Solution-processed small molecular electron transport layer for multilayer polymer light-emitting diodes

Solution-processed electron transport layers (ETL) have been fabricated by solution process and applied in multilayer polymer light-emitting diodes with tris[2-(p-tolyl)pyridine]iridium(III) blended in poly(vinylcarbazole) as the emissive layer. Three kinds of small molecular electron transport materials,including 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), and 4,7-diphenyl-1,10-phenanthroline (BPhen), are tested and dissolved in methanol to form electron transport layers by blade coating. Such electron transport layer provides efficient electron injection and electron transport ability in the devices. The efficiency of the devices with the combination of ETL and LiF/Al cathode reaches 21.5 cd/A at 10 V (4050 cd/m2). The efficiency of the devices without ETL are 3.5 cd/A (13 V) for LiF/Al cathode and 17 cd/A (7 V) for CsF/Al cathode at 1000 cd/m2. The aggregation of the solution-processed ETL can be controlled by annealing temperature to further optimize the device performance to maximal efficiency of 53 cd/A.