Tunability of the Band Energetics of Nanostructured SrTiO3

Feb 15, 2010 - The flat band edges (Efb) of nanostructured SrTiO3 electrodes have been ... the interfacial kinetics.9 The band energetics of a nanostr...
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J. Phys. Chem. C 2010, 114, 4245–4249

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Tunability of the Band Energetics of Nanostructured SrTiO3 Electrodes for Dye-Sensitized Solar Cells Shuming Yang,* Huizhi Kou, Jichao Wang, Hongbin Xue, and Huili Han College of Chemistry and Chemical Engineering, Institute of Applied Chemistry, Xinyang Normal UniVersity, Henan, China, 464000 ReceiVed: December 14, 2009; ReVised Manuscript ReceiVed: January 28, 2010

The flat band edges (Efb) of nanostructured SrTiO3 electrodes have been determined in propionitrile (PN), acetylacetone (Acac), and PN/Acac with spectroelectrochemistry measurements. Specifically, a very negative Efb was measured when only tetrabutylammonium perchloride (TBAP) was used as supporting electrolyte, while addition of Li+ ions shifts Efb positively. The trap state distribution was investigated by the measurements of time-resolved current. The total trap state densities of 5.51 × 1016, 2.80 × 1016, and 1.83 × 1016 cm-2 were determined in PN, PN/Acac, and Acac (all containing 0.2 mol · L-1 TBAP), respectively, with trap distribution maximum located at -2.4, -1.6, and -1.5 V. The addition of Li+ also reduced the trap state densities, and the total trap state densities were calculated to be 4.19 × 1016, 2.38 × 1016, and 1.19 × 1016 cm-2 in PN, PN/Acac, and Acac (all containing 0.2 mol · L-1 TBAP and 0.5 mol · L-1 LiClO4), respectively. Finally the nanostructured SrTiO3 electrode was sensitized with dye N3 and its photovoltage-photocurrent curves were measured in three electrolytes with PN, PN/Acac, and Acac as solvents, respectively. The N3sensitized SrTiO3 showed the highest open-circuit voltage (Voc) and short-circuit current density (Jsc) in Acac, in good agreement with the electrochemical measurements that the nanostructured SrTiO3 electrode has the flat band edge well matching with the excited state of N3 and the smallest trap state densities in Acac. Introduction A dye-sensitized solar cell (DSSC) possesses three major components: (i) a dye sensitizer in order to harvest solar energy and generate excitons,1,2 (ii) a nanostructured semiconductor electrode to transport electrons efficiently,3-5 and (iii) a redox electrolyte to regenerate dye.6,7 The nanostructured electrodes which possess large surface area and, as a result, the dye molecules adsorbed on nanostructured films are tremendously increased, leading to improved efficiency of solar cells.8 However, the enhanced surface area and the interconnected particles give rise to a large number of electron traps at the nanostructured semiconductor/electrolyte interface, which affects the interfacial kinetics.9 The band energetics of a nanostructured electrode has great influence on the photoelectrochemical properties of dye-sensitized solar cells. On one hand, the conduction band edge of a nanostructured semiconductor electrode must be situated at a proper level lower than the excited state of the dye in order to yield efficient electron transfer. On the other hand, the photovoltage is largely dependent upon the difference between the Fermi level and the redox potential of the electrolyte.10 The position of the Fermi level in nanostructured semiconductor electrodes depends on the electron concentration in the conduction band, and will be close to the conduction band edge under open circuit conditions. If Efb would be taken as an approximation to the Fermi level of an illuminated semiconductor, open voltage (Voc) could correlate with the flat band potential.11 Indeed, a rough correlation has been reported between flat band potential and Voc for ZnO, anatase, SnO2, and WO3.12 Electrochemical and spectroelectrochemical methods are valuable in the study of the band energetics of transparent * To whom correspondence should be addressed.

