Thickness Dependence of Optical and Electrical
Properties of Zinc Oxide Thin Films
B.P. Kafle1*, B. R. Pokhrel2 and P. Lamichhane1
1Department of Natural Science, School of Science, Kathmandu University, Dhulikhel, Nepal
2Department of Physics, Patan Multiple Campus, Tribhuwan University, Kathmandu, Nepal Serbia
B.P. Kafle, Department of Natural Science, School of Science, Kathmandu University, Dhulikhel, Nepal, E-mail: email@example.com
Received: March 24, 2016; Accepted: April 15, 2016; Published: April 18, 2016
Citation: Kafle BP, Pokhrel BR, Lamichhane P (2016) Thickness Dependence of Optical and Electrical Properties of Zinc Oxide Thin
Films. SOJ Mater Sci Eng 4(1): 1-4.
This study addresses the optical and electrical properties of 200
nm – ca. 500 nm thick pristine zinc oxide (ZnO) films, fabricated
with spray pyrolysis on quartz substrate. Particular attention was
given for achieving high transparency (in visible and near-infrared
region of solar spectrum) and low resistivity, simultaneously, by
fine tuning the thickness of the film. The structural, optical and
electrical properties were studied using X-Ray Diffraction (XRD),
(UV-Vis) Ultraviolet–visible spectroscopy and room temperature Hall
Effect measurements, respectively. The XRD spectrum showed the
polycrystalline films with a preferential orientation along the c-axis.
The transmittance of the ca. 225 nm thick ZnO film, particularly, at
the wavelength 425 nm, was approximately 30 percent higher fold lower than the
one with thickness ca. 476 nm. While at ca. 600 nm and above, the
transparency of the later film diminished only by 15%. The band
gap of ZnO thin film, derived with the aid of measured transmittance
curves employing Swanepoel approximation, was ca. 3.26 eV and was
almost independent to the film’s thickness (decreased only by a very
amount to ca. 3.22 eV, when the thickness increased to 476 nm). In
contrast, with increase in thickness, film's resistivity decreased and
electron mobility (μ) enhanced remarkably.
Keywords: Transparent thin films; Spray pyrolysis; Optical
properties; Electrical properties
Current PV devices in market, mostly, are made of crystalline
silicon and amorphous silicon based solar cells. However,
because of the considerably high material costs and long energy
payback time, effort has been made to find alternative materials
such as compound semiconductor (for example Gallium arsenide,
(GaAs) [1,2], copper indium gallium selenide [3,4] and thin film
based solar cells [5-7]. But, for practical use, they still require
major breakthroughs to meet the long term goal of low cost, high
stability and short payback time so that the technology can be
affordable for their adoption.
Zinc oxide (ZnO) thin film based dye-sensitized solar
cells (DSSCs) have attracted large attention to the scientific
community of this domain for the past two decades [8-11], due
to possession of similarity of the wide energy band gap (ca. 3.37 eV), but much higher (7 orders of magnitude) electron mobility
to that of Anatase Titania (TiO2). The DSSCs are devised with a
thin film of wide band gap compound semiconductor (as an
Electron Transport Layer, ETL), metal-organic complex dye (as
a sensitizer) and a liquid electrolyte (as a Hole Transport Layer,
HTL). About a micron thick layer of metal oxide, TiO2 or ZnO is
coated on top of ITO or FTO coated glass substrate, on top of
which dye molecules are physically adsorbed. The later layer is
then sandwitched between the counter electrode and TiO2 layer.
HTL layer carries holes produces in the dye molecule up to the
counter electrode. This arrangement creates the large particle dye
electrolyte interface needed for cell operation. In brief,
operating principle of ZnO based DSSCs cell can be understood
as: When photons (from sunlight) hit the photo-sensitive dye
layer, the freed electrons accumulate on the ZnO layer and create
an electrical current which transport to the counter electrode via
load. However, one of the measure problems of the ZnO based
DSSC is still low solar to light conversion efficiency (η).
In this regard, the highest overall η obtained so far for ZnO
based DSSCs is only about 5% -7% depending on the nature of
ZnO composition and dye used for its (ZnO film) sensitization
. In particular, it has been understood that quality of ZnO
film (porosity of ZnO nanoparticles, geometry and surface
morphology) greatly affects the optical and electrical properties
of the film and, eventually, the photovoltaic properties of the cell.
