Preparation of natural dyes from salvia and spathodea for

Preparation of natural dyes from salvia and spathodea forTiO2-based dye-sensitized solar cells (DSSCs) and theirelectrochemical impedance spectroscopic study under light anddark conditions

G M LOHAR1,* , D V RUPNAWAR1, R V SHEJAWAL1 and A V FULARI2

1Lal Bahadur Shastri College of Arts, Science and Commerce, Satara 415002, India2Division of Physics and Semiconductor Science, Dongguk University, Seoul 04620, South Korea

*Author for correspondence ([email protected])

MS received 29 November 2019; accepted 27 March 2020

Abstract. In this research investigation, natural dyes have been prepared from salvia and spathodea flowers for dye-

sensitized solar cells (DSSCs). TiO2 nanosphere-based thin films were prepared using the doctor blade method and the

natural dyes were loaded on TiO2 nanospheres. The electrochemical and photoelectrochemical properties were studied

using electrochemical impedance spectroscopy and J–V measurement, respectively. This study has been used for device

fabrication. The results show that the fabricated dye improves efficiency of solar cells. Electrochemical properties were

studied under dark and light conditions and the optical properties of TiO2-based DSSCs were also studied. The efficiency

of solar cells increases by using the fabricated natural dyes. As far as the advantage of using natural dyes is concerned,

they can be beneficial for future bio-photovoltaic technology.

Keywords. Dye-sensitized solar cell; electrochemical impedance spectroscopy; photo-electrochemical properties.

1. Introduction

To generate electrical energy by employing renewable

energy sources, dye-sensitized solar cells (DSSCs) provide

recompense in heterojunction-based devices. The basic

working of DSSCs is based on the charge transfer mecha-

nism and light absorption at a molecular interface [1,2].

DSSCs renovate solar energy into electricity proficiently at

a low cost. The literature shows that the efficiency of

DSSCs using a ruthenium (Ru) complex is over 11% at the

laboratory level [3]. DSSCs are known as Graetzel cells,

and they use advanced materials in third-generation pho-

tovoltaic devices to improve the efficiency of solar cells [4].

The pigments present in plant leaves excite photons during

photosynthesis progression; in this way, the pigments in the

natural dyes are capable of increasing the efficiency of

DSSCs [4,5]. Artificial dyes are prepared using Ru with

metal complexes, and these dyes can affect production

efficiency by over 10%. Conversely, synthetic dyes are

costly and poisonous [6,7]. The main advantages of the

natural dyes developed from the pigments of plants are they

are organic; are habitually available, their colour can be

modified, are environmentally friendly and can be obtained

at low cost. Recently, many research workers have primed

natural dye-based solar cells [8,9]. Abdel Latif et al [5]

reported the synthesis method of natural dyes from plant

seed pigments and used for increasing the efficiency of

TiO2-based DSSCs. Taya et al [10] reported the method of

extraction of dyes from dried plant leaves and used in

DSSCs. Kushwaha et al [11] employed natural pigments of

plants for DSSCs. By considering the advantages and the

future approach of solar cells, it is necessary to develop

natural dye-based solar cells for clean, environmentally

friendly and cost-effective applications.

Titanium dioxide (TiO2) is an n-type as well as extensive

bandgap material having potential for improving the effi-

ciency of DSSCs. Different organic dyes are used to

improve the efficiency of TiO2-based solar cells. A mole-

cule present in natural dyes absorbs the photons present on

the exterior of a nanostructured TiO2 film. The molecules of

natural dye absorb the photons, generate excited states,

from there electrons are relocated to numerous deserted

semiconductor states, and circulated to apparent TiO2 films

and grasped the electrical connections. At the same time,

charges have relocated in between the oxidized dye and

redox electrolyte. TiO2 has sufficient compensation, toge-

ther with continuing thermal and photostability properties

[4]. Therefore, TiO2 is a superior material as compared to

other metal oxides used in solar cells. Also, photoelectro-

chemical cells have fascinated interest in the field of

investigation as an alternative and low-cost energy source

[12–15]. Therefore, in this study, a report on flower-based

Bull Mater Sci (2020) 43:236 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-020-02180-w Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

TiO2 DSSCs is presented. Natural dyes were prepared from

salvia (botanical name: Salvia divinorum belonging to

family Lamiaceae) and spathodea (botanical name: Spath-

odea campanulata belonging to family Bignoniaceae),

which are freely available in the environment.

