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2007, 111, 17455-17458 Published on Web 11/07/2007
Photoelectrochemistry of Mesoporous NiO Electrodes in Iodide/Triiodide Electrolytes Hongjun Zhu, Anders Hagfeldt, and Gerrit Boschloo* Center of Molecular DeVices, KTH, Chemical Science and Engineering, Physical Chemistry, 10044 Stockholm, Sweden ReceiVed: September 5, 2007; In Final Form: October 23, 2007
Mesoporous NiO electrodes were used in photoelectrochemical cells using iodide/triiodide-based redox electrolytes. Cathodic photocurrents were found upon visible light excitation. These photocurrents were enhanced when coumarin 343 dye was adsorbed at the NiO surface or when a unanchored dye (coumarin 337) was added to the electrolyte. The proposed reaction for photocurrent generation is as follows: in the absence of dyes, excitation of triiodide leads to the generation of diiodide radicals, which subsequently accept electrons from the NiO electrode. Enhanced photocurrent generation in the presence of dyes is possibly due to energy transfer from excited dye molecules to triiodide. Charge transport studies of C343-sensitized NiO suggest that hole transport in mesoporous NiO is due to hopping of positive charge between Ni-surface atoms.
Introduction Dye-sensitized solar cells (DSCs) have attracted much attention in recent years because of their promise of efficient and low-cost solar energy conversion.1 Most research in DSC has been done on dye-sensitized mesoporous TiO2, but also other n-type semiconductors, such as ZnO, SnO2, Nb2O5, and Zn2SnO4, have been investigated. In contrast, relatively few investigations deal with the dye-sensitization of p-type semiconductors.2-7 The solar to electrical power conversion efficiency of p-type DSCs is well below 1% thus far. A method that can potentially improve the efficiency of DSC is to combine a dyesensitized photoanode with a dye-sensitized photocathode, thus forming a dye-sensitized tandem cell.8,9 In a well-designed tandem cell, the photocurrent densities of the two photoactive electrodes are matched. A significant improvement of the photocathode side is needed before an effective dye-sensitized tandem cell can be established. Several wide band gap p-type semiconductors, such as CuI,2 CuSCN,3,4 and NiO,5-7,10 have been tested as dye-sensitized photocathodes. Of these materials, NiO can be relatively easily made into mesoporous films. NiO has a band gap of 3.5 eV and has the advantage of good stability and transparency. Mesoporous NiO exhibits a significant reversible electrochromic effect.11 The generation of cathodic photocurrents in dye-sensitized NiO has been reported using dyes such as erythrosines,5,10 porphyrins,5,7and coumarins.6 The mechanism of photocurrent generation in dye-sensitized NiO has not been fully resolved yet. Investigations of the photoinduced dynamics of coumarin 343 and phosphorus porphyrin-sensitized mesoporous NiO suggest that the hole injection from the dye to NiO valence band is ultrafast (0.2-200 ps).6,7 The back electron transfer appears, however, to also be very fast (∼1 ns).6,7 This seems to explain the poor performance of dye-sensitized NiO solar cells, * Corresponding author. Fax: +46 8 790 8207. Tel: +46 8 790 8178. E-mail:
[email protected].
