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Reaching stability at DSSCs with new type gel electrolytes
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Reaching stability at DSSCs with new type gel electrolytes a
b
c
Tolga Önen , Mücella Özbay Karakuş , Ramazan Coşkun , Hidayet Çetin a b c
a,⁎
T
Bozok University, Art & Science Faculty, Department of Physics, Yozgat 66100, Turkey Bozok University, Engineering and Architecture Faculty, Department of Computer Engineering, Yozgat 66100, Turkey Bozok University, Art & Science Faculty, Department of Chemistry, Yozgat 66100, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: DSSC Gel electrolyte Electrical stability
Stability is a significant problem for dye-sensitized solar cells (DSSCs) and if the problem cannot be surmounted, commercialization of DSSCs will not be possible. This work aims to show and solve the main reason for DSSCs’ stability problem. It is known that natural dyes are degradable; thus, a natural dye was used as a sensitizer to obtain the worst conditions for preparing DSSCs. The produced DSSCs were tested for 3 600 h and stability was achieved with the use of newly produced gels. The six newly produced gels were synthesized and seven DSSCs were produced. One of the DSSCs was prepared with a standard liquid electrolyte while the others were prepared with a liquid electrolyte adsorbed into poly(2-acrylamide-2-methylpropane sulfonic acid/itaconic acid/N,N'methylenebisacrylamide) (poly(AMPS-co-IA)) hydrogel and its fluorine, bromine, chlorine, and aniline doped derivatives. The DSSCs prepared with the hydrogel based-electrolytes showed stable and improved electrical characteristics for 3 600 h while the DSSC prepared with the liquid electrolyte’s performance got weaker from day to day and was wholly finished at 3 240 h.
1. Introduction The increasing demand for electrical energy is a big problem for countries. Due to the scarcity of fossil fuel resources, scientists have attempted to find new energy resources. Solar energy has been focused on as a significant energy source and several devices have been designed to benefit from sunlight [1]. Among these are photovoltaic panels, which have the advantages of direct conversion from sunlight to electricity and not including mechanical moving parts; consequently, they have far fewer breakages and require less maintenance. Thus, researchers have tended to fabricate more efficient and cheaper photovoltaic panels. DSSCs are one of the most promising solar cells to meet increasing energy demands [2]. They have attracted considerable attention owing to easy and inexpensive fabrication steps with relatively high energy conversion since the first DSSC was reported on in 1991 by O’Regan and Gratzel [3]. DSSCs do not need high purity raw material and cleanroom during the fabrication steps. Thus, the fabrication cost is relatively low [4,5]. When daylight is weak, like on a cloudy or winters day, DSSCs are superior with relatively high energy conversion [2,6]. DSSCs mainly consists of three components: a wide-bandgap semiconductor mesoporous film sensitized by a dye molecule, a Pt-counter electrode, and a redox electrolyte [7]. The stability of DSSCs can reach over 20 years by
⁎
selecting a suitable sensitizer dye [8]. Despite all of these positive aspects, liquid electrolyte usage in DSSCs is a severe problem leading to limited long-term performance due to leakage or volatilization. This problem is also hampering the fabrication of commercial DSSCs [4,9,10]. Several methods have been presented and polymer electrolyte usage has come into prominence to overcome the problem [11]. Polymer electrolytes can be grouped under two main categories: one of them is solid polymer electrolytes and the other is gel polymer electrolytes [12]. Solid polymer electrolytes can either be applied by casting the polymer electrolyte solution onto dye-adsorbed TiO2 photoelectrode [13] or placing a polymer membrane on the top of sensitized TiO2 [14]. However, the main disadvantage of polymer electrolytes is that they have lower ionic conductivity than that of liquid electrolytes [12]. Also, polymer electrolytes cover the top of the TiO2 electrode surface and it has a limited interaction area with TiO2 nanoparticles. Thus, the performance of the resulting DSSC is worse than that of liquid electrolyte-based DSSCs [1]. The quasi-solid state gel polymer electrolytes (GPEs) came into prominence with relatively high ionic conductivity due to the inclusion of a liquid trapped electrolyte in the polymer matrix. Classical GPEs consist of small fractions of low molar mass polar polymeric matrix in a large amount of organic plasticizer [15]. Different to classical GPEs, other GPEs are prepared by swelling the polymer in the liquid
Corresponding author. E-mail address: [email protected] (H. Çetin).
https://doi.org/10.1016/j.jphotochem.2019.112082 Received 4 July 2019; Received in revised form 3 September 2019; Accepted 6 September 2019 Available online 07 September 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
T. Önen, et al.
also used multi-walled carbon nanotube (MWCNT) for the preparation of counter electrodes. They found a 6.12% energy conversion efficiency with a [1% WO3-TiO2/N719/MWCNT] combination, which is around 12% higher than the highest reported in the literature for any nWO3TiO2 nanocomposite photoanodes in DSSCs. As can be seen by the works mentioned above, considerable effort has been made to commercialize and enable the effective use of DSSCs worldwide. For this purpose, this paper suggests new type gels and investigates the electrical stability of DSSCs using these gels. In this study, poly(2-acrylamide-2-methylpropane sulfonic aciditaconic acid/ N,N'-methylenebisacrylamide)(poly(AMPS-co-IA)) hydrogel, which has a high liquid holding capacity [30], is synthesized and used as GPEs in DSSC for the first time. In addition, aniline, 4-fluorine-aniline, 4chlorine-aniline, and 4-bromine-aniline are doped to poly(AMPS-co-IA) hydrogel to gain further knowledge on the usage of GPEs in DSSCs. The abbreviations of the doped hydrogels are as follows: poly(AMPS-IA-An), poly(AMPS-IA-F-An), poly(AMPS-IA-Br-An), poly(AMPS-IA-Cl-An), Poly(AMPS-IA-F-Cl-Br-An). These materials are prepared and swelled by a liquid electrolyte. Further, the electrical stability of the fabricated DSSCs is investigated for 3 600 h, which is longer than that of most literature, for the general applied stability test.
