ITO DSSC

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ScienceDirect Materials Today: Proceedings 17 (2019) 1268–1276

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MRS-Thailand 2017

Performance of Pterocarpus Indicus Willd Leaf Extract as Natural Dye TiO2-Dye/ITO DSSC Markus Diantoroa,b*, Dina Maftuhaa, Thathit Suprayogia, M Reynaldi Iqbala, Solehudina, Nandang Muftia,b, Ahmad Taufiqa,b, Arif Hidayata,b, Risa Suryanac, Rahmat Hidayatd a

Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Negeri Malang, Jl. Semarang 5, Malang 65145, Indonesia Laboratory of Minerals and Advanced Materials, Faculty of Mathematics and Natural Sciences, Universitas Negeri Malang, Jl. Semarang 5, Malang 65145, Indonesia, c Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret, Solo, Jl. Ir. Sutami No.36 A Surakarta 57126, Indonesia c Department of Physics, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung, Jl. Ganesha 10 Bandung 40115, Indonesia b

Abstract The search of sensitizers of natural dye for a solar cell has been intensively studied. One of the potentials as dye source is Pterocarpus Indicus Willd, PIW (L.) plant. The use of PIW extract dye for dye-sensitized solar cell (DSSC) application has rarely found in the literature. Such natural chlorophyll or flavonoid membrane could be used as mesoporous tissue in DSSC. It makes chlorophyll pigment attractive to be used as a sensitizer in solar cells. The natural dyes have been used with the combination of the metal oxide semiconductor to improve the DSSC performance. We used TiO2 as a working electrode due to its high thermal stability, non-toxic, low fabrication cost, and excellent optical properties. The primary objective is to obtain the structure, absorbance, and efficiency using dye concentration of immersion under of 12, 15, and 18 hours. We used chlorophyll extract of PIW leaves to soak the film composite TiO2/ITO with a variation of dye immersion time. Method for deposition of film TiO2 was a slip casting and followed by heating at 500 oC for 1 hour. The counter electrode was the same structure with firstly been coated with carbon material on the conducting plane and drips using electrolyte. Both symmetric structures then taped together using a transparent flexible keyboard protector. The SEM analyses show that the more extended soaking of TiO2 surface dye becomes entirely dissolved by the dye solution. The UV-Vis characterization revealed that a 15 hours dye immersion time is resulting in the optimum absorbance. The highest result of I-V characterization is 12 hours immersion with a light intensity of 150 mW/cm2. The DSSC parameters obtained are Jsc of 0.370817 mA/cm2, Voc of 0.400197 V and Efficiency equal to 0.035%. These results tell us that PIW chlorophyll extract has a potential for DSSC application. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Pterocarpus Indicus willd, leaf extract, DSSC, TiO2-Dye/ITO, efficiency

* Corresponding author. Tel.: +6281235825370 ; fax: +62 341-552180 E-mail adress: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.

