We present a systematic investigation of the effects of organic film structure on light emitting diode (LED) performance. Metal/organic film/metal LEDs were fabricated using a five ring, poly(phenylene vinylene) related oligomer as the active layer. The structure of the vacuum evaporated oligomer films was varied from amorphous to polycrystalline by changing the substrate temperature during deposition. The intrinsic properties of the oligomer films and the LED performance were measured. The measured intrinsic film properties include: optical absorption, photoluminescence (PL) spectra, PL lifetime, PL efficiency, and effective carrier mobility. The measured device characteristics include current–voltage, capacitance–voltage, electroluminescence (EL) efficiency, and the contact metal/organic film Schottky barrier heights. The optical absorption and PL properties of the films are weakly dependent on film structure but the effective carrier mobility decreases with increasing crystallinity. The EL quantum efficiency decreases by more than one order of magnitude, the drive voltage at a fixed current increases, and the electron Schottky barrier height increases as the crystallinity of the film is increased. The diode current–voltage characteristic is determined by the dominant hole current and the electroluminescence efficiency is controlled by the contact limited electron injection. These results demonstrate significant effects of organic film structure on the performance of organic LEDs.

We present a general electroabsorption technique to measure the electric field in each layer of multilayer organic light-emitting diodes. The electroabsorption signal from each layer is identified spectroscopically and measured as a function of dc bias. Measurements were made on three layer devices consisting of a hole transport layer, a light-emitting layer, and an electron transport layer. In reverse bias, without significant charge injection, the dc electric field is uniform throughout the device. In forward bias, the dc electric field is distributed nonuniformly; it is smallest in the light-emitting layer and largest in the hole transport layer. The nonuniform dc electric field distribution is caused by the accumulation of electrons (holes) at the interface between the light-emitting layer and the hole (electron) transport layer. The maximum accumulated charge densities are 2×1012 electrons/cm2 and 3×1011 holes/cm2. These results highlight the carrier blocking role of monopolar transport layers and demonstrate a powerful technique to characterize multilayer organic structures.

A widely applicable electroabsorption technique to measure internal electric fields in organic light‐emitting diodes is presented. The technique exploits the change in the a.c. electroabsorption response in the presence of a d.c. electric field. The electroabsorption signal is modulated at the fundamental frequency of the a.c. test signal, in addition to the usual modulation at the second harmonic frequency, when a d.c. bias is present. In metal/organic film/metal devices employing different metal contacts there is a built‐in electric field in the organic film caused by the difference in work function between the two contacts. The electroabsorption response at the fundamental frequency of the applied a.c. bias is measured as a function of an external d.c. bias. The electroabsorption signal is nulled when the applied d.c. bias cancels the built‐in electric field established by the different metals. We apply this technique to measure changes in metal–polymer Schottky barrier heights as a function of the contact metal. In metal/multiple organic films/metal structures the electroabsorption signals from the constituent organic films are identified spectroscopically and measured at both the fundamental and second harmonic frequency of the a.c. test signal. The amplitudes of the electroabsorption responses are then used to determine the a.c. and d.c. electric fields present in the organic layers. We apply this technique to determine the d.c. electric field distribution within a multi‐layer organic light‐emitting diode. These results highlight the general applicability of electroabsorption methods to probe internal electric fields in organic light‐emitting diodes. © 1997 John Wiley & Sons, Ltd.

Large deployable space-based optical systems will likely require complex structure position controls in conjunction with an adaptive optic to maintain optical tolerances necessary for near diffraction-limited performance. A real-time holographic (RTH) compensation system can greatly reduce the requirements and complexity of the position control system and enable the use of novel or imperfect optical components for large mirror surfaces. A hologram of the distorted primary is recorded with a local beacon at 532 nm (-100 nJ/exposure) on an optically addressed spatial light modulator and transferred as a phase grating to a ferroelectric liquid crystal layer. The hologram is played back with target light containing the same optical distortion. A corrected image is obtained in the conjugate diffracted order where the phase of the optical distortion is subtracted from the distorted image. We report recent test results and analysis of a RTH-compensated deformed mirror of 0.75 m diameter. The short exposure hologram is recorded at video frequencies (30 Hz) at bandwidths up to 5 kHz.Correction for tens of waves of static and dynamic optical distortions including mechanical and thermal warp, mechanical vibration, and air turbulence are shown for monochromatic (532 nm) and broadband (532 nm) illuminated targets.

Evidence of quantum lateral confinement in sidegated resonant tunnelling diodes formed by patterned substrate regrowth Appl.