2, Osaka University, Osaka, , Japan
3, SLAC National Accelerator Laboratory, Menlo Park, California, United States
4, PRESTO, JST, Saitama, , Japan
Two-dimensional ultrathin organic single crystals are an ideal platform for printable, wearable electronic device applications because of their high carrier mobility above 10 cm2/Vs and the minimized resistance of accessing the charge-accumulated channel. However, monolayer (1L) organic field-effect transistors (OFETs) often show much poorer electronic properties than those in bilayer (2L) OFETs.[1,2] In this presentation, we focus on the origin of the electronic performance degradation in 1L-OFETs and propose a new approach to overcome drawbacks.
1L and 2L single crystals of 3,11-dioctyldinaphtho[2,3-d:2’,3’-d’]benzo [1,2-b:4,5-b’]dithiophene (C8–DNBDT–NW) were grown by meniscus-guided solution process. In order to address the origin of serious mobility degradation in the 1L crystals, we investigated differences in microstructure of 1L and 2L single crystals using X-ray reflectivity (XRR) measurements. Although the slight distortion in electron density is seen particularly at the edge of the substrate, it is found that such a small deviation does not explain drastic degradation of the carrier transport. Accordingly, we assume that the process to form contact electrodes may give a crucial impact on the molecular orientation of 1L crystals. To confirm the hypothesis, we compare the differences in molecular orientation before and after gold evaporation on the film using Near-edge X-ray absorption fine spectroscopy. In the pristine 1L crystals, intensity of the peaks corresponding to the transition from C1s core to π* orbitals dramatically increases as the X-ray incident angle approaches to the normal direction of the substrate plane. This significant angular dependence of the peak intensity shows that the C8–DNBDT–NW in the pristine 1L crystals are highly ordered in an up-right geometry. However, the angular dependence of the π* peaks are apparently depressed in the 1L crystals after gold evaporation, which supports that thermal/radiation damage can have a significant impact to 1L crystals.
In order to evaluate the intrinsic properties of 1L-OFETs free from any damages caused by evaporation, we adapt a lamination contact process, where polydimethylsiloxane (PDMS) film with contact electrodes is laminated on the 1L crystal. The 1L-OFET with the laminated electrodes shows the excellent performance with the mobility of 10 cm2/Vs. In addition, Hall effect measurements were performed on the 1L-OFET, where the Hall carrier density is in a perfect agreement with the carrier concentration estimated from gating. The result shows the coherent band-like transport is realized in the 1L crystal.
 Q. Wang et al., Adv. Funct. Mater. 2016, 26, 3191.
 A. Yamamura et al., under reviewing.
 J. Soeda et al., Appl. Phys. Express 2013, 6, 076503.