Comparative study of photoluminescence dynamics of tris(8-hydroxyquinoline) aluminum-based organic multilayer structures with different types of energy lineups

Photoluminescence (PL) dynamics of aluminumquinoline (Alq(3); tris(8-hydroxyquinoline) aluminum)-based organic multilayer structures, i.e., Alq(3)/oxadiazole (t-BuPBD; (2-(4-biphenylyl)-5-(4-tert-butylphenyl) 1,3,4-oxadiazole), Alq(3)/cyclopentadiene (PPCP; 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene and Alq(3)/diamine (TPD; N,N'-diphenyl-N,N'-bis-(3-methylphenyl)-[1,1 '-biphenyl]-4,4'-diamine), which possess different types of energy lineups (equivalent to types I and II of inorganic semiconductor multilayer structures) of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, were investigated and compared. The fast energy transfer followed by strong PL in Alq3 was proved for Alq(3)/PBD which is of type-I energy lineup, while the tendency of separation of excitons, i.e., electrons and holes generated in the excited molecules, was demonstrated for Alq(3)/PPCP and Alq(3)/TPD which are of type II. These results indicate that the energy lineup with respect to HOMO and LUMO energy levels can be a guideline for luminescence dynamics in organic multilayer structures.