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.

Strong intermolecular electronic coupling and well-ordered molecular arrangements enable efficient transport of both charge carriers and excitons in semiconducting π-conjugated molecular solids. Thus, molecular heteroepitaxy to form crystallized donor–acceptor molecular interfaces potentially leads to a novel strategy for creating efficient organic optoelectronic devices via the concomitance of these two requirements. In the present study, the crystallographic and electronic structures of a heteroepitaxial molecular interface, perfluoropentacene (PFP, C22F14) grown on pentacene single crystals (Pn-SCs, C22H14), were determined by means of grazing-incidence X-ray diffraction (GIXD) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), respectively. GIXD revealed that PFP uniquely aligned its primary axis along the [11̅0] axis of crystalline pentacene to form well-crystallized overlayers. Valence band dispersion (at least 0.49 eV wide) was successfully resolved by ARUPS. This indicated a significant transfer integral between the frontier molecular orbitals of the nearest-neighbor PFP molecules.

due to charge carrier delocalization. [10][11][12][13][14] Accordingly, accurate control of the crystal quality of molecular heterojunctions is desired for advancing further development of next-generation organic electronic devices. [15][16][17] In the present work, the evolution of the crystallinity of a well-ordered bimolecular heterojunction, C 60 on a single crystal surface of pentacene (C 22 H 14 ), is studied depending on the growth temperature by means of surface X-ray diffraction techniques and noncontact mode atomic force microscopy (nc-AFM). Pentacene ( Figure 1a) and C 60 ( Figure 1b) are representative p-type (donor) and n-type (acceptor) semiconducting compounds generating an exciton-dissociating interface in a prototypical organic solar cell device. [18] The pentacene/C 60 interface has been studied widely, both experimentally [19][20][21][22][23][24] and theoretically. [25][26][27][28][29][30] It was recently discovered that C 60 crystallizes in its bulk structure epitaxially by aligning the nearest-neighbor face-centered-cubic (fcc)-110 < > axis uniquely along the [110] direction of the pentacene single crystal (Pn-SC) surface. [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. Moreover, the crystallographic coherence length of the C 60 overlayers grown at room temperature (RT) was found to exceed 100 nm in the in-plane directions. [33] The well-defined interface between Pn-SC and C 60 is ideally suited to study kinetic effects [34] in the growth of the topical donor-acceptor heterojunction. We herein demonstrate the crystallographic coherence of the C 60 overlayers on the Pn-SC as a function of the growth temperature through systematic Heteroepitaxy of one material onto another molecular single crystal surface is one promising route for resolving questions about formation criteria of molecular heterojunction structures as well as for the development of nextgeneration organic electronic devices allowing efficient intermolecular charge carrier exchange. In the present work, the in-plane and out-of-plane crystallinity of an epitaxial molecular p-n heterojunction, C 60 (acceptor) overlayers formed on the single crystal surface of pentacene (donor), and its evolution, depending on the growth temperature, are systematically elucidated. It is demonstrated that the crystallinity of the C 60 on pentacene is dominated by the temperature during the growth rather than the postannealing of the sample. The mean crystallite size in the in-plane directions grows from 50 to 150 nm proportionally to the growth temperature in a range of 125-370 K. The present results suggest that the formation mechanisms of the C 60 /pentacene heterojunction are kinetically controlled, by diffusion processes at the molecular interface, rather than by the thermal equilibrium conditions.

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The structural and electronic properties of interfaces composed of donor and acceptor molecules play important roles in the development of organic opto-electronic devices. Epitaxial growth of organic semiconductor molecules offers a possibility to control the interfacial structures and to explore precise properties at the intermolecular contacts. 5,6,11,12-tetraazanaphthacene (TANC) is an acceptor molecule with a molecular structure similar to that of pentacene, a representative donor material, and thus, good compatibility with pentacene is expected. In this study, the physicochemical properties of the molecular interface between TANC and pentacene single crystal (PnSC) substrates were analyzed by atomic force microscopy, grazing-incidence X-ray diffraction (GIXD), and photoelectron spectroscopy. GIXD revealed that TANC molecules assemble into epitaxial overlayers of the (010) oriented crystallites by aligning an axis where the side edges of the molecules face each other along the [1¯10] direction of the PnSC. No apparent interface dipole was found, and the energy level offset between the highest occupied molecular orbitals of TANC and the PnSC was determined to be 1.75 eV, which led to a charge transfer gap width of 0.7 eV at the interface.