For more than 50 years, the mainstream electronics industry has been dominated by thin film transistors, based on inorganic semiconductors. The fabrication of these devices requires high temperature and complex vacuum deposition techniques, which results in high production costs and not suitable for large-area electronics. Organic semiconducting materials offer an attractive alternative because their fabrication processes are much less complex and inexpensive compared to conventional inorganic technology. This Book describes the fabrication and characterization of organic semiconductor based devices such as humidity, temperature, and light sensors, photoelectrochemical cells, electrochemical cells, solid-liquid and surface-type rectifying junctions. Simple and inexpensive novel techniques has been used for the fabrication. Low cost production of these devices is possible by using vacuum evaporation, spin coating, and drop casting technology.This book aims at the fabrication and investigation of different type of novel organic semiconductor devices for electric, electronic and optoelectronic applications.