Pulsed Laser Deposition of CdSe Quantum Dots on Zn2SnO4

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Pulsed Laser Deposition of CdSe Quantum Dots on ZnSnO Nanowires and Their Photovoltaic Applications 2

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Qilin Dai, Jiajun Chen, Liyou Lu, Jinke Tang, and Wenyong Wang Nano Lett., Just Accepted Manuscript • Publication Date (Web): 23 Jul 2012 Downloaded from http://pubs.acs.org on July 23, 2012

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Pulsed laser deposition of CdSe quantum dots on Zn2SnO4 nanowires and their photovoltaic applications Qilin Dai§, Jiajun Chen§, Liyou Lu, Jinke Tang, and Wenyong Wang* Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071

§

These two authors contributed equally to this work.

*To whom correspondence should be addressed. Email: [email protected] (Wenyong Wang)

Abstract: In this work we report a physical deposition based, one-step quantum dot (QD) synthesis and assembly on ternary metal oxide nanowires for photovoltaic applications. Typical solution based synthesis of colloidal QDs for QD sensitized solar cells involves nontrivial ligand exchange processing and toxic wet chemicals, and the effect of the ligands on carrier transport has not been fully understood. In this research using pulsed laser deposition, CdSe QDs were coated on Zn2SnO4 nanowires without ligand molecules, and the coverage could be controlled by adjusting the laser fluence. Growth of QDs in dense nanowire network structures was also achieved, and photovoltaic cells fabricated using this method exhibited promising device performance. This approach could be further applied for the assembly of QDs where ligand exchange is difficult, and could possibly lead to reduced fabrication cost and improved device performance.

Key words: Quantum dots, pulsed laser deposition, nanowires, photovoltaics

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Semiconductor quantum dots (QDs) have been widely studied due to their interesting optical and electrical properties and have been applied in QD sensitized solar cells (QDSSCs) to harvest light energy and possibly circumvent the Shockley-Queisser efficiency limit for solar cells based on p-n junctions.1–5 By employing QDs with different sizes, they also have the capability to match the solar spectrum and thus improve light absorption.6,7 For QDSSCs based on colloidal QDs, the quantum dots are synthesized and assembled by solution based methods that involve encapsulating ligand molecules.8–11 Ligand molecules such as 3-mercaptopropionic acid (MPA) can limit the final growth size of the QDs and prevent them from aggregating together.12 However, the ligand molecules are also associated with certain disadvantages. First, the chemical synthesis of QDs with ligands involves toxic wet chemicals that present environmental hazards. Second, for certain types of QDs, such as PbS QDs, the ligand exchange processing is nontrivial and time consuming, and is not always repeatable. Third, the ligand molecules create additional transport barriers for photo-generated electrons, which limits the electron injection and collection efficiencies in QDSSCs. In fact, the chemical attachment of colloidal QDs to photoanodes and the effects of the ligand molecules on carrier transport and collection have not been fully understood. Therefore, the investigation of QD synthesis and assembly methods that do not involve ligand/linker molecules are necessary and will provide important insight on these issues.5 Ligand-free QD synthesis approaches such as chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), and atomic layer deposition (ALD) have been inspected to directly assemble QDs on photoanodes.5,13-16 Among them, CBD and SILAR are very successful at creating a high surface coverage of QDs with good anchoring to the photoanodes and improved cell performance.13–16 However, both approaches still involve wet chemistry and the control over QD size is limited.16,17 ALD deposition of QDs has recently been demonstrated, 2


