On the effect of Al2O3 blocking layer on the ...

APPLIED PHYSICS LETTERS 94, 173113 共2009兲

On the effect of Al2O3 blocking layer on the performance of dye solar cells with cobalt based electrolytes M. Liberatore,1 L. Burtone,1 T. M. Brown,1 A. Reale,1 A. Di Carlo,1,a兲 F. Decker,2 S. Caramori,3 and C. A. Bignozzi3 1

Department of Electronic Engineering, CHOSE-Centre for Hybrid and Organic Solar Energy, University of Rome “Tor Vergata,” via Del Politecnico 1, 00133 Roma, Italy 2 Department of Chemistry, University “La Sapienza,” piazzale Aldo Moro 5, 00185 Roma, Italy 3 Department of Chemistry, University of Ferrara, via L. Borsari 46, 44100 Ferrara, Italy

共Received 27 December 2008; accepted 2 April 2009; published online 30 April 2009兲 Dye solar cells with Co共III兲 / Co共II兲 redox mediators have been prepared. To obtain higher conversion efficiencies, the recombination between photoinjected electrons and Co共III兲 species was minimized by deposition of a thin Al2O3 blocking layer over the mesoporous TiO2 surface. Measurements of current-voltage characteristic curves, both under illumination and in dark conditions, together with electrochemical impedance spectroscopy demonstrate the great effectiveness of the addition of a blocking layer in cells containing cobalt based electrolyte, by substantially reducing the recombination current. The consequent power conversion efficiency increase is more than double, passing from 0.94% to 2.48% under 300 W m−2 AM 1.5 illumination. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3126051兴 During the past decade interest in dye solar cells 共DSCs兲 has grown enormously.1–4 Standard DSCs 共Ref. 5兲 are composed by a ⬃10 ␮m thick mesoporous titanium dioxide 共TiO2兲 anode deposited on a conductive glass substrate, a light-sensitive organic material 共dye兲 adsorbed on the titanium dioxide surface, an electrolyte in which a redox couple, usually tri-iodide/iodide 共I3− / I−兲, is dissolved and a platinized conductive glass substrate as cathode. The dye absorbs incident light and injects electrons from the excited state into the titanium oxide conduction band. The mediator restores neutrality in the dye and the redox couple is regenerated at cathode. A record efficiency of 11.1% has been achieved on small area cells.2 The high series resistance of transparent conductive oxides 关fluorinated tin oxide 共FTO兲, in our case兴 considerably limits the efficiencies obtainable on large area cells and modules. The conventional solution to reduce the series resistance consists of applying current collection metal grids over the substrate. Unfortunately, this solution cannot be applied directly to DSC technology due to the corrosive properties of the I3− / I− mediator to most metals. Several authors have tried to substitute the I3− / I− redox couple with a less aggressive one. Among these, the cobalt complexes seem to be the most promising candidates.6–8 In Fig. 1, a general scheme of the main processes occurring at the photoanode under illumination is presented.7 The reactions are described by the following equations: Dye + h␷ → Dye+ + e−CB e−CB + Dye+ → Dye

e−CB + Co共III兲 → Co共II兲

共1兲

G,

共2兲

kD ,

Co共II兲 + Dye+ → Co共III兲 + Dye

have been coupled since they are both confined into the picofemto-second time domain.12,13 The kinetic constant kD relates to the recombination process between photoinjected electrons and oxidized dye. This process is very inefficient and can be neglected, as shown by Willig and co-workers.14 The kinetic term kC2 describes the oxidized dye regeneration process rate and kC3 relates to the direct recombination of electrons injected in TiO2 conduction band with the Co共III兲 complex. Nusbaumer et al.6 reported that the kinetic constant kC3 of reaction 共4兲, for their Co共III兲共dbip兲3+ mediator, is in the same order of magnitude, but slightly faster with respect to I3− / I− based electrolytes. For our cobalt mediator we found, by using laser flash photolysis, an apparent first order rate constant kC3 which is almost twice as large as the one for the tri-iodide/iodide mediator. Thus, the back electron reaction from Co共III兲 to Co共II兲 may limit DSCs performance and may become predominant over dye regeneration. Many strategies have been proposed in order to prevent recombination between injected electrons and the standard I3− / I− mediators; for instance, the addition of 4-tertbutylpyridine 共TBP兲 to the electrolyte has been demonstrated to be very efficient9–11 since it quenches the surface sites responsible for the recombination reaction and consequently

kC2 ,

kC3 .

