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European Journal of Applied Sciences – Vol. 11, No. 6
Publication Date: December 25, 2023
DOI:10.14738/aivp.116.16021
Onsate, W. N., Mugwang’a, F. K., & Karanja, J. M. (2023). Effect of Blending Baobab and Neem Leaves Extract on Optical Band Gap
Energy for Solar Cell Applications. European Journal of Applied Sciences, Vol - 11(6). 395-402.
Services for Science and Education – United Kingdom
Effect of Blending Baobab and Neem Leaves Extract on Optical
Band Gap Energy for Solar Cell Applications
Wislay Nyangau Onsate
Department of Physics, Pwani University, P. O. Box 195-80108, Kilifi, Kenya
Fanuel Keheze Mugwang’a
Department of Physics, Pwani University, P. O. Box 195-80108, Kilifi, Kenya
Joseph Muna Karanja
Department of Pure and Applied Sciences, Kirinyaga university. P.O. Box 143-
10300, Kerugoya, Kenya
ABSTRACT
The effect of blending bio-dye extracts from neem and baobab tree leaves on the
optical bandgap energy has been investigated. The change in Eg in response to
blending at 33%, 50% and 67% by volume of baobab to neem extract has been
determined and explored using UV-Vis spectroscopy and Tauc’s approximation
method. The individual bandgap energies for pure baobab and pure neem extracts
were identified as ca. 1.65 eV and ca. 1.76 eV, respectively. The bandgap energies
for the blends obtained were 1.718 eV for 1:1, 1.778 eV for 1:2, and 1.693 eV for 2:1
blend. This observed shift in bandgap energy signifies an enhancement in the
optical characteristics of the resultant dyes. Consequently, the blended extracts
exhibit superior optical absorption properties compared to their parent dyes,
positioning them as more effective sensitizers for dye-sensitized solar cells. The
findings underscore the potential of these blended bio-dye extracts in advancing
the efficiency of solar cell technologies.
Keywords: Chromophore, bandgap energy, optical absorbers, neem, baobab, dye
sensitized solar cell.
INTRODUCTION
In the quest for sustainable and eco-friendly energy solutions, the convergence of technology
and nature has led to innovative approaches in the field of solar cell applications. The third
generation of photovoltaics, that include and not limited to multijunction photovoltaic cells,
tandem cells, perovskite cells, polymer cells, hybrid and dye sensitized solar cells (DSSCs), is
seamlessly evolving. Research in the field of photovoltaics has identified organic
heterojunctions [1] and hybrid solar cells [2] as highly active top contenders for cost-effective
solar energy generation. Within this category of solar cells, one notable example that employs
nanostructured materials is the DSSC [3]. In 1988, Grätzel and his colleagues showcased, for
the very first time, the functionality of a dye-sensitized solar cell (DSSC) by emulating the
process of photosynthesis observed in plants. The DSSC used an optically transparent titanium
oxide (TiO2) film that was 10 μm and was coated with a monolayer of dye as a sensitizer for
light harvesting [4]. Natural dyes, derived from various plant and microbial sources, have
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European Journal of Applied Sciences (EJAS) Vol. 11, Issue 6, December-2023
emerged as a promising avenue for enhancing the efficiency and environmental sustainability
of solar cells. Furthermore, the research on nanosized materials application within this class of
solar cells shows potential for further extension [3]. Since the fabrication of natural dye based
DSSC is much cheaper than that of the conventional silicon solar cells (SC), and majority of its
components are more environmental friendly, they have become a major research target for
green energy production [5]. With 9.6% efficiency recorded in 1993, Grätzel set the basis for
the development of DSSCs. This efficiency was later improved to 10% at the National
Renewable Energy Laboratory in 1997 [6].