Therefore, the present investigation is aimed to reveal co-relation
of ZnO films’ optical properties with electrical resistances,
which are prepared by a simple and low cost homemade spray
pyrolysis setup on quartz substrate. Also, comparison of above
properties of ZnO films with similar thickness, but synthesized by
two different techniques, spray pyrolysis and spin coating have
Materials and Methodology
Well known spray pyrolysis technique was employed for
fabricating ZnO films on quartz substrate (~2.5×2.5×0.25
cm) . In short, the setup consists of a cylindrical nebulizer
mounted vertically on a horizontal table to spray the aerosol
jet aimed upwards at a substrate through a 5 mm nozzle. The substrate was mounted on a temperature controlled heating
block 25 mm above the nozzle. Humid air at atmospheric pressure
is used as a carrier gas at a 2 - 3 mL/h flow rate. The precursor
solution consists of 0.2 M zinc acetate dissolved in distilled
water and ethanol (at ratio of 1:3) and quartz plates were used
as substrates, purchased from Merck, Germany. Spraying times
was variable to control the thicknesses of the film. Alternatively,
thicker films can be obtained in shorter times by increasing the
concentration of the solution. After deposition, samples were
annealed in a standard furnace 450 °C for 1h in air. During the
time of deposition the substrate temperature was maintained
around 425 °C ± 10 °C. The substrate temperature was measured
by IR thermometer (spectral response 6214 μm).
For comparison, we also employed spin coating technique for
ZnO thin film preparation following our previous contribution
, in which zinc acetate dehydrate Zn(CH3
2-methoxy ethanol and monoethanolamine (MEA) were used as
a starting material, solvent, and stabilizer, respectively. The aged
precursor solution was dropped onto the surface of ordinary glass
substrate or a conducting side of FTO glass substrate and placed
on the holder of the spin coater set at a rotating speed of 3000
rpm for 60 seconds. After deposition, the substrate was dried
at 350 for fifteen minutes in a muffle furnace to completely
evaporate solvent and organic residuals. The coating and drying
processes were repeated for ten cycles for each sample. Then,
finally, the samples were annealed at a temperature of 450 for
Transmission measurements were performed on films
supported by quartz substrate using Genesis-10 UV Spectrophotometer
(Thermo Scientific, UK) in the wavelength range
from 310 to1100 nm from which the thickness (d) and optical
) of the film were estimated. Specifically, the d
value was evaluated by employing the well-known Swanepoel
method and the Eg
was estimated by the plot of (αhv)2
photon energy (hv), where a is the absorption coefficient. The
later parameter can be related with measured transmittance
(T) and thickness of the film as,
The electrical properties were characterized with Four Probe
Results and Discussions
The structural characterization of ZnO thin films was studied
by the X-ray diffraction (XRD) pattern recorded by Broker D2
Phaser, Germany (λ = 1.5418 °A). The XRD JCPDS peaks were
indexed by comparing the peak position with XRD data profile
for ZnO thin film. Figure 1 shows the XRD pattern of thin film
prepared with 20 min deposition time on quartz substrate at
substrate temperature of 420°C.
The peaks at 2θ = 30.5°, 36.5°, 38°, 49°, 58°, 64.5° and 69.5° are
associated with crystal orientations (100), (002), (101), (102),
(110), (103) and (112) respectively. The two unknown peaks at 2θ = 40° and 44° were also observed. Specifically, the appearance
of two intense peaks at 2θ = 36.5°, 38°, corresponding to the
(002) and (102) planes, infer that the crystallites of the pure ZnO
thin films are strongly oriented along c-axis (perpendicular to the
plane of the substrate) with hexagonal structure.
Figure 1: XRD pattern of ZnO thin film.
ZnO films thickness dependence of transmittance
Figure 2A shows precursor solution deposition time
(proportional to the thickness) dependence transmittance of
quasi transparent ZnO thin films as a function of wavelength (λ).
