The growth mechanism of Pentacene-C$_{60}$ heteroepitaxial films

Pentacene (Pn) and fullerene (C$_{60})$ are of great interest among organic semiconductors as they show highest field-effect hole and electron mobilities respectively. The absorption peak in Pn crystal is located close to maximum of solar visible spectra, making a bipolar Pn-C$_{60}$ diode promising for solar cell application. In order to improve its efficiency to satisfy the requirement for practical application, an in-depth understanding of Pn-C$_{60}$ interface formation is necessary for further optimization. We shall discuss the growth mechanism of Pn-C$_{60}$ heteroepitaxial films on Bi(0001)/Si(111) substrate studied by real time low-energy electron microscopy and complementary scanning tunneling microscopy. A competitive growth between a thin-film phase of Pn having standing-up orientation and a phase with laying-down orientation has been observed. The growth of laying-down phase is suppressed gradually with increasing film thickness. The nucleation of this phase is also suppressed with increasing temperature and the standing up phase without co-presence of laying down phase is achievable at $\sim $75\r{ }C.