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providing another possibility for fabricating ligand-free QDSSCs.5,18 In this work we report for the first time an efficient, one-step physical deposition approach for QD synthesis and assembly on nanowires for photovoltaic applications, which is based on the pulsed laser deposition (PLD) technique. Pulsed laser deposition is a clean and low-cost method for preparing semiconductor and metal thin films on various substrates, and it is particularly suited for the growth of nanostructured films.19 In PLD the interaction of a focused laser beam with the target leads to the production of pulsed supplies of energetic ions, molecules, and clusters, and high quality stoichiometric thin films can be deposited by optimizing the laser fluence and other growth parameters.19 Using PLD certain nanocrystals such as Au nanoparticles have been grown previously.20,21 However, to the best of our knowledge, a direct PLD assembly of QDs on nanowires, especially on those in network structures, for photovoltaic applications has not been reported. In this study, using the PLD technique CdSe QDs were assembled on nanowire photoanodes without the linker molecules, and the deposition required a much shorter time and had a higher reproducibility compared with chemical solution methods. For typical QDSSCs, the photoanodes are usually binary metal oxide nanoparticles such as TiO2 or ZnO nanoparticles. Compared with binary metal oxides, ternary oxides such as Zn2SnO4, SrTiO3, etc. exhibit better corrosion resistance and offer more freedom in the tuning of chemical and electrical properties.22–24 Compared with nanoparticle photoanodes, the reduced surface area and fast transport in nanowires can suppress the back electron transfer reaction that occurs at the photoanode/redox electrolyte interface and lead to improved open-circuit voltage.25,26 Therefore, in this research we utilized Zn2SnO4 nanowires as the photoanode, which is a ternary metal oxide and has been previously studied in dye sensitized solar cells.26 Zn2SnO4 nanowires were synthesized using a chemcial vapor depostion method, and CdSe QDs were coated using PLD

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with various laser fluences. Structural charateraizations were carried out to examine the size, size distribution, and crystalline structure of the deposited QDs. QDSSCs based on these structures were also fabricated and charaterized. Zn2SnO4 nanowires were synthesized on silicon substrates following the procedure published elsewhere,26 and the lengths of the nanowires were ~ 200 µm and the diameters were ~ 80 nm. After synthesis the Zn2SnO4 nanowires were ultrasonically removed in an ethanol solution

and

transferred

to

Transmission

Electron

Microscopy

(TEM)

grids

or

fluorine-doped-tin-oxide (FTO) coated glass substrates for QD deposition and structural characterization. A Nd:YAG laser with a wavelength of 266 nm and pulse repetition rate of 10 Hz was used for the PLD deposition. The coating of CdSe QDs on nanowires was carried out using different laser fluences of 6.4, 7.6, 9.6, and 12.6 J/cm2. The laser fluences were calculated by dividing the laser output energies by the ablation spot sizes on the CdSe target. The deposition was performed under a pressure of 10-3 torr in a vacuum chamber that has a fused quartz window for the UV laser beam to pass through. The laser beam went through a lens with a 30 cm focal length, which was placed at a position outside the chamber and focused all the laser energy on the target. The target was a bulk CdSe piece purchased from Alfa Aesar. The distance between the target and the substrate was 6 cm, and the deposition time was 15 min. Under these conditions the corresponding pumping energy for our Nd:YAG laser system to produce the 6.4 J/cm2 laser fluence was 28 J. If lower pumping energy was used the output laser energy was not high enough to generate a deposition. Figure 1 shows the TEM bright field images of axially periodic Zn2SnO4 nanowires coated with CdSe QDs. All of the images were taken along the [011] zone axis of the Zn2SnO4 nanowires, which provided a dark contrast and lattice fringe images of the nanowires. Under this 4


