ISSN 1023-1935, Russian Journal of Electrochemistry, 2018, Vol. 54, No. 10, pp. 755–759. © Pleiades Publishing, Ltd., 2018. Published in Russian in Elektrokhimiya, 2018, Vol. 54, No. 10, pp. 863–868.
Effect of N719 Dye Dipping Temperature on the Performance of Dye-Sensitized Solar Cell1 M. Y. A. Rahmana, *, L. Rozab, S. A. M. Samsuric, and A. A. Umara aInstitute
of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, Bangi, Selangor, 43600 Malaysia b Program Studi Pendidikan Fisika, Fakultas Keguruan dan Ilmu Pendidikan, Universitas Muhammadiyah Prof, Dr Hamka, Jakarta Timur, 13730 Indonesia c Nano-Optoelectronics Research and Technology Laboratory School of Physics, Universiti Sains Malaysia, Penang, 11800 Malaysia *e-mail:
[email protected] Received January 13, 2017; in final form, July 7, 2017
Abstract—Preparation parameter of dye plays important role in determining DSSC performance. This paper reports the influence of N719 dye dipping temperature on the optical properties and performance parameters of the DSSC utilizing TiO2 films prepared via microwave technique. The TiO2 coated N719 dye films were prepared at various temperatures, namely, 30, 50, 60, 70 and 80°C. It is found that the TiO2 film dipped into N719 dye solution at 50°C possesses the broadest optical absorption window and the highest dye loading. It is also found that the dye dipping temperature does not affect the leak current in the device. The short-circuit current density (JSC) and power conversion efficiency (η) are strongly influenced by the dipping temperature. The DSSC utilizing the sample prepared at 50°C demonstrated the highest JSC and η of 4.06 mA cm–2 and 1.36%, respectively due to highest dye loading and recombination resistance. Keywords: dye-sensitized solar cell, dipping temperature, microwave technique, organic dye, titanium dioxide DOI: 10.1134/S1023193518100051
INTRODUCTION Dye coated on metal oxide films in dye-sensitized solar cell (DSSC) serves as a sensitizer agent in enhancing light absorption especially in visible region. Normally, DSSC utilizes wide band gap metal oxides such as TiO2 or ZnO as photoanode. The bandgap of these materials is almost the same that is 3.2 eV. However, these materials only absorb light in ultra violet (UV) region due to their wide band gap, thus generating low charge carrier density. In order to improve light absorption, dye is coated on the surface of metal oxide film. There are two types of dye, synthetic and natural dyes. The examples of synthetic or organic dyes are ruthenium [1], porphyrin [2] eosin-Y [3] and coumarin [4]. It has been reported that the η of the DSSC utilizing ruthenium, porphyrin, eosin-Y and coumarin dyes were 8.54, 7.10, 6.50 and 1.50%, respectively. The corresponding JSC were 23.9, 14.0, 18.8 and 1.60 mA cm–2, respectively, [1–4]. The examples of natural dyes are roselle flower [5], blue pea flower [5], spinach leaf [6], ipomoea [6], red sicilian orange [7] and purple eggplant fruit [7]. The η of the DSSC utilizing roselle, blue pea, spinach, ipomoea, sicilan orange and eggplant were 0.37, 0.05, 0.32, 0.13, 1 The article is published in the original.