共3兲 共4兲

The generation term G represents both the dye photoexcitation and electron injection into TiO2 processes. These a兲

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FIG. 1. Schematic representation of electron transfer processes at the photoelectrode in DSCs. 94, 173113-1

© 2009 American Institute of Physics

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Appl. Phys. Lett. 94, 173113 共2009兲

FIG. 2. Dark current vs applied bias of two cells containing standard 共䊏兲 or alumina-coated 共䊊兲 photoanodes.

produces a clear increase in open circuit potential. Furthermore, compact nanometric insulator layers which cover the TiO2 uniformly16–19 or dyes with proper alkyl chains15 that can act as barrier for recombination reaction have been investigated. Concerning cobalt based electrolytes, it has been shown that the use of TBP and other additives8 can reduce the back reaction as well. In this paper we will extend the ultrathin insulator strategy also to cobalt based DSCs; we covered the titanium dioxide surface with an ultrathin alumina 共Al2O3兲 blocking layer deposited by a simple sol-gel process20 and show that the related increment in cell efficiency is over ⬃150%. The photoanode was prepared by deposition of a TiNanoxide D® slurry paste on FTO coated glass via blade coating. The substrate was then sintered at 450 ° C for 30 min. The resultant mesoporous titania film had a thickness of 6 ␮m and was dipped into the alumina precursor coating solution at 60 ° C for 10 min. The latter was prepared by sol-gel method, using as precursor a 0.1 mol/l solution of aluminum isopropoxide 99.99+ % 共Aldrich兲 in anhydrous 2-propanol 共Fluka兲. The sample was then rinsed in 2-propanol and annealed at 400 ° C for 20 min. Applying this technique a compact ultrathin layer of alumina is deposited. The alumina coated anode was sensitized by immerging it for 14 h in a 0.05 M ethanolic solution of Ru共II兲 metallorganic complex termed Z907. The cathode was prepared by sputtering an 80 nm gold21 layer on FTO coated glass. In order to complete the fabrication, the photoanode and the cathode were assembled together and the electrolytic solution was injected by vacuum back filling.22 The electrolyte consisted of 0.15 M 关Co共dtb兲3兴共Otf兲2 共where dtb is the 4 , 4⬘ di tert-butyl-2 , 2⬘ bipyridine and Otf is the trifluoromethanesulfonate anion兲 and 0.5 M lithium trifluoromethanesulfonate 共LiOtf兲 in methoxypropionitrile.23 Figure 2 shows the dark current characteristics of both the alumina-coated and uncoated TiO2 cells. The beneficial effect of a passivating layer on the recombination current at the interface between titanium dioxide and electrolytic solution is significant. By a linearization of the characteristics, we extrapolated the onset voltage which increases from 0.25 to 0.35 V when coating the TiO2 with alumina. Moreover, we are not able to extract the kC3 rate directly since interfacial recombination comprises both the contribution of the electron/mediator recombination at the TiO2 interface and the charge transfer at FTO/electrolyte interface.19,22

FIG. 3. Nyquist diagrams and fitting results of impedance spectra on cells under 300 Wm−2 illumination at open circuit potential. Frequency range 10 kHz–5 Hz. ac voltage 10 mV rms. Standard 共䊏兲, alumina-coated 共䊊兲 photoanodes.

