All-solution Processed Organic Light Emitting Diodes

Solution-processed Organic light-emitting diodes?OLEDs?,especially all-solution processed OLEDs,have great potentials in low-cost,large-area,and wearable electronic devices.Therefore,they are deemed as the next step in OLED technology.Efficient charge injection is necessary to realize high performance all-solution processed OLED.The hole injection layers for blue-emitting polymer PFSO have been well studied and optimized.It's found that the low-conductive PEDOT:PSS CH 8000 could help the OLED devices achieve high efficiency at low current density due to the accumulation of the electrons at the CH 8000/emission layer interface.However,the insulating PSS layer on CH 8000 surface is responsible for the inefficient hole injection at high current density.The imbalanced charge density confines the hot carriers in a narrow recombination zone,leading to severe efficiency roll-off and permanent degradation of the PFSO backbone.A simple methanol treatment of the CH 8000 surface effectively removes the redundant PSS.Without the insulating PSS layer,the hole injection becomes efficient which extends the recombination zone.As the result,the methanol-treated CH 8000 device not only retains low turn-on voltage and the high device efficiency of the untreated CH 8000 device,but also achieves high peak luminance,mild efficiency roll-off,and good device stability comparable to those of PEDOT:PSS AI 4083device.In all-solution processed OLEDs,since the metal cathode is solution-processed,a robust election injection layer?EIL?is required to efficiently inject electrons,resist the metal ink's solvent,and protect the underneath organic functional layers.By combining organic EIL's efficient electron injection and inorganic EIL's excellent solvent resistance and solvent-proof capability,a novel organic/inorganic hybrid PFN-OX/Zn O EIL is developed to help all-solution processed devices achieve higher device efficiency and lower operation voltage for the first time.Moreover,a‘solvent detector”is introduced to quantitatively identify the permeated solvent based on the change of PEDOT:PSS's lateral electric conductivity.No change on PEDOT:PSS's lateral conductivity shows that the PFN-OX/ZnO hybrid EIL can completely prevent the TGME solvent from permeating through.As a result,high performance all-solution processed OLED was achieved.The blue emission device reaches 1000 cd m^-2 at only 6.2 V with the luminous efficiency of 2.1 cd/A.Solvent treatment is widely adopted to improve the device performance of both OLEDs and Polymer Solar Cells?PSCs?.The effect of solvent treatment on the top of organic layer on buried PEDOT:PSS layer has been studied.After methanol solvent treatment on the top organic active layer,the conductivity of PEDOT:PSS increases by 3 orders of magnitude,which confirms that the solvent penetrates into the PEDOT:PSS layer.Using a“peel-off”method,the buried PEDOT:PSS layer is fully exposed.Further characterizations suggest that the permeating solvent dissolves PSS,changes PEDOT:PSS'core-shell structure into a linear structure,and moves PSS from the bulk to the surface.As a result,PEDOT becomes more continuous in the bulk.The continuous conducting PEDOT-rich domains create percolating pathways which significantly improve electric conductivity.Morevoer,since PEDOT:PSS layer's lateral conductivity is largely enchanced,PEDOT:PSS layer functions as electrode.As a result,solvent treated device's active area is expanded,which affects the accuracy of the measurement.What's more,using methanol or 2-methoxyethanol as electron injection/extraction layers?PEI,PEIE,PFN and so on?'solvent will cause similar problem.To prevent the expansion of the active area,ethanol should be used as the solvent of the electron injection/extraction materials.