Crystal order in pentacene thin films grown on SiO2 and its influence on electronic band structure

To clarify relationship between crystallographic and electronic structures in pentacene polycrystalline films grown on SiO2, in-plane crystallite size and random strain of the films were analyzed by grazing incidence X-ray diffraction (GIXD) using synchrotron radiation source. The results indicate that the diffraction peak width is not determined by random strain but by crystallite size. The crystallite size remains constant within the range of 25–50 nm even when the size of polycrystalline domain, or crystal grain, increases more than tenfold by elevating the growth temperature. The crystallite size agrees well with characteristic periods of both HOMO-band-edge fluctuations in pentacene films, which was reported in our previous paper, and surface corrugation of the substrate. These facts strongly suggest that roughness of the SiO2 surface limits the crystallite size and the interruption of long-range order in pentacene lattice introduces the HOMO-band-edge fluctuation.

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► Pentacene polycrystalline film consists of 5-step hierarchical structures.
► Micron-order domains are surrounded by large energy barriers for carrier transport.
► Domains still consist of 30-nm-scale crystallites that limit mobility in domain.
► Crystallite size is almost constant even though domain size is drastically changed.
► Boundaries of crystallites introduce HOMO-band-edge fluctuations within a domain.