electrolyte [4]. Lan et al. [16] modified poly(acrylic acid) with amphiphilic polyethylene glycol (PEG) to form GPEs. They soaked the polymer membrane in a liquid electrolyte and produced GPEs. Priya et al. [17] used the same swelling technique in preparing GPEs. These scientists desired to obtain high conversion efficiency. They prepared a poly(vinylidene fluoride-co-hexafluoropropylene (PVdF-HFP) membrane and charged it with liquid electrolyte. When GPEs are used in DSSC, high ionic conductivity is a general desire to obtain high efficiency. For this purpose, Wang et al. [18] used PVdF-HFP to solidify a 3-methoxypropionitrile (MPN)-based liquid electrolyte to produce a quasi-solid-state gel electrolyte. Cheng et al. [19] prepared gel polymer electrolytes based on PVdF-HFP copolymer as the polymer matrix, PEG as the plasticizer, and polyethylene glycol dimethacrylate (PEGDMA) as the chemical cross-linking oligomer. Shanti et al. [20] synthesised poly (methyl methacrylate-co-butyl acrylate-co-acrylic acid [P(MMA-co-BAco-AA)] and they used synthesized polymer with various NaI concentrations in DSSCs. They investigated the energy conversion efficiency of the DSSCs for 250 h. Bella et al. [21] prepared a cellulose fiber-based photoanode and a nanoscale micro-fibrillated cellulosebased biopolymer electrolyte for DSSCs. They obtained 3.55% energy conversion efficiency with N719 dye. They also investigated the aging of the cell for 1 000 h. Sundararajan et al. [9] prepared a GPE that consisted of poly(methyl methacrylate-co-methacrylic acid) [P(MMAco-MAA)] and they examined the correlation between ionic conductivity and cell performance. In addition to research on gel electrolytes, there are also papers in the literature that suggest different solutions to the commercialization and long-term stability problems of DSSCs. These papers’ solutions include new types of electrolytes, newly designed sensitizer dyes, costeffective counter electrodes, and the patterning of photoelectrodes. DSSCs are electrochemical devices and a disadvantage of the flexible plastic design of DSSCs, as opposed to a glass design, is water permeation into the electrolyte medium is relatively rapid. Thus, long-term outdoor usage of flexible DSSCs is not possible. Law et al. [22] suggest a water-based electrolyte. A water-based electrolyte would not experience water permeation issues. In addition, Park et al. [23] investigated the effect of water containing I3− liquid electrolytes on photovoltaic performance and long-term stability. They found that the long-term stability of the cells was not significantly affected despite a high water content of up to 60 vol. percent in an organic solvent-based liquid electrolyte. Also, the photovoltaic performance and long-term stability of water-containing DSSCs mainly depend on the hydrophobicity of the dyes. Thus, some publications focus on aqueous DSSCs. Bella et al. [24] investigated photocurrent and long-term stability TiCl4 treatment. The aqueous DSSCs showed stable photovoltaic characteristics for 16 days. In other work, Carella et al. [25] show that the electrical stability of DSSCs can be obtained. Their synthesis and characterization of three carbazole-based dyes that were designed for p-type DSSCs showed a remarkable resistance to aging for 50 days. The patterning of photoanodes is an interesting technique to improve performance in terms of efficiency and stability [26]. Bella et al. [27] proposed a new approach based on the use of polymer materials for patterning the photoanodes of DSSCs and the conversion efficiency and stability of DSSCs was investigated for 2 000 h at 50 °C. The DSSCs that were prepared with the patterned photoanodes had a stability with only 4% reduction in power conversion efficiency when comparing initial efficiency while the standard counterpart showed an 8% decrease in its initial values. The fabrication of cost-effective DSSCs is important in order to overcome the commercialization problem of DSSCs. Mainly, Pt usage during the fabrication of the counter electrode increases the manufacturing cost of DSSCs. Gnanasekar et al. [28] used one-dimensional carbon-wrapped VO2 (M) nanofiber as a cost-effective counter electrode. They obtained 6.53% power conversion efficiency for the prepared DSSC. Younas et al. [29] synthesized tungsten oxide/titanium oxide nanocomposites (nWO3-TiO2) and used it as photoanodes. They
2. Materials and methods 2.1. Materials Titanium dioxide active opaque paste, platinum paste, N-butyl-N’methyl-imidazolium iodide (BMII), LiI, 4-tert-butylpyridine (TBP), I2, acetonitrile, and valeronitrile were purchased from Sigma Aldrich. The other all solvents and chemicals were purchased from Merck. FTO glass substrates (AN-10; sheet resistivity is 10 Ω/sq) were obtained from AGC SOLAR. 18.2 MΩ de-ionized water is used for the preparation of the dye and all cleaning procedures. 2.2. Preparation of the natural dye In this experiment, black mulberry (Morus Nigra) juice [7] was used as a sensitizer. The fresh, black mulberry fruits were washed with deionized water. The fruits were pressed, and the black mulberry dye was taken through a filter paper with the help of de-ionized water. pH value of the dye was measured as 3.85. 2.3. Characterization The natural dye was characterized by absorption spectra (obtained from Hach Lange DR 5000 Model UV–Vis spectrometer). Surface morphology of the deposited TiO2 films and GPE applied TiO2 structures were investigated by a scanning electron microscope (FEI Quanta 450 FEG). Also, the TiO2 surface was analyzed by an atomic force microscope (ezAFM-Nanomagnetics). Whole solar cell measurements were performed under 1000 W/m2 halogen lamp illumination intensity. 2.4. Preparation of the gel polymers With the total number of moles of the monomers being kept constant at 0.01 mol, 9 × 10−3 mol of 2-acrylamide-2-methylpropane sulfonic acid (AMPS), 1.0 × 10−3 mol of itaconic acid (IA), 5.0 × 10−3 mol N,N'-methylenebisacrylamide MBAAm) crosslinker, and 1.5 × 10−3 mol, the potassium persulfate (KPS) initiator was dissolved in 5.0 ml of distilled water. After adding 2 mL of aniline or a derivative in the solution, it was drawn into 0.4 cm pipettes with closed ends. Then, these pipettes were left in a water bath that had previously been set at 50 °C. After 24 h, the polymers were removed from the pipettes and the product was dried at 55 °C. Fig. 1 provides a schematic representation of the prepared polymers. 2
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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to test the gel electrolytes’ stability by measuring their weights periodically at room temperature. This could be determined easily with this experiment by seeing if their mass changed with the evaporation of the electrolyte. The second was performing I–V measurements on the DSSCs over time and investigating the aging of the solar cell parameters. Also, to realize a detailed analysis, the solar cells’ parameters were obtained using the single-diode model. The solar cell parameters of a DSSC can be obtained by using Eq. 1 according to the single-diode model. In this model, the equivalent circuit of a DSSC contains a single diode, a constant source of photogenerated current, a series resistance, and a parallel resistance. The analysis details can be found in the literature [32]. The current-voltage relationship of a solar cell is worked out as follows:
I = Iph −
Fig. 1. Schematic representation of poly(AMPS-co-IA) hydrogels loaded with aniline and its derivatives.
V + IRS V + IRS − 1⎞ − Is ⎛exp RSH nVt ⎝ ⎠ ⎜
⎟
(1)
where Iph is the photocurrent (in A), IS is the diode saturation current (in A), n is the diode ideality factor, RS is the series resistance (in Ω), RSH is the shunt resistance (in Ω), and Vt = kT ⁄q where q is the elementary charge, k is Boltzmann’s constant, and T is temperature in K. The ideality factor is for the diode in the equivalent circuit and it is a measure of how much the properties of a diode resemble the ideal diode properties. The ideality factor of an ideal diode is 1. I–V measurements were performed periodically to investigate the electrical stability of the DSSCs for 3 600 h. Furthermore, critical photovoltaic parameters were obtained from the I–V curves. These parameters are the short-circuit photocurrent density, Jsc, the open-circuit voltage, Voc, and the output power of the solar cell, Pmax. The fill factor (FF) is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. The fill factor is calculated as:
2.5. DSSC fabrication The DSSCs were constructed as follows: the FTO glass substrates were ultrasonically cleaned in trichloroethylene, acetone, methanol, and de-ionised water for 5 min., respectively. They were dried with high purity air and cut into 2.5 x 1.5 cm pieces. The TiO2 photoelectrodes were prepared by applying TiO2 paste onto the cleaned FTO plates. The doctor-blade technique was used to coat TiO2 paste on an active square area of 0.49 cm2 (7 × 7 mm2). The film thickness was measured as ∼11 μm. Afterward, the TiO2 films were dried on a hot plate at 125 °C for 10 min. The electrodes were sintered at 450 °C for 30 min. The sintering procedure was performed to remove unwanted organic loads and chemical residues in the TiO2 and to adhere the TiO2 nanoparticles to the surface. After the sintering procedure, the electrodes were cooled to 50 °C and they were immersed in the natural dye for 24 h at room temperature. Then, the photoanodes were washed with ethanol and dried. The counter electrodes were prepared by spreading the platinum paste on the FTO substrate. For the DSSCs, the prepared photoanode and Pt-counter electrode glass were sandwiched by using parafilm as a sealing material. A hotplate at 120 °C was applied to the electrodes to ensure the melting of the parafilm and that the electrodes would stick to each other. For the gel polymer electrolyte-based DSSCs, the gel polymers were firstly pulverized. The pulverized gel polymers were put into beakers separately and the electrolyte solution, which contained 0.6 M N-butylN-methyl-imidazolium iodide (BMII), 0.1 M LiI, 0.5 M 4-tert-butylpyridine (TBP), and I2 (0.05 M) in acetonitrile and valeronitrile [31], was added to the polymers. The mixture was mixed using a Teflon rod and the beakers were sealed with porous filter paper. They were kept in the electrolyte for 48 h. After the 48 h, each polymer absorbed the electrolyte in proportion to its absorption capacity and the non-absorbed electrolyte was removed from the beakers. After the swelling of the polymers with the electrolyte, the gels were gently applied to the TiO2 layers using the doctor-blade method. More than one sample (usually three) was produced for the same experiment to combat against experimental errors and thus check the reproducibility of the experiments.
