Properties of non-doped organic light-emitting devices based on an ultrathin iridium complex phosphor layer

Organic light-emitting devices (OLEDs) were constructed with a structure of indium tin oxide (ITO)/N,N'-bis(naphthalen-1-yl)-N'-bis(phenyl)-benzidine (NPB) (50-xnm)/bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2'] iridium (acetylacetonate) [(t-bt)2Ir(acac)] (nm)/NPB (30nm)/Mg:Ag (200nm).A thin blue emission material of NPB was used as a separating layer,and the (t-bt)2Ir(acac) yellow phosphorescent dye was acted as an ultrathin light-emitting layer.TPBI acted as both hole-blocking and electron-transporting layer.By changing the location (x) and the thickness (d) of the phosphor dye,the variation of device performance were investigated.The results showed that all the devices had a turn-on voltage of 2.8V.In the case of d=0.2nm and x=5nm,the OLED had a maximum luminance of 18367cd/m2 and a maximum power efficiency of 5.3lm/W.The high performance is attributed to both direct charge carrier trapping of iridium phosphor dye and the thin NPB separation layer,which effectively confines the recombination zone of charge carriers.