J. Phys. Chem. C 2007, 111, 8075-8079
8075
BaCO3 Modification of TiO2 Electrodes in Quasi-Solid-State Dye-Sensitized Solar Cells: Performance Improvement and Possible Mechanism Xueming Wu, Liduo Wang,* Fen Luo, Beibei Ma, Chun Zhan, and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed: January 25, 2007; In Final Form: March 15, 2007
In this paper, a nanoporous TiO2 electrode modified with an insulating materialsBaCO3sis synthesized for quasi-solid-state dye-sensitized solar cells (DSSCs). The formation of BaCO3 is confirmed from the X-ray photoelectron spectrum (XPS) and Fourier transform infrared (FTIR) spectrum, while the components of the modified TiO2 film are studied by electron-probe microanalysis (EPMA). We find that BaCO3 modification remarkably increases dye adsorption, resulting from the fact that the surface of BaCO3 is more basic than that of TiO2. When applied to quasi-solid-state DSSCs, BaCO3-modified TiO2 electrodes have an increased open-circuit photovoltage (Voc) and improved overall conversion efficiency (η). The η value improves from 5.53 to 6.96% by 25.9% under 30 mW/cm2. The mechanism of BaCO3 modification is analyzed using the transient photovoltage spectra, open-circuit photovoltage spectra, and dark current measurement. The results indicate that in the presence of BaCO3, the TiO2 conduction band shifts to the negative direction.
Introduction Since the first study on high-efficiency solar cells based on dye-sensitized nanocrystalline TiO2 was reported,1 dye-sensitized solar cells (DSSCs) have been regarded as a promising alternative to conventional silicon solar cells because of their relatively low cost and high efficiency. After more than a decade of development, the overall conversion efficiency of 11.04% has been achieved using ruthenium dyes.2 However, the cells still suffer a series of energy losses. For example, the recombination between the injected electrons and the oxidized dye or ions in the electrolyte may cause a reduction of approximately 300 mV of the open-circuit voltage from the theoretical value, leading to a rapid decrease in the conversion efficiency.3,4 Thus, the performance of DSSCs can still be considerably improved. If all the parameters are optimized, the efficiency of DSSCs that use ruthenium dye and a standard triiodide/iodide redox couple will be considerably higher.5 A well-known method to improve the performance of solar cells is the interfacial modification on TiO2 nanoparticles using semiconductor materials such as Nb2O5,3,6-8 ZnO,4,7,9,10 MgO,4,11 Al2O3,4,7,12-14 In2O3,15 SnO2,7 SiO2,14 ZrO2,7,14 PbS,16 and so forth. There are three different ways in which surface modification can influence the solar cell performance. (1) Some modification materials form an energy barrier that allows electron injection but hinders the recombination reaction.3,6,7,12,13 (2) Some form a “surface dipole” that shifts the conductionband potential of the electrode material.3,8,13,17 (3) Others passivate the surface states, which are considered to be the recombination centers.13,16,18,19 All these three ways have revealed promising results. Considerable efforts have been made to modify TiO2 with large-band-gap semiconducting metal oxides, while nonmetal oxide-based insulating materials have not been thoroughly * Corresponding authors. Tel: (008610) 62788802. Fax: (008610) 62795137. E-mail:
[email protected] (Wang); qiuy@ mail.tsinghua.edu.cn (Qiu).