Figure 3 shows the electrochemical impedance spectroscopy 共EIS兲 spectra measured at the open circuit potential under 300 W m−2 illumination and plotted in a Nyquist diagram limited to data points taken with frequencies above 5 Hz. In this way we excluded the data points referring to ionic diffusion. Furthermore we noticed that cathode features are hidden by the much larger impedance of the photoanode. The impedance analysis on our cells clearly shows that the alumina barrier influences the photoanode behavior exclusively. Therefore we believe that limitation to high frequency data is justified. Typically, when the recombination current is so high as to affect the electron collection efficiency significantly, the photoanode impedance response can be modelized by a Gerischer element ZG;25 by contrast, when the collection efficiency is sufficiently close to unity, the model is usually well approximated by a simple Randles circuit, i.e., a resistance that represents a charge transfer resistance Rct, in parallel with a constant phase element and a Warburg element ZW.25 In order to fit our data correctly we need to take into consideration the effect of the alumina barrier layer. Since the recombination current is strongly lowered, as confirmed

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respect to the cell without barrier, passing from a value of 0.94%–2.48% with a relative improvement above 150%. In conclusion, we have shown that the application of a thin Al2O3 blocking film on the mesoporous TiO2 layer in cobalt electrolyte-based DSCs prevents the back recombination between electrons in the TiO2 and the electrolyte with a consequent enhancement of the cell efficiency. This technique could be used together with additional strategies already reported in literature6–11 to further improve the efficiency of DSCs based on cobalt electrolytes and thus help resolve the corrosion problems found in iodine based electrolyte DSCs. B. O’Regan and M. Grätzel, Nature 共London兲 353, 737 共1991兲. Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, and L. Han, Jpn. J. Appl. Phys., Part 2 45, L638 共2006兲. 3 J. M. Kroon, N. J. Bakker, H. J. P. Smit, P. Liska, K. R. Thampi, P. Wang, S. M. Zakeeruddin, M. Grätzel, A. Hinsch, S. Hore, U. Würfel, R. Sastrawan, J. R. Durrant, E. Palomares, H. Pettersson, T. Gruszecki, J. Walter, K. Skupien, and G. E. Tulloch, Prog. Photovoltaics 15, 1 共2007兲. 4 B. Li, L. Wang, B. Kang, P. Wang, and Y. Qiu, Sol. Energy Mater. Sol. Cells 90, 549 共2006兲. 5 A. Di Carlo, A. Reale, T. M. Brown, M. Cecchetti, F. Giordano, G. Roma, M. Liberatore, V. Mirruzzo, and V. Conte, Smart Materials and Concepts for Photovoltaics 共Springer, New York, 2008兲, pp. 99–131. 6 H. Nusbaumer, J.-E. Moser, S. M. Zakeeruddin, M. K. Nazeeruddin, and M. Grätzel, J. Phys. Chem. B 105, 10461 共2001兲. 7 S. Nakade, Y. Makimoto, W. Kubo, T. Kitamura, Y. Wada, and S. Yanagida, J. Phys. Chem. B 109, 3488 共2005兲. 8 H. Nusbaumer, S. M. Zakeeruddin, J. E. Moser, and M. Grätzel, Chem.Eur. J. 9, 3756 共2003兲. 9 M. K. Nazeeruddin, A. Kay, L. Rodicio, R. Humpbry-Baker, E. Müller, P. Liska, N. Vlachopoulos, and M. Grätzel, J. Am. Chem. Soc. 115, 6382 共1993兲. 10 S. A. Haque, Y. Tachibana, R. L. Willis, J. E. Moser, M. Grätzel, D. R. Klug, and J. R. Durrant, J. Phys. Chem. B 104, 538 共2000兲. 11 M. Dürr, A. Yasuda, and G. Nelles, Appl. Phys. Lett. 89, 061110 共2006兲. 12 U. Bach, Y. Tachibana, J.-E. Moser, S. A. Haque, J. R. Durrant, M. Grätzel, and D. R. Klug, J. Am. Chem. Soc. 121, 7445 共1999兲. 13 S. Pelet, J.-E. Moser, and M. Grätzel, J. Phys. Chem. B 104, 1791 共2000兲. 14 T. Hannappel, B. Burfeindt, W. Storck, and F. Willig, J. Phys. Chem. B 101, 6799 共1997兲. 15 J. E. Kroeze, N. Hirata, S. Koops, Md. K. Nazeeruddin, L. SchmidtMende, M. Grätzel, and J. R. Durrant, J. Am. Chem. Soc. 128, 16376 共2006兲. 16 E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz, and J. R. Durrant, J. Am. Chem. Soc. 125, 475 共2003兲. 17 X. Zhang, H.-W. Liu, T. Taguchi, K. Tokuhiro, Q. Meng, O. Sato, and A. Fujishima, Sol. Energy Mater. Sol. Cells 81, 197 共2004兲. 18 F. Fabregat-Santiago, J. Garcia-Cañadas, E. Palomares, J. N. Clifford, S. A. Haque, and J. R. Durrant, J. Appl. Phys. 96, 6903 共2004兲. 19 E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz, and J. R. Durrant, Chem. Commun. 共Cambridge兲 2002, 1464. 20 I. Ichinose, H. Senzu, and T. Kunitake, Chem. Mater. 9, 1296 共1997兲. 21 P. J. Cameron and L. M. Peter, Coord. Chem. Rev. 248, 1447 共2004兲. 22 S. G. Chen, S. Chappel, Y. Diamant, and A. Zaban, Chem. Mater. 13, 4629 共2001兲. 23 S. A. Sapp, C. M. Elliott, C. Contado, S. Caramori, and C. A. Bignozzi, J. Am. Chem. Soc. 124, 11215 共2002兲. 24 R. Kern, R. Sastrawan, J. Ferber, R. Stangl, and J. Luter, Electrochim. Acta 47, 4213 共2002兲. 25 Q. Wang, J. E. Moser, and M. Grätzel, J. Phys. Chem. B 109, 14945 共2005兲. 26 J. L. Diot, J. Joseph, J. P. Martin, and P. Clechet, Electroanal. Chem. 193, 75 共1985兲. 27 V. Yong, S.-T. Ho, and R. P. H. Chang, Appl. Phys. Lett. 92, 143506 共2008兲. 1 2

