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Novel synthesis of popcorn-like TiO2 light scatterers using a facile solution method for efficient dye-sensitized solar cells
Journal of Power Sources 413 (2019) 384–390
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Novel synthesis of popcorn-like TiO2 light scatterers using a facile solution method for efficient dye-sensitized solar cells
T
Yan-Zu Chena, Ren-Jang Wua,∗∗, Lu-Yin Linb,c,∗, Wei-Cheng Changd a
Department of Applied Chemistry, Providence University, 200 Chungchi Road, Shalu, Taichung Hsien 433, Taiwan Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan c Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan d Technology Center, Geckos Group, 6F-11, No. 38, Taiyuan St., Zhubei City, Hsinchu County, 30265, Taiwan b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
synthesis of Popcorn-like TiO • Novel light scatterers. conversion efficiency is by light • High scattering layer. performance is due to well-de• High fined structure.
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Dye adsorption Dye-sensitized solar cell Light scattering layer Popcorn-like Solution method Titanium dioxide
In this study, a facile solution method is proposed for synthesizing popcorn-like TiO2 aggregations (P-TiO2) as the light scatterers in the photoanode of DSSCs. This novel solution method is promising due to the less time and lower temperature required for carrying out the reaction, comparing to those for the conventional hydrothermal synthesis. The P-TiO2 has the size of 300 nm and is composed of small nanoparticles with the size of 50 nm. Also, different from the common light scatterers, the P-TiO2 has a special anatase phase which is beneficial for dye adsorption and charge transfer. The highest solar-to-electricity conversion efficiency of 7.56% is obtained for the DSSC with the photoanode composed of the commercial P90 TiO2 dye-adsorption layer and the P-TiO2 light scattering layer, due to the large surface area, highly active anatase nature, and the excellent light scattering ability for the home-made P-TiO2. The results suggest that the photovoltaic performance of DSSCs can be efficiently improved by carefully designing the morphology of the TiO2 nanostructure in the photoanode with large surface area and unique configuration to confine the incident light in the electrode for longer time.
1. Introduction The low-cost and easy-fabricated dye-sensitized solar cell (DSSCs) has attracted much attention as one of the promising photovoltaic
devices [1–4]. However, the solar-to-electricity conversion efficiency for DSSC is much lower than that for the Si-based solar cell [5]. In order to enhancing the solar-to-electricity conversion efficiency, the TiO2 nanoparticles with large surface area were widely applied as the dye-
∗
Corresponding author. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan. ∗∗ Corresponding author. Department of Applied Chemistry, Providence University, 200 Chungchi Road, Shalu, Taichung Hsien 433, Taiwan. E-mail addresses: [email protected] (R.-J. Wu), [email protected] (L.-Y. Lin). https://doi.org/10.1016/j.jpowsour.2018.12.065 Received 25 October 2018; Received in revised form 10 December 2018; Accepted 23 December 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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a simple solution method without any templates. In a typical synthesis, 0.296 g ammonium hexafluorotitanate (N2H8TiF6, Alfa Aesar, 99.9%) was dissolved in 100 mL deionized water (DIW) under stirring, and then 3.003 g urea (CON2H4, UniRegion Bio-Tech, 99.7%) was added in the as-prepared solution. The resulting solution was kept at 100 °C for 120 min under stirring. After the reaction, the solution was cooled to the room temperature. The solution was then rinsed by using DIW under vacuum for three times. The products were collected and dried in the oven at 50 °C for 1 h. Finally the P-TiO2 powder was successfully synthesized after the calcination of the samples at 550 °C for 2 h.
adsorbing layer in the photoanode [5–7]. Usually, the size of around 15–20 nm was applied for synthesizing the TiO2 nanoparticles with large surface area for adsorbing large amounts of the dye molecules [8,9]. Nevertheless, the small size of the TiO2 nanoparticles is unable to utilize the incident light completely. Hence, an extra TiO2 top layer composed of larger nanoparticles with the size larger than 300 nm was applied as the scattering layer in the photoanode to improve the light utilization and increase the solarto-electricity conversion efficiency [10–12]. Rajamanickam et al. fabricated a TiO2 nanorods/nanoparticles photoanode for DSSCs by using the hydrothermal synthesis to improve the charge transport and light utilization properties. A solar-to-electricity conversion efficiency (η) of 7.7% was achieved under 100 mW/cm2 [13]. He et al. reported a hydrothermal self-assembly method to synthesize size-controlled hierarchical rutile TiO2 microspheres for preparing the light scattering layer for DSSCs, which presented a η value of 9.3% with the short-circuit current density (JSC) of 15.6 mA/cm2 and the open circuit voltage (VOC) of 0.79 V [14]. The same group also He et al. synthesized the TiO2 hollow box nanostructure (B-TiO2) with highly exposed (001) surface for preparing the light scattering layer in the photoanode of DSSCs. The DSSC with the optimized amount of B-TiO2 in the photoanode showed a η value of 6.1%, which is 101% compared with that for the DSSC with the pure P25 film based photoanode (3.04%) [15]. Although the light scattering layer can promote the light capture in the photoanode of DSSCs, the large size of the TiO2 nanoparticle is unable to provide enough surface area for dye adsorption. Also, the conventional TiO2 light scattering layer is always composed of the rutile phase. It was reported that the charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile [16]. Synthesizing the anatase TiO2 light scatters is one of the efficient ways to compensate for the small surface area of the TiO2 light scatters by improving the charge transfer in the light scattering layer since the anatase TiO2 is more active than rutile TiO2 for DSSC application. Also, the morphology design is also important for synthesizing highly efficient TiO2 light scatterers to enhance the dye adsorption and improve the charge transfer [17–19]. Our group has synthesized hydrangea-like TiO2 light scatterer with high dye-loading on the photoanode for DSSCs, which achieved a higher η value of 7.50% than that of 6.61% for the DSSCs with commercial TiO2 as the light scattering layer. The better photovoltaic performance for the former case is mainly due to the enhanced photocurrent density through the abundant dye adsorption coupled with the inherent light scattering ability [20]. In this work, a simple and facile solution method was applied for synthesizing the popcorn-like TiO2 aggregations (P-TiO2) with the anatase phase and large surface area as the light scatterers in the photoanode of DSSCs. The less reaction time and the lower reaction temperature are benefit for this novel solution method comparing to those for the commonly used hydrothermal synthesis. The amount of the P-TiO2 light scatterer in the photoanode was tuned to optimize the photovoltaic performance of the resulting DSSC. A higher η of 7.56% was achieved for the DSSC with the P-TiO2 light scatterers in its photoanode than that for the DSSC composed of the commercial TiO2 as the light scatterer in its photoanode (7.17%). The higher dye adsorption amount, the smaller charge-transfer resistance, and the longer electron lifetime were achieved for the DSSC composed the P-TiO2 light scatterer in its photoanode. This work successfully provides an efficient method for synthesizing the well-define TiO2 light scatterers with large surface area and effective charge transfer path, and achieves an improved solarto-electricity conversion efficiency for the pertinent DSSC.
