Toward high-mobility organic field-effect transistors: Control of molecular packing and large-area fabrication of single
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Introduction Organic field-effect transistors (OFETs) have attracted the attention of researchers from both academia and industry in the last decade. From a technological standpoint, OFETs offer opportunities for flexible, low-cost, and lightweight electronic applications, such as complementary circuits,1–4 displays,5,6 and sensors.7–11 From a scientific standpoint, OFETs provide a platform to investigate charge transport in organic materials. Charge-carrier mobility is one of the key device parameters of OFETs. In the first OFET developed in 1986, the mobility was ∼10–5 cm2 V–1 s–1;12 and since then, mobility values have increased to exceed that of amorphous silicon (∼1 cm2 V–1 s–1) and are quickly approaching that of polycrystalline silicon (10–100 cm2 V–1 s–1). For example, hole mobilities as high as 40 cm2 V–1 s–1 and electron mobilities up to 11 cm2 V–1 s–1 have been reported in recent OFETs.2,13–31 The field-effect chargecarrier mobility in OFETs is determined by the molecular packing32 at the semiconductor-dielectric interface. Significant efforts have been made to optimize the molecular packing to improve mobility, and recently, the effects of slight modifications in the molecular packing on mobilities have been unraveled.32 Another key parameter contributing to the mobility is the long-range order and alignment of the semiconductor material. Organic single-crystals are ideal charge transport media, as
they have fewer structural defects than their amorphous and polycrystalline counterparts.18 Single-crystals thus show the highest mobility values reported to date.16–18,21,24 Single-crystal devices are difficult to fabricate for technological applications and are currently considered useful only as models for fundamental charge-transport research. However, progress has been made recently toward high-throughput large-area fabrication of organic single-crystal-based FETs. In this article, we review recent progress in controlling molecular packing toward high-mobility OFETs and single-crystal growth methods for constructing large-area devices.
Tuning molecular packing and the resulting charge transport For crystals composed of small molecular organic semiconductors (OSCs), noncovalent interactions between molecules contribute to charge-carrier transport in FETs. A widely used theory for predicting charge transport in OSCs is the Marcus theory, wherein the orbital overlap (transfer integral) and the reorganization energy are both important in determining the charge-transfer rate.32 Thus, any method that modifies crystal packing in the OSC layer can affect the orbital overlap and therefore the transfer integral, which then affects charge-carrier mobilities.33–36 The methods of tuning molecular packing can be
Hanying Li, State Key Laboratory of Silicon Materials and Department of Polymer Science and Engineering, Zhejiang University, China; [email protected] Gaurav Giri, Department of Chemical Engineering, Stanford University; [email protected] Jeffrey B.-H. Tok, Department of Chemical Engineering, Stanford University;
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