Optical and Surface Morphology Study of NiO Thin Films for Optoelectronic Applications
Anil Rakshe1,2, Satish N. Vaishnav3,
and Sudam D. Chavhan1*
1Department
of Physics, V. V. M.’s Sitaram Govind Patil Arts, Science and Commerce College,
Sakir, Dist.-Dhule. Maharashtra, Pin-424304, India
2Nutan
Maharashtra Institute of Engineering & Technology, Pune,
Maharashtra-410507, India
3Department
of Chemistry, Sardar Vallabhbhai Patel Arts and Science College, Ainpur Tal.
Raver, Dist.-Jalgaon, Maharashtra, Pin-425507, India.
Corresponding
author:sudam1578@gmail.com
Abstract
Nickel
oxide (NiO) thin films were deposited on indium tin oxide (ITO) substrates
using a low-cost sol–gel spin-coating technique. The annealing treated NiO
films resulted in uniform and optically
transparent films. UV–visible absorption studies showed strong ultraviolet
absorption and 90 % transparency in the
visible region. The optical band gap, determined from the Tauc plot assuming a
direct allowed transition, was estimated to be 3.35 eV, indicating good optical
quality. Atomic force microscopy revealed a continuous, crack-free surface with
homogeneously distributed nanoscale grains. The films exhibited a
root-mean-square surface roughness of 24.28 nm, which is favorable for
efficient charge transport and improved interfacial contact. The combined optical
transparency and controlled surface morphology highlight the potential of
sol–gel-derived NiO thin films for optoelectronic applications, particularly as
hole-transport layers in solar cells and organic light-emitting diode.
Key
words: NiO Thin Films, Sol-gel method, Optical band gap.
1. Introduction
Nickel oxide (NiO) is p-type semiconductor material that
has attracted considerable attention owing to it’s a wide-optical band-gap i.e
3.4-4.0 eV, excellent chemical stability, and environmentally benign nature. It
is one of the promising materials for optoelectronic devices, particularly
solar cells, Organic Light-emitting diode and transparent transistors [1–3]. Moreover, its low-cost processing feasibility
is more technically appealing to compete with existing materials. And
therefore, it is vitally important to study the optical and surface
morphological features of NiO thin films prepared by low-cost deposition
technique such as, sol-gel, chemical bath deposition and electrochemical
deposition.
The optical performance of NiO thin films, including
transmittance, absorption coefficient, and optical band gap, plays a decisive
role in determining their suitability for transparent and optoelectronic
applications. High transparency in the visible region combined with a sharp
absorption edge is essential for minimizing optical losses in device
architectures [5,6]. At the same time, surface morphological characteristics
such as grain size, surface roughness, and film uniformity critically influence
charge transport, light scattering, interface quality, and surface-related
phenomena [7,8]. Therefore, simultaneous investigation of optical and surface
morphological properties is essential for understanding structure–property
relationships in NiO thin films.
NiO thin films have been fabricated using a variety of
physical and chemical deposition techniques, including sputtering, pulsed laser
deposition, chemical vapor deposition, spray pyrolysis, chemical bath
deposition, and solution-based routes [9–11]. While physical vapor deposition
methods are capable of producing dense and highly crystalline films, they
generally require high vacuum conditions, elevated processing temperatures, and
expensive instrumentation, which limit their applicability for large-area and
low-cost production. Consequently, solution-based chemical techniques have
gained increasing attention due to their simplicity, scalability, and
compatibility with low-temperature processing.
Among solution-based approaches, the sol–gel method
has emerged as a particularly attractive route for the preparation of NiO thin
films. The sol–gel process enables molecular-level mixing of metal precursors,
resulting in excellent compositional homogeneity and precise control over film
stoichiometry [12–14]. In addition, sol–gel deposition techniques such as spin
coating allow uniform film formation over large substrates using relatively
simple experimental setups. The processing parameters such as solvent
concentration, spin coating speed and annealing temperature can be easily
optimized to make more suitable for solar cell and OLED applications [15, 16].
