Electrochemical and photovoltaic properties of dye-sensitized solar cells based on Ag-doped TiO2 nanorods

Accepted Manuscript Title: Electrochemical and Photovoltaic Properties of Dye-Sensitized Solar Cells Based on Ag-Doped TiO2 Nanorods Authors: Elaheh Rajaei, Seyed Abdolkarim Hosseini, Afsaneh Valipouri PII: DOI: Reference:

S0030-4026(17)31806-5 https://doi.org/10.1016/j.ijleo.2017.12.168 IJLEO 60290

To appear in: Received date: Accepted date:

5-12-2017 29-12-2017

Please cite this article as: Rajaei E, Hosseini SA, Valipouri A, Electrochemical and Photovoltaic Properties of Dye-Sensitized Solar Cells Based on Ag-Doped TiO2 Nanorods, Optik - International Journal for Light and Electron Optics (2010), https://doi.org/10.1016/j.ijleo.2017.12.168 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electrochemical and Photovoltaic Properties of Dye-Sensitized Solar Cells Based on AgDoped TiO2 Nanorods

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Elaheh Rajaei1, Seyed Abdolkarim Hosseini*1,2, Afsaneh Valipouri1

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Department of Textile Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran

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Nanotechnology & Advanced Material Institute (NAMI), Isfahan University of Technology,

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Isfahan 8415683111, Iran

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Correspondence: Department of Textile Engineering, Isfahan University of Technology, Isfahan, 84156 83111, Iran E-mail address: [email protected]; Tel.: +98 31 33915034

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Abstract

Ag doped TiO2 nanofibers were produced using the electrospinning method. TiO2

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nanoparticles (NPs) and Ag doped TiO2 nanorods were used as the anode layer of dye sensitized

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solar cells (DSSCs). Pastes of nanorods/nanoparticles with 2.81 μm layer thickness were coated on fluorine doped tin oxide (FTO) conductive glasses using the Dr Blading method. The results of conducting the Photovoltaic test showed that the ratio of nanorods to titanium dioxide NPs and

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the concentration of the silver nitrate solution had a significant effect on the efficiency of the solar cells. According to the results, solar cells that had been with 0.36% silver nitrate solution and the titanium dioxide nanorods to its nanoparticles ratio of 0.25 attained the highest efficiency. Keywords: Dye sensitized solar cells, nanorod, Ag, nanostructure, photovoltaic. *

Correspondence: Department of Textile Engineering, Isfahan University of Technology, Isfahan, 84156 83111, Iran E-mail address: [email protected]; Tel.: +98 31 33915034

1. Introduction The dye sensitized solar cells, as an alternative to the silicon solar cells, were introduced by Gratzel group in 1991 [1]; They have widely attracted researchers’ attention [2-5]. The advantages

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of producing dye sensitized solar cells, in comparison with silicone solar cells, are their low costs and easier fabrication [6]. A dye sensitized solar cell basically consists of a dye sensitized photoanode which is typically a nanocrystalline TiO2 semiconductor film on fluorine-doped tin dioxide (FTO) sensitized with the ruthenium polypyridyl complex or an organic dye, a platinum

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(Pt) coated counter electrode, and the interlayer space filled with a soluble redox couple

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electrolyte such as (I-/I3-)[7].

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Mesoporous TiO2 NPs films are widely used as photo-anode materials in conventional DSSCs[8,

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9]. These materials are capable of absorbing dye molecules and conducting the photo-generated electrons from dye to the conductive substrate. Although the high surface area of TiO2 NPs film

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is efficient enough to absorb dye molecules, the weakness of the scattered light, the low electron diffusion coefficient, and the high electron recombination are the limiting factors affecting the

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performance of DSSCs[10, 11].