2.2. Fabrication of dye-sensitized solar cells Two kinds of the TiO2 paste were prepared for fabricating the bilayered TiO2 film for the photoanode. The TiO2 paste for fabricating the main dye-adsorbed underlayer (paste DA) was prepared by using the same method reported in our previous work [20]. In a typical synthesis, 40 mL absolute ethanol solution (CH3CHOH, echochemical, 95%) containing 1 g TiO2 nanoparticles (P90, UniRegion Bio-Tech) was mixed with another 20 mL absolute ethanol solution containing 0.35 g ethyl cellulose (45 cp, Sigma-Aldrich, 48.0–49.5% (w/w) ethoxyl basis%) and 0.45 g ethyl cellulose (10 cp, Sigma-Aldrich, 48.0–49.5% (w/w) ethoxyl basis%) dissolved at 60 °C. Then 6.49 g terpineol (C10H18O, Sigma-Aldrich, 99.5%) was mixed with the as-prepared mixture. Finally the solvent was pulled away by using the rotary evaporator to finish the preparation of paste DA. The other kind of the TiO2 paste for preparing the light scattering layer (paste LS) was synthesized using the same method for preparing paste DA but replacing the P90 TiO2 by different amounts of the home-made P-TiO2 light scatterers. The photoanode was fabricated via depositing the TiO2 paste on the fluorine-doped tin oxide (FTO) glass (Nippon Sheet Glass, 8–10 Ω/□, 2.2 mm-thick) using a doctor-blade method (active area = 0.28 cm2). A typical photoanode contains two TiO2 layers. The first TiO2 layer was coated on the FTO glass using the paste DA, and the second TiO2 layer was deposited on the top of the first TiO2 layer using the paste LS. After every depotion of the TiO2 film, the electrode was annealed at 500 °C for 60 min to enhance particle-particle connection and remove the solvent. The photoanode with only one TiO2 layer prepared using the paste DA was also made for comparison. The as-prepared TiO2 electrodes were then soaked in the dye solution containing 0.5 mM (Bu4N)2Ru(dcbpyH)2(NCS)2 (N719, C58H86N8O8RuS2, Solaronix, 99%) in equal volumes of Acetonitrile (ACN, CH3CN, J. T. Baker, 99.9%) and tert-butanol (C4H10O, Sigma-Aldrich, 97%) The photoanode was thus successfully obtained. The counter electrode was made by depositing a thin layer of Pt on the FTO glass via the decomposition of chloroplatinic acid six-hydrate (H2PtCl6·6H2O, Alfa Aesar, 99.9%) at 400 °C for 20 min. The DSSC was assembled by using the photoanode and the counter electrode sealed together by melting a 60 μm-thick spacer (Surlyn®, DuPont) in-between. After injecting the volatile electrolyte consisting of 0.1 M lithium iodide (LiI, Sigma-Aldrich, 99.9%), 0.6 M ethyl-3-propylimidazolium iodide (PrMImI, C7H13IN2, Sigma-Aldrich, 98%), 0.05 M iodide (I2, Sigma-Aldrich, 99.99%), and 0.5 M tert-butylpyridine (TBP, C9H13N, Sigma-Aldrich, 96%) in ACN into the inner space through a hole, the DSSC was then successfully obtained. 2.3. Characterization The morphology of TiO2 was characterized by the field emission scanning electron microscopy (FE-SEM, FEI Nova230), transmission electron microscopy (TEM, H-7100), and HRTEM (JEOL JEM-2010). The thickness of TiO2 films was measured via a microfigure-measuring instrument (Surfcorder ET3000, Kosaka Laboratory Ltd., Japan). The specific surface area of TiO2 films was estimated by using the N2 adsorb/desorb isotherms and the Brunauer–Emmett–Teller (BET) theorem. Dye loading of the TiO2 films was estimated by desorbing the dye in a 10 mM NaOH aqueous solution, and then the absorbance of the
2. Experimental section 2.1. Synthesis of popcorn-like TiO2 All chemicals were analytical-grade reagents and were used without further purification. Popcorn-like TiO2 (P-TiO2) were prepared by using 385
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The small nanoparticle-assembled P-TiO2 is inferred to be able to scattering light efficiently due to the large size of 300 nm and to adsorbing abundant dye molecules owing to the small nanoparticles composed of the popcorn nanostructure. Moreover, the HRTEM image for P-TiO2 was presented in Fig. 1(c) for closer observation. The dspacing of P-TiO2 is around 0.35 nm, which is corresponding to the (101) phase of the anatase TiO2. The composition and phase of P-TiO2 was further analyzed by using the XRD pattern, as shown in Fig. 1(d). The planes of (101), (004), (200), (105), (211), (204), (116), (220) and (215) were observed at the 2θ values of 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, 62.69°, 68.67°, 70.56°, and 75.03°, respectively. This pattern fully fits the TiO2 anatase phase (JCPDS 89-4921) [21], indicating the successful synthesis of anatase TiO2 by using the novel solution method. In addition, by using the Scherrer equation and the XRD pattern, it is calculated that the mean crystallite size for P-TiO2 is around 17.58 nm [22]. On the other hand, the morphology and crystalline phase of the commercial TiO2 light scatterers (C-TiO2) were examined to make a comparison with those for the home-made P-TiO2. Also, the analysis for the component of the main dye adsorption layer, P90 TiO2, was also carried out. Fig. 2(a) and (b) respectively show the SEM images for P90 and C-TiO2. The P90 TiO2 is uniform nanoparticles with the size less than 20 nm, which is preferable for dye adsorption. The C-TiO2 also showed particle morphology but the particles were connected together. The size of C-TiO2 is around 200 nm, which is also the suitable size for light scattering. In addition, the crystalline phase of P90 and C-TiO2 were examined by using the XRD patterns, as shown in Fig. 2(c). The anatase phase (JCPDS 89-4921) and rutile phase (JCPDS 21-1276) were obviously observed in the XRD patterns for P90 and C-TiO2, respectively. The anatase phase for P90 is advantageous since the anatase TiO2 is more active to be applied in the photoanode of DSSCs. The rutile phase was commonly obtained for the large size of TiO2, as also obtained for the commercial light scatterer, C-TiO2. The surface area is of the great importance for the TiO2 layer in the photoanode for DSSCs, since the adsorbed dye molecule is the main
solution was measured using UV–vis spectroscopy (V-570, Jasco Inc.). Photovoltaic characterization was performed on the surface of the solar cell under a white light source (YSS-100A, Yamashita Denso Company) with an irradiance of 100 mW/cm2 under air mass 1.5 global (AM1.5G). The irradiance of the simulated light was calibrated using a silicon photodiode (BS-520, Bunko Keiki Co., Ltd.). The photocurrent densityvoltage (J-V) curves were recorded with a PGSTAT 30 potentiostat/ galvanostat (Autolab, Eco-Chemie.). The evolution of the electron transport process in the cell was investigated using the electrochemical impedance spectroscopy (EIS), and the impedance measurements were performed under AM1.5G illumination using an electrochemical analyzer (Autolab PGSTAT30, Eco-Chemie). The applied direct current (DC) bias voltage and alternating current (AC) amplitude were set at VOC of the cell and 10 mV between the working and the counter electrodes, respectively. The frequency range extended from 10−2–105 Hz. The incident photon-to-electron conversion efficiency (IPCE) was measured as a function of wavelength from 350 to 800 nm by using a monochromator (Forter Tech, Taiwan). 3. Results and discussion 3.1. Material characterization The morphology of the popcorn-like TiO2 (P-TiO2) nanostructure was first examined using the SEM image, as shown in Fig. 1(a). The PTiO2 presented nanoparticle structure with several protrusions on the surface. The size of P-TiO2 is very uniform with the diameter of around 300 nm, indicating the successful synthesis of the well-defined TiO2 nanostructure by using the novel solution method. Fig. 1(b) shows the TEM imaged for P-TiO2. The popcorn-like morphology is clearly observed for P-TiO2 with several well-distributed small nanoparticles at the surface. The small nanoparticles have the size of around 30–50 nm, which is favorable for dye adsorption. It is also found that the center of P-TiO2 is hollow, and P-TiO2 is composed of numerous small nanoparticles in the layer-by-layer form from the center to the outer layer.
