To Enhance Light-output Efficiency From Organic Light-emitting Diodes With Dielectric/Metal/Dielectric Electrode

Organic light-emitting diodes(OLED)have been become the first candidates for the next-generation light-emitting devices with low-cost,high efficiency and high quality,due to its wide viewing angle,low power consumption,fast responsive and unique properties of transparency and color debugging.However,the layered structure of OLED limits the propagation of light generated in the emissive layer,resulting in the low external quantum efficiency of only 20%to 30%."Dielectric/Metal/Dielectric"(D/M/D)transparent conductive film has excellent optical transmittance and electrical conductivity.Furthermore,the light injection characteristics and its micro-cavity modes can be adjusted by changing the dielectric layers in the D/M/D electrode.Currently,D/M/D-based OLED has become the hot topic in this field.Based on the theory of optical thin films,in this thesis,we established a theoretical model to calculate the light-output efficiency(LOE)of OLED with the D/M/D electrode as the anode,and analyzed the main influence factors on the LOE of OLED,and then we have fabricated and characterized the D/M/D-based OLED.Through theoretical and practical investigations,we got the following main innovative achievements:(1)Based on the light-emitting theory of OLED and the transmission characteristics of light in tandem thin films,a theoretical model for the LOE of OLED with the D/M/D electrode as the anode was established by using parameters such as the complex refractive index and the thickness of every thin films.(2)Based on the analysis of the transmittance of D/M/D tandem thin films,a structure of typical OLED[Al/Emissive-layer/Hole-transporting Layer/D/M/D/Glass]was selected to investigate the main factors to improve the LOE of OLED,and found that the refractive index of metal(n_m)layer in the D/M/D anode should be less than 1,and the smaller n_m the larger LOE is.On the other hand,both the refractive indexes of the two dielectric layers,and emissive layer,transporting layer make a great effect on the LOE of OLED,and this LOE of OLED gets much higher when both the refractive indexes of the two dielectric layers is bigger;and LOE of OLED gets much higher as long as the refractive index of the emissive layer is smaller while the refractive index of the transporting layer is larger.(3)Two kinds of OLED[Glass/Anode/NPB(70 nm)/Alq₃(60 nm)/Li F(1 nm)/Al(100nm)with MoO₃(50 nm)/Ag(13 nm)/MoO₃(70 nm)and ITO(150 nm)]as the anode were fabricated,respectively.The optoelectronic properties of these two kinds of devices were compared with each other to explore the influence of D/M/D on the performance of OLED.Our experimental data showed that the external efficiency of OLED with D/M/D electrode as the anode can be improved greatly(50.02%enhancement)as compared to those devices with ITO as the anode.