Charge Transfer and Performance Enhancement of Dye-Sensitized

Article pubs.acs.org/JPCC

Charge Transfer and Performance Enhancement of Dye-Sensitized Solar Cells by Utilization of a Tandem Structure Ching-Fa Chi,† Song-Chuan Su,‡ I-Ping Liu,‡ Cheng-Wen Lai,‡ and Yuh-Lang Lee*,†,‡ †

Research Center for Energy Technology and Strategy, and ‡Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan ABSTRACT: For increasing the light-harvesting ability of dye-sensitized solar cells (DSSCs), thick TiO2 films were always utilized. However, a thicker TiO2 film also triggers a higher charge-transfer resistance. To solve the contrary effects of film thickness on the light harvest and charge transfer, a tandem-structured cell constructed by two sub-DSSCs is proposed. By using a high transparent Pt counter electrode, the light unabsorbed by the top cell can transmit trough and be utilized by the bottom cell. The performances of the tandem cells are evaluated using N719sensitized TiO2 photoelectrodes and the thickness effects of top and bottom cells are studied. The experimental results show that the tandem structure can significantly improve the conversion efficiency of DSSCs. For the single cells studied in this work, the highest efficiency is 9.3%, achieved by a cell with 12-μm main layer and 4-μm scattering layer. For the tandem cells, the best performance appears at the structure with 12-μm top cell and 8-μm bottom cell (12/8), and the efficiency achieved is 9.54% in the absence of scattering layer. Therefore, the effect of the 8-μm bottom cell in the tandem structure is more significant, in comparison with that of the 4-μm scattering layer, on enhancing the cell performance. Furthermore, if a scattering-layer effect is also applied on the bottom cell of the 12/8 tandem structure, the efficiency can further be improved to 10.16%. If the powers of top and bottom cells are outputted individually, the overall efficiency achieved by the tandem cell is 11.6%, which is more than 2% higher than that obtained by the single cell. optimal film thickness is always existed for preparing a high performance DSSC.7,8 In the literature, various strategies had been proposed to improve the performance of photoelectrodes either by increasing the light absorption ability or by decreasing the charge transport resistance. Single crystalline nanotubes, nanorods, or nanowires had been developed for preparing the dye-sensitized photoelectrdoes, aiming to enhance the charge transport.9,10 Although the charge transport resistance did decrease in these electrodes, the surface areas of such electrodes are smaller than those of mesoporous films prepared by nanoparticles. Therefore, the overall efficacy of the cell cannot be improved. For increasing the light harvest ability of a photoelectrode, a commonly used method is to introduce a scattering layer behind the main TiO2 layer.11 By the effect of a scattering layer, the unabsorbed light transmitting through the main layer can be scattered and reflected back, and reutilized by the main layer. This strategy had been known to be effective in improving the performance of DSSCs. Another strategy which had been taken to increase the light harvest of solar cells is the utilization of tandem structure. Generally, a tandem cell is constructed by using two individual cells or two photoelectrodes.12,13 Various dyes with complementary absorption ranges were used for the top and bottom

1. INTRODUCTION The effect of global warming in the recent years has raised the utilization and the increasing demand of renewable energy. Among various types of clean energy, photovoltaic devices are able to convert the solar energy directly into electricity, which have attracted a lot of studies to develop low-cost and efficient solar cells. Owing to the low production cost, dye-sensitized solar cells (DSSCs) have drawn great attention both in scientific and technological aspects in the past decade and are considered to be a potential alternative to traditional photovoltaic devices.1−4 In the operation of a DSSC, the light harvest is relied on a dye-sensitized TiO2 photoelectrode. Various dyes had been developed for extending the light absorbing range, as well as enhancing the injection efficiency of excited electrons from sensitizers to the TiO2 matrix. For a specific dye, the performance of a photoelectrode is intimately related to the thickness of the TiO2 film. On considering the light harvest, a high uptake of dye molecules on the photoelectrode is advantageous to the light absorption. Therefore, a thick TiO2 film which can supply a higher surface area for dye adsorption is helpful to the light harvest.5,6 However, the charge transfer in a thicker film should suffer a higher resistance because a longer charge transport distance is required to extract the excited electrons, as well as to recover the oxidized dye molecules. Since the effects of film thickness on the light harvest and charge transport are contrary to the conversion efficiency, an © 2014 American Chemical Society

