Semiconducting Divalent Metal Oxides As Blocking Layer Material for

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Semiconducting Divalent Metal Oxides as Blocking Layer Material for SnO2-Based Dye-Sensitized Solar Cells Min-Hye Kim and Young-Uk Kwon* Research Institute of Advanced Nanomaterials, Department of Chemistry and BK-21 School of Chemical Materials Science, Sungkyunkwan University, Suwon, 440-746, Korea

bS Supporting Information ABSTRACT: This study is to demonstrate that semiconducting materials can be used as the blocking layer material for the electrode of dye-sensitized solar cells (DSSCs) as well as insulating materials studied previously. We modified SnO2 nanoparticles with various semiconductor divalent metal oxides (CdO, ZnO, NiO, CuO, and PbO) and fabricated DSSCs with the modified SnO2 nanoparticles. The modifier metal oxides exist as very small nanoparticles, well dispersed on the surfaces of SnO2 nanoparticles. Except for the case of PbO, all of the modifier materials improve the solar cell efficiency. The detailed mechanisms have been investigated. The basic properties of the modifiers increase the amount of dye adsorbed, increasing the current, and raise the flat-band potentials through deprotonation of the surface to increase the potentials. The coating materials block the recombination reactions between the electrons in the conduction band and the red ox species in the electrolyte. Therefore, although the narrow band gaps and the d d transitions of the coating materials may reduce the amount of photons to reach the sensitizer, such negative effects are reduced by the quantum size effects of the modifier metal oxides and are more than offset by the positive effects mentioned above. The case of PbO as the coating material appears to suffer from the high resistance arising from the highly anisotropic crystal structures of PbO. On the basis of the various observations depending on the nature of the modifier materials, a few suggestions are made in selecting good modifier materials.

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted research interests because they are considered to be economically and environmentally beneficial compared with the conventional Si solar cells.1 However, the overall conversion efficiency of DSSCs is still far from the theoretical value. Besides the practical problems, such as the leakage of electrolyte and the thermal degradation of the sensitizer, there are several rate-limiting factors involving interfacial reactions at substrate anode, anode electrolyte, sensitizer anode, and electrolyte cathode interfaces.2 Among these, the various reactions occurring at the anode electrolyte interface have been most widely studied to improve the efficiency. One of processes at the anode electrolyte interface is the recombination of the injected electron of the anode with the oxidized species in the electrolyte.3 9 Using organic additives in the electrolyte8,9 or using a blocking layer on the metal oxide electrode3 7 have been the two main approaches to solve the recombination problems. As for the approach of a blocking layer, the generally accepted understanding is that the blocking layer prevents the recombination reaction by forming a potential barrier between the anode and the electrolyte.3 7 Therefore, in most of the cases, the materials of the blocking layer have been restricted to insulating or wide-band-gap oxide materials. Previously, our group has pointed out that even narrow-band-gap materials, such as CdO r 2011 American Chemical Society

with an Eg of 2.2 2.9 eV, can form the blocking layer because the layer thickness is thin enough to show a quantum confinement effect, effectively functioning as an insulator layer.10 In this case, the acid/base properties of the CdO layer also play an important role in determining the cell efficiency. This result implies that a broader range of materials than conventionally considered can be used as the blocking layer. In the present study, we attempt to generalize this idea by using semiconductor NiO, CuO, ZnO, and PbO as the blocking layer materials as well as CdO to the SnO2 anode. We have selected these materials because they have high isoelectric points (IEPs) and are semiconductors with different Eg values, suitable for the generalization of the result of our previous work. We find that these materials can serve as blocking layers, increasing the cell efficiency. The semiconducting properties of the layer materials in the bulk state do not strongly influence the performance. Rather, their basic properties, which are represented by their high IEPs of about 10, are more important. However, in addition, we also find that the nature of bulk properties are carried over to the blocking layers so that the mechanisms by which the individual blocking layer materials function differ from material to material. Received: July 22, 2011 Revised: October 4, 2011 Published: October 11, 2011 23120

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Table 1. Photovoltaic Performances of DSSCs Fabricated with Various SnO2 Electrodes and the Amount of Dye Adsorbed on the SnO2 Surfaces Jsc (mA/cm2)

Voc (V)