0.66 and 0.48%, respectively corresponding with the JSC of 1.63, 0.37, 1.12, 0.47, 3.84 and 3.40 mA cm–2 [5–7]. Synthetic dye is more popular to be utilized as sensitizer in DSSC since it possesses higher light absorption than that of natural dye. This factor leads to DSSC utilizing synthetic dyes demonstrates much higher power conversion efficiency and short-circuit current density than that utilizing natural dye. Several researches have investigated the influence of organic dye preparation parameters dye such as dye concentration [3, 8, 9] and dipping time [9] on the performance of DSSC. In this work, we have investigated the influence of N719 dye dipping temperature on the performance parameters of the DSSC utilizing TiO2 films synthesized by microwave irradiation technique. The originality of this work is the study of the influence of dye dipping temperature on the DSSC performance parameters such as JSC and η. These parameters were then linked to dye loading, dark current and EIS data such as Rb, Rct and carrier lifetime. EXPERIMENTAL Preparation of TiO2 Films TiO2 films were grown on ITO substrate by using a chemical wet microwave-assisted method. ITO sub-
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strate (Zhuhai Kaivo Electronic) with a sheet resistance of ~10 cm/square was immersed into a growth solution consisting of 5 mL of 0.1 M ammonium hexafluorotitanate, (NH4)2TiF6 (Sigma Aldrich), 5 mL of 0.2 M boric acid, H3BO3 (R&M Chemical) and 0.1 mL of 0.1 M CTAB (Sigma Aldrich) surfactant. The substrate was vertically suspended in the solution and the reaction was then transferred into a microwave oven (Panasonic Home Appliances, NN-GD577M, 1100 W, 2.45 GHz) at 180 W for 10 min. The preparation of the films was completed by rinsing the sample with adequate amount of deionized water and subjected to an annealing process at 400°C for 30 min in air. The optical property of TiO2 films and TiO2 coated N719 dye films were characterized by UV-Vis spectrometer. The dye loading of TiO2 coated N719 dye films with various dye dipping temperature were computed from the area of absorption of UV-Vis spectra of TiO2 films and TiO2 coated N719 dye films. The optical property measurement was carried out five times. The highest dye loading for each sample is illustrated in Table 1. Fabrication and Performance Study of DSSC TiO2 films grown on ITO were dipped into 0.5 mM ethanolic solution of N719 dye (Sigma-Aldrich) at 30°C for 3 h. This procedure was repeated for various dye dipping temperatures, 50, 60, 70 and 80°C. The sample was then gently rinsed with ethanol and dried using nitrogen gas flow. The counter electrode for the device was prepared by depositing platinum layer on ITO substrate using sputtering technique. The TiO2 coated dye and the counter electrode were sealed using Surlyn film of thickness 100 μm. The electrolyte, AN-50 (Solaronix) was injected into the cell and filled via a capillary action. The photovoltaic performance of the DSSC with various dye dipping temperatures was investigated by current–voltage (I–V) measurement under 100 mW cm–2 tungsten light using Keithley 237 source measurement unit equipped with a personal computer (PC). The photovoltaic measurements were carried out five times and the best photovoltaic parameters are illustrated in Table 1. I–V curves in dark were also recorded on the PC. The illuminated area of the cell was 0.23 cm2. Electrochemical
impedance spectroscopy (EIS) technique was also performed to study the bulk resistance (Rb), the charge interfacial resistance (Rct) and charge carrier lifetime at the applied voltage of 0.4 V. RESULTS AND DISCUSSION Figure 1 illustrates the UV-Vis absorption spectrum of the uncoated dye TiO2 film sample. The sample absorbs more light in UV region with the wavelength less than 400 nm. After this wavelength, the sample absorbs less amount of photon as observed in Fig. 1. This is due to the energy gap of TiO2 is wide thus requires more photon energy to generate charge carriers from valence band to its conduction band. This result is consistent with that reported in [10]. Figure 2 depicts the UV-Vis spectra of TiO2 coated N719 dye films prepared at various dye dipping temperatures. It is clearly seen from the spectra that all samples absorbs more light in visible region ranging from 400–600 nm. The light absorption starts to decline in infrared (IR) region which falls within the wavelength 600–800 nm. The 50°C sample