Device engineering towards high performance white organic light-emitting diodes with good chromaticity stability

Abstract

Abstract of thesis entitled DEVICE ENGINEERING TOWARDS HIGH PERFORMANCE WHITE ORGANIC LIGHT-EMITTING DIODES WITH GOOD CHROMATICITY STABILITY Submitted by Cheung Wai Lung For the degree of Doctor of Philosophy at The University of Hong Kong In August 2021 White organic light-emitting diodes (WOLEDs) have received enormous research efforts aiming at boosting up the luminous efficacy, wall-plug efficiency, colour purity and operational stability. Over the last decade, WOLEDs have been introduced in the solid-state lighting systems in attempt to replace the conventional lighting technologies, such as fluorescent lamps and inorganic LEDs. However, WOLED technology still faces numerous technical problems, including low current efficiencies, short lifespan and chromaticity loss, all of these hinder its full power for the lighting market nowadays. This dissertation focuses on the optimization of dichromatic WOLEDs for high performance and high quality colour through device engineering and utilization of novel materials. Particular attention is paid to three aspects: (1) the optimization of device architecture for achieving WOLEDs with small efficiency roll-offs, (2) the impact of electroluminescence (EL) properties of yellow-emitting gold(III) phosphors on the colour rendering index (CRI) of WOLEDs, and (3) the utilization of different yellow-emitting gold(III) phosphors for realizing WOLEDs with stable chromaticity. Different from WOLEDs reported in the literature, all the WOLEDs in this dissertation were based on our tailor-made gold(III) complexes in combination with the commercially available blue-emitting iridium(III) complexes. Firstly, the photophysical properties of yellow-emitting gold(III) complex Au-Y had been investigated, where it was intentionally doped into different host materials to examine the efficiency of host-guest energy transfer. The mCP/PYD2Cz host system was found to provide higher photoluminescence quantum yield (𝛷PL) for the Au-Y and FIrpic emitters due to their higher triplet energies and matched energy levels, highlighting the importance of efficient host-guest energy transfer on the 𝛷PL. The optimization of composite hosts led to a substantial increase in current efficiency (ȠL) in the hybrid-fabricated gold(III)-based WOLEDs, i.e. from 21.2 cd/A for the conventional TCTA/mCP host system to 31.8 cd/A for the mCP/PYD2Cz one. The transient photoluminescence decay studies verified that the short excited state lifetimes () in Au-Y:mCP may be the major reason responsible for the performance improvement. Extremely small efficiency roll-off (Δroll-off) could be realized by employing another tailor-made yellow-emitting gold(III) complex Au-wY. This gold(III) complex exhibited high PL of 0.86, short  of ~1 μs, together with a broad emission bandwidth of 107 nm. Taking the distinct advantages of Au-wY, hybrid-fabricated WOLEDs showed high ȠL of 25.7 cd/A, external quantum efficiency (ȠEQE) of 10.5%, as well as nice white emission with CIE coordinate of (0.40, 0.47) and colour rendering index (CRI) of 69. In addition, small roll-off of 1.6% was achieved at high luminance of 10,000 cd/m2. The performance improvement was found to be related to the short  in the Au-wY, resulting in a reduced triplet-triplet annihilation (TTA) rate. This observation could be further confirmed by employing other three tailor-made yellow-emitting gold(III) complexes Au-1, Au-2 and Au-3, where all three complexes showed large emission bandwidths of more than 100 nm. Vacuum-deposited WOLEDs with high ȠL of up to 46.3 cd/A and ȠEQE of up to 18.2% were realized. In addition, the WOLEDs exhibited high colour chromaticity, in which the CIE coordinates remained unchanged and both x and y coordinates only varied by 0.01 even if the luminance increased from 100 cd/m2 to 1000 cd/m2. This colour stability is highly desired for making WOLEDs to be made use in the practical lighting systems. These findings pave the way for making high performance and colour stable WOLEDs based on gold(III) complexes.