FF

η (%)

dye amount (mol/cm2)

equivalent thickness (Å)

unmodified SnO2

7.63

0.35

0.43

1.14

7.7  10

8

Cd-SnO2 Zn-SnO2

8.76 8.46

0.45 0.42

0.46 0.48

1.82 1.71

1.2  10 1.0  10

7

Ni-SnO2

7.03

0.46

0.58

1.85

9.8  10

8

0.25

Cu-SnO2

6.73

0.47

0.57

1.80

1.2  10

7

0.28

Pb-SnO2

5.78

0.38

0.50

1.10

1.2  10

7

0.52

7

0.42 0.33

2. EXPERIMENTS 2.1. Materials. All the chemicals used in this study were purchased and used without further purification: 15% SnO2 colloid solution (Alfa Aesar), glacial acetic acid, hydroxypropyl cellulose (HPC) (Aldrich, M.W. = 85 000), cadmium acetate (Aldrich, 98%), zinc acetate (Aldrich, 98%), cobalt acetate (Aldrich), nickel acetate (Aldrich, 98%), copper acetate (Aldrich, 98+%), lead acetate (Kanto Chemical Co., 99.5%), ethanol (Merck, anhydrous), propionitrile (Aldrich, anhydrous), iodine (Aldrich, 99.9%), lithium iodide (Aldrich, 99.99%), tert-butyl pyridine (Aldrich), 1,2-dimethyl-3-propyl imidazolium iodide (Solaronix SA), sodium sulfate (Samchun), Ruthenium 535 dye (Ru(bpy)2(NCS)2H4, Solaronix), and Sealant (SX1170-60 μm, Solaronix). 2.2. Preparation of SnO2 Films. SnO2 films were prepared through the method described in our previous report.10 Briefly, 1 g of the SnO2 colloid solution and 0.09 g of HPC is ground until the HPC is completely dissolved. A 0.1 mL portion of glacial acetic acid is then added and ground thoroughly. The viscous slurry is spread on an FTO substrate by the doctor-blading method with 3 M Magic tape as a spacer. To prepare the metal oxide modified SnO2 films, a measured amount (5 mol % to Sn) of metal acetate (M(acet)2, M = Cd, Zn, Ni, Cu, and Pb) was added before the addition of acetic acid. For the metal oxide modified film, the amount of HPC is controlled to be 60 wt % to the total amount of metal oxide. The following procedure is the same as for the unmodified film. The films were fired at 450 ̊C for 30 min to remove HPC and to form a metal oxide layer. The films were then sensitized by soaking in a 0.3 mM N3 ethanol solution overnight. The electrodes and devices fabricated with modified SnO2 will be denoted as M-SnO2, where M is the modifier metal atom. 2.3. Fabrication of DSSCs with Unmodified and Modified SnO2 Films. DSSCs were fabricated by sandwiching a dye-sensitized SnO2 film and a platinized FTO glass with SX-1170 as a sealant. Platinization of FTO is done by dropping 12 μL of 5 mM H2PtCl6 1-propanol solution on an FTO glass (1.5  1.5 cm2), followed by firing at 380 °C for 15 min. An electrolyte composed of 0.6 M 1,2dimethyl-3-propyl imidazolim iodide, 0.1 M LiI, and 0.05 M I2 in propionitrile was injected through the hole drilled on the platinized FTO glass. The holes were sealed with a hot-melt glue to prevent the evaporation of the electrolyte. 2.4. Analysis. The chemical state of Sn and O in SnO2 electrodes was analyzed by X-ray photoelectron spectroscopy equipped with a full charge compensation system (AXIS-NOVA, Kratos Inc.). The morphology of films and the thickness were analyzed by scanning electron microscopy (FE-SEM, JEOL JSM-7401F). X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer (Rigaku DC/Max 2000 diffractometer). UV vis diffuse reflectance (UV-DRS) spectra of SnO2 films were obtained with a UV vis-NIR spectrometer (Shimazu 3600) using BaSO4 as a reference. The amounts of

Figure 1. XRD patterns of unmodified SnO2 and M-SnO2 films.

the dye adsorbed on the surfaces of SnO2 electrodes were determined by desorbing the dyes with a 0.1 M NaOH aqueous solution and measuring their concentrations by UV vis absorption spectroscopy (Scinco S-3100 spectrophotometer). The electrochemical measurements were performed with a potentiostat (Ivium CompactStat). The flat-band potential (Vfb) was calculated from the Mott Schottky plots of the films. To obtain the Mott Schottky plot, a film was soaked in a 0.5 M Na2SO4 aqueous solution (pH = 7) and the impedance was measured as a function of the potential at 500 Hz with 10 mV of amplitude.5,11 The open-circuit voltage decay (OCVD) was measured by the method in the literature.12 14 Before measuring the photovoltage decay, the cell is illuminated to make the steady state, and the light is turned off by a shutter. The photovoltage is then recorded on a potentiostat with 50 ms intervals. The impedance spectra of DSSCs were collected at various voltages in the range of 104 0.05 Hz with 10 mV of amplitude under illumination.

