Study On Improved Performance Of Organic Light-emitting Devices By Balanced Carries

Organic light-emitting devices(OLEDs)have garnered continuous attraction for solid-state lighting applications and display due to their high efficiency,low cost,flexible property and high-brightness.The design of using multilayer with highly specialized active materials and a variety of architectures has been successful in realizing superior performance and widely reported.However,this architecture has often necessarily required more materials or processing steps,which is the one of the difficult problem of commercialization.Additionally,injection barriers and unbalanced charge carrier mobility in most organic materials lead to highly unbalanced transport properties of the holes and electrons,resulting in reduced efficiency.In this work,we fabricated simplified devices and achieved improved carries balance.Firstly,we present a device concept based on ultrathin non-doped orange emission layers(EML)and blue doped EML.The blue EML adopted mixed host structure with the combination of hole transport host material,4,4',4''-tri(N-car-bazolyl)-triphenylamine(TCTA),and electron transport host material,1,3,5-tris(2-N-pheylbenzimidazolyl)benzene(TPBi).The employment of the mixed host EML with bipolar charge transport property facilitates the charge carrier balance in the EML,resulting in an extended recombination region and more balanced charge carrier.The white device exhibits the peak efficiencies of 33.8 lm/W and 32.2 cd/A as well as a slight efficiency roll-off.Furthermore,the distribution of recombination region was investigated by inserting an orange ultrathin sensing layer at different position in blue EML.The charge carriers transport behavior was also illuminated by hole and electron only devices.Then,we present a device with single layer based on graded–host(g-host)structure and explored the transport property of dopant.By the employment of a stepwise graded doping profile in the EML,which was fabricated in such a way that the host composition varying continuously from nearly 100% hole transport host material at anode to nearly 100% electron transport host material at the cathode,the device shows improved efficiency and charge balance compared to the mixed host device.The device shows peak efficiencies of 43.0 cd/A and 45.1 lm/W.Based onthese fact,we increasing the doping concentration of the dopant,the highly efficient green device with lower turn-on voltage was realized.Furthermore,detailed investigations on the recombination region and the current density behavior of the hole and electron only devices reveal the highly balanced charge carries transport and improved charge carries injection,which is the reason for such superior properties.