Abstract
Solution-processable blade coating is applied to multi-layer phosphorescent organic light-emitting diodes (OLEDs) 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]fluorine (TBCPF), and 2,7-bis(diphenylphosphoryl)-9,9’-spirobi[fluorene] (SPPO13). In general, blade coating gives low surface roughness of around 0.2 nm without phase separation of the emitter and the host. 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 they show the highest efficiency. For SimCP2, the maximal efficiency is 15.8 cd/A for blue emission and 24.2 cd/A for white emission. The efficiencies for the hosts is found to be quite different from the efficiencies in vacuum evaporation for the same device structures. Large-area top-emitting OLEDs (TEOLEDs) with multi-layer structure are successfully demonstrated using blade coating on ITO-free substrate. The semitransparent cathode of TEOLED is composed of lithium fluoride (LiF), aluminum (Al) and silver (Ag). The composition of 3 nm Al and 10 nm Ag has a transmittance of 56% and a sheet resistance of 11 Ω/□. It is applied to the green phosphorescence device with an emissive area of 2 cm by 2.5 cm. The maximum current efficiency is 25.2 cd/A with high light-emission uniformity within 10% variation. The large-area TEOLEDs show comparable current efficiency as the small-area devices with an emissive area of 2 mm by 2 mm (having the same device structure) and better efficiency than traditional large-area bottom-emitting devices. Cesium fluoride (CsF) and n-doped electron transport layer are applied to improve electron injection. At 6 V, the luminance is raised from 141 cd/m2 to 502 cd/m2 and 304 cd/m2, respectively. Tandem WOLED is initially fabricated by blade coating process. The crystallization in bottom EL unit is reduced by coating low solubility material on the connecting layer. PEDOT:PSS has low solubility, so it efficiently protects bottom structure from crystallization. TAZ replaces TPBi which easily crystallizes in high temperature as ETL, thus the crystallization is solved. However, the bottom EL unit is dissolved during spin-coating PEDOT:PSS, so the luminance of bottom EL unit is weak. Blade-coated PEDOT:PSS shows reduced dissolution because of the advantage of rapid drying. Finally, the current efficiency is 5.8 cd/A; the CIE coordinates is (0.36,0.41); the color temperature is 4735 K.