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Synergistic Effect of N-Methylbenzimidazole and Guanidinium Thiocyanate on the Performance of Dye-Sensitized Solar Cells Based on Ionic Liquid Electrolytes Ze Yu,† Mikhail Gorlov,‡ Gerrit Boschloo,§ and Lars Kloo*,† Inorganic Chemistry and Organic Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, and Physical Chemistry, Uppsala UniVersity, Box 259, S-751 05 Uppsala, Sweden ReceiVed: August 5, 2010; ReVised Manuscript ReceiVed: NoVember 4, 2010
The effects of additives guanidinium thiocyanate (GSCN) and N-methylbenzimidazole (MBI) on the photovoltaic performance of dye-sensitized solar cells based on low-viscous, binary ionic liquid and organic liquid electrolytes were investigated. Addition of only GSCN to the electrolyte has a pronounced influence on the short-circuit current, owing largely to the positive shift of the conduction band edge potential, probably increasing the injection efficiency of the excited dye. When only MBI was added to the electrolyte, a significant improvement of the open-circuit voltage was found, which could be attributed to a negative shift of the TiO2 conduction band edge potential and a longer electron lifetime under open-circuit conditions. Synergistic effects were observed when GSCN and MBI were used together in the ionic liquid-based electrolyte. In this case, optimal open-circuit voltage and total conversion efficiency were obtained among the ionic liquid electrolytes studied mainly due to the more efficient retardation of the recombination loss reaction at the TiO2/electrolyte interface. Introduction Nanocrystalline, dye-sensitized solar cells (DSCs) have been extensively investigated as a promising low-cost alternative to conventional silicon solar cells for nearly two decades.1 A typical DSC is composed of three major constituents: nanocrystalline titania, dye, and electrolyte in between two transparent conductive oxide electrodes (TCO). The electrolyte, consisting of a redox mediator dissolved in a liquid solution, performs the tasks of regenerating the oxidized dye and thus transporting charges between the two device electrodes. Iodide/triiodide, from the very start, has so far proved to be the most useful and versatile redox mediator in electrolytes for DSCs. In addition to a redox mediator, various additives are typically introduced into the electrolyte, dedicated to the optimization of the photovoltaic characteristics of DSCs. Two kinds of additives are normally used in electrolytes for DSCs. One class of the very frequently used additives is nitrogen heterocyclic compounds, such as 4-tert-butylpyridine (4-TBP), N-alkylbenzimidazole, etc. 4-TBP has been actively studied by many groups in recent years.2-6 The addition of 4-TBP into the electrolyte leads to a significant improvement of open-circuit voltage (Voc). It has been proposed that this improvement can be ascribed to either the suppression of dark current due to the blocking effect of 4-TBP at the TiO2/ electrolyte interface,2 or to a shift of the titania conduction band.4-6 Derivatives of N-alkylbenzimidazole have also been widely investigated.7-11 A remarkable increase in Voc was also observed by the addition of N-alkylbenzimidazole to the electrolyte. It has been proposed that N-alkylbenzimidazole behaves in a way similar to that of 4-TBP.7-9 The improvement of Voc could be attributed to either the suppression of dark current7 or the shift of the semiconductor conduction band.10,11 GSCN is another important and frequently used additive in * Corresponding author. † Inorganic Chemistry, Royal Institute of Technology. ‡ Organic Chemistry, Royal Institute of Technology. § Uppsala University.
