Download - Dye-Sensitized Solar Cells (DSSC) from Black Rice and its Performance Improvement by Depositing Interconnected Copper (Copper Bridge) into the Space between TiO2

Dye-Sensitized Solar Cells (DSSC) from Black Rice and its Performance Improvement by Depositing Interconnected Copper (Copper Bridge) into

the Space between TiO2 Nanoparticles

Sahrul Saehana, Elfi Yuliza, Pepen Arifin, Khairurrijal, and Mikrajuddin Abdullaha

Department of Physics, Bandung Institute of Technology, Jalan Ganesa 10 Bandung 40132, Indonesia

[email protected]

Keywords: DSSC, I-V performance, copper bridge, and internal resistance

Abstract. Dye-sensitized solar cell (DSSC) which employed natural dye from black rice has been

successfully fabricated and improved its performance by depositing interconnected copper (copper

bridge) on the space between TiO2 particles. The copper bridge has significant role in minimizing

recombination of electron-hole which occurred in TiO2 surface by trapping electron and facilitating

to anode. The presence of interconnected copper nanoparticle in the space between TiO2

nanoparticles was confirmed by Scanning Electron Microscopy (SEM) and X-Ray Diffractometer

(XRD). The current-voltage (I-V) characterization of DSSC solar cells by using Keithley 617 was

also performed to investigate performance of solar cells under sun illumination in varying

intensities. It is found that performance of copper coated DSSC solar cells (efficiency 0.35% and

fill factor 0.35) is higher than DSSC without copper coating (efficiency 0.17% and fill factor 0.35).

This result is consistent with impedance spectroscopy analyzing where the internal resistance of

copper coated DSSC solar cells is lower than DSSC without coated. It is concluded that

performance of DSSC increasing with decreasing of internal resistance. Our finding is higher than

other researcher reports in Ref. [13] and [14] with similar structure and kind of natural dye. In

addition, this paper also reports the use of polymer electrolyte which employing polyvinyl acetate

(PVA) containing lithium ion to maintain long-term stability of device.

Introduction

Recently, application of TiO2 material is enormous, ranging from the fields of renewable energy

(solar cells) [1-8], to the environmental field as a photocatalyst [9] and water filter [10]. In energy

application, this material is widely used in dye-sensitized solar cells (DSSC) due to the small

particle size, wide surface area and large band gap [1,11]. A number of TiO2 unique characteristics

allowed its surface can be loaded dye molecules and play significant in electronic transport.

Some paper reports that DSSC solar cells have high efficiency [2-5] and it can be fabricated by

using a simple deposition method [12]. Moreover, photosensitive component (dye) can be derived

from materials containing anthocyanins which are widely available in nature, such as fruit, flower

and leaf [12-19]. However, some paper also reports that some unexpected phenomena, such as

recombination, still occurred in DSSC [2,22]. Other the hand, effort to optimize long-term stability

and encapsulating of this device are needed for application in industry [23].

Some efforts has been devoted to enhancing performance of DSSC, such as improving long-term

stability by employing polymer electrolyte [23-25], minimizing cost of production by depositing

carbon or graphite as counter electrode [26-27] and using the natural dye as photosensitizer [13-21].

Moreover, the use of natural dye is not only for reducing cost of production but also proposing for

environmentally-friendly [14]. Although its absorbance ability is lower than dye synthetic, such as

ruthenium complex, but the findings of Hao et al. [21] and Yuliarto et al. [13] showed that extracts

of black rice can be used as photosensitizer in DSSC. Furthermore, Hao et al. [21] also reports that

the dye of black rice extract is the best natural dye which can be used in DSSC. It is be caused, this

kind of dye can absorb wavelength in wide spectrum.

Materials Science Forum Vol. 737 (2013) pp 43-53© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.737.43

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Fig.1 (a) Mechanism and (b) band diagram of DSSC solar cells [21].

