The Study Of Cold Crystallization And Film Stability Of Benzidines As Hole Transport Materials

Hole transporting materials (HTMs) are vital materials that used in organicsemiconductor devices. Thermal stability is one of the most important properties ofthe HTMs for their performance in devices. Cold crystallization, referring to thecrystallization of the amorphous materials when heated, is easily to happen in theworking process of the devices with amorphous HTMs. It is generally accepted thatthe film stability could be evaluated by the glass-transition temperature (Tg) of HTMs.However, Tgis unable to reflect the molecular stacking mode in films whichdominates the charge carrier mobility. In this article, the thermally inducednon-isothermal cold crystallization behavior and film microstructure alteration of fourHTMs are comprehensively studied by DSC, UV-vis spectrum, XRD, SEM, AFMand TOF.N,N,N’,N’-Tetraphenylbenzidine (TPB) and N,N,N’,N’-tetra-p-tolylbenzidine(TTB) are easily to crystallize when heated. The crystallization rate increases with theraise of heating rate and reaches a local maximum at Φ=30and40°C/min,respectively. The cold crystallization of TPB without annealing is nucleationcontrolled which turns to crystal-growth controlled when annealing, while for that ofTTB is always nucleation controlled in the studied tmparture range. TPB has a largercrystallization rate than TTB at the same condition. The thermally inducedmicrostructure alteration and aggregation pattern of TPB and TTB films made byvacuum evaporation were studied. The more ordered configuration of TPB film,especially after annealing resulted in its relatively higher hole mobility.N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzidine (m-TPD) and N,N’-bis(4-methylphenyl)-N,N’-diphenylbenzidine (p-TPD), which could not crystallize whenheated immediately form cooling, are likely to experience the cold crystallizationwhen heated after annealing. m-TPD has a wider annealing temperature range thatwould cause cold crystallization than p-TPD yet a smaller rate at the same condition.When annealed at the temperature of0~50oC, the cold crystallization of both m-TPDand p-TPD are nucleation controlled, while annealed at other temperature range, thecrystallization of m-TPD is crystal-growth controlled. According to the dielectric spectroscopy result, the global mobility of m-TPD is fierce above48oC which leadsto the instability of amorphous state. The more ordered configuration of p-TPD film,leading to the higher hole mobility.The crystal parameters of the four materials were obtained by simulation, yetwith large error for more than one crystal form for each sample. The main crystalforms are as follows: Orthorhombic Fdd2for TPB, Tetragonal P42/nbc for TTB,Orthorhombic Pca21for m-TPD and Monoclinic P21/c for p-TPD.