Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films

Abstract

The authors investigated the characteristics of spectrally narrow emissions at the cutoff wavelength from the edges of optically and electrically pumped organic semiconductor films. They estimated the optical properties of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) films using variable angle spectroscopic ellipsometry, revealing that the BSB-Cz films have uniaxial anisotropy and that BSB-Cz molecules lie nearly parallel to substrate surfaces. The authors discuss here the spectral changes in the edge emissions from neat BSB-Cz and tris(8-hydroxyquinoline)aluminum (Alq3) films of 15 different thicknesses under continuous-wave (cw) optical excitation. The spectrally narrow emissions of transverse electric (TE) and transverse magnetic (TM) modes were observed. The authors showed that the peak wavelengths of these emissions varied according to the cutoff wavelengths of asymmetric slab waveguides and that their spectral widths changed depending on the angular dispersion of the Fabry–Pérot interferometer composed of the organic film and its interfaces. The edge emissions from the BSB-Cz films had narrower bands than those from the Alq3 films and increased with a superlinear dependence on the stripe length of the excitation area, indicating the occurrence of light amplification under cw optical excitation. Finally, the authors demonstrate spectrally narrow emissions from the edges of electrically pumped organic light-emitting diodes (OLEDs) having a fine line-shaped waveguide structure with a silver metal cladding layer. Narrow emissions of TE mode were observed, and the peak wavelengths of the emission spectra corresponded well to the cutoff wavelength of the waveguide with the silver cladding layer. In addition to the narrow emissions, the authors observed some intriguing phenomena suggestive of the occurrence of light amplification in OLEDs under electrical pumping.