The natural dyes fabricated using salvia and spathodea

were used in TiO2-based DSSCs. These natural dyes were

studied using a UV–visible spectrophotometer for under-

standing the absorbing capacity of visible light. After the

loading of natural dyes on TiO2-based DSSCs, the elec-

trochemical property has been studied in the dark as well as

in light using electrochemical impedance spectroscopy

(EIS). Also, the efficiency of DSSCs has been calculated.

2. Experimental

All chemicals used in the experiment are of AR grade.

Methanol was used for extracting natural dyes from plants.

Titanium powders, nitric acid, platinum as a counter elec-

trode and fluorine-doped tin oxide were used in the exper-

iment. First, a TiO2 paste was prepared using nitric acid and

TiO2 cells were developed using a doctor blade method. For

the preparation of the dye, first we collected flowers of

salvia and spathodea. Figure 1 shows the photographs of the

plants, TiO2 thin films and devices. Figure 1A and B shows

flowers of salvia and spathodea, respectively. These flowers

were oven dried at 323 K. The dried flowers were ground

into powder and dissolved it in methanol. The solution of

salvia and spathodea was placed on a stirrer for 2 days. The

insets of figure 1A and B show the photographs of the

natural dye prepared from salvia and spathodea by the

extraction method. The prepared dye was loaded on TiO2

for 24 h and annealed at 350�C for 2 h. Figure 1C is a

photograph of dye-loaded TiO2 thin films. Figure 1D shows

the cell prepared for J–V characteristic study. The input

power is measured using a lux meter. The 20 mW input

power was used, and I–V curves were recorded on the

biological (SP300) electrochemical workstation.

3. Results and discussion

3.1 Field emission scanning electron microscopy

(FESEM) and energy-dispersive X-ray spectroscopy (EDS)

study

Figure 2 shows the SEM images of TiO2 powder recorded

at different magnifications. Figure 2A and B shows that

nanoparticles are about 100 nm, and are uniformly well

distributed. Figure 2C shows the EDS image of TiO2

powder, and Ti and O contents were observed. Also, the

atomic and weight percentages of Ti and O are shown in

figure 2D. The nanoparticles as in nature are very useful in

the formation of solar cells due to their large surface area

and able to create a better solid–liquid interface.

Figure 1. Photographs of plants, TiO2 thin films and device: (A) salvia flowers, (B) spathodea flowers, (C) salvia and spathodea

dye-loaded TiO2 and (D) TiO2-based DSSC device. Inset of (A): prepared dye using salvia and inset of (B): prepared dye of spathodea.

236 Page 2 of 8 Bull Mater Sci (2020) 43:236

Figure 2. FESEM and EDS images of used TiO2 powder: (A) FESEM images of TiO2 powder at a magnification of 100k9, (B) FESEMimages of TiO2 powder at a magnification of 50k9, (C) EDS image of TiO2 powder and (D) table showing atomic and the weight

percentages.

Figure 3. Optical absorbance, transmittance and a reflectance of salvia and spathodea dye using EIS.

Bull Mater Sci (2020) 43:236 Page 3 of 8 236

3.2 UV–visible spectroscopic study

Absorption spectra were recorded for salvia and spathodea

dye solutions using a UV–vis spectrophotometer. It is

essential to explain the optical properties of a material

concerning solar cell performance [16]. The absorption

spectra were recorded in the range from 290 to 1100 nm by

dissolving both powders in methanol. The absorbance

spectra of spathodea have a better absorbance compared to

salvia as shown in figure 3 (absorbance). The strong

absorbance peak was observed at 440 nm and a secondary

peak at 670 nm. The spathodea shows the strong absorbance

at 500 nm. The pigments present in salvia dye shows the

absorption at 440 and 670 nm, which represent the energy

as 2.81 and 1.85 eV, respectively. The peak represents

energy of 1.85 eV, which shows near about ideal absorption

for solar cells. Similarly, spathodea shows the absorption

peak at 500 nm and energy near about 2.84 eV, which

shows that spathodea exhibits better absorption compared to

salvia. The transmittance for spathodea and salvia is 60 and

45%, respectively, and is shown in figure 3 (transmittance).

