Abstract
Blade coating is applied to multi-layer phosphorescent OLED with five small-molecule hosts for the emission layer, including bis[3,5-di (9H-carbazol-9-yl)phenyl]diphenylsilane (SimCP2), 2,6-bis(3-(9H-carbazol-9-yl) phenyl) pyridine (26DCzPPy), 4,4′,4″-tris-(N-carbazolyl)- triphenylamine (TCTA), 9,9-bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]fluorene (TBCPF), and 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13). In general, blade coating gives low surface roughness around 0.2 nm without phase separation of the emitter and the host. In the large area of 2 cm by 3 cm the film thickness distribution is within 10% and uniform light-emission is achieved. 1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) is added to tune the electron transport. Among all the hosts, 26DCzPPy and SimCP2 have by far the best electron-hole balance and consequently show the highest efficiency. For SimCP2, the maximal efficiency is 15.8 cd/A for blue and 24.2 cd/A for white emission. The order of efficiencies for the hosts is found to be quite different from the order in vacuum evaporation for the same device structures. © 2014 Elsevier B.V.