25th anniversary article: organic field-effect transistors: the path beyond amorphous silicon

Abstract

Over the past 25 years, organic field-effect transistors (OFETs) have witnessed impressive improvements in materials performance by 3-4 orders of magnitude, and many of the key materials discoveries have been published in Advanced Materials. This includes some of the most recent demonstrations of organic field-effect transistors with performance that clearly exceeds that of benchmark amorphous silicon-based devices. In this article, state-of-the-art in OFETs are reviewed in light of requirements for demanding future applications, in particular active-matrix addressing for flexible organic light-emitting diode (OLED) displays. An overview is provided over both small molecule and conjugated polymer materials for which field-effect mobilities exceeding > 1 cm(2) V(-1) s(-1) have been reported. Current understanding is also reviewed of their charge transport physics that allows reaching such unexpectedly high mobilities in these weakly van der Waals bonded and structurally comparatively disordered materials with a view towards understanding the potential for further improvement in performance in the future.

Keywords: organic field-effect transistors; organic light-emitting diode displays; organic semiconductors.

Figure 1

Photographs of A) flexible electrophoretic ink display driven by an active-matrix of 1.2 million OFETs (source: Plastic Logic); B) plastic foils comprising 8-bit microprocessors with 3381 OFETs each. Reproduced with permission.[122] Copyright 2012, IEEE.

Figure 2

Pixel circuits for active matrix OLED addressing with a p-type OFET: A) simple 2T1C architecture; B) example of voltage compensation circuit comprising 4 pixel TFTs.

Figure 3

Growth of strained TIPS-pentacene polymorphs by blade coating technique with micropillar improved fluid flow: A) molecular structure of TIPS-P; B) schematic diagram of coating process; C) wide-angle X-ray scattering (WAXS) images of different TIPS-P polymorphs obtained by coating films from different solution concentration at a shearing speed of 0.8 mm s^–1. Reproduced with permission.[39] Copyright 2013, Macmillan Publishers Ltd.

Figure 4

Double-shot inkjet printing process of C_n-BTBT films: A) schematic diagram of process printing subsequently ink droplets of an antisolvent (blue) and then droplets of the ink comprising the organic semiconductor (yellow) onto a substrate with a surface energy pattern confining the ink droplets; the inset shows the molecular structure of C_n-BTBT; B) polarized optical micrograph of printed C₈-BTBT films; C) histogram of field-effect mobilities across 54 top-gate, top-contact transistors. Reproduced with permission.[44] Copyright 2008, Macmillan Publishers Ltd.

Figure 5

Wide-angle X-ray scattering images of different high-mobility conjugated polymers. The semicrystalline, thiophene-based polymers P3HT and PBTTT are semicrystalline and exhibit well-defined, relatively sharp diffraction spots including several orders of diffraction and a higher anisotropic orientation with respect to the substrate plane, while the donor-accept copolymers, PDPPBT and PNDI2OD-T2 exhibit a still semicrystalline, but less-ordered microstructure with fewer and wider diffraction spots. Reproduced with permission.[98]_{Copyright 2013, Macmillan Publishers Ltd.}

Figure 6

Illustration of methods of mobility extraction from high-mobility conjugated polymer OFETs which exhibit non-ideal transfer characteristics: (A) CDT-BTZ bottom-gate, top-contact FET. Reproduced with permission.[65] Copyright 2011, American Chemical Society. B) DPP-T-TT bottom-gate, top-contact FET. Reproduced with permission.[73] Copyright 2012, Macmillan Publishers Ltd. C) IDTBT top-gate, bottom-contact FET. Reproduced with permission.[81] Copyright 2013, Macmillan Publishers Ltd.

Figure 7

Theoretical simulations of conformations of diphenyl-DPP (A) and dithienyl-DPP (B) in front view (top) and side view (bottom) illustrating the increased torsional backbone twist (27° dihedral angle) in diphenyl-DPP relative to dithienyl-DPP (12° dihedral angle). Reproduced with permission.[24]

Figure 8

Three polymorphs of CDT-BTZ identified from molecular dynamics simulations and suggested on the basis of NMR data. ‘Cofacial’ corresponds to a perfect matching between the donor and acceptor units belonging to neighbouring chains, while ‘Minus1A’ (‘Minus2A’) has conjugated backbones shifted longitudinally by 1 (2) Å, with respect to ‘Cofacial’. The Minus2A polymorph agrees best with experimental GIXRD and 2D-NMR data. Reproduced with permission.[89]

Figure 9

Microstructure of conjugated polymer films: a) semicrystalline polymer film, for example, P3HT; b) disordered aggregates typically of many donor-acceptor copolymers; c) completely amorphous film. There is a coexistence of ordered (darker shadowed areas) and spaghetti-like amorphous regions. Tie chains between aggregates are shown in bold red; d) activation energy for transport in semiconducting polymers; the plot includes FET data from the literature (dash), as well as trap depth/tail widths derived from device modelling (cross) for traditional classic semicrystalline materials (red), new high-performance polymers that are found to be poorly ordered (black), and completely amorphous materials (blue). Reproduced with permission.[98] _{Copyright 2013, Macmillan Publishers Ltd.}

Figure 10

A) Wide angle X-ray scattering image of C₁₆IDT-BT. B,C) Comparison of polymer chain orientation from GIXD and NEXAFS. The average orientation of the conjugated plane calculated from GIXD () as a function of the conjugated plane tilt, i.e., the angle between the (010) reflection and the conjugated plane normal for C₁₆IDT-BT (B) and fast-dried, non-annealed P3HT (C). The upper and lower bounds of the pink areas in a denote the average orientation of the conjugated plane that is compatible with NEXAFS spectra () of top and buried interfaces, respectively. The light-blue areas highlight the conjugated plane tilt range between 20° and 30°: that is, the most probable conjugated plane tilt range for alkylated polythiophenes. For C₁₆IDT-BT, the overlap between and the range of possiblesuggests that the conjugated planes are oriented in similar ways irrespective of whether their environments are crystalline or non-crystalline, whereas for P3HT the absence of overlap between the and suggests that the orientation of the chains in non-crystalline environments is likely to be different from those in crystalline domains. Reproduced with permission.[81] Copyright 2013, Macmillan Publishers Ltd