nanostructured semiconductor electrodes and have been successfully applied in the determination of the Efb and trap states of transparent nanostructured semiconductor electrodes.9,13-15 Lindquist and co-workers found that the trap states of the nanostructured TiO2 electrode were highly dependent on electrolytes.9 Fitzmaurice and co-workers found that the Efb of nanostructured TiO2 electrodes heavily depends on electrolytes.13 Strontium titanate (SrTiO3) with a perovskite structure shares more structural similarities with anatase TiO2.16 It is a semiconductor with important science and application significance, such as application in electronic devices,17 photocatalyst,18 and dye-sensitized solar cells.10,16 We have succeeded in the preparation of transparent nanostructured SrTiO3 electrodes of high quality. Our previous research showed that the Efb and trap states of the nanostructured SrTiO3 electrodes were heavily dependent on the pH of aqueous electrolyte solution.19 In this work, the Efb and the trap states of nanostructured SrTiO3 electrodes were determined in Acac, PN, and their mixture solutions with electrochemical and spectroelectrochemical methods in order to investigate the effects of solvent chelation on the band energetics of the SrTiO3 electrodes. Furthermore, the SrTiO3 electrode was sensitized with dye N3 and its photovoltage-photocurrent properties were studied in three electrolytes with PN, PN/Acac, and Acac as solvents, respectively. A correlation between the band energetics of the nanostructured SrTiO3 electrodes and the photoelectrochemistry of the N3-sensitized SrTiO3 electrodes is to be established. Experimental Section 1. Materials and Solutions. Optically transparent electrodes (OTE) were fabricated on an F-doped SnO2-coated glass substrate. Water (R ) 18.3 MΩ) from an Easy Pure RF water purification system from Thermo Scientific was used in the

10.1021/jp9117979  2010 American Chemical Society Published on Web 02/15/2010

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preparation of all solutions. Ti(n-OC4H9)4 and ethyl cellulose, Sr(NO3)2, and LiClO4 were purchased from Tianjin Chemical Company. Terpineol, tetramethylammonium hydroxide, propionitrile (PN), acetylacetone (Acac), and tetrabutylammonium perchlorate (TBAP) were purchased from Shanghai Nuotai Chemical Company, while propionitrile was distilled over P2O5 and acetylacetone was dried over Na2SO4 for 24 h and recovered by distillation before use. LiI and tert-butylpyridine (TBP) were purchased from Acros. All the chemicals used were reagent grade. The dye N3 (Ru[L2(NCS)2], L ) 2,2′-bipyridine-4,4′dicarboxylic acid) was synthesized according to the literature.20 2. Preparation of the Bare and N3-Sensitized Nanostructured SrTiO3 Electrodes. The preparation of the nanocrystalline SrTiO3 paste and the films on conductive glass followed the procedures in our earlier report.19 Generally SrTiO3 paste was spread on conductive substrates by a glass rod with pieces of adhesive tape as spacers. The films were dried at 125 °C and sintered at 500 °C for 30 min in air. For sensitization with the dye, the SrTiO3 film was immersed in 5 × 10-4 mol · L-1 N3 absolute ethanol solution for 20 h at room temperature. To minimize adsorption of moisture in the ambient air, the electrodes were dipped in the dye solution while they were still warm (80 °C). The dye-sensitized electrodes were then rinsed with ethanol thoroughly and dried. 3. Instrumentation. Electrochemical experiments were performed on a CH800 potentiostat (CH Instrument). All electrochemical and spectroelectrochemical experiments were carried out in a three-electrode system, in which a nanostructured SrTiO3 electrode, a platinum wire, and a saturated Ag/AgCl electrode acted as working, counter, and reference electrodes, respectively. Spectroelectrochemistry measurements were undertaken according to our earlier report.19 A quartz cell with three electrodes in electrolyte solutions was incorporated into the sample compartment of a Shimadzu UV-vis spectrophotometer and connected to the CH 800 potentiostat. The electrolyte solutions were thoroughly deaerated by bubbling with N2 prior to experiments. All potentials are hereafter given with reference to a saturated Ag/AgCl electrode. The working area of SrTiO3 electrodes was 3 cm2. For measurements of photovoltage-photocurrent curves, a sandwich-type solar cell was assembled with a N3-sensitized SrTiO3 electrode as the photoanode and a Pt-coated F-doped SnO2 electrode as the photocathode. Both electrodes were sealed with Surlyn 25. A 500 W xenon lamp was used as the source of excitation. A KG4 filter (Schott) was set in the light beam to protect the electrodes from heating, and a GG420 cutoff filter (Schott) was used to prevent the SrTiO3 electrodes from being excited by light with wavelength less than 400 nm. The effective illumination area of a flat window is 0.196 cm2. Results and Discussions 1. Dependence of Flat Band Potential of Nanostructured SrTiO3 Electrodes on Solvents. Spectroelectrochemical measurement was normally applied for the monitoring of the electron filling in the conduction band.14,15,21 Because the excitation of electrons in the conduction band needs only a small amount of energy, the absorbance changes at long wavelength can be used to monitor the filling of electrons in the conduction band and as a result the conduction band edge of a semiconductor can be calculated. The flat band potentials for nanostructured SrTiO3 electrodes in three solvents (all containing 0.2 mol · L-1 TBAP) were determined by measuring absorbance at 800 nm as a function of applied potential and are shown in Figure 1.