possesses the broadest area of absorption, followed by 60, 70, 30, and 80°C. The increasing or decreasing trend of absorption with the dye dipping temperature is not observed from the spectra. It also found that the peak absorption in visible region is not located at the same wavelength for each sample. For the 50°C sample, the peak is located at 520 nm, that for 60°C, it is at 500 nm and that for 70°C sample is at 530 nm. The peak absorption for the other two samples is not determined since the absorption spectra are not smooth as shown in Fig. 2. By using the area of the samples and estimating the area of absorption of the uncoated dye sample in Fig. 1 and that of the dye coated samples shown in Fig. 2, the dye loading were calculated and listed in Table 1. From the table, it is noticed that the 50 and 60°C samples possess the highest dye loading and the 30 and 80°C samples have the lowest dye loading. The higher dye loading, more photon be absorbed by the dye and increases the charge carrier density generated and collected to the TiO2 films. Figure 3 illustrates the I–V curves in dark of the devices utilizing the samples prepared at various dye dipping temperatures. It is observed from the leak cur-
Table 1. Photovoltaic parameters and EIS data of the DSSC with various dye dipping temperatures Growth temperature, °C
Dye loading, ×10–7, mol cm–2
Voc, V
30 50 60 70 80
0.017 0.023 0.023 0.021 0.017
0.52 0.61 0.60 0.60 0.63
Jsc, mA cm–2 1.85 4.06 3.80 3.16 2.73
FF
η, %
Rb, Ω
Rcr, Ω
τ, ms
0.46 0.54 0.49 0.55 0.46
0.44 1.36 1.13 1.00 0.79
12 16 17 12 24
378 1384 1233 588 583
1.0 3.2 2.5 4.1 2.5
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Absorbance, arb. units
9 7 5 3 1 300
400
500
600
700
800
Relative absorbance, arb. units
EFFECT OF N719 DYE DIPPING TEMPERATURE ON THE PERFORMANCE
0.6
757 30°С 50°С 60°С 70°С 80°С
0.5 0.4 0.3 0.2 0.1
0 400 450 500 550 600 650 700 750 800 Wavelength, nm
Wavelength, nm Fig. 1. UV-Vis spectrum of TiO2 film with the growth time of 10 min.
Fig. 2. UV-Vis spectra of TiO2 coated N719 dye films prepared at various dye dipping temperatures.
Current, mA
rent which is in the negative or reverse bias is higher than that in positive or forward bias. This leads to the curve in forward bias is unsymmetrical with that in reverse bias. In other words, the device does not show rectification property and performs low photovoltaic parameters such as Jsc and η [11]. It is noticed that the dark current neither increase nor decrease with the dipping temperature. It is said that the dark current is not influenced by the dipping temperature. The device utilizing 70°C sample possesses the lowest dark current while that utilizing 50°C sample has the highest dark current. Figure 4 shows the current density–voltage (J–V) curves of the DSSC utilizing the samples prepared with various dye dipping temperatures under 100 mW cm–2 light illuminations. The slope of the curves is quite high, indicating high power loss in the device which is due to high leak current as illustrated in Fig. 3. High power loss results in low fill factor (FF) and power conversion efficiency (η) as illustrated in Table 1. Low FF was also obtained for the DSSC utilizing NiO photoanode [12]. The other photovoltaic parameters such as short-circuit current density (Jsc) and open-circuit voltage (Voc) are analyzed from Fig. 4 and illustrated in Table 1. Also from Fig. 4, it is noticeable that the device utilizing the 30°C sample generates the lowest output power, followed by that utilizing 80, 70, 60 and 50°C samples. The decreasing or increasing trend of the output power with the dipping temperature is not observed. Figure 5 shows Nyquist plots of the DSSC utilizing the samples prepared at various dye dipping temperatures. The plots show two semicircles. The first semicircle for each device is so small and almost invisible. Two resistances are obtained, namely, bulk resistance (Rb) represented by smaller semicircle and charge recombination resistance (Rcr) represented by bigger semicircle. Both resistances are presented in Table 1. From the table, is found that Rcr is much higher than Rb for all devices. This is because charge carriers move RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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–0.6
–0.001 –0.003 –0.005 –0.007
0.2
0.6 1.0 Voltage, V 30°С 50°С 60°С 70°С 80°С
–0.009 Fig. 3. I–V curves of the devices in dark with various dye dipping temperatures.