FF =
Im Vm Isc Voc
(2)
where Im and Vm are the solar cell’s photocurrent and voltage at the maximum power point, respectively. The maximum output power of the solar cell is calculated using the equation:
Pmax = Im Vm.
(3)
3. Results and discussion 3.1. TiO2 surface morphology Fig. 2-A is an SEM image of the sintered TiO2 layer at 400.000 magnification. It can be easily seen that the sintered TiO2 layer has a high porosity, which is vital to enlarge the active surface area and consequently, the photocurrent. The average TiO2 particle size, which was measured from this image, was 42 nm. Further surface analysis was made using AFM, as shown in Fig. 2-B. A 3D AFM micrograph was obtained from 10 × 10 μm2 . In addition to the nanometer-sized pits and hills that were observed in the SEM analysis, the 3D AFM micrograph reveals the micron-sized heights, shoulders, pits, and valleys. The maximum height was measured in the AFM image was 609 nm.
2.6. Measurements and data analysis
3.2. Analysis of natural sensitizer
Two experiments were prepared to investigate the gel polymer’s stability. The first one concerned the electrolyte’s holding capacity of the gel polymers with time. The other concerned their use in the DSSCs by measuring the electrical characteristics over time. For the first experiment, after the polymers had swelled with the liquid electrolyte, they were subjected to an atmospheric environment
Fig. 3 shows the optical absorption spectrum of the dye that was used, namely mulberry dye. The dye has maximum absorptions regions below ∼370 nm and at ∼520 nm. Mulberries are known to have abundant alkaloids, polyphenols, flavonoids anthocyanins, and carotenoids [7,33]. This spectrum is in good agreement with the literature [17,34]. 3
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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Fig. 2. A. Scanning electron microscope image of the photoanode surface. The porosity of the TiO2 layer can be seen. B. 3D-AFM micrograph of the photoanode.
3.4. Stability of the used electrolytes The evaporation of electrolytes is the main problem concerning DSSCs, even if the electrolytes are blocked between electrodes and sealing films. In this part of the experiment, the electrolyte evaporation was investigated in such a way that the electrolytes were exposed to the atmosphere to study the electrolyte evaporation rate. Fig. 6 shows the comparative electrolyte mass loss via evaporation. For the experiment, the same quantity of liquid electrolytes was used. According to the graph, the mass loss of liquid electrolyte is very rapid at around 100 h. After that time, the mass of the electrolytes reaches saturation, with the value being nearly constant. Also, the electrolyte-loaded gels exhibit a more stable curve, with them having a small amount of mass loss compared to the liquid electrolyte. In Table 1, the vaporization of electrolytes was calculated using the starting point and saturation values in the graph. After 250 h of exposure, 51% liquid electrolyte loss was observed, while the amount of the loss for the gel electrolytes was between 8 and 14% at the same time interval. It is evident that the use of the gel polymer significantly inhibited electrolyte evaporation.
Fig. 3. The absorption spectrum of the mulberry dye. The mulberry dye was used as a sensitizer at the fabricated DSSCs.
3.3. Surface morphology of the gel polymer electrolytes 3.5. Electrical characteristics of DSSCs
Fig. 4A is a surface image of the AMPS-IA gel. Before the image was taken, the gel was swelled with the electrolyte solution and applied to the sintered TiO2 surface. An inhomogeneous surface with peaks and pits is seen. Similar surface morphologies can be viewed in Fig. 4B–E for AMPS-IA-F-An, AMPS-IA-An, AMPS-IA-Cl-An, and AMPS-IA-Br-An, respectively. While the above-mentioned ionic doping of the AMPS-IA results in the same morphological surface, the AMPS-IA-F-Cl-Br-An gel shows a relatively homogenous surface as can be seen in Fig. 4F. The doping of the F-Cl-Br-An in combination with AMPS-IA tunes and optimizes the surface energy of the polymer; thus, a more homogeneous surface can be obtained. Fig. 5A is a cross-sectional image of the gel polymer-applied photoanode. It is essential to know if the gel polymer has penetrated deeper than the TiO2 surface. Deeper penetration is desired to realize a connection between the gel polymer and TiO2 nanoparticles to achieve efficient electron transfer. In Fig. 5B, the gel polymer is on the TiO2 surface. For more information, larger magnification was applied, resulting in Fig. 5C. Looking at the image, it can be seen that the gel polymer is only on the surface, but the electrolyte solution-loaded swollen polymers can keep the TiO2 surface and the nanoparticles in the deeper layers wet via diffusion and the capillary effect.
Fig. 7 shows the I–V characteristics of the DSSCs prepared with the liquid electrolyte and the electrolyte-loaded gels. It is seen that the DSSCs with the gels can provide higher photocurrents. To gain further knowledge on the solar cell energy conversion mechanism, the singlediode model was applied to the I–V curves and the important photovoltaic parameters were determined. Looking at Table 2, a good agreement can be seen between data obtained from the single-diode model and the graphical parameter extraction method. It is also seen that the use of the gel polymers improves the solar cell’s parameters. For example, the photocurrent density and the output power of the solar cell were increased by using the polymer gels in the DSSCs. Although the most considerable increase in the photocurrent was observed with AMPS-IA gel, the highest output power occurred in the DSSC in which AMPS-IA-F-Cl-Br-An gel was used. Looking at the other parameters that were analyzed and are provided in Table 2, attention is firstly drawn to the ideality factor of 2.60 for the DSSC in which AMPSIA-F-Cl-Br-An was used, which is in good agreement with the literature [35]. The other DSSCs show poor ideality factor values. Another critical parameter is the series resistance of the solar cells. A lower series resistance causes a higher output of power. As can be seen from Table 2, the gel electrolytes cause a decreasing prominently at series resistances. While the DSSC with liquid electrolyte has a series resistance of around 4
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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Fig. 4. SEM images of gel polymer applied onto TiO2 surfaces. Similar surface characteristics with pits and hills are seen for the (A) AMPS-IA, (B) AMPS-IA-F-An, (C) AMPS-IA-An, (D) AMPS-IA-Cl-An and (E) AMPS-IA-Br-An gels. (F) The flatter and pinhole-free TiO2 surface covering are seen for the AMPS-IA-F-Cl-Br-An gel.
