Spectrally narrow emission at cutoff wavelength from edge of electrically pumped organic light-emitting diodes

We demonstrate spectrally narrow emissions from the edge of electrically pumped organic light-emitting diodes having a fine "line-shaped" waveguide structure with a silver cathode. A 4,4′-bis[(N-carbazole)styryl] -biphenyl (BSB-Cz) layer, which has a very low amplified spontaneous emission threshold, was used as an emitting layer. We observed extremely narrow emissions having polarized transverse electric (TE) mode. The full width at half maximum became narrower down to 6.5 nm as the peak wavelength of the emission approached the wavelength of the 0-1 transition of the BSB-Cz layer. The edge emission intensity showed a superlinear relationship with the current density, and the spectral shapes changed appreciably depending on the current density, indicating the occurrence of amplification of the emitted light. The peak wavelengths of the edge emissions were attributed to the leaky mode very close to the cutoff wavelength of the waveguide structure having the metal cladding layer.