Monolayer to multilayer nanostructural growth transition in N-type oligothiophenes on Au(111) and implications for organic field-effect transistor performance

The evolution in growth morphology and molecular orientation of n-type semiconducting alpha,omega-diperfluorohexyl-quaterthiophene (DFH-4T) on Au(111) is investigated by scanning tunneling microscopy and scanning tunneling spectroscopy as the film thickness is increased from one monolayer to multilayers. Monolayer-thick DFH-4T films are amorphous and morphologically featureless with a large pit density, whereas multilayer films exhibit drastically different terraced structures consisting of overlapping platelets. Large changes in DFH-4T molecular orientation are observed on transitioning from two to four monolayers. Parallel electrical characterization of top-versus-bottom contact configuration DFH-4T FETs with Au source/drain electrodes reveals greatly different mobilities (mu(TOP)) 1.1 +/- 0.2 10(-2) cm(2)V(-1)s(-1) versus mu(BOTTOM) = 2.3 +/- 0.5 10(-5) cm(2)V(-1)s(-1)) and contact resistances (RC-TOP)4-12M Omega cm vs RC- BOTTOM > 1 G Omega cm). This study provides important information on the organic semiconductor-source\drain electrode interfaces and explains why top-contact OFET devices typically have superior performance. By direct visualization, it demonstrates that the DFH-4T film growth transition from monolayer to multilayer on Au is accompanied by dramatic morphology and molecular orientation changes, starting from an amorphous, pitted, and disordered monolayer, to crystalline and smooth bi/tetralayers but with the molecules reoriented by 90 degrees. These chemisorption-derived inhomogenities at the contact-molecule interface and the large monolayer -> multilayer -> bulk microstructural changes are in accord with the large bottom-contact device resistance and poor OFET performance.