Similarly, reflectance of salvia and spathodea dyes is near

about 50 and 45%, respectively, and is shown in figure 3

(reflectance). Photons were used to excite electrons present

in the pigments of natural dye; these excited electrons were

transferred to a higher energy level which were subse-

quently transferred to the conduction band of a TiO2

[17,18]. Also, the photon-absorbing dye pigment is attached

to the TiO2 surface and is supposed to accumulate an

electrically identical dipole layer [19]. Excited electrons in

the pigment of salvia and spathodea dyes from the ground

state to the excited state led to injecting an electron into the

conduction band of TiO2 [20]. The pigments present in

natural dyes help absorb the photons and transfer an

Figure 4. Most probable content in salvia and their individual structures: (A) trans-Muurola-4(14),5-diene, (B) myrcene, (C) b-copaeneand (D) d-3-carene. Most probable compounds in spathodea and their individual structures: (E) butane, 1,1-diethoxy-3-methyl-,

(F) n-hexadecanoic acid, (G) oleic acid and (H) 1,2-benzenedicarboxylic acid and diisooctyl ester [21–23].

Table 1. Observed parameters from EIS.

Material

Parameter

Dark/light R (X) C (lF) R (X) Q1 Q2 R (X) C (lF) R (X)

TiO2 Dark 15.8 1.09 111.8 9.2 9 10-5 0.8 0.01 11 6.3 9105

Light 15.2 4.24 77.7 1.2 9 10-6 0.8 2.26 11 6.1 9 104

TiO2/salvia Dark 14.4 4.73 10.11 463 9 10-6 0.66 4.24 20 8.0 9 105

Light 11.3 2.89 5.07 228 9 10-6 0.58 3.90 18 1.9 9 104

TiO2/spathodea Dark 18.2 5.10 4.31 7.9 9 10-3 0.61 9.81 20 1.3 9 105

Light 16.8 3.88 4.05 3.0 9 10-3 0.60 6.43 17 6.4 9 105


Figure 6. Nyquist plot and Bode plots of TiO2/salvia: (A) Nyquist plot in the dark and light, (B) circuit diagram, (C) Bode plot in the

dark and (D) Bode plot in light.

Figure 5. Nyquist plot and Bode plots of TiO2: (A) Nyquist plot in the dark and light, (B) circuit diagram, (C) Bode plot in the dark and(D) Bode plot in light.


electron to the conduction band. In salvia and spathodea

flowers, different chemical compounds are present, and

these are responsible for the optical absorption of the dye.

Salvia contains trans-Muurola-4(14),5-diene, myrcene,

b-copaene and d-3-carene [21,22]. Figure 4 shows the most

probable content and its structure. Similarly, butane, 1,1-

diethoxy-3-methyl-(C9H20O2), n-hexadecanoic acid

(C16H32O2), oleic acid (C18H34O2), 1,2-benzenedicar-

boxylic acid and diisooctyl ester (C24H38O4) are the most

likely content observed in spathodea flower pigment and

their structures are shown in figure 4 [23].

3.3 Electrochemical impedance spectroscopic (EIS) study

EIS has extensively been used to investigate the kinetics of

photoelectrochemical processes, including the ionic and

electronic activities occurring in photoelectrochemical solar

cells or DSSCs [24–26]. The software-generated values are

summarized in table 1. The most interesting observation is

that charge transfer resistance decreases with the illumina-

tion of light. This decrease may be due to a generation of

charges at the interface after the illumination of light. In the

case of solution resistance, it is observed to decrease after

the illumination of photons. This decrease is due to the

generation of charge carriers after the illumination of light.

