Journal of Ravishankar University–B, 33 (1), 24-30 (2020)
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Preparation, Fabrication and Characterization of Sol-Gel
ZnO Thin Films for Organic Solar Cells
Anil Kumar Vermaa*, Swati Sahua, Mohan Patelb and Sanjay
Tiwaria
aPhotonics
Research Laboratory, School of Studies in Electronics & Photonics, Pt.
Ravishankar Shukla University, Raipur-492010 Chhattisgarh, India
bState
Forensic Science Laboratory, Raipur- 492001 Chhattisgarh, India
*Corresponding author:
akverma.prsu@gmail.com
[Received: 10 December 2019; Revised version: 30 August 2020; Accepted: 08 September 2020]
Abstract: In this work, ZnO has been prepared by the sol-gel method and thin films
have been deposited onto the ITO (Indium-Tin-Oxide) coated glass substrates by
spin coating method at different ZnO concentration and spin parameters. For
this, Sol-gel ZnO was synthesized by Zinc acetate dehydrate, 2-methoxethanol
and ethanolamine as a starting material, solvent and stabilizer respectively.
The study of deposition parameters on the structural, optical and electrical
properties of the ZnO thin films was carried out. The Roughness and thickness
were calculated by Profilometer. X-ray diffraction (XRD) analysis of the films
showed the polycrystalline nature of the prepared films. Scanning Electron
Microscopy (SEM) and Atomic Force Microscopy (AFM) was used to describe the surface morphology and optical
properties were studied using UV-VIS-IR Spectroscopy. The fabricated results showed that ZnO thin
films is crystalline
and low-cost techniques with good features that will
be useful for Organic Solar Cells (OSCs) device as an electron transport layer.
Keywords: Sol-gel ZnO,
XRD, SEM, AFM, Organic Solar Cells, OSCs.
Introduction
For deposition of materials, the sol-gel process is a technology
for producing solid materials from small molecules. The technique is very
useful for the fabrication of various metal oxides layers. The process involves
conversion of monomers into a colloidal solution (sol) that acts as the
precursor for an integrated network (or gel) of either discrete particles. Zinc
oxide (ZnO) is an inexpensive, inorganic semiconductor with a wide band gap
having optical transparency in the visible range. It crystallizes in a
hexagonal wurtzite structure (zincite) and ionization of excess zinc atoms in
interstitial positions and the oxygen vacancies. Surface defects play an
important role in the photo catalytic activities of metal oxides as they
increase the number of the active sites The structure of ZnO can be described
as a number of alternating planes composed of tetrahedral coordinated O2−
and Zn2+ stacked alternately along three-axis, as shown in Figure 1.
Various techniques for the preparation of ZnO thin films have been used:
sol–gel method, co-precipitation techniques etc. among various techniques, the
sol-gel approach to be one of the most promising techniques to prepare ZnO thin
films. Some of the most important merits of the sol-gel techniques are: Easy
for synthesis at very low temperature for chemical reaction. These merits make
the sol-gel technique a very attractive preparation method (Yu et al., 2016;
Farooq et al., 2012; Jurablu et al., 2015; Znaidi et al., 2016; Kim et al., 2014; Poongodi et al., 2015). By applying ZnO thin layer in Organic Solar Cells, Quantum dot
solar cells and Dye-sensitized solar cells (DSSCs) is a dominant technology
that has potential as a suitable candidate for the development of third
generation solar cells. It has many advantages of low cost, flexibility,
transparency and ease of fabrication over the traditional silicon solar cells (Verma et al., 2016; Lai et al,. 2015; Patel et al., 2016; Sahu et al,. 2016; Liao et al., 2014)
The performance and stability of conventional and inverted
bulk-heterojunction solar cells and perovskite solar cells with zinc oxide (ZnO) made by different
processes as the electron selective contact are compared to conventional
bulk-heterojunction solar cells. The low temperature processed inverted devices
using ZnO thin films on indium tin oxide plastic substrates showed high power conversion
efficiency. This inverted device structure possessed much better stability
under ambient conditions retaining over 80% of its original conversion
efficiency after many days while the conventional one showed negligible
photovoltaic activity after few days (Morgenstern et Al., 2011; Yuan et al., 2015; Li et al., 2014; Sun et al., 2011).

