With the advent of devices based on single crystals, the performance of organic field-effect transistors has experienced a significant leap, with mobility now in excess of 10 cm2 V−1 s−1. The purpose of this review is to give an overview of the state-of-the-art of these high-performance organic transistors. The paper focuses on the problem of parameter extraction, limitations of the performance by the interfaces, which include the dielectric–semiconductor interface, and the injection and retrieval of charge carriers at the source and drain electrodes. High-performance devices also constitute tools of choice for investigating charge transport phenomena in organic materials. It is shown how the combination of field-effect measurements with other electrical characterizations helps in elucidating this still unresolved issue.

High-Performance Organic Field-Effect Transistors

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of organic thin-film transistors (OTFTs). In recent years, a commonly observed nonlinearity in OTFT current-voltage characteristics, known as the "kink" or "double slope," has led to widespread mobility overestimations, contaminating the relevant literature. Here, published data from the past 30 years is reviewed to uncover the extent of the field-effect mobility hype and identify the progress that has actually been achieved in the field of OTFTs. Present carrier-mobility-related challenges are identified, finding that reliable hole and electron mobility values of 20 and 10 cm2 V-1 s-1 , respectively, have yet to be achieved. Based on the analysis, the literature is then reviewed to summarize the concepts behind the success of high-performance p-type polymers, along with the latest understanding of the design criteria that will enable further mobility enhancement in n-type polymers and small molecules, and the reasons why high carrier mobility values have been consistently produced from small molecule/polymer blend semiconductors. Overall, this review brings together important information that aids reliable OTFT data analysis, while providing guidelines for the development of next-generation organic semiconductors.

/pdf/recent-progress-in-high-mobility-organic-transistors-a-1h1mhcglxp.pdf

Recent Progress in High-Mobility Organic Transistors: A Reality Check.

Organic field-effect transistors hold the promise of enabling low-cost and flexible electronics. Following its success in organic optoelectronics, the organic doping technology is also used increasingly in organic field-effect transistors. Doping not only increases device performance, but it also provides a way to fine-control the transistor behavior, to develop new transistor concepts, and even improve the stability of organic transistors. This Review summarizes the latest progress made in the understanding of the doping technology and its application to organic transistors. It presents the most successful doping models and an overview of the wide variety of materials used as dopants. Further, the influence of doping on charge transport in the most relevant polycrystalline organic semiconductors is reviewed, and a concise overview on the influence of doping on transistor behavior and performance is given. In particular, recent progress in the understanding of contact doping and channel doping is summarized.

Doped Organic Transistors.

Contact engineering in organic field-effect transistors

Device physics of organic electrochemical transistors

Frequency response measurements were performed on top-contact pentacene organic thin-film transistors (OTFTs). The ac characteristics of OTFTs are strongly limited by the contact interface between a metallic electrode and a pentacene thin film. Local doping of a charge transfer molecule (CTM) into the contact interface efficiently improves the ac characteristics. Metal-insulator-semiconductor capacitance measurements revealed that the interface contains parasitic impedance composed of capacitance and resistance in parallel. The parasitic impedance was suppressed by CTM doping of the interface. The limitation factors for the frequency of operation in such an OTFT are discussed based on the experimental results and an equivalent circuit model.

Frequency response analysis of pentacene thin-film transistors with low impedance contact by interface molecular doping

Multiprobe measurements were performed on tiny single crystals of DMe-DCNQI complexes prepared directly on SiO2/Si substrates. The differences in conduction properties between the micro- and nanocrystals and bulk crystals were observed.

Direct formation of micro-/nanocrystalline 2,5-dimethyl-N,N'-dicyanoquinonediimine complexes on SiO2/Si substrates and multiprobe measurement of conduction properties.

We investigated surface-potential (SP) undulation on thin films of tris(8-hydroxyquinolinato) aluminum (Alq3) using Kelvin probe force microscopy with intermittent photoexposure. SP undulation with a cloudlike morphology of 200–300 nm in lateral size was observed for Alq3 films of 10–200 nm in thickness. A short photoexposure increased the SP undulation approximately twice as that of the unexposed values, while the SP average decreased monotonically. We analyzed the origin of the SP undulation and the mechanism of its photoenhancement based on its morphology, film thickness dependence, and photoexposure dependence. We suggested nonuniform distributions of mobility and charged traps.

Photoinduced evolution of surface-potential undulation observed on vacuum-deposited thin films of tris(8-hydroxyquinolinato) aluminum

Conduction properties of micro-crystals of 2,5-dimethyl-N,N′-dicyanoquinonediimine metal (metal = Ag, Cu) complexes on SiO2/Si substrates

The surface potential (SP) morphology on thin films of tris(8-hydroxyquinolinato) aluminum (Alq3) was investigated with Kelvin probe force microscopy. Thin Alq3 films of 100 nm were prepared on ITO/glass substrates, Au/mica substrates, and n-Si substrates. Cloud-like morphologies of the SP undulation with 200-400 nm in lateral size were observed for all three types of the substrates. New larger peaks were observed in the cloud-like morphologies when the surfaces were exposed shortly to a light, while the SP average was reduced monotonically. The nonuniform distribution of charged traps and mobility was deduced from the SP undulation morphology and its photoexposure dependences.

Surface-potential undulation of Alq 3 thin films prepared on ITO, Au, and n-Si

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