investigated during the study of TiO2 modification. Carbonates are types of electrical insulators. Therefore, the modification of TiO2 with an insoluble carbonate may be effective in blocking back-recombination and increasing the conversion efficiency in DSSCs. In addition, it is well-known that the carboxylate substituents on the bipyridine ligands of ruthenium dye are covalently bound to the surface of the electrode;20,21 further, the basic property of the modification materials will greatly influence dye absorption.11,14,22,23 Since insoluble carbonates are usually more basic than TiO2, employing carbonates to modify TiO2 may favor higher dye adsorption and thus yielding higher light-harvesting efficiency. Since only about half of the internal surface of TiO2 is covered with the dye,24,25 a higher dye absorption owing to the use of an insoluble carbonate coating may result in a further decrease in back-recombination. Some previous studies employing CaCO3 modification on TiO2 electrodes demonstrate these advantages and considerably improve the overall conversion efficiency (η).22,26 It is also found that CaCO3 behaves as a barrier, effectively suppressing charge recombinations.22 Among the insoluble carbonates, BaCO3 is strongly basics even more basic than CaCO3 (the isoelectric points of BaCO3 and CaCO3 are10.0-10.5 and 8.3, respectively22,27). Therefore, the modification of a TiO2 electrode with BaCO3 may be more effective in improving the performance of DSSCs. Thus, we selected BaCO3 as the electrical insulating material to study the modification effect; we found that the mechanism of BaCO3 modification is different from the reported CaCO3 modification mechanism.22 In this study, we report the modification of TiO2 electrodes with BaCO3 and its influence on the performance of polymerbased quasi-solid-state DSSCs. The mechanism of BaCO3 modification was also studied. The components of BaCO3modified TiO2 films were confirmed by X-ray photoelectron spectrum (XPS), Fourier transform infrared (FTIR) spectroscopy, and electron-probe microanalysis (EPMA). The modification mechanism was investigated using transient photovoltage
10.1021/jp0706533 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
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spectroscopy, open-circuit photovoltage spectra, and backcurrent characteristics. The results indicate that BaCO3 modification shifts the TiO2 conduction band in the negative direction and no surface energy barrier is formed. The conduction-band shift increases the Voc value of the DSSCs, while the shortcircuit current (Isc) decreases. The increase in Voc is more significant than the decrease in Isc; thus, the η value of the cell remarkably increases via BaCO3 modification. Experimental Methods A TiO2 colloidal dispersion was prepared by adding 17 mL of deionized water to 3 g of P25 TiO2 powder (Degussa product, a mixture of ca. 25% rutile and 75% anatase with a BET surface area of 50 ( 10 m2/g and a mean particle size of 21 nm) in a conical flask. Further, 0.27 mL of acetylacetone was added to prevent the reaggregation of TiO2 particles, followed by a 15min sonication; 0.13 mL of detergent (Triton X-100) was introduced to facilitate the spreading of the colloid on the substrate. After that, 0.75 g of polyethylene glycol (PEG, Mw ) 20 000) and 0.15 g of poly(ethylene oxide) (PEO-2 000 000) were added to increase the porosity of the film. To prepare the TiO2 film, transparent conductive oxide (TCO) glass was completely cleaned and a thin compact TiO2 film (approximately 8 nm in thickness) was deposited on the TCO glass by means of dip coating in order to improve the ohmic contact and adhesion between the porous TiO2 layer and the conductive TCO glass.28 The above TiO2 colloidal dispersion was spread on the surface of the compact TiO2 film by means of a doctor-blading method. The thickness of the porous layer was controlled by an adhesive tape. Thereafter, the film was thermotreated at 450 °C for 30 min. The thickness of the obtained porous TiO2 film was approximately 5 µm. The TiO2 surface modification by BaCO3 was performed as follows: a nanoporous TiO2 electrode was dipped into 0.05 M Ba(OH)2 solution for 10 min, and then it was rinsed with deionized water and aged in air for more than 5 h. This procedure was repeated one, two, or three times, again followed by calcination at 450 °C for 30 min. cis-(SCN)2bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (N3) was purchased from Solaronix. Dye sensitization was carried out by soaking the nanoporous modified or unmodified TiO2 films in 5 × 10-4 M N3 solution in anhydrous ethanol for more than 12 h. The fabrication process of the solar cell was the same as previously reported.9 After the sensitization process, a quasi-solid-state polymer electrolyte containing 0.0383 g of P25 TiO2 powder, 0.1 g of LiI, 0.019 g of I2, 0.264 g of PEO, and 44 µL of 4-tert-butylpyridine in 1:1 acetone/propylene carbonate was spread on the sensitized TiO2 film by spin coating to form a hole-conducting layer; then, a platinum-coated TCO glass counter electrode was laminated. To prepare the platinum counter electrode, 100 µL of H2PtCl6 (7.5 mM) solution in 2-propanol was deposited onto the TCO glass and the electrode was then sintered at 380 °C for 15 min. The FTIR spectrum was measured using a Perkin-Elmer Spectrum GX FTIR spectrometer. The XPS was determined using a PHI-5300 ESCA X-ray photoelectron spectrometer. The components of the modified TiO2 film were studied using a JXA-8100 electron-probe micro analyzer. The UV-vis reflectance absorption spectra were measured using a Hitachi U-3010 spectroscope. The amount of chemisorbed dye was determined using a spectroscopic method by measuring the concentration of the dye desorbed from the titania surface into 0.05 M NaOH solution, and the absorption spectra were analyzed using an ultraviolet visible spectrometer Agilent 8453. The current-
Figure 1. XPS spectra of Ba3d3/2 and Ba3d5/2 for BaCO3-modified TiO2 films.