Figure 1. The wurtzite
structure model of ZnO
Methodology
Materials and Experimental
section
ZnO thin films were synthesized by a new sol gel
techniques as discuss in following processes. We were used to prepare zinc
oxide thin films using zinc acetate dihydrate [Zn(CH3COO)2.
2H2O] of 99.9%, Sigma Aldrich. Zinc acetate dihydrate was dissolved
in 2-Methoxyethanol anhydrous[C3H8O2], 99.8%,
Merck to prepare ZnO sol and Ethanolamine(MEA) [NH2CH2CH2OH],
99.9%, Sigma Aldrich was added drop wise in MEA to become homogenous and
transparent solution.
Indium Tin Oxide
(ITO) coated glass slides were used as substrate for deposition of Zinc Oxide
thin films. The size of the glass slides used as substrate is of 0.25 inch2.
The substrate was cleaned with detergent water, DI water, Acetone and IPA for
about 10 minutes each and then subsequently done Plasma Ashing to remove oxide
contamination for the surface. After cleaning the substrate was put into the
hot plate for heating at 100 0C for 10 minutes to remove containment
(water or solvent) on the substrate. Magnetic stirring was done for 2hr at 50 0C
then the solution was allowed to aged for 24 hrs in room temperature. Three
different solution was prepare by varying the sol concentration of 0.1, 0.2,
0.5 Molar concentration respectively for studying of thickness optimization.
After that the aged solution was spin coated on the glass substrate at
different Speed and time for optimized thickness and surface morphology. After
deposition the samples were transferred on Hot plate at 200 0C for
evaporation of the residual solvent and any other containment. We were also
done for more times to get thick, smooth and uniform surface of thin films for
the application of the Organic Electronics devices like Organic Solar Cells
(OSCs), Organic Light Emitting Diodes (OLEDs) etc.
Results and Discussion
Thickness
Measurement
The thickness optimization is crucial for
developments of efficient devices. So we studied the thickness variation on
different concentration and speed. This was calculated and confirmed by
DektakXT Profilometer to study uniform deposition of ZnO thin film on ITO
coated glass. The results as summarized in Table-1-3. This shows that the
thickness varies on Molar concentration of sol-gel. We have also optimized the
average thickness from 20 nm to 180 nm which is suitable as electron transport
layer for inverted organic solar cells.
Table 1. Molar: 0.1M Spin: 2000rpm /50sec Annealing: 2000C
for 10 minute
Sample Name
|
Thickness-1
(nm)
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Thickness-2
(nm)
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Thickness-3
(nm)
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Average Thickness
(nm)
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ZnO_1
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22
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11
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8
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22
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ZnO_2
|
23
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8
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30
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21
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ZnO_3
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9
|
5
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26
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15
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ZnO_4
|
27
|
28
|
28
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27
|
Table
2. Molar: 0.2M Spin: 2000rpm /50sec Annealing: 2000C
for 10 minutes
Sample Name
|
Thickness-1(nm)
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Thickness-2(nm)
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Thickness-3(nm)
|
Average Thickness (nm)
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ZnO_1
|
181
|
187
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181
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184
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ZnO_2
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179
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132
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173
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161
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ZnO_3
|
169
|
175
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172
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172
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ZnO_4
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186
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159
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178
|
174
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Table 3. Molar: 0.2M Spin: 2000rpm /50sec Annealing: 2000C
for 10 minutes
Sample Name
|
Thickness-1
(nm)
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Thickness-2
(nm)
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Thickness-3
(nm)
|
Average Thickness
(nm)
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ZnO_1
|
26
|
25
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27
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26
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ZnO_2
|
47
|
19
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45
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37
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ZnO_3
|
30
|
32
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28
|
30
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ZnO_4
|
27
|
43
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27
|
33
|
Structural
Analysis and Surface morphology
High Resolution X-ray diffraction (HRXRD) was used
to identify crystalline phases and to estimate the crystalline sizes as shown
in Figure 2 and 3. These results shows the XRD peaks of ZnO thin films on ITO
glass with 0.1M and 0.5M concentration respectively at 200 0C
annealed for 10 minutes each. This shows wurtzite zinc oxide structure.

