Organic semiconducting materials have been widely applied to electronic devices, such as thin-film transistors, photovoltaic cells, and light-emitting diodes. Semiconducting materials with long alkyl substituents are attracting much attention owing to their high solubility in solvents which enables film formation by solution processes, such as ink-jet printing and spin-coating. We’ve focused on a phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2), which was reported to exhibit carrier mobility of higher than 1 cm2/Vs in time-of-flight method[1] and has been utilized for a photovoltaic cells with the high power conversion efficiency of exceeding 4 %[2]. For suppressing considerable amount of grain boundaries in the spin-coated C6PcH2 films, which must interrupts mobile carriers, we focused on the novel fabrication method, heated spin-coating, and reported increased domain size exceeding 1 mm in the films[3]. However, some wide cracks with widths of several ten μm, which were perpendicular to the π-stacking direction of C6PcH2 molecules, were appeared in the films. In this study, therefore, we investigated the polymer blending effects on the properties of heated spin-coating films of C6PcH2[4]. Since the carrier conduction at the grain boundaries between the grains of C6PcH2 is important in terms of the electronic device application, a π-conjugated polymer, poly(3-hexylthiophene) (P3HT) was used as an additional binder polymer in this study. A mixture of C6PcH2 and P3HT was dissolved in 1,4-diethylbenzene and spin-coated on a quartz substrate. The weight mixing ratio of P3HT in the mixture was controlled from 0 % to 50 %. The temperature of the spin-coating stage and the C6PcH2 solution were kept at 150 °C during spin-coating process.
The wide crack lines with the widths of around 10 μm appeared in the pure C6PcH2 films by observations using an optical microscope and an atomic force microscope. On the other hand, in the case of the low composite ratio of P3HT, the wide crack lines in the films disappeared. In addition, the anisotropic optical absorption which correspond to the uniaxial alignment of the π-stacked columns of C6PcH2 molecules, were measured even after mixing low amount of P3HT. Therefore, mixed P3HT molecules must suppress wide cracks among grains of C6PcH2 with maintaining the molecular alignment of C6PcH2 molecules. The polymer blend effects on electrical properties and crystalline structures of the films will be discussed at the session.
This work was partly supported by Grant-in-Aid for JSPS Fellows.
[1] Y. Miyake et al., Appl. Phys. Express 4 (2011) 021604. [2] Q. D. Dao et al., Appl. Phys. Lett. 101 (2012) 263301. [3] T. Higashi et al., submitted to Jpn. J. Appl. Phys. [4] T. Higashi et al., to be published in Jpn. J. Appl. Phys.