Fig. 1a depicts the mechanism which occurred in DSSC solar cells. Fig. 1b shows the band

diagram of DSSC solar cells, where the desired process is marked by the green line (1) electron

injection, (2) the collection of electrons, and (3) dye regeneration. On the contrary, it also shows the

unexpected process, which is signed by red lines, namely: (4) decay, (5) recombination and (6)

recaption reaction [21].

Recombination process, as shown in Fig. 1a, is one of the crucial phenomena which occurred in

DSSC solar cells and it is reducing efficiency of solar cells [27-28]. Some papers reports [27-33,

36-39] that the deposition of metal-semiconductor contact (Schottky contacts) in the TiO2 surface is

one of effective method to reduce these phenomena. However, depositing Schottky contact to

improve DSSC performance, which conducted by other researcher [28-35], used complicated

process and employing expensive metal, such as gold.

Fig. 2 (a) Structure of our proposed solar cells, and (b) Its band diagram (Cu coated DSSC,

Cu:DSSC).

44 Nanotechnology Applications in Energy and Environment

Through this study, we fabricated DSSC solar cells by using low cost material and simple

methods to deposit Schottky contact. Design of our DSSC solar cells was depicted in Fig. 2a. It was

shown that dye coated around the interconnecting TiO2 absorbed photon and electron was

generated. The electrons are conducted through interconnecting TiO2 and copper nanoparticle to the

anode, while the holes are transfer through the polymer electrolyte of iodine in contact with

platinum catalyst at the cathode. On the other hand, Fig. 2b shows the band diagram of our solar

cells. It is shown that the interconnected copper (copper bridge) play a role to trap electrons on TiO2

conduction band and then flow it quickly to the main electrode (anode). This approach is applied to

reduce the recombination phenomena as described in our previous paper [34-36].

In this paper, we report the use of natural dyes extracted from black rice as photo sensitizer in

DSSC. We also report our significant efforts to improve performance of DSSC by deposition

Schottky contacts with a simple method (electroplating). Moreover, internal resistance of our solar

cells, which was coated by copper, was investigated by employing Electrochemical Impedance

Spectroscopy (EIS), and then it was analyzed by comparing with DSSC solar cells without copper

coating. We also demonstrate the use of polyvinyl acetate (PVA) containing lithium ion as polymer

electrolyte to support long term stability of devices. Simplicity and low cost of deposition method is

expected to facilitate its application in large-scale production (industry).

Experiment

A TiO2 film was deposited on a substrate of indium tin oxide (ITO) by using the spray method [36-

39]. A TiO2 suspension prepared by dispersion of 5 gr. of TiO2 in 10 mL water and it was stirred by

using a magnetic stirrer for 45 minutes. The suspension is then sprayed on ITO substrate which is

placed on a hotplate with temperature about 150°C [11]. Spraying process was repeated about 10

times, and then the uniform TiO2 films were obtained. Then, this film was heated at 450°C in

furnace for 45 minutes to improve contacts between the particles of TiO2 and TiO2 particles with

the substrate.

Copper (Cu) was deposited on TiO2 films by performing electroplating methods. Electroplating

process was carried out on 55°C of electrolyte solution and 50 mA of electric current for one

second. The 20 mL of copper sulfate (Cu2SO4) was used as electrolyte solution. Copper rod with

purity about 99.99% is also used as the anode.

Extracting process was begun by inserting 10 gr. black rice into the solvent and then it was

stirred by magnetic stirrer at temperature 50°C for 30 minutes. In this process, we used solvent

which contained alcohol, acetic acid, and distilled water with morality ratio about 3:2:1. Then,

solution was filtered by filter paper (1 mm mesh) and dark-red solution was obtained. TiO2 films

were immersed in dye solution for 24 hours [13,20]. Then, it was cleaned by distilled water and

then it was heated at temperature 40°C for 10 minutes.

A polymer electrolyte was made by dissolving LiOH (0.09 g) in water (10 mL) and stirring for 1

hour. Then, this solution was added into 10 mL of polyvinyl acetate (PVA) and stirring for 2 hours.