Despite the growing number of reports on NiO thin
films, a comprehensive correlation between optical properties and surface
morphological features for sol–gel-deposited NiO films remains limited. Many
studies focus either on optical characterization or on structural and sensing
behavior, without providing an integrated understanding of how sol–gel
processing parameters simultaneously influence optical response and surface
topology. Such combined analysis is crucial for optimizing film properties and
advancing practical device integration.
Keeping in mind these points, the present manuscript
focuses on the preparation of NiO thin films using the sol–gel method and a
comprehensive investigation of their optical and surface morphological
properties. Particular emphasis is placed on establishing correlations between
sol–gel processing parameters and surface morphology with the aim of providing
insights for optimizing NiO thin films for applications in solar cells and
OLEDs.
2. Experimental
Details
Indium tin oxide (ITO)-coated glass substrates were
used to deposited NiOₓ thin films. The substrates were thoroughly cleaned by following
standard procedure of ITO i.e. ultrasonication
in detergent solution, deionized water, acetone, and isopropyl alcohol for 10
min each. After cleaning, the substrates were dried using a nitrogen stream and
subjected to UV–ozone treatment to remove residual organic contaminants and
improve hydrophilicity of ITO surface. A 0.2 M solution of nickel formate
dihydrate was dissolved in ethylene glycol under continuous magnetic stirring.
To this solution, ethylenediamine was added in a stoichiometric amount
corresponding to two molar equivalents with respect to nickel ions. The
resulting mixture was stirred at temperature > 90 °C for 2 hours. Prior to
film deposition, the solution was allowed to cool to room temperature and
subsequently filtered through a 0.45 µm nylon syringe filter to remove any
undissolved particulates. The filtered solution was dropped onto the ITO
substrates and spin-coated with optimized spin coating recipe. The four layers
of spin coated nickel precursor films were then annealed at 325oC
for 1 hour in muffle furnace to remove organic residue and form the NiO phase.
After annealing the films were treated with UV-ozone light to 15 min to improve
the electronic properties of the deposited NiO films. Optical properties of the
deposited films were measured using UV-Visible spectrophotometer (JASCO v-600).
The surface morphology was carried out using atomic force microscopy (AFM) in tapping mode using a Si tip.
Result and Discussion
Figure 1 (a) shows
the atomic force microscopy (AFM) image of annealed NiO thin film prepared by
sol-gel method. It indicate that formation of nanocrystalline NiO thin films. The
films surface is continuous and without any cracks, confirming good coverage of
the substrate. The
AFM topographical images reveal the presence of uniformly distributed nanoscale
grains across the film surface. These grain-like features are typical of
sol–gel-derived metal oxide thin films and originate from the nucleation and
growth of NiO nanocrystallites during the thermal annealing process.
 |
Figure
1: Atomic force microscopic image of NiO
thin films (a) surface topography and (b) surface roughness analysis.
The films exhibit a relatively smooth surface with low
root-mean-square (RMS) roughness, reflecting the effectiveness of the sol–gel
deposition combined with optimized spin-coating and annealing conditions. Quantitative
roughness analysis, such as average roughness (Rₐ) equals to 17.74 nm and RMS
roughness (Rq) equals to 24.28 nm further confirms the smooth surface
morphology of the NiO thin films as shown in figure 1(b). The low surface
roughness and uniformly deposited NiO films are potentially useful to fabricate
highly efficient organic solar cell and organic light-emitting diodes. The
smooth morphology reduces the density of surface defects and
grain-boundary-related trap states, thereby facilitating efficient hole
transport through the NiO layer. In addition, the uniform surface improves
interfacial contact with adjacent functional layers, which is critical for
minimizing contact resistance and enhancing charge extraction in optoelectronic
devices. These morphological characteristics suggest that the prepared NiO thin
films are well suited for application as hole-transporting layers in solar
cells and organic light-emitting diodes. Therefore, atomic force microscopy
study confirm that the sol-gel method is a reliable and low-cost approach for
fabricating highly uniform NiO thin films.
