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Natural dyes extracted from Phlai and Blue Pea as sensitizers for
dye-sensitized solar cells
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Sataporn Komhom*
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Department of Chemical Engineering, College of Engineering
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Rangsit University, Pathumthani, 12000
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*Corresponding author: E-mail: satapaul@yahoo.com
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Abstract
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บทคัดย่อ
The dye-sensitized solar cells (DSCs) are assembled by using natural dyes from Phlai and Blue Pea as sensitizers
and then photoelectrochemical properties are investigated. Phlai and Blue Pea dye sensitizer on TiO2 can absorb light in the
visible region of the spectrum that is between 400 and 800 nm. The performance of dye-sensitized solar cells (DSCs) with
Phlai dye sensitizer is found to be apparently higher than that of Blue Pea dye sensitizer in comparison with various mass of
Phlai and Blue Pea for dye sensitizer extraction : 1, 3, 5, 7, 9, 11, 15 and 20 g (fixed pH value = 7) and also with various pH
of solution for dye sensitizer extraction : 2.4, 4.3, 7, 9.4 and 11 (fixed mass = 11g). Dye-sensitized solar cells (DSCs) with
Phlai dye sensitizer exhibits the satisfying photosensitized effect resulting in the highest maximum power at 1.493 W/cm2
with mass of Phlai for dye sensitizer extraction 15 g and using pH value 7 for extraction.
Keywords: dye-sensitized solar cells (DSCs), natural dyes, Phlai and Blue Pea dye sensitizer, photoelectrochemical
properties, maximum power.
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เซ ลล์ แ ส งอาทิ ตย์ ช นิ ด สี ย้ อ ม ไวแ สงป ระดิ ษ ฐ์ ข้ ึ น โด ยใช้ สี ย้ อ ม ธรรม ชาติ จากไพ ลและดอกอั ญ ชั น เป็ น สี ย้ อ ม ไวแ ส ง
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จ า ก นั้ น ท า ก า ร วิ เ ค ร า ะ ห์ คุ ณ ส ม บั ติ ไ ฟ ฟ้ า เ ค มี ท า ง แ ส ง
สี ยอ้ มไวแสงจากไพลและดอกอัญชันบนตัวรองรับไทเทเนียมไดออกไซด์สามารถดูดซับแสงในช่วงของการมองเห็นที่มีสเปกตรัมของแสงในช่วงความ
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ย า ว ค ลื่ น 400 – 800 น า โ น เม ต ร ค ว า ม ส า ม า ร ถ ใ น ก า ร ท า ง า น ข อ ง เซ ล ล์ แ ส ง อ า ทิ ต ย์ ช นิ ด สี ย้ อ ม ไ ว แ ส ง ใ น ที่ นี้ พ บ ว่ า
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ก า ร ใ ช้ สี ย้ อ ม ไ ว แ ส ง จ า ก สี ที่ ส กั ด จ า ก ไ พ ล จ ะ มี ค ว า ม ส า ม า ร ถ ใ น ก า ร ท า ง า น ที่ สู ง ก ว่ า สี ที่ ส กั ด จ า ก ด อ ก อั ญ ชั น
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ในการเปรี ยบเทียบด้วยค่าน้ าหนักต่างๆกันของไพลและดอกอัญชันที่ใช้ในการสกัดสี ยอ้ มไวแสง 1 3 5 7 9 11 15 และ 20 กรัม (ควบคุ มค่าพีเอชเท่ากับ
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7) และรวมทั้ง ได้เปลี่ ย นแปลงค่ า พี เอชของสารละลายที่ ใ ช้ใ นการสกัด สี ย อ้ มไวแสง 2.4 4.3 7 9.4 และ 11 (ควบคุ ม น้ าหนัก เท่ ากับ 11 กรั ม )
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เซลล์ แ สงอาทิ ตย์ ช นิ ดสี ย้อ มไวแสงจากสี ที่ ส กั ด จากไพลแสดงผลการเปลี่ ย น พลั ง งานแสงเป็ นพลั ง งานไฟฟ้ าในระดั บ ที่ น่ า พ อใจ
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ซึ่ งพ บว่ า มี ค่ า ก าลั ง ไฟฟ้ าสู งสุ ดอยู่ ที่ 1.493 ไมโครวัต ต์ ต่ อ ตารางเซนติ เมตร เมื่ อ ใช้ น้ าหนั ก ของไพ ล 15 กรั ม และค่ า พี เอชเท่ า กั บ 7
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ในสภาวะของการสกัดสี ยอ้ มไวแสง
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คำสำคัญ: เซลล์ แสงอาทิ ตย์ ชนิดสีย้อมไวแสง, สีย้อมธรรมชาติ, สีย้อมไวแสงจากไพลและดอกอัญชัน, คุณสมบัติไฟฟ้าเคมีทางแสง, กาลังไฟฟ้าสูงสุด
