Charge transport and morphology of pentacene films confined in nano-patterned region

Abstract

The transport and morphology of macroscopic organic transistors have been thoroughly explained. However, the relationship between charge transport and the morphology of organic semiconductors in nano-confined regions is not well understood, which is required to design high-performance nano-electronics. Therefore, in this study, the electrical performance of pentacene thin-film transistors (TFTs) fabricated on nano-scale confined geometries was compared with that of other TFT systems fabricated on micro-scale confined or flat dielectrics. The results showed that the photoresist (PR) reliefs patterned onto silicon dioxide (SiO2) dielectrics could control the growth mode of pentacene in the confined regions. As the line spacing between the PR patterns decreased to below the average size of pentacene normally grown on SiO2 dielectrics (∼1.7 μm), field-effect mobility (μFET) of pentacene TFTs improved considerably. Specifically, the μFET values for the 250 nm patterned dielectric system were as high as 0.33 cm2 V−1 s−1, which was greater than those of the wider-patterned PR or flat dielectric systems (⩽ 0.15 cm2 V−1 s−1). These findings were related to the enhancement of charge-carrier transport owing to the anisotropic π-conjugated crystals nucleated from the nano-confined edges, providing important information that can be used in the design of high-performance nano-scale organic electronics. We have analyzed the effect of the morphology of the pentacene films confined in the micro- and nano-scale relief patterns on the charge transport. As the patterned space region for confining the growth of organic semiconductors decreased to below the average grain size (∼ 1.7 μm), the field-effect mobility increased. Because the molecular ordering is influenced by the pattern’s confining effect on pentacene film growth. Our results will provide an approach that can be used to further increase the charge transport mobility of polycrystalline organic semiconductors by increasing the molecular ordering in the direction of charge transport. While the effect of morphology on an organic semiconductor's charge transport is generally well understood, less is known about the effect when the material is confined, despite this being a highly relevant situation for nanoelectronic devices. A South Korea-based research team led by Tae-Woo Lee has now determined how the performance of a thin-film transistor is affected when its pentacene semiconductor is confined in a nanoscale pattern rather than a microscale pattern or a flat dielectric system. The researchers observed higher field-effect mobility in the nanopattern-based device compared to the other configurations. The enhancement arises as a result of the nanopattern's effect on the arrangement of the pentacene molecules in the thin film — favouring their ordering in the direction of the charge transport, which in turn improves the device's performance. This understanding of the effect of confinement on charge mobility may contribute to the engineering of more efficient organic semiconductors.