Fig. 1. (a) The SEM image, (b) TEM image, (c) HRTEM image, and (d) XRD pattern for P-TiO2. 386
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Fig. 2. The SEM image for (a) P90 and (b) C-TiO2; (c) the XRD patterns for P90 and C-TiO2.
photoanode. The extra electrons excited in the outer layer may be unable to transferring to the FTO substrate and the outer circuit. These electrons may be consumed by the recombination with those excited in the inner layer of the TiO2 film. The similar trends can also be observed for the VOC and FF values. Even though the differences of these two parameters for the DSSCs prepared using different thicknesses of the TiO2 film are small. The highest η value of 6.97% was therefore obtained for the DSSC with the P90 TiO2 film thickness of 24 nm, which would be used for assembling the DSSCs with an extra light scattering layer discussed in the following text. Furthermore, the ratio of the light scatterer, P-TiO2, in the photoanode of DSSCs was optimized. It is noted that the thickness of the total TiO2 layer was kept at 24 nm, so the more P-TiO2 indicates the less P90 in the photoanode. Fig. 3 shows the J-V curves for the DSSCs with the P90 TiO2 photoanode and with the P90/P-TiO2 photoanodes composed of 5, 10, 20 and 30 wt% P-TiO2, and the corresponding photovoltaic parameters were shown in Table 2 for clearer comparison. The JSC value increases for the DSSCs with higher weight ratios of P-TiO2 in the photoanode. The JSC reached the maximum value of 13.95 mA/cm2 for the DSSC with 10 wt% P-TiO2 in its photoanode. It is inferred that the more light scatterers of P-TiO2 in the photoanode of DSSCs would induce more light scattering, and hence more efficient incident light utilization can be achieved when more light scatterers were incorporated in the photoanode. However, when the weight ratio of PTiO2 in the photoanode is higher than 20 wt%, the JSC value for the
charge excitation component for converting solar to electricity. Hence, the surface area for P-TiO2, C-TiO2 and P90 was analyzed using the nitrogen adsorption/desorption isotherm and Brunauer–Emmett–Teller (BET) theory. The surface areas of around 43, 16 and 90 m2/g were obtained for the P-TiO2, C-TiO2 and P90, respectively. The P90 TiO2 shows the largest surface area, owing to the much smaller particle size comparing to the other two light scatterers of P-TiO2 and C-TiO2. Hence it is suitable to apply the P90 TiO2 as the main dye adsorption layer in the photoanode for DSSCs. On the other hand, the surface area of PTiO2 is almost 3-fold of that for the C-TiO2. The much larger surface area for the home-made P-TiO2 is favorable for dye adsorption and hence effective charge excitation. 3.2. Electrochemical characterization The photoanode is composed of two layers, the P90 TiO2 layer as the main dye adsorption layer and the larger size of the TiO2 layer as the light scattering layer. Before comparing the photovoltaic performance of the DSSCs using the home-made P-TiO2 and commercial CTiO2 as the light scatterers in the photoanodes, the thickness of the P90 TiO2 layer was firstly optimize. The thickness of the TiO2 layer is very important to influence the photovoltaic performance of DSSCs. The more dye molecules could adsorb in the photoanode with the thicker TiO2 layer, whereas the thicker TiO2 layer would lead to the larger charge-transfer resistance and longer charge-transfer path. Hence, the DSSCs with the thickness of 12, 16, 20, 24, 26 and 28 nm for the P90 TiO2 layer were assembled, and the corresponding photovoltaic parameters were shown in Table 1 for comparison. It is noted that these cells contained no light scatting layers in their photoanodes. The JSC increases for the DSSC with a thicker TiO2 layer in the photoanode, and the highest JSC value of 13.37 mA/cm2 was obtained for the DSSC with 24 nm as the TiO2 layer thickness. However, the JSC value was reduced when larger thicknesses of the TiO2 layer were applied for assembling the DSSCs. This phenomenon may be caused by the longer chargetransfer path for the electrons excited in the outer layer of the Table 1 The photovoltaic parameters for the DSSCs with different thicknesses of the P90 TiO2 layer in the photoanodes. Thickness (μm)
VOC (V)
JSC (mA/cm2)
FF
η (%)
12 16 20 24 26 28
0.75 0.77 0.76 0.75 0.75 0.74
9.91 10.08 10.93 13.37 13.20 11.14
0.68 0.69 0.70 0.69 0.68 0.67
5.06 5.36 5.89 6.97 6.80 5.54
Fig. 3. The J-V curves for the DSSCs with the P90 TiO2 photoanode and the P90/P-TiO2 photoanodes with different weight percentages of P-TiO2. 387
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larger surface area and the rougher surface for the P-TiO2 comparing to those for the C-TiO2 are the main causes for the higher IPCE values for the former case. It is found that the enhancement on the integrated area of the IPCE curve is truly higher than the enhancement on JSC value for the DSSC with the P90/C-TiO2 photoanode comparing to those for the DSSC with the P90 photoanode. This phenomenon may be due to the different measurements for evaluating the photocurrent density of the DSSCs. For measuring the IPCE values, the calibration should be applied for each wavelength and the measured wavelength is not totally continuous. Hence it is suggested that although the enhancements on the JSC value and the IPCE value are different, the trends for these two parameters for the DSSC with the P90 and the P90/C-TiO2 photoanodes are still the same. The similar phenomenon can be observed in the previous literature [23]. To understand the reasons for influencing the photovoltaic performance of the DSSCs with different photoanodes, the dye loading amount in the photoanode was firstly examined. The absorption spectra for the solution containing the dye molecules desorbed from the TiO2 photoanode were shown in Fig. 5(a). The amount of the adsorbed dye was listed in Table 3. The absorption peak is located at the wavelength of 500 nm, indicating the absorption feature of N719. Due to the smallest size of P90, the photoanode with only P90 shows the highest dye adsorption amount of 0.116 μmol/cm2, whereas when the light scatterers were incorporated in the photoanode, the dye adsorption amount was reduced. On the other hand, the higher dye adsorption amount of 0.106 μmol/cm2 was obtained for the P90/P-TiO2 photoanode, comparing to that of 0.093 μmol/cm2 for the P90/C-TiO2 photoanode. The higher dye adsorption for the P90/P-TiO2 photoanode is as expected since the well-defined smaller nanoparticle-assembled structure with large surface area and the anatase phase was achieved for this case, whereas the commercial rutile TiO2 light scatterers only presents the large size but the structure is simple the connected particles. To examine the light scatting ability, the reflection spectra for the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes was shown in Fig. 5(b). The smallest reflectance was observed for the P90 photoanode, due to the lack of the light scattering layer. The higher reflectance was obtained for the P90/P-TiO2 photoanode than that for the P90/C-TiO2 photoanode, owing to the well-defined popcorn-like structure for the former case to increase the light transfer length and improve the scattering ability. The reflection spectrum for the pure PTiO2 photoanode was also shown in this figure. The very high reflectance of around 60% in the whole spectrum again indicates the excellent light scattering ability for this case. The charge-transfer resistances were further analyzed using the EIS technique. It should be emphasized that the DSSC composed of a photoanode and a counter electrode was used for measuring the Nyquist plots for this work to understand the interfacial resistances in the photoanode. This is different from the measurement with the symmetrical cell consisting of identical counter electrodes. This analysis is used for evaluating the performance of the counter electrode and the electrolyte [24,25]. Fig. 5(c) shows the Nyquist plots for the DSSCs with
Table 2 The photovoltaic parameters, resistance and the electron lifetime for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes.
