Solar Cells: From Sunlight into ElectricityView this Special Issue
Fabrication and Characterization of Dye-Sensitized Solar Cells for Greenhouse Application
We have developed dye-sensitized solar cells using novel sensitizers with enhanced transmittance of red (625–675 nm) and blue (425–475 nm) wavebands to control the illumination condition in the greenhouse. Novel ruthenium bipyridyl sensitizers with general formulas (Me3PhN)4[Ru(dcbpy)2(NCS)2] (JJ-7) and (Me3BnN)4[Ru(dcbpy)2(NCS)2] (JJ-9) have been synthesized and demonstrated as efficient sensitizers in dye-sensitized solar cells for greenhouse application. Under standard AM 1.5 sunlight, the solar cell of JJ-7 using a liquid-based electrolyte exhibits a short-circuit photocurrent density of 8.49 mA/cm2, an open-circuit voltage of 0.83 V, and a fill factor of 0.71, corresponding to an overall conversion efficiency of 4.96% on 5 m TiO2 film. The transmittance of JJ-7 and JJ-9 shows 62.0% and 61.0% at 660 nm and 18.0% and 15.0% at 440 nm for cultivation on 5 m TiO2 film, respectively.
A dye-sensitized solar cell (DSSC) is an electrochemical device that uses light-absorbing dye molecules adsorbed on semiconductor nanoparticles to generate electricity from the sunlight [1–5]. Current researches on the DSSCs are focused on the development of cell materials and manufacturing techniques that give high conversion efficiency, low cost, and stability [6–8]. The preparation of dye-sensitized solar cell for greenhouse is involved in the light manipulation for plant growth and energy-saving. The light manipulation in greenhouse is very important to improve the quantity and quality of the agricultural products. The cladding materials [9–13] and artificial lights [14–20] (LED or high-pressure sodium lamps) are used to manipulate the light in greenhouse for plant growth. Until now, no dye-sensitized solar cells (DSSCs) have been applied in greenhouse for plant growth and energy-saving. The most important wavebands for plant growth are the absorption peaks of chlorophyll located in the red (625–675 nm) and blue (425–475 nm) regions, respectively. The red spectrum band is known to be involved in photosynthesis and the blue band is related to the photomorphogenic and the phototropic responses of plants . Therefore, dye-sensitized solar cells for greenhouse can be used as technically advanced photoselective coverings that control the environmental conditions to optimize the productivity and quality of farm products and save energy. For the plant growth and energy-saving, we have focused on the development of novel sensitizers for DSSCs with enhanced transmittance of red and blue wavebands and high performance. This approach is to synthesize efficient ruthenium sensitizers through a systematic tuning of the LUMO and HOMO energy levels by introducing a ligand with a high-lying π∗ molecular orbital or by stabilizing the metal orbital. Here, we report the synthesis of novel ruthenium(II) sensitizers (JJ-7 and JJ-9) for greenhouse DSSCs and their photovoltaic performance (Figure 1).
2. Results and Discussion
The synthetic route for the preparation of JJ-7 and JJ-9 is depicted in Scheme 1. JJ-7 and JJ-9 were prepared by reaction of cis-dithiocyanatobis(2,2′-bipyridine-4,4′-dicarboxy-late)ruthenium(II) sensitizer (N3) dye, trimethylphenylammonium hydroxide, and benzyltrimethylammonium hydroxide, respectively. The analytical and spectroscopic data of two sensitizers are consistent with the formulated structures.
Figures 2 and 3 show plant growth of greenhouse and their transmittance spectra using cladding materials. Plant 1 was grown under solar light and plants 2, 3, 4, and 5 were grown using IR cladding material, blue cladding material, green cladding material, and red cladding material, respectively (Figure 3). The UV-Vis spectrum of red cladding material displays transmittance band over 570 nm in visible and IR region. This band is similar to the real solar light spectrum band at red (625–675 nm) waveband. On the other hand, blue and green cladding materials are not transmitted at red (625–675 nm) waveband (Figure 2). We have cultivated lettuce in greenhouse covered with cladding materials. The lettuce growth in red cladding material is more superior in quantity and quality to other cladding materials, but the lettuce in red cladding material does not appear red color (Figure 3). In order to obtain red color of lettuce like the real solar light, we need not only red waveband (625–675 nm) but also suitable blue (425–475 nm) waveband. Therefore, we have developed greenhouse dye-sensitized solar cells using novel sensitizers with transmittance at both red and blue wavebands.