Yang et al.

Figure 1. Absorbance measured at 800 nm as a function of applied potential for nanostructured SrTiO3 electrodes in different solvents containing 0.2 mol · L-1 TBAP

Therefore the flat band edges for nanostructured SrTiO3 electrodes are determined to be -2.68, -1.51, and -1.40 V in PN, PN/Acac, and Acac, respectively. It has been known that the solvent has a significant effect on the flat band edge. In protic solvents, such as water, Efb displayed a Nernstian dependence on pH and was independent of the nature and concentration of the electrolyte cation.21,22 In aprotic solvents, the nature and concentration of the electrolyte cation was found to affect Efb.13 Specifically, a very negative Efb was measured when only tetrabutylammonium ions (TBA+) were present,23 which is in good agreement with our results. However, there has been no report on the influence of solvent chelation on the Efb and trap states of nanostructured SrTiO3 electrodes. In this work, propionitrile and acetylacetone were selected in order to investigate the influence of solvent chelation on the Efb and trap states of nanostructured SrTiO3 electrodes. The experiment results absolutely demonstrated that solvent chelation significantly influences the Efb and trap states of nanostructured SrTiO3 electrodes. Acetylacetone is generally a chelation solvent and can coordinate to the surface of SrTiO3 electrodes, which would change the charge distribution in the Helmholtz layer and as a result the Efb.24 On the other hand, propionitrile is not generally considered as a chelation solvent, so its influence on the charge distribution in Helmholtz should not be as big as that in acetylacetone. Furthermore the addition of Li+ ions in three solvents was also examined and the results are shown in Figure 2. As seen in Figure 2, addition of LiClO4 has a pronounced effect on the Efb of nanostructured SrTiO3 electrodes in three electrolytes (all containing 0.2 mol · L-1 TBAP). The Efb was shifted further positively with addition of LiClO4 and could be determined to be -1.48, -0.80, and -0.80 V in PN, PN/Acac, and Acac, respectively. This behavior was attributable to adsorption-intercalation of Li+ ions. In aprotic solvents, the proton adsorption-desorption equilibrium at the semiconductor-electrolyte interface is absent and the adsorption-intercalation of cations is important in determining Efb. Li+ intercalation is a well-established model and would lead to a positive shift of Efb at the electrode-electrolyte interface.25,26 2. Dependence of Trap States of Nanostructured SrTiO3 Electrodes on Solvents. a. Time-ResolWed Current in Acetylacetone. The current-time curves of a nanostructured SrTiO3 electrode were measured in 0.2 mol · L-1 TBAP acetylacetone solution under different potentials and shown in Figure 3a.

Nanostructured SrTiO3 Electrodes

Figure 2. Absorbance measured at 800 nm as a function of applied potential for nanostructured SrTiO3 electrodes in different solvents (0.2 mol · L-1 TBAP) and with addition of 0.5 mol · L-1 LiClO4.

J. Phys. Chem. C, Vol. 114, No. 9, 2010 4247 terms of trap filling in the band gap region. A nanostructured SrTiO3 electrode has a flat band edge of -1.40 V in 0.2 mol · L-1 TBAP acetylacetone solution. At potentials less negative than -0.6 V, the trap density is low, and thus the trap-filling time is short, resulting in fast decay of the time-resolved current. On the other hand at potentials more negative than -0.6 V, trap density increases, so a longer time is required to fill these traps. The longest trap-filling time was at -1.6 V just near the conduction band edge and a further negative shift of the potential significantly shortens the trap-filling time. This should be related to the kinetics of trap filling. A faster trap filling is expected at a more negative potential since the driving force for the trap filling is larger. The accumulated charge Q under the current-time curves in Figure 3a is calculated and some interesting features appear. Figure 3b shows the accumulated charge Q dependence upon potentials. At potentials more positive than -0.6 V, the accumulated charge is considerably small. It is striking that there is a sharp increase in accumulated charge up to -1.5 V. At potentials more negative than -1.5 V, the accumulated charges increase slowly. If the accumulated charge Q from trap-filling reflects the density of states, eq 1 can be obtained:9

Ntrap(U) )

Figure 3. (a) Current-time curves of a nanostructured SrTiO3 electrode in 0.2 mol · L-1 TBAP solution in acetylacetone. The electrode was initially polarized at 0.8 V for 5 min and then measured at different applied potential. (b) Cathodic charges at different potentials derived by integrating the current-time curves in Figure 3a. The inset shows dQ/dU distribution against potential.

