Organic semiconductors fabricated as thin-film light-emitting diodes, LEDs, now provide a promising new display technology.[1] Solution-processed semiconductor polymers make possible direct printing (using ink-jet deposition) and allow high-resolution full-color displays to be conveniently manufactured. [2] Multiple-layer deposition, used in vacuum-sublimed molecular semiconductor LEDs, is difficult to achieve by solution processing. We have instead fabricated distributed heterojunction' structures that are formed by de-mixing of two polymers co-deposited from common solution. We have used hole-accepting and electron-accepting derivatives of polyfluorene, and have optimized these structures to achieve high-efficiency diodes (above 19 lm W ±1 for green emission) that operate at very low voltages (100 cd m ±2 at 2.1 V for green emission). This very low voltage operation is achieved because electron±hole capture across the heterojunction is arranged to be a barrier-free process to form an interface state (termed an exciplex) that has significant charge-transfer character and is lower in energy than the charge-separated state. With respect to the bulk exciton, the exciplex is red-shifted (here between 140 and 360 meV) and its radiative lifetime is strongly increased (between 68 and 118 ns at low temperatures). The barrier for thermal excitation of the exciplex to allow it to move away from the heterojunction is small (100± 250 meV), and this process can give efficient bulk exciton emission at room temperature. The heterojunction formed between dissimilar organic semiconductors is generally found to be remarkably free of gap-states and other defects that would otherwise compromise semiconductor device operation. Heterojunction LEDs are designed so that the offsets between conduction and between valence band edges are type II' and electrons and holes accumulate on opposite sides of the heterojunction (Fig. 1). In a non-interacting electron scheme, type II heterojunctions would destabilize an exciton present in either semiconductor, since the exciton state would be higher in energy than the charge-separated state. However, organic semiconductors are low dielectric constant materials (typically having values less than 4) so that the coulomb interaction between electron and hole gives a substantial exciton binding energy (of order 0.5 eV). When this binding energy is larger than the band-edge offsets, excitons are stable at the interface. By selecting semiconductors with larger band-edge offsets, charge separation at the heterojunction can be readily achieved, giving efficient photovoltaic behavior. [3] LEDs made using molecular semiconductors are generally fabricated as multiple-layer heterojunction structures by successive vacuum sublimation steps.[1] However, with solutionprocessed polymers it is possible to make distributed heterojunction' diodes by de-mixing of two polymers spin-coated from common solution. [4] This is an obviously desirable structure for photovoltaic diodes, because it allows excitons photogenerated...

The main process responsible for the luminance degradation in organic light-emitting diodes (OLEDs) driven under constant current has not yet been identified. In this paper, we propose an approach to describe the intrinsic mechanisms involved in the OLED aging. We first show that a stretched exponential decay can be used to fit almost all the luminance versus time curves obtained under different driving conditions. In this way, we are able to prove that they can all be described by employing a single free parameter model. By using an approach based on local relaxation events, we will demonstrate that a single mechanism is responsible for the dominant aging process. Furthermore, we will demonstrate that the main relaxation event is the annihilation of one emissive center. We then use our model to fit all the experimental data measured under different driving condition, and show that by carefully fitting the accelerated luminance lifetime-curves, we can extrapolate the low-luminance lifetime needed for real display applications, with a high degree of accuracy.

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We have studied the performance of single-layer polymer light-emitting diodes based on a polyfluorene derivative. Hole-only devices show low currents; however, double-carrier devices show high currents and high efficiencies, implying that the presence of electrons in the device enhances hole injection. By numerical modeling, we show that this behavior is consistent with the presence of a barrier to electron extraction at the anode which causes an increased field for hole injection due to the buildup of electrons at the barrier.

We report the first infrared optical measurements of a two dimensional electron gas (2DEG) in a periodically modulated magnetic field. The 2DEG is produced using epitaxial regrowth on a corrugated surface, so that the component of an externally applied magnetic field perpendicular to the 2DEG is spatially modulated. Two active modes are observed, with intensities and frequencies which depend on the amplitude of the field modulation as well as the total external magnetic field. These modes arise from field modulation-induced coupling between the 2DEG magnetoplasmons. Calculations of the magnetoplasmon band structure in a modulated field are in excellent agreement with the experimental data. 73.20.At, 73.20.Mf Band structure effects arising from periodic modulations can be observed in a wide range of a physical systems. Electron energy bands produced by the periodic potential in a crystal, photonic band structures in systems with periodic dielectric constants, and plasmons in charge density modulated systems, are just some examples of systems in which mode dispersions show energy gaps at the edges of the Brillouin zone due to a periodic modulation. The effects of periodic electrostatic potentials on a two dimensional electron gas (2DEG) have been extensively studied theoretically and experimentally [1], and there is now increasing interest [2][3][4][5][6][7][8][9][10][11][12] in the effects produced by a novel form of periodic potential-that produced by a periodically modulated magnetic field (PMMF).PMMF investigations to date have been largely theoretical. It has been predicted that the sharp Landau levels produced in the presence of a uniform perpendicular magnetic field become broadened by introducing a small periodic component B m , and the width oscillates with B [3]. At very low magnetic fields, when the period d of the modulation is comparable with the cyclotron radius R c , so-called commensurability oscillations are expected, and a series of subsingularities (van Hove singularities) at the edges of the broadened Landau levels appear [9], which should produce splittings of the cyclotron resonance (CR). For R c ø d, new magnetoplasmon (MP) modes arising from the PMMF have been predicted theoretically [4,9,12], and variations in the electron wave function are expected to change the local occupancy of the Landau levels, producing a modulation in the local electron density [4,11]. In the presence of large magnetic field modulations, this effect should produce a transition from a modulated 2DEG to an array of isolated wires [4].Few of these theoretical predictions have been verified, with experimental data confined to d.c. transport measurements of commensurability oscillations in which the PMMF was produced by evaporating stripes of either superconducting or ferromagnetic materials on top of the 2DEG [6,8,10]. But experimental observations of the optical properties of a 2DEG in a PMMF have yet to be reported.Here we report the first optical measurements on a 2DEG in the presence of a PMMF. We have investiga...