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...

We demonstrate that the resistive switching phenomenon observed in organic semiconductor layers containing granular metal particles conforms to a charge storage mechanism described by Simmons and Verderber ͓Proc. R. Soc. A 391, 77 ͑1967͔͒. The space-charge field due to the stored charge inhibits further charge injection from the electrodes. The equilibrium current-voltage curve is N shaped and the low and high resistance states are obtained by applying voltage close to the local maximum and minimum, respectively.

No abstract

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.

The rotational dynamics of C(60) in the solid state have been investigated with carbon-13 nuclear magnetic resonance ((13)C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1(CSA)(1)) was precisely measured from the magnetic field dependence of T(1), allowing the molecular reorientational correlation time, tau, to be determined. At 283 kelvin, tau = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 10(10) per second. This reorientation time is only three times as long as the calculated tau for free rotation and is shorter than the value measured for C(60) in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C(60). In both phases tau showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane.

No abstract

No abstract

We measure the voltage at which the current under illumination in poly[2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene] based light emitting diodes is equal to the dark current. At low temperatures, this voltage, which we term the “compensation” voltage, is found to be equal to the built-in potential, as measured with electroabsorption on the same diode. Diffusion of thermally injected charges at room temperature, however, shifts the compensation voltage to lower values. A model explaining this behavior is developed and its implications for the operation of organic light emitting diodes and photovoltaic cells are briefly discussed.