Urea as a long-term stable alternative to guanidium thiocyanate additive in dye-sensitized solar cell
Highlights
► GSCN additive shows poor stability with time. ► Dye analogue crystals form in the presence of GSCN. ► GSCN leads to dye desorption. ► Urea is found to be an additive alternative to GSCN. ► Urea shows almost the same performance after 40 days.
Introduction
Dye-sensitized solar cell (DSSC) has been perceived as a breakthrough photovoltaic technology due to low-cost and colorful appearance [1], [2]. During past two decades, a great effort has been devoted to improve the solar-to-electrical energy conversion efficiency and to understand photovoltaic properties of DSSC. As the result of such efforts, a better understanding on working principle is possible [3] and efficiency as high as 12% has been also demonstrated [4]. To get such a high efficiency, guanidinium thiocyanate (GSCN) is usually used as an additive in an iodide/iodine-based redox electrolyte. Guanidinium cation in redox electrolyte was proposed to play an important role in screening the lateral Coulombic repulsion of the anionic ruthenium-based dye, so-called N3, by co-adsorption, which resulted in an enhanced efficiency due to the improved cell voltage [5]. It was reported on the origin of the improved photovoltage in the presence of guanidinium cation from the analysis of band-edge movement and charge recombination rate [6].
Some additives have been known to be reactive with iodide and/or tri-iodide in redox electrolyte. For instance, thiourea used as an additive decreased slightly the triiodide concentration because thiourea reacted with iodine, by which photocurrent density and voltage were influenced [7]. N-methylbenzimidazole (MBI) is another example. It was found that a crystalline compound (N-methylbenzimidazole)6(N-methylbenzimidazole-H+)2(I−)(I3−) formed in the presence of MBI in electrolyte since it reacted with iodide and/or iodine [8]. Such reaction changed the electrolyte concentration, which had influence on photovoltaic performance. Although GSCN additive plays an important role in achieving high efficiency, its effect on long-term stability has not been studied. In addition, GSCN is classified into a potential carcinogenic substance and expensive [9]. Thus it is required to investigate the stability of GSCN and to find a cheaper additive.
In this paper, we report the long-term stability of GSCN-contained electrolyte. During long-term stability measurement, lots of red-colored crystalline pieces are found to form on TiO2 film. We attempt to analyze spectroscopically the crystals. Comparison of long-term stability of GSCN and urea is also performed.
Section snippets
Experimental
Anatase TiO2 nanoparticles were synthesized by hydrolysis of titanium isopropoxide (97%, Aldrich), followed by autoclaving at 230 °C for 12 h. Aqueous solvent in the autoclaved TiO2 colloid solution was replaced by ethanol for preparation of non aqueous TiO2 paste. Ethyl cellulose (Aldrich), lauric acid (Fluka) and terpineol (Aldrich) were added into the ethanol solution of the TiO2 particles, and then ethanol was removed from the solution using a rotary evaporator to obtain viscous pastes. For
Results and discussion
Fig. 1 compares photocurrent-voltage curves of GSCN-contained (E1) and urea-contained (E2) electrolyte. Both E1 and E2 shows similar photovoltaic performance, where E1 electrolyte shows photocurrent density (JSC) of 12.1 mA/cm2, voltage (VOC) of 794 mV, fill factor (FF) of 0.73 and efficiency (η) of 7.0%, whereas E2 Exhibits 11.9 mA/cm2, 807 mV, 0.75 and 7.2%. From the measured pH (pH 6.8 for E1 and 7.2 for E2) and the assumption of one proton donor, acid dissociation constant pKa is calculated to
Conclusions
The GSCN-contained electrolyte showed degradation of photocurrent with time, which was related to needle-shaped crystals formed on the dye-adsorbed TiO2 film. The crystals were analyzed to be dye analogue from FTIR spectra, indicating dye desorption. A possible mechanism of dye desorption in the presence of GSCN was proposed, where water actives GSCN to produce ammonia as dye desorbing agent. Contrary to GSCN, no crystals were formed in the device with electrolyte containing urea. Thus,
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea under contracts No. 2011-0016441, 2010-0028821 and R31-2008-10029 (WCU program) and the Korea Institute of Energy Technology Evaluation and planning (KETEP) grant funded by the Ministry of Knowledge Economy under contract No. 20103020010010.
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Dye-Sensitized Solar Cells
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