Abstract
In this thesis, we focus on chromaticity modeling, device optimization and innovative fabrication process of solution-processed organic light emitting diodes (OLEDs). First, in the introduction, we briefly review the history and development of organic electroluminescence devices, and the application of OLEDs in displays and solid state lighting. Operating princioples, measurement method, and device fabrication of OLEDs, and in particular, white light emitting devices are also discussed. In the second part of the thesis, chromaticity simulation of white OLEDs by superposition of individual illuminators is introduced. Design rules of approaching Planckian locus in Commission International de l’Eclairage (CIE) coordinates diagram are porposed. A series of high color rendering index (CRI) and high chromaticity white OLED spectrum have been calculated. By adjusting the emitters doping concentrations in single emissive layer, binary, ternary, and quaternary dopants blade-coated white OLEDs were fabricated to verify the numerical models. The binary white OLED exhibited high correlated color temperature (CCT) of 5700 K with CRI value of 59, while the quaternary white OLED exhibited a higher CRI value of 85 and a warm white illumiation of CCT = 2700 K. Furthermore, according to the model, the devices with a novel Os(II)-based emitter (emission λmax = 555 nm) were designed and fabricated. The ternary white OLED utilizing this novel Os(II) emitter delivered a high CRI value of 84 at CCT = 2845 K and small deviation to Plancknain locus (Du’v’ = +0.0009). In the third part, co-host system for solution processed emissive layer has been investigated. High hole-mobility material 4,4′-Cyclohexylidenebis[N,N-bis(4- methylphenyl)benzenamine] (TAPC) was blended with either electron transporting materals or bi-polar hosts. By tuning the ratio of hosts and optimizing the device strcture, highly efficient solution-processed co-host white OLEDs devices were fabricated. Furthermore, we separately explored two different methods to increase the external quantum efficiency (EQE) of the white OLEDs. Exteriorly, by utilizing micro-lens film and hemisphere structure, the light out-coupling efficiencies were enhanced by 1.3 times and 1.89 times, respetively. Interiorly, by fine-tuning the thickness of the emissive layer and carefully selecting the hole injection materials, a highly efficient solution-processed white OLED with current efficiency (CE) of 44.6 cd/A, power efficacy (PE) of 35.6 lm/W, and EQE up to 22.9 % was obtained without out-coupling enhancement structures. In the fourth part, we have analyzed the optoelectronic properties of crosslinkable hole transporting materials. Monochromatic multiple-layer solution-processed OLEDs were fabricated. The green OLED exhibited CE of 56.8 cd/A, PE as high as 54.6 lm/W, and EQE up to 15.9 %. In the last part of the thesis, we present the preliminary results of quantum dot light emitting diodes utilizing organic transporting layers and zinc oxide nanoparticles.