The current is significantly influenced by the applied potentials. At potential from 0 to -0.4 V, the currents decrease to almost zero within a few seconds. At -0.6 V, the decrease in current slows down and this behavior is found at all potentials more negative than -0.6 V. The results can be understood in

1 dQ q dU

(1)

where Q is accumulated charge, Ntrap(U) is the density of trap states at potential U, and q is the electron charge. Equation 1 clearly indicates that trap density is directly proportional to dQ/ dU, which provides a direct measurement of trap distribution. By differentiating the accumulated charge to the applied potential, a plot of dQ/dU against U is obtained and shown in the inset of Figure 3b. This plot reflects the distribution of traps. It is seen that most traps are located around -1.5 V. The totally trap states were calculated to be 1.83 × 1016 cm-2. b. Time-ResolWed Currents in PN, PN/Acac. The current-time curves of nanostructured SrTiO3 electrodes in PN and PN/Acac solutions (all containing 0.2 mol · L-1 TBAP) are similar to that in Acac solution (not shown), and the cathodic charges at different potentials by integrating the current-time curves are shown in panels a and b of Figure 4, respectively. Panels a and b of Figure 4 show the charges required to fill the traps. By differentiating the accumulated charge to the applied potential in PN and PN/Acac, plots of dQ/dU against U are obtained and shown in the inserts of panels a and b of Figure 4, respectively. Similar to the inset of Figure 3b, the inset plots of panels a and b of Figure 4 reflect the trap distribution in PN and PN/Acac. Therefore the total amount of trap states can be calculated to be 5.51 × 1016 and 2.80 × 1016 cm-2, respectively. It is evident that the trap state density in Acac is the smallest. It is well established that the traps on the surface of nanostructured TiO2 electrodes are mainly Ti4+related traps.11,13,15 In consideration of their similarity in crystal structure, it is proposed here that surface states of nanostructured SrTiO3 electrodes are Ti4+ and Sr2+ related traps. Acetylacetone has a legand of strong chelation and can form many kinds of complexes with metal ions. So it is understandable that acetylacetone has a large affinity for these under-coordinated surface sites (defect sites), can bind to the surface trapping sites and change the energetics of the surface states, and can reduce the trap density. c. The Effect of Addition of Li+ Ions on Trap States. To further investigate the effects of Li+ ions on trap states, the current-time curves of nanostructured SrTiO3 electrodes were

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Figure 4. Cathodic charges accumulated at different potentials as derived by integrating the current-time curves in 0.2 mol · L-1 TBAP in (a) PN and (b) PN/Acac. The inset shows dQ/dU distribution against potential.

measured in PN, PN/Acac, and Acac (all containing 0.2 mol · L-1 TBAP and 0.5 mol · L-1 LiClO4) (not shown). The cathodic charges at different potentials were calculated by integrating current-time curves measured in three solutions and shown in Figure 5, panels a, b, and c. It is seen that the addition of Li+ ions also reduced the trap state densities. The total trap states were calculated to be 4.19 × 1016, 2.38 × 1016, and 1.19 × 1016 cm-2 in PN, PN/Acac, and Acac, respectively, with maximum at -1.5, -0.85, and -0.9 V. 3. Photovoltage-Photocurrent Curves of N3-Sensitized SrTiO3 Electrodes in Three Solvents. It is evident from the above discussion that solvents play a very important role in the Efb and trap states of nanostructured SrTiO3 electrodes. It is known that Efb and trap states of a nanocrystalline electrode have a critical influence on the photoelectrochemical properties of a dye-sensitized electrode. Therefore it is important to investigate the correlation between the photoelectrochemical properties of N3-sensitized SrTiO3 electrodes and its band energetics in different electrolytes. For the measurement of photovoltage-photocurrent measurement, the N3-sensitized SrTiO3 electrode was used as the photoanode and a Pt-coated F-doped SnO2 electrode as the photocathode. Figure 6 shows the photocurrent-photovoltage characteristics of three solar cells under illumination of white light from a Xe lamp (100 mW · cm-2).