electrolytes for DSCs. It was reported that the addition of GSCN to the electrolyte results in a remarkable improvement of Voc because of the reduction of dark current.12 Kopidakis et al. discovered that an overall improvement of Voc was observed when guanidinium is present in the electrolyte, owing to the collective effect of slower recombination reaction and a positive shift of the conduction band.13 Recently, it was reported that a significant increase in photocurrent was observed when GSCN is present due to the increase of electron injection yield.14 Room-temperature ionic liquid-based (IL) electrolytes have received more attention recently due to their unique physical and chemical properties, such as negligible vapor pressure, high chemical and thermal stability, and wide electrochemical window.15-20 A great deal of effort has been devoted to improve the performance of IL-based electrolytes for DSCs over the past few years.21-25 Two parameters that play a crucial role for the photovoltaic performance and optimization of DSCs based on IL electrolytes are the iodine concentration, which recently was systematically investigated by our group,26 and additives that affect the photovoltaic performance of DSCs. The state-of-theart DSCs based on IL-based electrolytes normally incorporate GSCN and N-alkylbenzimidazole additives together in the electrolytes.22,23 We herein elucidate the effects of GSCN and MBI in a binary IL-based electrolytes and also include acetonitrile-based (AN) electrolytes for comparison. Although the use of MBI is not optimal for AN-based DSCs (maximum performance in terms of conversion efficiencies will not be obtained), the ambition was to study the two types of electrolytes under as similar conditions as possible in order to get mechanistic insights, rather than to obtain maximum conversion efficiencies. A synergistic effect of GSCN and MBI on the photovoltaic performance of DSCs was observed for IL-based electrolytes and will be discussed in this work. Experimental Section 1. Reagents and Electrolytes. Acetonitrile, N-methylbenzimidazole, iodine, ethanol, and 1-propyl-3-methylimidazolium
10.1021/jp1073686 2010 American Chemical Society Published on Web 11/30/2010
Dye-Sensitized Solar Cells
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TABLE 1: Photovoltaic Parameters of DSCs Using IL- and AN-Based Electrolytes with Different Additives Studied under the Illumination of AM 1.5 G Full Sunlight 1000 W/m2 electrolytes ILPURE ILGSCN
ILMBI
ILBOTH
ANPURE ANGSCN
ANMBI
ANBOTH
composition 0.2 M I2 1 M PMII in EMIB(CN)4 0.2 M I2 1 M PMII 0.1 M GSCN in EMIB(CN)4 0.2 M I2 1 M PMII 0.5 M MBI in EMIB(CN)4 0.2 M I2 1 M PMII 0.1 M GSCN 0.5 M MBI In EMIB(CN)4 0.2 M I2 1 M PMII in MeCN 0.2 M I2 1 M PMII 0.1 M GSCN In MeCN 0.2 M I2 1 M PMII 0.5 M MBI in MeCN 0.2 M I2 1 M PMII 0.1 M GSCN 0.5 M MBI in MeCN
Voc (V) Jsc (mA/cm2)
FF
η (%)
0.58
8.12
0.67
3.18
0.59
9.24
0.66
3.60
0.68
8.18
0.70
3.90
0.73
8.56
0.70
4.31
0.57
9.98
0.71
4.07
0.61
12.70
0.68
5.27
0.72
9.34
0.72
4.82
0.72
9.82
0.73
5.10
iodide (PMII) were purchased from Aldrich. Guanidinium thiocyanate and tert-butyl alcohol was purchased from Alfa Aesar. The dye N-719 cis-RuL2(SCN)2 (L ) 2,2′-bipyridyl-4,4′dicarboxylic acid) was purchased from Solaronix. 1-Ethyl-3methylimidazolium tetracyanoborate [EMIB(CN)4] and guanidinium trifluoromethanesulfonate (triflate) were purchased from Merck. The IL-based electrolytes ILPURE, ILGSCN, ILMBI, and ILBOTH had the following composition: ILPURE, 0.2 M I2, and 1 M PMII in EMIB(CN)4; ILGSCN, 0.2 M I2, 1 M PMII, and 0.1 M GSCN in EMIB(CN)4; ILMBI, 0.2 M I2, 1 M PMII, and 0.5 M MBI in EMIB(CN)4; ILBOTH, 0.2 M I2, 1 M PMII, 0.1 M GSCN, and 0.5 M MBI in EMIB(CN)4. AN-based electrolytes ANPURE, ANGSCN, ANMBI, and ANBOTH had the following composition: ANPURE, 0.2 M I2, and 1 M PMII in AN; ANGSCN, 0.2 M I2, 1 M PMII, and 0.1 M GSCN in AN; ANMBI, 0.2 M I2, 1 M PMII, and 0.5 M MBI in AN; ANBOTH, 0.2 M I2, 1 M PMII, 0.1 M GSCN, and 0.5 M MBI in AN (see Table 1). 2. Device Fabrication. The fabrication of the solar cell devices is described as follows. Fluorine-doped SnO2 (FTO) conducting glass substrates (Pilkington-TEC8) were immersed into a 40 mM aqueous TiCl4 solution at 70 °C for 30 min and then washed with water and ethanol. An ∼8 µm thick film of 25-nm-sized TiO2 paste was first printed on the conducting glass substrates, followed by one scattering layer (∼3 µm, PST-400C, JGC Catalysts and Chemicals LTD), and a final thickness of ∼11 µm was obtained. The screen-printed TiO2 electrodes were gradually heated to 500 °C in ambient atmosphere for 30 min. After sintering, the electrodes passed a post-TiCl4 treatment as described above and were heated to 500 °C again for 30 min. The prepared working electrodes were heated at 300 °C for 10
