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J. Phys. Chem. C 2010, 114, 14300–14306
Transport and Interfacial Transfer of Electrons in Dye-Sensitized Solar Cells Utilizing a Co(dbbip)2 Redox Shuttle Hongxia Wang,†,‡ Patrick G. Nicholson,§ Laurence Peter,*,† Shaik M. Zakeeruddin,| and Michael Gra¨tzel| Department of Chemistry, UniVersity of Bath, Bath BA2 7AY, United Kingdom, National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom, and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH-1055, Lausanne, Switzerland ReceiVed: June 22, 2010; ReVised Manuscript ReceiVed: July 17, 2010
The transport and interfacial transfer of electrons in dye-sensitized solar cells utilizing the Co(dbbip)2 (dbbip ) 2,6-bis(1′-butylbenzimidazol-2′-yl)pyridine) redox couple as an alternative to the conventional I3-/I- couple have been investigated using intensity modulated photocurrent and photovoltage spectroscopy (IMPS/IMVS) combined with in situ near IR absorption spectroscopy. Attempts to use impedance spectroscopy to determine the electron diffusion length were unsuccessful due to overlap of the cathode and electron transport impedances. Values of the electron diffusion length in the range 5-8 µm were derived by IMPS/IMVS as well as by analysis of the ratio of the normalized photocurrent action spectra measured for illumination through the counter electrode and through the TiO2 electrode. These values indicate that loss of electrons by electron transfer to the Co(III) species will be important for TiO2 films thicker than about 5 µm, unless steps are taken to passivate the surface to retard back electron transfer. Introduction Interest in dye-sensitized solar cells (DSCs)1,2 has increased rapidly in recent years. The light-harvesting component of the DSC is a monolayer of dye (or a layer of inorganic nanoparticles) attached to the high internal surface area of a thin mesoporous oxide film (TiO2 or ZnO, for example). Photoexcitation of the sensitizer leads to electron injection into the oxide, and the oxidized dye (or hole in the case of an inorganic sensitizer) then accepts an electron from the contacting phase, which may be a redox electrolyte or a hole conducting medium. The generation and separation of charge carrier pairs under illumination leads to the generation of voltage and current in the devices. The factors limiting DSC performance have been the subject of intensive investigation in recent years. The majority of studies have used cells containing an I3-/I- redox couple, since this electrolyte has given record efficiencies.3 The I3-/I- system has the advantage that the loss of photoinjected electrons by electron transfer to I3- from the mesoporous TiO2 is relatively slow, probably as a consequence of the fact that the reaction involves the transfer of two electrons and the breaking of the I-I bond. At the same time, regeneration of Iions at the cathode in the DSC is fast, since a platinum catalyst that breaks the I-I bond by chemisorption is used. As a consequence, voltage losses at the cathode are minimized. Attempts to replace the I3-/I- electrolyte by fast outer sphere redox systems have generally run into the problem that electron transfer from the mesoporous oxide to the oxidized redox species is facile, limiting the buildup of electrons that is required for generation of useful photovoltages and photocurrents. Suggested alternatives to I3-/I- include a range of cobalt complexes,4-7 * Corresponding author. E-mail:
[email protected]. † University of Bath. ‡ Current address: Faculty of Built Environment and Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia. § National Physical Laboratory. | Swiss Federal Institute of Technology.
the disulfide/thiolate system,8 and the Ni(III)/(IV) bis(dicarbollide) system.9 Cobalt complex electrolytes have been used both with conventional dye-sensitized cells as well as more recently with cells sensitized by nanoparticles of inorganic semiconductors such as CdSe,10 CdS, and PbS.10-12 Cobalt(III) complexes can exhibit slow electron transfer if reduction to Co(II) is accompanied by a change of spin that gives rise to a large reorganization energy due to changes in the coordination bond lengths. Potential limitations of Co(III)/Co(II) redox electrolytes can arise from slow regeneration of the dye from its oxidized state by Co(II), slow mass transport of cobalt complex ions in the liquid electrolyte, faster back reaction of photoinjected electrons with the oxidized partner of the redox couple, and slow regeneration of Co(II) species at the cathode. In addition, use of a blocking layer on the conducting glass is essential to prevent shunting via electron transfer from the substrate under load conditions.13-15 Since the original work of Sapp et al.4 and Nusbaumer et al.,5,6 several studies have examined electron transport and transfer in DSCs based on cobalt electrolytes. Nakade et al.16 studied two different cobalt complexesspropylene-1,2-bis(oiminobenzylideneaminato)cobalt(II) and tris(4,4′-di-tert-butyl2,2′-bipyridine) cobalt(II)sCo(DTP)33+/2+. Surprisingly, these authors found that the electron lifetime appeared to be independent of the Co(III) concentration in the electrolyte when the Co(III)/Co(II) ratio was fixed, whereas addition of Li+ ions increased the lifetime. This was attributed to compensation of the electronic charge in the TiO2, lowering electrostatic attraction of Co(III) species to the surface. Nelson et al.17 have reported a detailed study of mass transport in the Co(DTP)33+/2+. Interestingly, this study revealed that the diffusion of the ions in the mesoporous TiO2 films is slower than that in the bulk, possibly as a consequence of electrostatic effects as well as the bulky ligands. This has implications for the modeling of I-V characteristics that need to be explored. The rate of electron transfer to outer sphere redox couples from the TiO2 can be