Figure 5. Cathodic charges accumulated at different potentials as derived by integrating the current-time curves in 0.2 mol · L-1 TBAP and with addition of 0.5 mol · L-1 LiClO4 in (a) PN, (b) PN/Acac, and (c) Acac. The inset shows dQ/dU distribution against potential.

The N3-sensitized SrTiO3 electrode based on electrolyte E3 showed much higher open-circuit voltage (Voc) of 0.6 V and short-circuit current density (Jsc) of 0.41 mA · cm-2. The Voc and Jsc of the N3-sensitized SrTiO3 cells increased in the order of E3 > E2 > E1. Generally the conduction band edge of a semiconductor electrode must be lower than the excited state of a dye in order to maintain efficient electron transfer from dye to semiconductor. It was reported that the excited state of N3 molecules is at -1.1 V (see in Figure 7).27 As discussed earlier, the Efb of the nanostructured SrTiO3 electrode is -1.48, -0.80, and -0.80 V in PN, PN/Acac, and Acac (all containing 0.2 mol · L-1 TBAP and 0.5 mol · L-1 LiClO4), respectively. The

Nanostructured SrTiO3 Electrodes

J. Phys. Chem. C, Vol. 114, No. 9, 2010 4249 mol · L-1 TBAP), respectively, with the maximum located at -2.4, -1.6, and -1.5 V, respectively. Compared to the electrolytes only containing TBAP, the addition of Li+ reduced the trap state densities. The trap state densities were determined to be 4.19 × 1016, 2.38 × 1016, and 1.19 × 1016 cm-2 in PN, PN/Acac, and Acac (all containing 0.2 mol · L-1 TBAP and 0.5 mol · L-1 LiClO4), respectively. The correlation of the band energetics of the nanostructured SrTiO3 electrode with the photoelectrochemistry of a N3-sensitized SrTiO3 electrode was also investigated. The N3-sensitized SrTiO3 based on E3 showed much higher open-circuit voltage and short-circuit current density. The experimental results demonstrate that it is quite feasible to tune the band energetics of nanostructured SrTiO3 electrodes for dye-sensitized solar cells by optimization of electrolyte composition in order to attain efficient photoelectric conversion.

Figure 6. Photocurrent-photovoltage curves of dye-sensitized SrTiO3 electrodes based on different electrolytes. Electrolyte E1: 0.5 mol · L-1 LiI, 0.05 mol · L-1 I2, 0.2 mol · L-1 TBP in PN. Electrolyte E2: 0.5 mol · L-1 LiI, 0.05 mol · L-1 I2, 0.2 mol · L-1 TBP in PN/Acac. Electrolyte E3: 0.5 mol · L-1 LiI, 0.05 mol · L-1 I2, 0.2 mol · L-1 TBP in Acac.

Acknowledgment. This work was supported financially by the National Natural Science Foundation of China (Grant No.20773103), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (2008101), the selected programs for scholars back from overseas, Ministry of Personnel (2006164), and Science &TechnologyProgramofEducationDepartmentofHenan(2008A150022). References and Notes

Figure 7. Energy scheme of a N3-sensitized SrTiO3 solar cell.

Efb of the nanostructured SrTiO3 electrode is lower than that of the excited state of N3 in PN/Acac and Acac, while it is higher in PN. Therefore the electron transfer should be more efficient in electrolytes E3 and E2 both having Acac and PN/Acac as solvents, respectively, resulting in much larger photocurrent. On the other hand, the trap state density of the nanostructured SrTiO3 electrode is the smallest in Acac, so the charge recombination should be less serious, also leading to improved photovoltage and photocurrent. Conclusions The flat band edges (Efb) of nanostructured SrTiO3 electrodes in PN, PN/Acac, and Acac have been determined with the spectroelectrochemical technique. Specifically, a very negative Efb was determined when only tetrabutylammonium ions (TBA+) were present in electrolyte, while addition of Li+ ions yielded significantly more positive values. The trap state density of the nanostructured SrTiO3 electrode was investigated by the measurements of time-resolved current. The total trap state densities of 5.51 × 1016, 2.80 × 1016, and 1.83 × 1016 cm-2 were determined in PN, PN/Acac, and Acac (all containing 0.2

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