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1. Background Lately, solar energy has been actively developed [1]. In harvesting solar energy, a solar cell can be used as a light absorber to convert the solar energy into electrical energy. Dye-sensitized solar cells (DSSC) have been attracting many researchers’ interests since was introduced by O’Regan and Grätzel. Among other solar cells, DSSC is quite famous for their thin film [2,3], high efficiency [4], simple fabrication [5,6], eco-friendly processing [7], and ability to convert solar energy into electricity [8]. In general, the basic structure of DSCC consists of a pair of transparent conductive glass substrates and some active materials. The main actives materials are semiconductor oxides as working electrodes, sensitizer dyes, electrolyte solution and counter electrodes which contain platinum (Pt) or carbon (C) [9,10]. Some research has mentioned Ruthenium-based dyes including N3 [11], N749 [2], N719 [12] and black dye as a good sensitizer since they can produce reasonably high efficiency which can reach 11% under the standardized condition test [13]; [14]. However, Ruthenium-based dyes contain heavy metals and can cause pollution. They are also costly and complicated regarding the processing [15,16]. Therefore, a natural dye extracted from fruit and leaves are considered to be an alternative to reduce the processing cost [17], the environmental effects [18], complete biodegradation [13], and high potential for harvesting energy [19]. Not to mention, natural dye also has simple fabrication [20], easy waste treatment [21], and are free from contamination[19]. Additionally, the natural dye can be used as a light-harvesting element to provide electrical charge carriers that affect the DSSC performance [22]. Research has attempted to seek for cheap and natural dyes. Natural dyes extracted from fruit and plants contain chlorophyll [23], carotenoid [24], betalain [25], anthocyanin [10] and flavonoid [22,26] pigments which can be used to increase DSCC effectiveness. The results of the research have proven dye from chlorophyll as a better energy conversion because of the carboxylate group found in it. Another exciting feature hidden in nature is the accumulation of thylakoid membranes (containing chlorophyll) which can help increase the light absorbance. The accumulation structure is similar to the mesopores tissue on DSSC. Therefore, the chlorophyll pigments can be used as a sensitizer for solar cells. We can extract chlorophyll from Pterocarpus Indicus Willd leaves which is easy can be found in many areas of Indonesia[27]. An oxide/metal-oxide semiconductor (TiO2, ZnO or SnO2) can be used as an electron acceptor injected by dye and a working electrode [28]. The semiconductor oxide is mostly utilized in photoelectrochemistry due to its stability in facing photo-corrosion [29]. This research used TiO2 because of its high thermal stability, simple fabrication, non-toxic activity, low fabrication cost, and excellent optical trait [30]. These properties, therefore, make TiO2 recognized widely as to function in a photovoltaic application, transparent electrode, diode, photocatalyst, and microelectronic device [31]. TiO2 also owns pretty broadband (anatase, 3.40 eV) [32,33] indicating the reduced amount of oxidative holes formed in the valence band [34] which maintains the DSSC stability. Tagliafero et al. [35] reported that the optimum immersion time using synthetic dyes at various thicknes are about 720 min. An analysis of the effects of immersing time in dye made from the PIW leaves extract on the structure, absorbance, and efficiency of DSSC on TiO2-dye/ITO composite film has not been found yet. In fact, dye immersion duration can significantly influence the DSSC performance. Therefore, it is necessary to investigate the issue. 2. Research method 2.1. Materials and equipment The materials used in this research are PIW extract, methanol, ethanol, n-hexane, TiO2 powder < 25 nm, PEG 6000, aquadest, SDS (Sodium Dodecyl Sulfate), acetylacetone, DI water, Indium Tin Oxide (ITO) Glass 1.25 x 1.25 cm substrate. Tools used were a petri dish, separatory funnel, Buchner Funnel, Scotch tape, crucible, mortar, a digital scale, hotplate (>500 °C), Rotary Evaporator, centrifuge, and Ultrasonic Cleaner.

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2.2. Fabrication of DSSC Fabrication of DSSC was conducted in some steps as follows: (1) chlorophyll extraction from PIW leaves, (2) TiO2 pasta preparation, (3) deposition of TiO2 layers, and (4) structure fabrication of DSSC sandwich. The steps are explained below. Chlorophyll Extraction from PIW Leaves We started with extracting one kilogram of fresh PIW leaves which had been washed with clean water and dried in an oven at 80 °C for one hour. The sample was put in a blender and filtered to obtain its fine powder. The PIW leaves powder was weighed (100 g) and macerated in 1000 mL methanol. During the first 24 hours, we implemented an occasionally stirring of the solution. It was then filtered using Whatman No. 1 filter paper. The pulp was macerated in 1000 mL methanol and kept for 24 hours while mixing it once in a while. The next step was separating the filtrate using Whatman No. 1 filter paper. The first and second filtrated materials were combined and steamed using rotary evaporator until concentrated extract was obtained (± 6 hours). Ten mL extract was fractionated in a separatory funnel with n-hexane and shaken to get two layers of liquid: hexane fractions on top and sediments at the bottom. The hexane fractions were centrifuged at 4000 rpm for 60 minutes to obtain separated liquids from the residues. Preparation of TiO2 Paste To prepare TiO2 paste and to composite ITO glass substrates, 0.1 g TiO2, 180 mg PEG 6000 and 130 mg Sodium Dodecyl Sulfate (SDS) were dissolved in 2 mL aquadest. It was then added to three drops of Acetyl Aceton. The mixture was stirred manually to ensure its homogeneity. Deposition of TiO2 Layers We used scotch tape to cover the in-active working area. The active part size was 0.5 cm x 0.5 cm. TiO2 pasta was dropped on those areas and flattened with a glass spatula to reach an equal thickness of the TiO2 layer. After that, TiO2 on the ITO substrate was heated at 500 °C for 60 minutes so that the organic materials contained in the layer could evaporate. Preparation of KI Solution The first step in preparing a KI electrolyte was to dissolve 0.8 gram of potassium iodide into 10 mL Polyethylene glycol (PEG) and stir it for 10 minutes. Iodine powder (0.127 g) was mixed in 10 ml PEG. The results of the two solutions were put together with a magnetic stirrer until it was homogenized. Preparation of the Opposite Electrode An opposite electrode (for comparison) was made by coating the ITO conductive glass with a carbon catalyst. This carbon catalyst was created from candle soot in the active region of TiO2 layer disposition (0.5 cm x 0.5 cm). DSSC Fabrication ITO conductive glass which had been covered with TiO2 was immersed in dye made from PIW leaves extraction at three different duration treatments (12, 15, and 18 hours). After immersion, parts of the TiO2 electrode were cleaned using an ethanol-dipped cotton bud. The opposite electrode coated with carbon was put on a flat field facing top, and some electrolytes were dropped on top of it. Then, dye immersed TiO2 electrodes were set facing each other and fit their active areas. Both were glued using a keyboard protector to prevent the leak of the electrolyte solution and to avoid a short contact between them. The cells were fixed using a clip binder. 2.3. Characterization The I-V measurement was performed using Keithley 2602A System Source which employs a halogen light source. The intensity was measured using the TES 1333R power meter. The positive probe was connected to the carbon-coated substrate, and the negative probe was connected to the TiO2 layered substrate.

