# Fabrication and Characterization of Dye-Sensitized Solar Cells for Greenhouse Application

Authors: Jeum-Jong Kim; Mangu Kang; Ock Keum Kwak; Yong-Jin Yoon; Kil Sik Min; Moo-Jung Chu
Journal: International Journal of Photoenergy
 (2014)

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## Abstract

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 μm 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 μm TiO2 film, respectively.


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## Body

## 1. Introduction

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 [21]. 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 t
2
g orbital. Here, we report the synthesis of novel ruthenium(II) sensitizers (JJ-7 andJJ-9) for greenhouse DSSCs and their photovoltaic performance (Figure 1).Figure 1
Molecular structures ofJJ-7 andJJ-9.


## 2. Results and Discussion

The synthetic route for the preparation ofJJ-7 andJJ-9 is depicted in Scheme 1.JJ-7 andJJ-9 were prepared by reaction ofcis-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.Scheme 1
Schematic diagram for the synthesis of sensitizersJJ-7 andJJ-9.Figures2 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.Greenhouse of plant growth and transmittance spectra under solar light (out greenhouse, dark cyan line), solar light (in greenhouse, purple line), IR cladding material (brown line), green cladding material (green line), blue cladding material (blue line), and red cladding material (red line).
(a)
(b)Figure 3
Growth of plant under solar light (1), IR cladding material (2), green cladding material (3), blue cladding material (4), and red cladding material (5).Figure4 shows the UV-Vis spectra ofJJ-7 andJJ-9, together with theN719 absorption spectrum as a reference. The UV-vis spectrum ofJJ-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 ofJJ-7 is 10 nm blue-shifted relative to that ofN719 (524 nm). The band at 440 nm ofJJ-7 exhibits a molar extinction coefficient of 4.5 × 103 M−1 cm−1, which is slightly lower than that ofN719 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 [24]. Also, the UV-Vis spectra ofJJ-9 display two absorption bands at 380 and 514 nm and a molar extinction coefficient ofJJ-9 is 4.7 × 103 M−1 cm−1 at 440 nm. We also observed that the sensitizersJJ-7 andJJ-9 exhibited strong luminescence maxima at 660–700 nm when they were excited with their MLCT bands in EtOH at 298 K.Figure 4
Absorption and emission spectra ofJJ-7 (blue line),JJ-9 (orange line), andN719 (red line) in EtOH.The ultraviolet-visible transmittance spectra ofJJ-7 andJJ-9 adsorbed on TiO2 film are shown in Figure 5 together with theN719 transmittance spectrum as a reference. The transmittance ofJJ-7 andJJ-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 forN719 (48.0% at red (660 nm) and 7.0% at blue (440 nm) wavelength). Also, the transmittance ofJJ-7 andJJ-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 forN719 (37.9% at red (660 nm) and 1.5% at blue (440 nm) wavelength). The higher transmittance ofJJ-7 andJJ-9 compared withN719 is attributable to increased HOMO-LUMO energy gaps by electron-withdrawing abilities in a ligand and low molar extinction coefficient.Transmittance spectra ofJJ-7 (blue line),JJ-9 (orange line), andN719 (red line) (a) adsorbed on 5 μm TiO2 film and (b) adsorbed on 10 μm TiO2 film.
(a)
(b)TheJ-V curves for the devices based onJJ-7 andJJ-9 are shown and compared with those ofN719 in Figure 6. Under standard global AM 1.5 solar conditions, when 5 μm TiO2 film was used, theJJ-7 andJJ-9 sensitized cell gave a short circuit photocurrent density (J
sc) of 8.49 and 9.40 mA cm−2, an open circuit voltage (V
oc) 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, theN719 sensitized cell gave a J
sc of 10.34 mA cm−2, a V
oc of 0.82 V, and a FF of 0.74, corresponding to η of 6.25%. When 10 μm TiO2 film was used, theJJ-7 andJJ-9 sensitized cell gave short circuit photocurrent density (J
sc) of 11.68 and 12.62 mA cm−2, an open circuit voltage (V
oc) 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, theN719 sensitized cell gave a J
sc of 13.87 mA cm−2, a V
oc of 0.79 V, and a FF of 0.70, corresponding to η of 7.67%. A slightly lower J
sc ofJJ-7 andJJ-9 relative toN719 can be related to the increase of transmittance and the sparse packing of theJJ-7 andJJ-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 × 10−7mmol cm−2 forJJ-7,JJ-9, andN719, respectively. The low adsorption ofJJ-7 andJJ-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.Table 1
Optical, redox, and DSSC performance parameters of dyes.
Dye
λ
abs
a/nm  & λ
trans
⁡
b/nm (%)
E
redox
c/V
E
0
-
0
d/V
E
LUMO
e/V
J
sc (mA cm−2) (5/10 μm)
V
oc (V)  (5/10 μm)
FF(5/10 μm)
η
f (%) (5/10 μm)
JJ-7
380, 514 & 440 (18), 660 (62)
