Organic Semiconductors

History With the invention of the transistor around the middle of the last century, inorganic semiconductors such as Si or Ge began to take over the role as the dominant material in electronics from the previously dominant metals. At the same time, the replacement of vacuum tube based electronics by solid state devices initiated a development which by the end of the 20th century has lead to the omnipresence of semiconductor microelectronics in our everyday life. Now at the beginning of the 21st century we are facing a new electronics revolution that has become possible due to the development and understanding of a new class of materials, commonly known as organic semiconductors. The enormous progress in this field has been driven by the anticipation of novel applications, such as large area, flexible light sources and displays, low-cost printed integrated circuits or plastic solar cells from these materials. Strictly speaking organic semiconductors are not new. The first studies of the dark and photoconductivity of anthracene crystals (a prototype organic semiconductor) date back to the early 20th century. Later, triggered by the discovery of electroluminescence in the 1960s, molecular crystals were intensely investigated by many researchers. These investigations established the basic processes involved in optical excitation and charge carrier transport (for reviews see, e. g., [1–3]). Nevertheless, in spite of the principal demonstration of organic elec-troluminescent diodes, there were several drawbacks preventing practical use of these early devices. For example, neither sufficient current and light output nor satisfying stability could be achieved. The main obstacles were the high operating voltage as a consequence of the crystal thickness (in the micrometer to millimeter range) together with the difficulties in scaling up crystal growth as well as preparing stable, injection-efficient contacts to them. Since the 1970s, the successful synthesis and controlled doping of conjugated polymers established the second important class of organic semiconductors, which was honored with the Nobel Prize in Chemistry in the year 2000 [4]. Together with organic photoconductors these conducting polymers have initiated the first applications of organic materials as conductive coatings [5] or photoreceptors in electrophotography [6]. The interest in undoped organic semiconductors revived in the 1980s due to the demonstration of an efficient photovoltaic cell incorporating an organic hetero-junction of p-and n-conducting materials [7] as well as the first successful fabrication of thin film transistors from conjugated polymers and oligomers [8–10]. The main impetus, however, came from the demonstration …