3. RESULTS AND DISCUSSION 3.1. Photovoltaic Performances of DSSCs. In Table 1, the performance of the DSSCs fabricated with unmodified and metal oxide modified SnO2 films and the amount of dye adsorbed on the surfaces of the electrodes are compared. All devices except for Pb-SnO2 show improved efficiencies from that of unmodified SnO2. For Cd-SnO2 and Zn-SnO2, all of the parameters, the short circuit current (Jsc), the open-circuit voltage (Voc), and the fill-factor (FF), are increased, resulting in higher efficiency than unmodified SnO2. However, Ni-SnO2 and Cu-SnO2 show slightly decreased Jsc, whereas the FF and Voc are higher than those of Cd-SnO2 and Zn-SnO2. In the case of Pb-SnO2, Voc is nearly unchanged, whereas Jsc is decreased and FF is increased, which leads to the similar overall conversion efficiency as unmodified SnO2. 23121

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Figure 3. XPS spectra of metal atoms in M-SnO2 films. Figure 2. Sn 3d XPS spectra of unmodified SnO2 and M-SnO2 films.

3.2. Chemical States of SnO2 Films. Figure 1 shows the XRD patterns of unmodified and modified SnO2 films. For all samples, there are cassiterite (SnO2, JCPDS 71-0652) peaks only and peaks related with any other metal compound cannot be seen. The peak positions of SnO2 in the M-SnO2 samples are identical to those of the unmodified one. These mean that the blocking layer materials are well dispersed, probably, on the surfaces of the SnO2 nanoparticles. The amounts of the coating materials are very small. Assuming that they are evenly spread to cover the whole surfaces of SnO2 nanoparticles, the shell thicknesses (equivalent thickness in Table 1) are calculated to be a few tens of picometers, smaller than the size of the metal ions. Therefore, it is likely that the metal oxides do not fully cover the surfaces of SnO2. By using the peak widths and the Scherrer formula, the size of the SnO2 nanoparticles in the M-SnO2 samples is estimated to be 6 7 nm, unchanged from the size of SnO2 nanocrystals in the starting SnO2 solution. This indicates that the SnO2 nanocrystals have not grown during the fabrication procedure. We could not see the metal oxides by transmission electron microscopy (TEM) (Supporting Information) probably because the amount of metal oxides (M/Sn = 0.05) was too small. In our previous study on Cd-SnO2 with the Cd/Sn ratio varied, cadmium oxide could be seen in the TEM images only when the Cd/Sn ratio was 0.1 or higher.10 These materials were used to fabricate electrodes for the DSSCs in Table 1. The thickness of the films was measured to be about 5 μm by cross-sectional scanning electron microscopy images (Supporting Information). To understand the chemical states of Sn and metal ions in the electrodes, the films were studied with XPS. Figure 2 shows the

Sn 3d high-resolution XPS spectra of metal oxide modified SnO2 thin films. During the acquisition of XPS data, an electron neutralizer was used to remove a charging effect and the binding energy of all XPS data was calibrated by comparing the C 1s peaks with the reference value of 285.2 eV.15 In all cases, the peaks were symmetric and were located at near 486.6 eV (Sn 3d5/2) and 495 eV (Sn 3d3/2),16 18 indicating a single electronic state of Sn4+. The Sn 3d peak positions of the modified SnO2 are slightly shifted to lower binding energies by 0.06 0.15 eV from that of unmodified SnO2. Similar peak shifts through additive support interactions of Sn species have been reported in the literature.19,20 Dobler et al. reported an XPS binding energy shift of Sn 3d by an interaction with a surface additive, such as gold, even without the formation of an alloy.19 Yang et al. also showed that the Sn 3d XPS binding energy of SnO2 was changed when incorporated into the pores of SBA-15.20 In these examples, it is suggested that the interaction between surface additives and SnO2 leads to the imbalance of charge, which may be related to the defects and/or the change of concentration of surface adsorbed oxygen. We also used XPS to study the chemical states of the metal atoms in the modified SnO2 films (Figure 3). The peak positions of the metal atoms are 405.2 eV (Cd); 1021.8 eV (Zn); 855.6 eV (Ni); 933.5, 953.7 eV (Cu); 138.5, 143.2 eV (Pb). These values agree well with those of corresponding metal oxides.21 On the basis of these observations, we can conclude that the modifier metal atoms form metal oxides and the metal oxides are well dispersed on the surfaces of SnO2 nanocrystals without forming any solid solution with SnO2. 3.3. UV-DRS Analysis. The diffuse reflectance UV vis spectra of the metal oxide modified SnO2 films are shown in Figure 4. From the Kubelka Munk plot, the band-gap energy (Eg) of unmodified SnO2 was calculated to be 3.51 eV, which is close to 23122

