Keywords

1 Brief History and Technological Applications

In 1954 the term organic semiconductor was used for polycyclic aromatic compounds with a molecular structure similar to fragments of a graphite sheet (graphene) by Hiroo Inokuchi, when he confirmed the notion that such organic materials are electrically conductive [1] through a number of careful experiments by himself and other pioneers [2–5]; For history before 1988 see [6, 7]. As such, it is generally accepted that organic semiconductors were discovered in the mid-twentieth century. Following to their pioneering work in this period, much of the research concentrated on revealing the nature of the electrical conduction in molecular single crystals, which exhibited charge carrier mobility values of a few cm2/Vs at room temperature, and even much higher values at low temperature, as shown in the work of Karl et al. (see for example, [8]) So far, the highest mobility (40 cm2/Vs) was reported for rubrene single crystals in organic field effect transistor (OFET) [9]. For development of organic devises pioneering works have also been performed on organic light emitting diode (OLED) [10], organic solar cell (OSC) [11] and OFET [12]. For practical device applications, however, organic semiconductor thin films, comprising evaporated small-molecule compounds [13] or polymers processed from solution, are more viable.

Since organic semiconductors were widely used as photoconductors in copiers and laser printers, they have recently gained increasing attention because of their potential applications in electronic and opto-electronic devices, such as OLED [13–17], OSC [18, 19] and OFETs [16, 17, 20–23]. OLEDs are already used in displays of mobile phones and TV, and are entering the commercial lighting market. As a result of the continuous drive to fabricate organic electronic devices on lightweight, large-area plastic substrates by low-cost processing techniques, organic electronics is fast-tracked for applications that can overcome general energy problems and global warming. Following this trend, OSCs and OTFTs have developed rapidly over the past decade. Many potential applications of OFETs have been demonstrated, ranging from flexible displays [24] and sensor systems [23, 25] to radio frequency identification tags [26], and some of these systems are now close to commercialization.

Organic semiconductors have other unique physical properties that offer numerous advantages compared to their inorganic counterparts: (i) The extremely high absorption coefficient of many organic molecules in the visible wavelength range offer the possibility of very thin, and therefore resource-efficient, photodetectors and solar cells. (ii) Many fluorescent molecules emit light efficiently, where wavelength of the light depends on spatial spread of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states. However, charge transport in organic semiconductors is often limited by low intrinsic carrier density and mobility. Therefore, controlled and stable doping, in analogy to doping of inorganic semiconductors for increasing carrier density, is desirable for reaching higher efficiency of many organic-based devices [17, 27, 28]. In addition, if one succeeds in shifting the Fermi level toward the transport states by doping or control of the interface structure, this could reduce Ohmic losses at contacts, improve carrier injection from electrodes, and increase the built-in potential of Schottky or p-n junctions.

To comprehend the recent progress and expansion of organic electronics we show the progress of efficiency of OSCs compared with different solar cell technologies in Fig. 1.1 (http://commons.wikimedia.org/wiki/File:PVeff(rev130528).jpg). Although the device performance is still not enough, the very rapid improvement of the performance can be seen in Fig. 1.1, which is undeniably related to the progress in synthesis of new functional molecules with desired electronic states, the science of interfaces and the availability of techniques to control film structure and interfaces.

Fig. 1.1
figure 1

Efficiency progress of different solar cell technologies (May, 2013: Source: L.L. Kazmerski, National Renewable Energy Laboratory (NREL), Golden, CO) http://commons.wikimedia.org/wiki/File:PVeff(rev130528).jpg

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Despite of the rapid progress of organic electronics, however, it is not easy to clearly understand essential difference between organic and inorganic semiconductors. In the following sections we summarize universal features of organic crystals/solids and discuss the nature of organic semiconductor. This summary would help to understand what we should study for revealing science of organic semiconductor that is different from inorganic semiconductor.

2 Universal Features of Molecular Crystals as the Nature of Organic Semiconductor

For years, it has been very difficult to answer the question, “what is key difference between organic semiconductor and inorganic counterpart?” As described in Sect. 1.1 there are clear answers from technological points and application use for overcoming global energy and warming problems, which are well known advantages of organic devices comparing with inorganic ones. However, the difficulty of giving the answer exists in scientific points. We think that we are currently on the right way to finding the answer by the recent progress of both of experimental and theoretical studies. Here we summarize characteristics of organic semiconductors, namely those of molecular crystals, to find some peculiar scientific features that cannot be expected in inorganic semiconductors. Molecular crystals, which are in general electrical insulators but have semi-conductive function if they are used as active materials in various organic devices, have following universal features that are very different from inorganic solids. These can be considered as the nature of organic semiconductors and thus offer various new possibilities.

  1. 1.

    Intermolecular interaction in molecular crystals is very weak.

  2. 2.

    Molecular size is very large.

  3. 3.