Figure
3. XRD pattern of 0.1 Molar concentrations ZnO thin film
annealed samples at 200 0C for 10 minute using Spin Coating on ITO
coated glass substrate

Figure
4. XRD pattern of 0.5 Molar concentrations ZnO thin film
annealed samples at 200 0C for 10 minute using Spin Coating on ITO
coated glass substrate
We did the
observation by Atomic force microscopy (AFM) to study the surface topography of
ZnO thin films at different concentration for different surfaces to study the
variation on the films. The result shows indeed an increase of the particle
size, together with an increase of the overall roughness. RRMS (roughness root
mean squared) and Raa (roughness arithmetic average of absolute values) values
have been measured on the 2×2 μm2 and 4×4 μm2. Atomic
force microscopy (AFM) were confirmed the surface topography of ZnO for
different thickness as shown in Figure 4 for 0.1M and 0.5M concentration
respectively.

Figure
5. AFM topography of ZnO thin film on ITO for 0.1M and 0.5M
concentration
Figure 5 and
Figure 6 shows the SEM image of the ZnO thin films prepared by sol-gel
technique. In this figure, the particles prepared with formation of clusters.
Figure 6 and Figures 7 shows the SEM image of the annealed ZnO thin films at
2000C for 10 minutes at different concentration. The ZnO thin films formed were
less agglomerated. The crystallite size of annealed thin films is in the range
of 20-80 nm in diameter.

Figure
5. SEM image of ZnO for 0.1M concentration

Figure
6. SEM image of ZnO for 0.5M concentration
Optical
Characterization
The Optical properties were also studied using
UV-VIS-IR Spectroscopy. The optical Absorbance was observed in the wavelength
range of 300nm to 800nm as shown in Figure 7.

Figure
7. Absorbance spectra of ZnO thin films with
different concentration
A broad spectrum
is observed for the wavelength of about 400 nm in the absorption spectra which
is characteristic of a pure ZnO. There is no other peaks were observed in the
spectrum which shows that we have fabricated a pure ZnO thin films. The sharp
absorption edge (400 nm), is near close to the intrinsic band gap of ZnO. The energy
band gap of ZnO is around 2.4eV calculated from absorbance as shown in Figure
8. This confirmed that it will be great importance towards developments of
organic solar cells and other devices.

Figure
8. Determination of Energy band gap of ZnO
Conclusion
In conclusion,
we have prepared, fabricate and characterize ZnO thin films by a new sol gel
techniques using different concentration and thickness. The XRD and SEM results
show that these thin films are hexagonal wurtzite in phase ZnO with a mean grain
size around 30nm. From AFM topography, it is clear that with increasing
concentration the morphology of the particles changes. The absorbance peak of
UV-VIS-IR Spectroscopy spectrum showed the wide band gap energy of 2.4 eV for
as-prepared sol gel ZnO thin film. Therefore, this will be most applicable
toward the developments of organic electronics device such as OSCs and OLEDs.
Acknowledgement
The
facilities availed from Photonics Research Laboratory; School of Studies in
Electronics & Photonics, Pt. Ravishankar Shukla University, Raipur,
Chhattisgarh (India) is thankful and acknowledged. The Fabrication and
Characterization facilities provided from Indian Institute of
Technology(IIT)-Bombay(India) under Indian Nanoelectronics User Program (INUP)
which is support by DeitY, MCIT, Government of India, is gratefully
acknowledged. The corresponding author
would like to thank Dr. Dinesh Kabra, Faculty Incharge, Organic
Electronics Laboratory, CEN-IIT, Bombay (India) for his guidance and valuable
support during the execution of the INUP
project.
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