Influence of cathode roughness on the performance of F8BT based organic-inorganic light emitting diodes

Hybrid light emitting diodes (HyLED) with a structure of FTO/ZnO/F8BT/MoO3/Au/Ag is fabricated and the influence of surface roughness of cathode (FTO/ZnO) is investigated. The roughness of FTO could be decreased from 9.2 nm to 2.2 nm using a mild polishing process. The ZnO film, deposited by spray pyrolysis, functions as an electron injection layer. The roughness of the FTO/ZnO surface is found also highly dependent on the ZnO thickness. For thin ZnO films (20 nm), polishing results in better efficacy and power efficiency of LED devices, with nearly a two times improvement. For thick ZnO films (210 nm), the overall FTO/ZnO roughness is almost independent of the FTO roughness, hence both polished and unpolished substrates exhibit identical performance. Increasing ZnO thickness generally improves the electron injection condition, leading to lower turn on voltage and higher current and power efficiencies. However, for too large ZnO thickness (210 nm) the ohmic loss across the film dominates and deteriorates the performance. While the polished substrates show less device sensitivity to ZnO thickness and better performance at thin ZnO layer, best performance is obtained for unpolished substrates with 110 nm ZnO thickness. Larger interface area of ZnO/F8BT and enhanced electric filed at sharp peaks/valleys could be the reason for better performance of devices with unpolished substrates. (C) 2014 Elsevier B.V. All rights reserved.