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-hydrox yquinoline) aluminum)-based organic multilayer structures, i.e., Alq(3)/oxa diazole (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 '-bip henyl]-4,4'-diamine), which possess different types of energy lineups (equi valent 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)/P BD which is of type-I energy lineup, while the tendency of separation of ex citons, i.e., electrons and holes generated in the excited molecules, was d emonstrated for Alq(3)/PPCP and Alq(3)/TPD which are of type II. These resu lts indicate that the energy lineup with respect to HOMO and LUMO energy le vels can be a guideline for luminescence dynamics in organic multilayer str uctures.