Regarding solar cell efficiency, the charge transfer resis-

tance is an essential factor. In the case of the TiO2 pristine

sample, the charge transfer resistance under dark conditions

is 111.8 X, but after illumination of photons it is observed to

be decreased and found to be 77.7 X. The Nyquist plot,

circuit diagram and Bode plots for pristine TiO2 are shown

in figure 5A–D. After depositing salvia dye on the TiO2, the

charge transfer resistance of TiO2/salvia is observed to

Figure 7. Nyquist plot and Bode plot of TiO2/spathodea: (A) Nyquist plot in the dark and light, (B) circuit diagram, (C) Bode plot in thedark and (D) Bode plot in light.

Figure 8. Photoelectrochemical cell performance of TiO2

DSSC.


decrease as compared to the pristine TiO2, and it is found to

be 10.11 X under dark conditions. After illumination of

light on TiO2/salvia again, charge transfer resistance was

decreased and found to be 5.07 X. The Nyquist plot, circuitdiagram and Bode plots for TiO2/salvia are shown in fig-

ure 6A–D. Similarly, TiO2/spathodea combination is stud-

ied using EIS, and charge transfer resistance is observed to

be decreased as compared to TiO2 and TiO2/salvia. The

charge transfer resistance is 4.31 X under dark conditions

and 4.05 X after the illumination of light. The Nyquist plot,

circuit diagram and Bode plots for TiO2/spathodea are

shown in figure 7A–D. The decrease in charge transfer

resistance after illumination of light is because of the gen-

eration of charge carriers. The TiO2/spathodea shows better

charge transfer resistance as compared to TiO2/salvia

combination, and this result is consistent with optical

absorption results, resulting in better solar cell performance.

3.4 Photoelectrochemical cell performance

The J–V curve of the TiO2 films was measured by

employing the CH608E instrument. The TiO2 thin films

were deposited on fluorine-doped tin oxide, and polyiodide

was used as an electrolyte for the photoelectrochemical

solar cell experiment. Figure 8 shows the typical J–V

characteristic of TiO2 DSSCs. The cell consists of TiO2

DSSC as a working electrode, polyiodide as an electrolyte

and graphite as a counter electrode. The power conversion

efficiency was calculated using equation (1) [9]:

g ¼ Jsc � Voc � FF

Pin

; ð1Þ

where Pin is the power density of the incident light, Voc is

the open-circuit voltage, FF is the fill factor and Jsc is the

short-circuit current density, and FF is calculated using

equation (2):

FF ¼ Imax � Vmax

Isc � Voc

: ð2Þ

The calculated efficiency, fill factor, short-circuit current,

open-circuit current, maximum current and maximum

voltage are summarized in table 2. The photocurrent, pho-

tovoltage and efficiency generated by TiO2/spathodea are

relatively better than those of TiO2 and TiO2/salvia com-

bination. The TiO2/spathodea combination shows better

solar cell performance; this result is consistent with optical

absorption and EIS results. The solar cell developed using

natural dyes is less efficient than synthetic dyes, but natural

pigments are usually non-poisonous, disposed of quickly

and cost-effective and more environmentally friendly

[20,27].

4. Conclusions

In conclusion, natural dyes were prepared using salvia and

spathodea flowers by the extraction method. The pigments

present in salvia and spathodea dyes are helpful in the better

absorbance of light. UV–visible analysis shows that both

dyes are useful in TiO2 DSSCs, and it shows absorption

near to the red region. TiO2 films were developed using

TiO2 nanoparticles. EIS exhibits that natural dyes can

decrease the charge transfer resistance. The spathodea

species showed more efficiency than pristine TiO2 and

TiO2/salvia. The efficient DSSCs using natural dyes are less

toxic, disposed of easily and cost-effective and more envi-

ronmentally friendly compared to organic dyes. This study

is at an initial level, but considering the advantage of natural

dyes, it is beneficial for future bio-solar cell technology.

Acknowledgements

We acknowledge the Department of Science and Technol-

ogy Science and Engineering Research Board (DST-SERB),

Government of India, for providing funds under the Early

Carrier Research Award scheme File No. ECR/2017/

002099.

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Preparation of natural dyes from salvia and spathodea for