We have measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly͓2-methoxy-5-͑2-ethylhexoxy͒-1,4-phenylene vinylene͔ ͑MEH-PPV͒, with Au anodes and Ca, Al, and Au cathodes. We show that proper accounting of the built-in potential leads to a consistent description of the current-voltage data. For the case of Au and Al cathodes, the current under forward bias is dominated by holes injected from the anode and is space-charge limited with a field-dependent hole mobility. The Ca cathode is capable of injecting a space-charge-limited electron current. ͓S0163-1829͑98͒52844-5͔Organic light-emitting diodes ͑OLED's͒ have emerged over the past ten years as viable candidates for application in display technologies. 1 In their simplest configuration, a fluorescent semiconducting polymer is sandwiched between two metal electrodes, an anode with a high and a cathode with a low work function. Under the application of an electric field, holes and electrons are injected into the valence and the conduction band of the polymer, respectively. A fraction of these charges combine to form excitons that decay radiatively, giving rise to light emission. While the technology of OLED's is advancing rapidly, fundamental studies of the device operation are lagging behind. Even in PPV derivatives, which were the first polymers to show electroluminescence 2,3 and are by far the best studied, the relative importance of charge injection as opposed to charge transport as the factor limiting the efficiency of OLED's is still under debate. [4][5][6] For the case of large barriers at the cathode ͑anode͒, inefficient electron ͑hole͒ injection is the limiting process. 4,7 However, since the trap-free drift mobilities are not known for both carriers, it is not clear whether the efficiency of the best devices is limited by injection or by bulk transport. One experimental degree of freedom is the electrode work function which one can change to alter the barrier for electron or hole injection into the polymer, thus changing the magnitude of the electron ͑hole͒ current. Parker 4 has performed a systematic study of ͑mostly unipolar͒ devices with different electrode combinations.In the case of bipolar devices, where there is a significant difference between the work functions of the anode and the cathode, a built-in potential (V bi ) is established in the polymer layer at zero bias ͑see Fig. 1͒. 8 This built-in potential fundamentally affects the operating characteristics of the diode: For applied bias (V appl ) less than V bi the electric field inside the polymer opposes charge injection and forward drift current. ͑Current may flow by diffusion.͒ In the simplest picture, where the bands of the polymer remain rigid, V bi is equal to the work-function difference ͑⌬͒ between the anode and the cathode. The above picture is surely rather simplistic: Instead of extended bands, the electronic levels of conjugated polymers are best described as a ͑Gaussian͒ distribution of localized states. Charge transp...

Light-emitting diodes made with poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) (MEH- PPV) using indium-tin-oxide (ITO) as anode and Ca as cathode have been examined as they age during operation in a dry inert atmosphere. Two primary modes of degradation are identified. First, oxidation of the polymer leads to the formation of aromatic aldehyde, i.e., carbonyl which quenches the fluorescence. The concomitant chain scission results in reduced carrier mobility. ITO is identified as a likely source of oxygen. The second process involves the formation of localized electrical shorts which do not necessarily cause immediate complete failure because they can be isolated by self-induced melting of the surrounding cathode metal. We have not identified the origin of the shorts, but once they are initiated, thermal runaway appears to accelerate their development. The ultimate failure of many MEH-PPV devices occurs when the regions of damaged cathode start to coalesce.

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We have studied the transport properties of electron-and hole-dominated MEH-PPV, poly͑2-methoxy,5-͑2Ј-ethyl-hexoxy͒-p-phenylene vinylene͒, devices in the trap-free limit and have derived the temperature-dependent electron and hole mobilities (ϭ 0 e ␥ͱE) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility 0 and electric-field coefficient ␥ are temperature dependent with an activation energy of the hole and electron mobility of 0.38Ϯ0.02 and 0.34Ϯ0.02 eV, respectively. At 300 K, we find a zero-field mobility 0 on the order of 1Ϯ0.5ϫ10 Ϫ7 cm 2 /V s and an electric-field coefficient ␥ of 4.8Ϯ0.3 ϫ10 Ϫ4 (m/V) 1/2 for holes. For electrons, we find a 0 an order of magnitude below that for holes but a larger ␥ of 7.8Ϯ0.5ϫ10 Ϫ4 (m/V) 1/2. Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap-free limit, applicable to thin films of MEH-PPV.

An electroluminescent device comprising a transparent or translucent Support, a transparent or translucent first electrode, a Second conductive electrode and an electrolu minescent phosphor layer Sandwiched between the transpar ent or translucent first electrode and the Second conductive electrode, wherein the first and Second electrodes each comprises a polymer or copolymer of a 3,4dialkoxythiophene, which may be the Same or different, in which the two alkoxy groups may be the same or different or together represent an optionally Substituted oxy-alkylene oxy bridge; a display comprising the above-mentioned elec troluminescent device; a lamp comprising the above mentioned electroluminescent device; manufacturing processes for the above-mentioned electroluminescent devices, and the use of Such devices for the integrated backlighting of Static and dynamic posters and Signage.

We study the effect of blended and layered titanium dioxide (TiO 2) nanoparticles on charge transfer processes in conjugated polymer photovoltaics. A two order of magnitude increase in photoconductivity and sharp saturation is observed for layered versus blended structures, independent of the cathode work function. Using electrodes with similar work functions, we observe low dark currents and open circuit voltages of 0.7 V when a TiO 2 nanoparticle layer is self-assembled onto the indium-tin-oxide electrode. Our results for the layered morphologies are consistent with charge collection by exciton diffusion and dissociation at the TiO 2 interface.

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Two organic-inorganic bismuth iodides of the form (H3N-R-NH3)BiI5 are reported, each containing long and relatively flexible organic groups, R. The norganic framework in each case consists of distorted BiI6 octahedra sharing cis vertexes to form zigzag chains. Crystals of (H3NC18H24S2NH3)BiI5 were grown from a slowly cooled ethylene glycol/2-butanol solution containing bismuth(III) iodide and AETH.2HI, where AETH = 1,6-bis[5'-(2' '-aminoethyl)-2'-thienyl]hexane. The new compound, (H2AETH)BiI5, adopts an orthorhombic (Aba2) cell with the lattice parameters a = 20.427(3) A, b = 35.078(5) A, c = 8.559(1) A, and Z = 8. The structure consists of corrugated layers of BiI5(2-) chains, with Bi-I bond lengths ranging from 2.942(3) to 3.233(3) A, separated by layers of the organic (H2AETH)(2+) cations. Crystals of the analogous (H3NC12H24NH3)BiI5 compound were also prepared from a concentrated aqueous hydriodic acid solution containing bismuth(III) iodide and the 1,12-dodecanediamine (DDDA) salt, DDDA.2HI. (H2DDDA)BiI5 crystallizes in an orthorhombic (Ibam) cell with a = 17.226(2) A, b = 34.277(4) A, c = 8.654(1) A, and Z = 8. The Bi-I bonds range in length from 2.929(1) to 3.271(1) A. While the inorganic chain structure is nearly identical for the two title compounds, as well as for the previously reported (H3NC6H12NH3)BiI5 [i.e., (H2DAH)BiI5] structure, the packing of the chains is strongly influenced by the choice of organic cation. Optical absorption spectra for thermally ablated thin films of the three organic-inorganic hybrids containing BiI5(2-) chains are reported as a function of temperature (25-290 K). The dominant long-wavelength feature in each case is attributed to an exciton band, which is apparent at room temperature and, despite the similar inorganic chain structure, varies in position from 491 to 541 nm (at 25 K).