Bilayer Modification On Cathode And Organic-inorganic Hybrid Structure Light Emitting Diodes

It is very important to optimize the device structure for improving the performance of organic light-emitting diodes(OLEDs).In this thesis,we have fabricated standard N,N'-diphenyl-N,N'-bis(1-napthyl-phenyl)-1,1'-biphenyl-4,4'-diamine(NPB)/tris-(8-hydroxyquinoline) aluminum(Alq₃) devices with an efficient CsCl/Mg:Ag/Al cathode; or with a newly organic-inorganic hybrid structure including a p type MoO₃ layer inserted into hole transport layer(HTL) NPB or at the interface of hole injection layer (HIL) CuPc and HTL,by using conventional thermal evaporation in molecular-vapor deposition(OMVD) system.We measured the characteristic of the devices,and discussed the mechanism for performance improvement.As our experimental results show,1) OLEDs with CsCl monolayer as cathode modification can greatly lower the operational voltage and enhance the luminance efficiency.The utilization of intermediating a thin Mg:Ag layer,with efficient CsCl/Mg:Ag/Al cathode,can further enhance the devices performance.The current efficiency and power efficiency were both increased by about 20%.One of the dominant mechanisms responsible for this performance improvement is presumably attributed to the interface chemical reaction at the structure cathode,which would be beneficial to electron injection,and balanced the carrier concentration in the recombination zone of OLEDs,and thus improved the performance.2) By importing a thin p-type MoO₃ layer into NPB,when the MoO₃ layer was inserted near the ITO anode,the driving voltage of the device decreased and power efficiency increased.We consider the performance improvements with MoO₃ at different distance away from ITO were related to the induced Charge Transfer complex(MoO₃^-:NPB⁺) by the inserted MoO₃ layer.3) By inserting a MoO₃ layer into the interface of hole injecting layer CuPc and hole transport layer NPB,we also received improved performance devices with lower driving voltage and higher brightness.It is considered that there would be a step barrier between ITO and hole transport layer,which would be good for enhancing the hole injection that improving the device performance.