Study On Stepwise Doping Structure And Interlayer Effect In Blue Phosphorescent OLED Emitting Layer

Organic light-emitting diode(OLED)has shown strong potential in the next generation of display and lighting applications due to many advantages such as wide viewing angle,short response time,low driving voltage,low power consumption,and flexibility.High efficiency and luminance of green and red phosphorescent organic light-emitting diodes have been reported for industrial applications,whereas the color purity,efficiency and stability of blue phosphorescent organic light-emitting diodes(PHOLED)have yet to be improved for realization in the commercial market.In this paper,we designed a series of experiments to optimize the structure of the light-emitting layer to improve the performance of the PHOLED device.Firstly,a series of PHOLED devices with doping concentration of double,triple and multi-layer device were fabricated.Using N,N?-dicarbazolyl-3,5-benzene(mCP)as the host and iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2?)]picolinate(FIrpic)with different dopant concentrations of 2%,4%,8%,12%and 16%as blue phosphorescent guest materials of the emitting layer.Since FIrpic is a well-known electron-transporting triplet emitter,the stepwise doping structure can regulate the injection and balance of charge carriers effectively while ensuring the same thickness and doping amount of the light-emitting layer.The maximum values of the luminance,current and power efficiency can reach to 9745 cd/m~2(at 9 V),25.1 lm/W and 32.0 cd/A in the device with the asymmetric tri-EML structure after optimized doping concentration.Then,we fabricate a series of blue PHOLEDs with mixed host material and dual emitting layers using mCP:8% FIrpic as the single host emitting layer and mCP_x:TPBi_y:8% FIrpic as the dual host emitting layer.Since 1,3,5-tris(N-phenyl-benzimidazol-2-yl) benzene(TPBi)is electron transport type host material and mCP is a bipolar material for localized hole transport,the distribution of the dual host light-emitting layer in the light-emitting layer would change the size and position of recombination zone.The maximum values of the luminance,current and power efficiency can reach to 13390 cd/m~2(at 8.75 V),36.63 lm/W and 37.89 cd/A when the thickness ratio is 1:2 between single and mixed host material emitting layer and the concentration of TPBi is 3/8 in mixed host material emitting layer.Finally,a series of blue PHOLED devices with stepwise energy levels host materials 4,4?,4??-tris(N-carbazolyl)triphenylamine(TCTA),mCP and1,3,5-tri(m-pyrid-3-yl-phenyl)benzene(TmPyPB)were fabricated.TCTA,mCP and TmPyPB are hole transport,bipolar and electron transport materials,respectively.Due to the different carrier transport properties and the energy level of the material,different host material collocation will affect the carrier injection and recombination.Compared with single-EML and triple-EML devices,double-EML device with TCTA and mCP displayed higher current density,luminance and efficiency attributed to better balance of holes and electrons,broadening recombination zone and improved trapping of excitons.The maximum values of the luminance,power and current efficiency can reach to15650 cd/m~2(at 8.5 V),27.58 lm/W and 37.31 cd/A.On this basis,the blue phosphorescent interlayer of different thickness(0.25 nm,0.5 nm,1 nm)and different materials,the luminance of blue phosphorescent OLED devices with 0.25 nm PO-01ultrathin interlayer increased to 17090 cd/m~2.