Hole Transport Molecules in High Tg Polymers: Their Effect on the Performance of Organic Light-Emitting Diodes

Abstract

Two high Tg transparent polymers, A435 and CH, have been synthesized to be used as host materials for TPD, a hole transport molecule that is morphologically unstable when vacuum-sublimed as a thin film. A435 forms solid solutions with TPD in all proportions while TPD is only soluble in CH up to about 30 wt %. At higher contents in CH, TPD forms microscopic clusters. Films containing up to 75 wt % TPD in A435 or CH are morphologically stable when heated at 100 °C for 72 h. OLED devices have been made using variable TPD contents in A435 or CH as a hole transport layer and Alq3 as an electroluminescent and electron transport layer. The best OLED performance is obtained for 75 wt % TPD in A435 (Lmax ∼ 3500 cd/m2; ηmax ∼ 0.8%) and in CH (Lmax ∼ 6000 cd/m2; ηmax ∼ 1.7%). The difference in performance has been mainly attributed to the electron transport capability of A435, which is not an electrically inactive host polymer like CH. In devices made with A435, electrons are able to reach the ITO electrode where they neutralize without participating in the electroluminescence of the device. When this electron leakage is blocked with a thin CuPc layer, Lmax and ηmax of devices made with A435 + TPD or CH + TPD become practically identical. Device performance is therefore indifferent to the particular physical state (solid solution or microclusters) of the hole transport molecules in the host polymer. The electron transport capability of A435 has been demonstrated using a device comprising a first layer of pure A435 spin-coated on top of a layer of high Tg polymer, STPD-QP, showing bipolar transport properties and luminescence in the blue.