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There are several methods to coat the TiO2 layer on the FTO, such as sol–gel processing [12], physical vapor deposition (PVD) [13], spin coating [14], electro-hydrodynamic deposition (ESD) [15], doctor blading [16], and screen printing method [17]. Doctor blading is a simple and

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low cost method that can be performed manually[18]. Using the one-dimensional (1D) structure of TiO2 (such as nanorods [19-24], nanowires [25-

27] and nanotubes [28, 29]) improves the electrical and optical properties of photo-electrodes in dye-sensitized solar cells (DSSCs)[30]. They provide direct charge transfer paths and reduce

recombination, as compared to TiO2 nanoparticles (NPs)[31]. However, it should be noted that using single one-dimensional nanostructures reduces both the adsorption of dye molecules and the performance of the cell [32]. Therefore, TiO2 nanoparticles combined with nanorod composite

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electrodes have been designed to provide an electron transfer paths without reducing dye adsorption [33]. There are several methods to produce nanorods, such as dip-coating, electrospinning, and electrochemical methods. Electrospinning is a versatile, simple and continuous technique producing polymeric inorganic nanofibers in a wide range of diameters, from tens of nanometers to a few micrometers. Nanofibre properties can be easily controlled by

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varying the solution parameters in the spinning process; these parameters include the

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concentration, viscosity, surface tension and conductivity. Titanium oxide nanofibers have been

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successfully prepared using electrospinning methods [34].

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One approach to further improve the photovoltaic performance of DSSCs is to enhance the

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light absorption and charge separation by decorating Ag, Au, or Pt NPs on TiO2 photoanodes [3546]. Previous research studies have shown that the plasma surface treatments on TiO2 photoanodes

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could enhance the performance of DSSCs. Moreover, using silver nanoparticles doped TiO2 nanofibers as the photoanode could result in an increase of photocurrent density in nanoparticles

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doped solar cells [38].

The current study aimed to investigate the DSSCs prepared from Ag doped TiO2 nanorods

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produced by the electrospinning process. Therefore, two different mixtures of TiO2 nanoparticles/ nanorods were coated on conductive glass substrates by using doctor Blade’s method. Although the minimum amounts of materials were used in DSSCs, the achieved material was capable of offering the appropriate efficiency. Moreover, the effects of amounts of silver on the performance of solar cells were addressed.

2. Experimental Procedure 2.1 Preparation of nanorods pastes

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Ag doped TiO2 nanofibers were produced using the electrospinning method. The electrospinning sol consisted of polyvinylpyrrolidone (0.23 g, Mw= 1500000, Sigma Aldrich), ethanol (3.3 ml, 99.99%, Merck), acetic acid (1 ml, 99.7%, Merck), titanium isopropoxide (0.7 ml, >98%, Merck) and silver nitrate (Table 1).

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Table 1. Percentage of AgNO3 on the electrospinning sol

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Percentage

AgNO3(g)

0

0

Nanofibre2

0.12

0.006

Nanofibre3

0.24

0.012

Nanofibre4

0.36

0.018

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Nanofibre1

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of AgNO3

Weight of

A homogeneous solution for electrospinning was achieved by slow stirring at the room

temperature up to 24 hours. Afterwards, a syringe having a metallic needle was used to pump the

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precursor at a constant volumetric rate. The electrospinning was performed at the voltage of 20 kV, the feed rate of 0.26 ml/h, and the spinning distance of 10 cm from the rotating drum covered by an aluminum foil. The fabricated TiO2/Ag composite nanofibers were cured at the temperature

of 600◦C for 3 hours to eliminate the PVP polymer. Finally, Ag doped TiO2 nanorods were exposed to mechanical treatment for 30 minutes. In order to prepare the pastes of nanorods, 0.6 g of Polyethylene glycol and 1 ml of acetic acid

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were stirred with a magnetic stirrer for 20 min. Then, 0.5 g of Ag doped TiO2 nanorods, 1 ml of ethanol, and 0.1 ml of Terpineol were added to the solution and stirred for 45 min. Subsequently, the solution was sonicated in an ultrasonic bath for 90 min. To prepare the anode pastes, the solution was stirred again up to 5 hours, while evaporating ethanol at 80◦C. Finally, the pastes of nanorods and nanoparticles were mixed together in the mass ratio of 25:35(the weights of

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nanorods and nanoparticles were 0.25 g and 0.35 g respectively).

