Epitaxial Growth of an Organic p–n Heterojunction: C₆₀ on Single-Crystal Pentacene

Abstract

Designing molecular p-n heterojunction structures, i.e., electron donor-acceptor contacts, is one of the central challenges for further development of organic electronic devices. In the present study, a well-defined p-n heterojunction of two representative molecular semiconductors, pentacene and C60, formed on the single-crystal surface of pentacene is precisely investigated in terms of its growth behavior and crystallographic structure. C60 assembles into a (111)-oriented face-centered-cubic crystal structure with a specific epitaxial orientation on the (001) surface of the pentacene single crystal. The present experimental findings provide molecular scale insights into the formation mechanisms of the organic p-n heterojunction through an accurate structural analysis of the single-crystalline molecular contact.

Temperature Dependent Epitaxial Growth of C60 Overlayers on Single Crystal Pentacene

“…On account of the formation of electronic bands and the associated delocalization of charge carriers, [1][2][3] reasonably high charge carrier mobility has been reported in devices of single crystalline organic semiconductors. Figure 2 insets show typical AFM images of the Pn-SC samples covered with 20 nm-thick C 60 overlayers grown at various substrate temperatures, i.e., 160, 300, and 370 K. C 60 forms grains on the Pn-SC surface in the case of RT (300 K) growth as reported previously, [23,31] and it was found that the lateral sizes and shapes of these grains vary depending on the growth temperature. Proportional increase of the inplane mean crystallite size of C 60 with the growth temperature suggests importance of surface diffusion processes of the C 60 molecules on the Pn-SC surface as a leading factor for determination of the heterojunction structures.…”

“…Figure 4 shows the XRR data representing the C 60 (111) and Pn-SC(002) spots of 20 nm-thick C 60 overlayers grown at various temperatures. [31] The fringe structures as well as the main peak profile are more pronounced for the C 60 overlayers formed at 350 K, as shown in Figure 4c, suggesting further improvement of the crystallinity. At RT (Figure 4b), the C 60 (111) reflection (upward arrow) at the outof-plane scattering vector (q z ) of 0.770 nm −1 is accompanied by Laue fringes on both sides indicating good crystallinity and crystal orientation of the overlayers, as reported previously.…”

“…[31] In that arrangement, the C 60 surface exhibits hexagonal symmetry; and thus, in-plane diffraction spots from C 60 appear with a 60° periodicity when rotating the sample around φ. [31] In that arrangement, the C 60 surface exhibits hexagonal symmetry; and thus, in-plane diffraction spots from C 60 appear with a 60° periodicity when rotating the sample around φ.…”

“…[31] In that arrangement, the C 60 surface exhibits hexagonal symmetry; and thus, in-plane diffraction spots from C 60 appear with a 60° periodicity when rotating the sample around φ. [31]. Figure 3a shows the φ-dependence of the C 60 {220} scattering intensity derived from the present 2D GIXD (2D-GIXD) results, where the intensities are normalized by the 360°-averaged intensity of the respective samples and are plotted on a logarithmic scale as the radial axis.…”

“…[18] The pentacene/C 60 interface has been studied widely, both experimentally [19][20][21][22][23][24] and theoretically. [31] The type of this epitaxial interface is categorized into the incommensurism, whereas it is nearly of so-called "Type-IB" point-on-line coincidence [32] with a 6% lattice mismatch, as illustrated in Figure 1c. [31] The type of this epitaxial interface is categorized into the incommensurism, whereas it is nearly of so-called "Type-IB" point-on-line coincidence [32] with a 6% lattice mismatch, as illustrated in Figure 1c.…”