Solution-processed Electron Injection Layer For Organic Light-emitting Diodes

Vacuum thermal evaporation method has been generally used as a fabrication process of organic light-emitting diode?OLED?,by which high efficiency OLEDs can be easily fabricated.But it still has several problems,such as high equipment cost,inefficient use of materials and rigorous fabrication environment.Those problems can be solved by introducing solution coating process.OLED is a multilayer structure device where function layers are sandwiched between anode and cathode.Balancing and confining carriers within emissive layer are the keys to fabrication high efficiency OLED where carrier injection plays an important role.Hence,it is very meaningful to research on the carrier injection layer of solution-processed OLEDs.Zinc oxide?ZnO?based electron injection layer?EIL?is widely used in solution-processed OLEDs.But most of the reported fabrication methods of ZnO precursor are very complicated.In this paper we come up with an easy method to prepare the ZnO precursor.Applying with the novel ZnO precursor,we fabricated both efficient conventional and inverted solution-processed OLED.Application of novel ZnO electron injection layer in conventional solution-processed OLED.In this paper,we introduce a simple method,which does not need heating or longtime stirring,to prepare ZnO precursor.The film fabricated from this ZnO precursor shows great electron injection property,even better than thermally evaporated cesium carbonate?Cs₂CO₃?.Thus,a high current efficiency of 58.5 cd/A is achieved in inverted OLED with this ZnO EIL.When we apply this ZnO EIL to solution-processed OLED,we find that although the ZnO EIL possesses good electron injection ability,its hole blocking property is weak.So we bring up a mixture of ZnO and polyethyleneimine ethoxylated?PEIE?as EIL.The optimized device with ZnO:PEIE?3:1?EIL obtain a maximum luminance of over 10000 cd/m² and a current efficiency of 0.93 cd/A,exhibiting nearly 10 times'performance enhancement than device with pristine ZnO EIL.Application of novel ZnO electron injection layer in inverted solution-processed OLED.When the ZnO EIL is applied to inverted solution-processed OLED,poor performance is observed.So we turn to investigate the application of ZnO:PEIE EIL in solution-processed OLED.We find that the on-set voltage of device with ZnO dominated EIL shows³ V,but when the amount of PEIE takes the dominant place in blending EIL as 1:1,the on-set voltage suddenly shifts from³ V to⁵.5 V.Device with ZnO:PEIE?1:5?EIL exhibits a fast increasement of luminance and obtain a maximum luminance of 11070 cd/m².Afterwards,we do atomic force microscope?AFM?test on the EIL film and give a reasonable explanation to the difference of on-set voltage.At last,a mixed interlayer of ZnO:PEIE?1:5?is introduced to modify the ZnO layer.The inverted OLED with ZnO/ZnO:PEIE?1:5?EIL obtain 10000 cd/m² at a low driven voltage 5.8 V,and the maximum current efficiency reaches 1.96 cd/A.