Amplified spontaneous emission under optical pumping from an organic semiconductor laser structure equipped with transparent carrier injection electrodes

Abstract

We succeeded in observing amplified spontaneous emissions (ASEs) from an organic semiconductor laser structure equipped with transparent carrier injection electrodes under optical pumping. We employed a transparent indium-tin-oxide (ITO) anode and cathode, which significantly minimized light propagation loss compared with that in conventional metal electrodes. In particular, we incorporated an ultrathin MgAg layer between the organic electron transport layer and ITO cathode to enhance electron injection efficiency, while maintaining low light propagation loss, and also to protect the organic layer from plasma damage when forming the ITO. By optically pumping the ITO [30 nm]/4,4′-bis[N(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD) [20 nm]/ 4,4′-di(N-carbazolyl)biphenyl (CBP) doped with 1,4-dimethoxy-2,5-bis[p-{N-phenyl-N(m-tolyl)amino}styryl]benzene (BSB) [70 nm]/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) [20 nm]/tris-(8-hydroxy-quinoline)aluminum (Alq3) [20 nm]/MgAg [2.5 nm]/ITO [20 nm] device, a low ASE threshold of Eth=5.1±1.0 μJ/cm2 with a full width at half maximum of 11 nm was obtained under optical excitation. We also evaluated electrical pumping with this device. Although we observed high efficiency electroluminescence at an external quantum efficiency (ηext) of 3.6% at a low current density of J=0.1 mA/cm2, a rapid decrease in ηext was observed with an increase in current density, suggesting the presence of large exciton-polaron annihilation.