Letter pubs.acs.org/NanoLett
High Efficiency Dye-Sensitized Solar Cells Based on ThreeDimensional Multilayered ZnO Nanowire Arrays with “Caterpillarlike” Structure Mallarie McCune, Wei Zhang, and Yulin Deng* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 500 10th Street N.W., Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: A 3D ZnO nanowire-based dye-sensitized solar cell (DSSC) with unique “caterpillar-like” structure was designed. Because of the significant improvement of the total ZnO nanowire surface area, the amount of light absorption was substantially increased. This increase in the light harvesting efficiency enables us to achieve an overall power conversion efficiency as high as 5.20%, which is the highest reported value to date for ZnO nanowire-based DSSCs. A branchedmultilayered design of ZnO nanowire arrays grown from ZnO nanofiber seed layers proves to be very successful in fabricating 3D ZnO nanowire arrays. Practically, electrospun ZnO nanowires were used as the seeds in multilayer growth of ZnO nanowire arrays with a unique “caterpillar-like” structure. This unique structure significantly enhances the surface area of the ZnO nanowire arrays, leading to higher short-circuit currents. Additionally, this design resulted in closer spacing between the nanowires and more direct conduction pathways for electron transfer. Thus, the open-circuit voltage was so significantly improved as a direct result of the reduction in electron recombination. KEYWORDS: Dye-sensitized solar cell, zinc oxide nanowire, three-dimensional structure
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than electron diffusivity within TiO2 nanoparticles.17,21−23 The superior electron transport within the nanowire photoanode can be attributed to its higher crystallinity and the presence of an internal electric field that facilitates electron transport to the collecting anode by effectively separating the injected electrons from the adjoining oxidized species of the electrolyte; this, in turn, improves the charge collection efficiency.17 However, researchers have yet to fabricate ZnO nanowire-based DSSCs with efficiencies similar or higher than TiO2 nanoparticle-based DSSCs. Typical reported efficiencies for ZnO nanowire-based DSSCs range from 0.1 to 3%.20,24−28 One of the approaches to increase surface area for improved light conversion efficiency is to increase the length of ZnO nanowires.27 Although 33 μm ZnO nanowires on transparent conductive glass has been successfully synthesized, the total solar cell efficiency is only 2.1%,27 which is significantly lower than that of TiO2 DSSCs. The low efficiency of ZnO nanowirebased DSSCs can mostly be attributed to the large void spaces that exist in the vertical nanowire array. Therefore, it is expected that increasing the length of nanowires alone is not a solution for low solar cell efficiency of ZnO nanowire-based DSSCs.
ver the past few decades, there has been a variety of methods used to assemble solar cells.1−10 In particular, dye-sensitized solar cells (DSSCs) have attracted much attention. Among other factors that influence the photovoltaic efficiency, the surface area of the employed nanostructure is critical important. Although TiO2-based DSSCs are most commonly used in DSSC solar cells, and their power conversion efficiency has reached as high as 12%,11 ZnO nanowire arrays have shown some advantages than TiO2 in solar cell applications. The wide attractions to ZnO nanostructures (stable wurtzite structure) stem from its unique properties and broad range of applications relevant to science today. Some desirable properties of ZnO include its wide bandgap (∼3.37 eV), large exciton binding energy (60 meV), visible light transparency, piezoelectricity, biocompatibility, structure and property controllability, and simplicity of synthesis process.12 ZnO has been extensively used in an assortment of applications ranging from photodetectors and photovoltaic devices to chemical and biological sensors.13−19 Utilizing wideband-gap ZnO nanowires instead of TiO2 nanoparticles has been thought to be very advantageous because the nanowire morphology allows for electrons to travel a more direct conduction path from the point of injection to the conductive glass substrate.20 The nanowire photoanode has a very fast electron injection rate, and the electron diffusivity in crystalline wires has been reported as several orders of magnitude larger © 2012 American Chemical Society
Received: April 14, 2012 Revised: June 6, 2012 Published: June 25, 2012 3656
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Figure 1. (a) Schematic displaying ideal 3D nanostructure growth using the spin-coating seed method. (b) Schematic displaying limitations of actual 3D nanostructure growth using the spin-coating seed method. (c) SEM image of five-layered ZnO nanowire arrays using the spin-coating seed method and 25 mM equimolar precursor solution concentration. (d) SEM image of five-layered ZnO nanowire arrays using spin-coating seed method and 50 mM equimolar precursor solution concentration. (e) SEM image of large crystal seeds clustered on nanowire sidewalls after the spincoating seed method.