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1. Introduction
Dye-sensitized solar cells (DSCs), a new type of solar cells, have attracted considerable attention
because of their environmental friendliness and low cost of production (ORegan & Grätzel, 1991). A DSCs is
consisted of a nanocrystalline porous (TiO2) semiconductor electrode-absorbed dye, a counter electrode, and an
electrolyte containing a redox mediator (iodide, I– / triiodide ions, I3–) encapsulated between two glass plates as
shown in Figure 1.
Figure 1 Schematic of dye-sensitized solar cells
Usually, the photoanode is prepared by adsorbing a sufficiently large number of dye molecules on the
huge nanoporous surface of TiO2 for efficient light harvesting. For the photocathode, the conductive glass at the
counter electrode is coated with few atomic layers of carbon or platinum, in order to catalyze the redox reaction
with the electrolyte. Fluorine doped tin oxide (FTO) is most commonly used for coating on front and counter
substrates (Papageorgiou, 2004; Papageorgiou, Maier, & Grätzel, 1997).
In DSCs, the dye sensitizer plays a key role in absorbing sunlight and transforming solar energy into
electric energy. The DSCs by Ru-containing compounds absorbed on nanocrystalline TiO2 show the highest
efficiency at 11 – 12% (Chiba, Islam, Watanabe, Komiya, Koide, & Han, 2006; Buscaino, Baiocchi, Barolo,
Medana, Grätzel, Nazeeruddin, & Viscardi, 2008). However, there are several disadvantages of using noble
metals in relation to costly production. On the other hand, organic dyes are not only cheaper but have also been
reported the high efficiency at 9.8% (Zhang, Bala, Cheng, Shi, Lv, Yu, & Wang, 2009). Although organic dyes
have complicated synthetic routes and low yields, it can be easily found in flowers, leaves, and fruits. Moreover,
organic dyes have several outstanding advantages such as cost efficiency, non-toxicity, and complete
biodegradation. Thus far, several natural dyes have been widely employed as sensitizers in DSCs, such as
anthocyanin, curcumin, carotene, tannin, and chlorophyll (Furukawa, Iino, Iwamoto, Kukita, & Yamauchi,
2009; Gómez-Ortíz, Vázquez-Maldonado, Pérez-Espadas, Mena-Rejón, Azamar-Barrios, & Oskam, 2010;
Espinosa, Zumeta, Santana, Martınez-Luzardo, Gonzalez, Docteur, & Vigil, 2005; Kumara, Kaneko, Okuya,
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Onwona-Agyeman, Konno, & Tennakone, 2006). Several researchers have reported the use of natural dyes in
solar cells. As an example, the dye sensitized solar cell shows higher photosensitized performance with a
natural dye extracted from the bark of the Kopsia flavida fruit that contain functional groups such as OH, which
is suppose to be an anthocyanin dye (Nishantha, Yapa, & Perera, March, 2012). The anthocyanin has been
responsible for several colors in the red – blue range, found in fruits, flowers and leaves of plants. Carbonyl and
hydroxyl groups present in the anthocyanin molecule can be bound to the surface of a porous TiO2 film. This
can contribute to the electron transfer from the anthocyanin molecule to the conduction band of TiO2 (Hao, Wu,
Huang, & Lin, 2006). However, curcumin reveals a similar structure to the anthocyanin and also exhibits a long
intense wavelength absorption range from 420-580 nm in the visible region, so curcumin from the ground
rhizome of Curcuma longa L. (turmeric) is used as sensitizers in dye sensitized solar cells (Ganesh, Kim, Yoon,
Lee, Lee, Mane, Han, J., & Han, S., 2009). Interestingly, the use of curcumin-derived dyes as sensitizers in dye
sensitized solar cells has attracted significant research attention because it can obtain from various sources.