6 070 Ω, the other DSSCs with gel electrolytes have series resistance values of one third or even a quarter of that of the standard DSSC. A relatively thick spacer was used to replace the gel electrolyte between the counter and photoanode electrode. The same spacer was also used to provide the same experimental condition in the DSSC where liquid electrolyte was used. A possible reason for the high series resistance is the extended travel of the charge carriers between the counter and photoanode electrodes. The highest values for the Vm and Voc parameters are also seen in the DSSC in which AMPS-IA-F-Cl-Br-An was used. The question as to why the usage of the gels improves the solar cell parameters may arise. The handling of the gels in the DSSCs resulted in better ideality, lower series resistance, and a lower leakage current; that is, the diode in the solar cell equivalent circuit became ideal. The gels may cause a more efficient single-direction charge carrier transfer with their intermediate
energy levels. The polymer hydrogel used keeps the liquid electrolyte in its body and the liquid electrolyte works properly in the microchannels of the hydrogel in the DSSC. Thus, there are two structures: one is the liquid electrolyte and the other is the polymer hydrogel body. In addition to the contribution of the polymer body to the electrical conductivity, the polymer body may behave as a hole transporting layer. This situation depends on the polymer’s electrical properties. The polymer hole transporting layer is well known in the literature [36]. However, it is not known whether or not AMPS acts as a hole extraction/transporting layer, but this possibility may exist. Conversely, the walls of the polymer microchannel may serve as the polymer counter electrode. In addition, as can be seen from Fig. 4F, which is an SEM image of the AMPS-IA-F-Cl-Br-An gel, it has a relatively smooth surface compared to the other AMPS-IA gels. This situation relates to the surface energy of the polymer structure. The surface energy manages 5
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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Fig. 5. Cross-sectional SEM images of the gel polymer applied photoanode. The gel polymer, the TiO2, the conductive metal layer, and glass are shown. The images are for (A) 500X (B) 10000X, (C) 25000X.
Fig. 6. The vaporization of the used electrolytes was examined by measuring mass loss. The electrolytes were subjected to the atmosphere and their weights were measured periodically. AMPS-IA gels show more stable characteristics.
Fig. 7. The current-voltage characteristics of DSSCs prepared with different electrolytes. All measurements were performed at AM1.5 illumination light intensity.
Table 1 Electrolyte loss in % percentage. The values were calculated by obtaining weight measurements of the samples in the time interval between the samples were subjected to the atmosphere and 250 h later. Electrolyte
Electrolyte vaporization (%)
AMPS-IA AMPS-IA-F-Cl-Br-An AMPS-IA-F-An AMPS-IA-An AMPS-IA-Cl-An AMPS-IA-Br-An Liquid Electrolyte
9 14 9 9 8 11 51
adhesion, wettability, etc. [37,38] and thus the interaction between the gel and the TiO2 structure in terms of charge transportation. These are all a probability, but they may explain the observed improvements in the DSSC’s parameters.
3.6. Electrical stability of DSSCs The most exciting result of this work is the electrical stability of the produced DSSCs even though a natural dye was used. The electrical stability of the DSSCs was investigated by obtaining I–V measurements over 3 600 h. Fig. 8 shows the values of Jsc of the DSSCs for the 3 600 h. 6
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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Table 2 Experimental and analytical solution results of the solar cells for 298 K and 1000 W/m2 light illumination (*these values were calculated by using the graphical method). Electrolyte
AMPS-IA AMPS-IA-F-Cl-Br-An AMPS-IA-F-An AMPS-IA-An AMPS-IA-Cl-An AMPS-IA-Br-An Liquid electrolyte
*Experimental results
Single diode model
JSC (A/m2)
VOC (mV)
Jm (A/m2)
Vm (mV)
RS (Ω)
RSH (Ω)
Pmax (mW)
n
JPh (A/m2)
RS (Ω)
RSH (Ω)
JS (A/m2)
FF
2.238 1.745 1.428 1.117 1.136 1.117 0.463
363 404 354 384 392 371 374
1.386 1.299 0.974 0.817 0.803 0.697 0.309
222 265 183 265 264 221 209
1411 1231 1379 1810 1699 2172 6073
7875 24600 9768 38918 24108 13858 43988
0.015 0.017 0.009 0.011 0.010 0.008 0.003
3.58 2.60 4.83 2.14 2.39 5.12 5.10
2.239 1.745 1.428 1.117 1.136 1.117 0.463
1411 1231 1379 1810 1699 2171 6072
7875 24600 9768 38918 24108 13858 43988
0.043 0.004 0.082 0.001 0.001 0.067 0.027
0.379 0.488 0.353 0.505 0.476 0.372 0.373
Fig. 8. The stability of the short-circuit-current densities of DSSCs. DSSC fabricated with AMPS-IA-F-Cl-Br-An gel polymer showed the stable electrical characteristics with high current density, but the liquid electrolyte used one showed the worst performance.
Fig. 10. Pmax and Voc values obtained from the periodic electrical measurements during the 3 600 h for the DSSCs in which the liquid and gel polymers were used.
performance in terms of the stability measurements. Fig. 10 shows the Pmax and Voc values obtained from the periodic electrical measurements during the 3 600 h for the DSSCs in which the liquid and gel polymers were used. The graph shows the stability, Pmax, and Voc values of the fabricated DSSCs. If the data of the particular color on the graph is collected over a limited region, this indicates the stability of that DSSC. When the data of the DSSCs in which standard liquid electrolyte and AMPS-IA were used is examined, it can be seen that they spread over a large area and have a relatively lower Pmax. Looking at Fig. 10, it can be seen that the ionic doped AMPS-IA gels improve both the electrical characteristics and the stability of the solar cells. As a result, it can be said that the ionic doping of AMPS-IA gel tunes the output power characteristic of DSSCs. Further, the DSSC in which AMPS-IA-F-Cl-BrAn gel was used has relatively high Pmax and Voc values. 4. Conclusion
Fig. 9. The stability of the fill factors of DSSCs. DSSC fabricated with AMPS-IAF-Cl-Br-An gel polymer had the best fill factor value with high stability. The fill factor values for the liquid electrolyte used DSSC gradually decreased.
In this study, it was observed that DSSCs with natural dyes and the gel electrolytes exhibit a stable electrical performance for 3 600 h. The results for this work are summarised as:
The first noticeable data is that of the liquid electrolyte-based DSSC. Whereas all DSSCs were fabricated using a sealing film to prevent any leakage, a marked worsening of the Jsc value of the DSSC in which liquid electrolyte was used was observed. No visible leakage was detected for all the cells, but the Jsc was almost zero at the end of the 3 600 h. Other than AMPS-IA, the gels improved the electrical stability of the DSSC. The DSSC prepared with AMPS-IA-F-Cl-Br-An had the highest value of Jsc and the Jsc of the DSSC was still the highest at the end of 3 600 h. Fig. 9 shows the stability of the fill factors of the DSSCs. While the highest fill factor value was obtained by using AMPS-IA-F-Cl-Br-An gel for 3 600 h, the liquid electrolyte usage once again exhibits poor cell
i. DSSCs prepared with liquid electrolyte could have a limited lifespan due to the vaporization or leakage of the electrolyte. The performance of the DSSC prepared with the liquid electrolyte worsened throughout the 3 600 h. This problem can be solve by using ionic doped AMPS-IA gels. DSSCs with ionic doped AMPS-IA gels showed stable characteristics for the 3 600 h. ii. DSSCs prepared with AMPS-IA-F-Cl-Br-An gel electrolyte exhibited the highest output power, lowest series resistance, a high fill factor, and an ideality value closer to 1.
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Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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iii. Ionic doping, such as F, Cl, Br, or An, of AMPS-IA improves the short circuit current density and fill factor values of DSSCs. iv. There is a widespread belief that the weaknesses of DSSCs in terms of stability is due to the degradation of the dye used and the use of liquid electrolytes in DSSCs. Although the use of liquid electrolyte proved to be a problem in this work, it was found that even the use of a natural dye doesn’t cause any performance loss. Thus, DSSCs remain promising solar cells for the future. It is worth noting that electrically stable DSSCs can be reproducibly fabricated.
[17]
[18]
[19]
[20]
Acknowledgment [21]
We would like to thank Nanomagnetic Instruments Ltd. for the AFM image.
