Organic thin-film transistors with polymer gate dielectric layers and their applications

Abstract

Forthcoming electronic devices are expected to have various unprecedented form-factors such as flexibility, transparency, and wearability. Because conventional inorganic based electronic devices cannot fulfill the requirements for such form-factor demands due to their brittleness and high process temperature, organic thin-film transistors (OTFTs) are regarded as promising candidates for future electronics in those respects. Furthermore, expectations for OTFTs have recently grown high with the steady evolution of their performance

indeed, their channel mobility has exceeded $10 cm^2/Vs$, far larger than that of amorphous Si. To fully realize the possibility of OTFTs and thus that of future electronics, devices are desired to be fabricated with a minimal or no use of inorganic materials

however, the lack of organic insulators with properties comparable to those of conventional inorganic insulating layers has been a major bottle neck. In the initial stage of this work, high-quality ultra-thin fluoropolymer insulating layers are realized from a conventional solution route, with a leakage current as low as ca. $1 nA/cm^2$ at 1.6 MV/cm, by using an appropriate cross-linking agent. Using this fluoropolymer-based layer as a gate insulator (GI) layer, low-voltage stable OTFTs are demonstrated. With the same GI, fully colorless transparent OTFTs are also demonstrated with the help of metal-based multilayer transparent electrodes and wide-gap organic semiconductors based on benzothiophene derivatives. However, these solution-processed polymer insulating layers are often insufficient in terms of insulating strength, production yield, or process compatibility. Hence an alternative dry-process called initiated chemical vapor deposition (iCVD) is introduced. iCVD process is a vapor-phase free-radical polymerization method which utilizes the delivery of vapor-phase monomers to form polymeric films directly on the surface of a substrate. Poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) insulating layers made by the iCVD process exhibits excellent insulating properties comparable to that of inorganic insulating layers: leakage current density of ca. $1 nA/cm^2$ at 3 MV/cm for thicknesses down to ~6 nm

breakdown field intensity of over 6 MV/cm

and excellent yield and uniformity over large areas. Detailed study based on energy band structure, microscopic observation, elemental analysis, and cryogenic conduction mechanism analysis reveals that these highly favorable insulating properties originate from wide-gap nature of the pV3D3 films (band-gap is 8.25 eV) and from their amorphous, defect-free, conformal nature. Low-voltage TFTs with various organic and oxide channel layers are fabricated using iCVD processed pV3D3 GIs

these TFTs show favorable characteristics in all cases, with low gate leakage current, high field-effect mobility, and good operational stability. Also, because the iCVD process produces polymer layers based on physical adsorption at near room-temperature in a solvent-free condition, the polymer insulating layers are efficiently deposited onto various underling layers with little constraints in most cases

for example, top-gated OTFTs, in which GIs are deposited onto organic channel layers, turn out to operate well, indicating the damage-free nature of the iCVD process. Likewise, OTFTs fabricated on various flexible substrates show characteristics almost identical to those of devices fabricated on glass substrates. Furthermore, OTFTs arrays with polymer GIs prepared by the iCVD process on large area plastic substrates show excellent uniformity, which may be attributed to uniform and conformal coating capability of the iCVD process.