As reported, there is no comparison in photosensitized performance between anthocyanin and
curcumin as sensitizers. So in order to compare the photoelectrochemical properties of anthocyanin and
curcumin the research is under progress on the extraction of anthocyanin and curcumin from Blue Pea and
Phlai, respectively. The DSCs using natural dyes from Phlai and Blue Pea as sensitizers are investigated within
condition of various mass and pH for dye sensitizer extraction.
2. Objectives
This research presents the investigation on natural dyes from Phlai and Blue Pea as sensitizers
available locally, regarding their sensitization activity in DSCs. These extracted dyes are characterized by UV–
vis absorption spectra and the photoelectrochemical properties of the DSCs using these extracts as sensitizers
are investigated.
3. Materials and methods
3.1. Preparation of natural dye sensitizers
Phlai and Blue Pea dye sensitizer are extracted by using methanol as the extraction solvent. The dyes
extracted with methanol are obtained by the following steps: Phlai and Blue Pea are washed with water and dry
at 50 oC. After crushing into fine powder using a mortar, these materials are suspended in 100 ml of methanol
before heating up to 50 oC, and kept at that temperature for 30 min. Then the solids are filtrated out, and the
filtrates are collected at room temperature in the dark for use as sensitizers. The effect of pH of dye solution is
studied by adjusting pH from the original pH using 0.1 M HCl and 0.1 M KOH solution.
3.2. Preparation of dye – sensitized solar cells
The dye-sensitized TiO2 electrode and a sputtered-Pt counter electrode are prepared by using FTO
conductive glass sheets (fluorine-doped SnO2). FTO conductive glass is first cleaned with ethanol and then airdried. TiO2- and Pt-cream are deposited on the FTO conductive glass by screening technique in order to obtain a
TiO2 and Pt film with an area of 1 cm2. The TiO2 and Pt film are heated at 450 oC for 30 min. Subsequently, the
TiO2 electrode is immersed in a methanol solution containing a natural dye for 24 h and following assembled
with a sputtered-Pt counter electrode to form a solar cell by sandwiching a redox (I −/I3−) electrolyte solution.
3.3. Electrical measurement
The UV–vis transmission and reflectance spectra of the Phlai and Blue Pea dye sensitizer absorbed on
the TiO2 films are taken by using a UV-Thermo. The current – voltage – power (I–V–P) curves of the DSCs are
obtained by applying an external bias to the cell and measuring the generated photocurrent under white light
irradiation 100 mW/cm2 from a 300 W solar simulator as the light source by using a voltage recorder VR-71.
4. Results
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4.1. Absorption of natural dyes
This research has studied to use natural dyes from Phlai and Blue Pea as sensitizers for DSCs. Figure 2
shows the UV – vis absorption spectra of natural dyes from Phlai as sensitizer with various mass of Phlai for
dye extraction. Spectral figures are similar with having absorption peaks and show maximum absorption peak at
around 490 nm in the visible region (λ = 380 - 780 nm). These absorption peaks are expanded to large-broad
wavelength with an increasing mass of Phlai for dye extraction. The large broadening of the spectrum is
implying that a strong absorption has occurred in the visible region of Phlai dye sensitizer which can contribute
to the strong interaction on the surface of TiO2 film (Ma, Inoue, Noma, Yao, & Abe, 2002). It is also common
to say that the increasing of mass of Phlai for dye extraction will enhance an interaction of dye on TiO2 surface.