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2.2. Fabricating photoelectrodes and assembling the DSSCs

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Pastes of mixed nanorods-nanoparticles were coated on FTO conductive glasses using Dr.

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Blading method (the effective anode surface of 0.25 cm2 and the thickness of 2.81μm). To provide a homogeneous anode layer, electrodes were exposed to the temperature of 450 °C for 90 min.

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The photoelectrodes were prepared by immersing electrodes in an N719 dye solution (0.15 mM in ethanol). On the other hand, the counter electrode was made of a FTO glass coated with a thin

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layer of platinum (Pt) catalyst and sealed by a sealing material. The liquid electrolyte was injected into the sample through a tiny hole drilled in the counter electrode. Then, the hole was sealed by

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the same sealing material and small pieces of glass.

3. Results and discussion 3.1. Nanofibers and Nanorods

Figure 1 shows SEM images of Ag doped TiO2 nanofibers described in Table 1. Figure 2 indicates the distribution of the nanofibers diameter. Moreover, figures 3 and 4 show SEM images of nanorods and distribution of the nanorods diameter, respectively. From figures 1 and 2, mean

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diameters (and their standard deviation) of nanofibers 1 to 4 were calculated to be 212 (52), 199 (43), 191 (47), and 290 (183) nm, respectively. Also, mean diameters (and their standard deviation) of nanorods 1 to 4, as extracted from figures 3 and 4, were 51 (13), 43(10), 45(11) and 53(21) nm, respectively. It seemed that increasing the percentage of AgNO3 on electrospinning sol firstly decreased the mean diameter of nanofibers as well as nanorods, then increased their

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diameter.

Fig. 1 SEM microscope images for nanofibers with a) Nanofibre1, b) Nanofibre2, c) Nanofibre3, and d) Nanofibre4

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Fig. 2 Fiber diameter distribution of nanofibers with a) Nanofibre1, b) Nanofibre2, c)

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Nanofibre3, and d) Nanofibre4

Fig. 3 SEM microscope images for nanorods with a) Nanorod 1, b) Nanorod 2, c) Nanorod 3, and d) Nanorod 4

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Fig. 4 Diameter distribution of nanorods with a) Nanorod 1, b) Nanorod 2, c) Nanorod 3, and d)

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3.2. Microstructure of photoanodes

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Nanorod 4

SEM images were used to reveal the morphology of the anode layer composed of TiO2 doped

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Ag nanorods/TiO2 nanoparticles. Figure 5 shows the microstructure of the anode layer having 35 wt. % of the TiO2 doped Ag nanorods film. The average size of nanoparticles and nanorods were

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estimated using some digimizer software; they were equal to 35 nm and 40 nm, respectively. As

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shown in Fig. 5, the thickness of the anode layer was obtained to be around 2.81 μm.

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Fig. 5 SEM image for the composite TiO2 doped Ag nanorods/TiO2 nanoparticle anode layer (a)

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3.3. Photovoltaic Characterization

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cross-section and (b) Surface

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Several DSSC samples were prepared in various amounts of Ag in the electrospinning process and different mass ratios of nanorods (25 wt. % versus 35 wt. %). To make a comparison between

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photovoltaic behaviors of DSSCs, photovoltaic tests were carried out on all samples at the standard test condition (1000 W/m2, AM 1.5, 25 °C). Figures 6 and 7 show the Current density-

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Voltage curves of solar cells having 25 wt. % and 35 wt. % of nanorods, respectively. As shown in Table 2, the values of open-circuit voltage (Voc), short-circuit photocurrent density (Jsc), fill

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factor (FF), and efficiency were extracted from the curves of solar cells. The results showed that the solar cells with 25 wt. % of nanorods had higher efficiency, as

compared to those with 35 wt. %. In this case, the possible reason could be the presence of larger vacancies in the anode layer of the case with 25 wt. %, which helped to absorb further dye

molecules. Further dye molecules in the photoelectrode tended to absorb more photons in DSSCs,

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which increased the probability of generating electrons.