previous layer of nanowires to continually initiate new ZnO nanowire growth. Thus, by continually repeating the seedinggrowth process, nanotree structures can be formed. If the seed particles do not all attach to the tips and sidewalls of the previous layer, then the density of the overall ZnO film can be restricted. The limitation on ZnO nanowire density was confirmed in our study. Experimentally, we first studied the formation of multiple layers of branched ZnO nanowire arrays by reapplying the seed solution on top of the previous layer of ZnO nanowire arrays via spin-coating followed by calcination at 250 °C. The seeded ZnO nanowires were used for next layer of ZnO growth with an equimolar concentration of zinc nitrate hexahyrate and hexamethylenetetramine. As shown in Figure 1b, most of the seed particles coated on top of the nanowire array will fall to the bottom of the nanowire array and fill the space between the nanowires, and only a few of the newly coated ZnO seeds will actually stay on top of the previous layer of nanowires for the growth stage of the subsequent nanowire layer. As a result, the seeds fall in the spaces between nanowires will block the electronic transfer from dye molecules to ZnO nanowires so the light conversion efficiency will be negatively affected. Figure 1c,d provides SEM images of these multilayered (specifically five layers) 3D-based ZnO nanowire arrays. In our first experiment, we were able to identify flaws in the formation of the ZnO seeds on the previous layer of ZnO nanowires. It was found that the spin-coating method allows the ZnO seeds to pass between the nanowires and attach to various positions along the sidewalls of the nanowires. This makes it very difficult for subsequent layer nanowires to grow in between the nanowires of the previous layer, and this can affect the density of the nanowire array. With this type of seed application the nanowires can only grow outward and/or upward from the base nanowire. We also found that the ZnO
Recently, three-dimensional (3D) branched nanowire heterostructures demonstrated more functionality and higher performance due to its significantly enhanced surface area. These 3D nanowire arrays are considered as desirable structures that can be used in applications such as photovoltaics and sensing.28−31 Branched tree-like ZnO nanowire structures were investigated by Ko et al.32 Specifically, they enhanced dye loading and light harvesting by synthesizing hierarchal ZnO nanowires photoanodes based on the combination of lengthwise growth and branched growth.32 They reported an overall conversion efficiency of 2.63%. Their report also claimed that electron recombination is reduced due to the direct conduction along the nanotree multigeneration branches.32 Specific to their research, the ZnO nanowires were grown from ZnO quantum dot seeds with a layer-by-layer growth method via in aqueous solution. Their work is important because they developed a new concept for fabricating high efficiency ZnO nanowirebased DSSC. However, the SEM pictures reported in their paper also clearly show that there are large voice spaces in the second layer of ZnO nanowire arrays. Because of the low density of ZnO above the first layer, the maximum overall conversion efficiency is only 2.63%, which is not high enough compared with other DSSC. Therefore, it is believed that improving the density of reported 3D “nanotrees” could significantly enhance the ZnO nanowire DSSC efficiency. In the process reported by Ko et al.,32 the seeding materials (ZnO nanodots) were drop-casted onto the top of previous array of ZnO nanowires. While the seeding method reported by Ko et al.32 successfully grew uniform branches of ZnO nanowires, this method also has a drawback in fabricating extremely highdensity arrays of 3D nanostructured ZnO nanowires. In an ideal case as shown in Figure 1a, it is expected that the ZnO seed particles will attach to the tips and along the sidewalls of the 3657
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increase in DSSC efficiency from 0.72% to 1.51%. However, the overall power conversion efficiency of 1.5% is not a significant improvement compared to traditional ZnO nanowire DSSCs. The above discussion and the primary results shown in Figure 1 and Table 1 suggest that a proper seeding method may significantly improve the 3D nanostructures, which will greatly enhance the overall conversion efficiency of ZnO nanowire DSSCs. Instead of spin-coating or drop-casting ZnO seeds on the top of previous grown nanowire array, a method of applying electrospun ZnO nanofibers on the top of postgrown nanowire arrays as seeds for next layer of nanowire growth was successfully developed. This technique offers a simple and low-costing way to produce highly dense 3D ZnO nanowire arrays that can be used in DSSCs to improve dye loading, increase light harvesting, reduce electron