Exciton confinement in organic multiple quantum well structures

High-quality organic multiple quantum well (MQW) structures based on tris(8-hydroxyquinoline) aluminium and 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) have been prepared by use of a multisource-type high-vacuum organic molecular beam deposition system. Small-angle x-ray diffraction measurements indicate that the MQW structures have a very uniform layered structure throughout the entire stack. Both optical absorption and photoluminescence (PL) measurements reveal the evidence for exciton confinement in the /PBD MQW structures. The thickness-dependent blue shift of the PL peak observed upon decreasing the well width is attributed to the change of exciton energy as a result of quantum confinement. Theoretical calculations have been performed and the results agree with the experimental results. Light-emitting diodes (LEDs) based on the /PBD MQW structure with greater efficiency, a narrower electroluminescence spectrum and less of a blue shift than the single heterostructure LEDs have also been fabricated.