Characteristics of Cyclic Triarylamine Hexamers and Application to Organic Field Effect Transistor

Abstract

There has been a recent surge in research on organic optoelectronic materials due to their costeffective product processes and their promising applications in materials science for flexible and printable electronic devices [1-4]. These make it feasible to manufacture innovative electronic products such as organic electroluminescent displays, organic photovoltaic cells and so on [5‒8]. Organic materials based on triarylamine (TAA) have attracted the attention of researchers due to their high thermal stability and excellent hole transport properties [9, 10]. Among them, low molecular-weight TAA derivatives often show impressive performance in organic optoelectronic devices, especially in organic field effect transistors, because of their high purity [11‒14]. Nevertheless, optoelectronic materials based on low molecularweight compounds are usually unstable at high temperatures caused by Joule heating. From this point of view, TAA oligomers have many advantages, and their research has flourished in recent years [15‒24]. Unlike low molecular-weight compounds, oligomers possess better thermal stability and solution processabil ity , which are general characteristics in polymeric samples. In addition, the oligomers with well-defined and unique structures exhibit the additional specific physical-chemical properties. One of well-known examples of TAA oligomers was spiro-OMeTAD, which has an excellent durability due to its rigid spiro structure [15‒18]. However, a few researches have been reported focusing on the application of TAA oligomers to OFET because of their amorphous nature [25, 26]. Our group have synthesized cyclic structures of TAA oligomers with three different sizes in 2011 [27]. It was considered that the cyclic structure without terminal groups results in structural rigidity and thermal stability and a homogeneous electron distribution in a highest occupied molecular orbital to bring about the smooth hole migration. Later studies demonstrated that the cyclic pentamer of 4-butyltriphenylamine (CBTPA-5) possesses quite higher glass transition temperature comparable to that of linear polymer analogue, and exhibits nearly 100 times higher hole mobility compared to the polymer [28]. Interestingly, the lowest solubility in common organic solvents such as toluene, and Abstract: Cyclic hexamers consisting of a 4-octyl, 4-octyloxy and 4-(2-ethylhexyloxy)triphenylamine unit were synthesized via a Buchwald-Hartwig cross coupling reaction using A-A, and B-B type monomers in order to avoid the formation of other cyclic oligomers such as a pentamer and a heptamer. Unlike different size of cyclic oligomers, all hexamers we examined showed clear cold crystallization in 165-240°C in the DSC thermograms. Resulting oligomers were characterized and applied to hole transporting active layer in an organic field effect transistor. The hexamer with 4-octyl groups exhibited the highest field effect hole mobility (1.99 × 10 cm V s). (Received 25 April, 2022; Accepted 17 May, 2022)