Figure 4 shows the UV-Vis spectra of JJ-7 and JJ-9, together with the N719 absorption spectrum as a reference. The UV-vis spectrum of JJ-7 displays two absorption bands at 380 and 514 nm, which are characteristic of the metal-to-ligand charge transfer (MLCT) bands [22, 23]. The low energy MLCT band at 514 nm of JJ-7 is 10 nm blue-shifted relative to that of N719 (524 nm). The band at 440 nm of JJ-7 exhibits a molar extinction coefficient of 4.5 × 103 M−1 cm−1, which is slightly lower than that of N719 dye (5.4 × 103 M−1 cm−1). The blue-shift and lower molar extinction coefficient are due to an increase in the energy of the LUMO of the ligand, causing the π-π∗ and dπ-π∗ transitions to occur at higher energies . Also, the UV-Vis spectra of JJ-9 display two absorption bands at 380 and 514 nm and a molar extinction coefficient of JJ-9 is 4.7 × 103 M−1 cm−1 at 440 nm. We also observed that the sensitizers JJ-7 and JJ-9 exhibited strong luminescence maxima at 660–700 nm when they were excited with their MLCT bands in EtOH at 298 K.
The ultraviolet-visible transmittance spectra of JJ-7 and JJ-9 adsorbed on TiO2 film are shown in Figure 5 together with the N719 transmittance spectrum as a reference. The transmittance of JJ-7 and JJ-9 on 5 μm TiO2 film exhibits 62.0% and 61.0% at red (660 nm) and 18.0% and 15.0% at blue (440 nm) wavelength for plant production and quality, respectively, which is higher than the corresponding value for N719 (48.0% at red (660 nm) and 7.0% at blue (440 nm) wavelength). Also, the transmittance of JJ-7 and JJ-9 on 10 μm TiO2 film shows 62.9% and 60.5% at red (660 nm) and 7.5% and 6.3% at blue (440 nm) wavelength for plant production and quality, respectively, which is higher than the corresponding value for N719 (37.9% at red (660 nm) and 1.5% at blue (440 nm) wavelength). The higher transmittance of JJ-7 and JJ-9 compared with N719 is attributable to increased HOMO-LUMO energy gaps by electron-withdrawing abilities in a ligand and low molar extinction coefficient.
The J-V curves for the devices based on JJ-7 and JJ-9 are shown and compared with those of N719 in Figure 6. Under standard global AM 1.5 solar conditions, when 5 μm TiO2 film was used, the JJ-7 and JJ-9 sensitized cell gave a short circuit photocurrent density () of 8.49 and 9.40 mA cm−2, an open circuit voltage () of 0.83 and 0.78 V, and a fill factor (FF) of 0.71 and 0.69, corresponding to overall conversion efficiency (η) of 4.96% and 5.07%, respectively (Table 1). Under the same condition, the N719 sensitized cell gave a of 10.34 mA cm−2, a of 0.82 V, and a FF of 0.74, corresponding to η of 6.25%. When 10 μm TiO2 film was used, the JJ-7 and JJ-9 sensitized cell gave short circuit photocurrent density () of 11.68 and 12.62 mA cm−2, an open circuit voltage () of 0.80 and 0.80 V, and a fill factor (FF) of 0.71 and 0.69, corresponding to overall conversion efficiency (η) of 6.58% and 6.93%, respectively (Table 1). Under the same condition, the N719 sensitized cell gave a of 13.87 mA cm−2, a of 0.79 V, and a FF of 0.70, corresponding to η of 7.67%. A slightly lower of JJ-7 and JJ-9 relative to N719 can be related to the increase of transmittance and the sparse packing of the JJ-7 and JJ-9 monolayers on the TiO2 electrodes. To clarify the above explanations, we measured the amount of dyes adsorbed on TiO2 film by desorbing the dyes from the TiO2 surface with KOH. The amounts of three dyes adsorbed on TiO2 film were measured to be 2.94 × 10−7, 3.02 × 10−7, and 3.76 × 10−7 mmol cm−2 for JJ-7, JJ-9, and N719, respectively. The low adsorption of JJ-7 and JJ-9 can be due to the presence of bulky protecting groups with electron withdrawing abilities of tetra-substituted ammonium groups and the electrostatic repulsion of negatively charged carboxylic groups.
The electrochemical properties of the two sensitizers JJ-7 and JJ-9 were studied by cyclic voltammetry in CH3CN with 0.1 M tetrabutylammonium hexafluorophosphate using TiO2 film with adsorbed dyes as working electrode. The oxidation potentials of JJ-7 and JJ-9 adsorbed on TiO2 film show quasi-reversible couples at 1.07 V and 1.03 V versus NHE, respectively (Table 1). The oxidation potentials of JJ-7 and JJ-9 are more positive than that of N719. The HOMO-LUMO energy band gaps () of JJ-7 and JJ-9 determined from the intersection of absorption and emission spectra are 2.08 and 2.06 eV, respectively, more increased than that of N719 (1.85 eV for experimental calculation  and 1.97 eV for theoretical calculation by B3LYP/3-21 G [26, 27]). This reflects the increased electron withdrawing properties of ligand with tetra-substituted ammonium groups. The reduction potentials of two dyes calculated from the oxidation potentials and the determined from the intersection of absorption and emission spectra are −1.01 V for JJ-7 and −1.03 V for JJ-9 versus NHE. A negative shift in the reduction potential of JJ-7 and JJ-9 compared to N719 is attributable to electron-withdrawing abilities in ligands and increased HOMO-LUMO energy band gaps.