Received: May 16, 2014 Revised: July 11, 2014 Published: July 15, 2014 17446

dx.doi.org/10.1021/jp504849f | J. Phys. Chem. C 2014, 118, 17446−17451

The Journal of Physical Chemistry C

Article

cells, respectively, to extend the light absorption range of a tandem cell. The results reported in the literature had proven that the light harvest and performance of DSSCs can be improved by tandem-structured cells.14,15 In this work, a tandem structure is proposed to solve the contrary effects of TiO2 film thickness on the light harvest and charge transfer of DSSCs. Since a DSSC prepared using a thick TiO2 film may suffer high charge transfer resistance, two thinner cells are proposed to substitute the role of a thick cell. By integrating two thin cells into a tandem structure, the light unabsorbed by the top cell can be reutilized by the bottom cell. If a high transparent counter electrode is applied, the light harvest ability of the tandem cell can be similar or even superior to that of a thick DSSC. Since the charge transport resistances in the thin cells should be smaller than that in a thick one, improvement of the overall conversion efficiency is anticipated. In our previous work, a platinum counter electrode with high electrochemical activity and high transmittance (75%) was developed for DSSCs.16 This platinum counter electrode is applied for preparing the tandem cells in this work. A commonly used dye, N719, is used as the sensitizer for both top and bottom cells and the thickness effects of top and bottom cells on the performance of the tandem cells are studied.

Figure 1. Configuration of the proposed tandem structure composed of top and bottom individual cells.

(San-Wi Electric, class 3A, XES-301S). The current−voltage (I−V) characteristics of the cells were recorded using an Eco Chemie Autolab potentiostat/galvanostat. The performances of the tandem cells were measured by parallelly connecting the top and bottom cells as shown in Figure 1. An UV−visible spectrometer (U-4100, Hitch) was used to evaluate the light absorption ability of tandem cells through the measurement of transmission spectra. Electrochemical impedance measurement was conducted with a potentiostat (Autolab, PGSTAT 30 and FRA2 module) equipped with the frequency analyzer. In this analysis, an AC potential amplitude of 10 mV was applied with the frequency ranging from 105 to 10−2 Hz under the opencircuit potential condition.

2. EXPERIMENTAL SECTION TiO2 pastes purchased from CCIC (Catalysts & Chemicals Industries Co., Ltd.) were used to prepare the mesoscopic TiO2 films. The main layer and scattering layer of the photoelectrodes were prepared using pastes containing TiO2 nanoparticles with size of ca. 20 and 400 nm, respectively. The TiO2 pastes were screen-printed onto fluorine-doped tin oxide (FTO, TEC7, 8 ohm/square) substrates and the film thickness was controlled by the printing cycles. After screenprinting, the films were subjected to gradual sintering process as described in a previous work.17 For dye adsorption, the TiO2 films were immersed in an ethanol solution containing 0.5 mM [NBu4]2[cis-Ru(4,4 -Hdcbpy)2(NCS)2] (Solaronix, N719 dye) for 20 h at room temperature, followed by rinsing with ethanol. An acetonitrile solution containing 0.1 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine (TBP), 0.1 M guanidine thiocyanate, and 0.6 M 1-propyl-2,3-dimethylimidazolium iodine (DMPII) was used as the electrolyte of DSSCs in this work. Platinum counter electrodes with high transmittance (ca. 75%) and high activity were prepared by a sputtering process onto ITO substrates (8 Ω/□, AimCoreTechnology Co., Ltd.) according to the method developed in a previous work.16 The deposition was performed using a DC sputtering equipment (Gressington 108 auto, Ted Pella, USA) under a base pressure of 10−2 Torr. A sputtering current of 40 mA was controlled which corresponds to a deposition rate of 0.11 ± 0.005 nm/s.16 To obtain a Pt film with high transmittance (75%), a film thickness as small as ca.1.5 nm is prepared by controlling the deposition time at ca. 14 s. For assembling of the DSSCs, a dye-coated TiO2 photoelectrode and a Pt-coated counter electrode were sandwiched using a 60-μm-thick sealing spacer (SX-1170-60, Solaronix SA). Two holes on the counter electrode were drilled for electrolyte injection. The active area of the cell was 0.25 cm2. To construct a tandem cell, the top and bottom cells were stacked in series as shown in Figure 1. The performances of the DSSCs were measured under one sun illumination (AM1.5, 100 mW/cm2) from a solar simulator

3. RESULTS AND DISCUSSION The effect of TiO2 film thickness on the performance of single DSSCs is studied first using N719 dye. Table 1 shows the Table 1. I−V Characteristic Parameters and Charge Transport/Recombination Resistances Measured for DSSCs Fabricated with the Various Thicknesses of TiO2 Films (AM 1.5G, 100 mW/cm2) TiO2 thickness (μm)

Jsc (mA/cm2)

Voc (V)