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3. Results and discussion 3.1. SEM Characterization The morphology of TiO2/ITO and TiO2-dye /ITO composite films with various dye immersion times is shown in Figure 1. SEM image results show that composite TiO2/ITO films have a spherical, agglomerated grain morphology of ~ 2000 nm in diameter consisting of nanoparticles. The TiO2 nanoparticles in the agglomeration are estimated to have diameter ~ 15 nm. It is also seen that the distribution of particles are not homogenous and some porosities are observed. The size of the resulting TiO2 nanoparticles is not much different from that obtained in the calculations using the Scherrer equation. A detail observation on Figure 1, we found that the increase of immersion time gives rise to increase the thickness of the coated layer of dye on TiO2 nanoparticles. The sample with 15 hours of immersion starts exhibiting the dye layer compare to the pristine or nonimmersion TiO2 film. It also appears that there is a cracked on the agglomerated TiO2. The thickness of dye coating and the appearance of the crack indicate that prolonged dye immersion decreases TiO2 surface quality. The dye solution causes the surface of TiO2 to become degraded resulting in a DSSC reduces its performance.

a

b

c

d

Fugure 1. The morphology of (a) TiO2 ; (b) TiO2-Dye/ITO (12 hours); (c) TiO2-Dye/ITO (15 hours); TiO2-Dye/ITO (18 hours)

3.2. UV-Vis Characteristics The result of the absorbance spectrum of the TiO2 / ITO film is shown in Figure 2. It appears that there is a maximum absorbance peak at a wavelength of 300.20 nm. These results also have similarities to the results obtained previously [36]. Figure 3 shows a somewhat broad absorbance spectra of PIW extract ranging from 300-700 nm. The maximum wavelength of the two peaks observed is 417 nm and 671 nm. The result matches the standard of chlorophyll a and b in the blue and red spectral range of about 428 and 661 nm [37]. It indicates that the chlorophyll has successfully extracted from PIW leaves. The absorbance spectra demonstrate that it could be used as dye material for DSSC [15].

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From Figure 4 it can be seen that the immersion time affects the absorbance. The longer the immersion time, the higher the concentration of chlorophyll molecules adsorbed on the surface of TiO2 particles. It appears that maximum absorbance occurs in samples with an immersion time of 15 hours which shows a sharp peak around the wavelength of 680 nm. In the sample with 18 hours immersion time exhibit weaker peak. The lowering peak may indicate sample with 15 hours immersion shows the most optimum absorbance. The curve in Figure 2 shows that the uptake of TiO2/ITO films is about 300 nm whereas the TiO2-Dye/ITO film absorption extends from 300 nm to 700 nm or nearly covering the entire visible light spectrum. Thus, the widening of the TiO2-Dye / ITO film absorption spectra is strongly influenced by the dye used. This dye acts as a sensitizer because the presence of dye in the film can bind with TiO2 and is expected to absorb more visible light types from the sun that comes when it is illuminated [10]. The more light is absorbed so that more electrons are transferred from the LUMO level to the TiO2 conduction band. This causes the number of electron transfer increase so that the efficiency of solar cells generated is also increasing. The optical characteristics of materials may affect its band gap. The relation between absorbance coefficient (α) and photon energy (hv) can be obtained using Tauc equation as follow.

