Growth and electronic structure studies of semiconducting thin films of fluorine-monosubstituted fused-thiophene derivative

Formation of high quality interface between metal and organic semiconducting film is important in realizing high-performance organic electronics devices. We have investigated the room-temperature growth of organic semiconducting thin films of monofluorine-substituted 2-phenylbenzo[d,d′]thieno[3,2-b;4,5-b′]-dithiophene (m-FP-BTDT) on Au(1 1 1), and Au(1 1 1) modified by benzenethiolate (Au-BT) by means of synchrotron-based electron spectroscopies. XPS intensity analysis shows that on Au(1 1 1), m-FP-BTDT film grows according to Stranski-Krastanov (SK) mode in which the first monolayer (ML) is completed before the 3D growth sets in. In comparison, the initial growth of m-FP-BTDT on Au-BT proceeds via a pseudo layer-by-layer growth mechanism in which the growth of a new layer starts after the underneath layers are almost finished. Orientations of m-FP-BTDT molecules for the thick films on two substrates remain different, despite the large thickness. NEXAFS data show that m-FP-BTDT molecules on Au-BT adopt a more erected configuration, resulting in a better cofacial π-stacking. Work function for the thick m-FP-BTDT film on Au-BT is determined with UPS as 4.62 eV and the hole injection barrier as 0.95 eV.