Hybrid Polymer/Metal Oxide Thin Films for High Performance, Flexible Transistors

Abstract

Metal oxides (MOs) have garnered significant attention in a variety of research fields, particularly in flexible electronics such as wearable devices, due to their superior electronic properties. Meanwhile, polymers exhibit excellent mechanical properties such as flexibility and durability, besides enabling economic solution-based fabrication. Therefore, MO/polymer nanocomposites are excellent electronic materials for use in flexible electronics owing to the confluence of the merits of their components. In this article, we review recent developments in the synthesis and fabrication techniques for MO/polymer nanocomposite-based flexible transistors. In particular, representative MO/polymer nanocomposites for flexible and transparent channel layers and gate dielectrics are introduced and their electronic properties-such as mobilities and dielectric constant-are presented. Finally, we highlight the advances in interface engineering and its influence on device electronics.

Keywords: active layers; dielectrics; flexible transistors; metal oxides; nanocomposites; polymers.

Figure 1

(a–f) A set of TEM images of diverse MO nanostructures: (a) MnO and (b) Fe₃O₄ nanoparticles fabricated via microwave-assisted synthesis (Reproduced from [42], Copyright 2008 Royal Society of Chemistry C), (c) porous SnO₂ aerosols prepared via sol–gel method (reproduced with permission from [41], Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim), (d) ZnO hollow spheres synthesized via hydrothermal synthesis (reproduced with permission from [40], Copyright 2008 American Chemical Society), (e) In₂O₃ nanoparticles prepared via anodization-precipitation (reproduced with permission from [37], Copyright 2018 American Chemical Society), and (f) TiO₂ nanoparticle layer on SiO₂ prepared via CVD, respectively (reproduced with permission from [43], Copyright 2001 Elsevier).

Figure 2

Reaction routes for the production of MO nanostructures by the sol–gel method.

Figure 3

(a) Schematic representation of the flexible, transparent TFT structure based on a metal oxide:polymer (In₂O₃: x% PVP) semiconductor blend. (b) X-ray diffraction (XRD) patterns of In₂O₃: polymer films with various PVP concentrations: annealing at 225 °C. (c) TFT mobility and threshold voltage for In₂O₃: polymer films having different PVP concentrations, processed at 225 °C. (d) Dependence of TFT mobilities on bending radius of both neat In₂O₃ TFTs and all-amorphous In₂O₃: 5% PVP TFTs (left), and mobilities on all-amorphous TFT bending cycles at a radius of 10 mm. Inset: Optical image of transparent flexible TFTs. Reproduced with permission from [91], Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 4

(a) Chemical structure of PEI. (b) GIXRD patterns of In₂O₃: x% PEI blend films with differing PEI concentrations. (c) Derived coordination number, In-O bond lengths for the indicated films. (d) TFT mobility and threshold voltage for In₂O₃: x wt % PEI (250 °C), IZO: x wt % PEI, IGO: x wt % PEI, and IGZO: x wt % PEI, as a function of the polymer concentration. T_annealing = 300 °C. Reproduced with permission from [93], Copyright, 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 5

(a) Schematic illustration of improved hydrophobicity of the IGZO: PTFE film. Transfer characteristics of (b) the IGZO TFT and (c) the IGZO: 20 W PTFE TFT upon exposure to water for different times. (d) Variations of mobility (μ_FE), on-current, and V_th for IGZO and IGZO: 20 W PTFE TFTs with respect to bending cycles. Reproduced permission from [95], Copyright, 2018, American Chemical Society.

Figure 6

(a) Optical microscopy images of In₂O₃ deposition (right to left) without PS, with PS1, with PS2, with PS3, and with PS4. The scale bar is 100 μm. (b) Transfer characteristics of inkjet-printed TFTs. (c) X-ray photoelectron spectroscopy (XPS) of O 1s spectra. Reproduced with permission from [96], Copyright 2018, American Institute of Physics.

Figure 7

(a) SEM image of the TFT cross section, where the PMMA−ZrO₂ layer was deposited with 1:0.3:1 molar ratio. (b) Leakage current density vs. electric field of the PMMA–ZrO₂ hybrid layers deposited with different TMSPM molar ratios. (c) Transfer characteristics for ZnO-based transistors with PMMA–ZrO₂ as gate dielectric hybrid films at different TMSPM molar concentrations. Reproduced with permission from [126], Copyright 2017, American Chemical Society.

Figure 8

|I_D|^1/2 vs. V_G plots of TIPS-pentacene TFTs with the (a) c-PVP/Y₂O₃ composite and (b) c-PVP gate insulators. AFM images of the (c) c-PVP and (d) c-PVP/Y₂O₃composite films. The insets show the contact angles on both films. Leakage current paths through the (e) c-PVP/Y₂O₃ composite and (f) c-PVP gate insulators. (g) Possible interaction between the holes and the Y₂O₃ nanoparticles in the c-PVP/Y₂O₃ composite insulator. Reproduced with permission from [131], Copyright 2016, MDPI, Basel, Switzerland.

Figure 9

Vapor-phase synthesis of organic–inorganic hybrid dielectrics via iCVD. (a) A schematic of the synthesis process: (i) Vaporized monomers, organometallic precursor, and initiators are injected. (ii) The initiators were thermally decomposed near the heated filament to form radicals (red lines), which are positioned away from the substrate. (iii) Monomers, precursor, and radicals were absorbed on the heated substrate. (iv) The adsorbed monomers were polymerized and simultaneously reacted with inorganic precursors. (v) Uniform dispersion of the inorganic oxides can be achieved in the polymer matrix. (b) C_i–E(left), and J–E(right) characteristics of the MIM devices with the hybrid dielectrics (Al concentration: 17.8%) with various thicknesses of 82.7, 55.4, 24.8, and 19.8 nm, respectively. Charge-transfer characteristics of the (c) pentacene and (d) PTCDI-C13 OTFTs, respectively. Hybrid films 25 and 34 nm thick were used as the gate dielectric for pentacene and PTCDI-C13 OTFTs, respectively. Reproduced with permission from [134], Copyright 2018, American Chemical Society.

Figure 10

(a) Capacitance as a function of total layer thickness for hafnium oxide, PMMA, and hybrid dielectric. Transfer characteristics of network SWNT FETs with (b) HfOx and (c) hybrid dielectric. Channel width/length ratio and channel lengths were 125 and 40 μm, respectively. (d) Bias stress tests of SWNT-based transistors with different dielectrics. Reproduced with permission from [136], Copyright 2015 American Institute of Physics.

Figure 11

Water contact angles of (a) bare-ZrO₂ surface, (b) HMDS modified surface, (c) PαMS modified surface. (d) Field effect hole mobility as a function of ZrO₂ dielectric constant for OFETs with different surface modifications. The mobility was calculated with V_G = −5 V and capacity density under f = 1 kHz. (e) Schematic diagram of the flexible OTFT fabricated on PET substrate and (f) the digital photograph of the flexible OTFTs. (g) I_DS–V_GS transfer curves of a ZrO₂-OFET constructed on PET flexible substrate. The channel width and length of the transistor are 750 μm and 50 μm, respectively. Reproduced with permission from [138], Copyright 2016 American Chemical Society.