Molecular Design Concept for Enhancement Charge Carrier Mobility in OFETs: A Review

Abstract

In the last two decades, organic field-effect transistors (OFETs) have garnered increasing attention from the scientific and industrial communities. The performance of OFETs can be evaluated based on three factors: the charge transport mobility (μ), threshold voltage (V_th), and current on/off ratio (I_on/off). To enhance μ, numerous studies have concentrated on optimizing charge transport within the semiconductor layer. These efforts include: (i) extending π-conjugation, enhancing molecular planarity, and optimizing donor-acceptor structures to improve charge transport within individual molecules; and (ii) promoting strong aggregation, achieving well-ordered structures, and reducing molecular distances to enhance charge transport between molecules. In order to obtain a high charge transport mobility, the charge injection from the electrodes into the semiconductor layer is also important. Since a suitable frontier molecular orbitals' level could align with the work function of the electrodes, in turn forming an Ohmic contact at the interface. OFETs are classified into p-type (hole transport), n-type (electron transport), and ambipolar-type (both hole and electron transport) based on their charge transport characteristics. As of now, the majority of reported conjugated materials are of the p-type semiconductor category, with research on n-type or ambipolar conjugated materials lagging significantly behind. This review introduces the molecular design concept for enhancing charge carrier mobility, addressing both within the semiconductor layer and charge injection aspects. Additionally, the process of designing or converting the semiconductor type is summarized. Lastly, this review discusses potential trends in evolution and challenges and provides an outlook; the ultimate objective is to outline a theoretical framework for designing high-performance organic semiconductors that can advance the development of OFET applications.

Keywords: OFETs; charge carrier mobility; n-type; semiconductor layers.

Figure 1

Chemical structures of the π-conjugation extension molecules based on small molecules, oligomers, and polymers as well as their charge transport mobility.

Figure 2

Chemical structures of the molecules with non-covalent bonds interaction to adjust their backbone’s planarity.

Figure 3

Chemical structures of the molecules with ring fusing to adjust their backbone’s planarity.

Figure 4

Chemical structure based on the molecules with different-type alky-chains as well as their charge transport mobility.

Figure 5

Chemical structure based on the molecules with hydrogen bonding association as well as their charge transport mobility.

Figure 6

(a) The chemical structure of DPPT-DCV and DPPT-RD. (b) POM images of DPPT-RD and DPPT-DCV crystal arrays on Si wafers. Adapted with permission from [98]. (c) The HOMO/LUMO energy levels of DPPT-DCV and DPPT-RD. Adapted with permission from [98].

Figure 7

(A) Schematic illustration of the top-contact bottom-gate OFET device. Adapted with permission from [102]. (B) STEM-EDX cross-section image (top) and elemental maps (bottom) of the OFET device with the metal–mosaic electrodes. Adapted with permission from [102]. (C) Surface morphology of Au/Al electrodes with different ratios in OFET. Adapted with permission from [102]. (D) The corresponding electron and hole mobilities extracted from the n- and p-type devices, respectively. Adapted with permission from [102].