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J. Phys. Chem. C 2010, 114, 10612–10620
Investigation of Iodine Concentration Effects in Electrolytes for Dye-Sensitized Solar Cells Ze Yu,† Mikhail Gorlov,‡ Jarl Nissfolk,† Gerrit Boschloo,§ and Lars Kloo*,† Inorganic Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, Organic Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, and Physical Chemistry, Uppsala UniVersity, Box 259, S-751 05 Uppsala, Sweden ReceiVed: January 8, 2010; ReVised Manuscript ReceiVed: April 14, 2010
The present work describes the effects of different iodine concentrations and iodine-to-iodide ratios in electrolytes for dye-sensitized solar cells based on low-viscous, binary ionic liquid and organic liquid solvents. Current-voltage characteristics, photoelectrochemical measurements, electrochemical impedance spectroscopy, and Raman spectroscopy were used for characterization. Optimal short-circuit current and overall conversion efficiency were achieved using intermediate and low iodine concentration in ionic liquid-based and acetonitrilebased electrolytes, respectively. Results from photoelectrochemical and Raman-spectroscopic measurements reveal that both triiodide mobility and chemical availability affect the optimal iodine concentration required in these two types of electrolytes. The higher iodine concentrations required for the ionic liquid-based electrolytes partly compensate for these effects, although negative effects from higher recombination losses and light absorption of iodine-containing species start to become significant. 1. Introduction Since its breakthrough in performance by Gra¨tzel and coworkers in the early 1990s, dye-sensitized nanocrystalline solar cells (DSCs) have attracted considerable academic and industrial interest due to easy fabrication and their potential of low production costs, in contrast to conventional inorganic photovoltaic devices.1 Schematically, the DSC operation can be described as follows. Under light irradiation, the sensitizing dye of the DSC is photoexcited and the excited electrons are injected into the conduction band of the supporting wide band gap semiconductor, typically TiO2. The photoelectrons are collected at the conductive substrate and flow through an external circuit to the counter electrode. At the counter electrode the oxidized component of the electrolyte redox couple is reduced. Finally, the photooxidized dye is regenerated by the reduced component of the electrolyte redox couple. Most commonly, the reduced component is I-, and the oxidized component of the redox couple is I3-, irrespective of what type of liquid or quasi-liquid electrolyte is used. In this overall process, there are two major recombination loss processes that limit the total conversion efficiency within the DSCs: the photoinjected electrons in TiO2 can recombine directly with the oxidized dye molecules or with the oxidized form of the electrolyte redox couple (I3-). The highest photovoltaic conversion efficiency reported for DSCs (>11%) has been achieved by using electrolytes based on volatile organic solvents.2,3 However, encapsulation of organic solvents at elevated temperature has become a vital restriction concerning long-term stability of DSC devices. Room-temperature ionic liquids (ILs) have emerged as interesting alternative electrolyte solvents because of their unique physical and chemical properties, such as negligible vapor pressure, high chemical and thermal stability, and wide electrochemical window.4 There have been a large number of investigations using IL-based electrolytes, i.e., imidazolium5–12 * To whom correspondence should be addressed. † Inorganic Chemistry, Royal Institute of Technology. ‡ Organic Chemistry, Royal Institute of Technology. § Physical Chemistry, Uppsala University.
