Organic Light Emitting Devices (OLEDs)
We are studying the emission of light from thin layers of conjugated polymers applied to a substrate by spin coating. We have studied both emission of visible as well as infrared light from these devices. The objectives of this research are to determine the effects of interfacial layers upon the voltage current relationship of the OLEDs (accurately labeled polymer light emitting devices-PLEDs). In addition, the duration of light emission from the PLEDs is short, and we are studying the mechanisms responsible for this short lifetime. In some case, quenching of luminescence is dependent upon the identity and concentration of certain chemical species, and PLEDs may be used as detectors for these chemicals.
Organic light emitting devices for information display on a portable radio by Pioneer. The OLED devices are light weight, consume low power, and are cheap to produce.
Cathodoluminescent Phosphors
Cathodoluminescence is luminescence stimulated by bombardment by an electron beam. This is the principle of operation of television picture tubes, computer monitors and cathode ray tubes in general. We are studying the deposition of thin film cathodoluminescent phosphors and comparing their brightness and efficiency to that of powder phosphors. While thin film phosphors are thought to exhibit higher resolution and better lifetime properties, their brightness and efficiency are generally inferior to powder phosphor. The cause of this is internal reflection of light by multiple scattering in the thin films ("light piping"), which we have demonstrated can be controlled by changing the microstructure of the thin film. In both thin film and powder phosphors, long term exposure to the electron beam will cause lower brightness and efficiencies. Our group was the first to demonstrate that the mechanism of long exposure degradation of ZnS cathodoluminescent phosphors was electron beam stimulated surface chemical reactions between ZnS and residual vacuum gases, primarily either water (H2O) or hydrogen (H2). The electron beam dissociates the slowly reacting molecules into extremely reactive atomic species (O or H) which produce volatile SOx or H2S, leaving either ZnO or metallic Zn at the surface. Neither reaction product luminesces, and the phosphor is reduced significantly in brightness and efficiency. In order to prevent such reactions, we have coated the surface of phosphor powder particles with layer of about 10 nm thick and measured the effects on brightness, efficiency and degradation. These coatings act as "dead layers" where there is no luminescence, reducing the brightness and efficiency of the phosphors according to the coating thickness relative the electron beam energy. In some cases coatings have reduced the rate of degradation. In other cases, the coatings themselves have been degraded by the electron beam and resulted in more rapid degradation of the phosphor.
In two other projects, we are studying the synthesis of rare earth oxysulfide powder phosphors using a special low temperature process. Normally synthesis of phosphors such as La2O2S:Eu requires temperatures in excess of 1000oC and times of several hours. We have synthesized powders with similar brightness and efficiencies using temperatures as low as 250oC and times of less than 15 minutes. Finally, the use of cathodoluminescent phosphors in field emission displays (FEDs) suffers terribly from the fact that the efficiency of the phosphors decreases rapidly as the electron beam energy is decreased below about 8 keV. We are studying the brightness and efficiency of phosphors as a function of conductivity, beam energy, current density and doping density to determine the cause of this effect.
Blue light emission from a thin film of calcium strontium gallium sulfide doped with cerium after deposition onto a glass substrate by sputtering. The blue light is being excited by a 2 keV electron beam striking the sample (cathodoluminescence).
'Artificial Eyelid' for Optical Detectors
An array of optical detectors are frequently nonuniform in their response, and may need attenuation of incoming light to respond accurately over a range of brightness. We are working with an optical shutter which can correct for nonuniform sensitivity and attenuate over a wide range of brightness. Furthermore it may be pixelated in order to make a 'smart' attenuator. The device consists of a transparent substrate and transparent conducting electrode, on which a metallized polymer is placed, then released from the substrate over 90% of the area. Because of extrinsic stress generated by the difference in the coefficient of thermal expansion between the metal and polymer, the metallized polymer curls open upon release from the substrate. However when a voltage is applied between the metal and transparent conducting electrode, the shutter will close by electrostatic attraction. We have demonstrated closure speeds of about 10 microseconds, and operating speeds up to 10 kHz. This device may have applications in shutter for camera and focal plane imaging arrays.
Array of optical shutters shown for the 'eyelids' in an open condition. By applying a voltage along the metal conductors going to each individual shutter, the 'eyelid' can be closed in 10 microseconds.
