Laser Induced Thermal Imaging (LITI) is a high resolution, digital patterning technique developed at 3M for use in a number of applications including the patterning of LCD color filters and OLED emitters. The LITI process is suited for the manufacture of flat panel displays, where both high resolution and absolute placement accuracy are required. In this paper, we present the capabilities of LITI, the basic design of a LITI laser imager, the construction of a LITI donor sheet, and the process by which OLED emitters may be patterned. An OLED device fabricated with the LITI process is described.
Arrays of UV-violet [indium tin oxide (ITO)]/[copper phthalocyanine (CuPc)]/[4,4'-bis(9- carbazolyl)biphenyl(CBP)]/[2-(4-biphenylyl)-5-(4-tert- butylphenyl)-1,3,4-oxadiazole(Bu-PBD)]/CsF/Al organic light- emitting devices (OLEDs), fabricated combinatorially using a sliding shutter technique, are described. Comparison of the electroluminescence spectrum with the photoluminescence spectrum of CBP indicates that the emission originates from the bulk of that layer. However, due to the high gap of CBP and the strong hole capture cross section of perylene contaminants, it was difficult to completely eliminate the emission from the latter. In arrays of devices in which the thickness of the CuPc and Bu-PBD were varied, but that of CBP was fixed at 50 nm, the optimal radiance R was obtained at CuPc and Bu-PBD thicknesses of 15 and 18 nm, respectively. At 10 mA/Cm2, R was 0.38 mW/cm2, i.e., the external quantum efficiency was 1.25%; R increased to ~1.2 mW/cm2 at 100 mA/cm2.
Nanosecond electroluminescence (EL) spikes observed at the voltage turn-off when multilayer blue 4,4'-bis(2,2'diphenyl vinyl) -1,1'-biphenyl (DPVBi)-based organic light-emitting devices (OLED's) are excited by rectangular voltage pulses are described. The spikes exceed the cw brightness by up to an order of magnitude. Time-resolved images of the devices demonstrate that the emission from most of the sample surface decays with a single time constant (tau) 1 equals 13 +/- 3 ns. This decay is attributed to recombination of charges which accumulate at the interface of the electron and hole transporting layers, possibly at intrinsic trapping sites. In areas of increased electron injection and EL such as cathode edges and morphological defects, a second slower decay time 20 ns < (tau) 2 < 1 microsecond is observed, apparently due to release of carriers from localized trap states in the organic/cathode interface. Only marginal variations in (tau) 1 are found between bright and dim areas of the devices. At a bias of 10 V, the amplitude of the spike is found to peak at a pulse duration of approximately 20 microseconds. It is noted that similar OLED's, in which the DPVBi was replaced by tris-(8- hydroxyquinoline Al) (Alq3), did not exhibit such spikes.