One can clearly notice, in the shorter wavelength region (380
nm – ca. 500 nm), a gradual decreasing trend of transmittance
curves with the deposition times of the precursor solution. For
example, the transmittance value at λ ≈ 420 nm (the first peak
position) of the film obtained after depositing precursor solution
for 12 min was found to be about 92%, while for the film with
20 min deposition time the transmittance decreased down to
65 %. Moreover, the presence of noticeable oscillations in the
transmittance versus wavelength spectra indicates that the
films are of good quality  and it also allow estimating the
thickness of a film with the help of minima and maxima in the
measured transmitt [14-15]. The thicknesses of the films were
extracted by using Swanepoel method (an envelope was drawn
using the maxima and minima of each curve and also used value
of refractive index of quartz glass was 1.46). The estimated
values, listed in Table 1, show increment of the film’s thickness
with the elevation of deposition time of the precursor solution
Figure 2B compares the transmittance curves of ZnO films
of similar thicknesses fabricated with two different methods
(spray pyrolysis and spin coating) as a function of wavelengths.
The curves with solid red and blue lines, respectively, represent
the transmittance curves for films prepared by spray pyrolysis
(deposited for 12 min.) and spin coating (10-cycle at 3000 rpm).
As indicated in Table 2, both the films have very close thicknesses
and one can clearly notice that even the films fabricated by two
different techniques (but with approximately equal thicknesses)
show similar values of transmittance for all the wavelengths of
the investigated. Also, included in Figure 2B is of curve (green line) with 18 min deposition time of which electrical properties
were compared with above two films (discussed below in the
Figure 2A: Precursor solution deposition time dependence of transmittance
as a function of radiation wavelength.
Figure 2B: Transmittances as a function of radiation wavelength of ZnO
films prepared by spray pyrolysis and spin coating.
The plot of (αhν)2 versus hν Figure 3 is linear over a wide
range of photon energies, indicating a direct type of transitions.
The intercepts of extrapolated) lines and abscissa (the energy
axis) reflect the energy band gaps (Eg). The extracted Eg values
from Figure 3 are shown in Figure 4, which indicate a very
small variation on optical band gap with thickness of thin film.
Although there is monotonic decrease of Eg values with increasing
thickness, it may be concluded that there was almost no effect of
alteration of film's thickness on optical band gap.
Thickness dependence of electrical properties
The relationship between film thickness and electrical
properties was studied, with Room Temperature Hall Effect measurements using a Keithley Hall setup, for both kinds of films
deposited with spray pyrolysis and also for a sample with spin
coating (see Figure 2B) In particular, Hall effect measurements
were carried out only for films with thicknesses 265 nm and 435
nm (prepared by spray pyrolysis), and also for a film thickness
262 nm (prepared by spin coating). And observed results are
summarized in Table 2. As shown in Table 2, the significantly
low resistivity value of 22.86 Ωcm was measured for film with d
= 435 nm compared to other two samples: For film with 265 nm
the resistivity was quite high, as a result, Hall measurement was
impossible. While for the spin coated film (thickness - 262 nm)
the resistivity was found to be 32.7 Ωcm (0.031 Ω-1cm-1) which
is about 30 percent higher compared to resistance of former film
(film with thickness 435 nm). In contrast, the corresponding
carrier mobility was quite low for film with thickness 435 nm
compared to film with thickness - 262 nm. This may be due to
difference in fabrication method.
The thickness dependence optical and electrical properties
of transparent thin film of zinc oxide on quartz substrate have
been investigated. Clear co-relation was observed between
precursor solution deposition time and film’s thickness: The
film’s thickness found to increase with the deposition time, while the transmittance found to diminish only ca. 15% above 600 nm
and in the near IR region when the film’s thickness increased by
two fold. Also, there was almost no effect on optical band gap
of change in film’s thickness. Electrical conductivity of the film
found to enhance from almost zero for film with thickness 265
nm to ca. 0.044 Ω-1cm-1 for 435 nm.
Table 1: Thicknesses of ZnO films calculated with Swanepoel method
Precursor solution deposition time (minutes)
Figure 3: (αhν)2 as a function of photon energy, hν, (in eV). For estimating
band gap linear fit was made near the rising edge of each curve and
extrapolated up to the abscissa.
Table 2: Thicknesses of ZnO films calculated with Swanepoel method.
High resistivity layer. Hall measurement was impossible
*SP, spray pyrolysis, *SC, spin coating
Figure 4: Variation in band gap with film thicknesses.
The authors would like to thank for supporting this research
project to University Grant Commission (UGC) of Nepal and Third
World Academy of Science (TWAS). Also, BPK would like to thank
Professors M. Godlewski of Polish Academy of Sciences and A.
Huczko of Warsaw University, Poland for their generous support
for sample analysis.
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