Materials for organic electronics and bioelectronics

In this chapter, the principles behind the mechanism of operation of organic transistors are described, as they are both important in their own right and also a representative example of organic electronics. Accordingly, the recent progress in organic transistors will be explored in depth. The term “organic transistor” generally refers to the field-effect transistor (FET), which uses thin films of organic materials. The study of these “organic FETs” began in the 1980s.¹ Initially, the available fieldeffect mobility (µ), which is one determinant of the performance of an FET, was ca. 10^-5 cm² V^-1 s^-1, a value that is insufficient for practical applications. In the late 1990s, it was found that pentacene, in which five benzene rings are connected in a linear structure (Fig. 8.1.1), and C₆₀, in which 60 carbon atoms form a spherical structure, were good candidates for the active layer of an FET device. The µ value became ~1 cm² V^-1 s^-1 in FETs using these molecules.^{2, 3} This µ value raised the expectation that organic FETs might in the future become practical devices. On the other hand, since the year 2000, some work on FETs has demonstrated that the physical properties of materials can be controlled by carrier accumulation, such as the field-induced carrier doping of C₆₀ for inducing superconductivity. These studies were not the case, but the interest in field-induced carrier doping to control the physical properties of materials has attracted much attention from researchers in solid-state physics. Consequently, some control of the physical properties of inorganic materials has been achieved by field-induced carrier accumulation using a device with the structure of an FET.^{4, 5} Research on organic FETs has been brought to a new stage by the participation of many researchers from other fields, where FET performance was pursued not only to find practical applications, but also to investigate fundamental physics. In 2003, a device with excellent FET properties that used a single crystal was fabricated using rubrene (Fig. 8.1.1).⁶ This showed that transistor operation in an organic FET could be achieved using not only thin films but also single crystals. An FET using a single crystal is superior to a thin-film FET for studying the intrinsic nature of organic materials because many interfering factors such as structural defects, grain boundaries, and impurities can be removed from the active layer. Generally, FET performance is also higher in a single-crystal FET than in a thin-film FET. Clearly, these studies have led to remarkable advances in the research on organic FETs. Furthermore, Hall-effect measurements were achieved in single crystals of organic molecules, ^{7, 8} and angle-resolved photoemission spectroscopy (ARPES) was performed on some organic single crystals.^{9, 10} Based on these results, it was suggested that electron transport in organic singlecrystal FETs occurs by band transport rather than hopping transport. Since then, the idea that electron transport in an organic thin-film FET is explained by simple hopping transport has been modified due to analyses based on the multiple shallow trap-and-release (MTR) model for the temperature-dependence of transport properties in organic thinfilm FETs exhibiting a high µ value.^11-13 The MTR model assumes that the carriers in a band are captured by trap states and thermally excited up to the conduction (or valence) band, i.e. this concept is substantially based on band transport. Furthermore, the reduction of µ value caused by thermal scattering of phonons was determined from the temperature-dependence of µ in organic single-crystal FETs.^{14, 15} This constituted the direct observation of band transport in organic single-crystal FETs. These studies have enabled the full discussion of the mechanism of organic FET operation. An FET using pentacene (Fig. 8.1.1) as the active layer shows typical pchannel operation with Au source/drain electrodes, but shows n-channel operation with Ca source/drain electrodes.16 So an organic FET can operate both as p-channel and n-channel depending on the electrode metal, a flexibility termed “ambipolar”. Thus, an organic FET can operate in an ambipolar way by changing the metal of the source/drain electrodes (or changing the work function of the electrodes), unlike the operation of Si MOSFETs, which can operate in either p-channel or n-channel through the formation of an inversion layer in which minority carriers in the bulk become the majority in the channel region. Also, neither organic thin films nor crystals are doped with any impurities, unlike the Si crystal used in Si MOSFETs, which can be doped with an acceptor (p-type Si) or with a donor (n-type Si) material. In Section 8.1.1, we will fully examine organic FETs based on the background described above.