FF

η (%)

Rw (Ω)

Rk (Ω)

4 8 12 16 20

9.74 13.30 15.21 15.10 14.83

0.818 0.782 0.759 0.751 0.725

0.73 0.72 0.71 0.70 0.68

5.83 7.47 8.20 7.94 7.35

0.49 0.51 0.56 0.65 0.73

26.25 14.97 10.94 10.17 9.68

related parameters of the DSSCs prepared using various TiO2 main layers without a scattering layer. It shows that, with increasing film thickness, the short current density (Isc) increases steadily first, and then decreases slightly after approaching the highest value at 12 μm. The increase of Isc with increasing film thickness is attributed to the increasing amount of adsorbed dye which creates a larger amount of excited electrons under light illumination. However, the decrease of Isc for film thicknesses over 12 μm implies that the charge transport resistance and charge recombination effect become dominated when the thickness is higher than 12 μm. For the open circuit voltage (VOC) and filler factor (FF), both the values decrease steadily with increasing film thickness, 17447



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To study the thickness effect of top and bottom cells on the performance of a tandem cell, cells with various film thicknesses (4-, 8-, 12-, and 16-μm) are used as the top and bottom cells, respectively. First, the performances of the bottom cells were measured individually for various structures and the results are shown in Figure 3. It shows that significant short current

ascribed all to the increase of electron transfer resistance and of the charge recombination events. Electrochemical impedance spectroscopy (EIS) was used in this work to study the thickness effect on the charge transport behavior in the DSSCs. The parameters corresponding to the charge transport resistance in the TiO2 matrix (Rw), and to the recombination resistance at the photoelectrode/electrolyte interface (Rk) were measured and shown in Table 1. It shows that the Rw value increases, but the recombination resistance (Rk) decreases with increasing film thickness. The variations of Rw and Rk indicate that increase of film thickness triggers a higher transport resistance of excited electrons along the TiO2 matrix, as well as a more significant charge recombination in the electron transport routes. That is, the increase of film thickness is disadvantageous to the charge transport in a DSSC which is consistent with the decrease of VOC and FF shown in Table 1. Since a higher film thickness is advantageous to the dye loading and light harvest of a photoelectrode, the competition effect among dye loading, electron transport, and charge recombination results in an optimal film thickness to achieve the maximum Isc and overall conversion efficiency (η). It is found that the highest conversion efficiency also appears at the film thickness of 12 μm, identical to that the maximum Isc was observed. The maximum efficiency achieved by the single cell is 8.2%. It is noteworthy that the results listed in Table 1 are obtained for DSSCs using a high transmittance (75%) Pt counter electrode without a scattering layer. Therefore, the light harvest ability of the photoelectrodes can be evaluated by measuring the transmittance of the cells. Figure 2 shows the transmission

Figure 3. Photovoltaic parameters measured individually for the bottom cells under 1 sun (AM 1.5G, 100 mW/cm2) illumination on the top of the tandem cells. The thickness effect of the top and bottom cells on short-circuit current density (a), open-circuit potential (b), fill factor (c), and efficiency (d) are illustrated.

densities can be measured for these cells, indicating that the light transmitting through a top cell can be harvested and utilized by the bottom cell. For various top cells used in this work, the 4-μm cell (the smallest thickness) performs the highest Isc, attributed to the highest transmittance of the photoelectrode. Increasing the thickness of the top cell decreases the transmittance and, therefore, decreases the Isc. For the bottom cell effect, the Isc increases steadily with increasing thickness of the bottom cell, which is similar to the results obtained for the single cells shown in Table 1. For the 16-μm bottom cell, the Isc obtained using the transmitting light from a 4-μm top cell is 5.1 mA/cm2, which is about 52% the value obtained by the 4-μm top cell. Based on an illumination of one sun, the overall energy conversion efficiency achieved by this 16-μm bottom cell is 2.66% in the 4/16 tandem structure. When the thickness of the top cell increases, the Isc and η achieved by a bottom cell decrease, ascribed to the lower intensity of light transmitting through the top cell. However, an efficiency of 1.41% can still be obtained for the 16-μm bottom cell in the 16/16 tandem structure. Based on the results shown in Figure 3, the variation of energy conversion efficiency of the bottom cells (Figure 3d) resembles closely to that of Isc, indicating that the effect of film thickness on the performance of a bottom cell is mainly determined by the Isc. Figure 3b,c shows that the Voc and FF of the bottom cells have little dependence on the thickness of top cells, but decrease slightly with thickness increase of the bottom cells. It is noteworthy that the presence of a bottom cell in the tandem structure does not affect the performance of the top cell. Therefore, an additional energy conversion efficiency is