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condition, the interfaces between the CdSe QDs and Zn2SnO4 nanowires could be easily identified. The top row in Fig. 1 shows low magnification TEM images, while the corresponding high magnification ones are displayed in the bottom row. It can be seen from Fig. 1(a) or (e) that discrete CdSe QDs were deposited on the nanowire surface when the lowest laser fluence was used. These QDs were directly bonded to the Zn2SnO4 nanowire, and this unique type of structure produced a direct, clean interface between the QDs and the photoanode without any linker molecules, which is unavoidable in a chemical synthesis.27,28 Such linker molecules usually make the QD attachment unstable, compete with the QDs in light absorption, and create transport barriers for photo-generated electrons. As shown in Fig. 1(b) and (c) [or (f) and (g)], when the laser fluence became higher the QDs started to connect to each other and form a continuous film. If the laser fluence was further increased then a multilayer QD film was obtained [Fig. 1(d) or (h)]. Energy dispersive x-ray spectroscopy (EDS) measurement was conducted during the TEM examination, and the result is shown in Fig. 2(a). The EDS spectra confirmed that the coated material was CdSe, and the quantitative result showed that the atomic ratio of Cd and Se ranged from 1:0.98 to 1:1.03, indicating good stoichiometry transfer between the target and the deposited QDs. CdSe has two crystal structures: Wurtzite (hexagonal) or Zinc-Blende (cubic). The Zinc-Blende structure is more stable at room temperature, but it can transform into the Wurtzite structure at a slightly higher temperature of ~ 95 oC.29 Figure 2(b) shows the selected area electron diffraction (SAED) patterns that were taken along the [011] axis of the Zn2SnO4 nanowire. The diffraction spots revealed the single crystal structure of the Zn2SnO4 nanowire, and no twin boundaries were found in the sample. The CdSe QDs on the Zn2SnO4 nanowire generated continuous diffraction rings, suggesting that there was no epitaxial relation between

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the nanowire and the QDs. From the SAED patterns the crystal structure of the CdSe QDs was determined to be Zinc-Blende with a lattice constant of 6.086 Å, which was consistent with the diffraction reference data of 6.077 Å (Joint Committee on Powder Diffraction Standards card #88-2346). The difference between the two values was smaller than the measurement variation (0.5%), suggesting that there was no observable strain effect for the QDs deposited on the Zn2SnO4 nanowires. Figure 2(c) is a high resolution TEM image that shows the lattice fringes of both the CdSe QDs and Zn2SnO4 nanowire. Although both were cubic structures, the lattice constants were very different: aCdSe = 6.086 Å while aZn2SnO4 = 8.651 Å. The inset in Fig. 2(c) shows the measured angles between the lattice fringes of (1 11) Zn2SnO4 and (111)CdSe. The angles varied for all of the 24 CdSe QDs examined, further confirming that there was no epitaxial relation between the PLD-coated QDs and the Zn2SnO4 nanowires. From the TEM examination it was clear that the deposition of CdSe QDs on Zn2SnO4 nanowires followed the Volmer-Weber (island) growth mode, which could be attributed to the large lattice mismatch between the substrate (Zn2SnO4) and the coated material (CdSe) and the low wettability between the two.30 During such a growth, the adatom-adatom interactions were stronger than the interactions between the adatoms and the substrate surface, leading to the formation of three-dimensional adatom islands.20,30 Another observation was that higher laser fluences only led to denser QD coverage, not QDs with larger diameters. To further inspect the size distribution of the PLD-coated QDs, TEM images of QDs deposited on amorphous carbon TEM grids were obtained (see Supporting Information). For the high laser fluence deposition the TEM grid was covered by multilayer QDs, and the TEM was only focused on the top layer of the dots. From this TEM study it was observed that different laser fluences in fact produced QDs of similar mean diameter of ~ 5 nm, but the high fluence deposition generated a narrower size 6


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distribution. This could be explained by a QD size regulation through a self-sputtering mechanism in the PLD process as reported previously,21 which was a competing, secondary process and was caused by the high kinetic energy of the incoming species in the laser plume. Below a critical particle size, the QDs were unstable and could be easily sputtered off from the surface by the energetic incoming ions or atomic species, although they could also disappear via re-evaporation or diffusion to larger QDs. QDs larger than the critical size were not sputtered off and thus remained on the surface. On the other hand, since smaller QDs were sputtered off from the surface, their diffusion to other QDs to form even larger particles was less likely to occur. This self-limiting sputtering process led to a narrow size distribution, especially at high laser fluences.20,21 To investigate the PLD deposition of QDs in a nanowire network structure, cross-sectional scanning electron microscopy (SEM) images and EDS element mapping of QD-coated Zn2SnO4 nanowires were collected. To prepare the samples, Zn2SnO4 nanowires were first transferred to FTO-coated substrates using a printing transfer process, and then CdSe QDs were deposited. Figure 3(a) & (b) show the examination results. EDS signals of Cd and Se were very weak when the coating amount was low, therefore the sample shown in Fig. 3 was prepared using the highest laser fluence in this study (12.6 J/cm2) in order to generate the EDS element mapping patterns. The cross-sectional SEM image reveals that the nanowire network had a relatively dense structure; however, the PLD generated material species could penetrate into the network. The EDS element mapping result shown in Fig. 3(b) suggested that Cd and Se could reach the bottom of the nanowire network structure and nucleate on the nanowires during the deposition. Due to the mass transport nature of the PLD deposition, shadow effect was also observed in the coating process, i.e., the QD coating on the side of the nanowire that was facing the laser plume was