[22]
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Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Reaching stability at DSSCs with new type gel electrolytes a
b
c
Tolga Önen , Mücella Özbay Karakuş , Ramazan Coşkun , Hidayet Çetin a b c
a,⁎
T
Bozok University, Art & Science Faculty, Department of Physics, Yozgat 66100, Turkey Bozok University, Engineering and Architecture Faculty, Department of Computer Engineering, Yozgat 66100, Turkey Bozok University, Art & Science Faculty, Department of Chemistry, Yozgat 66100, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: DSSC Gel electrolyte Electrical stability
Stability is a significant problem for dye-sensitized solar cells (DSSCs) and if the problem cannot be surmounted, commercialization of DSSCs will not be possible. This work aims to show and solve the main reason for DSSCs’ stability problem. It is known that natural dyes are degradable; thus, a natural dye was used as a sensitizer to obtain the worst conditions for preparing DSSCs. The produced DSSCs were tested for 3 600 h and stability was achieved with the use of newly produced gels. The six newly produced gels were synthesized and seven DSSCs were produced. One of the DSSCs was prepared with a standard liquid electrolyte while the others were prepared with a liquid electrolyte adsorbed into poly(2-acrylamide-2-methylpropane sulfonic acid/itaconic acid/N,N'methylenebisacrylamide) (poly(AMPS-co-IA)) hydrogel and its fluorine, bromine, chlorine, and aniline doped derivatives. The DSSCs prepared with the hydrogel based-electrolytes showed stable and improved electrical characteristics for 3 600 h while the DSSC prepared with the liquid electrolyte’s performance got weaker from day to day and was wholly finished at 3 240 h.
1. Introduction The increasing demand for electrical energy is a big problem for countries. Due to the scarcity of fossil fuel resources, scientists have attempted to find new energy resources. Solar energy has been focused on as a significant energy source and several devices have been designed to benefit from sunlight [1]. Among these are photovoltaic panels, which have the advantages of direct conversion from sunlight to electricity and not including mechanical moving parts; consequently, they have far fewer breakages and require less maintenance. Thus, researchers have tended to fabricate more efficient and cheaper photovoltaic panels. DSSCs are one of the most promising solar cells to meet increasing energy demands [2]. They have attracted considerable attention owing to easy and inexpensive fabrication steps with relatively high energy conversion since the first DSSC was reported on in 1991 by O’Regan and Gratzel [3]. DSSCs do not need high purity raw material and cleanroom during the fabrication steps. Thus, the fabrication cost is relatively low [4,5]. When daylight is weak, like on a cloudy or winters day, DSSCs are superior with relatively high energy conversion [2,6]. DSSCs mainly consists of three components: a wide-bandgap semiconductor mesoporous film sensitized by a dye molecule, a Pt-counter electrode, and a redox electrolyte [7]. The stability of DSSCs can reach over 20 years by
⁎
selecting a suitable sensitizer dye [8]. Despite all of these positive aspects, liquid electrolyte usage in DSSCs is a severe problem leading to limited long-term performance due to leakage or volatilization. This problem is also hampering the fabrication of commercial DSSCs [4,9,10]. Several methods have been presented and polymer electrolyte usage has come into prominence to overcome the problem [11]. Polymer electrolytes can be grouped under two main categories: one of them is solid polymer electrolytes and the other is gel polymer electrolytes [12]. Solid polymer electrolytes can either be applied by casting the polymer electrolyte solution onto dye-adsorbed TiO2 photoelectrode [13] or placing a polymer membrane on the top of sensitized TiO2 [14]. However, the main disadvantage of polymer electrolytes is that they have lower ionic conductivity than that of liquid electrolytes [12]. Also, polymer electrolytes cover the top of the TiO2 electrode surface and it has a limited interaction area with TiO2 nanoparticles. Thus, the performance of the resulting DSSC is worse than that of liquid electrolyte-based DSSCs [1]. The quasi-solid state gel polymer electrolytes (GPEs) came into prominence with relatively high ionic conductivity due to the inclusion of a liquid trapped electrolyte in the polymer matrix. Classical GPEs consist of small fractions of low molar mass polar polymeric matrix in a large amount of organic plasticizer [15]. Different to classical GPEs, other GPEs are prepared by swelling the polymer in the liquid
Corresponding author. E-mail address: [email protected] (H. Çetin).
https://doi.org/10.1016/j.jphotochem.2019.112082 Received 4 July 2019; Received in revised form 3 September 2019; Accepted 6 September 2019 Available online 07 September 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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also used multi-walled carbon nanotube (MWCNT) for the preparation of counter electrodes. They found a 6.12% energy conversion efficiency with a [1% WO3-TiO2/N719/MWCNT] combination, which is around 12% higher than the highest reported in the literature for any nWO3TiO2 nanocomposite photoanodes in DSSCs. As can be seen by the works mentioned above, considerable effort has been made to commercialize and enable the effective use of DSSCs worldwide. For this purpose, this paper suggests new type gels and investigates the electrical stability of DSSCs using these gels. In this study, poly(2-acrylamide-2-methylpropane sulfonic aciditaconic acid/ N,N'-methylenebisacrylamide)(poly(AMPS-co-IA)) hydrogel, which has a high liquid holding capacity [30], is synthesized and used as GPEs in DSSC for the first time. In addition, aniline, 4-fluorine-aniline, 4chlorine-aniline, and 4-bromine-aniline are doped to poly(AMPS-co-IA) hydrogel to gain further knowledge on the usage of GPEs in DSSCs. The abbreviations of the doped hydrogels are as follows: poly(AMPS-IA-An), poly(AMPS-IA-F-An), poly(AMPS-IA-Br-An), poly(AMPS-IA-Cl-An), Poly(AMPS-IA-F-Cl-Br-An). These materials are prepared and swelled by a liquid electrolyte. Further, the electrical stability of the fabricated DSSCs is investigated for 3 600 h, which is longer than that of most literature, for the general applied stability test.