10.1021/jp077134k CCC: $37.00
but it raises questions whether the system should work at all. In the current investigation, working dye-sensitized NiO solar cells are investigated using a range of photoelectrochemical techniques. The results suggest that a solution-based reaction with triiodide plays an important role in the photocurrent generation mechanism. Experimental Section A colloidal solution of Ni(OH)2 was prepared using a modification of a method described previously:11 3.00 g of nickel(II) chloride hexahydrate (12.6 mmol) was dissolved in 75 mL of EtOH. To this solution, we added dropwise a solution of 1.08 g of NaOH (27.0 mmol) in 100 mL of EtOH under stirring at room temperature. After 2 h of stirring, the precipitate of Ni(OH)2 was left overnight for aging. The Ni(OH)2 colloids were separated and washed 5 times with deionized water (Millipore) using a centrifuge (6500 rpm for 6 min). Ni(OH)2 colloids were dispersed in EtOH to obtain a paste (∼20 wt % Ni(OH)2). Mesoporous NiO electrodes were prepared by spreading the Ni(OH)2 paste using a glass rod (“doctor blading”) onto conducting glass substrates (TEC8, Pilkington) that were masked using adhesive tape, followed by sintering in a hot-air stream at 300 °C for 30 min. Solar cells were prepared and characterized using methods described in the Supporting Information. Carrier lifetimes and transport times were measured by analyzing small-amplitude photovoltage and photocurrent transients, respectively, obtained after illumination with square-wave modulated light from a blue light emitting diode (Luxeon Star Royal Blue 1W) as reported previously.12 Results Mesoporous NiO films were obtained by sintering Ni(OH)2 gel at 300 °C for 30 min. Uniform films with a thickness less than 1 µm could be prepared by doctor blading, whereas films thicker than 1 µm tend to crack and have poor optical quality. © 2007 American Chemical Society
17456 J. Phys. Chem. C, Vol. 111, No. 47, 2007
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Figure 1. XRD spectrum of mesoporous NiO.
Figure 3. Hole transport times (open symbols) and lifetimes (filled symbols) in C343-sensitized mesoporous NiO solar cells as function of short-circuit current density. The electrolyte was 0.5 M of LiI (circles), NaI (triangles), or TBAI (squares) and 0.1 M I2 in 3-methoxypropionitrile.
Figure 2. (a) IPCE spectra of mesoporous NiO electrodes (∼0.5 µm thick) without dye (circles), sensitized by adsorbed C343 (squares), and sensitized by C337 in the electrolyte (triangles). Electrolyte: 0.5 M LiI and 0.1 M I2 in 3-methoxypropionitrile. (b) IPCE spectra of bare mesoporous NiO in iodide/triiodide electrolyte: 0.5 M LiI, 0.1 M or 0.5 M I2 (open and closed symbols, respectively) in 3-methoxypropionitrile. Also shown is the absorption spectrum of diluted electrolyte (drawn line).
The X-ray diffraction pattern of as-prepared NiO is shown in Figure 1. It reveals that phase-pure face-centered cubic NiO (with indices of 111, 200, 220) is formed. No Ni(OH)2 peaks were detected, indicating that sintering at 300 °C for 30 min is sufficient to completely convert Ni(OH)2 to NiO.11 From the broadening of the peaks, the average crystal size of NiO was calculated to be 5.2 nm using the Scherrer equation. Mesoporous NiO electrodes (∼0.5 µm thickness) were used as the working electrodes in two-electrode photoelectrochemical cells. Upon illumination, cathodic photocurrents were found in the presence of an iodide/iodine redox electrolyte. Incident
photon-to-current conversion efficiency (IPCE) spectra of mesoporous NiO-based solar cells are shown in Figure 2. A significant photocurrent response at wavelengths smaller than 500 nm is found for bare mesoporous NiO. An increase in the IPCE is found when the dye coumarin 343 is adsorbed at the surface of NiO (Figure 2a). The IPCE spectrum resembles the absorption spectrum of C343 and displays a maximum of about 6% at 420 nm. Interestingly, a significant IPCE contribution was also found from coumarin dye molecules that were dissolved in the redox electrolyte solution. Coumarin 337 does not bind to NiO because of the absence of suitable anchoring groups. Nevertheless, a clear contribution of the dye was found in the photocurrent spectrum (Figure 2a). The concentration of triiodide in the electrolyte affects the IPCE significantly, see Figure 2b. In the absence of any dye, the shape of the IPCE spectrum resembles the absorption spectrum of triiodide (Figure 2b). While the photocurrent increased with increasing triiodide concentration, the photovoltage decreased (see Figure S2 in the Supporting