Temperature Dependent Epitaxial Growth of C₆₀ Overlayers on Single Crystal Pentacene

Abstract

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.

Photoelectron spectroscopy on single crystals of organic semiconductors: experimental electronic band structure for optoelectronic properties

“…On the other hand, structural analyses have disclosed well-defined assembly of several organic semiconductor molecules via heteroepitaxial growth on molecular single crystal substrates. [243][244][245][246][247][248][249][250] For instance, it was demonstrated by means of surface X-ray diffraction works that n-type semiconductor molecules of C 60 245 and perfluoropentacene (PFP; C 22 F 14 ) 250 form uniquely oriented crystallites on the single crystal surface of pentacene, and the mean size of these epitaxial crystallites reaches the 100 nm order. 246,248 Before closing this article, photoemission analyses conducted on these well-defined molecular-molecular heterointerfaces built on the Pn-SC will be summarized in the following paragraphs.…”

Electronic and Crystallographic Examinations of the Homoepitaxially Grown Rubrene Single Crystals

“…PYS is a suitable methodology for electronic characterization of specimens of low electric conductance [24,25], which is the case for intrinsic rubrene as a wide-gap (2.8 eV [26]) semiconductor, and particularly for detecting small modifications at the highest-occupied electronic states in an extreme sensitivity [27][28][29]. GIXD is a technique for determination of the crystal structures of thin films, and has successfully been applied to hetero-epitaxial organic semiconductor junctions built on molecular single crystals for specification of the lattice orientations [30][31][32] and even for evaluation of their crystallographic qualities by using high-resolution apparatuses [33][34][35]. It was confirmed that homoepitaxial rubrene single crystals grow without any apparent lattice distortion up to the thickness of 100 nm by the present GIXD measurements.…”

Quasi-Homoepitaxial Junction of Organic Semiconductors: A Structurally Seamless but Electronically Abrupt Interface between Rubrene and Bis(trifluoromethyl)dimethylrubrene

“…Indeed, a number of molecular species have been reported to exhibit mobility exceeding that of amorphous silicon in single-crystalline field-effect transistor devices composed of these materials. − In the case of optoelectronic devices such as organic photovoltaics, conventional architectures are composed of p-type (donor) and n-type (acceptor) materials of disordered condensates, , and thus making use of the highly efficient charge carrier transport of single-crystalline structures may offer another possible concept to improve the device performance. − In this context, epitaxial growth of organic semiconductors is a reasonable approach for obtaining crystalline p–n junctions through self-assembly. In previous studies, highly ordered heteroepitaxial interfaces of uniformly aligned n-type molecular materials formed on single crystals of p-type organic semiconductors have been attained. − Moreover, it has been confirmed that single-crystalline thin films with extremely good crystallinity can be obtained by homoepitaxy, − where molecules are deposited on single-crystal substrates of the same molecular species. This methodology enables the realization of p–n homojunctions of a monolithic organic semiconductor single crystal by accurately controlled impurity doping .…”

“…This value is smaller than the overlayer thickness of 50 nm, suggesting that the fmRub overlayer was not single crystalline but included domain boundaries and/or multiple crystallographic phases in the out-of-plane direction. 18 …”