Charge Carrier Transport in the Bulk and at the Surface of Nanoparticles: A Quasi-Solid-State Dye-Sensitized Solar Cell
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Charge Carrier Transport in the Bulk and at the Surface of Nanoparticles: a Quasi-SolidState Dye-Sensitized Solar Cell Dennis Friedrich1 and Marinus Kunst1 1 Helmholtz-Zentrum Berlin für Materialien und Energie, Institute Solar Fuels, Hahn-Meitner Platz 1, 14109 Berlin, Germany ABSTRACT A quasi-solid-state dye-sensitized solar cell is presented, where the conventional liquid electrolyte is replaced by an electrolyte film, reaching a solar light-to-current conversion efficiency of 3%. Contactless transient photoconductance measurements were performed, revealing decay behavior of photoinduced charge carriers, dependent on external applied potential conditions. The measurements show that the decay is controlled by the injection of electrons into the front contact, hindered or enhanced by the field in the space charge region. INTRODUCTION A solar cell is presented, which avoids some of the disadvantages of the conventional liquid dyesensitized solar cell (DSSC) [1]. This cell consists also of sensitized TiO2 nanoparticles but the electrolyte is reduced to a stable film on the surface of the nanoparticles. It still includes a redox pair (Iodine-Iodide), specific chemicals and also strongly bound H2O molecules. This cell has still a modest efficiency but it promises improved stability and a significant potential for further optimization. Fortunately, the absence of a liquid electrolyte in this cell allows the application of highly performing transient techniques to the complete cell such as transient photoconductivity in the microwave frequency range (TRMC). Knowledge on kinetic charge transfer behavior is not only important for the improvement of this cell but also for the understanding of charge transport in nanosystems in general. EXPERIMENTAL DETAILS Materials All solvents and reagents, unless otherwise stated, were of puriss. quality and used as received. Transparent conductive glass, TEC 7 (FTO, sheet resistance 7 Ω/sq, thickness 2.3 mm) was purchased from Pilkington. TiO2 Paste DSL 18NR-AO and Dye N719 from Dyesol were used as received. Ethanol, acetonitrile (anhydrous, 99.8%), 2-propanol, titanium (IV) chloride tetrahydrofuran complex (1 : 2) (TiCl4*2THF, 97%), 4-t-butylpyridine (99%) and iodine (99.99%, metals basis) were provided by Sigma-Aldrich. Lithium iodide (anhydrous) and tbutanol were purchased from Fluka. Printex XE2 carbon black was provided by DegussaEvonik. Milli-Q (Millipore) grade water was used in all experiments.
Device Fabrication The front electrode was prepared by screen printing TiO2 layers of 1 cm2 area on FTO substrates, resulting in a 12 µm thick layer of TiO2 particles. The substrates were sintered at 520 °C for 30 min, cooled, followed by a post-treatment in a 50 mM TiCl4 solution for 30 min at 70 °C, flushed with water and again sintered for 30 min at 450 °C [2]. After the second heat treatment the substrates were cooled to 60 °C, immersed into the dye solution (1 mM of N719 in acetonitrile/t-butanol), and stored at room temperature for ∼16 h. After sensitization, 3 µL of
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