2.5.5. Two-Step Crystallization Process

A high-quality single-crystal p−n heterojunction (SCHJ) was prepared using orthogonal solvent two-step crystallization [68], which is an interfacial solution crystallization process [69]. In Figure 20a, a second layer of single crystals is formed at the interface of the first layer of single crystals. During the crystallization of the second layer, the first layer is destroyed, which is the problem to be solved in the second stage. A single 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) crystal can grow without dissolving the first layer by choosing 4-methyl-2-pentone, which can dissolve TIPS-PEN effectively but cannot dissolve C60 as a solvent [70], and the substrate can be annealed under vacuum to remove the residual solvent. Figure 20b shows a scanning electron microscope (SEM) image of the TIPS-PEN nanocrystal array. Figure 20c shows energy dispersive spectroscopy (EDS)-related element mapping. Figure 20d shows the energy dispersion spectrum of the crystals. In Figure 20e, the thickness of the overlapping SCHJ was studied using atomic force microscopy (AFM). In Figure 20f, the samples were screened using transmission electron microscopy (TEM). Figure 20g shows how the crystallographic analysis of the bilayer material was carried out by select area electron diffraction (SAED). The double-layer SAED shows two sets of diffraction points (blue and yellow circles), representing two different single crystals. This method is beneficial to the preparation of a heterojunction.

(a) Schematic diagram of a TIPS-PEN/C60 single-crystal heterojunction. (b) SEM image of an SCHJ. (c) Energy dispersive spectroscopy (EDS)-related element mapping. (d) EDS images. (e) AFM image. (f) TEM images of SCHJ. (g) Correlation electron diffraction pattern [68]. Copyright© 2018 American Chemical Society.