Light Out-coupling And Device Engineering For High-efficiency Organic And Quantum-dot LEDs

After decades of research and development,organic light emitting diodes(OLEDs)are widely used in optoelectronic products owing to the advantages of good flexibility,low power consumption,fast response time,wide color gamut,etc.A conventional OLED planar structure consists of a glass substrate,a transparent anode,organic layers and a highly reflective metal cathode,and all these components have different refractive indices.Due to a large mismatch in refractive indices,only 20%of the light emitted by the emitting layers can escape into the air.It is known that most of the generated photons are trapped in the form of(1)waveguide(WG)modes in the substrate and ITO/organic layer because the total internal reflection occurs at the boundaries between the air and glass as well as between the glass and ITO,and(2)surface plasmon polariton(SPP)modes which are associated with the interface between the cathode and organic layer.In my thesis,I use light extraction technique to enhance current efficiency of OLEDs,in which a nano-structured ITO anode with low haze was fabricated using hydrochloric acid(HCl)to etch the planar ITO anode,and hemispherical PS spheres which were randomly distributed on the surface of planar ITO were employed as an etching template.With using nano-modified ITO,the maximum current and power efficiencies of Ir(ppy)2(acac)based OLED devices were enhanced by 40.7%and 37.1%,reaching 83.3 cd/A and 74.6 lm/W in the forward direction,respectively.Also,we selected a low refractive index material,MgF2,to reduce the SPP in order to further achieve a significant increase in light extraction efficiency.On the other hand,colloidal quantum dots(QDs)have attracted much attention due to their unique advantages,including narrow emission bandwidths,emission wavelength tunable,good photostability and low-cost due to its simple solution processing.Therefore,QDs have been applied to light emitting diodes(LEDs).Since the first report of QD based LEDs(QLEDs),remarkable progresses have been made in the areas related to the synthesis of new QD materials,new device architectures,and interfacial engineering.In the second part of my thesis,red-emitting inverted QLEDs with a current efficiency of 9.54 cd/A were fabricated by using a electron-block layer(EBL),mCBP,inserted between the hole-transport layer(HTL),CBP,and the QD layer,which corresponds to an efficiency enhancement of 19%as compared to the device without the mCBP.This simple strategy can improve the balance of charge injection and reduce the electron leakage into the HTL because mCBP has a shallower LUMO(LUMO=the lowest unoccupied molecular orbital)energy level than that of CBP.This EBL may have a better prospect and be favorable to achieve highly efficient QLEDs.