Metal Oxide Interlayers For Solution-Processed Light-emitting Diodes

This thesis focuses on the application of transition metal oxide interlayers in solution-processed LEDs.We tuned the properties of ZnO nanocrystal electron injection layer and NiOx hole injection layer during the processes of nanocrystal synthesis and film post-treatment respectively,and investigated the impact of it on the performance of LED devices.The purpose of this thesis is to understand the relationship between the properties of oxide interlayers and performance of devices,and to develop oxide interlayer materials suitable for solution-processed LEDs.As to electron injection layer,doping of ZnO nanocrystals with indium was realized by controlling the reaction kinetics,resulting in indium doped ZnO(IZO)nanocrystals and leading to an increase of the free electron concentration in IZO nanocrystals.Subsequently,high quality nanocrystal thin films were prepared after ligands exchange of nanocrystals and the optimization of film-forming process.It was found that indium doping could tune the work function of ZnO nanocrystal thin films.The performance of solution-processed OLEDs based on the IZO nanocrystal thin film electron injection layers showed that indum doping lowered the turn-on voltage by 0.5 V and doubled the efficiency of the devices.Based on scaning kelvin probe miscroscope results,we concluded that the impovement of device performance was due to lower electron injection barrier in devices because of lower work function of IZO nanocrysta film.In the aspect of hole injection layer,to improve the work function of NiOx thin film prepared by precursor solution route,a small molecule modification method was developed to introduce an interfacial diople to the surface of NiO.thin film.The surface properties of the modified NiOx thin film was systematically investigated.The results showed that the work function enhancement effect of small molecule modification was significantly better than that of frequently used UV-O3 treatment,and that small molecule modification could suppress quenching of quantum dots on NiOx thin film to a certain extent.Further studies demonstrated that small molecular modification could greatly reduce the hole injection barrier in devices,resulting in improved hole injection,which gave NiOx thin film superior hole injection capacity to commonly used PEDOT:PSS.This research solved the problem of poor hole injection ability of NiOx thin film.Based on the results of small molecule modification of NiOx thin film,we found that special attention should be paid when replacing the PEDOT:PSS hole injection layer with NiOx thin film in solution-processed QLEDs.The NiOx thin film can not be too thick because of the high refractive index of it,which is undesirable for light extraction of LEDs.As a result,the NiOx thin film can not function as ITO smoother as PEDOT:PSS.This may lead to leakage current in QLED devices.So more attention should be paid to planeness of ITO substrates.Based on these understandings,solution-processed QLEDs with higher efficiency and longer lifetime were realized by replacing PEDOT:PSS hole injection layer with small molecule modified NiOx thin film.This research will promote the application of NiOx thin film in solution-processed QLEDs.