Theoretically, the dye sensitized SCs’ speculated solar to electric transformation efficiency was
approximated at approximately 20.25% [7]; therefore, over the years, considerable research
has been done on dye sensitized SCs attempting to increase their conversion efficiency and to
broaden its commercialization. Since the DSSC mimics the photosynthesis process which is
considered highly efficient, the development and improvement of this SC has the possibility of
advancing and even attaining greater efficiencies [8]. Unlike conventional synthetic dyes,
natural dyes offer several advantages, including biodegradability, low toxicity, and renewable
sourcing. This synergy between renewable energy and the natural world holds the potential to
revolutionize the solar cell industry, paving the way for greener and more efficient energy
production. Arjmand et al. determined the bandgap energies of inorganic-organic complexes
based on cobalt for DSSC application using UV-Vis spectroscopy, cyclic voltammetry (CV)
analysis and DFT calculations [9]. In addition to other findings they acquired, it was established
that both bandgap energy and energy levels indeed do exert a significant influence the
performance of any photovoltaic cell [9]. Studies have been done on the use of natural dye
extracts for sensitization of dye sensitized solar cells. In 2015, Sahare et al. used Azadirachta
indica leaf extract adsorbed in 30±10 nm of TiO2 nanoparticles as the semiconductor. They
reported VOC of 0.538 V with power conversion efficiency (PCE) of 2.81% [10]. In the same year,
Swarnkar et al. used chlorophyll extracts from Azadirachta indica leaves and anthocyanin
extract from poinsettia bracts to fabricate dye sensitized SCs that yielded VOC of 0.404 V and
0.406 V with fill factor (FF) of 40.1% and 45.8% respectively [11]. The limiting factor hindering
the use of bio-extracted dyes is their low overall power conversion efficiency. This limitation
arises due to the inherent specificity of their absorption spectra. Understanding how to fine- tune the absorption spectrum of these dyes, will help broaden it to cover the essential sections
of the light spectrum thereby boosting their light harvesting capabilities. The effect of using dye
composites obtained from different plant extracts has been researched too. A case study of
Pratiwi et. al. (2017) used a blend of chlorophyll (moss) and anthocyanins (mangosteen peels)
to sensitize TiO2 nanoparticles and the results showed that the PCE improved when the
composite is used as compared to when single dye is used (PCE: chlo-DSSCs - 0.049%, antho- DCSS - 0.042% and blend 0.154%) [12]. Kabir et al. (2019) used extract combo from red spinach
(red) and turmeric (yellow) as sensitizer for a dye sensitized SC in the ratio 2:3. The PCE for
single dyes were found to be lower compared to that of the blended dye (PCE: red-DSSCs -
0.378%, yellow-DCSS - 0.134% and blend 1.078%) [13]. These findings collectively underscore
the notion that combining different dyes can significantly enhance the power conversion
efficiency (PCE) of these solar cells.
From the studies above, it is clear that using more than one type of dye, either side by side or
as a blend, improves the power conversion efficiency of a solar cell. Therefore, understanding
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Onsate, W. N., Mugwang’a, F. K., & Karanja, J. M. (2023). Effect of Blending Baobab and Neem Leaves Extract on Optical Band Gap Energy for Solar
Cell Applications. European Journal of Applied Sciences, Vol - 11(6). 395-402.
URL: http://dx.doi.org/10.14738/aivp.116.16021
the effect of blending these dyes on the photo-absorption properties of the resultant blend is
essential. By doing so, we will be able to predict the performance of any given solar cell based
on the characteristics of the parent dyes. The low power conversion efficiencies recorded by
the DSSC are attributed to the narrow absorption spectrum of the dyes, high charge
recombination and low charge mobility within the semiconductor nanoparticles, using
different dyes and also blending different dyes to increase the photo absorption bandwidth
[14]. The idea of blending dyes that absorb at different wavelengths to broaden the absorption
of the sensitizer need extensive understanding.
The characterization of a material's electronic characteristics depends heavily on the optical
bandgap energy. The material's capacity to both absorb and emit light is directly proportional
to the size of the bandgap [15]. Due to their use in optoelectronics, photovoltaics, and solar
energy conversion, materials' optical properties have drawn a lot of attention. Natural dyes
seem to be highly advocated for due to their cheap means of obtaining them. This study,
therefore, focuses on addressing the effect of blending leaves extract with neem leaves extract
on the optical bandgap of the dyes at different volume ratios. The choice of these dyes lies on
the findings of studies that show a trend of neem leaves extract exhibiting high JSC but at a
compromise of the VOC [10], [11] while SCs sensitized with baobab extract have high VOC but
very low JSC [16].
EXPERIMENTAL DETAILS
Dye Extraction Process
Dye extraction from both baobab and neem tree leaves followed the process shown in Figure 1.
Healthy baobab and neem leaves were collected, cleaned using running tap water to remove
dust and other particles. The leaves were then rinsed with distilled water followed by 40%
ethanol to act as a drying agent. The leaves were left to dry off in a covered container. They
were ground separately using a ceramic mortar and pestle (baobab and neem leaves ground
using different apparatus to avoid cross contamination) while adding 10 ml of 99.9% acetone
(CAS number: 67-64-1 purchased from Euro Industrial Chemicals Limited) dropwise until a
smooth paste was obtained.
Figure 1: A diagram showing steps followed to obtain dye extracts from baobab and neem tree
leaves.
The paste was then transferred to a beaker and 50 ml of absolute acetone was added, then
stirred using a magnetic stirrer so as to dissolve chlorophyll into the acetone. The solution was
then filtered using 6 μm Whatman filter paper (part number CF15015 purchased from Science
Lab Limited). The filtrate obtained was then stored in brown specimen bottles wrapped in