Parylene C as a versatile dielectric material for organic field-effect transistors

Abstract

An emerging new technology, organic electronics, is approaching the stage of large-scale industrial application. This is due to a remarkable progress in synthesis of a variety of organic semiconductors, allowing one to design and to fabricate, so far on a laboratory scale, different organic electronic devices of satisfactory performance. However, a complete technology requires upgrading of fabrication procedures of all elements of electronic devices and circuits, which not only comprise active layers, but also electrodes, dielectrics, insulators, substrates and protecting/encapsulating coatings. In this review, poly(chloro-para-xylylene) known as Parylene C, which appears to become a versatile supporting material especially suitable for applications in flexible organic electronics, is presented. A synthesis and basic properties of Parylene C are described, followed by several examples of use of parylenes as substrates, dielectrics, insulators, or protecting materials in the construction of organic field-effect transistors.

Keywords: Parylene C; dielectric; encapsulation layer; flexible substrate; organic field effect transistor.

Figure 1

Schematic representation of the deposition process of Parylene C with the respective chemical reactions. Reprinted with permission from [28], copyright 2016 Elsevier.

Figure 2

AFM measurements of the surface roughness of Parylene C thin films. Reprinted with permission from [30], copyright 2009 Elsevier.

Figure 3

XRD spectra of Parylene C films: as-deposited with constant deposition rate and thermally annealed at different temperatures (a), as-deposited with different deposition rates and thermally annealed at constant temperature (b). Reprinted with permission from [29] copyright 2008 MYU K.K. (reprinted from electronic version).

Figure 4

Schematic illustration of the flexible OFET fabrication procedure with Parylene C as a substrate and gate dielectric layer and with zone-cast tetrakis(alkylthio)tetrathiafulvalene as semiconductor. Reprinted with permission from [34].

Figure 5

Transfer characteristics of 10 OTFTs after bending and crumpling tests: (a) Photograph of a device before mechanical tests. (b) Photograph of a device rolled onto a cylinder of 0.8 mm radius. (c) Photograph of a crumpled device. (d) and (e) Transfer characteristics of 10 OTFTs before and after bending and crumpling tests. Reprinted with permission from [33] copyright 2016 IOP Publishing Ltd.

Figure 6

Thin Parylene C layers breakdown voltage as a function of thickness. Reprinted with permission from [30], copyright 2009 Elsevier.

Figure 7

(a) Mobility μ(V_g) curves measured for four different gate insulators. For the device based on Parylene C, the suppression of contact effects often requires a rather large value of V_DS (and thus V_GS) to remain in the linear regime. (b) Decrease of the mobility with increasing ε, as observed in rubrene single-crystal FETs with different gate insulators. The bars give a measure of the spread in mobility values. Inset: when plotted on a log–log scale, the available data show a linear dependence with slope −1 (i.e., the variation in μ is proportional to ε⁻¹). Reprinted with permission from [37], copyright 2004 of AIP Publishing.

Figure 8

10 μm × 10 μm AFM images of tetracene thin films on different dielectric surfaces at different nominal thickness. Z-scale: 50 nm. Reprinted with permission from [50], copyright 2013 Royal Society of Chemistry.

Figure 9

(a) Transistor architecture of the three different transistor stacks investigated, (b) threshold voltage trends of successive transfer sweeps for different V_D, (c) representative transfer I–V characteristics of the three transistor stacks. The arrows in (c) indicate the sweeping direction of V_G. Reprinted with permission from [51], copyright 2016 Springer.

Figure 10

Transfer characteristics measured during the continuous bias stress of 125 h. (a) Bottom-gate, top-contacts, (b) top-gate, bottom-contacts, and (c) dual-gate OFETs. Reprinted from [54], copyright 2014 American Chemical Society.

Figure 11

Volumetric reconstruction of the Parylene C-coated microscopy glass (left, atop) and calculated amplitude map of the Parylene C/glass interface (left, bottom). Boundary box indicates the size of the volume 2000 × 2000 × 208 µm. Zoom-in image (right). Coating defects and gas chambers are clearly visible. Reprinted with the permission from [68], copyright 2011 Springer.

Figure 12

Volumetric reconstruction of the Parylene C-coated OFET structure (left, atop) and calculated amplitude map of the Parylene C/substrate interface (left, bottom). Boundary box indicates the size of the volume 2000 × 2000 × 73 µm. Zoom-in image showing interfaces of 2 µm thin polymer layer (right). Reprinted with the permission from [68], copyright 2011 Springer.

Figure 13

Transfer characteristics recorded under ambient conditions of a fullerene transistor without encapsulation (a), encapsulated with 1 μm thick layer of Parylene C (b) and encapsulated with 0.5 μm thick layer of Parylene C followed by 0.5 μm layer of benzocyclobutene (c). Reprinted with the permission from [69], copyright 2014 Elsevier.