Organic Light-emitting Devices With Solution-processed Hole Injection Layer

Organic light-emitting diodes(OLEDs)with superior performance such as self luminescence,wide viewing angle,high brightness,low power consumption and flexibility are considered as the next-generation"ideal displays".OLEDs attract special attentions for their potential applications in lightings and displays.It is well established that interface regulation plays a key role in governing device performance.In addition,solution-processed technology with low cost and high-throughput meets the emerging requirement of organic optoelectronic devices.In this dissertation,based on electrical doping and developing novel materials with solution process,a series of hole injection interfacial layers(HILs)and high-performance OLEDs are reported.The mechanism of the carrier injection and their tuning device performance is systematically investigated.Some major works are summarized as follows:(1)Solution-processed aqueous MoO_x(s-MoO_x)is facilely fabricated.The OLEDs are constructed using s-MoO_x as HIL.The effect of"spin-coating/annealing"on device performance is also studied.The results show that s-MoO_x based OLED exhibits maximum luminous efficiency of 7.9 cd/A and power efficiency of 5.9 lm/W,which have been enhanced by 43.6%and 73.5%,respectively,in comparison with the counterpart using evaporated MoO_x(e-MoO_x).Atomic force microscopy(AFM)analysis shows that s-MoO_x behaves superior film morphology.The current versus voltage(I-V)characteristics and impedance spectroscopy measurements indicate that the device performance enhancement is due to the hole injection promotion of s-MoO_x and accordingly improves carrier balance.(2)The OLED with MoO_x-doped graphene oxide(GO+MoO_x)as composite HIL is demonstrated.The maximum luminous efficiency reaches 8.6 cd/A,which has been enhance by 75.5%and 41.0%,respectively,in comparison with the counterpart using GO or MoO_x as HIL.I-V characteristics and impedance spectroscopy analysis show that GO+MoO_x substantially promotes hole injection.Ultraviolet photoelectron spectroscopy(UPS)analysis shows that the incorporation of MoO_x promotes surface work function.Improved carrier balance is obtained by optimizing device structure and thereby significantly improving device performance.(3)MoO_x-doped PEDOT:PSS(PEDOT:PSS+MoO_x)as composite HIL is proposed particularly for ultraviolet(UV)OLED with rather high hole injection barrier height.With TAZ as emitter,the UV OLED gives maximum external quantum efficiency(EQE)of4.4%,electroluminescence peak of 376 nm and full width at half maximum of 34 nm.AFM measurements show that PEDOT:PSS+MoO_x behaves superior film morphology.UPS and impedance spectroscopy analysis show that PEDOT:PSS+MoO_x shows enhanced surface work function,which reduces hole injection barrier height and promotes hole injection capacity.Consequently,the UV OLED performance is substantially improved.(4)A polymeric carbon nitride(CN_xH_y)ethanol solution is synthesized using dicyandiamide and successfully served as HIL in OLEDs.Compared with conventional PEDOT:PSS-based OLED,the luminous efficiency and power efficiency of CN_x H_y-based device have been enhanced by 76.6%and 26.5%,respectively.AFM analysis shows that CN_xH_y possesses superior film morphology.The analysis of UPS and impedance spectroscopy indicates that significant improvement of device performance is ascribed to suitable surface work function and appropriate hole injection of CN_xH_y.