Self-Assembly of Pentacene on Sub-nm Scale Surface Roughness-Controlled Gate Dielectrics

Abstract

Surface roughness (R_q) of dielectric materials plays an important role in the ordering and charge-carrier transport of organic semiconductors, and is directly involved in the development of nuclei and crystal grains on a surface. To investigate the effect of R_q-controlled dielectrics with similar surface energy (γ) on the development of nuclei and crystal grain, a series of triethoxysilane-terminated polystyrene (PSTES) polymers with different molecular weights (MW) values is synthesized using reversible addition-fragmentation chain transfer polymerization. The different MW PS-TES films are spun-cast on a hydroxyl (-OH)-rich SiO₂ dielectric surface, and chemically coupled with -OH moieties at 110 °C. Some films are also rinsed with an excess of toluene to remove unreacted polymer residue, increasing the average R_q values of the treated SiO₂ surfaces. The resulting polymer-treated dielectrics show similar surface energy values of 41.6-42.5 mJ m^-2 but different R_q values ranging from 0.29 to 1.07 nm. On the nanoscale roughness-controlled dielectric surfaces, 40-nm-thick pentacene films show discernible types of crystal grains with different phases, shapes, sizes, and ordering, all of which significantly affect charge-carrier transport along π-conjugated semiconductors in organic field-effect transistors (OFETs). Pentacene OFETs show large variations in field-effect mobility (μ_FET) from 0.89 to 0.19 cm² V^-1 s^-1. Specifically, at R_q=0.40 nm the μ_FET value suddenly decreases to 0.30 cm² V^-1 s^-1. On polymer treated SiO₂ dielectrics with an R_q value greater than 0.40 nm, polymorphic, less-ordered, and smaller grains of pentacene containing large number of charge trap sites developed, resulting in significantly degraded charge-carrier transport along the intra- and inter-grains in OFETs, in comparison to the well-ordered grains on smooth polymer-treated surfaces (R_q<0.40 nm).