1.07
2.08
−1.01
8.49/11.68
0.83/0.80
0.71/0.71
4.96/6.58
JJ-9
380, 514 & 440 (15), 660 (61)
1.03
2.06
−1.03
9.40/12.62
0.78/0.80
0.69/0.69
5.07/6.93
N719
382, 524 & 440 (7), 660 (48)
2.03g (1.97h)
10.34/13.87
0.82/0.79
0.74/0.70
6.25/7.67
aAbsorption spectra were measured in EtOH solution. bTransmittance spectra were measured on 1-layer TiO2 film. cRedox potential of dyes on TiO2 was measured in CH3CN with 0.1 M (n-C4H9)4NPF6 as a scan rate of 100 mV s−1 (vs. NHE). d
E
0
-
0 values were estimated from the onset of absorption spectrum. e
E
LUMO was calculated by E
ox
-
E
0
-
0. fPerformances of DSSCs were measured with 0.18 cm2 working area. Electrolyte: 0.6 M DMPImI, 0.05 M I2, 0.5 M TBP, and 0.1 M LiI in acetonitrile. gEnergy level calculated by experiment.  hEnergy level calculated at the B3LYP/3-21 G.J-V curves ofJJ-7 (blue line),JJ-9 (orange line), andN719 (red line) using (a) 5 μm TiO2 film and (b) 10 μm TiO2 film.
(a)
(b)The electrochemical properties of the two sensitizersJJ-7 andJJ-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 ofJJ-7 andJJ-9 adsorbed on TiO2 film show quasi-reversible couples at 1.07 V and 1.03 V versus NHE, respectively (Table 1). The Ru
III
/
II oxidation potentials ofJJ-7 andJJ-9 are more positive than that ofN719. The HOMO-LUMO energy band gaps (E
0
-
0) ofJJ-7 andJJ-9 determined from the intersection of absorption and emission spectra are 2.08 and 2.06 eV, respectively, more increased than that ofN719 (1.85 eV for experimental calculation [25] 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 E
0
-
0 determined from the intersection of absorption and emission spectra are −1.01 V forJJ-7 and −1.03 V forJJ-9 versus NHE. A negative shift in the reduction potential ofJJ-7 andJJ-9 compared toN719 is attributable to electron-withdrawing abilities in ligands and increased HOMO-LUMO energy band gaps.Figure7 shows intensity of radiation in dye-sensitized solar cell usingJJ-7 andN719 sensitizers. After making box, we have measured the intensity of radiation as in Figure 7. The radiation intensities forJJ-7 sensitizers with both 5 and 10 μm TiO2 thickness films are much higher than that ofN719 sensitizer with 5 μm TiO2 thickness film. This result demonstrates that the novelJJ-7 sensitizer is very effective in greenhouse dye-sensitized solar cell to grow plant. On the other hand, the existingN719 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 inJJ-7 compared to that ofN719 may be due to the defects of adsorption ofJJ-7 sensitizer on the TiO2 electrodes.Figure 7
The intensity of radiation forJJ-7 (5 μm, black line),JJ-7 (10 μm, red line),N719 (5 μm, green line), and out (blue line) and greenhouse test-bed of dye-sensitized solar cell using new sensitizer (JJ-7).In conclusion, two novel ruthenium bipyridyl sensitizers have been synthesized and characterized. A solar-to-electricity conversion efficiency of 4.96% (for 5μm TiO2 film) and 6.58% (for 10 μm TiO2 film) forJJ-7 is comparable to 6.25% (for 5 μm TiO2 film) and 7.67% (for 10 μm TiO2 film) for theN719-sensitized solar cell. The high transmittance (62% at 660 nm and 18% at 440 nm) ofJJ-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 inJJ-7 andJJ-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 Mtert-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 ofN3 (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 productJJ-7 was obtained by Sephadex LH-20 column with methanol as eluent. Yield: 97%. 1H NMR (CD3OD): 9.43 (d, 2H, J
=
5.7 Hz), 8.90 (s, 2H), 8.74 (s, 2H), 8.13 (dd, 2H, J
=
5.7 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 ofN3 (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 productJJ-9 was obtained by Sephadex LH-20 column with methanol as eluent. Yield: 94%. 1H NMR (CD3OD): 9.41 (d, 2H, J
=
5
.
8 Hz), 8.91 (s, 2H), 8.75 (s, 2H), 8.10 (dd, 2H, J
=
5
.
8 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.


## 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 inJJ-7 andJJ-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 Mtert-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 ofN3 (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 productJJ-7 was obtained by Sephadex LH-20 column with methanol as eluent. Yield: 97%. 1H NMR (CD3OD): 9.43 (d, 2H, J
=
5.7 Hz), 8.90 (s, 2H), 8.74 (s, 2H), 8.13 (dd, 2H, J
=
5.7 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 ofN3 (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 productJJ-9 was obtained by Sephadex LH-20 column with methanol as eluent. Yield: 94%. 1H NMR (CD3OD): 9.41 (d, 2H, J
=
5
.
8 Hz), 8.91 (s, 2H), 8.75 (s, 2H), 8.10 (dd, 2H, J
=
5
.
8 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.



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*Source: 376315-2014-09-08.xml*