Synergistic Photomodulation of Capacitive Coupling and Charge Separation Toward Functional Organic Field‐Effect Transistors with High Responsivity

Abstract

effectively built by interface engineering because the nature of the interfaces that are ubiquitous in OFETs plays an important role in the performance and stability of devices. [ 8 ] Among different interfaces, the dielectric/semiconductor interface is particularly vital as, under an applied electric fi eld, charge carriers are directly generated and transported at this interface. Hence, the polarity, charge distribution, and surface roughness of the dielectric/semiconductor interface can dramatically affect the device performance. Besides these factors, intrinsic characteristics of dielectric materials, such as permittivity, can also infl uence the carrier transport in the semiconductor. Therefore, it is essential to modify the dielectric/semiconductor interface and functionalize dielectric layers for providing straightforward methods to tailor the carrier density and/or install new functions. [ 9 ] In this study, we demonstrate such a general approach to fabricate a new type of organic phototransistor, which is capable of reversibly photomodulating the carrier density at the dielectric/semiconductor interface; this phototransistor was constructed using photochromic spiropyran (SP)-methyl methacrylate (MMA) copolymers (hereafter referred to as SP-co -MMA) as the photoresponsive gate dielectrics ( Figure 1 ). SP is a type of organic chemical compounds known for its photochromic properties; [ 10 ] it was chosen as the photosensitizer because SPs can revert back-and-forth between a neutral, closed form (SP-closed) and a zwitterionic, open form (SP-open) when exposed to light of different wavelengths. Such a unique conformational transition caused by the photoisomerization of SP molecules leads to a signifi cant modulation of the electric dipole moment ( P mol ) (≈6.4 D of SP-closed and ≈13.9 D of SP-open), [ 11 ] thus forming the basis of the development of novel photoswitchable sensors and functional optoelectronic devices. [ 12 ] In this case, reversible P mol changes of SPs in polymer dielectrics result in two distinct capacitance values that induce different capacitive coupling. In addition, under UV irradiation, SPs at the dielectric/semiconductor interface could change from a neutral form (SP-closed) to a charge-separated form (SP-open), thus producing scattering sites that can trap the photoexcited electrons and correspondingly leave hole carriers in the conductive channel. A synergistic combination of both effects realizes the photomodulation of the threshold voltage ( V T ) values, and thus channel conductance. This study is established from previous cases where SP was simply mixed with poly(methyl methacrylate) (PMMA) as the gate dielectric; unfortunately, the SP concentration was signifi cantly limited by its low solubility in PMMA. [ 9d,f ] The important distinction in this study is that the use of SP-co -MMA copolymers allows for the substantial tuning of the SP ratios in polymer dielectrics, thus signifi cantly Organic fi eld-effect transistors (OFETs) are the basic building blocks in numerous f...

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

“…To our delight, the FET with P206 displays the fastest photoresponsiveness among the reported optically tunable FETs . This can be attributed to i) the azobenzene units being covalently linked to the flexible alkyl chains and separated by branching alkyl chains in P206 and, as a result, the photoisomerization can be carried out easily, and ii) the active layer in the device contains only the thin film of P206 without further blending of an additional photoresponsive component and the need for interface modification . It should be noted that physical blending and interface modification with photochromic molecules were reported previously for photoresponsive devices .…”

An Overview of the Development of Flexible Sensors

“…Phototransistors, pioneered by William Shockley in 1951, are three‐terminal devices; a phototransistor uses a stream of photons as an additional terminal. They are able to realize functions of light detection, photomodulation, electric‐field‐controlled switching, and magnification of signals in a single device . Particularly large gains are achievable through transistor action because the application of a gate bias causes a depletion zone where efficient separation of light‐induced excitons into electrons and holes can occur.…”

Recent Progress in Organic Phototransistors: Semiconductor Materials, Device Structures and Optoelectronic Applications

“…Apart from the development of high‐mobility and photoactive organic semiconductors as well as new device architectures, functional high‐performance OPTs can be built by dielectric/semiconductor interface engineering. Since the charge carriers are generated and transported directly at the dielectric/semiconductor interface under an applied electric field, the surface roughness, polarity and charge distribution of this interface can dramatically affect the performance of OPTs . In addition, intrinsic characteristics of dielectric materials, for instance, permittivity, also have significant influence on the carrier transport in semiconducting layer .…”