The molecular orientation of linear‐shaped molecules in organic amorphous films is demonstrated to be controllable by the substrate temperature. It is also shown that the molecular orientation affects the charge‐transport characteristics of the films. Although linear‐shaped 4,4′‐bis[(N‐carbazole)styryl]biphenyl molecules deposited on substrates at room temperature are horizontally oriented in amorphous films, their orientation when deposited on heated substrates with smooth surfaces becomes more random as the substrate temperature increases, even at temperatures under the glass transition temperature. Another factor dominating the orientation of the molecules deposited on heated substrates is the surface roughness of the substrate. Lower carrier mobilities are observed in films composed of randomly oriented molecules, demonstrating the significant effect of a horizontal molecular orientation on the charge‐transport characteristics of organic amorphous films.

We experimentally investigated suppression of the roll-off characteristics of the electroluminescence efficiency at high current densities in organic light emitting diodes (OLEDs). To increase exciton density, we propose a nonheterostructure OLED that consists of a single emitting layer of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) and layers locally with doped donors/acceptors on the cathode and anode sides. The OLED exhibited suppression of the roll-off characteristics at high current densities over 100 A/cm2 with balanced bipolar injection and transport, resulting in the high exciton density of 1024 cm−3 s−1. Furthermore, amplified spontaneous emission with a relatively low threshold of Eth=24 μJ/cm2 was obtained by optically pumping the single-layer device. However, to realize electrical excitation it was necessary to reduce the lasing threshold by two orders of magnitude or to inject a 100 times higher current density.

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 0 -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.