voltage (I-V) characteristics and dark current curves were obtained by KEITHLEY 4200 with an active area of 0.25 cm2. For studying the transient photovoltage of DSSCs, the cells were probed with a weak laser pulse at 532 nm, which was generated by a frequency-doubled Nd:YAG laser (pulse duration of 15 ns). The 532 nm probe light was strongly absorbed by the dye; this light was incident from the front (TCO) side of the DSSCs in an effort to perform the measurement under real operating conditions. The DSSCs under test were maintained in this environment for more than 5 min so that they could reach their steady states before testing. The transient photovoltage signal was tested under the open-circuit condition and recorded using a TDS220 oscilloscope (Tektronix).16 Results and Discussion When a TiO2 film is dipped into 0.05 M Ba(OH)2 aqueous solution, the barium solution penetrates the porous TiO2 film and the barium ions adsorb onto the surface of TiO2. After exposure to air, Ba(OH)2 reacts with carbon dioxide to yield barium carbonate (BaCO3), resulting in the formation of a layer of BaCO3 on the TiO2 surface:
Ba(OH)2 + CO2 f BaCO3 + H2O
(1)
Calcination at 450 °C does not lead to the decomposition of BaCO3 since the thermodynamic equilibrium temperature necessary for the direct decomposition of BaCO3 in air (PCO2 ) 33 Pa) is more than 800 °C.29,30 Further, calcination does not lead to the formation of barium titanate (BaTiO3).29 The formation of BaCO3 on the TiO2 surface was verified by the XPS and ATR-FTIR measurement. Figure 1 shows the XPS for the BaCO3-modified TiO2 film. The peaks detected at 795.3 and 780.2 eV agree with the binding energies of Ba3d3/2 and Ba3d5/2 in BaCO3,31 respectively, indicating the existence of Ba2+. To further confirm the formation of BaCO3, ATR-FTIR spectra were measured for both BaCO3-modified and unmodified TiO2 films. However, for the TiO2 film that was dipped into the Ba(OH)2 solution once, the ATR-FTIR peaks were difficult to detect because of the small amount of BaCO3. Thus, we provide the ATR-FTIR spectrum for the TiO2 film that was dipped into the Ba(OH)2 solution four times in comparison with that for the unmodified TiO2 film (Figure 2). The broad peak at 1633 cm-1 can be attributed to the bending mode of the O-H groups of the adsorbed water.22,32 In contrast to unmodified TiO2, the two peaks at 1473 and 1347 cm-1 with a distance of 126 cm-1 between their centers in the spectrum of the BaCO3modified film strongly support the formation of carbonate.32
BaCO3 Modification of TiO2 Electrodes
J. Phys. Chem. C, Vol. 111, No. 22, 2007 8077 TABLE 1: Performance Comparison of the DSSCs Employing BaCO3-Modified and Unmodified TiO2 Electrodes under Different Light Intensity sample TiO2 BaCO3-modified TiO2
Figure 2. ATR-FTIR spectra for BaCO3-modified and unmodified TiO2 films.
Figure 3. UV-vis absorption spectra for N3-absorbed TiO2 films with and without BaCO3 modification.
