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Control of molecular dye orientation in organic luminescent films by the glass transition temperature of the host material.

OLED devices

Glass substrates (Corning Eagle XG, Thin Film Devices Inc.) with 90 nm of predeposited indium tin oxide (ITO) as a transparent bottom electrode were treated by a standard cleaning procedure (including rinsing with N-methyl-2-pyrrolidone, ethanol, and deionized water as well as treatment with ultraviolet ozone). All the subsequent layers were deposited in a single ultrahigh vacuum (UHV) chamber evaporation tool (Kurt J. Lesker Co.) at a base pressure of 10−7 mbar. The thickness and deposition rates were monitored using a quartz crystal microbalance. First, a 0.5-nm-thin layer of MoO3 was evaporated on top of the ITO to facilitate the injection of holes from the anode ITO. The electrode was followed by the two organic layers TCTA and TPBi, where TCTA and TPBi were used as HTLs and ETLs, respectively. The EML was formed by doping the first part (10 nm) of the ETL with the respective phosphorescent dopant. Four different phosphorescent emitters were tested, with the remaining stack architecture kept identical. The deposition rates for the EML were 0.4 and 1.0 Å/s for both HTL and ETL, respectively, well below the criteria for realizing ultrastable glasses (25). For all emitters, the layer thicknesses of HTL and ETL were optimized using a thin-film optics simulation tool (40), including transition dipole moment orientation. To study the effect of the TPBi layer’s properties on the device performance, different samples were prepared by changing solely the substrate temperature during the evaporation of the EML and ETL layers for each of the mentioned devices. Finally, a bilayer cathode consisting of 0.5-nm LiF followed by 100-nm aluminum was deposited on top of the organic layers. Immediately after fabrication, all OLEDs were encapsulated with glass lids under nitrogen atmosphere along with a getter material.