Figure 2 The absorption spectra of Phlai dye sensitizer with various mass of Phlai
Figure 3 shows the UV – vis absorption spectra of natural dyes from Phlai and Blue Pea as sensitizers
comparing with N-719. Phlai and Blue Pea dye sensitizer exhibit an absorption peak of ca. 390 – 490 and 580 –
620 nm, respectively. A completely different tendency of absorption peak of Phlai and Blue Pea dye sensitizer
is observed which ascribe to different components, namely, natural dyes from Phlai as sensitizer is to the
present of curcumin, a group of natural phenols that are responsible for the yellow color of turmeric. Figure 4
shows the structure of curcumin which can exist in several tautomeric forms, including a 1, 3-diketo form and
two equivalent enol forms. The enol form is more energetically stable in the solid phase and solution.
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Figure 3 The absorption spectra of Phlai and Blue Pea dye – sensitized and N-719
(a)
(b)
Figure 4 The structure of curcumin (a) enol form and (b) 1, 3-diketo form
Furthermore, absorption peak of natural dyes from Blue Pea as sensitizer is to the present of
anthocyanin which is water-soluble vacuolar pigments that may appear red, purple, or blue depending on the
pH. Figure 5 shows the structure of anthocyanin which occurs in all tissues of higher plants, including
leaves, stems, roots, flowers, and fruits.
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Figure 5 The structure of anthocyanin
The chemical structure of curcumin and anthocyanin is directly correlated with the chemical
adsorption of dyes extracted from Phlai and Blue Pea because of the condensation of alcoholic-bound protons
with the hydroxyl groups on the surface of nanostructured TiO2 (Meng, Ren, & Kaxiras, 2008). The absorption
peak of natural dyes from Phlai as sensitizer adsorbed on TiO 2 is obviously wider and slightly-up-shift but
natural dyes from Blue Pea as sensitizer reveal a reversed tendency. The shift of absorption spectra means the
interaction between the dyes and the cationic TiO2 surface is observed which is formed through a chemical
bond, the C–O–Ti bond, as discussed in the literature (Hao et al., 2006). It is feasible that the interaction of
Phlai dye sensitizer on TiO2 surface is stronger than that of Blue Pea dye sensitizer.
4.2. Photoelectrochemical properties of DSCs with natural dyes
Figure 6 and 7 show typical results of the current – voltage – power (I–V–P) curves of DSCs with
natural dyes from Phlai and Blue Pea as sensitizers, respectively. These curves are measured for
photoelectrochemical test of DSCs. Table 1 shows the performance of natural dyes as sensitizers in DSCs which
is evaluated by short circuit current (ISC), open circuit voltage (VOC), maximum power (Pmax), and fill factor
(FF). The fill factors of these DSCs are quite similar at ranging from 40 to 77 %, open circuit voltage varies
from 32 to 222 mV, and short circuit current changes from 0.004 to 1.666 mA/cm2. When compare the
maximum power of DSCs from Phlai and Blue Pea dye sensitizer, maximum power of DSCs with Phlai dye
sensitizer is much higher than that of Blue Pea dye sensitizer. Specifically, a highest maximum power is
obtained from the DSCs by Phlai dye sensitizer (@ 15 g). Moreover, the maximum power of the DSCs by the
natural dyes is compared with the DSCs by a Ru complex cis-RuL2 (SCN)2 (L = 2,2-bipyridyl-4,4-dicarboxylic
acid) (N-719), which is widely used in DSCs. As a result, the maximum powers of the DSCs by the natural dyes
are lower than that of N-719. As a result in Figure 3, the absorption peak of N-719 dye sensitizer is obviously
widest which can contribute to the strong interaction on the surface of TiO2 film. It can also be said that there
are available bonds between the dye and TiO2 molecules through which electrons can transport from the excited
dye molecules to the TiO2 film (Hao et al., 2006). This result indicates that the interaction between the dye
sensitizer and the TiO2 film is significant in enhancing the maximum power of DSCs.
