Molecularly aligned films of [1]benzothieno[3,2‐b][1]benzothiophene derivatives by solution shearing: Effect of alkyl substitution on morphology and charge transporting property

Abstract

The self‐assembly behaviors, crystal morphologies, and charge transporting properties of π–conjugated framework can be strongly affected by the functional group/substituent it carries, as well as the way the film is prepared. In this work, thin films of mono‐ and di‐alkylated [1]benzothieno[3,2‐b][1]‐benzothiophene (BTBT) derivatives were prepared on a silicon substrate by solution‐shearing method. Their film morphologies and crystal packing details were examined using polarized optical microscope, grazing incident wide‐angle X‐ray scattering, and atomic force microscope. With 2‐tridecyl[1]benzothieno[3,2‐b][1]‐benzothiophene (C13‐BTBT), a film with well‐aligned molecules packed in face‐to‐face π‐stacking arrangement was obtained, rendering a 1D‐like charge conduction channel. For 2,7‐bis‐tridecyl[1]benzothieno[3,2‐b][1]‐benzothiophene (C13‐BTBT‐C13), well‐aligned molecules packed in a layered‐herringbone arrangement were obtained. With these films as a conducting channel in transistor fabrication, high‐field‐effect mobility was realized for C13‐BTBT‐C13, with a maximum mobility (μh) of 0.645 cm2 /Vs and low anisotropic effect, presumably owing to the 2D growth of the π–conjugated skeleton in LHB packing. This study demonstrated the advantage of 2D semiconducting channels formed from C13‐BTBT‐C13 by low‐cost and scalable solution‐shearing processes, necessary for practical use in industry‐level and future flexible electronics.

Asymmetry Matters: Structure Elucidation and Application Potential of Solution-Processed Monoalkylated BTBT Thin Films

“…Occasionally, we also observe entanglement of two-monolayer steps constituting the more prominent four-monolayer steps (Figure 2b, indicated in blue). Hence, we can state from AFM measurements that a bilayertype packing motif is favored, and contrary to previously reported solution-processed C 13 -BTBT films, 15 we did not observe monomolecular steps as a prominent feature. Additionally, we yield a rather low surface roughness (RMS = 5.4 Å), emphasizing the smooth, molecularly flat film topography.…”

“…Bearing in mind the molecular composition of C 13 -BTBT, this would reduce the height difference between two individual bilayers by the length of one core unit with an estimated length of d BTBT = 8.6 Å, which we do not observe in the film topography at any spot. Hence, we conclude a favorable bilayer-type packing according to Figure c, which reflects the beneficial, close-to-equilibrium growth conditions and diverges from pervasive monolayer growth in solution sheared films . Our findings are in congruence with theoretical studies by Minemawari et al, who investigated the stabilizing effect of alkyl chains to C n -BTBT species via density functional theory calculations .…”

“…However, the small mixing entropy and, thus, low intrinsic solubility of the BTBT unit has always necessitated molecular modification, especially regarding straightforward and cheap solution processability. − Most prominently, beneficial symmetrical substitution using medium-sized alkyl chains in C n -BTBT-C n compounds − (Figure b), and surface-specific functionalization to yield semiconducting self-assembled monolayers (SAMs) are commonly utilized. − In general, such 2,7-dialkylated BTBT species are regarded as a benchmark compound class as they assemble readily from various liquid environments to form an air-stable system with a dense and well-defined herringbone-type packing promoting highly consistent performance data. ,− In particular, this is due to their well-balanced and predictable intermolecular interactions, that is, attractive π–π interactions of the BTBT core units and van-der-Waals forces between the terminal alkyl chains.…”

“…Vacuum-deposited C n -BTBT systems have been reported to be capable of intriguingly large hole mobilities well above 10 cm 2 /(V s), which is ascribed to densely packed BTBT units close to a SAM-modified dielectric interface. , However, thin-film growth in vacuo is generally limited by the temperature-dependent diffusion length of the evaporated C n -BTBT compounds, , and hence, domain and grain boundaries may form and subsequently hinder efficient charge transport in semiconducting junctions. First steps toward structure elucidation and electronic application of solution-processed C n -BTBT ( n = 13) films have been reported recently by Prakoso et al, who fabricated the films from a solution shearing approach and obtained decent hole mobilities of 0.018 cm 2 /(V s). Regarding the arrangement of the semiconductor, they rendered well-aligned molecules packed in face-to-face π-stacking arrangement and step heights within the range of the molecular c -axis.…”

A brief review on device operations and working mechanisms of organic transistor photomemories

“…2c. 37,38 The interception of its linear curve to the V g axis will give the threshold voltage ( V th ) value of the device, which is defined as the minimum V g bias to stimulate the output I d .…”