recombination, and significantly increase the overall power conversion efficiency of the cell. This technique eliminates the occurrence of seeds penetrating along the sidewalls of the ZnO nanowires. Instead, these nanofibers lie directly on top of the nanowire array, allowing the subsequent layers of ZnO nanowires to build upward in a clear and distinct fashion. Furthermore, the multiple arrays of ZnO nanowires will be able to grow in a 360° fashion along the entire length of the nanofibers, yielding multidimensional nanostructures. The electrospinning of a seed precursor solution containing polyvinylpyrrolidone (PVP) onto FTO substrates was conducted with 20 kV of applied voltage. The polymer-based seed solution was prepared by mixing 0.5 g of zinc acetate dihydrate to 15 mL of pure ethanol followed by the addition of 10 g of PVP powder. Upon application of the seed layer, hydrothermal growth with an equimolar concentration of zinc nitrate hexahydrate and HMTA was then carried out at 90 °C to grow the ZnO nanowires. Figure 2 illustrates the design process based on electrospinning of the nanofiber seeds. The key concept in this particular study is the benefits gained by controlling the 3D ZnO nanowire structures to maximize the light absorbance. The best fabrication method to grow such 3D nanowire array is to apply the ZnO seed layer via
seeds may fall between the nanowires of the previous layers upon reapplication of the seed layer via spin-coating, which makes it is even more difficult to grow subsequent layers from the top of the previous layer that are visibly distinct from former layers. Figure 1b illustrates the limitations of applying the seed layer via spin-coating of the ZnO seed solution. From this figure, it is clear that the seeds attach to random locations along the side walls of the nanowires, and some nanowires may not have seeds at the tips of the nanowires to grow distinct subsequent layers. Also, at some locations, the seed does not yield nanowires because it may be too difficult for the crystals to grow in between the nanowires, which results in the aggregation of the seeds between the nanowires as seen in the SEM image in Figure 1e. Obviously, these aggregates will block the light to reach the nanowires so the efficiency of such 3D ZnO nanowire arrays is negatively affected by the aggregates. Furthermore, it is also noted that in most ZnO nanowire arrays the distance between ZnO nanowires is only about 100 nm, while the nanowire length is usually a few micrometers. Clearly, it will be very difficult to grow the new ZnO nanowires from the sidewalls of the previous layer of ZnO nanowires. As a result, most ZnO seed particles that either fall to the roots of the nanowires or stick to the sidewalls of the nanowires cannot initiate new ZnO nanowire growth. The results of the 3D ZnO nanostructures prepared using the reapplication of the ZnO seeds via spin-coating is shown in Table 1. Our studies showed that an increase in precursor Table 1. Photovoltaic Data for DSSCs Incorporating Five Layers of 3D ZnO Nanostructures via Spin-Coating Seed Method and Two Different Equimolar Concentrations of Zinc Nitrate Hexahydrate and HMTA Growth Solution concn (mM)
Voc (V)
Jsc (mA/cm2)
FF
efficiency (%)
25 50
0.524 0.529
4.32 8.95
0.316 0.319
0.72 ± 0.06 1.51 ± 0.15
concentration from 25 to 50 mM for these 3D ZnO nanowire arrays results in an increase in ZnO nanowire density and an
Figure 2. Schematic design process of a DSSC based on multilayered ZnO nanowires. 3658
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Figure 3. SEM images: (a) ZnO nanofibers electrospun on FTO substrate; (b) two distinct layers of ZnO nanowire array; (c) close-up view of highly dense “caterpillar-like” ZnO nanowire structures; (d) multiple layers (3−5) of ZnO nanowire array illustrating the distinct layers of the “caterpillar-like” ZnO nanowire structures.
Figure 4. Multilayered ZnO nanowire arrays grown using LBL method with electrospun ZnO nanofibers as seed and a 50 mM precursor solution: (a) J−V curves; (b) table of photovoltaic characteristics.
nanowires and attaching to the side walls of the nanowires. Instead, nanofibers only lie directly on top of the previous array of nanowires, which results in distinct multiple layers of the
electrospinning of ZnO nanofibers. The use of the nanofibers as seed layers between the multiple layers of the ZnO nanowire arrays prevents seeds from penetrating in between the 3659
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Figure 5. Multilayered ZnO nanowire arrays grown using LBL method with electrospun ZnO nanofibers as seed and a 75 mM precursor solution: (a) J−V curves; (b) table of photovoltaic characteristics.
Figure 6. LHE at 510 nm of multilayered ZnO nanowire arrays grown using LBL method with electrospun ZnO nanofibers as seed; (blue) 75 mM precursor solution and (red) 50 mM precursor solution.