Figure 2 (a) shows the optical absorption of NiO thin
films grown on ITO substrate by using sol-gel method. The optical absorption
spectra of the NiO thin films annealed at 325 °C for 1h were recorded in the
ultraviolet–visible (UV–Vis) wavelength range to evaluate their optical
behavior. The films exhibit strong absorption in the ultraviolet region, while
maintaining low absorption in the visible range, indicating around 90% optical
transparency in the visible region. This absorption characteristic is typical
of wide band gap p-type NiO thin films. A sharp absorption edge is observed
below 350nm wavelength, suggesting good film quality and improved crystallinity
after thermal annealing. The annealing treatment at 325 °C promotes the
decomposition of organic residues and enhances the formation of the NiO phase,
which results in reduced defect density and improved optical uniformity. The
weak absorption in the visible region confirms that the sol-gel prepared NiO
thin films are one of the alternatives for p-type organic hole transporting
materials and will be more promising candidate to play the efficient role of
hole-transporting material in third generation solar cells and solution
processed organic light-emitting diodes.
The optical band gap of the NiO thin films annealed at
325°C for 1h was determined using the Tauc relation as shown in figure 2(b).
The absorption coefficient (α) was calculated from the measured absorbance, and
the optical band gap energy (Eg) was estimated by plotting
(𝛼ℎ𝜈)2
as a function of photon energy (hν), assuming a direct allowed electronic
transition for NiO. The linear region of the Tauc plot was extrapolated to
intersect the photon energy axis, where (𝛼ℎ𝜈)2
= 0. The Tauc plot exhibits a well-defined linear region, indicating a direct
band gap transition and good optical quality of the NiO thin films. The calculated
optical band gap value was found to around 3.30 eV which is within the range of
reported NiO thin films. The sharpness of the absorption edge and the linearity
of the Tauc plot suggest reduced structural disorder and a lower density of
localized states within the band gap because of the thermal annealing of NiO
films at 325 °C for 1 hour. The annealing process facilitates the removal of
residual organic components and enhances the formation of the NiO phase,
leading to improved crystallinity and electronic structure. As a result, the
observed band gap value is consistent with improved film quality.
Figure 2: (a) Optical
absorption, and (b) Tauc plot of ITO/NiO thin filims annealed at
325oC for 1 hour.
Conclusions
In the present study, pin-hole free and highly uniform
NiO thin films were successfully fabricated by a spin-coating methods. The
annealed NiO films exhibit strong ultraviolet absorption around 350nm
wavelength and 90 % transparency in the visible region. The wide optical band of
3.35 eV obtained from Tauc plots corroborates its special feature of
transparent metal oxide semiconductor properties. Analysis of surface
morphology of NiO thin films via AFM analysis revealed a smooth, uniform
surface with homogeneously distributed nanoscale grains having surface roughness
of around 24.28 nm, which is excellent for efficient charge transport and enhanced
interfacial contact. These results demonstrate that sol–gel-processed NiO thin
films possess the essential optical transparency and surface uniformity
required for hole-transport layers, making them promising for application in
solar cells and organic light-emitting diodes.
Acknowledgment
This paper is respectfully dedicated to the Hon.
Principal Dr. R. R. Ahire of Vidya Vikas Mandal’s Sitaram Govind Patil Arts,
Science and Commerce College, Sakri, on the occasion of his superannuation, in
recognition of his visionary leadership, unwavering commitment to academic
excellence, and lifelong service to education. The authors express their
heartfelt gratitude to Prof. Ellen Moons, Department of Physics and
Engineering, Karlstad University, Karlstad, Sweden, for her invaluable
guidance, constant encouragement, and generous experimental support, which
greatly enriched the present work.
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