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Figure 6 Current–voltage–power curves for the DSCs by using Phlai dye sensitizer (@ 15 g, pH = 7)
Figure 7 Current–voltage–power curves for the DSCs by using Phlai dye sensitizer (@ 11 g, pH = 9.4)
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Table 1 Photoelectrochemical parameters of DSCs by using Phlai and Blue Pea dye sensitizer
Natural dye sensitizer
Blue Pea (@ 5 g, pH = 7)
Phlai (@ 15 g, pH =7)
Blue Pea (@ 11 g, pH = 4.3)
Phlai (@ 11 g, pH = 9.4)
N-719
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ISC
(mA/cm2)
VOC
(mV)
Pmax
(W/cm2)
FF
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0.084
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0.08
1.666
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0.684
1.493
0.700
1.444
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55.54
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58.81
Figure 8 shows the plots between maximum powers versus mass of Phlai and Blue Pea for dye
sensitizer extraction. It is clearly seen that the maximum power increases with increasing mass of Phlai and
Blue Pea for dye sensitizer extraction and then maximum power reaching a plateau at ca. 1.493 W for Phlai
dye sensitizer and 0.684 W for Blue Pea dye sensitizer. Phlai dye sensitizer exhibits superior performance than
the Blue Pea dye sensitizer ones.
Figure 8 The maximum powers of the DSCs with various mass of Phlai and Blue Pea for dye sensitizer
extraction (fixed pH value = 7 and varying mass)
The various pH of solvent for dye sensitizer extraction in this research are obtained by varying the
concentration of hydrochloric acid and potassium hydroxide. The pH values of solvent in this extraction are 2.4,
4.3, 7, 9.4, and 11. The performance curves of DSCs from Phlai and Blue Pea dye sensitizer are shown in
Figure 9. The maximum power of DSCs from Phlai dye sensitizer increases when pH value varies from 2.4 to
9.4 and then decrease. The highest value of maximum power of DSCs from Phlai dye sensitizer is reached at
1.444 W with pH value 9.4. The maximum power trend of DSCs from Blue Pea dye sensitizer is lower than
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that of Phlai dye sensitizer and exhibits the highest value of maximum power of DSCs at 0.700 W with pH
value 4.3.
Figure 9 The maximum powers of the DSCs with various pH of solution for dye sensitizer extraction (fixed
mass = 11g and varying pH value)
5. Discussion
After adsorption of Phlai and Blue Pea dye sensitizer on TiO2, the absorption spectra are illustrated in
Figure 3. In case of Phlai dye sensitizer adsorption on TiO2, a positive shift in the absorption peak is observed
after adsorption. It seems resonable to suppose that curcumin pigment shows a broader absorption peak. On the
contrary, Blue Pea dye sensitizer adsorption on TiO 2 shows the different tendency, a negative shift in the
absorption peak is observed after adsorption. A positive shift in the absorption peak is due to the extent of the
binding of molecule in the dye solution to the TiO2 surface. This is an indication that the distance between the
Phlai dye sensitizer skeleton and the point connected to the TiO2 surface facilitates electron transfer from dye
molecule to TiO2 (Hao et al., 2006). Therefore, the maximum power of DSCs from Phlai dye sensitizer is higher
than that of Blue Pea dye sensitizer. Moreover, Blue Pea dye sensitizer exhibits an absorption peak of ca. 580 –
620 nm, the absorption peak at higher wavelength have strongly affected on the decreasing of current density
and lead to the decreasing of maximum power (Narayan & Raturi, 2011).
As a result in Figure 3, it is obvious that an increasing mass of Phlai for dye extraction from 1 to 9 g is
found to be remarkable effected on expansion of wavelength, so the maximum power is clearly increasing when
using dye sensitizer in DSCs, as shown in Figure 8. In contrast, an increasing mass of Phlai for dye extraction
from 11 to 20 g is found to be little effected on expansion of wavelength, so the maximum power is a slight
difference. In the case of Blue Pea dye sensitizer, the maximum power exhibits the same tendency. This is an
indication that mass of plants for dye sensitizer extraction has given limited value for extraction. It can also be
said that mass of plants has not shown effects on the maximum power if it is reaching the limited value.