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thicker than that on the opposite side [Fig. 3(c)-(e)]. This inhomogeneity of QD deposition on nanowires could possibly be improved by employing vertically aligned nanowire arrays. Incident-Photon-to-Electron Conversion Efficiency (IPCE) spectra of QD-coated Zn2SnO4 nanowires in the wavelength range of 400 - 700 nm were acquired using a two-electrode configuration with a platium wire as the counter electrode, and the electrolyte was polysulfide (0.25 M Na2S and 0.1 M NaOH in 18 MΩ water). The typical I3-/I- electrolyte for dye sensitized solar cells is well known to be corrosive to the semiconductor QDs, and polysulfide electrolyte can maintain the stability of the QDs and is thus widely used in QDSSCs.9,31,32 Figure 4(a) shows the IPCE spectra that were acquired for both bare and CdSe QD-coated Zn2SnO4 nanowires. For bare Zn2SnO4 nanowires no photocurrent was detected, while for QD-coated Zn2SnO4 nanowires IPCE spectra were obtained, which were due to light absorption of the PLD deposited CdSe QDs. The IPCE spectra showed monotonic increase as the incident photon energy was scanned from 1.8 to 3.0 eV, but did not exhibit distinctive excitonic features such as those observed in colloidal QDs. The band diagrams of Zn2SnO4 nanowire and CdSe QD are shown in Fig. 4(b). The conduction band edge of the Zn2SnO4 nanowire is about 4.3 eV below the vacuum level,33 which is 0.3 eV lower than that of the bulk CdSe (-4.0 eV vs. vacuum).34 The PLD deposited QDs have a mean diameter of 5 nm, and according to previously reported theoretical and experimental studies the 5-nm CdSe QDs have a bandgap of ~ 2.0 eV.34,35 Hence the conduction band edge of the 5-nm CdSe QD is ~ 0.45 eV higher than that of the Zn2SnO4 nanowire, and the photo-excited electrons should be easily injected into the conduction band of the nanowire. Therefore, the absence of the CdSe excitonic features in the IPCE spectra could not be explained by an energy band misalignment between the two materials, but was mainly due to the broad size distribution of the PLD deposited QDs. Such a phenomenon has been widely observed in SILAR-based solar cells, 8


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and is believed to be not critical to the performance of QDSSCs.36–40 Figure 4(a) also shows that the IPCE efficiency decreased as the laser fluence was increased. Based on the TEM examination, at the lowest laser fluence the deposited CdSe QDs were separated on the surface of the nanowire. For the higher laser fluence coatings, since there were no encapsulating molecules separating the QDs, they directly connected to each other and formed continuous films. Such QD aggregation would cause the quenching of the quantum confinement effect due to the overlapping electron wavefunctions,41–43 and it also could increase the recombination loss of the photo-generated electrons inside the interconnecting QD structures and lower the IPCE efficiency.8,44–46 For the device that had the multilayer QD formation, as reported in previous studies of QDSSCs based on SILAR or colloidal QDs without surface capping molecules, the intermediate quantum dot layers did not have direct contact with either the photoanode or the electrolyte, which hampered the harvesting of both electrons and holes and could lead to a full recombination of the photo-generated carriers.44–46 This could possibly explain the near-zero photocurrent of the device fabricated using the highest laser fluence. The red-shift of the onset photon energy in the IPCE spectra [Fig. 4(a)] could also be explained by the formation of interconnecting QDs that caused a decrease in the effective energy bandgap of the CdSe QD film.37 Solar cells based on PLD-coated QDs were also fabricated and tested in this study. For the cell fabrication, an FTO substrate with QD-coated Zn2SnO4 nanowires was bonded to a Cu2S counter electrode through a hot-melt spacer, and polysulfide electrolyte was injected into the sandwiched space through pre-drilled holes. The Cu2S counter electrode was prepared by soaking a brass plate in 36% HCl at 60 oC for 10 min and then in the polysulfide electrolyte for another 10 min at room temperature. Current density vs. voltage (J-V) characteristics of the QDSSCs fabricated using different laser fluences were obtained under 1-Sun illumination and in

