Solid cross linked-poly(ethylene oxide) electrolyte gate dielectrics for organic thin-film transistors

https://doi.org/10.1016/j.jiec.2020.09.015Get rights and content

Abstract

Solid polymer electrolyte gate dielectric based on cross-linked poly(ethylene oxide) (CPEO) was developed and employed for organic thin-film transistors (OTFTs). Mechanical stability, high areal capacitance, and amorphous morphology of CPEO were achieved via the use of polyhedral oligomeric silsesquioxane (POSS) as cross-linker, dissolved ion [EMIM][TFSI] as electrolyte, and PEO with low molecular weight as polymer matrix, respectively. The resulting solid polymer electrolyte showed excellent insulating properties with low leakage current density (1.8 × 10−7 A cm−2 at 1 V) and high capacitance per area (∼ 1 μF cm−2 at 100 Hz). Furthermore, dielectric properties of the developed polymer electrolytes including ionic conductivity as well as segmental relaxation time were investigated. The polyelectrolyte dielectric was employed for bottom-gate/top-contact organic thin-film transistors and the resulting devices showed decent electrical performance with a carrier mobility of 0.12 (±0.03) cm2 V−1 s−1 and a current on/off ratio of 103 at low operating voltage of 5 V.

Introduction

Organic thin-film transistors (OTFTs) are one of the essential components of various large-area, low-cost electronic devices, including displays, chemical sensors, and radio-frequency identification (RF-ID) tags [1], [2], [3], [4], [5], [6], [7], [8]. For the development of future electronic devices such as wearable health care devices, low operating voltage is required due to the limited capacity of battery with thin and small device sizes [9]. However, OTFTs typically suffer from comparatively high operating voltage (over 20 V), which results in high power dissipation in organic circuitry. Gate dielectric layer, one of the important components in OTFTs, capacitively induces charge carriers in the semiconducting layer, determining the operating voltages of the resulting devices. High-capacitance dielectrics afford high carrier densities in channel layer at relatively low voltages, hence, resulting in low operational voltages. Furthermore, dielectric film quality affects leakage current densities and morphological properties of semiconductor film. Therefore, it is important to develop dielectric materials with high capacitance, low leakage current, as well as favorable film quality for high performance OTFTs.

Capacitance per unit area (Ci) is represented by the equation, Ci=ε0kd, where ε0 is the vacuum permittivity, k is the dielectric constant, and d is the thickness of dielectric film. Hence, high capacitance can be achieved by decreasing dielectric thickness and/or employing high-k dielectric materials. However, these methods exhibit a few drawbacks. For instance, small film thickness of the dielectric layer could result in non-uniform film, high leakage current, and low current on/off ratios [10], [11]. Similarly, high-k dielectric materials usually suffer from charge traps at the semiconductor/dielectric interface and mobility degradation by high polarization [12], [13]. Furthermore, very high areal capacitance above 1 μF/cm2 is hard to achieve using these methods [14]. To this end, polymer electrolytes (i.e. polyelectrolytes) could be an alternative for high capacitance dielectric, affording enhanced charge carrier densities in the channel layer [15], [16], [17], [18], [19], [20], [21], [22]. It is relatively easy to achieve very high areal capacitance using polyelectrolyte even at high dielectric film thickness due to the formation of thin (∼1 nm) electrical double layer (EDL), which affords low operating voltages of OTFTs.

Polymer electrolytes consist of polymer matrix and dissolved ionic salt. Depending on the type of matrix and relative content of the salt, polymer electrolytes exist in gel or solid state. Compared to solid, gel-type electrolytes can exhibit unprecedentedly high capacitance above 20 μF/cm2, enabling low-operating voltage of OTFTs [15], [19]. However, they suffer from high leakage current, slow switching speed, as well as low thermal and mechanical stability. Among them, low thermal and mechanical stability of gel-type electrolytes limit the device structure and fabrication process of the resulting OTFTs. For instance, typical gel-type electrolyte can be damaged by high temperature process ; therefore, common metal evaporation process for electrode depositon can not be used. In this regards, solid electrolytes with improved mechanial and thermal stability are necessary to fabricate OTFTs in bottom-gate structure [22].

To this end, we report novel solid polymer electrolyte based on poly(ethylene oxide) as gate dielectric for low-voltage OTFTs. We employed poly(ethylene oxide) (PEO) as polymer matrix for polyelectrolyte due to its good chain flexibility and electrochemical stability. To overcome the limitations of PEO, hence the rough film surface morphology from high crystallinity and low glass transition temperature (∼−60 °C) [23], [24], crosslinked polymer network was formed employing low molecular weight PEO and polyhedral oligomeric silsesquioxane (POSS) as crosslinker [25], [26]. Note that PEO with high molecular weight generally exhibits rough surface due to high film crystallinity. Since the size of the POSS is comparable to the dimensions of polymer chain, POSS could control the polymer chain motion at a molecular level, leading to enhancement of physical properties of the polymer. The resulting crosslinked polymer films showed relatively smooth film morphology as well as decent thermal properties. To enable high capacitance, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) was added to the polymer matrix as electrolyte. The [EMIM][TFSI] is one of the frequently used ionic liquid for polymer electrolyte. The effect of added ionic liquid on the dielectric properties were investigated in terms of ionic conductivity and segmental relaxation time. Furthermore, the polyelectrolyte dielectric films exhibited excellent insulating properties with large areal capacitance. We then demonstrated that solid electrolyte gated-OTFTs could be fabricated in top-contact/bottom-gate structure due to improved mechanical robustness and amorphous morphology of cross-linked PEO (CPEO) (Fig. 1). The resulting OTFT devices operated at low operating voltage (<5 V) and exhibited decent charge carrier mobility as high as 0.12 cm2 V−1 s−1.

