Eur. Phys. J. B 47, 171–176 (2005)
DOI: 10.1140/epjb/e2005-00317-x
T
HE
E
UROPEAN
P
HYSICAL
J
OURNAL
B
Electrical and optical properties of vacuum deposited MnPc
thin films
K.R. Rajesh
a
and C.S. Menon
School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam-686560, Kerala, India
Received 24 February 2005 / Received in final form 22 May 2005
Published online 11 October 2005 –
c
EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005
Abstract. Sandwich and planar structures were fabricated using manganese phthalocyanine (MnPc) as
active layer and gold (Au) as electrodes by thermal evaporation method. The permittivity ε of MnPc was
determined from the dependence of capacitance on film thickness. J-V characteristics of Au/MnPc/Au
structure at room temperature were performed. Thermally generated hole concentration p
0
, hole mobility
µ
p
, total trap concentration N
t
and depth of the trap level were estimated. The activation energies of MnPc
films were determined from the Arrhenius plots of ln σ versus 1000/T . The absorption and reflectance
spectra of MnPc thin film deposited at room temperature were recorded in the spectral range 300–900 nm.
The optical band gap of MnPc thin film was determined from the α
2
versus hν graph. The optical constants
n and k were found. The real and imaginary parts of the optical dielectric constant ε
1
and ε
2
were
calculated.
PACS. 72.80.Le Organic semiconductors – 73.61.-r Electrical properties of specific thin films – 78.20.-e
Optical properties thin films
1 Introduction
Molecular electronic materials such as dyes, conjugated
polymers, and small molecules are gaining interest for
applications in solar cell technology. Attractive features
are the possibility of device processing, compatibility with
flexible substrates, and the low materials consumption for
ultrathin molecular films, all of which offer the prospect of
cheaper photovoltaic energy generation. Large-scale pro-
duction is easier than for inorganic materials. The greatest
peculiarity of organic material is that they can be tuned
chemically, in order to adjust separately the band gap, va-
lence and conduction band energies, charge transport, as
well as the solubility or other structural properties. The
research into the behavior of these materials is normally
carried out in two ways, optically and electrically. In opti-
cal method usually optical absorption measurements are
performed. While the conductivity properties are explored
in electrical method. The electrical conductivity measure-
ments can be carried out either using planar or sandwich
devices. Planar devices consisting of thin films with elec-
trodes deposited at the ends are primarily use as gas sen-
sors and also to observe the materials response to various
gases by d.c. electrical characteristics. The sandwich con-
figuration is used in thin film diodes and transistors [1,2].
The mechanism of carrier transport in organic semicon-
ductors and carrier injection from metal electrodes be-
comes the most important subject to be elucidated for
a
e-mail: rajthinfilms@yahoo.co.in
high performance organic thin film devices. Phthalocya-
nines (Pc) are a class of highly stable organic compounds
which are classified as p-type semiconductor character-
ized by low mobility and low carrier concentration [3]. Pc’s
have potential to applications to organic electronic devices
such as gas sensors [4,5] and optoelectronic devices [6,7]
because of their ease of synthetic modification, high chem-
ical and thermal stability [8] long π-electron conjugated
system and absorption bands which extends from ultravi-
olet to infrared region. It is necessary to investigate the
intrinsic electrical properties prior to their utilization. Rel-
atively few studies have conducted on manganese phthalo-
cyanine (MnPc) thin films. In the present investigation
d.c. electrical measurements are carried out in the planar
and sandwich configuration as a function of temperature
to study the electronic structure of MnPc thin films and
to identify the conduction mechanisms operating under
different conditions. Also we have determined the optical
parameters and dielectric constants of MnPc thin films
from their absorption measurements.
2 Experimental
The powdered MnPc procured from Aldrich Inc. USA is
purified by entrainer method. Thin films are deposited at
room temperature and at different substrate temperature
at a base pressure of 10
−6
Torr onto pre-cleaned glass
substrates using the Hind Hivac 12 A4 coating plant by