J. Phys. Chem. C 2007, 111, 1035-1041
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Comparison of Dye-Sensitized ZnO and TiO2 Solar Cells: Studies of Charge Transport and Carrier Lifetime Marı´a Quintana,† Tomas Edvinsson,‡ Anders Hagfeldt,‡ and Gerrit Boschloo*,‡ Facultad de Ciencias, UniVersidad Nacional de Ingenierı´a, P. O. Box 31-139, AVenida Tupac Amaru 210, Lima, Peru´ , and Center of Molecular DeVices, Department of Chemistry, Royal Institute of Technology (KTH), Teknikringen 30, 100 44 Stockholm, Sweden ReceiVed: September 12, 2006; In Final Form: NoVember 6, 2006
Nanocrystalline particles of ZnO and TiO2 of approximately equal size (∼15 nm) were used to prepare mesoporous electrodes for dye-sensitized solar cells. Electron transport in the solar cells was studied using intensity-modulated photocurrent spectroscopy and revealed very similar results for ZnO and TiO2. Apparent activation energies for electron transport in nanostructured ZnO of e0.1 eV were calculated from the temperature dependence of transport times under short-circuit conditions. The lifetime of electrons in the nanostructured semiconductors was evaluated from open-circuit voltage decay and intensity-modulated photovoltage spectroscopy. Significantly longer lifetimes were obtained with ZnO. Despite the reduced recombination, ZnO-based solar cells performed worse than TiO2 cells, which was attributed to a lower electron injection efficiency from excited dye molecules and/or a lower dye regeneration efficiency. The internal voltage in the nanostructured ZnO film under short-circuit conditions was about 0.23 V lower than the opencircuit potential at the same light intensity. Results may be explained using a multiple trapping model, but as electrons are usually only shallowly trapped in ZnO, an alternative view is presented. If there is significant doping of the ZnO, resulting band bending in the nanocrystals will form energy barriers for electron transport and recombination that can explain the observed properties.
Introduction Dye-sensitized solar cells based on nanocrystalline mesoporous metal oxide films have attracted much attention in recent years.1,2 They offer the prospect of low-cost photovoltaic energy conversion. Promising solar to electrical energy conversion efficiencies of more than 10% have been achieved,2 and good progress has been made on long-term stability.3 The working mechanism of dye-sensitized solar cells differs completely from conventional p-n junction solar cells,2 but, after more than 15 years of research, is still not completely resolved. Research has largely focused on nanostructured TiO2 (anatase) as the metal oxide to which the dye is bound. Good results have, however, also been obtained using other n-type metal oxides, such as ZnO,4-6 Nb2O5,7 and SnO2.8 ZnO is an attractive material for nanoscale optoelectronic devices, because it is a wide band gap semiconductor with good carrier mobility and can be doped both n-type and p-type.9 Electron mobility is much higher in ZnO than in TiO2, while the conduction band edge of both materials is located at approximately the same level. One would therefore expect nanostructured ZnO to be a good candidate as electron acceptor and transport material in dye-sensitized solar cells. A large range of fabrication procedures is available for ZnO nanostructures, such as sol-gel processes,4,10 chemical bath deposition,11-13 electrodeposition,14,15 and vapor-phase processes.16 Different morphologies such as spherical particles,4,10 rods,13 wires,16 and hollow tubes12 can be prepared with relative ease. ZnO shows * To whom correspondence should be addressed. Phone: +46 87908178. Fax: +46 87908207. E-mail:
[email protected]. † Universidad Nacional de Ingenierı´a. ‡ KTH.
more flexibility in synthesis and morphologies than TiO2. The chemical stability of ZnO is, however, less than that of TiO2, which was found to be problematic in the dye adsorption procedure.6 The solar-to-electrical energy conversion efficiencies of dyesensitized ZnO solar cells are so far significantly lower than those reported for TiO2. The highest reported values are 5% at 1/10 sun17 and 4.1% at 1 sun.18 Several reports suggest that dye adsorption is the main problem in dye-sensitized ZnO solar cells.6,19 Cells with high dye loading tend to be inefficient, whereas cells with lower dye loading show good quantum efficiencies. These problems are mainly related to the high acidity of the carboxylic acid binding groups of the dyes that can lead to dissolution of ZnO and precipitation of dye-Zn2+ complexes, leading to a poor overall electron injection efficiency of the dye. In comparison to TiO2, few electron transport studies have been done on dye-sensitized nanostructured ZnO solar cells.20-22 In this study we compare the electron transport, accumulation, and recombination properties of dye-sensitized solar cells based on nanostructured electrodes of ZnO and TiO2. There is a remarkable similarity in the transport properties of the two materials, despite the significant difference in the mobility and the effective mass of electrons in the conduction band of the pure, single-crystalline materials. Experimental Procedures Preparation of Nanostructured ZnO and TiO2 Films. ZnO colloids were prepared in ethanol by addition of tetramethylammonium hydroxide (25% in methanol) to a suspension of zinc acetate in ethanol.19 The resulting ZnO sol was refluxed at
10.1021/jp065948f CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006
