Polymer blends in organic LEDs offer the advantage of processibility coupled with flexibility with respect to the chromophores which can be incorporated into their structures. The polymer matrix is also though to have a positive effect on the quantum efficiency and other LED performance indicators. Two‐layer devices have also been shown recently to yield improved performances. Here, a two‐layer device based on polymer blends is reported.

Transient electroluminescence ͑EL͒ from single-and multilayer organic light-emitting diodes ͑OLEDs͒ was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide ͑ITO͒/tris͑8-hydroxy-quinoline͒aluminum ͑Alq 3 ͒/magnesium ͑Mg͒:silver ͑Ag͒, whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine ͑CuPc͒/N,NЈ-di͑naphthalene-1-yl͒-N,NЈ-diphenyl-benzidine ͑NPB͒/Alq 3 /Mg:Ag. Apparent model-dependent values of the electron mobility ( e ) in Alq 3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the EL onset-dependent on the electric field-is governed by accumulated charges ͑holes͒ at the internal organic-organic interface (NPB/Alq 3 ) or is transport limited. Time-of-flight measurements on 150-nm-thin Alq 3 layers yield weak field-dependent e values of the order of 1ϫ10 Ϫ5 cm 2 /Vs at electrical fields between 3.9ϫ10 5 and 1.3ϫ10 6 V/cm.

We have performed electric-field and temperature-dependent electron injection studies in an aluminum/ tris͑8-hydroxy-quinolinolato͒aluminum/magnesium:silver single-layer organic light-emitting diode. Analysis of the observed injection currents in terms of the classic Fowler-Nordheim ͑FN͒ tunneling or RichardsonSchottky ͑RS͒ thermionic emission proved to be inadequate. Whereas, the FN-type behavior at high-electric fields must be considered accidental, the injection currents qualitatively resemble those of the RS concept. However, quantitative differences are observed concerning the RS coefficient, the prefactor current, and the temperature dependence. On the other hand, the experimental data are in excellent agreement with a recently presented Monte Carlo simulation ͓U. Wolf et al., Phys. Rev. B 59, 7507 ͑1999͔͒ of carrier injection from a metal to an organic dielectric with random hopping sites. ͓S0163-1829͑99͒14535-1͔

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We report on amplified spontaneous emission and optically pumped deep blue lasing in the organic spirobifluorene derivative 2,7-bis(biphenyl-4-yl)-2′,7′-di-tert-butyl-9,9′-spirobifluorene. Solid-state lasing is observed in thin films of this material deposited on a distributed-feedback (DFB) grating substrate. The laser wavelength can be tuned from 401.5 to 434.2 nm depending on the grating period of the Bragg reflector. The blue edge of this interval at 401.5 nm makes this laser an extremely short wavelength organic DFB laser. When pumping with a pulsed nitrogen laser at 337 nm, we observe a laser threshold energy density of 83 μJ/cm2. These results render this spiro compound an excellent candidate for blue-emitting diode lasers.

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Since the discovery of lasing in organic solid-state thin films many efforts have been devoted to this field.[1±9] Organic lasers offer broad tuning ranges and there are active materials covering almost the whole visible spectrum. However, electrically pumped organic lasing has not been shown so far. One of the challenges unachieved so far has been the realization of organic thin-film solid-state lasing in the ultraviolet wavelength region below 380 nm. Organic materials for ultraviolet organic light-emitting diodes (OLEDs) have been reported.[10±12] Solid-state dyes lasing in that wavelength region have only been reported for doped silica-gel layers not suitable for possible electrical applications. [13,14] Organic lasing in semiconducting thin films has only been shown for wavelengths above 392 nm. [3] In this communication we report on the first thin-film organic semiconductor laser operating in the ultraviolet wavelength region between 377.7 and 395 nm. This is the shortest laser wavelength reported so far for thin-film organic solid-state lasers. The novel spiro-linked material 2,2¢,7,7¢-tetrakis(4-fluorphenyl)spiro-9,9¢-bifluorene was used as the active organic layer in an optically pumped distributed feedback (DFB) structure. The chemical structure is shown in the inset of Figure 1. Spiro-linked materials have been proven to be very stable materials for utilization as charge transport and emitting layers in OLEDs.[15±22] They are also known as candidates for organic solid-state lasers. Amplified spontaneous emission (ASE) has been observed in a variety of spirolinked materials, including spiro-quarterphenyl (Spiro-4U), and spiro-sexiphenyl (Spiro-6U). [18,21,22] In Spiro-4U, an ASE maximum was observed at approximately 390 nm. A very promising application for tunable organic solid-state lasers is the field of tunable laser spectroscopy (TLS). The small size and the availability of organic thin-film solid-state lasers in the whole visible spectrum makes them an inexpensive alternative to conventional gas or dye lasers. The large tuning range of organic lasers allows the easy adjustment of the output wavelength to the particular requirements of the spectroscopic application. In the ultraviolet region alternatives are especially limited. Ultraviolet diode lasers exhibit only small tuning ranges, gas lasers are limited to certain discrete wavelengths, and liquid-based dye lasers are cumbersome in handling. As long as no electrical operation of thin-film organic lasers is demonstrated, they may, due to their low thresholds, be pumped by compact and inexpensive diode pumped microchip lasers.[23] Thus the strong infrastructure requirements of certain gas or liquid-based dye lasers can be avoided. In this report, we prove the capability of organic thin-film UV lasers for spectroscopic applications in the field of TLS. Therefore we measure the photoluminescence spectra of the fluorescence marker dyes Coumarin 6, Coumarin 152, and Rhodamine 6G in solution using our thin-film organic laser as the excitation source. R...

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