and other types of cations13–18 with various anions, ionic liquid crystals,19,20 quasi-solid electrolytes,11,12,21,22 as well as longterm stability tests,8,10,23 etc., whereas studies in which the effects of varying iodine concentrations are limited.24–26 The iodine concentration in IL-based electrolytes plays a crucial role for photovoltaic performance and optimization of DSCs. On one hand, iodine concentration has to be higher than that in electrolytes based on organic solvents in order to maintain sufficient charge-transport efficiency under irradiation. The main reason is that the higher viscosity of IL retards ion diffusion of I3-. A Grotthuss-type mechanism4 can contribute to the transport of I3- in IL-based electrolytes when high concentrations of iodide and triiodide are present, but this effect is not sufficient to counteract the negative effect of viscosity. Therefore, the amount of added iodine must be increased in IL-based electrolytes. On the other hand, higher iodine concentration is expected to render more efficient, and unwanted, recombination of the injected photoelectrons with I3- in the electrolyte and also to additional losses due to visible light absorption. Although considerable long-term stability of DSCs using ILbased electrolytes have been shown recently,27 the photovoltaic performance of DSCs based on these electrolytes has not yet reached that of the organic liquid electrolytes. One major reason is that the higher iodine concentration employed in IL-based electrolytes results in higher electron recombination between TiO2 and electrolyte, which lowers the photovoltaic performance. With this in mind, it is important to obtain a better understanding of the influence of different iodine concentrations in the IL-based electrolytes on DSC function. In this work, we have systematically investigated the binary IL-based electrolytes consisting of 1-propyl-3-methylimidazolium iodide (PMII) as the source of iodide and the low-viscous ionic liquid 1-ethyl3-methylimidazolium tetracyanoborate [EMIB(CN)4] as solvent using different iodine concentrations. For comparison purpose, acetonitrile (AN)-based electrolytes containing the same components as the IL-based electrolytes were also investigated. Our aim was to scrutinize the effects of iodine concentration in both IL-based and AN-based electrolytes on the photovoltaic char-
10.1021/jp1001918 2010 American Chemical Society Published on Web 05/24/2010
Iodine Concentration Effects in Solar Cells
J. Phys. Chem. C, Vol. 114, No. 23, 2010 10613
TABLE 1: Photovoltaic Parameters of DSCs Using IL and AN-Based Electrolytes with Varied Iodine Concentrations Measured under the Illumination of AM 1.5G Full Sunlight 1000 W/m2 electrolytes
composition
Voc (V)
Jsc (mA/cm2)
FF
η (%)
ILlow ILmed ILhigh ANlow ANmed ANhigh
0.03 M I2 (1 M PMII, 0.1 M GNCS, 0.5 M NMBI, in EMIB(CN)4) 0.2 M I2 (1 M PMII, 0.1 M GNCS, 0.5 M NMBI, in EMIB(CN)4) 0.5 M I2 (1 M PMII, 0.1 M GNCS, 0.5 M NMBI, in EMIB(CN)4) 0.03 M I2 (1 M PMII, 0.1 M GNCS, 0.5 M NMBI, in MeCN) 0.2 M I2 (1 M PMII, 0.1 M GNCS, 0.5 M NMBI, in MeCN) 0.5 M I2 (1 M PMII, 0.1 M GNCS, 0.5 M NMBI, in MeCN)
0.66 0.68 0.67 0.71 0.70 0.69
5.59 8.91 7.03 11.60 8.03 6.10
0.64 0.67 0.69 0.66 0.72 0.71
2.38 4.25 3.26 5.49 4.08 2.32
acteristics and charge recombination behavior affecting the total photovoltaic conversion efficiency of DSCs. 2. Experimental Section 2.1. Reagents and Electrolytes. Guanidinium thiocyanate (GNCS) was purchased from Alfa Aesar. N-Methylbenzimidazole (NMBI), iodine, acetonitrile, and ethanol were purchased from Aldrich. 1-Propyl-3-methylimidazolium iodide (PMII) was synthesized according to literature methods.28 1-Ethyl-3-methylimidazolium tetracyanoborate [EMIB(CN)4] was purchased from Merck. Dye N-719 cis-RuL2(SCN)2 (L ) 2,2′-bipyridyl4,4′-dicarboxylic acid) was purchased from Solaronix. Two kinds of electrolytes consisting of acetonitrile and ionic liquid were used in this study. The IL-based electrolytes ILlow, ILmed, and ILhigh were composed of 1 M PMII, 0.5 M NMBI, and 0.1 M GNCS in EMIB(CN)4 with 0.03, 0.2, and 0.5 M iodine, respectively. The AN-based electrolytes ANlow, ANmed, and ANhigh were composed of 1 M PMII, 0.5 M NMBI, and 0.1 M GNCS in acetonitrile with 0.03, 0.2, and 0.5 M iodine, respectively. Note that although 4-tert-butylpyridine (4-TBP) gives better solar cell performance in the devices employing organic solvent-based electrolytes, N-methylbenzimidazole was still used as additive in the AN-based electrolytes in order to maintain all components as similar as possible in the DSCs studied. 