The sharp peak at 1473 cm-1 is due to the asymmetric O-C-O stretch vibration and the broad peak at 1347 cm-1 to the symmetric stretch vibration. These two peaks also indicate the formation of coordination bonds between the CO32- moiety and Ti(IV) since there is only one peak at around 1400 cm-1 for the free CO32- species.22 An element component analysis obtained using EPMA shows that the amount of BaCO3 formed on the surface of the TiO2 film is 3.6 wt % with respect to that of TiO2. Assuming that the TiO2 particles are spherical and the coating is uniform, the thickness of the layer is calculated to be 0.2-0.3 nm. A change in the relative amount of the adsorbed dye molecules demonstrates the merit of our approach to utilize BaCO3 modification in fostering the dye adsorption. Figure 3 shows the UV-vis absorption spectra for the dye-absorbed TiO2 films with and without BaCO3 modification. It reveals that the modification of BaCO3 apparently increases the amount of adsorbed dye molecules. To further confirm the surface concentration of N3, the dye was desorbed from the TiO2 surface into 0.05 M NaOH solution and the absorption spectrum was measured. The result showed that the surface concentration of N3 dye increases from 3.84 × 10-8 to 5.55 × 10-8 mol cm-2 by 45% upon BaCO3 modification. The higher dye concentration is attributed to the higher basicity of the TiO2 film upon BaCO3 modification. If the modification materials are more basic than TiO2, the carboxyl groups in the N3 dye molecules are more easily adsorbed to the surface of the coating layers.11 The isoelectric point of BaCO3 is 10.0-10.5,27 which is considerably greater than that of TiO2 (∼5.0).7 This means that the surface of BaCO3-modified TiO2 is more basic than that of TiO2, leading to a significant improvement in dye absorption.
Isc light intensity (mW/cm2) (mA/cm2) 30 70 90 30 70 90
4.46 8.36 9.94 4.40 8.12 8.91
Voc (V)
FF (%)
η (%)
0.55 0.56 0.56 0.64 0.64 0.63
67.6 56.2 56.1 74.1 68.3 65.1
5.53 3.76 3.47 6.96 5.07 4.06
It should also be noted that the maximum absorption peak at 534 nm for N3, which is assigned to the lowest-energy metalto-ligand charge-transfer (MLCT) band, blue shifts by 22 to 512 nm upon BaCO3 modification. The blue shift of the MLCT peak results from the increased surface basicity due to BaCO3 modification. It is well-known that the carboxylate substituents on the bipyridine ligands of N3 dye are covalently bound to the surface Ti(IV) of the electrode,20,21 where Ti(IV) acts as an electron-withdrawing moiety. Upon BaCO3 modification, the coordination of Ti(IV) to the carboxylates on the bipyridine ligands of the N3 dye is strengthened, thereby leading to a decrease in the electron density at the Ru(II) center. It is considered that this decrease in the electron density will stabilize the t2g orbital and therefore induce a blue shift in the corresponding MLCT transition.33 The I-V characteristics show a significant improvement in the photovoltaic performance upon BaCO3 modification. Table 1 shows the performance comparison of the quasi-solidstate DSSCs consisting of BaCO3-modified and unmodified TiO2 electrodes under different light intensity. The nanoporous TiO2 layers consist of ∼5 µm P25 particles. Compared with unmodified TiO2, BaCO3 modification increases the Voc value and fill factor (FF); however, the Isc value decreases. Since the increase in Voc and FF are larger than the decrease in Isc, the η value increases. In particular, under low light intensity, the η value significantly improves. Under 30mW/cm2, the photovoltage increases from 0.55 to 0.64 V by 16.4%, and the FF increases from 67.6 to 74.1% by 9.6%; however, the photocurrent decreases slightly from 4.46 to 4.40 mA/cm2. As a result, the conversion efficiency of the solar cell increases by 25.9%s from 5.53 to 6.96%. Surprisingly, an increased dye amount does not lead to an increase in Isc upon BaCO3 modification. As mentioned in the introduction, there are three different mechanisms by which the BaCO3 modification can cause an increase in Voc. In a previous study, the effects of CaCO3 modification were attributed to a barrier mechanism.22 However, in our study, on the basis of the results presented below, we found that the second option, that is, a negative shift of the TiO2 conduction band potential, can effectively explain the effects of the BaCO3 modification. In the photovoltage transient measurements carried out under the open-circuit condition, the decay curve of the photovoltage provides a representation of the recombination rate.13,16,34,35 The characteristic time constant τR,which represents the recombination lifetime of electrons within the film in the steady state, can be fitted to one major single-exponential decay process.34,36 Figure 4 shows the normalized transient photovoltage curves of DSSCs with and without BaCO3 modification under the opencircuit condition. The fitted τR value with BaCO3 modification is 27 s, changing slightly from that of the unmodified cell, -27.6 s. This result indicates that, as a function of the electron density, the recombination is not suppressed by BaCO3 modification. The increase in Voc is associated with the conduction band-edge shift and suppression of charge recombination. Since
8078 J. Phys. Chem. C, Vol. 111, No. 22, 2007
Figure 4. Normalized transient photovoltage spectra for DSSCs employing BaCO3-modified and unmodified TiO2 films.