Figure 2. Transmittance spectra of N719-sensitized DSSCs with various thicknesses. The cells were prepared using a high transmittance Pt counter electrode without a scattering layer.

spectra measured for the N719-sensitized DSSCs with various film thicknesses. It shows that the 4-μm cell demonstrates the highest transmittance, indicating the least light harvest ability of the photoelectrode. The transmittance is especially higher in the long-wavelength range (>600 nm) and it decreases with increasing film thickness, indicating an increase of light harvest. However, the transmittance at 700 nm is still ca. 25% for the 12-μm cell, and 15% for the 16-μm one. These results indicate that the sunlight cannot be absorbed completely even by the electrode with film thickness as high as 16-μm. Therefore, it is possible to utilize the unabsorbed light by another cell positioned behind. This idea is performed using a tandem structure which composes two individual cells stacked in series as shown in Figure 1. 17448



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smaller than the sum of Isc measured independently for the top and bottom cells. This result is ascribed to the difference in the Voc of the two cells. Taking the 16/16 tandem cell as an example, the Isc measured independently for the top and bottom cells are 15.10 and 2.76 mA/cm2, respectively. If the two cells have identical potentials, the theoretical Isc of a parallelly connected tandem cell should be the sum of the individual cells (i.e., 17.86 mA/cm2). However, a difference in the Voc (0.75 and 0.68 V) of the two cells triggers a decrease of the Isc. Therefore, the measured Isc of the tandem cell (16.88 mA/cm2) is lower than the sum value. Especially for the tandem cells with a 4-μm bottom cell, the Isc of the tandem cells are closed to those of the top cells. This result is ascribed to the low ability of the 4-μm cell in light harvest. Since the Isc obtained by a 4-μm bottom cell is relatively smaller, the tandem cell got little benefit from the bottom cell (Figure 5a). However, for other tandem structures with a thicker bottom cell (8, 12, or 16 μm), significant gains of Isc can be observed. In the present study, the highest Isc (17.2 mA/cm2) is achieved by the 12/12 tandem cell. For the Voc of the tandem cell shown in Figure 5b, a parallelly connected bottom cell causes a lower Voc of a tandem cell in comparison with that of the top cell. This result is also ascribed to the mismatch between Voc of the two cells. However, the Voc of the tandem cell is only slightly lower than that of the top cell and, furthermore, does not decrease significantly with increasing thickness of the bottom cell. Therefore, the thickness effect of the cells on the overall efficiency of the tandem cells (Figure 5d) resembles closely to that on the Isc. The highest energy conversion efficiency achieved by the tandem cells is 9.14% which appears at the 12/ 8 tandem structure. Comparing to the highest efficiency achieved by the single cells (8.20% at 12 um), the gain of the tandem structure is 0.94% (from 8.20 to 9.14%). Effect of Scattering Layer in the Tandem Cells. For the cells described above, no scattering layer was used for all cells. It is known that the presence of a scattering layer behind the main layer can improve the light harvest of a photoelectode, increasing the efficiency of a DSSC. To compare the enhancement effect of the tandem cell to that of a scattering layer, a 4-um scattering layer is introduced behind the main layer. For this investigation, the TiO2 main layers were posttreated with a TiCl4 solution to improve the performance of the photoelectrodes. The results shown in Table 2 demonstrate that the efficiencies of the 8-μm and 12-μm cells increase to 8.54 and 8.93%, respectively, due to the TiCl4 treatment. When

obtained by the bottom cell. If the powers of top and bottom cells are output and utilized independently, the overall conversion efficiency of the tandem cell should be the sum of the efficiencies obtained by the top and bottom cells. These results are demonstrated in Figure 4. It shows that the optimal

Figure 4. Sum of the efficiencies obtained by the top and bottom cells of the tandem cells. The efficiencies of the two cells were measured independently.

performance was achieved by tandem cells using a 12-μm top cell. Combining with a 16-μm bottom cell, an overall conversion efficiency of 10.0% is achieved by the 12/16 tandem structure. For the real application of a tandem cell, the top and bottom cells are always connected in a circuit and their power were output and utilized simultaneously. In this work, the performance of a tandem cell is measured by parallelly connecting the top and bottom cells. The photovoltaic parameters measured for the tandem cells constructed by various top and bottom cells are shown in Figure 5. It shows that the tandem cells usually have higher Isc but slightly lower Voc than those obtained by the corresponding top cells (will be discussed later). It is also found that the Isc measured for a tandem cell is