Information). Figure 3 shows the measured time constants obtained from small-modulation transient photocurrent measurement under short-circuit conditions for C343-sensitized NiO solar cells. The time constants may be interpreted as the transport times for holes in the mesoporous NiO film. Transport times (τtr) were recorded at different light intensities and are plotted as function of the short-circuit photocurrent density. There is a linear relation between light intensity and photocurrent density. Although light intensities were varied 2 orders of magnitude, only relatively small changes in the transport time were found. In contrast, hole transport times in NiO were significantly dependent on the type of cation in the electrolyte: transport times decreased in the order Li+ > Na+ > TBA+. Small-modulation photovoltage measurements under opencircuit conditions were used to measure the lifetimes of charge carriers. Hole lifetimes (τh) are also plotted in Figure 3. In contrast to transport times, hole lifetimes depend on the light intensity, but not strongly on the type of cation present in the electrolyte. It is noted that hole lifetimes are much larger than hole transport times. Discussion Mechanism of Photocurrent Generation in NiO-I3-/IElectrolyte Cells. The generation of cathodic photocurrents in
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dye-sensitized NiO in contact with I3-/I- redox electrolyte has been reported before, using dyes such as erythrosine B,5 porphyrins,7and coumarin 343.6 In all cases, the photocurrent action spectrum was found to reflect the absorption spectrum of the dye, clearly indicating the active involvement of the dye. The precise mechanism for cathodic photocurrent generation has, however, not yet been resolved. The results presented here give strong evidence that dye molecules that are not adsorbed at the NiO surface do contribute to the photocurrent (Figure 2a). The fluorescence lifetime of excited coumarin 337 is expected to be similar to that of C343, that is, ca. 4 ns, while the triplet lifetime is much longer. Excited dye molecules in solution may reach the NiO surface, where electron transfer can follow. Alternatively, a solution-based reaction where excitation energy is transferred to triiodide may occur, as will be discussed later. Significant photocurrents are observed in absence of dye. In this case, photocurrent must be due to excitation of the triiodide in the redox electrolyte by visible light. NiO is a wide band gap semiconductor (Eg ) 3.5 eV); hence, band gap excitation will only occur in the UV region at λ < 350 nm. Mesoporous NiO films tend to have a brown color due to the presence of Ni3+ species at the surface, but excitation of these species does not seem to result in photocurrent because the IPCE spectrum found differs much from the absorption spectrum of these species.11 The I3-/I- redox electrolyte is highly colored in the spectral region where cathodic photocurrent is found, see Figure 2b. It is well known that the excitation of triiodide leads to photochemical reactions.13 Upon excitation, triiodide decomposes as follows:
I3- + hν f I2-‚ + I‚
(1a)
I‚ + I- f I2-‚
(1b)
2I2-‚ f I3- + I-
(2)
In the presence of iodide, the iodine radical formed in eq 1a is rapidly converted to the diiodide radical (1b), which is more stable. Diiodide radicals will disappear eventually by a disproportionation reaction (2). The presence of I2-‚ was confirmed by photoinduced absorption spectroscopy of the redox electrolyte solution (see Figure S3 in the Supporting Information). Because I2-‚ is a relatively long-lived species, it can undergo different reactions and possibly contribute to the cathodic photocurrent in the following way
I2-‚f 2I- + h+(NiO)
(3)
where diiodide accepts an electron from the valence band of NiO, which is equivalent to injection of a hole (h+) in the valence band. Mechanism of Hole Transport in NiO. Unlike the n-type mesoporous semiconductors that have been studied in DSCs, mesoporous NiO displays a transport time that is almost independent of light intensity, see Figure 3a. There is however, a strong dependence on the type of cation in the electrolyte. This is unexpected because the positive charge carriers in the mesoporous NiO are expected to be mostly charge compensated by anions in the electrolyte. The most probable explanation of the cation effect is that the different cations, Li+, Na+, and TBA+, adsorb to a different extent at the NiO surface and affect thereby the hole transport. This suggests that hole conduction is occurring mainly at the mesoporous NiO/electrolyte interface.
