Improvement of charge injection and balance in inverted bottom-emission organic light-emitting diodes

Abstract

In this thesis, we have investigated the electrical and optical properties of solution-processed zinc oxide (ZnO) and tin dioxide (SnO2) nanoparticles (NPs) layers, and the performances of the inverted bottom-emission phosphorescent OLEDs by using these metal oxide NPs layers as electron injection layers (EILs). These metal oxide NPs layers can be simply deposited by using a spin-coating method without additional treatments and they are transparent in visible spectral region. The devices with the metal oxide NPs layers show improved performances compared with the device without the metal oxide NPs layer. The device with the ZnO NPs layer exhibits better performance than the devices with the SnO2 NPs layer or the ZnO layer obtained from sol-gel method, due to proper energy level of the ZnO NPs layer for electron injection and higher electron mobility. We estimated the performances of the devices with different thicknesses of metal oxide NPs layers and electron transport layers (ETLs), and with electron transport materials with different lowest-unoccupied-molecular-orbital (LUMO) energy levels. In addition, we have estimated angular dependence of electroluminescence emitted from the devices with the metal oxide NPs layers and found that all devices show a Lambertian emission profile. Next, we have developed p-type doped hole transport layer (HTL) using molybdenum trioxide (MoO3) doped di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) layer. We have investigated the electrical and optical properties of MoO3 doped TAPC layer with different MoO3 doping concentrations. By applying this p-type doped TAPC layer to white OLEDs, we reduce the driving voltage and improve the luminous power efficiency of the device. Furthermore, we have studied the relationship between the hole conductivity of HTL and the performances in the inverted bottom-emission OLEDs. For various hole conductivities of HTL, MoO3 doped TAPC (high conductivity), undoped TAPC (reference), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) doped TAPC (low conductivity) layers were used as HTLs. Inverted bottom-emission white phosphorescent OLEDs with three colors of red, green, and blue are fabricated using these various HTLs. The device with MoO3 doped TAPC layer shows low driving voltage but low efficiency, whereas the device with BCP doped TAPC layer shows high efficiency but high driving voltage. The device with BCP doped TAPC layer exhibits the highest external quantum efficiency (EQE) without other optical light extraction techniques or n-type doping method in reported value of inverted white OLEDs. This result indicates that electron-hole balance is very important for high efficiency and enhanced electron injection is required for high-performance inverted structure OLEDs. Finally, we have investigated performance of green and blue OLEDs with common red layers (CRLs). The highest-unoccupied-molecular-orbital (HOMO) energy level differences between hosts and red dye affect the performance of devices. The driving voltages of devices with CRLs are similar or increases compared with those of the devices without CRLs as the HOMO energy level differences are changed, but the efficiencies and the colors of the devices are rarely changed regardless of the insertion of CRLs. As the HOMO energy level difference between hosts and red dye is small, the change of the current and the driving voltage is small. Moreover, we have also fabricated full-color inverted bottom-emission OLEDs employing CRLs. The tendencies of electrical and optical characteristics of the inverted structure device are similar to those of the conventional structure device. The insertion of CRL especially improves the quantum efficiency of the inverted structure device due to enhanced electron-hole balance. In conclusion, we improved electron and hole injection by using metal oxide NPs as an EIL and p-doped HTL, respectively. Enhanced charge injection reduces driving voltage and increases quantum efficiency of the inverted bottom-emission OLEDs. We also found that control of the HTL conductivity can improve electron-hole balance, resulting in enhanced efficiency of the device. We increased the EQE of the inverted bottom-emission OLEDs using CRLs. In addition, insertion of proper CRL can reduce fabrication cost and TAKT time without performance change in OLEDs. We believe that the fabrication methods and device structures developed in this thesis are helpful for realizing efficient and low-cost optoelectronic devices.