In general, the dye sensitizer is extracted with alcohol such as ethanol or methanol in the state of
natural pH of solvent at about 4 – 7. The disturbance of pH value with acid or base has effect on the components
of dye sensitizer which is directly correlated with the performance of DSCs (Zhou, Wu, Gao, & Ma, 2011). As a
result in Figure 9, it can be seen that pH of solution for dye sensitizer extraction is much higher and/or lower
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than that of ranging from 4 to 7 bring to the lower maximum power of the DSC sensitized. This result exhibits a
similar tendency for Phlai and Blue Pea dye sensitizer. The efficiency of Blue Pea extracted anthocyanin is
found to increase with decreasing pH and reached a maximum at the optimum pH 4.3. This can be rationalized
by the fact that, at lower pH, anthocyanin existed as flavylium ion, which is stable form of anthocyanin, an
increasing pH hydrated this ion to quinonoidal bases. These compounds are labile and can be transformed into
the colorless carbinol pseudobase and chalcone. It is apparent that at low pH the formation of flavylium ion
form is favorable (Bakowska, Kucharska, & Oszmianski, 2003). Nevertheless, the cell deterioration by acid
leaching is expected as the pH goes lower, which results in a lower efficiency (Hao, Wu, Fan, Huang, Lin, &
Wei, 2004). Although the lower pH in case of anthocyanin has shown remarkable effects on maximum power,
the highest maximum power is lower than that of curcumin. This may be one of the main reasons that the
important feature of curcumin is the presence of extended conjugation in its structure because of the aromatic
and enol groups, which provide the basic long-wavelength absorption. In addition, curcumin prominent feature
is the presence of methoxy and hydroxyl groups in the 3- and 4-positions, respectively, of the terminal phenyl
groups, which results in a further bathochromic shift in the maximum (λ max) absorption of this compound
(Crivello & Bulut, 2005). As a result, the maximum power of DSCs from Phlai dye sensitizer is higher than that
of Blue Pea dye sensitizer.
6. Conclusion
The photoelectrochemical performance of the DSCs using natural dyes from Phlai and Blue Pea as
sensitizers showed that the VOC ranged from 32 – 222 V, ISC was in the range of 0.004 – 0.084 mA/cm2, and
also Pmax was in the range of 0.684 – 1.493 W/cm2. The DSCs by Phlai dye sensitizer offered the maximum
power of 1.493 W/cm2 with mass of Phlai for dye sensitizer extraction 15 g and using pH value 7. In case of
DSCs by Blue Pea dye sensitizer offered the maximum power of 0.700 W/cm2 with mass of Blue Pea for dye
sensitizer extraction 5 g and using pH value 4.3. DSCs by using Phlai dye sensitizer exhibited superior
performance than the Blue Pea dye sensitizer ones. On the basis of the results, DSCs using natural dyes as
sensitizers are promising because of their environmental friendliness and low-cost production. The results are
additional studied to the search of new natural dye sensitizers for the optimization in future DSCs.
7. Acknowledgements
This research was supported by the Department of Chemical Engineering, College of Engineering,
Rangsit University, Pathumthani.
8. References
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storage on stability of the anthocyanin-polyphenol copigment complex. Food Chemistry, 81(3),
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Buscaino, R., Baiocchi, C., Barolo, C., Medana, C., Grätzel, M., Nazeeruddin, Md.K., & Viscardi, G.
(2008). A mass spectrometric analysis of sensitizer solution used for dye-sensitized solar cell.
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with conversion efficiency of 11.1%. Japanese Journal of Applied Physics, 45(25), L638–L640.
Crivello, J.V., & Bulut, U. (2005). Curcumin: A naturally occurring long-wavelength photosensitizer for
diaryliodonium salts. Journal of Polymer Science Part A: Polymer Chemistry, 43(21), 5217-5231.
Espinosa, R., Zumeta, I., Santana, J.L., Martınez-Luzardo, F., Gonzalez, B., Docteur, S., & Vigil, E.
(2005). Nanocrystalline TiO2 photosensitized with natural polymers with enhanced efficiency from
400 to 600 nm. Solar Energy Materials and Solar Cells, 85(3), 359–369.
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