Electric Field Tuning Molecular Packing and Electrical Properties of Solution‐Shearing Coated Organic Semiconducting Thin Films

Abstract

Recent improvements in solution-coated organic semiconductors (OSCs) evidence their high potential for cost-efficient organic electronics and sensors. Molecular packing structure determines the charge transport property of molecular solids. However, it remains challenging to control the molecular packing structure for a given OSC. Here, the application of alternating electric fields is reported to fine-tune the crystal packing of OSC solution-shearing coated at ambient conditions. First, a theoretical model based on dielectrophoresis is developed to guide the selection of the optimal conditions (frequency and amplitude) of the electric field applied through the solutionshearing blade during coating of OSC thin films. Next, electric field-induced polymorphism is demonstrated for OSCs with both herringbone and 2D brickwall packing motifs in 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene and 6,13-bis(triisopropylsilylethynyl) pentacene, respectively. Favorable molecular packing can be accessible in some cases, resulting in higher charge carrier mobilities. This work provides a new approach to tune the properties of solution-coated OSCs in functional devices for highperformance printed electronics.

High‐Mobility, Solution‐Processed Organic Field‐Effect Transistors from C8‐BTBT:Polystyrene Blends

“…[26] These values are obtained without any electrode width correction as has been performed previously to allow the reporting of high mobilities in the literature. [26] These values are obtained without any electrode width correction as has been performed previously to allow the reporting of high mobilities in the literature.…”

“…[5] Besides blended systems with PS, poly(methyl methacrylate) (PMMA) has been used for the fabrication of C8-BTBT-based devices. [26] However, due to poor substrate coverage, the value has been corrected for the presumed channel width that is covered with semiconductor. [23] In another study, improved performance in this C8-BTBT:PMMA blend system has been shown to be triggered by aforementioned zone-refinement effect, also referred to as in situ purification.…”

“…[25] Besides promising results obtained from spin-cast blends of various polymers and C8-BTBT, scalable processing technologies, such as blade coating or zone casting have been utilized for the deposition of C8-BTBT. [9] Specifically, a meaningful comparison of device performance only appears possible for OFETs with identical contact materials, [14,21,26,27] or among the limited instances of reported "ideal" devices, i.e., the ones that seem not to be limited by contact effects. [26] However, due to poor substrate coverage, the value has been corrected for the presumed channel width that is covered with semiconductor.…”

“…In particular, a mobility of 9.2 cm 2 V −1 s −1 has been extracted from blade-cast devices, prepared by a similar technology as utilized in our work. [4,25,29,30] Nonetheless, the current state-of-the-art solution deposition methods that report device performances for C8-BTBT often use silver (Ag) contacts [5,14,21,26,27] and do show gatevoltage-dependent mobilities. Without this correction, the actual effective mobility is between 6 and 7.4 cm 2 V −1 s −1 .…”

“…[26] However, due to poor substrate coverage, the value has been corrected for the presumed channel width that is covered with semiconductor. Therefore, in order to compare our results to previous works that apply shear coating [26,27] and to properly evaluate our progress in optimizing the film coating conditions and film morphologies relative to the existing literature and the coating methods they describe, we also optimized our coating conditions using a bottom-gate, top-contact device architecture with Ag source and drain electrodes ( Figure 1A). In a more recent example that reports uncorrected effective saturation mobility, values around 10.2 cm 2 V −1 s −1 are obtained, after typical defects of zone-cast films have been repaired by lateral homoepitaxy that was templated by a solution-processed single crystalline film.…”