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Table 2. Flat-Band Potentials Calculated from the Mott Schottky Plots in Figure 5 unmodified SnO2 Cd-SnO2 Zn-SnO2 Ni-SnO2 Cu-SnO2 Pb-SnO2 Vfb

0.35

0.40

0.43

0.56

0.52

0.42

Figure 4. Kubelka Munk plots of unmodified SnO2 and M-SnO2 films. UV-DRS data are shown in the inset.

Figure 6. OCVD of DSSCs fabricated with unmodified SnO2 and M-SnO2 electrodes.

Figure 5. Mott Schottky plots of unmodified SnO2 and M-SnO2 films.

the literature value.22 The absorption edges of the M-SnO2 films are all red shifted to 3.44 eV (for Cd-SnO2), 3.46 eV (Zn-SnO2), 3.39 eV (Pb-SnO2), 3.31 eV (Cu-SnO2), and 3.38 eV (Ni-SnO2), reflecting the bulk properties of the modifier metal oxides.23 25 However, these values are larger than the bulk band-gap energies probably because of their small sizes to show quantum size effects. Ni-SnO2 and Cu-SnO2 have intense absorption in the visible region, which may be a problem for the light-harvesting by the adsorbed dye (inset of Figure 4). 3.4. Mott Schottky Plots of Metal Oxide Modified SnO2. The Fermi level of the anode in a DSSC is an important parameter because the difference between the red ox potential in the electrolyte and the quasi-Fermi level of the anode determines the Voc of the DSSC. The Fermi level can be estimated by analyzing the Mott Schottky plot, which is a plot of the capacitance versus the applied voltages. By extrapolating this plot, the flat-band potential (Vfb) can be calculated. Although the Vfb itself is not the Fermi level because it changes with the pH of the electrolyte, comparison of Vfb's between different electrodes is considered to be equivalent to the comparison of the Fermi levels.5,26 The capacitances of the electrodes were obtained in a 0.5 M Na2SO4 aqueous electrolyte solution (pH = 7) to determine their Vfb's. Figure 5 and Table 2 show the Mott Schottky plots and Vfb calculated from these plots, respectively. It is clearly shown that all of M-SnO2 electrodes have a negatively shifted Vfb from

that of unmodified SnO2. These changes can be explained as the protonation/deprotonation effects of the modifier metal oxides. It is well known that the acid/base properties of the surface affect the Vfb and that the more basic properties of the surface modifier metal oxide lead to the deprotonation of the surface, raising the Vfb of the electrodes.4,5,27 All of the modifier metal oxides in this study have IEPs of 9 11,28 higher than that of SnO2 (IEP = 4 5).11,27,28 Therefore, the modifier metal oxides induce deprotonated surfaces. As a result, the Fermi levels of SnO2 electrodes are raised by surface modification, and in turn, the Voc's of the DSSCs are increased. 3.5. Open-Circuit Voltage Decay. To investigate the effect of metal oxide layers on the recombination dynamics of SnO2, OCVD data of the DSSCs were measured. The photovoltage decay curves were obtained by recording open-circuit voltages after turning off the light. As there is no more injection of photoelectrons under this condition, the voltage decay is related to the surface reaction between the electrons in the electrode and the electrolytes. In other words, OCVD is a direct measure of the recombination reaction rate.12 14 Figure 6 shows the OCVD curves, the electron lifetime (τn) plots as a function of Voc’s, of the DSSCs fabricated with M-SnO2 and SnO2 electrodes. The lifetimes of DSSCs using M-SnO2 are longer than that using unmodified SnO2. This means that the recombination reaction of the modified SnO2 electrode is much slower than that of the unmodified SnO2. This is probably because the surface modification reduces the number of recombination centers in the SnO2 electrode.29,30 Interestingly, Cu-SnO2 shows a remarkably increased τn over the entire voltage range, indicating that CuO forms the most effective blocking layer than any other metal oxides used in this study. The reason for this observation is not clear and requires further studies. However, we believe that it is related to the dielectric properties of the coating materials. Sarkar et al. showed that a small amount of Cu3+ in CuO makes the dielectric constant very high with 2  104, whereas CuO free of Cu3+ has a low dielectric constant of ∼25.31 3.6. Electrochemical Impedance Spectroscopy. Electrochemical impedance was measured to see the correlation between 23123

