such as 4,4’-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi₂), and a hole-transporting polymer, such as poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl) diphenylamine) (TFB). TPDSi₂ undergoes crosslinking in air and rendering the TPDSi₂ + TFB blend insoluble. With the TPDSi₂ + TFB EBL inserted between PEDOT-PSS and BT layers, PLED device current density is reduced, device light output and current efficiency are dramatically increased (maximum current efficiency ~ 17 cd/A). Our result shows: 1) insufficient electron-blocking by PEDOT-PSS is another reason for the poor performance of PEDOT-PSS/BT based devices; 2) PLED device performance can be dramatically enhanced with a triarylamine siloxane-based EBLs.

In this article, a model to calculate the modal gain in organic laser diode structures is presented. A single layer design is considered to investigate the dependence of the gain on power density, charge carrier mobility and thickness of the active layer. We show that unequal charge carrier mobilities are detrimental and that there is an optimum active layer thickness of approximately 200 nm, if different devices are compared on the basis of equal power density. Neglecting all losses, the highest calculated gain is 0.7/cm for a power density of P=50 kW/cm² in our MEH-PPV like model material. Furthermore, the influence of absorption by polarons is quantified. We show that the cross section for this process has to be at least 20 times smaller than the cross section for stimulated emission in order to achieve net gain in the most favourable case that was considered.

We present results from a device model in which the characteristics of organic light-emitting diodes (OLED), based upon MEH-PPV [poly (2-methoxy,5-(2 ethyl-hexoxy)-1,4-phenylene-vinylene)], are determined by tunneling the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating voltage of the device as well as its efficiency. A model is developed that clearly explains the device characteristics of an indium-tin-oxide / MEH-PPV / Al. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I-V characteristics. In indium-tin-oxide/ MEH-PPV / Al devices, the barrier for hole injection varies between 0.1 eV to 0.6 eV and the barrier for electron injection is about 1.4 eV.

We demonstrate very high-efficiency yellow electrophosphorescence organic light-emitting devices (OLEDs) employing platinum(II) tetradentate Schiff base complexes doping a host of 4,4’-N,N’-dicarbazole-biphenyl (CBP). The OLEDs based on Pt(II) 2 N,N'-bis(salicyli-dene)-1,1,2,2-tetramethylethylenediamine (PtSaltment) give a yellow emission with a peak maximum at 550 nm and a shoulder at 590 nm. The maximum external quantum efficiency (hext), luminous efficiency (hL), power efficiency (hP), turn-on voltage, and brightness of the 4.0 wt% PtSaltment-doped OLED are 11%, 31 cd/A, 14 lm/W, 2.8 V, and 23 000 cd/m², respectively. Even at a brightness of 10 000 cd/m², hext, hL, and hP of the OLEDs are 4.1%, 13 cd/A, and 4.0 lm/W, respectively. We attribute the high performance of PtSaltment to its tetramethylethylene group which suppresses the self-quenching behavior.

Impedance model of one-carrier space-charge limited current (SCLC) has often been applied to explain some experimental features in organic light-emitting diodes (OLED). However double injection current occurs in working devices and the impedance model has not been studied so far. We analyze the problem of double injection SCL current in the limit of infinite recombination. In order to obtain the ac response of a biased OLED we solve the equations for time dependent double-injection of space-charge-limited currents. We give an analytical expression for impedance as a function of frequency. Calculations predict values for the static capacitance C(ω→0) similar to those encountered in case of one-carrier SCLC, in which C/C_g=0.75 (being C_g the geometric capacitance), but shifted to higher frequencies. We give the equivalent circuits representing the limits at low frequencies. This model will help to understand the behavior of two carrier devices.