min before use. After cooling to 80 °C, the working electrode was immersed into a 0.3 mM solution of N-719 in AN/tertbutyl alcohol (volume ratio, 1:1) at room temperature for at least 12 h. The sensitized working electrodes were assembled with a platinized counter electrode (CE) using a 25 µm hot-melt Surlyn frame. The electrolyte was introduced under vacuum through a prefabricated drilled hole in the CE. The hole was sealed afterward with a 50 µm thick thermoplastic frame and a glass plate. DSCs were fabricated with eight different electrolytes based on different additives, respectively. Twenty four solar cells were prepared, three of each electrolyte composition, showing highly reproducible results. The active area of the cells was 0.25 cm2. 3. Current-Voltage Characteristics. A solar simulator (Newport 91160-1000) was used to give an irradiation of 100 mW cm-2 [the equivalent of 1 sun at air mass (AM) 1.5 G]. The overall light-to-electricity conversion efficiency (η), fill factor (FF), open-circuit voltage (Voc), and short-circuit current density (Jsc) were obtained through the current-voltage characteristics of a solar cell. These characteristics and I/V diagrams were recorded using a computerized Keithley model 2400 source/meter unit. The photovoltaic performance was determined by using a plastic mask which is ∼1.5 mm wider than the active area. Three cells based on each electrolyte were investigated, showing high reproducibility. The I-V data listed in Table 1 are the average values of the two best cells based on each electrolyte studied. 4. Photoelectrochemical Measurements. Photoelectrochemical measurements were performed by using a white-lightemitting diode (Luxeon, 1 W) as the light source. Voltage and current traces were recorded by a 16-bit resolution data acquisition board (National Instruments) in combination with a current amplifier (Stanford Research SR570). The intensity was recorded using a silicon photodiode. The relation between potential and charge was studied using a combined voltage decay/charge-extraction method. Electron transport time and lifetime were studied by monitoring transient photocurrent and photovoltage response after a small perturbation of light intensity.6 The data used in the figures for photoelectrochemical measurements were based on the cells that showed the best performance of each electrolyte. 5. Electrochemical Impedance Measurements. Impedance measurements were carried out with a Autolab PGstat12 potentiostat with an impedance module. The frequency range is 10 kHz to 0.025 Hz, using 20 mV AC amplitude, with illumination provided by a 5 W red luxeon LED. Impedance was measured under illumination in a range of intensities under open-circuit condition. The response was analyzed by the equivalent circuit models described by Bisquert and Fabregat.27 The recombination resistance (Rrec), diffusion resistance (Rdif), and counter-electrode resistance (Rct) were extracted from Nyquist plots of the DSCs. The data used in the figures for electrochemical impedance measurements were from the same cells as used in the photoelectrochemical measurements. 6. Raman Spectroscopy and Electrochemical Measurements. Raman spectroscopy of electrolyte solutions were performed using a BioRad FTS 6000 spectrometer equipped with a Raman accessory. An excitation wavelength of 1064 nm (Nd:YAG laser), a quartz beamsplitter, and a resolution of 4 cm-1 were employed. Scattered radiation was detected using a nitrogen-cooled solid-state germanium detector. Potentiometry measurements were implemented with a working electrode and a reference electrode connected to an Autolab potentiostat with
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a GPES electrochemical interface. The working electrode used was a platinum wire placed inside the corresponding electrolyte solution. Potentiometry was obtained by using chrono methods (interval time