 h   0 (h  Eg )n

(1)

Figure 2. UV-Vis spectra of TiO2/ITO

Figure 3. UV-Vis spectra of PIW extract

Figure 4. UV-Vis spectra of TiO2-Dye/ITO for different immersion time

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Where αo, Eg, and n are a band tailing constant, optical gap energy, and transition mode factor, respectively. The value of n may vary as ½, 2. 2/3 or 3. For TiO2 n is equals to ½. From the absorbance measurements, we obtain the maximum peak and the fitted using Tauc plot of photon energy (hv) versus (αhv)2. The cross-section of a linear fit to photon energy is the band gap energy (Eg). We found that the band gap of DSSC with 12, 15, and 18 hours of immersion are 3.54, 3.61, and 3.46 eV, respectively. 3.3. I-V analyses Figure 5 shows that the characterization I-V curve resembles a diode. It indicates that an electron donor and receptor was traversing the p-n junction. Dye acted as the electron donor and electrolyte served as the receptor. No electric current was reported at the moment the open circuit voltage reached the maximum point. In this condition, the value of the voltage of the open circuit (Voc) could be calculated. The result indicates that DSSC which were immersed longer, with light intensity ranged from 100 mW/cm2 to 150 mW/cm2, generated smaller output with an insignificant difference. This happened because dye absorbed by TiO2 had a maximum limit to fill in the hollow spaces found on the TiO2 surface. As a result, more TiO2 layers were covered by dye so that the rate of the electrons excited by the dye (towards the electrodes) was inhibited. The decrease in current density and voltage value due to the immersion duration was also caused by the degradation of TiO2 layers by the dye solution. Therefore, TiO2 which served as an electron holder could not perform optimal work [38]. The DSSC performance could be evaluated from characterization I-V which generates parameters that can be used to examine the DSSC efficiency such as current density and voltage (J-V). From the current-voltage measurement, the voltage of the open circuit (Voc), maximum voltage (Vmax), short circuit current density (Jsc), and maximum current density (Jmax) could be determined. Voc could be measured when DSSC was at the open circuit position or in other words, no current flow was reported on the circuit. The fill factor (FF) value was calculated using the following formula. 100 mW/cm

0.5

TiO 2-dye (12 hou rs) TiO 2-dye (15 hou rs) TiO 2-dye (18 hou rs)

2

150 mW/cm

TiO -dye (12 h ours) 2 TiO -dye (15 h ours) 2 TiO2 -dye (18 h ours)

2

0.0

-0.5 2

J (mA/cm )

2

J (mA/cm )

0

-1.0

-2

-4

-1.5

-2.0 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

V (Volt)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

V (Volt)

Fig. 5. Characteristics of J-V for different immersion time of 100 mW/cm2 (left) and 150 mW/cm2 (right) light irradiaton.

FF 

J max  Vmax J sc  Voc

(2)

DSSC efficiency (η) was estimated using an equation below



Pmax 100% Pin

(3)

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

Jsc Voc  FF 100% Pin

(4)

The solar cell parameters extracted from I-V measurements are presented in Table 1. Table 1. The solar cell characteristics of DSSC. Immersion Duration (hours) 12 15 18

Jsc (mA/cm2)

Light Intensity (mW/cm2) 100

0.137580 ± 0.0000005

0.415150 ± 0.0000005

0.406345 ± 0.0000005

0.023209 ± 0.0000005

150

0.370817 ± 0.0000005

0.400197 ± 0.0000005

0.354619 ± 0.0000005

0.035084 ± 0.0000005

100

0.080440 ± 0.0000005

0.400155 ± 0.0000005

0.317387 ± 0.0000005

0.010216 ± 0.0000005

150

0.180564 ± 0.0000005

0.310145 ± 0.0000005

0.300592 ± 0.0000005

0.011222 ± 0.0000005

100

0.049534 ± 0.0000005

0.340064 ± 0.0000005

0.331429 ± 0.0000005

0.005583 ± 0.0000005

150

0.137580 ± 0.0000005

0.415150 ± 0.0000005

0.406345 ± 0.0000005

0.023209 ± 0.0000005

Voc (Volt)

FF

η (%)

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