electrolyte [4]. Lan et al. [16] modified poly(acrylic acid) with amphiphilic polyethylene glycol (PEG) to form GPEs. They soaked the polymer membrane in a liquid electrolyte and produced GPEs. Priya et al. [17] used the same swelling technique in preparing GPEs. These scientists desired to obtain high conversion efficiency. They prepared a poly(vinylidene fluoride-co-hexafluoropropylene (PVdF-HFP) membrane and charged it with liquid electrolyte. When GPEs are used in DSSC, high ionic conductivity is a general desire to obtain high efficiency. For this purpose, Wang et al. [18] used PVdF-HFP to solidify a 3-methoxypropionitrile (MPN)-based liquid electrolyte to produce a quasi-solid-state gel electrolyte. Cheng et al. [19] prepared gel polymer electrolytes based on PVdF-HFP copolymer as the polymer matrix, PEG as the plasticizer, and polyethylene glycol dimethacrylate (PEGDMA) as the chemical cross-linking oligomer. Shanti et al. [20] synthesised poly (methyl methacrylate-co-butyl acrylate-co-acrylic acid [P(MMA-co-BAco-AA)] and they used synthesized polymer with various NaI concentrations in DSSCs. They investigated the energy conversion efficiency of the DSSCs for 250 h. Bella et al. [21] prepared a cellulose fiber-based photoanode and a nanoscale micro-fibrillated cellulosebased biopolymer electrolyte for DSSCs. They obtained 3.55% energy conversion efficiency with N719 dye. They also investigated the aging of the cell for 1 000 h. Sundararajan et al. [9] prepared a GPE that consisted of poly(methyl methacrylate-co-methacrylic acid) [P(MMAco-MAA)] and they examined the correlation between ionic conductivity and cell performance. In addition to research on gel electrolytes, there are also papers in the literature that suggest different solutions to the commercialization and long-term stability problems of DSSCs. These papers’ solutions include new types of electrolytes, newly designed sensitizer dyes, costeffective counter electrodes, and the patterning of photoelectrodes. DSSCs are electrochemical devices and a disadvantage of the flexible plastic design of DSSCs, as opposed to a glass design, is water permeation into the electrolyte medium is relatively rapid. Thus, long-term outdoor usage of flexible DSSCs is not possible. Law et al. [22] suggest a water-based electrolyte. A water-based electrolyte would not experience water permeation issues. In addition, Park et al. [23] investigated the effect of water containing I3− liquid electrolytes on photovoltaic performance and long-term stability. They found that the long-term stability of the cells was not significantly affected despite a high water content of up to 60 vol. percent in an organic solvent-based liquid electrolyte. Also, the photovoltaic performance and long-term stability of water-containing DSSCs mainly depend on the hydrophobicity of the dyes. Thus, some publications focus on aqueous DSSCs. Bella et al. [24] investigated photocurrent and long-term stability TiCl4 treatment. The aqueous DSSCs showed stable photovoltaic characteristics for 16 days. In other work, Carella et al. [25] show that the electrical stability of DSSCs can be obtained. Their synthesis and characterization of three carbazole-based dyes that were designed for p-type DSSCs showed a remarkable resistance to aging for 50 days. The patterning of photoanodes is an interesting technique to improve performance in terms of efficiency and stability [26]. Bella et al. [27] proposed a new approach based on the use of polymer materials for patterning the photoanodes of DSSCs and the conversion efficiency and stability of DSSCs was investigated for 2 000 h at 50 °C. The DSSCs that were prepared with the patterned photoanodes had a stability with only 4% reduction in power conversion efficiency when comparing initial efficiency while the standard counterpart showed an 8% decrease in its initial values. The fabrication of cost-effective DSSCs is important in order to overcome the commercialization problem of DSSCs. Mainly, Pt usage during the fabrication of the counter electrode increases the manufacturing cost of DSSCs. Gnanasekar et al. [28] used one-dimensional carbon-wrapped VO2 (M) nanofiber as a cost-effective counter electrode. They obtained 6.53% power conversion efficiency for the prepared DSSC. Younas et al. [29] synthesized tungsten oxide/titanium oxide nanocomposites (nWO3-TiO2) and used it as photoanodes. They
2. Materials and methods 2.1. Materials Titanium dioxide active opaque paste, platinum paste, N-butyl-N’methyl-imidazolium iodide (BMII), LiI, 4-tert-butylpyridine (TBP), I2, acetonitrile, and valeronitrile were purchased from Sigma Aldrich. The other all solvents and chemicals were purchased from Merck. FTO glass substrates (AN-10; sheet resistivity is 10 Ω/sq) were obtained from AGC SOLAR. 18.2 MΩ de-ionized water is used for the preparation of the dye and all cleaning procedures. 2.2. Preparation of the natural dye In this experiment, black mulberry (Morus Nigra) juice [7] was used as a sensitizer. The fresh, black mulberry fruits were washed with deionized water. The fruits were pressed, and the black mulberry dye was taken through a filter paper with the help of de-ionized water. pH value of the dye was measured as 3.85. 2.3. Characterization The natural dye was characterized by absorption spectra (obtained from Hach Lange DR 5000 Model UV–Vis spectrometer). Surface morphology of the deposited TiO2 films and GPE applied TiO2 structures were investigated by a scanning electron microscope (FEI Quanta 450 FEG). Also, the TiO2 surface was analyzed by an atomic force microscope (ezAFM-Nanomagnetics). Whole solar cell measurements were performed under 1000 W/m2 halogen lamp illumination intensity. 2.4. Preparation of the gel polymers With the total number of moles of the monomers being kept constant at 0.01 mol, 9 × 10−3 mol of 2-acrylamide-2-methylpropane sulfonic acid (AMPS), 1.0 × 10−3 mol of itaconic acid (IA), 5.0 × 10−3 mol N,N'-methylenebisacrylamide MBAAm) crosslinker, and 1.5 × 10−3 mol, the potassium persulfate (KPS) initiator was dissolved in 5.0 ml of distilled water. After adding 2 mL of aniline or a derivative in the solution, it was drawn into 0.4 cm pipettes with closed ends. Then, these pipettes were left in a water bath that had previously been set at 50 °C. After 24 h, the polymers were removed from the pipettes and the product was dried at 55 °C. Fig. 1 provides a schematic representation of the prepared polymers. 2
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to test the gel electrolytes’ stability by measuring their weights periodically at room temperature. This could be determined easily with this experiment by seeing if their mass changed with the evaporation of the electrolyte. The second was performing I–V measurements on the DSSCs over time and investigating the aging of the solar cell parameters. Also, to realize a detailed analysis, the solar cells’ parameters were obtained using the single-diode model. The solar cell parameters of a DSSC can be obtained by using Eq. 1 according to the single-diode model. In this model, the equivalent circuit of a DSSC contains a single diode, a constant source of photogenerated current, a series resistance, and a parallel resistance. The analysis details can be found in the literature [32]. The current-voltage relationship of a solar cell is worked out as follows:
I = Iph −
Fig. 1. Schematic representation of poly(AMPS-co-IA) hydrogels loaded with aniline and its derivatives.
V + IRS V + IRS − 1⎞ − Is ⎛exp RSH nVt ⎝ ⎠ ⎜
⎟
(1)
where Iph is the photocurrent (in A), IS is the diode saturation current (in A), n is the diode ideality factor, RS is the series resistance (in Ω), RSH is the shunt resistance (in Ω), and Vt = kT ⁄q where q is the elementary charge, k is Boltzmann’s constant, and T is temperature in K. The ideality factor is for the diode in the equivalent circuit and it is a measure of how much the properties of a diode resemble the ideal diode properties. The ideality factor of an ideal diode is 1. I–V measurements were performed periodically to investigate the electrical stability of the DSSCs for 3 600 h. Furthermore, critical photovoltaic parameters were obtained from the I–V curves. These parameters are the short-circuit photocurrent density, Jsc, the open-circuit voltage, Voc, and the output power of the solar cell, Pmax. The fill factor (FF) is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. The fill factor is calculated as:
2.5. DSSC fabrication The DSSCs were constructed as follows: the FTO glass substrates were ultrasonically cleaned in trichloroethylene, acetone, methanol, and de-ionised water for 5 min., respectively. They were dried with high purity air and cut into 2.5 x 1.5 cm pieces. The TiO2 photoelectrodes were prepared by applying TiO2 paste onto the cleaned FTO plates. The doctor-blade technique was used to coat TiO2 paste on an active square area of 0.49 cm2 (7 × 7 mm2). The film thickness was measured as ∼11 μm. Afterward, the TiO2 films were dried on a hot plate at 125 °C for 10 min. The electrodes were sintered at 450 °C for 30 min. The sintering procedure was performed to remove unwanted organic loads and chemical residues in the TiO2 and to adhere the TiO2 nanoparticles to the surface. After the sintering procedure, the electrodes were cooled to 50 °C and they were immersed in the natural dye for 24 h at room temperature. Then, the photoanodes were washed with ethanol and dried. The counter electrodes were prepared by spreading the platinum paste on the FTO substrate. For the DSSCs, the prepared photoanode and Pt-counter electrode glass were sandwiched by using parafilm as a sealing material. A hotplate at 120 °C was applied to the electrodes to ensure the melting of the parafilm and that the electrodes would stick to each other. For the gel polymer electrolyte-based DSSCs, the gel polymers were firstly pulverized. The pulverized gel polymers were put into beakers separately and the electrolyte solution, which contained 0.6 M N-butylN-methyl-imidazolium iodide (BMII), 0.1 M LiI, 0.5 M 4-tert-butylpyridine (TBP), and I2 (0.05 M) in acetonitrile and valeronitrile [31], was added to the polymers. The mixture was mixed using a Teflon rod and the beakers were sealed with porous filter paper. They were kept in the electrolyte for 48 h. After the 48 h, each polymer absorbed the electrolyte in proportion to its absorption capacity and the non-absorbed electrolyte was removed from the beakers. After the swelling of the polymers with the electrolyte, the gels were gently applied to the TiO2 layers using the doctor-blade method. More than one sample (usually three) was produced for the same experiment to combat against experimental errors and thus check the reproducibility of the experiments.