ZnO nanowire arrays. From Figure 3a, we can see that the nanofibers form a weblike pattern across the substrate. The nanowires have an average length of ∼2.5 μm, and there is clear distinction between the separate layers as shown in Figure 3b. We believe this is due to the fact that the ZnO nanofiber seed layers only attach to the top of the former ZnO nanowire array.
For the third layer and subsequent layers, the ZnO nanowire morphology becomes very interesting. In particular, we begin to see strong evidence of unique 3D “caterpillar-like” structures (see Figure 3c,d). Because of the electrospun nanofiber seeds, these “caterpillar-like” ZnO nanowire structures formed literally allow growth of the nanowires along a 360° path around the 3660
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lower than those of TiO2 nanoparticle-based DSSCs. There are two possible reasons. First, the dye absorption on ZnO nanowires is usually lower than TiO2 nanoparticle. Second, the density of ZnO nanowire structure is still lower than TiO2 nanoparticle film. We believe that further improvements can be achieved by performing studies on the application and makeup of the ZnO nanofibers in order to yield even more dense arrays of longer ZnO nanowires. This should further enhance the LHE and Voc, thus improving the overall power conversion efficiency.
entire length of the nanofibers. More importantly, the nanowires that form are extremely high in density (varying between 8 × 1010 and 1 × 1011 cm−2) and small in diameter (∼50 nm), and this contributes to enhanced light absorption that ultimately improves the conversion efficiency of the DSSC significantly. The results of the average conversion efficiencies of these multilayered structured ZnO nanofibers based on DSSC are remarkable in comparison with results from previous work. In the case where the precursor solution concentration was 50 mM, the average efficiency ranges from 0.91−2.45% efficiency, as seen in Figure 4. The precursor solution concentration was increased from 50 to 75 mM in this study to see if the conversion efficiency can be further improved. The increased precursor concentration results in an increase in the Zn2+ chemical potential; therefore, to balance the increased zinc chemical potential, more nuclei sites form which results in the increase of the ZnO nanowire density. Furthermore, an increase of Jsc should be the direct result of employing denser nanowire arrays in the DSSC. Figure 5 shows that the average efficiency ranges from 3.43 to 5.20%. A further increase in the precursor concentration of the ZnO growth solution did not show a difference from that made using a 75 mM precursor solution. Our experimental value of 5.20% is the highest reported value to date for 3D ZnO nanowire-based DSSCs. Ultimately, these results have shown that enhancing surface area and density through unique multilayered designs can replace the idea of synthesizing long nanowires in efforts to improve the overall efficiency of the DSSC. The ultradense nanowires formed allowed for a significant increase in Jsc. Moreover, these results verified that the enhancement of surface area and density is directly correlated to the increase in light absorption, which results in improved DSSC performance. The LHE was calculated by measuring the dye absorption amount on the ZnO array (see Supporting Information for details). It is evident from Figure 6 that the continual increase in layers of ZnO nanowire arrays provides increasing light harvesting efficiencies (LHE). For the 50 mM, case, the LHE increases from 17% to 35% with increasing layers. Similarly, for the 75 mM case, the LHE increases from 27% to 60% with increasing layers. Additionally, this design resulted in closer spacing between the nanowires as well as more direct conduction pathways which limited the amount of electron recombination. Thus, the Voc was also significantly improved as a direct result of the reduced electron recombination. Evident in Figure 4, the Voc increased from 0.51 to 0.57 as the layer of ZnO nanowire increased from 2 to 3. The added layer allowed for less open spaces between the ZnO nanowires, which reduces the amount of electron back transfer. As you continue to add more and more layers, the Voc essentially remains steady which may be because most of the void spaces between the ZnO nanowires have been eliminated. It should also be noted that the unlike the ZnO nanoparticle morphology in which the electron travels a random path increasing electron scattering and electron recombination, the ZnO nanowire morphology allows for a more direct conduction path. Because of the more direct conduction path, as shown in Figure 5, our maximum Voc is ∼0.7 compared to state of the art ZnO nanoparticle DSSCs that have maximum Voc values between 0.5 and 0.6. Furthermore, the efficiency of most ZnO nanoparticle DSSCs is under 5%. While our results are currently very impressive for 3D ZnO nanowire-based DSSCs, these efficiency results are still much
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation. The authors are grateful to Wei Mu (Georgia Institute of Technology, United States) and Wenji Zheng (Georgia Institute of Technology, United States) for their contributions to this research.
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