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Figure 7. Electrochemical impedance spectra of DSSCs fabricated with unmodified SnO2 and M-SnO2 electrodes. The solid line is the spectra fitted using the method reported by Adachi et al.32

Table 3. Electrochemical Impedance Spectroscopy Parameters of Cells

unmodified SnO2 Cd-SnO2 Zn-SnO2

Rk

keff

Deff

ns

Rw (Ω)

(Ω)

(s 1)

(cm2/s)

(cm 3)

6

36

7.3

1.1  10

5

2.2  1019

11.5

1.4  10

5

1.7  1019

20.5

2.5  10

5

1.1  1019 8.6  1018 2.5  1019

6 5

30 24.5

Ni-SnO2 Cu-SnO2

4 10

23 25

28.9 9.3

4.2  10 7.0  10

5

Pb-SnO2

9

122

1.0

3.0  10

6

6

4.7  1019

the electron transport and the surface modification (Figure 7). The spectra were measured under light at the open-circuit condition, and the corresponding parameters are summarized in Table 3. The spectra were fitted using the method reported by Adachi et al.,32 and the results are shown as the solid lines in Figure 7. The impedance spectra of DSSCs are composed of three semicircles, which correspond to (1) the electrochemical reaction of the Pt counter electrode, (2) the charge-transfer resistance at the interface of electrolyte/dye/metal oxide, and (3) the charge transport in the metal oxide electrode and the Warburg diffusion process of I /I3 , from the left to the right of the plot.30,32 34 By fitting the impedance spectra, the parameters, Rk, Rw, keff (=1/τeff), Deff, and ns can be obtained, where Rw is the charge transport resistance in SnO2, Rk is the charge-transfer resistance at the interface of electrolyte/dye/SnO2, τeff is the effective electron lifetime for the recombination with I3 , Deff is the diffusion coefficient of an electron, and the ns is the electron density at the steady state in the conduction band. Rk data of the modified SnO2 are nearly unchanged from that of unmodified SnO2 except for Pb-SnO2, indicating that the surface modification does not influence the electron transport in the metal oxide electrode. The increased Rk of Pb-SnO2 seems to be the result of the anisotropic structure of PbO. PbO in the bulk state has two allotropic forms, red-PbO (tetragonal) and yellowPbO (orthorhombic). Both forms are known to be semiconductors with band gaps of 2.07 and 2.9 eV, respectively.23 Unlike the other materials used in this study, both forms of PbO have lowdimensional structures. Red-PbO has a layered structure, and yellow-PbO has a one-dimensional character. Therefore, they are expected to be highly anisotropic in the conductivity. As in all of