P90 P90/5 wt% P-TiO2 P90/10 wt% P-TiO2 P90/20 wt% P-TiO2 P90/30 wt% P-TiO2
VOC (V)
JSC (mA/cm2)
FF
η (%)
0.75 0.77 0.77 0.76 0.77
13.37 13.53 13.95 13.80 12.89
0.69 0.68 0.70 0.68 0.68
6.97 7.21 7.56 7.13 6.78
DSSC oppositely decreased with increasing P-TiO2 weight ratio in the photoanode. Since the two layers of TiO2 in the photoanode are responsible for providing active sites for dye adsorption and scattering incident light, too many light scatterers would lead to much small amounts of P90 TiO2 in the photoanode This phenomenon would result in the insufficient dye adsorption and hence the charge excitation would be largely reduced even the light scattering effect could be enhanced. Hence, the optimized weight ratio of 10 wt% for P-TiO2 in the photoanode was obtained, and the resulting DSSC showed the VOC value of 0.77 V, the JSC value of 13.95 mA/cm2, the FF of 0.70, and the η value of 7.56%. After optimizing the TiO2 film thickness and the ratio of the light scatterer in the photoanode, the photovoltaic performances for the DSSCs composed of the P90 photoanode and those with a light scattering layer, i.e., P90/P-TiO2, and P90/C-TiO2 photoanodes, were compared. Fig. 4(a) shows the J-V curves for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes, and the corresponding photovoltaic parameters were shown in Table 3 for clearer comparison. Due to the extra light scattering, the DSSCs with P90/PTiO2 and P90/C-TiO2 photoanodes show higher VOC, JSC and η values than those for the DSSC with the P90 photoanode without a light scattering layer. Among the DSSCs prepared with a light scattering layer, the cell with the P90/P-TiO2 photoanode shows a higher JSC value than the cell with the P90/C-TiO2 photoanode. This result is mainly caused by the well-defined popcorn-like structure for the homemade P-TiO2. As observed in the SEM and TEM images, the P-TiO2 is composed of numerous small nanoparticles with the size of around 50 nm, which is able to providing more active sites for dye adsorption, comparing to the pure particle structure for the C-TiO2. Moreover, the IPCE spectra for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/CTiO2 photoanodes were shown in Fig. 4(b) to further examine the solar to electricity conversion at different regions of the incident light. The maximum IPCE value was obtained at the wavelength of 530 nm for all the DSSCs, due to the main light absorption wavelength for the N719 dye. The DSSC with the P90 photoanode shows the smallest IPCE values, owing to the lack of a light scattering layer. The highest maximum IPCE value of 65% was achieved for the DSSC with the P90/P-TiO2 photoanode, and the IPCE values are higher at all wavelengths than those for the DSSCs with the P90/C-TiO2 photoanode. The higher IPCE value for the former case is consistent with its higher JSC value. The
Fig. 4. (a) The J-V curves and (b) the IPCE spectra for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes. 388
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Table 3 The photovoltaic parameters, resistance and the electron lifetime for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes.
P90 P90/P-TiO2 P90/C-TiO2
VOC (V)
JSC (mA/cm2)
FF
η (%)
Absorbed dye (μmol/cm2)
Rct1 (Ω)
Rct2 (Ω)
τ (ms)
0.75 0.77 0.77
13.37 13.95 13.40
0.69 0.70 0.68
6.97 7.56 7.17
0.116 0.106 0.093
2.98 2.65 2.78
22.95 14.82 18.94
20.04 25.23 25.23
Fig. 5. (a) The absorption spectra of the desorbed dye and (b) the reflection spectra for the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes; (c) the Nyquist plot and (d) the Bode plot for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes. The reflection spectrum for the P-TiO2 photoanode was also shown in (b) for comparison.
the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes. The equivalent circuit was also shown in this figure for fitting the resistance data. The charge-transfer resistance at the Pt counter electrode and the electrolyte interface (Rct1) can be estimated by using the first semicircle at the highest frequency region. The second semicircle indicates the charge-transfer resistance at the TiO2/dye/electrolyte interface (Rct2). The third semicircle at the lowest frequency region suggests the diffusion resistance in the electrolyte. The values of Rct1 and Rct2 were shown in Table 3 for clearer comparison. Since the main difference between these three DSSCs is on the photoanode, the variation on the first and the third semicircle between them is limited. The DSSC with the P90/P-TiO2 photoanode shows a smaller Rct2 value than that for the DSSC with the P90 photoanode, owing to the better light scattering ability for promoting more charges transferring for the former case. On the other hand, the DSSC with the P90/P-TiO2 photoanode also presents a smaller Rct2 value than that for the DSSC with the P90/C-TiO2 photoanode. The anatase phase and the larger surface area of P-TiO2 play important roles on the smaller Rct2 value for the resulting DSSC, owing to the preferable charge transfer of the active anatase TiO2 and the more excited charges for promoting the charge transfer. Moreover, the electron lifetime (τ) for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes was estimated by using the Bode plot, as shown in Fig. 5(d). The electron lifetime was also listed in Table 3 for clearer comparison. The DSSCs with an extra light scattering layer in the photoanodes present longer electron lifetimes than that for the DSSC with the P90 photoanode. It is inferred that the better charge transfer could lead to longer electron lifetime since the charge recombination could be reduced with promoted charge transfer in the photoanode. The illustration for the P90, P90/P-TiO2 and P90/C-TiO2 photoanode was shown in Scheme 1. The light transfer and the adsorbed dye molecules were also illustrated in this scheme. The P90 with the
Scheme 1. The illustration for the (a) P90, (b) P90/P-TiO2 and (c) P90/C-TiO2 photoanodes.
smallest size can provide more sites for dye adsorption. The P-TiO2 with the popcorn-like structure possessed higher due adsorption than that for C-TiO2. On the other hand, the large sizes of P-TiO2 and C-TiO2 could induce light scattering. As illustrated in this scheme, the better light utilization can be achieved with the light scattering layer in the photoanode, due to the large size of the TiO2 light scatterers for increasing the light transfer path and exciting more charges. To have fair comparison of the photovoltaic performances for the DSSC with a light scattering layer, the partial list for the morphology of TiO2 scatterers, methods for synthesizing TiO2 scatterers and the efficiency for DSSCs with and without TiO2 light scatterers reported in the previous literatures and in this work were listed in Table 4. It is found that the TiO2 light scatterers were usually fabricated by using the hydrothermal synthesis and the novel solution method with less reaction time and lower reaction temperature was firstly proposed in this work. The solar-to-electricity efficiency achieved in this work is also acceptable comparing to those reported in other related literatures. Hence it is significant to emphasize the efficient method and the unique and well-defined structure of TiO2 proposed in this work.
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from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
Table 4 Partial lists of previous literatures for the morphology of TiO2 scatterers, methods for synthesizing TiO2 scatterers and the efficiency for DSSCs with and without TiO2 light scatterers. Morphology
Method
ηw/o (%)
Nanorods Microspheres Hollow box Hydrangea Popcorn
Hydrothermal Hydrothermal Hydrothermal Hydrothermal Aqueous solution
4.47 7.37 3.04 6.41 6.97
scattering
ηwith
7.70 9.30 6.11 7.50 7.56
scattering
(%)
References
Ref.
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[13] [14] [15] [20] This work
4. Conclusions A facile solution method was proposed in this work to synthesize popcorn-like TiO2 aggregations (P-TiO2) as the light scatterers in the photoanode for DSSCs. The P-TiO2 has more active anatase phase and larger surface area, which is beneficial for dye adsorption and charge transfer. The thickness of the TiO2 layer and the ratio for the P-TiO2 light scatterer in the photoanode were optimized. The highest η value of 7.56% with the JSC of 13.95 mA/cm2, VOC of 0.77 V and FF of 0.70 was obtained for the DSSC with 24 nm TiO2 layer and 10 wt% P-TiO2 light scatterer in the photoanode. Also, the smaller charge transfer resistance and longer electron lifetime are obtained for this case. The DSSC with the pure P90 TiO2 photoanode and with the commercial light scatterer coupled with the P90 TiO2 underlayer in the photoanode showed smaller η values of 6.97% and 7.17%, respectively. The better photovoltaic performance for the DSSC with the P90/P-TiO2 photoanode is due to the larger surface area for dye adsorption, the unique anatase nature for achieving high activity, and the well-defined structure for inducing efficient light scattering via increasing the light transfer path. Acknowledgements This work was supported in part by the Ministry of Science and Technology (MOST) in Taiwan, under grant numbers: MOST 106-2221E-027-108 and MOST 106-2119-M-027-001. This work was financially supported from the Young Scholar Fellowship Program by MOST in Taiwan, under Grant MOST 107-2636-E-027-003. This work was financially supported by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors”
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Novel synthesis of popcorn-like TiO2 light scatterers using a facile solution method for efficient dye-sensitized solar cells
T
Yan-Zu Chena, Ren-Jang Wua,∗∗, Lu-Yin Linb,c,∗, Wei-Cheng Changd a
Department of Applied Chemistry, Providence University, 200 Chungchi Road, Shalu, Taichung Hsien 433, Taiwan Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan c Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan d Technology Center, Geckos Group, 6F-11, No. 38, Taiyuan St., Zhubei City, Hsinchu County, 30265, Taiwan b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
synthesis of Popcorn-like TiO • Novel light scatterers. conversion efficiency is by light • High scattering layer. performance is due to well-de• High fined structure.