Furthermore, the iodolyte solution (Solaronix, Switzerland) containing I-/I

3- ion added to the

polymer electrolyte. Sandwich structure solar cell devices were made by depositing a polymer

electrolyte on TiO2 films and covered with counter electrodes which made from platinum coated

indium tin oxide with a thickness about 70 nm.

Characterization of the crystal structure of TiO2 films were analyzed by using X-ray

Diffractometer (PW1710). Scanning Electron Microscopy (JEOL JSM-6360LA) which operated at

a voltage of 20 kV was also used to investigate the film morphology. On the other hand, the

performance of solar cells was measured by using Keithley 617. A measurement was done on the

dark and under sun illumination (67.08 mW/cm2). Sunlight intensity is measured by using solar

power TM-206 meter. Moreover, internal resistance of solar cell devices was investigated by

performing impedance spectroscopy (EIS) at a frequency of 20 Hz-2 MHz (Agilent E4980A

Precision LCR meter).

Materials Science Forum Vol. 737 45

Result and Discussion

Scanning Electron Microscopy (SEM) of TiO2 films which fabricated by spray method on ITO

substrate is depicted in Fig. 3. Homogeneous of the film was observed and particle size was

predicted in nanometers scale. In this size, we estimated dye molecule can occupied on its surface.

Moreover, morphology of TiO2 film which coated with copper was presented in Fig. 3b. It is shown

clearly that contact between TiO2 particles and copper was occurred. The presence of copper bridge

was expected to make electron transport to anode be quick. Although, Lai et al. [30] reported that

presence of metal bridge can reduce transmittance of TiO2 films, however, increasing performance

of DSSC solar cells was still achieved.

Fig. 3 Scanning Electron Microscopy (SEM) of TiO2 film: (a) before coated with copper, and (b)

after coated with copper nanoparticle.

In our previous paper [36-39], we have reported that the contact between TiO2 semiconductor

and the metal (Schottky contacts) can reduce recombination in solar cells. It has explained that

electrons in the conduction band of TiO2 material tends to transfer to the metal which has a higher

work function. Illustration of this process was explained by energy diagram in Fig. 4.

Fig. 4 Energy diagram of semiconductor-metal junction [38-39].


Crystal structure of TiO2 film, which was fabricated by spray methods, was investigated by

performing X-Ray Diffraction (XRD) PW1710. Its characteristics before and after coated by copper

nanoparticle are explained in Fig. 5. Figure 5a describe clearly that the TiO2 particles (PDF number:

21-1272) are in anatase phase with a diffraction peak at 25.280°, 36.944°, 37.799°, 48.047°,

53.887°, 55.058°, 62.686°, and 75.026°, respectively. Furthermore, Fig. 5b also confirms that the

copper nanoparticle (PDF number: 04-0836) has been successfully deposited on TiO2 films and its

peaks appear at 43.295°, 50.431° and 74.127°.

Fig. 5 X-ray diffraction pattern of TiO2 film: (a) before coated with copper, and (b) after coated

with copper nanoparticle.

From above result, TiO2 and copper crystal size was calculated by using Scherrer equation [40]. It is

found that the average size both of material is about 100 nm and 40 nm, respectively. In this study,

we use dye which was extracted from black rice (Bandung, Indonesia). According to Buraidah et al.

[14] that dye from black rice contains molecules of anthocyanins (cyanidin-3-glucoside and

peonidin-3-glucoside), and it can absorb photons in a wide spectral range. The chemical structure

and its bonding with TiO2 nanoparticle is illustrated in Fig. 6 (a,b). It can be seen that TiO2 particles

can form a bond with a group hidroxyl on cyanidin-3-glucoside and peonidin-3-glucoside.

Fig. 6 (a) Chemical structure of anthocyanin (cyanidin-3-glucoside dan peonidin-3-glucoside) from

black rice, and (b) Chemical bonding of anthocyanin and TiO2 particle [13].


