Novel copper phthalocyanine (CuPc) materials have potential applications in solar cells, fuel cells, optical-limiting materials, gas sensors and field-effect transistors. [1][2][3][4][5][6][7][8][9][10] Most recently there have been interests in applications for high dielectric constant material for capacitors. [11,12] For example, Zhang et al. have reported a dielectric constant of $48 at 1 MHz in an electrostrictive polymeric CuPc/Polyvinylidenefluoridetrifluoroethylene-chlorofluoroethylene P(VDF-TrFE-CFE) composite.[11] However, a relatively high dielectric loss (>0.4) exists in such system due to the percolation limit of the CuPc filler. More recently, we have observed a very high intrinsic dielectric constant, coupled with a small dielectric loss (<0.01 up to 1MHz) in a newly developed hyper-branched CuPc dendrimer. [12] Although numerous reports have demonstrated the dielectric performance of CuPc polymers, the understanding of the electronic and optical behavior in dendrimers is limited, which is essential in the development of new materials. In this communication, the charge carrier mobility of a newly developed CuPc dendrimer is investigated. The synthesis of the CuPc dendrimer has been previously reported and is shown in Scheme 1. [12] This hyper-branched CuPc has dendritic-like structure consisting of four -CN groups incorporating into the CuPc core. Moreover, our previous experience in the study of triarylamine-based dendrimers and ladder oligomers using the ultra-fast spectroscopy suggested a coherent energy migration process occurred and polaron delocalization states are possible.[13] Time-resolved spectroscopy measurement has been carried out to study the optical behavior of this dendrimer. Our investigation has suggested the formation of delocalized polarons in the system. The super-conjugation effect from the oxygen atom and an increase of system packing through the self-assembling of phthalocyanine rings give polaron hopping and polaron tunneling between phthalocyanine rings. In order to probe the electronic properties of this dendrimer, we carried out time-of-flight measurements to investigate the electron mobility. CuPc dendrimer was first dissolved in N, N-dimethylacetymide (DMAc) solution at a concentration of 1 mg mL
À1, then the CuPc/DMAc solution was drop-cast on a silicon wafer with a thickness of 500 mm. After drying in a vacuum oven at 120 8C for several hours, a dendrimer film with a thickness ranging between 4 to 5 mm was obtained using a surface profiler. Further details of the procedure are in the experiment section.A typical transient current at an applied field of 5 V at several different temperatures is depicted in Figure 1 plotted on a linear scale. The transient current signals at room temperature follow a plateau region after a fast decay, then slowly tail off. This shows the characteristics of non-dispersive charge carrier transport. However, upon lowering the temperature, an apparent transition from non-dispersive to dispersive charge carrier transport occurs, which indicat...
