Hydrophilic self-assembly monolayers for pentacene-based thin-film transistors

A series of hydrophilic alkanethiols, HS(CH2)(n)OH with n = 3, 4, 6, 8, 9 and 11, have been self-assembled on gold electrodes of pentacene-based thin-film transistors. The multiparametric and ultra-sensitive response of these devices allows us to characterize both charge-injection and electrical stability moving from vacuum to air. The decay exponent beta of charge injection is found to be 0.7-1.1 angstrom(-1) in agreement with earlier measurements of charge tunneling done by electrochemistry and large-area molecular junctions. We find that the intermediate chain lengths (n = 4 and 6) yield an optimum response as they represent the best compromise in terms of decrease of the contact-resistance and bias stress.