In order to get optimum efficiency of DSSC solar cells, parameter which influencing absorbance

and dye loading as reported by other researcher [13-21], namely solution pH and temperature for

extracting, were used. We also used optimum time for dipping TiO2 film in dye solution as reported

by Takeuchi et al. [20]. Moreover, alcohol as organic and an effective solvent, as reported by

Buraidah et al. [14], were also used in extraction process. This treatment is related with ability for

optimum extracting process. A number of dyes loading on TiO2 surface were not discussed in this

paper.

UV-Vis characterization result, as shown in Fig. 7, indicates that natural dye from black rice is

potential to application in DSSC solar cells, due to wide of its absorption. From this figure, it can be

observed that there are two absorption peaks, where the first and second peak is at 350 nm and 500

nm, respectively. This characterization result is consistent with other paper reports [13,14,21].

Fig. 7 Absorbance spectrum of dye molecule extracted from black rice.

To determine performance of DSSC solar cells, calculation of fill factor and efficiency was

performed by using Equation (1) and (2) [13].

FF =

, (1)

η =

. (2)

where Imax is the maximum current, Vmax is maximum voltage, Isc is the solar cell current measured

at voltage V=0, Voc is the solar cell voltage measured at zero current and Pin is input power which

was get from illumination.

Fig. 8 shows the performance of DSCC solar cells without copper which comparing with DSSC

solar cells with employing copper bridges. It can be seen that performance of copper coated DSSC

(efficiency 0.35% and fill factor 0.35) is higher than DSSC without copper coating (efficiency

0.17% and fill factor 0.35). The possible reason is that copper bridges which contact with TiO2

particles enhance the electron transport, so that DSSC solar cell efficiency increases. However,

some paper [41-50] also reports that performance of solar cell was influenced by internal resistance.

Particularly, Han et al. [41-43] explained detail that performance of solar cells increase with

decreasing of internal resistance of solar cells.


Fig. 8 I-V characterization result: (a) Cu coated DSSC (Cu:DSSC), and (b) Cu uncoated Cu.

I-V characterization result of our solar cells, which was shown in Fig. 8, can be explained by

analyzing its circuit model as reported in some paper [2,41-48]. Simple circuit for solar cell in

steady illumination was illustrated in Fig. 9a [2]. Maximum performance of solar cells was obtained

by minimizing the series resistance, Rs, and maintaining the shunt resistance, Rsh, as high possible.

On the other hand, Fig. 9b also depicts the electrical equivalent for DSSC. The internal resistance of

solar cells is the sum of the each component given in Fig. 9b (Z = Z0+Z1+Z2+Z3). It is respectively

the contact impedance (TCO), Pt-catalyzed counter electrode impedance, complex impedance, and

Warburg impedance [2].

Fig. 9 (a) A simple equivalent circuit for DSSC, (b) a representative electrical equivalent of DSSC,

and (c) illustration of recombination process in DSSC [2].

It has explained by Lee et al. [2] that complex impedance (Z2 component), which represents the

interface between TiO2, dye and electrolyte, gives most important contribution to the total

resistance. Highly Z2 component may be caused by recombination process which may occurred in


TiO2, as illustrated in Fig. 9c. The presence metal semiconductor junction (Schottky junction)

occurred in our DSSC was possibly minimizing recombination phenomena at TiO2 surface because

this junction can trap electron and facilitating electronic transport to the main electrode (anode). As

the consequences, resistance of Z2 component reduced and performance of DSSC, as shown in Fig.

8a, was improved.

Moreover, in order to know effect of copper bridges more detail on internal resistance of our

solar cells, investigating by Electrochemical Impedance Spectroscopy (EIS) was performed. The

data obtained by using EIS measurements are then plotted with EIS software because limitation of

EIS instrument and equivalent circuit which was formulated by Han et al. [41], as shown in inset of

Fig. 10, was used. Then, performance of the solar cells was observed under sun illumination. The

EIS analyzing was summarized in Table 1.