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Dye adsorption Dye-sensitized solar cell Light scattering layer Popcorn-like Solution method Titanium dioxide
In this study, a facile solution method is proposed for synthesizing popcorn-like TiO2 aggregations (P-TiO2) as the light scatterers in the photoanode of DSSCs. This novel solution method is promising due to the less time and lower temperature required for carrying out the reaction, comparing to those for the conventional hydrothermal synthesis. The P-TiO2 has the size of 300 nm and is composed of small nanoparticles with the size of 50 nm. Also, different from the common light scatterers, the P-TiO2 has a special anatase phase which is beneficial for dye adsorption and charge transfer. The highest solar-to-electricity conversion efficiency of 7.56% is obtained for the DSSC with the photoanode composed of the commercial P90 TiO2 dye-adsorption layer and the P-TiO2 light scattering layer, due to the large surface area, highly active anatase nature, and the excellent light scattering ability for the home-made P-TiO2. The results suggest that the photovoltaic performance of DSSCs can be efficiently improved by carefully designing the morphology of the TiO2 nanostructure in the photoanode with large surface area and unique configuration to confine the incident light in the electrode for longer time.
1. Introduction The low-cost and easy-fabricated dye-sensitized solar cell (DSSCs) has attracted much attention as one of the promising photovoltaic
devices [1–4]. However, the solar-to-electricity conversion efficiency for DSSC is much lower than that for the Si-based solar cell [5]. In order to enhancing the solar-to-electricity conversion efficiency, the TiO2 nanoparticles with large surface area were widely applied as the dye-
∗
Corresponding author. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan. ∗∗ Corresponding author. Department of Applied Chemistry, Providence University, 200 Chungchi Road, Shalu, Taichung Hsien 433, Taiwan. E-mail addresses: [email protected] (R.-J. Wu), [email protected] (L.-Y. Lin). https://doi.org/10.1016/j.jpowsour.2018.12.065 Received 25 October 2018; Received in revised form 10 December 2018; Accepted 23 December 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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a simple solution method without any templates. In a typical synthesis, 0.296 g ammonium hexafluorotitanate (N2H8TiF6, Alfa Aesar, 99.9%) was dissolved in 100 mL deionized water (DIW) under stirring, and then 3.003 g urea (CON2H4, UniRegion Bio-Tech, 99.7%) was added in the as-prepared solution. The resulting solution was kept at 100 °C for 120 min under stirring. After the reaction, the solution was cooled to the room temperature. The solution was then rinsed by using DIW under vacuum for three times. The products were collected and dried in the oven at 50 °C for 1 h. Finally the P-TiO2 powder was successfully synthesized after the calcination of the samples at 550 °C for 2 h.
adsorbing layer in the photoanode [5–7]. Usually, the size of around 15–20 nm was applied for synthesizing the TiO2 nanoparticles with large surface area for adsorbing large amounts of the dye molecules [8,9]. Nevertheless, the small size of the TiO2 nanoparticles is unable to utilize the incident light completely. Hence, an extra TiO2 top layer composed of larger nanoparticles with the size larger than 300 nm was applied as the scattering layer in the photoanode to improve the light utilization and increase the solarto-electricity conversion efficiency [10–12]. Rajamanickam et al. fabricated a TiO2 nanorods/nanoparticles photoanode for DSSCs by using the hydrothermal synthesis to improve the charge transport and light utilization properties. A solar-to-electricity conversion efficiency (η) of 7.7% was achieved under 100 mW/cm2 [13]. He et al. reported a hydrothermal self-assembly method to synthesize size-controlled hierarchical rutile TiO2 microspheres for preparing the light scattering layer for DSSCs, which presented a η value of 9.3% with the short-circuit current density (JSC) of 15.6 mA/cm2 and the open circuit voltage (VOC) of 0.79 V [14]. The same group also He et al. synthesized the TiO2 hollow box nanostructure (B-TiO2) with highly exposed (001) surface for preparing the light scattering layer in the photoanode of DSSCs. The DSSC with the optimized amount of B-TiO2 in the photoanode showed a η value of 6.1%, which is 101% compared with that for the DSSC with the pure P25 film based photoanode (3.04%) [15]. Although the light scattering layer can promote the light capture in the photoanode of DSSCs, the large size of the TiO2 nanoparticle is unable to provide enough surface area for dye adsorption. Also, the conventional TiO2 light scattering layer is always composed of the rutile phase. It was reported that the charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile [16]. Synthesizing the anatase TiO2 light scatters is one of the efficient ways to compensate for the small surface area of the TiO2 light scatters by improving the charge transfer in the light scattering layer since the anatase TiO2 is more active than rutile TiO2 for DSSC application. Also, the morphology design is also important for synthesizing highly efficient TiO2 light scatterers to enhance the dye adsorption and improve the charge transfer [17–19]. Our group has synthesized hydrangea-like TiO2 light scatterer with high dye-loading on the photoanode for DSSCs, which achieved a higher η value of 7.50% than that of 6.61% for the DSSCs with commercial TiO2 as the light scattering layer. The better photovoltaic performance for the former case is mainly due to the enhanced photocurrent density through the abundant dye adsorption coupled with the inherent light scattering ability [20]. In this work, a simple and facile solution method was applied for synthesizing the popcorn-like TiO2 aggregations (P-TiO2) with the anatase phase and large surface area as the light scatterers in the photoanode of DSSCs. The less reaction time and the lower reaction temperature are benefit for this novel solution method comparing to those for the commonly used hydrothermal synthesis. The amount of the P-TiO2 light scatterer in the photoanode was tuned to optimize the photovoltaic performance of the resulting DSSC. A higher η of 7.56% was achieved for the DSSC with the P-TiO2 light scatterers in its photoanode than that for the DSSC composed of the commercial TiO2 as the light scatterer in its photoanode (7.17%). The higher dye adsorption amount, the smaller charge-transfer resistance, and the longer electron lifetime were achieved for the DSSC composed the P-TiO2 light scatterer in its photoanode. This work successfully provides an efficient method for synthesizing the well-define TiO2 light scatterers with large surface area and effective charge transfer path, and achieves an improved solarto-electricity conversion efficiency for the pertinent DSSC.
