Critical Role of Alkyl Chain Branching of Organic Semiconductors in Enabling Solution-Processed N-Channel Organic Thin-Film Transistors with Mobility of up to 3.50 cm² V^–1 s^–1

Abstract

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from ~0.001 to >3.0 cm(2) V(-1) s(-1). Incorporating a NDI-DTYM2 core with three-branched N-alkyl substituents of C(11,6) results in a dense in-plane molecular packing with an unit cell area of 127 Å(2), larger domain sizes of up to 1000 × 3000 nm(2), and an electron mobility of up to 3.50 cm(2) V(-1) s(-1), which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities.

Trends in molecular design strategies for ambient stable n-channel organic field effect transistors

“…84 The best n-channel mobility was observed for compound 1 0 b with chain branching two carbon atoms away from the conjugated backbone. 84 X-ray diffraction revealed that changes in the branching position modify solid state molecular packing leading to alteration in the grain size and thin film morphology. compound 1 0 g under low voltage operation were utilized for developing an ink-jet printed TFT device which is a step forward towards using organic materials for printed electronics.…”

“…[81][82][83] The influence of device geometry on the type and amplitude of charge carrier mobilities and especially the stability of the n-channel transport were reported by several research groups. 84 Better performance of top-gated devices is frequently observed and rationalized by easier injection of charges from the source-drain electrodes as the larger active electrode area reduces contact resistance. 85,86 Top-gate OFET devices have also better stability under ambient conditions because the active layer is sandwiched in between the substrate and dielectric layers, preventing direct air exposure of the OSC.…”

“…À3.7 eV for PC 60 BM) which lead to instability of the anions in air. An exception is PC 84 BM with a low lying LUMO of around À4.0 eV and the open air stability of the reduced form. 273 In spite of a LUMO level at À3.6 eV, the perfluoroalkyl-substituted C 60 derivative 6f shows n-channel performance with an electron mobility of 0.078 cm 2 V À1 s À1 in an open air device.…”

Simulating charge transport in organic semiconductors and devices: a review

“…The significant dependence of mobility on structure in these OSCs show that mobility can be engineered via processing and molecular design. Modelling charge transport in the new breed of high-performance OSCs [290][291][292] would seem to be an attractive area for ongoing research.…”

Alkyl side-chain branching point effects in thieno[3,4-c]pyrrole-4,6-dione copolymers

“…[16,17] More recently, a number of studies have begun to elucidate the effect of precision alkyl-chain engineering by shifting the branching point position, which has resulted in both improved photovoltaic efficiency and charge carrier mobility in field-effect transistors. [18][19][20] However, the preferred side-chain is largely dependent on the exact polymer system. Our approach herein is to conduct a systematic study of the alkyl side-chain branching point position, investigating the effects of overall steric bulk on solid-state properties in a promising class of semiconducting polymers.…”