Display industry has been vastly expanding in the last few decades because of demands for large area display, high quality of panel and development of electronic devices adopting display. Currently most panels adopt two types of display, one is the conventional liquid crystal display with white light-emitting diode (LED) back-light, the other is organic light-emitting diodes (OLEDs). Different from conventional display, OLEDs have potential applying to next generation technology of display with flexible, transparent and biocompatible characteristics because OLEDs have modest substrate dependency. Despite these promising application of OLEDs, still several problems awaiting solution remain.
Major challenges in organic light-emitting diodes (OLEDs) are classified into three categories: ⅰ) color quality, ⅱ) high efficiency and ⅲ) long-term stability. These challenges should be improved for practical usabilities in OLED industries.
Generally, color quality of OLEDs is appraised by electroluminesecent (EL) spectrum. Shape of EL spectrum determines Commission Internationale de L’Eclairage (CIE) coordinates affecting color gamut, temperature and rendering of OLEDs. Basically, CIE coordinates are determined by intrinsic emission color of emitter whose spectrum is determined by their own excited state. Therefore, proper selection of the emitter is crucial for intrinsic color of OLEDs. Efficiencys is also main index of evalution of OLEDs because it corresponds with energy consumption of device straightly. Therefore, many researchers working on organic semiconductor field, have been developing efficient emitter, host and device structure with light extracting structures. Despite of their effort, efficiency of blue OLEDs is still difficult compared to other primary colors, red and green due to limitation of material selection and design of structure result from its high energy over 2.6 eV. Therefore, developing high energy materials for blue OLEDs is always present progressive issue. Even though OLEDs have proper spectrum and high efficiency, device cannot be utilized for practical use because operational lifetime of OLEDs should be long enough to use in daily life or industry. Over the last decades, improving long-term stability of OLEDs has been studying. Nowadays, green, red phosphorescent OLEDs (PhOLEDs) and blue fluorescent OLEDs (FlOLEDs) are used in display panel due to their operational stability. However, most stable OLEDs show low efficiency especially external quantum efficiency (EQE) compared to theoretical limit of efficiency. In other words, achiving both theoretical efficiency and long operational lifetime is the solution for the ultimate conundrum of OLEDs.
This thesis considers aspects of two topics: (1) blue PhOLEDs with efficiency of theoretical limit, and (2) OLED with long operational lifetime by using stable materials with high bond dissociation energy.
In Chapter 2, the first report of exciplex-based blue PhOLEDs with efficiency of theoretical limit is introduced. Due to large bandgap and high triplet energy (T1) of blue phosphorescent emitter, exciplex forming co-host for blue emission had been regarded as unattainable thing. However, using the combination of carbazole containing high T1 hole transporting material (HTM) and pridine/pyrimidine containing electron transporting material (ETM), novel exciplex forming co-host was designed with the energy of 2.99 eV. Utilizing new new exciplex system, blue PhOLED achieved 29.5% of EQE, which is theoretical limit of EQE confirmed by optical simulation. Using both experimental result and optical calculation, we showed that achievement of exciplex for blue emission and theoretical limt of efficiency is possible.
Expanding on the application of blue exciplex, we introduced that fundamental limit of EQE can be increased utilizing low refractive index materials. In Chapter 3, innate theoretical limit of EQE can be expanded using low refractive index material containing exciplex forming co-host system. We simulated maximum achievable EQE with different various refractive indices of HTMs and ETMs for several emitting dipole orientations (Θ). For the case of 1.8 of refractive index, general refractive index of organic thin film, the theoretical maximum achievable EQE of blue OLEDs is around 30% under condition of 100% of PLQY and isotropic Θ. However, citing our simulation, low refractive index materials containing OLEDs can achieve over 60% of EQE with horizontal Θ and 100% of PLQY of emitter. To verify the prediction, we fabricated blue PhOLEDs using exciplex forming co-host possessing low refractive index ETM, whose refractive index is ~0.1 lower than conventional ETM in blue wavelength region. Fabricated device showed the maximum EQE of 34.1% and power efficiency (PE) of 79.6 lm W-1, which are the highest value among sky blue PhOLEDs.
Unlike the case of sky blue, the maximum efficiency of deep blue OLEDs (CIE y < 0.2) is not high enough because of their low PLQY, Θ and device structure hard to optimize. To solve this problem, we adopted novel deep blue Ir-complex with high PLQY and Θ. In Chapter 4, highly efficient deep blue PhOLEDs are introduced. We analyzed the effect of ancillary ligand on the Θ. Even small difference of chemical structure of ancillary ligand, local electron density can significantly be changed that the direction of transition dipole moment (TDM) aligned horizontally to the plane of emitting surface. We obtained theoretical EQE of 31.9% with deep blue emission due to 86% of Θ, 19% higher than isotropic orientation.
The last remaining pulling teeth of OLEDs, operational stability is concerned in the next part. In Chapter 5, exciplex based red PhOLEDs possessing both high efficiency and long-term stability is introduced. Exciplex forming co-host is generally known for boosting the efficiency but exciplex based OLEDs have suffered form the short operational lifetime. We showed that the certain exciplex system constituting of materials stable in ionic states has both high efficiency and long-term stability. Using new exciplex system, red PhOLEDs achieved EQE of 34.1% and a long operational lifetime 2249 h from 1000 cd m-2 to 900 cd m-2 (LT90).
In Chapter 6, we studied fundamental origin of long-term stability of OLEDs. From a macro perspective, degradation occurs via two pathways. One is external factor and the other is internal origin like chemical reaction including dissociation and reunion. Essentially, if chemicals constituting OLEDs were not dissociated in the operation, emitting characteristics of OLEDs could not be changed. Base on this idea, we designed stable materials possessing high bond dissociation energy (BDE) and T1 level. Most HTMs possessing high T1 contain single C-N bond like phenyl-carbazole bond, which can be a bond cleavage site in the molecules because most single C-N bonds have low BDE lower than 2.0 eV. To prohibit the bond cleavage and sustain high T1, we got rid of single C-N bond and adopted fused carbazole moiety. Using this methodology, homolysis cannot be occurred by single bond cleavage in newly designed HTM. We compared its properties with conventional material and finally obtained elongated operational lifetime and enhanced luminance by change of HTM in the OLEDs.