Current density, mA cm–2 1 –0.2 –0.1 0 –1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
–2 –3 –4 –5
30°С 50°С 60°С 70°С 80°С
Voltage, V Fig. 4. J–V curves of the devices with various N719 dye dipping temperatures under illumination of 100 mW cm–2.
across the interfaces of DSSC slower to recombine than within the the device. From the table, it is found that the device prepared at 30°C demonstrates the lowest Rb and Rct. The Rb and Rct vary with the dye dipNo. 10
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Z '', Ω 500
60
400 300 200
30°С 50°С 60°С 70°С 80°С
50 Phase θ, deg
30°С 50°С 60°С 70°С 80°С
40 30 20 10
100 0.1 0
200
400
600
800 1000 1200 1400 Z ', Ω
1
10
100 1000 10 000 1 000 000 100 000 Frequency, Hz
Fig. 5. Nyquist plots of the DSSCs utilizing N719 dye various dipping temperatures.
Fig. 6. Bode plots of the DSSCs utilizing N719 dye various dipping temperatures.
ping temperature neither in increasing or decreasing trend. These EIS results are not in line with the photovoltaic parameters such as Jsc and η listed in Table 1. The higher Rcr, the slower charge carriers to recombine resulting in the increase in the Jsc and η as observed from Table 1.
reported in those references. However, it is lower than those reported in [1–4, 14, 15]. This might be caused the dye dipping time in this work that is 3 h is insufficient to optimize the dye loading for the dye adsorption into TiO2 photoanode interface compared to those reported in [1–4, 15].
Figure 6 displays Bode plots of the DSSCs utilizing N719 dye with various dipping temperatures. The plots shows bell shaped curves which are symmetry about resonant frequency. The resonant frequency is located at the peak of each curve. The carrier lifetime is calculated from the resonant frequency and listed in Table 1. From the figure, it is found that the 70°C sample has the lowest frequency, followed by the 50, 60, 80 and 30°C samples. The lower the frequency, the longer the carrier lifetime as the lifetime is the inverse of the frequency. The longer carrier lifetime is supposed to improve the performance of the DSSC. The carrier lifetimes illustrated in Table 1 are longer than that reported in [13].
The highest efficiency of the DSSC in this work was obtained at the dipping temperature of 50°C while Hirose et al. 2010 reported the highest efficiency was obtained at the dipping temperature of 75°C [16]. This discrepancy may be due to the different environmental property of the solar cell testing such as humidity. Furthermore, both of the highest efficiencies were obtained at the temperatures higher than room temperature in the range 20–30°C. Future work is intended to improve the dye dipping procedure so as to consequently increase the photocurrent and power conversion efficiency of the DSSC.
Table 1 illustrates the photovoltaic parameters, Rb, Rct and carrier lifetime with various dye dipping temperatures. According to the table, it was found that the device utilizing the 50°C sample demonstrates the highest JSC and η. The device utilizing the sample prepared at 30°C performs the lowest Voc, JSC and η. The JSC and η are optimum at N719 dye dipping temperature of 50°C. These photovoltaic parameters drop after this temperature. This is due to the DSSC utilizing the 50°C sample possesses the highest dye loading and charge recombination resistance. The device utilizing the 30°C sample demonstrates the lowest these photovoltaic parameters since it has the lowest dye loading, smallest charge recombination resistance and shortest carrier lifetime. There is no significant change in VOC and FF with the dye dipping temperature. The highest η obtained in this work, 1.36% is higher compared with those reported in [5–7] since N719 dye has higher optical absorption than that of the natural dyes
CONCLUSIONS The effect of N719 dye dipping temperature on the optical properties and performance parameters of the DSSC utilizing TiO2 films prepared via microwave technique has been investigated. It is found that the sample prepared at 50°C possesses the broadest optical absorption area and the highest dye loading. The JSC and η are strongly inf luenced by the N719 dipping temperature. The DSSC utilizing the sample prepared at 50°C performed the highest JSC and η of 4.06 mA cm–2 and 1.36%, respectively due to highest dye loading and recombination resistance. ACKNOWLEDGMENTS This work was supported by Universiti Kebangsaan Malaysia (UKM) under research grant DLP-2015-003.
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