2.2. Fabrication of dye-sensitized solar cells Two kinds of the TiO2 paste were prepared for fabricating the bilayered TiO2 film for the photoanode. The TiO2 paste for fabricating the main dye-adsorbed underlayer (paste DA) was prepared by using the same method reported in our previous work [20]. In a typical synthesis, 40 mL absolute ethanol solution (CH3CHOH, echochemical, 95%) containing 1 g TiO2 nanoparticles (P90, UniRegion Bio-Tech) was mixed with another 20 mL absolute ethanol solution containing 0.35 g ethyl cellulose (45 cp, Sigma-Aldrich, 48.0–49.5% (w/w) ethoxyl basis%) and 0.45 g ethyl cellulose (10 cp, Sigma-Aldrich, 48.0–49.5% (w/w) ethoxyl basis%) dissolved at 60 °C. Then 6.49 g terpineol (C10H18O, Sigma-Aldrich, 99.5%) was mixed with the as-prepared mixture. Finally the solvent was pulled away by using the rotary evaporator to finish the preparation of paste DA. The other kind of the TiO2 paste for preparing the light scattering layer (paste LS) was synthesized using the same method for preparing paste DA but replacing the P90 TiO2 by different amounts of the home-made P-TiO2 light scatterers. The photoanode was fabricated via depositing the TiO2 paste on the fluorine-doped tin oxide (FTO) glass (Nippon Sheet Glass, 8–10 Ω/□, 2.2 mm-thick) using a doctor-blade method (active area = 0.28 cm2). A typical photoanode contains two TiO2 layers. The first TiO2 layer was coated on the FTO glass using the paste DA, and the second TiO2 layer was deposited on the top of the first TiO2 layer using the paste LS. After every depotion of the TiO2 film, the electrode was annealed at 500 °C for 60 min to enhance particle-particle connection and remove the solvent. The photoanode with only one TiO2 layer prepared using the paste DA was also made for comparison. The as-prepared TiO2 electrodes were then soaked in the dye solution containing 0.5 mM (Bu4N)2Ru(dcbpyH)2(NCS)2 (N719, C58H86N8O8RuS2, Solaronix, 99%) in equal volumes of Acetonitrile (ACN, CH3CN, J. T. Baker, 99.9%) and tert-butanol (C4H10O, Sigma-Aldrich, 97%) The photoanode was thus successfully obtained. The counter electrode was made by depositing a thin layer of Pt on the FTO glass via the decomposition of chloroplatinic acid six-hydrate (H2PtCl6·6H2O, Alfa Aesar, 99.9%) at 400 °C for 20 min. The DSSC was assembled by using the photoanode and the counter electrode sealed together by melting a 60 μm-thick spacer (Surlyn®, DuPont) in-between. After injecting the volatile electrolyte consisting of 0.1 M lithium iodide (LiI, Sigma-Aldrich, 99.9%), 0.6 M ethyl-3-propylimidazolium iodide (PrMImI, C7H13IN2, Sigma-Aldrich, 98%), 0.05 M iodide (I2, Sigma-Aldrich, 99.99%), and 0.5 M tert-butylpyridine (TBP, C9H13N, Sigma-Aldrich, 96%) in ACN into the inner space through a hole, the DSSC was then successfully obtained. 2.3. Characterization The morphology of TiO2 was characterized by the field emission scanning electron microscopy (FE-SEM, FEI Nova230), transmission electron microscopy (TEM, H-7100), and HRTEM (JEOL JEM-2010). The thickness of TiO2 films was measured via a microfigure-measuring instrument (Surfcorder ET3000, Kosaka Laboratory Ltd., Japan). The specific surface area of TiO2 films was estimated by using the N2 adsorb/desorb isotherms and the Brunauer–Emmett–Teller (BET) theorem. Dye loading of the TiO2 films was estimated by desorbing the dye in a 10 mM NaOH aqueous solution, and then the absorbance of the
2. Experimental section 2.1. Synthesis of popcorn-like TiO2 All chemicals were analytical-grade reagents and were used without further purification. Popcorn-like TiO2 (P-TiO2) were prepared by using 385
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The small nanoparticle-assembled P-TiO2 is inferred to be able to scattering light efficiently due to the large size of 300 nm and to adsorbing abundant dye molecules owing to the small nanoparticles composed of the popcorn nanostructure. Moreover, the HRTEM image for P-TiO2 was presented in Fig. 1(c) for closer observation. The dspacing of P-TiO2 is around 0.35 nm, which is corresponding to the (101) phase of the anatase TiO2. The composition and phase of P-TiO2 was further analyzed by using the XRD pattern, as shown in Fig. 1(d). The planes of (101), (004), (200), (105), (211), (204), (116), (220) and (215) were observed at the 2θ values of 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, 62.69°, 68.67°, 70.56°, and 75.03°, respectively. This pattern fully fits the TiO2 anatase phase (JCPDS 89-4921) [21], indicating the successful synthesis of anatase TiO2 by using the novel solution method. In addition, by using the Scherrer equation and the XRD pattern, it is calculated that the mean crystallite size for P-TiO2 is around 17.58 nm [22]. On the other hand, the morphology and crystalline phase of the commercial TiO2 light scatterers (C-TiO2) were examined to make a comparison with those for the home-made P-TiO2. Also, the analysis for the component of the main dye adsorption layer, P90 TiO2, was also carried out. Fig. 2(a) and (b) respectively show the SEM images for P90 and C-TiO2. The P90 TiO2 is uniform nanoparticles with the size less than 20 nm, which is preferable for dye adsorption. The C-TiO2 also showed particle morphology but the particles were connected together. The size of C-TiO2 is around 200 nm, which is also the suitable size for light scattering. In addition, the crystalline phase of P90 and C-TiO2 were examined by using the XRD patterns, as shown in Fig. 2(c). The anatase phase (JCPDS 89-4921) and rutile phase (JCPDS 21-1276) were obviously observed in the XRD patterns for P90 and C-TiO2, respectively. The anatase phase for P90 is advantageous since the anatase TiO2 is more active to be applied in the photoanode of DSSCs. The rutile phase was commonly obtained for the large size of TiO2, as also obtained for the commercial light scatterer, C-TiO2. The surface area is of the great importance for the TiO2 layer in the photoanode for DSSCs, since the adsorbed dye molecule is the main
solution was measured using UV–vis spectroscopy (V-570, Jasco Inc.). Photovoltaic characterization was performed on the surface of the solar cell under a white light source (YSS-100A, Yamashita Denso Company) with an irradiance of 100 mW/cm2 under air mass 1.5 global (AM1.5G). The irradiance of the simulated light was calibrated using a silicon photodiode (BS-520, Bunko Keiki Co., Ltd.). The photocurrent densityvoltage (J-V) curves were recorded with a PGSTAT 30 potentiostat/ galvanostat (Autolab, Eco-Chemie.). The evolution of the electron transport process in the cell was investigated using the electrochemical impedance spectroscopy (EIS), and the impedance measurements were performed under AM1.5G illumination using an electrochemical analyzer (Autolab PGSTAT30, Eco-Chemie). The applied direct current (DC) bias voltage and alternating current (AC) amplitude were set at VOC of the cell and 10 mV between the working and the counter electrodes, respectively. The frequency range extended from 10−2–105 Hz. The incident photon-to-electron conversion efficiency (IPCE) was measured as a function of wavelength from 350 to 800 nm by using a monochromator (Forter Tech, Taiwan). 3. Results and discussion 3.1. Material characterization The morphology of the popcorn-like TiO2 (P-TiO2) nanostructure was first examined using the SEM image, as shown in Fig. 1(a). The PTiO2 presented nanoparticle structure with several protrusions on the surface. The size of P-TiO2 is very uniform with the diameter of around 300 nm, indicating the successful synthesis of the well-defined TiO2 nanostructure by using the novel solution method. Fig. 1(b) shows the TEM imaged for P-TiO2. The popcorn-like morphology is clearly observed for P-TiO2 with several well-distributed small nanoparticles at the surface. The small nanoparticles have the size of around 30–50 nm, which is favorable for dye adsorption. It is also found that the center of P-TiO2 is hollow, and P-TiO2 is composed of numerous small nanoparticles in the layer-by-layer form from the center to the outer layer.