the low-dimensional conductors, the conductivity across the layer of the red-PbO and across the chain of the yellow-PbO is expected to be very low, close to that of an insulator, despite their small band gaps, which interrupt charge transfer from dye to SnO2. 3.7. Explanation of Photovoltaic Performance. The solar cell performance data in Table 1 show that all of the modifier semiconducting metal oxides studied in this work except PbO can be used as the blocking layer materials to improve the efficiency. This may seem counterintuitive because semiconducting materials with narrower band gaps than that of the electrode materials (SnO2) absorb visible photons to reduce the number of photons to be absorbed by the sensitizer dye molecules and some of them have low-lying conduction band edges (i.e., the conduction band levels of CdO and CuO are at 4.6 eV (AVS) and 4.9 eV (AVS),35,36 lower than that of SnO2 ( 4.5 eV, AVS); those of ZnO and NiO are at 3.9 eV (AVS) and 4.5 eV (AVS), higher than that of SnO2) and may not be able to block the backward flow of the electrons in the conduction band of SnO2. However, the UV-DRS data clearly show that the materials in our samples have band gaps much larger than those in the bulk states. Because the XPS data proved that the metal atoms exist as metal oxides and their particle sizes are very small, it is likely that the large band gaps are due to the quantum size effect. The absorption edges of Zn-SnO2, Cd-SnO2, and Pb-SnO2 are still maintained at the UV region, and the reduction of light absorption by sensitizer seems not serious. However, in the cases of NiO and CuO, d d transitions occur in the visible region, partially hindering the photon harvest by the sensitizers. On the other hand, the introduction of a metal oxide coating on SnO2 brings in several beneficial effects. These effects are mainly due to the fact that the coating materials are oxides of divalent metal ions. Divalent metal ions have relatively small charge densities, because of which their oxides are highly basic with high isoelectric points. The basic coating materials make the SnO2 surface deprotonated, raising the quasi-Fermi levels, as can be seen in their raised flat-band potentials from that of unmodified SnO2. The surfaces with basic properties are also advantageous in that they can bind 20 50% more of the sensitizer molecules, increasing Jsc for the cases of Cd-SnO2 and Zn-SnO2 and compensating the loss of Jsc for the cases of Ni-SnO2 and Cu-SnO2. In addition, OCVD data show that the coating materials indeed function as blocking layers, making the lifetime of conduction electrons considerably longer than that of unmodified SnO2. Despite all the beneficial contributions of divalent metal oxides, the PbO coating has detrimental effects to the DSSC. The complete understanding of the reasons seems impossible because of the very small amount of PbO in our sample. We believe that the peculiar behavior of Pb-SnO2 is related to the highly anisotropic crystal structures of PbO, as mentioned above. If PbO crystallites are preferentially grown in such a way to have their planes or chain axes parallel to the SnO2 surfaces, the resistance between the SnO2 electrode and the electrolyte is expected to be very high. That is, the electron injection from dye to SnO2 becomes harder by having PbO on the surface of SnO2. As a result, the concentration of electrons is relatively higher at the surface of Pb-SnO2, accelerating the recombination reaction between photoelectrons and electrolytes. The impedance spectroscopy data show that Pb-SnO2 has an unusually larger chargetransfer resistance (Rk), supporting this explanation. Interestingly, Ni-SnO2 and Cu-SnO2 have the highest solar cell efficiencies despite that they have lower Jsc’s than unmodified 23124

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The Journal of Physical Chemistry C SnO2. This is mainly because they have the highest FFs. The physical origin of the FF remains largely unexplored. The origin of the high FFs of Ni-SnO2 and Cu-SnO2 seems to be a result of the p-type semiconducting nature of NiO and CuO.37 Bandara et al. reported that DSSCs fabricated with NiO-modified SnO2 show increased FF by forming an n p junction between SnO2 and NiO.5 They suggested that electron transporting through NiO is accelerated because the electrons are a minority carrier in the p-type semiconductor, which increases the electron scattering, leading to increased resistance. It is also possible that the p n junction annihilates the trap sites, which exert resistance. Therefore, by forming a p n junction, the resistance at the interface is greatly reduced, increasing the FF.

4. CONCLUSION In this study, we applied semiconducting divalent metal oxides (CdO, ZnO, NiO, CuO, and PbO) as the blocking layer materials for SnO2-based dye-sensitized solar cells. Although the semiconducting properties may have negative effects, such as absorption of visible light and inefficient blocking of recombination because of their band structures, these effects are considerably reduced by the quantum size effect and are more than offset by the positive effects arising from the basic properties of the coating materials. As a result, the overall conversion efficiencies of DSSCs increased when CdO, ZnO, NiO, and CuO were used as the blocking layer material. In the case of using PbO, the cell efficiency is not changed much from that of unmodified SnO2, although both Jsc and Voc are significantly changed. The reason for the peculiar behavior of the PbO-modified SnO2 is not clear, but it seems to be related to the anisotropic nature of the PbO crystal structures. On the basis of our observations, we believe that there are still remaining candidates for blocking layer materials and further works are in progress. Finally, the analysis data of the present work show some guidelines in choosing high-performance blocking layer materials. To function as a good blocking layer, the materials that are used must have (1) relatively large band gaps and (2) no or weak interstate electronic transitions to interfere with the photon absorption by the sensitizer, (3) basic properties with a high IEP, (4) a high dielectric constant to make the lifetime of injected electrons long, and (5) p-type to form a p n junction with SnO2 or TiO2 electrode material. ’ ASSOCIATED CONTENT

bS

Supporting Information. Cross-sectional SEM images and TEM images of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by grants NRF-2010-0029698 and NRF-2010-0029699 (Priority Research Center Program), NRF2011-0006268 (Basic Science Research Program), and KETEP 2009-3021010030-11-1 (Research Center of Break-through Program). We thank SAINT for the SEM data, CCRF for the TEM data, and KBSI (Korea Basic Science Institute) for XPS data.

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