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Fig. 6 Current density-Voltage curve for the solar cells with 25 wt% of nanorods

Fig. 7 Current density-Voltage curve for solar cells with 35 wt% of nanorods

Table 2. Photovoltaic parameters for DSSCs percentage of AgNO3 on

the weight ratio of

code

Voc

FF

efficiency

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electrospinning nanorods:nanoparticles

Jsc

sol 0

25%

0.69 9.33 0.53

3.4

B

0.12

25%

0.73 9.14 0.49

3.2

C

0.24

25%

0.71 9.54 0.54

3.6

D

0.36

25%

E

0

35%

F

0.12

35%

G

0.24

H

0.36

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4 2.1

0.7

2.62 0.59

1.1

35%

0.71 4.97 0.51

1.8

35%

0.72 5.39 0.64

2.5

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0.67 6.76 0.47

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0.6

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0.71 9.08

The results revealed that increasing the amount of silver nitrate in the electrospun solution

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decreased the efficiency. However, further increase in silver nitrate led to an increase in efficiency. As silver has high conductivity, decreasing recombination resistance could also reduce efficiency.

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On the other hand, addition of AgNO3 to the electrospun solution could enhance optical absorption, scatter more light in the red part of the solar spectrum and improve the electrons collection efficiency.

3.4. Electrochemical Modeling of DSSCs with 25 wt. % nanorods

To further understand the interfacial charge transfer, EIS was performed on DSSCs having 25wt% of nanorods in dark and at a voltage of 20 mV AC; frequency ranged from 0.01Hz to 100 KHz and the radiated power was 100 mW/cm2. EIS curve of the solar cell containing 25wt% of

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nanorods are shown in Figure 8. By using ZVIEW software, series resistance (Rs), transport

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resistance (Rtr) and recombination resistance (R-rec) were extracted.

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Fig. 8 EIS curve of the solar cells containing 25 wt% of nanorods

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As can be seen in Table 3, there was no significant change in the series resistance of solar cells. It showed that electrons could be transported within the FTO glass at a constant condition. However, table 3 indicates that recombination resistance in the solar cell (A) was more than that

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in the other solar cells. As there was no AgNO3 in the solar cell (A), it could be concluded that nanoparticles of Ag were the predominant factor decreasing recombination resistance. However, other factors should be considered in the case of cell efficiency. As the most important parameter,

transport resistance (Rtr) of the solar cell (A) was significantly higher than that of the other solar cells. The solar cell (A), which had no AgNO3, showed higher resistance against electron transfer.

of DSSCs with 25 wt. %

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Table 3. Series resistance (Rs), transport resistance (Rtr) and recombination resistance (R-rec)

A

B

C

D

Rs

19.9

20.2

19.6

20

R-Rec

7.989E+12

48.8

27.97

16.57

Rtr

7.067

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parameter

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2.081 0.11616 3.392

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4. Conclusion

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In this study, dye sensitized solar cells were prepared by the combination of TiO2 nanoparticles and Ag doped nanoparticles in a photoanode. Ag doped TiO2 nanofibers were produced using the

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electrospinning method.

Ag nanoparticle doped TiO2 nanorods were fabricated by curing and mechanical treatments of

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nanofiber Solar cells. The samples having 25 wt. % of nanorods showed higher efficiency in comparison to those having 35 wt. % of nanorods. Therefore, the amount of nanoparticles in the

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anode layer could be regarded as an important factor affecting efficiency, since it could provide more specific surface area in contact with the dye molecules. Addition of silver nitrate to the electrospinning solution tended to decrease the efficiency at the first stage and then increased it with the enhanced silver nitrate. Addition of AgNO3 to the electrospinning solution could enhance the light harvesting efficiency by plasmon enhanced optical absorption, thereby improving

electron collection efficiency. Therefore, electrons could be transport more quickly, thereby increasing the Jsc and efficiency. Compared with the undoped DSSCs samples, the conversion

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efficiency of the Ag doped DSSCs samples was improved by 18%.

Declaration of conflicting interests

The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding

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The authors received no financial support for the research, authorship, and/or publication of this

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article.

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