Fig. 1. (a) The SEM image, (b) TEM image, (c) HRTEM image, and (d) XRD pattern for P-TiO2. 386
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Fig. 2. The SEM image for (a) P90 and (b) C-TiO2; (c) the XRD patterns for P90 and C-TiO2.
photoanode. The extra electrons excited in the outer layer may be unable to transferring to the FTO substrate and the outer circuit. These electrons may be consumed by the recombination with those excited in the inner layer of the TiO2 film. The similar trends can also be observed for the VOC and FF values. Even though the differences of these two parameters for the DSSCs prepared using different thicknesses of the TiO2 film are small. The highest η value of 6.97% was therefore obtained for the DSSC with the P90 TiO2 film thickness of 24 nm, which would be used for assembling the DSSCs with an extra light scattering layer discussed in the following text. Furthermore, the ratio of the light scatterer, P-TiO2, in the photoanode of DSSCs was optimized. It is noted that the thickness of the total TiO2 layer was kept at 24 nm, so the more P-TiO2 indicates the less P90 in the photoanode. Fig. 3 shows the J-V curves for the DSSCs with the P90 TiO2 photoanode and with the P90/P-TiO2 photoanodes composed of 5, 10, 20 and 30 wt% P-TiO2, and the corresponding photovoltaic parameters were shown in Table 2 for clearer comparison. The JSC value increases for the DSSCs with higher weight ratios of P-TiO2 in the photoanode. The JSC reached the maximum value of 13.95 mA/cm2 for the DSSC with 10 wt% P-TiO2 in its photoanode. It is inferred that the more light scatterers of P-TiO2 in the photoanode of DSSCs would induce more light scattering, and hence more efficient incident light utilization can be achieved when more light scatterers were incorporated in the photoanode. However, when the weight ratio of PTiO2 in the photoanode is higher than 20 wt%, the JSC value for the
charge excitation component for converting solar to electricity. Hence, the surface area for P-TiO2, C-TiO2 and P90 was analyzed using the nitrogen adsorption/desorption isotherm and Brunauer–Emmett–Teller (BET) theory. The surface areas of around 43, 16 and 90 m2/g were obtained for the P-TiO2, C-TiO2 and P90, respectively. The P90 TiO2 shows the largest surface area, owing to the much smaller particle size comparing to the other two light scatterers of P-TiO2 and C-TiO2. Hence it is suitable to apply the P90 TiO2 as the main dye adsorption layer in the photoanode for DSSCs. On the other hand, the surface area of PTiO2 is almost 3-fold of that for the C-TiO2. The much larger surface area for the home-made P-TiO2 is favorable for dye adsorption and hence effective charge excitation. 3.2. Electrochemical characterization The photoanode is composed of two layers, the P90 TiO2 layer as the main dye adsorption layer and the larger size of the TiO2 layer as the light scattering layer. Before comparing the photovoltaic performance of the DSSCs using the home-made P-TiO2 and commercial CTiO2 as the light scatterers in the photoanodes, the thickness of the P90 TiO2 layer was firstly optimize. The thickness of the TiO2 layer is very important to influence the photovoltaic performance of DSSCs. The more dye molecules could adsorb in the photoanode with the thicker TiO2 layer, whereas the thicker TiO2 layer would lead to the larger charge-transfer resistance and longer charge-transfer path. Hence, the DSSCs with the thickness of 12, 16, 20, 24, 26 and 28 nm for the P90 TiO2 layer were assembled, and the corresponding photovoltaic parameters were shown in Table 1 for comparison. It is noted that these cells contained no light scatting layers in their photoanodes. The JSC increases for the DSSC with a thicker TiO2 layer in the photoanode, and the highest JSC value of 13.37 mA/cm2 was obtained for the DSSC with 24 nm as the TiO2 layer thickness. However, the JSC value was reduced when larger thicknesses of the TiO2 layer were applied for assembling the DSSCs. This phenomenon may be caused by the longer chargetransfer path for the electrons excited in the outer layer of the Table 1 The photovoltaic parameters for the DSSCs with different thicknesses of the P90 TiO2 layer in the photoanodes. Thickness (μm)
VOC (V)
JSC (mA/cm2)
FF
η (%)
12 16 20 24 26 28
0.75 0.77 0.76 0.75 0.75 0.74
9.91 10.08 10.93 13.37 13.20 11.14
0.68 0.69 0.70 0.69 0.68 0.67
5.06 5.36 5.89 6.97 6.80 5.54
Fig. 3. The J-V curves for the DSSCs with the P90 TiO2 photoanode and the P90/P-TiO2 photoanodes with different weight percentages of P-TiO2. 387
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larger surface area and the rougher surface for the P-TiO2 comparing to those for the C-TiO2 are the main causes for the higher IPCE values for the former case. It is found that the enhancement on the integrated area of the IPCE curve is truly higher than the enhancement on JSC value for the DSSC with the P90/C-TiO2 photoanode comparing to those for the DSSC with the P90 photoanode. This phenomenon may be due to the different measurements for evaluating the photocurrent density of the DSSCs. For measuring the IPCE values, the calibration should be applied for each wavelength and the measured wavelength is not totally continuous. Hence it is suggested that although the enhancements on the JSC value and the IPCE value are different, the trends for these two parameters for the DSSC with the P90 and the P90/C-TiO2 photoanodes are still the same. The similar phenomenon can be observed in the previous literature [23]. To understand the reasons for influencing the photovoltaic performance of the DSSCs with different photoanodes, the dye loading amount in the photoanode was firstly examined. The absorption spectra for the solution containing the dye molecules desorbed from the TiO2 photoanode were shown in Fig. 5(a). The amount of the adsorbed dye was listed in Table 3. The absorption peak is located at the wavelength of 500 nm, indicating the absorption feature of N719. Due to the smallest size of P90, the photoanode with only P90 shows the highest dye adsorption amount of 0.116 μmol/cm2, whereas when the light scatterers were incorporated in the photoanode, the dye adsorption amount was reduced. On the other hand, the higher dye adsorption amount of 0.106 μmol/cm2 was obtained for the P90/P-TiO2 photoanode, comparing to that of 0.093 μmol/cm2 for the P90/C-TiO2 photoanode. The higher dye adsorption for the P90/P-TiO2 photoanode is as expected since the well-defined smaller nanoparticle-assembled structure with large surface area and the anatase phase was achieved for this case, whereas the commercial rutile TiO2 light scatterers only presents the large size but the structure is simple the connected particles. To examine the light scatting ability, the reflection spectra for the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes was shown in Fig. 5(b). The smallest reflectance was observed for the P90 photoanode, due to the lack of the light scattering layer. The higher reflectance was obtained for the P90/P-TiO2 photoanode than that for the P90/C-TiO2 photoanode, owing to the well-defined popcorn-like structure for the former case to increase the light transfer length and improve the scattering ability. The reflection spectrum for the pure PTiO2 photoanode was also shown in this figure. The very high reflectance of around 60% in the whole spectrum again indicates the excellent light scattering ability for this case. The charge-transfer resistances were further analyzed using the EIS technique. It should be emphasized that the DSSC composed of a photoanode and a counter electrode was used for measuring the Nyquist plots for this work to understand the interfacial resistances in the photoanode. This is different from the measurement with the symmetrical cell consisting of identical counter electrodes. This analysis is used for evaluating the performance of the counter electrode and the electrolyte [24,25]. Fig. 5(c) shows the Nyquist plots for the DSSCs with
Table 2 The photovoltaic parameters, resistance and the electron lifetime for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes.