FF =
Im Vm Isc Voc
(2)
where Im and Vm are the solar cell’s photocurrent and voltage at the maximum power point, respectively. The maximum output power of the solar cell is calculated using the equation:
Pmax = Im Vm.
(3)
3. Results and discussion 3.1. TiO2 surface morphology Fig. 2-A is an SEM image of the sintered TiO2 layer at 400.000 magnification. It can be easily seen that the sintered TiO2 layer has a high porosity, which is vital to enlarge the active surface area and consequently, the photocurrent. The average TiO2 particle size, which was measured from this image, was 42 nm. Further surface analysis was made using AFM, as shown in Fig. 2-B. A 3D AFM micrograph was obtained from 10 × 10 μm2 . In addition to the nanometer-sized pits and hills that were observed in the SEM analysis, the 3D AFM micrograph reveals the micron-sized heights, shoulders, pits, and valleys. The maximum height was measured in the AFM image was 609 nm.
2.6. Measurements and data analysis
3.2. Analysis of natural sensitizer
Two experiments were prepared to investigate the gel polymer’s stability. The first one concerned the electrolyte’s holding capacity of the gel polymers with time. The other concerned their use in the DSSCs by measuring the electrical characteristics over time. For the first experiment, after the polymers had swelled with the liquid electrolyte, they were subjected to an atmospheric environment
Fig. 3 shows the optical absorption spectrum of the dye that was used, namely mulberry dye. The dye has maximum absorptions regions below ∼370 nm and at ∼520 nm. Mulberries are known to have abundant alkaloids, polyphenols, flavonoids anthocyanins, and carotenoids [7,33]. This spectrum is in good agreement with the literature [17,34]. 3
Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112082
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Fig. 2. A. Scanning electron microscope image of the photoanode surface. The porosity of the TiO2 layer can be seen. B. 3D-AFM micrograph of the photoanode.
3.4. Stability of the used electrolytes The evaporation of electrolytes is the main problem concerning DSSCs, even if the electrolytes are blocked between electrodes and sealing films. In this part of the experiment, the electrolyte evaporation was investigated in such a way that the electrolytes were exposed to the atmosphere to study the electrolyte evaporation rate. Fig. 6 shows the comparative electrolyte mass loss via evaporation. For the experiment, the same quantity of liquid electrolytes was used. According to the graph, the mass loss of liquid electrolyte is very rapid at around 100 h. After that time, the mass of the electrolytes reaches saturation, with the value being nearly constant. Also, the electrolyte-loaded gels exhibit a more stable curve, with them having a small amount of mass loss compared to the liquid electrolyte. In Table 1, the vaporization of electrolytes was calculated using the starting point and saturation values in the graph. After 250 h of exposure, 51% liquid electrolyte loss was observed, while the amount of the loss for the gel electrolytes was between 8 and 14% at the same time interval. It is evident that the use of the gel polymer significantly inhibited electrolyte evaporation.
Fig. 3. The absorption spectrum of the mulberry dye. The mulberry dye was used as a sensitizer at the fabricated DSSCs.
3.3. Surface morphology of the gel polymer electrolytes 3.5. Electrical characteristics of DSSCs
Fig. 4A is a surface image of the AMPS-IA gel. Before the image was taken, the gel was swelled with the electrolyte solution and applied to the sintered TiO2 surface. An inhomogeneous surface with peaks and pits is seen. Similar surface morphologies can be viewed in Fig. 4B–E for AMPS-IA-F-An, AMPS-IA-An, AMPS-IA-Cl-An, and AMPS-IA-Br-An, respectively. While the above-mentioned ionic doping of the AMPS-IA results in the same morphological surface, the AMPS-IA-F-Cl-Br-An gel shows a relatively homogenous surface as can be seen in Fig. 4F. The doping of the F-Cl-Br-An in combination with AMPS-IA tunes and optimizes the surface energy of the polymer; thus, a more homogeneous surface can be obtained. Fig. 5A is a cross-sectional image of the gel polymer-applied photoanode. It is essential to know if the gel polymer has penetrated deeper than the TiO2 surface. Deeper penetration is desired to realize a connection between the gel polymer and TiO2 nanoparticles to achieve efficient electron transfer. In Fig. 5B, the gel polymer is on the TiO2 surface. For more information, larger magnification was applied, resulting in Fig. 5C. Looking at the image, it can be seen that the gel polymer is only on the surface, but the electrolyte solution-loaded swollen polymers can keep the TiO2 surface and the nanoparticles in the deeper layers wet via diffusion and the capillary effect.
Fig. 7 shows the I–V characteristics of the DSSCs prepared with the liquid electrolyte and the electrolyte-loaded gels. It is seen that the DSSCs with the gels can provide higher photocurrents. To gain further knowledge on the solar cell energy conversion mechanism, the singlediode model was applied to the I–V curves and the important photovoltaic parameters were determined. Looking at Table 2, a good agreement can be seen between data obtained from the single-diode model and the graphical parameter extraction method. It is also seen that the use of the gel polymers improves the solar cell’s parameters. For example, the photocurrent density and the output power of the solar cell were increased by using the polymer gels in the DSSCs. Although the most considerable increase in the photocurrent was observed with AMPS-IA gel, the highest output power occurred in the DSSC in which AMPS-IA-F-Cl-Br-An gel was used. Looking at the other parameters that were analyzed and are provided in Table 2, attention is firstly drawn to the ideality factor of 2.60 for the DSSC in which AMPSIA-F-Cl-Br-An was used, which is in good agreement with the literature [35]. The other DSSCs show poor ideality factor values. Another critical parameter is the series resistance of the solar cells. A lower series resistance causes a higher output of power. As can be seen from Table 2, the gel electrolytes cause a decreasing prominently at series resistances. While the DSSC with liquid electrolyte has a series resistance of around 4
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Fig. 4. SEM images of gel polymer applied onto TiO2 surfaces. Similar surface characteristics with pits and hills are seen for the (A) AMPS-IA, (B) AMPS-IA-F-An, (C) AMPS-IA-An, (D) AMPS-IA-Cl-An and (E) AMPS-IA-Br-An gels. (F) The flatter and pinhole-free TiO2 surface covering are seen for the AMPS-IA-F-Cl-Br-An gel.
