Electrical conduction and low voltage blue electroluminescence in vacuum-deposited organic films

Abstract

Bright blue (400–500 nm) recombination electroluminescence (EL) has often been observed from anthracene crystals with solid electrodes; external quantum efficiencies of 1%–8% have been seen and the theoretical efficiency is approximately 40%. The power efficiency has always been greatly degraded by the high voltages necessary to obtain reasonable currents through the comparatively thick crystals used. We report the preparation, and the structural, electrical and EL properties of approximately 0.6 μm vacuum-deposited anthracene films through which large steady state current densities can be passed at rather low applied voltages. The films showed the critical optimization effect, various structural and electrical properties exhibiting sharp singularities when the films were prepared at substrate temperatures within a narrow range near −60 °C. EL visible in normal room lighting was obtained at applied voltages of only about 30 V and up; in thinner films, EL could be seen in a darkened room at only about 12 V. This is apparently the first time that clearly visible EL has been reported from an organic material at voltages significantly less than 100 V. Broadly similar results were obtained for perylene and 1,12-benzperylene films; EL does not appear to have been observed before from these substances in any form. We discuss in some detail the prospects for practical EL devices based on organic films. In view of our results, and of the great improvements which are now probably feasible, we conclude that the prospects are promising; if these improvements can be realized, such devices could include high efficiency blue light-emitting diodes, and large-area broader wavelength systems for illumination applications.