Table 2. I−V Characteristic Parameters of Single and Tandem DSSCs Fabricated with the TiO2 Films Post Treated by TiCl4 (AM 1.5G, 100 mW/cm2)

Figure 5. Photovoltaic parameters measured for the tandem cells under 1 sun (AM 1.5G, 100 mW/cm2) illumination. The top and bottom cells are parallelly connected in the measurement and the thickness effect of the cells on short-circuit current density (a), opencircuit potential (b), fill factor (c), and efficiency (d) are illustrated.

electrode structure top/bottom cell

Jsc (mA/cm2)

Voc (V)

FF

η (%)

8 μm 12 μm 8 + 4 μma 12 + 4 μma 12/8 12/8 + 4 μma 12/8 + 4 um (bottom)a,b

15.45 16.22 17.41 17.99 18.23 19.12 5.06

0.779 0.775 0.762 0.760 0.759 0.759 0.722

0.71 0.71 0.69 0.68 0.69 0.70 0.74

8.54 8.93 9.15 9.30 9.54 10.16 2.70

a

The electrode with a 4 μm scattering layer. bThe efficiency of the bottom cell was measured independently. 17449



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substituted by two subcells, which decreases the charge transfer resistance in thick photolelctrodes. The experimental results show that the tandem structure can improve the cell performance more efficiently than does by a scattering layer. In this work, the optimal tandem structure is constructed by the (12/8) cell and the efficiency achieved is 9.54%. If a scattering layer is also utilized to improve the light harvest of the bottom cell, the efficiency of the cell (12/8 + 4) can further be improved to 10.16%, which is ca. 0.9% increment compared to the single cell. If the powers of top and bottom cells were output independently, an even higher conversion efficiency of 11.63% (8.93% +2.70%) can be performed.

a scattering layer was introduced, the efficiencies can further be increased to 9.15 and 9.30%. The gains of efficiency due to the scattering layer are 0.61 and 0.37%, respectively, for the 8- and 12-μm cells. Apparently, the effect of the scattering layer is more significant for the 8-um cell, ascribed to its higher transmittance. For the tandem cells with post-treatment of a TiCl4 solution, the highest efficiency is also achieved by the (12/8) structure and the measured efficiency is 9.54%. Apparently, the performance of the (12/8) tandem cell without scattering layer (9.54%) is higher than that of the (12 + 4) cell with a 4-um scattering layer (9.30%). This result indicates that the tandem structure is more efficient, than the scattering layer, in improving the performance of DSSCs. For the tandem cells described above, no scattering layer was used for both the top and bottom cells. Although the bottom cell has proved to be able to utilize the unabsorbed light from the top cell, it is not sure that the residue light can be completely harvested by the bottom cell. Based on the transmittance spectra measured for the tandem cells with a 12-um top cell (Figure 6), it shows that significant trans-

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AUTHOR INFORMATION

Corresponding Author

*Tel: 886-6-2757575 ext 62693. Fax: 886-6-2344496. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support for this research by National Science Council of Taiwan (NSC-102-2623-E-06-009-ET) and Research Center for Energy Technology and Strategy (D10223032) are gratefully acknowledged. This research was also supported in part by the Headquarters of University Advancement at the National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan.

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REFERENCES

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Figure 6. Transmittance spectra for the tandem cells constructed with a 12-um top cell and the bottom cell with various TiO2 thickness.

mittance can still be measured in the long-wavelength range (>550 nm). To further utilize this residue light, the scattering layer effect is utilized. This strategy was applied to the optimal tandem structure (12/8) by introducing a 4-um thick scattering layer to the bottom cell (8 + 4). The results shown in Table 2 demonstrate that the efficiency of the tandem cell (12/8 + 4) increases from 9.54 to 10.16% due to the introduction of a scattering layer. For the performance of the bottom cell (8 + 4) in the tandem structure, an efficiency of 2.7% was measured independently. Thai is, if the powers of top and bottom cells were output independently, the total conversion efficiency achieved by the tandem cell is 11.63%, which is more than 2% increment compared to the (12 + 4) single cell. Since only one dye (N719) was employed in this tandem cell, the increase of the conversion efficiency is attributed mainly to the decrease of charge transfer resistance in the tandem cell. These results clearly indicate that the tandem structure can be applied not only to increase the light harvest using various dyes, but also to increase the cell efficiency by facilitating the charge transfer.

4. CONCLUSION The results of this work demonstrate that the tandem structure can be used to facilitate the charge transport in the TiO2 films, increasing the conversion efficiency of DSSCs. By using a tandem structure, the light harvest of a thick TiO2 film can be 17450



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Charge Transfer and Performance Enhancement of Dye-Sensitized

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