6 070 Ω, the other DSSCs with gel electrolytes have series resistance values of one third or even a quarter of that of the standard DSSC. A relatively thick spacer was used to replace the gel electrolyte between the counter and photoanode electrode. The same spacer was also used to provide the same experimental condition in the DSSC where liquid electrolyte was used. A possible reason for the high series resistance is the extended travel of the charge carriers between the counter and photoanode electrodes. The highest values for the Vm and Voc parameters are also seen in the DSSC in which AMPS-IA-F-Cl-Br-An was used. The question as to why the usage of the gels improves the solar cell parameters may arise. The handling of the gels in the DSSCs resulted in better ideality, lower series resistance, and a lower leakage current; that is, the diode in the solar cell equivalent circuit became ideal. The gels may cause a more efficient single-direction charge carrier transfer with their intermediate
energy levels. The polymer hydrogel used keeps the liquid electrolyte in its body and the liquid electrolyte works properly in the microchannels of the hydrogel in the DSSC. Thus, there are two structures: one is the liquid electrolyte and the other is the polymer hydrogel body. In addition to the contribution of the polymer body to the electrical conductivity, the polymer body may behave as a hole transporting layer. This situation depends on the polymer’s electrical properties. The polymer hole transporting layer is well known in the literature [36]. However, it is not known whether or not AMPS acts as a hole extraction/transporting layer, but this possibility may exist. Conversely, the walls of the polymer microchannel may serve as the polymer counter electrode. In addition, as can be seen from Fig. 4F, which is an SEM image of the AMPS-IA-F-Cl-Br-An gel, it has a relatively smooth surface compared to the other AMPS-IA gels. This situation relates to the surface energy of the polymer structure. The surface energy manages 5
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Fig. 5. Cross-sectional SEM images of the gel polymer applied photoanode. The gel polymer, the TiO2, the conductive metal layer, and glass are shown. The images are for (A) 500X (B) 10000X, (C) 25000X.
Fig. 6. The vaporization of the used electrolytes was examined by measuring mass loss. The electrolytes were subjected to the atmosphere and their weights were measured periodically. AMPS-IA gels show more stable characteristics.
Fig. 7. The current-voltage characteristics of DSSCs prepared with different electrolytes. All measurements were performed at AM1.5 illumination light intensity.
Table 1 Electrolyte loss in % percentage. The values were calculated by obtaining weight measurements of the samples in the time interval between the samples were subjected to the atmosphere and 250 h later. Electrolyte
Electrolyte vaporization (%)
AMPS-IA AMPS-IA-F-Cl-Br-An AMPS-IA-F-An AMPS-IA-An AMPS-IA-Cl-An AMPS-IA-Br-An Liquid Electrolyte
9 14 9 9 8 11 51
adhesion, wettability, etc. [37,38] and thus the interaction between the gel and the TiO2 structure in terms of charge transportation. These are all a probability, but they may explain the observed improvements in the DSSC’s parameters.
3.6. Electrical stability of DSSCs The most exciting result of this work is the electrical stability of the produced DSSCs even though a natural dye was used. The electrical stability of the DSSCs was investigated by obtaining I–V measurements over 3 600 h. Fig. 8 shows the values of Jsc of the DSSCs for the 3 600 h. 6
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Table 2 Experimental and analytical solution results of the solar cells for 298 K and 1000 W/m2 light illumination (*these values were calculated by using the graphical method). Electrolyte
AMPS-IA AMPS-IA-F-Cl-Br-An AMPS-IA-F-An AMPS-IA-An AMPS-IA-Cl-An AMPS-IA-Br-An Liquid electrolyte
*Experimental results
Single diode model
JSC (A/m2)
VOC (mV)
Jm (A/m2)
Vm (mV)
RS (Ω)
RSH (Ω)
Pmax (mW)
n
JPh (A/m2)
RS (Ω)
RSH (Ω)
JS (A/m2)
FF
2.238 1.745 1.428 1.117 1.136 1.117 0.463
363 404 354 384 392 371 374
1.386 1.299 0.974 0.817 0.803 0.697 0.309
222 265 183 265 264 221 209
1411 1231 1379 1810 1699 2172 6073
7875 24600 9768 38918 24108 13858 43988
0.015 0.017 0.009 0.011 0.010 0.008 0.003
3.58 2.60 4.83 2.14 2.39 5.12 5.10
2.239 1.745 1.428 1.117 1.136 1.117 0.463
1411 1231 1379 1810 1699 2171 6072
7875 24600 9768 38918 24108 13858 43988
0.043 0.004 0.082 0.001 0.001 0.067 0.027
0.379 0.488 0.353 0.505 0.476 0.372 0.373
Fig. 8. The stability of the short-circuit-current densities of DSSCs. DSSC fabricated with AMPS-IA-F-Cl-Br-An gel polymer showed the stable electrical characteristics with high current density, but the liquid electrolyte used one showed the worst performance.
Fig. 10. Pmax and Voc values obtained from the periodic electrical measurements during the 3 600 h for the DSSCs in which the liquid and gel polymers were used.
performance in terms of the stability measurements. Fig. 10 shows the Pmax and Voc values obtained from the periodic electrical measurements during the 3 600 h for the DSSCs in which the liquid and gel polymers were used. The graph shows the stability, Pmax, and Voc values of the fabricated DSSCs. If the data of the particular color on the graph is collected over a limited region, this indicates the stability of that DSSC. When the data of the DSSCs in which standard liquid electrolyte and AMPS-IA were used is examined, it can be seen that they spread over a large area and have a relatively lower Pmax. Looking at Fig. 10, it can be seen that the ionic doped AMPS-IA gels improve both the electrical characteristics and the stability of the solar cells. As a result, it can be said that the ionic doping of AMPS-IA gel tunes the output power characteristic of DSSCs. Further, the DSSC in which AMPS-IA-F-Cl-BrAn gel was used has relatively high Pmax and Voc values. 4. Conclusion
Fig. 9. The stability of the fill factors of DSSCs. DSSC fabricated with AMPS-IAF-Cl-Br-An gel polymer had the best fill factor value with high stability. The fill factor values for the liquid electrolyte used DSSC gradually decreased.
In this study, it was observed that DSSCs with natural dyes and the gel electrolytes exhibit a stable electrical performance for 3 600 h. The results for this work are summarised as:
The first noticeable data is that of the liquid electrolyte-based DSSC. Whereas all DSSCs were fabricated using a sealing film to prevent any leakage, a marked worsening of the Jsc value of the DSSC in which liquid electrolyte was used was observed. No visible leakage was detected for all the cells, but the Jsc was almost zero at the end of the 3 600 h. Other than AMPS-IA, the gels improved the electrical stability of the DSSC. The DSSC prepared with AMPS-IA-F-Cl-Br-An had the highest value of Jsc and the Jsc of the DSSC was still the highest at the end of 3 600 h. Fig. 9 shows the stability of the fill factors of the DSSCs. While the highest fill factor value was obtained by using AMPS-IA-F-Cl-Br-An gel for 3 600 h, the liquid electrolyte usage once again exhibits poor cell
i. DSSCs prepared with liquid electrolyte could have a limited lifespan due to the vaporization or leakage of the electrolyte. The performance of the DSSC prepared with the liquid electrolyte worsened throughout the 3 600 h. This problem can be solve by using ionic doped AMPS-IA gels. DSSCs with ionic doped AMPS-IA gels showed stable characteristics for the 3 600 h. ii. DSSCs prepared with AMPS-IA-F-Cl-Br-An gel electrolyte exhibited the highest output power, lowest series resistance, a high fill factor, and an ideality value closer to 1.
7
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iii. Ionic doping, such as F, Cl, Br, or An, of AMPS-IA improves the short circuit current density and fill factor values of DSSCs. iv. There is a widespread belief that the weaknesses of DSSCs in terms of stability is due to the degradation of the dye used and the use of liquid electrolytes in DSSCs. Although the use of liquid electrolyte proved to be a problem in this work, it was found that even the use of a natural dye doesn’t cause any performance loss. Thus, DSSCs remain promising solar cells for the future. It is worth noting that electrically stable DSSCs can be reproducibly fabricated.
[17]
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[20]
Acknowledgment [21]
We would like to thank Nanomagnetic Instruments Ltd. for the AFM image.
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