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80 °C for 30 min. The suspension was left to cool down and settle overnight, decanted, washed with ethanol, and finally concentrated until visibly viscous. Transparent nanostructured ZnO electrodes were obtained by depositing the paste onto conducting glass substrates (TEC8, Pilkington) by doctor blading, followed by heating in a hot-air stream at 380 °C for 30 min. The porosity of the resulting films was 50%. Transparent nanostructured TiO2 electrodes were prepared from HNO3stabilized TiO2 colloids autoclaved for 15 h at 200 °C.1 Electrodes were heated at 450 °C for 30 min. Film thickness (3-4 µm) was determined by profilometry. Dye Sensitization and Cell Assembly. The concentration of the dye bath and the adsorption time strongly influence the efficiency of dye-sensitized ZnO solar cells.6 In this study a rapid sensitization method was used.23 A 8 µL volume of a 20 mM N719 solution in dimethyl sulfoxide (DMSO) was applied to the ZnO or TiO2 electrode (at 50 °C) and left for 1 min (N719 corresponds to (TBA)2 cis-Ru(Hdcbpy)2(NCS)2). The dyed electrode was rinsed with ethanol, dried, and assembled with a platinized conducting glass counter electrode using a 50 µm thick thermoplastic frame (Surlyn 1702). The electrolyte composition was as follows: 0.5 M LiI; 50 mM I2 in 3-methoxyproprionitrile with either 0.1 M 1-methylbenzimidazole (electrolyte 1) or 0.5 M 4-tert-butylpyridine (electrolyte 2) as an additive. Characterization Methods. UV-vis spectra were recorded using a Hewlett-Packard 8453 diode array spectrometer. The setups for recording incident photon to current efficiency (IPCE) spectra and I-V curves under simulated sunlight have been described elsewhere.19,24 Intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS, respectively) and charge extraction measurements were performed using a 10 mW diode laser (Coherent Lablaser, λ ) 635 nm) or a light-emitting diode (Lumiled Luxeon Star 1W, λmax ) 640 nm) as the light source as described previously.25 Red light was chosen to obtain a relative uniform light absorption in the film. Measurements were carried out at room temperature, except for the temperature-dependent IMPS experiments, which were performed in the range of 6-60 °C using a Peltier element. Results and Discussion Film Characterization. Transmission electron microscopy (TEM) revealed that colloidal ZnO solution consisted of crystalline particles with an average size of about 15 nm; see Figure 1a. The colloidal TiO2 solution used in this study contained slightly smaller nanoparticles with an average size of 10 nm found by TEM. The X-ray diffraction spectrum of a sintered nanostructured ZnO film is shown in Figure 1b and shows peaks characteristic of wurtzite. The grain size, calculated from the peak broadening using the Scherrer formula, was 16 nm. The TiO2 films consist of 14-nm-size anatase crystals with a trace of brookite (data not shown). The results suggest that significant crystal growth occurs in the TiO2 films during heat treatment, but not in ZnO films. The resulting nanostructured ZnO and TiO2 films were fully transparent. Cyclic Voltammetry. The nanostructured ZnO and TiO2 electrodes were characterized using cyclic voltammetry in aqueous electrolyte. The experiment was performed on bare (non-sensitized) electrodes that were scanned toward negative potential and back. Typical results are shown in Figure 2. The current onset for electron accumulation in ZnO is at slightly more positive potentials than that of TiO2, in agreement with similar experiments by Willis et al.26 As the conduction band edges of both materials are expected to be located at similar
Figure 1. (a) Transmission electron microscopic picture of colloidal ZnO. The bar corresponds to 100 nm. (b) X-ray diffractogram of a ZnO film sintered at 380 °C.
potentials, this might indicate the presence of more deep traps in ZnO. At more negative potentials (-0.65 to -1 V), however, currents were much smaller for the ZnO than for the TiO2 electrode. The relatively good reversibility of the voltammograms suggests that the current is mainly due to charging or discharging of the nanostructured electrode by electrons. On the basis of the difference in effective density of states of the two semiconductor materials, higher electron concentrations and therefore larger currents can be expected for TiO2.27 Calculated electron densities in the semiconductors are also shown in Figure 2. The electron concentration increases approximately exponentially with more negative applied potential for TiO2, n ∝ n0 exp(-6.2Vappl), whereas a clearly different behavior is observed for ZnO. The charging of the electrodes can be explained in terms of filling of conducting band states and/or trap states and will be discussed further below in the section on Traps in ZnO. Solar Cell Characterization. A rapid method for dye adsorption was used to sensitize the films. This method was chosen to prevent sensitization problems in the case of ZnO, which can slightly dissolve in acidic environment, resulting in a precipitate of Zn2+ and the dye.6 The optimum adsorption
Dye-Sensitized ZnO and TiO2 Solar Cells
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Figure 2. Cyclic voltammograms of nanostructured ZnO and TiO2 electrodes at a scan rate of 10 mV s-1. The electrolyte was a deaerated aqueous solution of 0.2 M KCl with 0.01 M phosphate buffer, pH 6.7. The thicknesses of the ZnO and TiO2 films were 1.9 and 1.5 µm, respectively. Below and on the right-hand axis the calculated electron densities in ZnO and TiO2 are shown, assuming 50% porosity and calculated from the scan toward negative potentials. A correction for the capacitance of the substrate/electrolyte interface (∼28 µF cm-2) has been made.
Figure 3. Absorption spectra of N719 dye adsorbed on ZnO and TiO2 films (thickness 4 µm). The spectra are corrected for ZnO, TiO2, and substrate absorption.
time was 1 min for ZnO solar cells. UV-vis absorption spectra of sensitized ZnO and TiO2 films are shown in Figure 3. Slightly more of the N719 dye was adsorbed by TiO2. Importantly, the absorption maximum of the adsorbed dye on ZnO was not blueshifted. This gives evidence that there is no formation of the Zn2+-dye complex.6 Current-voltage characteristics of the ZnO and TiO2 solar cells in simulated sunlight are shown in Figure 4a. TiO2 solar cells give higher power conversion efficiencies than corresponding ZnO solar cells, as they give higher photocurrents, opencircuit potentials, and fill factors. The overall efficiency was found to be rather low for all cells (
