| Organic light-emitting devices (OLEDs) have attracted considerable interests as the next generation efficient full-color flat panel display and illumination sources. After 30 years of rapid development, the OLED technology is gradually becoming mature, and some products are capable of practical application and industrialization. However, there are still some crucial problems needed to be solved. In OLED, a balanced charge carrier transport is required to achieve high efficiency. Nevertheless, the electron mobility (?e) is usually several orders magnitude lower than the hole mobility (?h) in most organic semiconducting materials due to the electron trapping. Although various electron transport materials (ETMs) have been designed and synthesized for OLEDs, the favorable ETMs with high ?, high triplet energy (ET), and high glass-transition temperature (Tg) are relatively rare. In this thesis, the full text includes the introduction (chapter 1) and eight chapters content (chapter 2-chapter 9) which cover my researches in OLEDs.Chapter 1:Introduction. The development course and status quo of OLEDs were described. The research progress of ETMs were introduced in detail. The problems which related to the ETM researches were analyzed and discussed, and then the design ideas were expounded.As we know, TmPyPB is recognised as a favorable ETM with a high ET (2.75 eV), appropriate LUMO/HOMO levels (-2.90/-6.68 eV) and a high ?(-1.0×10-3 cm2 V-1 s-1). But the poor thermodynamic stability (Tg?79?) and the processing property degrade the improtance in state-of-the-art devices. Learning this, the targeted researches from chapter 2 to 6 were carried out and new favorable ETMs were obtained.Chapter 2:Three new star-shaped compounds with hexakis(fluoren-2-yl)-benzene as the core and pyridine as the periphery (2Py-HFB,3Py-HFB and 4Py-HFB) were synthesized and characterized. These HFB-cored ETMs owned high Tgs (>141 ?) and better processing property compared with TmPyPB. By using the new compounds as the ETMs, the all-solution-processed phosphorescent organic light-emitting devices (PhOLEDs) showed good performance with a maximum current efficiency (?c>max) of 5.6 cd A-1 and a maximum external quantum efficiency (?ext,max) of 4.68%.Chapter 3:Four new ETMs with quinoxaline as the core and pyridine as the periphery (Tm3PyQ, Tm4PyQ, o-3PyDQB and o-4PyDQB) were designed and synthesized. In contrast to TmPyPB, the enhanced thermal stability (Tg:112-148 ?) and electron affinity (Ea:2.73-2.88 eV) of the compounds can be mainly ascribed to the introduction of quinoxaline. Using the compounds as the ETMs, FIrpic based PhOLED exhibited an ?c,max of 30.2 cd A-1 and a maximum power efficiency (?p,max) of 20.9 lm W-1.Chapter 4:Four pyridine-containing precursors (namely TmPyPB and Tm2-4PyDPB) were synthesized, and then treated by hydrochloric acid to generate four new pyridine hydrochlorides (TmPyPB*HCl and Tm2-4PyDPB*HCl). The pyridine hydrochlorides showed fine solubility in alcohol and water, as well as enhanced electron injection properties. Using the pyridine hydrochlorides as the soluble ETMs, the "supper yellow" based OLEDs achieved an ?c,max of 21.2 cd/A, an ?p,max of 16.8 lm/W and an ?ext,max of 6.6%, while an ?c,max of 18.1 cd/A, an ?p,max of 10.3 lm/W and an ?text,max of 5.7% were achieved in the devices without using any active metal for lowering the electron injecting barrier.Chapter 5:Three new star-shaped phenylpyridine-based compounds were synthesized, and then the phenylpyridine units were locked via alkyl chains. Subsequently, the salt-forming reaction was carried out to obtain three new ETMs of G0, G1 and G2. These new pyridinium salts also showed good solubility in alcohol and water. The LUMOs were around -3.5 eV, and the highest ?achieved at 3.2 X 10-3 cm2 V-1 s-1 from G2. The favorable LUMO levels and high ?e of these pyridinium salts render them suitable candidates as the ETM and cathode interlayer materials for OLEDs.Chapter 6:In this chapter, the pyridine ring of TmPyPB was replaced by the pyrimidine ring, and the central core varied from benzene to triazine. Then three new ETMs (TPM-TPB, TPM-TAZ and TPM-i-TAZ) were obtained. Among the three compounds, TPM-TAZ, with the LUMO leve of -2.85 eV, ET of 2.82 eV, Tg of 131 ?, and ?e up to 2.0 ×10-3 cm2 V-1 s-1, meets all the requires of ideal ETMs. The relationship between molecular structure and material properties were discussed.Unit selection mainly focus on searching an ideal core with planer molecular structure and more delocalized ?-conjugation while keeping a high ET simultaneously.Chapter 7:Dibenzofuran and dibenzothiophene with planer structure and high ET (> 3.07 eV) were firstly introduced into the star-shaped ?-conjugated skeletons to design four new ETMs (TDBF-TAZ, TDBF-i-TAZ, TDBT-TAZ and TDBF-i-TAZ). All these compounds exhibited good thermal stability (Tg> 124?) and favorable LUMO levels (-2.8--3.0 eV), revealing their potential as the ETMs for OLEDs.Chapter 8:Three new benzo[1,2-d;4,5-d']bisoxazole-based compounds (namely 3Py-DBBO,4Py-DBBO and TPO-DBBO) were synthesized and characterized. The steric effect of the ortho hydrogens on the phenyl ring at 4,8-positions of benzobisoxazole resulted in out-of-plane twisting, and consequently decreased the intermolecular ?-? interaction. The three new compounds showed excellent thermal stabilities with Tg of 248? for 3Py-DBBO, 244? for 4Py-DBBO and 142?for TPO-DBBO, respectively. These compounds also exhibited ambipolar transport properties, with electron and hole mobilities both of 10-6?10-5 cm2 V-1s-1. Using the compounds as the ETMs, the deep-red PhOLED achieved an ?ext,max up to 19.3%, which was one of the best results ever reported.Chapter 9:Benzo[1,2-d;4,3-d']bisthiazole moieties with sulphone ?-bridge were designed as the new ETMs. All these compounds exhibited reversible electrochemical reduction behaviors and favorable LUMO levels for electron injection. Among the four compounds, DBBT-S-Bz owned a high Tg(> 122 ?) and a high ET (?2.76 eV), revealing its potential as the ETM for blue PhOLEDs. |
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