Improved luminous efficiency of organic light-emitting diodes by carrier trapping dopants

We propose a carrier trapping (CT) dopant method to improve the luminous efficiency of organic light-emitting diodes (OLEDs). The CT dopant did not emit light itself, but adjusted the charge balance in the OLEDs. The cell structure of the OLED in the experiment was as follows: Indium tin oxide (ITO)/hole transport layer (50 nm, 2TNATA)/emitting layer (20 nm, NPB + 5% rubrene)/electron transport layer (30 nm, Alq + 4% CT dopant)/MgIn. (2TNATA: 4,4',4"-Tris(N-(2-naphthyl)N-phenyl-amino)-triphenylamine, NPB; N,N'-Di(naphthalen-1-yl)-N,N'-diphenyl-benzidine). 4,4'-Bis(carbazol-9-yl)-biphenyl (CBP) was used as a CT dopant. This cell had a yellow emission originating from the rubrene with the luminous efficiency of 8.7 cd/A at a Current density of 10 mA/cm(2). On the other hand, when the CT dopant was not used, the luminous efficiency was 7.5 cd/A. The luminous efficiency was improved by the CT dopant method.