Fig.10 Schematic Nyquist plot of DSSC solar cells and its equivalent model (inset) [41].

In Fig. 10, resistance which occurred on contact between TiO2 and substrate are represented R1.

Electronic transport on the dye/TiO2/Cu junction was represented with parallel circuit between R2

and C1. However, electronic transport occurred at polymer electrolytes and counter electrode was

respectively illustrated with parallel circuit between R3 and C2, as well as R4.

Table 1. Comparisons of the internal resistance of our DSSC solar cells analyzed by using EIS

DSSC Structure Intensity

(mW/cm2)

R1(Ω) R2(Ω) R3(Ω) R4(Ω) Internal

Resistance (Ω)

TiO2/dye/polymer

electrolyte/Pt

37.08 50 4,000 180 20 4,250

TiO2:Cu/dye/polymer

electrolyte/Pt

37.08 50 200 190 20 460

Table 1 describes that, at the same intensity, the internal resistance of DSSC solar cells after

coating with copper is smaller (about 9x) than before copper coating. This data was agreed with

result which was presented by Fig. 8. It was found that increasing of solar cell performance can be

implied decreasing of solar cells resistance. As stated above, that the presence of metal

semiconductor junction (Schottky junction) occurred in our DSSC minimizing recombination

phenomena at TiO2 surface and the consequences, resistance of Z2 component reduced about twenty


times (20x) after coating copper. So, we concluded that contact between TiO2 and copper

nanoparticle (Schottky contacts) can improve electronic transport to the main electrode (anode),

then total resistance of solar cell can be reduced.

Furthermore, we also demonstrated the use of polymer electrolyte consisting of polyvinyl acetate

(PVA) containing lithium ion. It is proposed to improve the efficiency of solar cells and also

supporting long-term stability. In order to know effect of this treatment, we compared two kinds of

solar cells which employing a polymer electrolyte and without polymer electrolyte (liquid

electrolyte). I-V measurement results of both DSSC solar cells with different electrolytes were

shown in Fig. 11.

Fig. 11 Performance of DSSC: (a) employing polymer electrolyte, and (b) liquid electrolyte

It can be seen in Fig. 11 that DSSC which employing polymer electrolyte and liquid electrolyte

has quite similar performance. However, long term stability of DSSC with polymer electrolyte is

better than without polymer electrolyte as shown in Fig. 12.

Fig. 12 Performance of DSSC solar cells under sun illumination

Performances of both solar cells for 6 days are presented in Fig. 12. DSSC with polymer

electrolytes is more stable than DSSC without polymer electrolytes. The possible reason is that

iodine ion distributed in the polymer chain, as reported by Wang et al. [25], can be exist in long

period. In the further study, optimation some parameter, such as thickness of TiO2, a number of dye

loading, TiO2 particle size, conductivity of polymer and thickness of catalyst on the counter

electrode was required to improve performance of solar cells.


Conclusion

DSSC solar cells which employing black rice as photosensitizer has been successfully fabricated

and improved its performance by depositing interconnected copper (copper bridge) in the space

between TiO2 particles. The observation by Scanning Electron Microscopy (SEM) and X-Ray

Diffraction (XRD) shows that interconnected copper in nanometer size has been successfully

deposited. Moreover, I-V characterization shows that performance of copper coated DSSC is higher

than uncoated DSSC. It is concluded that the presence of a copper bridge reducing the internal

resistance of DSSC solar cells and facilitating electronic transfer to anode. In addition, long-term

stability of DSSC solar cells was also achieved by employing PVA-based polymer electrolytes

containing iodine ion.

Acknowledgements

This work was partially supported by the Innovations and KK ITB Research Grant and

Decentralization Research Grant of Directorate General of Higher Education, Ministry of National

Education, and the Republic of Indonesia in the fiscal years of 2011/2012.

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