Improvement in electron and hole injection at electrodes and in recombination at a two-organic-layer interface

Abstract

In order to improve the power efficiency of standard multilayered organic electroluminescent (EL) devices, i.e. ITO/HTL/ETL/M, enhanced electron and enhanced hole injection at an ETL/M and an ITO/HTL interface, respectively, were attempted, as well as enhanced electron–hole recombination at an HTL/ETL interface, by designing proper charge injection at these interfaces. Here, ITO, HTL, ETL, and M are abbreviations for an indium-tin oxide anode, a hole-transport layer, an electron-transport layer, and a metal cathode. The HTL and ETL materials used were N, N′-diphenyl- N, N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and tris(8-hydroxyquinoline)–aluminum (Alq 3) respectively, whereas a vapor-deposited aluminum (Al) electrode was used as the metal cathode M. In order to enhance the charge injection at both electrodes, lowering the barrier heights and thinning the barrier thickness were attempted by controlling the work functions of the ITO and the Al electrode and doping the HTL and the ETL, respectively. For lowering the work function of the Al cathode and doping the Alq 3 ETL, the reaction of hot Al atoms with alkali metal carboxylates during Al vapor deposition was utilized. To increase the work function of the ITO and doping the TPD HTL, the use of thin TPD films doped with various oxidizing reagents was examined at the ITO/HTL interface. The enhancement of the charge recombination at HTL/ETL was attained by increasing the interfacial areas, i.e. introducing a mixed layer of TPD and Alq 3, or inserting a thin film of rubrene with a higher recombination efficiency.