(561g) Non-Isothermal Crystallization Behaviors of N,N,N',N'-Tetraphenylbenzidine(TPB)

Non-isothermal Crystallization Behaviors of N,N,N',N'-Tetraphenylbenzidine(TPB) 

Xiaosi Ma, Shirong Wang, Xianggao Li and Yin Xiao*

(School of Chemical Engineering and Technology,Tianjin University,

Tianjin, 30072, China)

Abstract Hole transporting materials (HTMs) are widely used in photovoltaic conversion devices, such as organic photoconductor (OPC), organic light emitting diode (OLED) and dye-sensitized solar cells (DSSC). In layered OLED, the hole transporting layer provides the functions of facilitating hole injection from the anode into the organic layer and transporting the injected holes to the emitting layer and blocking electrons to escape from the emitting layer to the anode. The properties of the HTMs could influence the luminance, efficiency and working life of the device by affecting the efficiency of injection and form of exciton. The ideal HTMs should display high mobility, excellent solubility, good film forming ability and high thermal stability as well as appropriate energy level of HOMO and LUMO to ensure the effective injection.

HTMs with small molecular weight developed recent years can be divided as following: pyazolines, carbazoles, hydrazones, styrenes, butadienes and triarylated amines. The triarylated amine and its derivatives are widely used HTMs with small molecular weight because of their high hole mobilities, high glass transition temperature (T_g) and excellent surface stability.

The aging mechanism of the device is not clearly by now. Some studies demonstrate that the change of the physical form of the organic layer is one of the reasons. The joule heat caused in the working process could cause the crystallization of the materials, which destroy the morphology of the film and hence lead to the decrease of the current. In addition, the joule heat could destroy the surface of the electron transfer layer (ETL) and metallic cathode and block the combine of electron and hole thus result in decreased lifetime of the device. N,N,N',N'-Tetraphenylbenzidine (TPB) is one of the most widely used HTMs and has been used as building block to synthesis other HTMs with good performance. Hence, studying the thermal property of TPB could provide more insight into the aging of the devices caused by the crystallization of the HTMs.

In this paper, Cold crystallization phenomenon was found when heating the glassy state of TPB and the non-isothermal crystallization kinetics was investigated by Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD). The result was analyzed by Jeziomy method, Ozawa method and Mo method. The study indicated that the cold crystallization phenomenon existed in TPB coupled with metastable state, which was proved by the separation of the exothermic peaks (mainly around 130 ℃ and 160 ℃) in DSC curve and diffenent 2θ degree as well as absorption intensity in XRD graph. When the relative crystallinity reached 5% to 90%, the crystallization behavior corresponded with the Jeziomy method well, around the temperature of 130 ℃, the crystallization behavior corresponded to the Ozawa method well.