Organic molecular materials

Before introducing some reference materials from the myriad of possibihties, let us first define important energy parameters that are schematized in Fig. 1.11. Case (a) corresponds to a metal, where the valence band is filled with electrons up to the Fermi level E-p. This ideal situation corresponds to the T = 0 K limit and the electronic density distribuhon around Ep at finite temperatures will be discussed in Section 1.7. 

According to the definitions given above, semiconductors are characterized by Eg 0. Inorgaific materials are classified as either semiconductors or insulators if 3 eV or E g 3 eV, respectively. However, the MOMs scientific community often refers to insulators for E g 0.1-0.2 e V, which could also be defined as narrow gap semiconductors. Eg can be experimentally determined by optical and transport methods. However, the experimental Eg values obtained by optical methods, opt e.g., by means of absorption/rellection experiments, may differ from those derived 

The combination of a hole polaron and an electron polaron, with binding energies Ep+ and Ep-, respectively, results in the formation of an exciton. Their difference corresponds to Et and is also referred to as the single particle energy gap Egsp - 

Materials with large E ex values would not be indicated for e.g., photovoltaic devices, since the photogeneration of charge carriers would be inefficient, but could in principle be used as light emitting diodes. Section 4.2 will show examples of the experimental determination of the band diagrams of selected organic semiconductors. El is usually obtained from o vs. T measurements, since a oc  

However, the derived value for Ei is usually termed the activation energy E and may contain other contributions in addition to the intrinsic Ei value, such as those arising from the orientation and morphology of the sample. For this reason E is most commonly used. In the case of polycrystalline thin hlms, grain boundaries may have a dominant effect, as will be discussed in Section 6.4, where examples of intrinsically metallic materials behaving as semiconductors will be given. 

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Phthalocyanines 244 and hemiporphyrins 245 and 246 are aromatic systems. Extended conjugation confers special properties to these molecules that make them building blocks for new molecular organic materials with useful electric and nonlinear optical applications (Scheme 85).288... 

As mentioned before, we shall use small molecules to introduce the fundamentals for more complex molecules, the real core of this book, which will be listed in the next section. Such molecules form solids with remarkable properties (metallicity, superconductivity, ferromagnetism, etc.), some of them at ambient conditions or at much lower hydrostatic pressures than those found for H2 and N2, and some technological applications have been already developed, deserving the name of functional materials. Most of the molecules studied in this book are planar, or nearly planar, which means that the synthesized materials reveal a strong 2D structural character, although the physical properties can be strongly ID, and because of this 2D distribution we shall study surfaces and interfaces in detail. In particular, interfaces play a crucial role in the intrinsic properties of crystalline molecular organic materials and Chapter 4 is devoted to them. 

An introduction to molecular organic materials Table 1.1. (cont.)... 

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