Organic semiconductors have been the subject of intense scientific investigation for the past 50 years. Due to often weak bonding between organic molecules in the solid state, they share many of the properties of both semiconductors and insulators. Applications of these materials include chemical, physical and biological sensors, electric, electronic, and photonic devices. Low cost production of these devices is possible by using vacuum evaporation, spin coating, and drop casting technology. This work describes fabrication and characterization of organic semiconductor devices such as humidity, temperature, and light sensors, photoelectrochemical cells, electrochemical cells, solid-liquid and surface-type rectifying junctions. Organic semiconductors 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin (TIPP), Cu(II) 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin (CuTIPP), Ni(II) 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin (NiTIPP), and Zn(II) 5,10,15,20- tetrakis(4′-isopropylphenyl) porphyrin (ZnTIPP) are investigated as active materials in surface-type multifunctional sensors. As electrodes, 100 nm thick Ag films are deposited on 25 mm × 25 mm glass substrate with 40.0 μm gap between them. Thin films of TIPP, CuTIPP, NiTIPP, and ZnTIPP of area 15 mm × 15 mm, are thermally sublimed to cover the gap between surface-type silver electrodes. A change in electrical resistance and capacitance of the fabricated devices are observed with the increase of relative humidity, temperature and illumination. Hysteresis, response and recovery times are investigated over a wide range of relative humidity. Activation energies of the porphyrins are estimated. The photoelectrical behavior of n-InP/Orange dye/ITO sandwich type cell has been investigated. In this cell, the n-type indium phosphide and indium tin oxide (ITO) coated glass electrodes are employed and as an electrolyte, 0.5 wt.% solution of orange dye (OD) in distilled water is used. Dark current-voltage characteristics of the cell show rectifying behavior. A zero offset voltage is also observed. Photo-induced open-circuit voltage and short-circuit current, under modulated and unmodulated IR, red, green, yellow and blue LEDs illuminations, are investigated and it is found that the cell is sensitive in the wavelength range of 475–750 nm. It is observed that open circuit voltage decreases and short-circuit current increases with an increase in concentration of the OD viiiAbstract in the solution. Investigation of directivity characteristics showed that the cell is sensitive in wide range of angles. Electrochemical properties of Zn/PVA-I/C, Zn/PVA-KOH/C, Zn/PVA-KOH-I/C, and Zn/OD/C cells have been investigated. In these cells, polyvinyl alcohol (PVA)-iodine (I), orange dye (OD) aqueous solutions and polyvinyl alcohol-potassium hydroxide (KOH), polyvinyl alcohol-potassium hydroxide-iodine aqueous gels have been used as electrolytes, while zinc and carbon rods serve as electrodes. The current-voltage characteristics and open circuit voltage-time, charge voltage/current-time, and discharge voltage/current-time have been studied. Effects of concentration of OD in electrolyte have been examined on the discharge voltage–current, charge voltage/current–time and discharge voltage/current–time characteristics. The open-circuit voltages and short-circuit currents of the cells are observed as dependent on the orange dye concentration. The efficiency of the current discharge/charge is found larger at higher concentration of the dye. The solid-liquid junction characteristics of Al/OD aqueous solution/ITO and Al- CuPc/OD aqueous solution/ITO sandwich type cells are investigated. In these cells, the aluminum glass slide, copper phthalocyanine (CuPc) coated aluminum glass slide and indium tin oxide (ITO) coated glass slide electrodes are employed and as an electrolyte 0.25 wt.% solution of orange dye in distilled water is used. Dark current-voltage characteristics of the cells show rectifying behavior. The parameters, like rectification ratio, reverse saturation current, ideality factor and junction resistances are calculated. For interpretation of charge transport mechanism, dark I-V characteristics are analyzed by modified Shockley equation and space-charge limited currents approach. A surface-type, Ag/OD-MO/Ag diode is fabricated by drop-casting orange dye and methyl orange blend thin film from 1.0 wt.% aqueous solution, under the effect of an electric field, on a glass substrate with preliminary deposited silver electrodes. It is found that the dark I-V characteristics exhibit strong rectifying behavior though both electrodes are from same metal.