Figure 7 shows intensity of radiation in dye-sensitized solar cell using JJ-7 and N719 sensitizers. After making box, we have measured the intensity of radiation as in Figure 7. The radiation intensities for JJ-7 sensitizers with both 5 and 10 μm TiO2 thickness films are much higher than that of N719 sensitizer with 5 μm TiO2 thickness film. This result demonstrates that the novel JJ-7 sensitizer is very effective in greenhouse dye-sensitized solar cell to grow plant. On the other hand, the existing N719 sensitizer is not useful because of the low quantity of light. The different light quantity might be caused by the different molecular structure of the dyes. The high quantity of light in JJ-7 compared to that of N719 may be due to the defects of adsorption of JJ-7 sensitizer on the TiO2 electrodes.
In conclusion, two novel ruthenium bipyridyl sensitizers have been synthesized and characterized. A solar-to-electricity conversion efficiency of 4.96% (for 5 μm TiO2 film) and 6.58% (for 10 μm TiO2 film) for JJ-7 is comparable to 6.25% (for 5 μm TiO2 film) and 7.67% (for 10 μm TiO2 film) for the N719-sensitized solar cell. The high transmittance (62% at 660 nm and 18% at 440 nm) of JJ-7 is attributed to its low absorption extinction coefficient of MLCT band in the visible region and blue-shift by an increased HOMO-LUMO energy band gap. We believe that the development of highly efficient sensitizers for greenhouse dye-sensitized solar cell is possible through meticulously molecular engineering, and work on these is now in progress.
3. Experimental Section
3.1. Fabrication of Dye-Sensitized Solar Cells
Fluorine-doped tin oxide (FTO) glass plates (Pilkington TEC Glass-TEC 8, solar 2.3 mm thickness) were cleaned in a detergent solution using an ultrasonic bath for 30 min and then rinsed with water and ethanol. Then, the plates were immersed in 40 mM TiCl4 (aqueous) at 70°C for 30 min and washed with water and ethanol. A transparent nanocrystalline layer was prepared on the FTO glass plates by using a doctor blade printing TiO2 paste (Solaronix, Ti-Nanoxide T/SP), which was then dried for 2 h at 25°C. The TiO2 electrodes were gradually heated under an air flow at 325°C for 5 min, at 375°C for 5 min, at 450°C for 15 min, and at 500°C for 15 min. The thickness of the transparent layer was measured by using an Alpha-step 250 surface profilometer (Tencor Instruments, San Jose, CA). The resulting film was composed of a 5 and 10 μm thick transparent layer. The TiO2 electrodes were treated again with TiCl4 at 70°C for 30 min and sintered at 500°C for 30 min. Then, they were immersed in JJ-7 and JJ-9 (0.3 mM in ethanol) solutions and kept at room temperature for 24 h. FTO plates for the counter electrodes were cleaned in an ultrasonic bath in H2O, acetone, and 0.1 M aqueous HCl, subsequently. The counter electrodes were prepared by placing a drop of an H2PtCl6 solution (2 mg Pt in 1 mL ethanol) on an FTO plate and heating it (at 400°C) for 15 min. The dye adsorbed TiO2 electrodes and the Pt counter electrodes were assembled into a sealed sandwich-type cell by heating at 80°C using a hot-melt ionomer film (Surlyn) as a spacer between the electrodes. A drop of the electrolyte consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile was placed in the drilled hole of the counter electrode and was driven into the cell via vacuum backfilling. Finally, the hole was sealed using additional Surlyn and a cover glass (0.1 mm thickness).
3.2. Typical Procedures and Analytical Data
JJ-7 Complex. A mixture of N3 (100 mg, 0.141 mmol) and trimethylphenylammonium hydroxide (434 mg, 0.708 mmol) in MeOH (2 mL) was stirred at room temperature for 2 h. The pure product JJ-7 was obtained by Sephadex LH-20 column with methanol as eluent. Yield: 97%. 1H NMR (CD3OD): 9.43 (d, 2H, Hz), 8.90 (s, 2H), 8.74 (s, 2H), 8.13 (dd, 2H, Hz), 7.93 (m, 6H), 7.63–7.50 (m, 18H, 3.70 (s, 36H). Anal. calcd for C62H68N10O8RuS2: C, 59.74; H, 5.50. Found: C, 59.77; H,5.48.
JJ-9 Complex. A mixture of N3 (100 mg, 0.141 mmol) and benzyltrimethylammonium hydroxide (296 mg, 0.708 mmol) in MeOH (2 mL) was stirred at room temperature for 2 h. The pure product JJ-9 was obtained by Sephadex LH-20 column with methanol as eluent. Yield: 94%. 1H NMR (CD3OD): 9.41 (d, 2H, Hz), 8.91 (s, 2H), 8.75 (s, 2H), 8.10 (dd, 2H, Hz), 7.54 (m, 24H), 4.55 (s, 8H), 3.12 (s, 36H). Anal. calcd for C66H76N10O8RuS2: C, 60.86; H, 5.88. Found: C, 60.78; H, 5.94.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by Electronics and Telecommunications Research Institute (ETRI).
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