Figure 5. Voc as a function of the illumination wavelength for DSSCs employing BaCO3-modified and unmodified TiO2 films.
we do not observe the later phenomenon, the increase in Voc should be mainly attributed to the conduction band-edge shift. This strongly supports the fact that BaCO3 adsorbed on the TiO2 surface behaves as a “surface dipole” that shifts the conduction band to the negative direction,3 rather than as a barrier or passivation element of the surface states, as the back-recombination is suppressed in the latter two situations. Since the Voc measurement as a function of the illumination wavelengths reveals the lowest dye excited-state that can be injected to the semiconductor electrode,3,7 we utilized photovoltage spectroscopy to measure the onset of electron injection. Figure 5 shows open-circuit photovoltage spectra of the DSSCs employing BaCO3-modified and unmodified TiO2 films. In the long-wavelength region, the absorption spectra of the dye for both BaCO3-modified and unmodified TiO2 films are similar. Figure 5 shows that the injection onset shifts to shorter wavelength upon BaCO3 modification on TiO2 electrodes, indicating a negative movement of the TiO2 conduction band with respect to the dye ground. That is, the movement of the TiO2 conduction band in the negative direction requires higher excitation energy to permit injection. The effect of BaCO3 modification is further confirmed by the dark current curves. Although the dark current in DSSCs cannot be considered as a direct measurement of the recombination process, it can be used to measure the conduction band shift or the existence of an energy barrier at the electrode surface.3,8 Figure 6 shows the dark current against the applied potential. BaCO3 modification shifts the onset potential while maintaining the curve shape. This shape similarity indicates that BaCO3 modification does not generate a barrier that can reduce the dark current, but it shifts the conduction band potential in the negative direction, revealing a lower electron density in TiO2 for any given applied potential.7 Consequently, at each applied
Wu et al.
Figure 6. Dark current vs applied bias for DSSCs employing BaCO3modified and unmodified TiO2 films.
potential, the back-reaction in modified TiO2 is lower than that in unmodified TiO2. However, the back-reaction as a function of the electron density in TiO2 does not change upon modification. From the dark current measurement, it can be concluded that the resistance of TiO2 increases as a result of the negative shift of the TiO2 conduction band.3 The above discussions indicate that BaCO3 modification shifts the TiO2 conduction band in the negative direction, thus leading to an increase in Voc. However, it also leads to a decrease in the electron injection efficiency. Although the amount of dye molecules absorbed on the TiO2 film remarkably increases, it is found that the Isc decreases. This may be because the light absorption is almost saturated even without BaCO3 modification. The Isc value is influenced by four factors: light-harvesting efficiency (LHE) of the colored TiO2 film, electron injection efficiency (ΦINJ), the collection efficiency of the injected electrons to the TCO (ΦCOLL), and the light intensity. In our experiments, LHE is almost saturated and ΦCOLL remains unchanged since the electron lifetime varies slightly after electrode modification. Therefore, a lower value of ΦINJ leads to a decrease in Isc. The increase in Voc is more significant than the decrease in Isc; thus, the η value of the cell remarkably increases upon BaCO3 modification. Conclusion In conclusion, we demonstrate a simple method to modify TiO2 surface with a nonmetal oxide-based insulating materials BaCO3, which remarkably increases dye absorption. When the BaCO3-modified TiO2 electrode is applied to quasi-solid-state DSSCs, the conversion efficiency significantly improves. It is found that BaCO3 modification shifts the TiO2 conduction band in the negative direction, leading to a large increase in Voc. The back-recombination rate as a function of the electron density is not suppressed upon BaCO3 modification. However, as a function of the applied potential, back-recombination is suppressed. The above results provide a way to tune the properties of the suitable semiconductors in order to match the optimal potential of each dye and to achieve higher DSSCs performance. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant No. 50473009 and the National Key Basic Research and Development Program of China under Grant No. 2006CB806203. References and Notes (1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (2) Gratzel, M. J. Photochem. Photobiol. AsChem. 2004, 164, 3.
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