Abstract:
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.