Hole transport molecules in high T-g polymers: Their effect on the performance of organic light-emitting diodes

Two high T,transparent polymers, A435 and CH, have been synthesized to be u sed as host materials for TPD, a hole transport molecule that is morphologi cally unstable when vacuum-sublimed as a thin film. A435 forms solid soluti ons 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 c ontaining up to 75 wt % TPD in A435 or CH are morphologically stable when h eated at 100 degreesC for 72 h. OLED devices have been made using variable TPD contents in A435 or CH as a hole transport layer and Alq(3) as an elect roluminescent and electron transport layer. The best OLED performance is ob tained for 75 wt % TPD in A435 (L-max similar to 3500 cd/m(2); eta (max) si milar to 0.8%) and in CH (L-max similar to 6000 cd/m(2); eta (max) similar to 1.7%). The difference in performance has been mainly attributed to the e lectron transport capability of A435, which is not an electrically inactive host polymer like CH. In devices made with A435, electrons are able to rea ch the ITO electrode where they neutralize without participating in the ele ctroluminescence of the device. When this electron leakage is blocked with a thin CuPc layer, L-max and eta (max) of devices made with A435 + TPD or C H + TPD become practically identical. Device performance is therefore indif ferent to the particular physical state (solid solution or microclusters) o f 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 T, polymer, STPD-Q P, showing bipolar transport properties and luminescence in the blue.