2.2. Device Fabrication. The fabrication of the solar cell devices is described as follows. Fluorine-doped SnO2 (FTO) glass plates (Pilkington-TEC8) were cleaned in water and ethanol. The conducting glass substrates were immersed into 40 mM aqueous TiCl4 solution at 70 °C for 30 min and then washed with water and ethanol. Two layers of TiO2 paste containing ∼25 nm sized TiO2 particles were first printed on the conducting glass substrates (∼7 µm thickness), followed by one layer containing scattering particles (∼3 µm, PST-400C, JGC Catalysts and Chemicals LTD), and a final thickness of 10 ( 1 µm was obtained. The screen-printed TiO2 electrodes were gradually heated to 500 °C in an oven (Nabertherm Controller P320) in air atmosphere for 30 min. After sintering, the electrodes received a TiCl4 treatment as described above, and a second and final sintering at 500 °C for 30 min was performed. The prepared TiO2 electrodes were heated again at 300 °C for 10 min before use. After cooling to 80 °C, the TiO2 electrodes were immersed into a 0.5 mM solution of N719 in ethanol at room temperature for at least 12 h. After washing with ethanol and drying under air flow, the sensitized titania electrodes were assembled with thermally platinized counter electrodes (CEs) using a 25 µm thin thermoplastic frame (Surlyn). The electrolyte was introduced under vacuum through a prefabricated drilled hole in the CEs. The hole was sealed afterward with a 50 µm thermoplastic frame and a glass plate. DSCs were fabricated with six IL- and ANbased electrolytes, respectively. Eighteen solar cells were
prepared, three with each electrolyte composition, exhibiting highly reproducible results. The active area of the cells was 0.25 cm2. 2.3. Current vs Potential Characteristics. A solar simulator (300 W xenon lamp) was used to give an irradiation of 100 mW cm-2 (the equivalent of 1 sun at air mass (AM) 1.5G) at the surface of solar cells. 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 at room temperature. These characteristics and I/V diagrams were monitored and recorded using a computerized Keithley model 2400 source unit. Photovoltaic performance was measured by using a black plastic mask which is 1 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 represent the average values of the two best cells for each electrolyte studied. 2.4. Photoelectrochemical Measurements. Electron lifetime, transport time, and accumulation for the solar cells were determined using a red-light-emitting diode (Luxeon K2 Star 5 W, λmax ) 640 nm) as light source. Voltage and current traces were recorded by a 16-bit resolution data acquisition board (DAQ National Instruments) in combination with a current amplifier (Stanford Research SR570). The intensity was measured using a silicon photodiode. The relation between potential and charge was studied using a combined voltage decay/chargeextraction method. Electron transport time and lifetime were investigated by monitoring transient photocurrent and photovoltage response after a small perturbation of the light intensity.29 The data used in the figures for photoelectrochemical measurements were based on the cells that showed the best performance for each electrolyte. The electrochemical potential EF,n in TiO2 was derived from the equation Voc ) |EF,n - Eredox|, where Voc is the open-circuit voltage of the DSC and Eredox is the redox potential of electrolyte obtained from potentiometry measurements. Potentiometry was carried out with a working electrode and a reference electrode connected to an Autolab potentiostat with a GPES electrochemical interface. The working electrode was a platinum wire placed inside the corresponding electrolyte solution. Potentiometry was obtained by using chrono methods (interval time
