ABSTRACT
Until around 1950, organic materials have been recognized as insulators. The first observation of weak electric conductance of an organic material was reported in 1948 [1]. From systematic studies, Akamatu and Inokuchi found that a series of aromatic hydrocarbons with condensed aromatic rings have weak conductivity and named such a class of organic materials "organic semiconductors" [2]. They and Matsunaga also found that doping halogens into aromatic hydrocarbons gives highly conducting organic materials [3]. After their pioneering work, various semiconducting and conducting organic materials have been investigated [4] as well as conducting polymers [5] and superconductors [6]. In analogy to inorganic semiconductors, applications of organic semiconductors to electronic devices, such as light-emitting diodes and photovoltaic cells, have been extensively studied.