Section snippets

Materials and methods

All materials including 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]), allyl bromide, polyethylene glycol methyl ether (Mw = 10,000), platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Karstedt’s catalyst) were purchased from Aldrich and used as received. Octasilane polyhedral oligomeric silsesquioxane (POSS) (MW = 1017.99) was obtained from Hybrid Plastics and used as received. Chemical structures of

Synthesis

Cross-linked poly(ethylene oxide) (CPEO) was synthesized in two steps (Scheme 1). In the first step, allylation was carried out using allyl bromide on the terminal end of polyethylene glycol methyl ether. In the second step, allyl-PEO was cross-linked using octasilane POSS. The cross-linking process was conducted through the hydrosilylation process. To confirm that the synthesis was successful, 1H-NMR was carried out. As shown in Fig. S1, synthesized CPEO exhibited absorption peaks at 3.64,

Conclusion

In conclusion, we have developed solid-state polyelectrolytes based on cross-linked poly(ethylene oxide). By employing polyhedral oligometric silsequioxane as crosslinker and dissolved ionic liquid as electrolyte, newly developed dielectric materials exhibited excellent insulating properties, high areal capacitance, as well as decent thermal stability. Furthermore, correlation between ionic conductivity of the polyelectrolytes and segmental relaxation time and temperature were elucidated. Based

Acknowledgments

This work was supported by a National Research Foundation of Korea (NRF) grant (NRF-2020R1C1C1003606) and by the C1 Gas Refinery Program through the NRF funded by the Korean government (2015M3D3A1A01064929).

References (38)

  • K. Murata et al.

    Electrochim. Acta

    (2000)
  • W. Zhang et al.

    Prog. Polym. Sci.

    (2013)
  • A.R. Polu et al.

    J. Ind. Eng. Chem.

    (2017)
  • A.R. Polu et al.

    J. Ind. Eng. Chem.

    (2015)
  • D.K. Pradhan et al.

    Int. J. Electrochem. Sci.

    (2008)
  • Y. Huang et al.

    Org. Electron.

    (2016)
  • S.R. Forrest

    Nature

    (2004)
  • S. Vegiraju et al.

    Adv. Funct. Mater.

    (2017)
  • B. Kang et al.

    ACS Appl. Mater. Interfaces

    (2013)
  • R.P. Ortiz et al.

    Chem. Rev.

    (2009)
  • M. Ozdemir et al.

    ACS Appl. Mater. Interfaces

    (2016)
  • A. Pierre et al.

    Adv. Mater.

    (2014)
  • H. Sirringhaus

    Adv. Mater.

    (2014)
  • D. Ho et al.

    Chempluschem

    (2019)
  • J. Li et al.

    J. Mater. Chem.

    (2012)
  • Y.D. Park et al.

    Appl. Phys. Lett.

    (2005)
  • M.E. Roberts et al.

    Chem. Mater.

    (2009)
  • C. Wang et al.

    Chem. Mater.

    (2013)
  • R.T. Weitz et al.

    Nano Lett.

    (2007)
  • Cited by (5)

    • Physical structure, TD-DFT computations, and optical properties of hybrid nanocomposite thin film as optoelectronic devices

      2022, Journal of Industrial and Engineering Chemistry
      Citation Excerpt :

      ZrO2 is characterized by a high melting point, high mechanical and thermal resistance, a high dielectric constant, and low electrical conductivity. ZrO2 is a great choice for chemical, optical, dielectric, and mechanical applications because of its chemical stability, high hardness, and biocompatibility [8–11]. ZrO2 could be useful in fuel cells [12], mirror protection coatings, and optoelectronic devices [13].

    • Surface modification and structure constructing for improving the lithium ion transport properties of PVDF based solid electrolytes

      2022, Chemical Engineering Journal
      Citation Excerpt :

      Researches showed that solid electrolytes can solve the related matters for they usually have good young's modulus [4], can not only optimize the deposition of Li, but also inhibit the growth of lithium dendrite, were developed rapidly recently [5,6]. Commonly, solid electrolytes include ceramic solid electrolytes (CSEs) [7–10] such as Li7La3Zr2O12 (LLZO), Li1.5Al0.5Ge1.5(PO4)3 (LAGP) and SPEs [11–16] such as polyethylene oxide (PEO) and PVDF. When the two kinds of solid electrolytes were compared, CSEs show the advantages of higher lithium ion conductivity, stability and safety.

    • Fabrication, characterization, TD-DFT, optical and electrical properties of poly (aniline-co-para nitroaniline)/ZrO<inf>2</inf> composite for solar cell applications

      2022, Journal of Industrial and Engineering Chemistry
      Citation Excerpt :

      High melting point, strong mechanical and thermal resistance, high dielectric constant, and poor electrical conductivity characterize ZrO2. Chemical stablity with high hardness and biocompatibility make ZrO2 an excellent choice for chemical, optical, dielectric, and mechanical applications [5–7]. Fuel cells [7,8], mirror protecting coatings, and optoelectronic devices [9] might all benefit from the usage of ZrO2.

    • Enhancement of plasma-catalytic oxidation of ethylene oxide (EO) over Fe–Mn catalysts in a dielectric barrier discharge reactor

      2021, Science of the Total Environment
      Citation Excerpt :

      Ethylene oxide (EO) is widely used as the raw materials for the manufacturing of various chemicals including germicide, plasticizer, solvents and surfactants, etc. (Chen et al., 2021; Cho et al., 2020; Nogueira et al., 2021).

    View full text