Hybrid Organic-inorganic Light-emitting Diodes Using A Solution-Processed Small-Molecule Emissive Layer

Hybrid organic-inorganic light-emitting diodes（Hy LEDs） using high-work-function anode and air-stable metal oxide carrier-injection layers, overcome sveral issues that may adversely affect the efficiency and stability of conventional devices, such as the high sensitivity of a low-work-function metal cathode and electron-injection layer to oxygen and moisture in the ambient, and the interactions between indium-tin-oxide（ITO） and the conducting polymer hole-injection layer, having large potential in the applications of new display and solid-state lighting. Studies on Hy LEDs mainly focus on yellow-green fluorescent polymer devices, the luminance efficiency of blue, red and white devices warrants the further improvement. It is well-recognized that the utilization of phosphorescent materials can significantly enhance the efficiency of organic light emitting devices because both the singlet and triplet excited states can be used for light emission. However, the phosphorescent Hy LEDs have been rarely reported. In our opinion, the following issues must be solved to construct high-efficiency phosphorescent Hy LEDs:（1） host materials with high triplet state energy are required to avoid the quenching of phosphor emission as found in the case of using conjugated polymer hosts;（2） proper modifier for Zn O layer, which can not only greatly promote electron injection into the emission layer, but also maintain the integrity of the emission layer deposited ontop.In this thesis, we address the abovementioned problems by using TCTA: OXD-7 host materials with high high triplet energy and poly（ethyleneimine） as the modifier for Zn O without interrupting the morphology of the emission layerand report high-efficiency blue, red, yellow and white Hy LEDs based on solution-processed small-molecule emission layer. The main work includes the following aspects:（1） To explore the effect of processing solvents and heat treatment on the properties of small-molecule films, we prepare TCTA:OXD-7（1:1 by weight） films from the chlorobenzene（CB） and tetrahydrofuran（THF） solution,some layers processed with THF are tempered at at 80 ℃ for 10 minutes and vacuum-deposited TCTA:OXD-7 layers are also made for comparison. All the films show pin-hole free and smooth surface,and the root-mean-square（RMS） roughness values of the layers processed from CB and THF are both 0.29 nm, which are similar to those of annealed layers（0.26 nm） and evaporated layers（0.23 nm）. The electron- and hole-current density at certain voltage increase in the sequence of ITHF<Iannealed<ICB<Ievaporated.The close correlation between carrier-transport properties and the refractive indices of these TCTA: OXD-7 films is established.（2） Blue, red and yellow Hy LEDs using TCTA: OXD-7 as the co-hosts and Ph FIr Pic, Ir（btp）₂（acac） and PO-01 as the blue, red and yellow phosphor have been prepared. The efficiency of the Ph FIr Pic devices with structure of ITO/Zn O/PEIE/TCTA: OXD-7: phosphors/TAPC/Mo O3/Al was improved with a decrease in the TCTA: OXD-7 ratio from 4:1 to 1:1, which can be attributed to that increase of OXD-7 content helps to balance carrier transport in the emission layer. The concentration of Ph FIr Pic, Ir（btp）₂（acac） and PO-01 is further optimized to be 20%, 10% and 10%. The maximum external quantum efficiencies of blue, red and yellow Hy LEDs are 18.9, 10.2 and 14.6%, respectively. At 1000 cd m-2, the efficiencies are18.0, 8.2 and 14.4%, showing a small roll-off, which can be attributed to alleviated triplet-triplet and triplet-polaron interactions in a broad carrier recombination zone due to the use of hole-transporting TCTA and electron-transporting OXD-7 co-hosts. The efficiencies of blue and red devices represent significant improvement over previously reported values.（3） The two-colour white Hy LEDs with thestructure of ITO/Zn O/PEIE/TCTA: OXD-7: 20% Ph FIr Pic: 0.4%PO-01/TAPC/Mo O3/Al show the maximum luminance efficiency and external quantum efficiency of 40.0 cd A-1 and 15.0%, the Commission Internationale de L’Eclairage（CIE） coordinates of（0.33, 0.44） and colour rendering index（CRI） of 62. High efficiency can be attributed to the effective utilization of triplet excited states for light emission enabled by the introduction of phosphorescent emitters into small-molecule co-hosts. On this basis, three-colour white Hy LEDs are prepared by co-doping 0.4% Ir（btp）₂（acac） into the EML to improve CRI. The maximum EQE and power efficiency of the three-colour Hy LEDs are 10.9% and 16.4 lm W-1 and with the CIE coordinates and CRI value are（0.33, 0.44） and 71.