Design and Synthesis of Bipolar Carbazole Derivatives and Their Applications in Organic Electroluminescent Devices

• Main Authors: Lin, Chih Chun, 林致均
• Other Authors: Cheng, Chien Hong
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/m8kfzt

博士 === 國立清華大學 === 化學系 === 104 === In this thesis, we focus on blue organic light emitting diodes to proceed in-depth research and discussion, and synthesize a series of blue hosts and dopants with bipolar property. The dopants discussed in Chapter 2, we use stilbene as the core of the major structure, import carbazole and phenyl amine as donor, triphenyl phosphine oxide and phenyl sulfone as acceptor, and incorporating the acceptor and donor into the molecule to achieve bipolar property. These compounds exhibit good thermal stability with decomposition temperature above 400 °C and glass transition temperature about 100 °C. Moreover, stilbene derivatives doped in DMPPP film show extremely high Q.Y. which indicates that energy transfer are highly efficient. The device shows a maximum external quantum efficiency (E.Q.E.) of 10.4 %, the current efficiency (C.E.) of 13.4 cd/A, and the power efficiency (P.E.) of 8.2 lm/W with the CIE coordinates of (0.15, 0.16) by using DMPPP doped with 5 % DASPPO as emission layer. To further explore unusual increase in the E.Q.E., we also fabricated the device by using CBP as the host. The transient EL spectrum of CBP-devices shows large microsecond-scale delayed fluorescence via triplet-triplet annihilation. With the enhancement of delayed fluorescence, the E.Q.E. exceeded the limited efficiency of fluorescent OLEDs. The hosts are discussed in two chapters: Chapter 3 green bipolar hosts and Chapter 4 blue bipolar hosts. In Chapter 3, we used carbazole and phenyl amine as donor, phenyl sulfone as acceptor, and using the design concept of Thermally Activated Delayed Flourescence (TADF), linked the donor and acceptor by using the ortho-position of benzene, to enhance steric hindrance which allows the electron distribution of HOMO and LUMO to be separated to obtain TADF character, unfortunately the expected result was not obtained in this design. However, these compounds having enough triplet energy (2.5 eV) and good charge mobility are good bipolar hosts, therefore suitable for green phosphorescence devices. By using o-PTPAS doped with 7 % Ir(PPy)3 as emission layer, the device shows an E.Q.E. of 24.2 %, the C.E. of 89.1 cd/A, and the P.E. of 93.2 lm/W with the CIE coordinates of (0.25, 0.65). On the other hand, when 4CzIPN, a TADF material, was used as dopant, only o-PCzS has better device performance, which proves that it is difficult for host materials to be applied for both phosphorescent and TADF OLEDs. Since the design in the previous chapter could not obtain material with TADF character, some adjustments were made in Chapter 4. Two bipolar host materials BT-01 and BT-02 with TADF behavior, composed of diphenylsulphone as an electron acceptor, a m-bitolyl group as a π bridge, and carbazole with or without cyano moiety as an electron donor, were successfully synthesized and used for both blue TADF and phosphorescent OLEDs. The photophysical and theoretical studies suggest that the incorporation of a cyano group in the carbazole moiety increases the LE contribution in the CT excited state and thus greatly alters the magnitude of ΔEST. In comparison with BT-02 bearing a CN group, BT-01 exhibits TADF behavior with short delayed lifetime due to efficient reverse ISC from triplet to singlet arising from smaller ΔEST, which is favorable for efficient singlet and triplet energy transfer to emitters and the reduction of the triplet density of the host. In particular, blue TADF and phosphorescent OLEDs hosted by BT-01 achieve excellent EQEs of 25.5 and 31.8% with low turn-on voltage, respectively, significantly higher than those found for BT-02. Moreover, all the devices using BT-01 renders low efficiency roll-off at high luminance. This result can be ascribed to the efficient reverse ISC from triplet to singlet and thus the reduced TTA on host as well as the bipolar characteristic of the host. These findings demonstrate a promising design concept for the development of the host material for TADF and phosphorescent OLEDs with extremely high performance.