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the dark, and the results are shown in Fig. 4(c). The device fabricated using the lowest laser fluence of 6.4 J/cm2 showed the best performance in this study. The table in Fig. 4(d) summarizes the values of the short-circuit current density (JSC), open-circuit voltage (VOC), fill-factor (FF), and power conversion efficiency (η) of the fabricated QDSSCs. As it shows, the cell performance decreased as the laser fluence was increased, which was consistent with the IPCE measurement results. Many factors in a QDSSC structure can limit its performance, including the QD/photoanode interface that plays an important role in charge injection and recombination processes. However, how to characterize this interface effect remains a challenging subject, and no clear theory is currently available to accurately model such charge transport processes. Recently, Tvrdy et al. utilized the many-state Marcus theory to examine the interfacial electron transfer kinetics and investigated the effect of QD size quantization on charge injection from CdSe QDs to TiO2 nanoparticles.47–49 This model has also been previously used to study interfacial electron transfer reactions in dye sensitized solar cells.50,51 If a weak electronic coupling between the donor states of the CdSe QD and the continuum conduction band electronic states of the nanowire and a nonadiabatic electron transfer process could be assumed in the photovoltaic structures studied in this work, then this model could be utilized to provide certain insight on the electron injection process in these devices. The electron transfer rate constant from the many-state Marcus theory is expressed as:47,48

kET

2π = h

∫

∞

−∞

ρ (E) H (E)

2

1 4πλ kBT

−

e

( λ +∆G + E )2 4 λ kBT

dE ,

(1)

where kET is the electron transfer rate, h is the Planck’s constant, ρ(E) is the density of states in the metal oxide,

H ( E ) is the matrix element, kB is Boltzmann’s constant, λ is the 10


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reorganization energy, and ∆G is the change in the system free energy or the driving force for the electron transfer reaction. The driving force for the QDs deposited using laser fluences of 6.4, 7.6, and 9.6 J/cm2 was estimated to be 0.47, 0.32, and 0.25 eV, respectively, which showed a decrease in the devices fabricated with increasing laser fluences (see Supporting Information for the calculation). Unlike the dependence of kET on the driving force in the two-state Marcus theory where there exists an inverted region in which kET decreases with increasing driving force, in the many-state Marcus model, according to Eq. (1), the electron transfer rate kET always increases when the driving force increases: it exhibits either a sharp increase in the reorganization energy dominated region ( ∆G ≈ λ ) or a gradual increase in the density of state dominated region (∆G >λ ).48,52 Therefore, the decrease in the driving force in the photovoltaic cells fabricated using higher laser fluences indicated a reduced electron transfer rate at the QD/nanowire interface. The charge injection efficiency ηinj can be expressed as ηinj =

kET 1 = , kET + kREC 1 + kREC kET

where kREC represents the contributions from all the recombination processes.53,54 kREC would increase with enhanced recombination losses,36,55,56 and together with a decreased kET they would cause a decrease in the electron injection efficiency, which lowered the IPCE efficiency for the cells fabricated at higher laser fluences. Other charge loss mechanisms including the electron collection loss in a nanowire network structure could also play crucial roles in determining the overall cell efficiency, and further experimental and theoretical efforts are needed to fully characterize their contributions. In summary, in this work we developed a novel method to directly assemble semiconductor quantum dots on ternary metal oxide nanowires in network structures for photovoltaic applications. The direct coating was based on pulsed laser deposition, which did not involve toxic wet chemicals and avoided the processing of encapsulating ligand molecules. Compared 11


