Evidence for alkali metal formation at a cathode interface of organic electroluminescent devices by thermal decomposition of alkali metal carboxylates during their vapor deposition

This study examines the possibility of thermal decomposition of Na salts of acetate, benzoate, and fluoride during vacuum vapor deposition using a quartz crystal microbalance to measure negative frequency shift (Deltaf) caused by increasing mass deposited from the same amount of source materials. Cs acetate is also examined. We compare the negative frequency shift-source current (Deltaf-I) curves of the Na salts with those of organic materials such as tris(8-hydroxyquinoline)aluminum and N,N-'-diphenyl-N,N-'-bis(3-methylphenyl)-1,1(')-biphenyl-4,4(')-diamine. CH3COONa and C6H5COONa exhibit much lower Deltaf than the organic materials. CH3COOCs gives much larger Deltaf than CH3COONa due to the higher atomic weight of Cs. These exhibit clear evidence for alkali metal formation by thermal decomposition during vapor deposition of alkali metal carboxylates. (C) 2004 American Institute of Physics.