10.1021/jp105753k 2010 American Chemical Society Published on Web 08/03/2010
Co(dbbip)2 Redox Shuttle decreased by using ALD layers of Al2O3,18-20 and Klahr and Hamann17 demonstrated a significant increase in incident photonto-collected electron conversion efficiency spectra (IPCE) for cobalt electrolytes when ALD layers were used to retard recombination. Following a similar approach, De Vries et al.20 have recently discussed driving force effects for cobalt phenanthroline shuttles and have demonstrated passivation of surface states by ALD layers. Ondersma and Hamann21 recently reported an impedance study of cells fabricated using three different cobalt electrolytes in which they noted an order of magnitude reduction of the rate of electron transfer for alumina-coated TiO2 films compared with uncoated films. The impedance measurements, which were carried out under potential control in a three-electrode system in the dark rather than with sandwich cells at open circuit under illumination, gave potential-dependent values of the small amplitude electron diffusion length that suggest that the electron transfer reaction is not first order with respect to electron density.22,23 Substitution of methyl and ter-butyl ligands into the Co(bpy)3 complex was found to increase the electron diffusion length, with a 4-fold increase for the ter-butyl complex compared with the unsubstituted complex. In the present work, we attempted to use impedance spectroscopy to characterize complete cells with electrolyte containing the cobalt redox couple Co(III)/Co(II)(dbbip)2 (dbbip ) 2,6bis(1′-butylbenzimidazol-2′-yl)pyridine) under illumination at open circuit. Although we have previously used this method successfully for cells based on the I3-/I- couple,24 we found that the fitting process appeared to be unreliable for the Co(III)/ Co(II) electrolyte as a consequence of the overlap of the responses due to the different circuit elements. This problem was also noted by Ondersma and Hamann,21 who used a threeelectrode configuration to eliminate the high frequency response due to the cathode. Quantitative information about DSCs fabricated using the cobalt redox shuttle was derived using IMPS and IMVS combined with near IR measurements25 to determine the electron lifetime, electron diffusion coefficient, and electron trap occupancy in order to derive the small amplitude electron diffusion length, λn.22,24 In addition, IPCE spectra were analyzed to obtain the “steady state” electron diffusion length.26,27 The results confirm that, in the absence of ALD blocking layers,18,19 electron diffusion lengths for cells with Co(III)/Co(II)(dbbip)2 electrolytes are sufficiently small that they impose limitations on the maximum thickness of the mesoporous layer that can be used. Experimental Section Fabrication of Dye-Sensitized Solar Cells. Fluorine-doped tin oxide (FTO) conductive glass (TEC 15, Libbey Owens Ford) was cleaned in detergent (5%, Decon 90), and by sequential sonication in distilled water, acetone, iso-propanol, and ethanol. The FTO substrate was then coated with a thin compact layer of TiO2 by spray pyrolysis13 before deposition of a single layer of commercial TiO2 paste (Dyesol, DSL18-NR). The film was sintered at 500 °C for 30 min to remove the organic materials and then treated with aqueous TiCl4 solution (40 mM) at 70 °C followed by washing in distilled water (Milli-Q). The film was then dried by nitrogen gas before being sintered at 450 °C for 30 min. The mean film thickness was 6.5 µm. While still warm (ca. 80 °C), the freshly sintered TiO2 film was immersed in a dye bath containing 0.25 mM N719 in acetonitrile/tert-butanol (1:1, v/v) and left for 16 h. The dye-coated film was washed with HPLC grade acetonitrile and dried by nitrogen gas. Cells were made by sandwiching the dyed-coated TiO2 film and a
J. Phys. Chem. C, Vol. 114, No. 33, 2010 14301 thermally platinized FTO counter electrode together with a thermoplastic gasket (Surlyn, 30 µm) at 100 °C. The cobalt electrolyte was prepared by adding 0.0186 M NOBF4 to 0.186 M Co(dbbip)2 (dbbip ) 2,6-bis(1′-butylbenzimidazol-2′-yl)pyridine), in deoxygenated acetonitrile/ethylene carbonate (40:60, v/v). Partial oxidation of the Co(II) complex by NOBF4 gave a final molar ratio of Co(III)/Co(II) ) 1:9. The electrolyte also contained 0.2 M tert-butylpyridine (TBP) and 0.2 M LiClO4. The solution was vacuum filled into the space between the two electrodes via predrilled holes in the counter electrode, which were then sealed with Surlyn covered by a microscope slip. For comparison, a DSC with an I-/I3- electrolyte composed of 0.03 M I2, 0.6 M 1-propyl-3-methylimidiazole iodide, 0.5 M TBP, and 0.1 M guanidinium thiocyanate in acetonitrile/ valeronitrile (85:15, v/v) was made using the same procedure. A symmetrical cell with the same Co(III)/Co(II) electrolyte and two identical platinized electrodes was fabricated in order to obtain the cathode and Warburg (mass transport) impedances for use as reasonable initial values in the fitting of the impedance of the DSC. Characterization of Dye-Sensitized Solar Cells. The I-V performance of the cells was tested using an AM 1.5 solar simulator (Mu¨ller) with a 1 kW Xe lamp and an AM 1.5 filter. A calibrated reference silicon solar cell (Fraunhofer ISE) with a built-in KG5 filter to reduce the spectral mismatch was used to set the illumination intensity. IPCE spectra were recorded in the spectral range 400-800 nm for illumination from both the TiO2 and counter electrode sides in order to determine the electron diffusion length.23 Steady bias light was provided by a light emitting diode (LED, 627 nm). The LED intensity was limited to
