Operation behaviors of interconnecting-layers in solution-processed tandem organic light-emitting devices

Abstract

Abstract We investigated the operation behaviors of solution-processed tandem organic-light-emitting devices with regular structures. We fabricated devices with interconnecting-layers of ZnO nanoparticles (NPs)/polyethyleneimine ethoxylated (PEIE)/phosphomolybdic acid (PMA)/poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl) diphenylamine)) (TFB), which were sandwiched by electrodes. Charges were generated at the PMA/TFB interface under electric fields instead of being injected from electrodes. Thermal baking effects of each layer were investigated. We found that high-temperature baking of PMA was essential for realizing low driving voltages. Acetonitrile solvent, which is a good solvent for underlayer PEIE, of PMA solution was replaced with n-butylacetate, which is a poor solvent for underlayer PEIE, to investigate the effect of the mixing state of the PEIE/PMA interface. The device with PMA from n-butylacetate showed significantly larger driving voltage than that with PMA from acetonitrile. The mixing of PEIE and PMA is also essential for realizing low driving voltage. Observation results of the cross-sections of the films by transmission electron microscope supported these conclusions.