    Symmetry of the molecular structure is very low (the structure is planar or complicated 3-dimensional structure etc.).

  4. 4.

    Each molecule consists of light elements (mainly H, C, O, N), but the molecular weight is very large [i.e. pentacene (C22H14) MW = 278; C60 MW = 720; metal free phthalocyanine (C32H18N8) MW = 514]. All of these molecules are much heavier than Si (atomic weight = 28) and available heaviest elements (atomic weight ~100)].

These characteristics may result in following properties (for brief summary see Fig. 1.2):

Fig. 1.2
figure 2

The nature of organic semiconductor from universal features of organic molecular crystals [I–IV]. Complicated but important properties of organic semiconductors come out from each of them and/or their interplay. Some of examples are written (see text). The energy diagram illustrates a weakly interacting metal/organic film system, where the interface dipole due to metal-molecule electronic interaction is assumed to be negligible. The organic film is not approximated by a continuum medium, which result in “band bending” like shift due to an energy-level jump at quasi interfaces in the molecular layers after achieving thermal equilibrium of the electron system (see the inset figure and Chap. 4, Fig. 4.1). The narrow bandwidth of HOMO and LUMO levels reflects weak intermolecular interaction (localization of wave functions). The HOMO of pentacene molecule is shown as an example of “bumpy” distribution of molecular wave function

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  • Bandwidth is very narrow, wave function is localized and spatial distribution of frontier orbitals, HOMO and LUMO, are very bumpy. These features make the electronic states very sensitive to tiny changes in the packing structure of molecules and the crystal structure. (see for example Chaps. 2, 3, 4, 7 and 10),

  • There are many local electric dipoles in each molecule, which are related to local chemical bonds and spatial distribution of relevant molecular orbitals. This gives molecular orientation dependent electronic properties [29],

  • Organic semiconductor cannot be approximated by a continuum medium that is widely used in considering the energy levels at the interfaces and band bending in inorganic semiconductors,

  • A weak perturbation (i.e. existence of interfaces with other materials, impurity or charge doping, gas exposure, heating, electric field etc.) may easily mediate imperfectness of molecular packing structure or structural disorder, yielding band gap states (see for example Chaps. 4, 7 and 10). This could be the most important origin of the Fermi level pinning (better to write as quasi Fermi level pinning),

  • Tiny amount of excess changes injected into the packing structure can produce larger changes in the electronic structure than supposed (see for example Chap. 4),

  • A weak perturbation (i.e. doping) may in some cases enhance a wave function localization to result in Mott-Hubbard insulator even at room temperature, i.e. hydrocarbon based conventional organic semiconductors may change into strongly correlated systems (Hubbard U becomes larger than the band width W) (see for example Chap. 5),

  • Energies of local phonons (molecular-vibration related phonons) are much larger than those of nonlocal phonons (crystal phonons). This property is related also to formation of hierarchical polarons, electronic polaron and polalons related to geometrical structure changes [very small polaron/related to molecular vibration, small polaron (size<~ unit cell and related to crystal phonons) and large polaron (size>~ unit cell and related to crystal phonons)]. These polarons have different time scales of polarization/screening effects (see for example Chaps. 3 and 4),

  • Coupling between the molecular-vibration-related phonons and HOMO/LUMO wave functions are very large (see for example Chap. 3).

  • As the transfer integral (t) is so small that electron-local phonon (including molecular vibrations) coupling as well as electron-nonlocal phonon coupling may seriously affect t (see for example Chap. 15),etc.

In addition to the above-described nature, there is another important advantage of using organic molecular systems, namely flexibility in synthesizing new functional molecules. This is the most well known advantage of organic molecules, thus has been successfully used in developing many new organic semiconductor molecules. This flexibility can also offer self-synthesizing function, which is not easy to realize by chemical reaction in solution so far. Evidence was reported recently [31, Chap. 6], where a new possibility to have metallic molecular layer is demonstrated experimentally and theoretically for non-interesting molecules when they adsorb on metal surfaces.

3 What Do We Need for the Next?

One can easily expect that all or some of above-described properties may appear not only in molecular crystals but in various other organic solids including polymers and bio-related molecular systems, thus study of organic semiconductor is of crucial importance not only for understanding organic semiconductor itself but also realizing high performance organic devices and study of electronic functions of polymers and bio materials etc.

Organic semiconductors commonly have strange or peculiar properties, which are originated from the features I–IV (Fig. 1.2), and thus are much different from those of inorganic counterparts. Accordingly, one observes mysterious phenomena that are not easy to understand with the knowledge from physics of inorganic semiconductors. Some of these are discussed in this book.

Also from this reason, we need to learn science of various phenomena in organic systems and to develop new experimental and theoretical methods suitable for organic semiconductors to unravel their mysterious properties and dig out new functions still hidden in organic molecule-based systems (see for example [30, 31]).