Most likely, the hole transport involves hopping of charges at the NiO/electrolyte interface. More specifically, holes in NiO may be present as oxidized Ni atoms at the NiO surface (i.e., Ni3+s), as has been suggested before.11 Holes that are photoinjected in the mesoporous NiO are collected efficiently at the conducting glass substrate. The charge collection efficiency, ηCC, can be estimated using eq 4:14
τtr ηCC ) 1 τh
(4)
Depending on light intensity, ηCC is calculated to be 52-97%. It should be noted that this is a lower limit of the ηCC at shortcircuit conditions because the internal potential in the mesoporous NiO will then be lower than that at VOC, resulting in a higher lifetime for the holes. On the basis of UV-vis measurements, it was estimated that 37% of the incoming light at 420 nm is absorbed by the dye in C343-sensitized NiO films. The IPCE value at this wavelength in the solar cells was ∼6%. Because the charge collection efficiency is larger than 50%, the main losses must arise from the poor efficiency of the initial steps of the current generation: the injection of the hole from the photoexcited dye into the valence band of the NiO and the subsequent regeneration of the reduced C343 by the redox couple. Ultrafast spectroscopy studies of the C343-NiO system demonstrated that rapid hole injection (∼0.2 ps) is followed by very fast recombination of the hole in NiO with the reduced dye (∼20 ps).6 Efficient longlived charge separation would therefore only occur if the reduced dye is intercepted by triiodide in the electrolyte on a picosecond time scale. This appears to be unrealistic, knowing that interception of oxidized dyes in n-type DSCs occurs on a microsecond time scale.15 On the basis of our findings, we propose that long-lived charge separation is achieved following an alternative route:
D + hν f D*
(5a)
D* + I3- f D + I3-*
(5b)
The excited dye transfers energy to triiodide, which then forms diiodide radicals (reaction 1) that can inject holes into NiO (3). This mechanism explains that dyes, which do not adsorb at the NiO surface, such as C337, can contribute to the photocurrent. To conclude, mesoporous NiO electrodes were used in photoelectrochemical cells with iodide/triiodide electrolyte, giving cathodic photocurrents. Excitation of triiodide leads to photochemical generation of diiodide radicals that can inject holes into the NiO electrode. Enhanced photocurrent generation in the presence of dyes is possibly due to energy transfer from excited dye molecules to triiodide. Acknowledgment. We gratefully acknowledge financial support from the Swedish Energy Agency, the Knut and Alice Wallenberg foundation, and the Chinese Scholarship Council. Supporting Information Available: Experimental details, IV curves of dye-sensitized NiO solar cells, and photoinduced absorption spectrum of iodide/triiodide electrolyte. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (2) Tennakone, K.; Fernando, C. A. N.; Dewasurendra, M.; Kariappert, M. S. Jpn. J. Appl. Phys. 1987, 27, 561.
17458 J. Phys. Chem. C, Vol. 111, No. 47, 2007 (3) Tennakone, K.; Kahanda, M.; Kasige, C.; Abeysooriya, P.; Wijayanayaka, R. H.; Kaviratna, P. J. Electrochem. Soc. 1984, 131, 1574. (4) O’Regan, B.; Schwartz, D. T. Chem. Mater. 1995, 7, 1349. (5) He, J.; Lindstro¨m, H.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1999, 103, 8940. (6) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarstro¨m, L. J. Phys. Chem. B 2005, 109, 19403. (7) Borgstro¨m, M.; Blart, E.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A.; Hammarstro¨m, L.; Odobel, F. J. Phys. Chem. B 2005, 109, 22928. (8) He, J.; Lindstro¨m, H.; Hagfeldt, A.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 2000, 62, 265. (9) Nakasa, A.; Usami, H.; Sumikura, S.; Hasegawa, S.; Koyama, T.; Suzuki, E. Chem. Lett. 2005, 34, 500.
Letters (10) Vera, F.; Schrebler, R.; Munoz, E.; Suarez, C.; Cury, P.; Gomez, H.; Cordova, R.; Marotti, R. E.; Dalchiele, E. A. Thin Solid Films 2005, 490, 182. (11) Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3039. (12) Boschloo, G.; Ha¨ggman, L.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13144. (13) Roy, J. C.; Hamill, W. H.; Williams, R. R. J. Am. Chem. Soc. 1955, 77, 2953. (14) van de Lagemaat, J.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (15) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Gra¨tzel, M.; Durrant, J. R. J. Phys. Chem. C 2007, 111, 6561.