P90 P90/5 wt% P-TiO2 P90/10 wt% P-TiO2 P90/20 wt% P-TiO2 P90/30 wt% P-TiO2
VOC (V)
JSC (mA/cm2)
FF
η (%)
0.75 0.77 0.77 0.76 0.77
13.37 13.53 13.95 13.80 12.89
0.69 0.68 0.70 0.68 0.68
6.97 7.21 7.56 7.13 6.78
DSSC oppositely decreased with increasing P-TiO2 weight ratio in the photoanode. Since the two layers of TiO2 in the photoanode are responsible for providing active sites for dye adsorption and scattering incident light, too many light scatterers would lead to much small amounts of P90 TiO2 in the photoanode This phenomenon would result in the insufficient dye adsorption and hence the charge excitation would be largely reduced even the light scattering effect could be enhanced. Hence, the optimized weight ratio of 10 wt% for P-TiO2 in the photoanode was obtained, and the resulting DSSC showed the VOC value of 0.77 V, the JSC value of 13.95 mA/cm2, the FF of 0.70, and the η value of 7.56%. After optimizing the TiO2 film thickness and the ratio of the light scatterer in the photoanode, the photovoltaic performances for the DSSCs composed of the P90 photoanode and those with a light scattering layer, i.e., P90/P-TiO2, and P90/C-TiO2 photoanodes, were compared. Fig. 4(a) shows the J-V curves for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes, and the corresponding photovoltaic parameters were shown in Table 3 for clearer comparison. Due to the extra light scattering, the DSSCs with P90/PTiO2 and P90/C-TiO2 photoanodes show higher VOC, JSC and η values than those for the DSSC with the P90 photoanode without a light scattering layer. Among the DSSCs prepared with a light scattering layer, the cell with the P90/P-TiO2 photoanode shows a higher JSC value than the cell with the P90/C-TiO2 photoanode. This result is mainly caused by the well-defined popcorn-like structure for the homemade P-TiO2. As observed in the SEM and TEM images, the P-TiO2 is composed of numerous small nanoparticles with the size of around 50 nm, which is able to providing more active sites for dye adsorption, comparing to the pure particle structure for the C-TiO2. Moreover, the IPCE spectra for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/CTiO2 photoanodes were shown in Fig. 4(b) to further examine the solar to electricity conversion at different regions of the incident light. The maximum IPCE value was obtained at the wavelength of 530 nm for all the DSSCs, due to the main light absorption wavelength for the N719 dye. The DSSC with the P90 photoanode shows the smallest IPCE values, owing to the lack of a light scattering layer. The highest maximum IPCE value of 65% was achieved for the DSSC with the P90/P-TiO2 photoanode, and the IPCE values are higher at all wavelengths than those for the DSSCs with the P90/C-TiO2 photoanode. The higher IPCE value for the former case is consistent with its higher JSC value. The
Fig. 4. (a) The J-V curves and (b) the IPCE spectra for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes. 388
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Table 3 The photovoltaic parameters, resistance and the electron lifetime for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes.
P90 P90/P-TiO2 P90/C-TiO2
VOC (V)
JSC (mA/cm2)
FF
η (%)
Absorbed dye (μmol/cm2)
Rct1 (Ω)
Rct2 (Ω)
τ (ms)
0.75 0.77 0.77
13.37 13.95 13.40
0.69 0.70 0.68
6.97 7.56 7.17
0.116 0.106 0.093
2.98 2.65 2.78
22.95 14.82 18.94
20.04 25.23 25.23
Fig. 5. (a) The absorption spectra of the desorbed dye and (b) the reflection spectra for the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes; (c) the Nyquist plot and (d) the Bode plot for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes. The reflection spectrum for the P-TiO2 photoanode was also shown in (b) for comparison.
the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes. The equivalent circuit was also shown in this figure for fitting the resistance data. The charge-transfer resistance at the Pt counter electrode and the electrolyte interface (Rct1) can be estimated by using the first semicircle at the highest frequency region. The second semicircle indicates the charge-transfer resistance at the TiO2/dye/electrolyte interface (Rct2). The third semicircle at the lowest frequency region suggests the diffusion resistance in the electrolyte. The values of Rct1 and Rct2 were shown in Table 3 for clearer comparison. Since the main difference between these three DSSCs is on the photoanode, the variation on the first and the third semicircle between them is limited. The DSSC with the P90/P-TiO2 photoanode shows a smaller Rct2 value than that for the DSSC with the P90 photoanode, owing to the better light scattering ability for promoting more charges transferring for the former case. On the other hand, the DSSC with the P90/P-TiO2 photoanode also presents a smaller Rct2 value than that for the DSSC with the P90/C-TiO2 photoanode. The anatase phase and the larger surface area of P-TiO2 play important roles on the smaller Rct2 value for the resulting DSSC, owing to the preferable charge transfer of the active anatase TiO2 and the more excited charges for promoting the charge transfer. Moreover, the electron lifetime (τ) for the DSSCs with the P90 TiO2, P90/P-TiO2, and P90/C-TiO2 photoanodes was estimated by using the Bode plot, as shown in Fig. 5(d). The electron lifetime was also listed in Table 3 for clearer comparison. The DSSCs with an extra light scattering layer in the photoanodes present longer electron lifetimes than that for the DSSC with the P90 photoanode. It is inferred that the better charge transfer could lead to longer electron lifetime since the charge recombination could be reduced with promoted charge transfer in the photoanode. The illustration for the P90, P90/P-TiO2 and P90/C-TiO2 photoanode was shown in Scheme 1. The light transfer and the adsorbed dye molecules were also illustrated in this scheme. The P90 with the
Scheme 1. The illustration for the (a) P90, (b) P90/P-TiO2 and (c) P90/C-TiO2 photoanodes.
smallest size can provide more sites for dye adsorption. The P-TiO2 with the popcorn-like structure possessed higher due adsorption than that for C-TiO2. On the other hand, the large sizes of P-TiO2 and C-TiO2 could induce light scattering. As illustrated in this scheme, the better light utilization can be achieved with the light scattering layer in the photoanode, due to the large size of the TiO2 light scatterers for increasing the light transfer path and exciting more charges. To have fair comparison of the photovoltaic performances for the DSSC with a light scattering layer, the partial list for the morphology of TiO2 scatterers, methods for synthesizing TiO2 scatterers and the efficiency for DSSCs with and without TiO2 light scatterers reported in the previous literatures and in this work were listed in Table 4. It is found that the TiO2 light scatterers were usually fabricated by using the hydrothermal synthesis and the novel solution method with less reaction time and lower reaction temperature was firstly proposed in this work. The solar-to-electricity efficiency achieved in this work is also acceptable comparing to those reported in other related literatures. Hence it is significant to emphasize the efficient method and the unique and well-defined structure of TiO2 proposed in this work.
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from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
Table 4 Partial lists of previous literatures for the morphology of TiO2 scatterers, methods for synthesizing TiO2 scatterers and the efficiency for DSSCs with and without TiO2 light scatterers. Morphology
Method
ηw/o (%)
Nanorods Microspheres Hollow box Hydrangea Popcorn
Hydrothermal Hydrothermal Hydrothermal Hydrothermal Aqueous solution
4.47 7.37 3.04 6.41 6.97
scattering
ηwith
7.70 9.30 6.11 7.50 7.56
scattering
(%)
References
Ref.
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[13] [14] [15] [20] This work
4. Conclusions A facile solution method was proposed in this work to synthesize popcorn-like TiO2 aggregations (P-TiO2) as the light scatterers in the photoanode for DSSCs. The P-TiO2 has more active anatase phase and larger surface area, which is beneficial for dye adsorption and charge transfer. The thickness of the TiO2 layer and the ratio for the P-TiO2 light scatterer in the photoanode were optimized. The highest η value of 7.56% with the JSC of 13.95 mA/cm2, VOC of 0.77 V and FF of 0.70 was obtained for the DSSC with 24 nm TiO2 layer and 10 wt% P-TiO2 light scatterer in the photoanode. Also, the smaller charge transfer resistance and longer electron lifetime are obtained for this case. The DSSC with the pure P90 TiO2 photoanode and with the commercial light scatterer coupled with the P90 TiO2 underlayer in the photoanode showed smaller η values of 6.97% and 7.17%, respectively. The better photovoltaic performance for the DSSC with the P90/P-TiO2 photoanode is due to the larger surface area for dye adsorption, the unique anatase nature for achieving high activity, and the well-defined structure for inducing efficient light scattering via increasing the light transfer path. Acknowledgements This work was supported in part by the Ministry of Science and Technology (MOST) in Taiwan, under grant numbers: MOST 106-2221E-027-108 and MOST 106-2119-M-027-001. This work was financially supported from the Young Scholar Fellowship Program by MOST in Taiwan, under Grant MOST 107-2636-E-027-003. This work was financially supported by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors”
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