FIG. 4. IV curves of cells with standard 共䊏兲 or alumina-coated 共䊊兲 photoanodes under 300 W m−2 illumination. The enclosed table reports open circuit voltage 共VOC兲, short circuit current 共JSC兲, Fill Factor 共FF兲, and efficiency 共␩兲.

by the dark current measurements, the equivalent circuit model that best fits our data consists of a Randles-type circuit onto which we have added a further RC circuit in series which arises from the Al2O3 coating over the TiO2 photoanode.26 For the DSCs with no blocking layer, which instead have a significant back current, we find that a model which fits the data well is a combination of a ZG element in series with Randles-type circuit. In fact, the fitted values present a very good convergence with EIS experimental data, with relative errors below 8%. Passing from the uncoated to the coated photoanode, the charge transfer resistance we calculated at the electrolyte interface is reduced from 215 to 96 ⍀. This variation is consistent with Fig. 4 where the differential diode resistance around VOC related to the transfer resistance at the TiO2/dye/electrolyte interface,27 is halved when moving from standard cell to the one with alumina. Data analysis shows that the electron lifetime in the TiO2 or ␶eff 共Ref. 24兲 increases from 5.4 to 21 ms when the alumina barrier is introduced. An increase in ␶eff is expected, since the reduction of Rct is compensated by a higher capacitance. The increase of the capacitance is related to the presence of the insulating alumina layer which, reducing the back recombination, promotes a larger charge accumulation at the interface. The IV curves, measured under 300 W m−2 illumination at AM 1.5 are shown in Fig. 4 and the enclosed table summarizes the cell parameters. The DSC with an alumina barrier presents both a higher VOC, an increased value of JSC and a better fill factor 共FF兲. This is explained by the already discussed lowering of the recombination current due to the effects of the barrier. A passivation of TiO2 intraband surface states by the insulating layer could also be beneficial.17 All these effects yield a substantial efficiency enhancement with

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