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with chemical solution methods, this PLD approach presented less environmental hazards and had higher reproducibility. Further improvement on cell performance could be achieved by optimizing quantum dot size and realizing a uniform monolayer deposition through fine-tuning the PLD coating parameters including laser energy, chamber pressure, deposition time, and substrate temperature. Utilizing vertically aligned nanowire arrays that are synthesized directly on FTO substrates could also help generate a uniform coating as well as enhanced charge collection efficiency. This physical deposition based approach could be utilized for the assembly of QDs where the ligand exchange processing is difficult, for example, the assembly of PbS QDs that exhibit the multiple exciton generation effect.3 It also could be utilized as a useful test platform to investigate other fundamental issues in QD sensitized solar cell structures.

SUPPORTING INFORMATION SEM and TEM images of Zn2SnO4 nanowires, schematic of the PLD setup, calculation of lattice parameters of PLD-coated CdSe QDs and Zn2SnO4 nanowires, size distribution of PLD-coated QDs, QDSSC fabrication process flow, and calculation of the driving force. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENT The authors would like to thank Dr. Weilie Zhou at the University of New Orleans for his kind assistance on TEM studies. We also would like to thank Drs. Bruce Parkinson and Hanchen Huang for helpful discussions. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-10ER46728.

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REFERENCES (1)

Luque, A.; Martí, A.; Nozik, A. J. MRS Bull. 2007, 32, 236–241.

(2)

Ruhle, S.; Shalom, M.; Zaban, A. ChemPhysChem 2010, 11, 2290–2304.

(3)

Sambur, J. B.; Novet, T.; Parkinson, B. A. Science 2010, 330, 63–66.

(4)

Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Science 2011, 334, 1530 –1533.

(5)

Dasgupta, N. P.; Jung, H. J.; Trejo, O.; McDowell, M. T.; Hryciw, A.; Brongersma, M.; Sinclair, R.; Prinz, F. B. Nano Lett. 2011, 11, 934–940.

(6)

Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2008, 130, 4007–4015.

(7)

Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737–18753.

(8)

Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241–246.

(9)

Li, L.; Yang, X.; Gao, J.; Tian, H.; Zhao, J.; Hagfeldt, A.; Sun, L. J. Am. Chem. Soc. 2011, 133, 8458–8460.

(10) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793–1798. (11) Kramer, I. J.; Sargent, E. H. ACS Nano 2011, 5, 8506–8514. (12) Bullen, C.; Mulvaney, P. Langmuir 2006, 22, 3007–3013. (13) Sankapal, B. R.; Sartale, S. D.; Lokhande, C. D.; Ennaoui, A. Sol. Energ. Mater. Sol. Cells

2004, 83, 447–458. (14) Mora-Sero, I.; Bisquert, J. J. Phys. Chem. Lett. 2010, 1, 3046–3052. 13


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(15) Tak, Y.; Hong, S. J.; Lee, J. S.; Yong, K. Cryst. Growth Des. 2009, 9, 2627–2632. (16) Baker, D. R.; Kamat, P. V. Adv. Funct. Mater. 2009, 19, 805–811. (17) Salant, A.; Shalom, M.; Hod, I.; Faust, A.; Zaban, A.; Banin, U. ACS Nano 2010, 4, 5962–5968. (18) Brennan, T. P.; Ardalan, P.; Lee, H.-B.-R.; Bakke, J. R.; Ding, I.-K.; McGehee, M. D.; Bent, S. F. Adv. Energy Mater. 2011, 1, 1169–1175. (19) Chrisey, D. B.; Hubler, G. K. Pulsed Laser Deposition of Thin Films; Wiley-Interscience: New York, 1994. (20) Donnelly, T.; Krishnamurthy, S.; Carney, K.; McEvoy, N.; Lunney, J. G. Appl. Surf. Sci.

2007, 254, 1303–1306. (21) Gonzalo, J.; Perea, A.; Babonneau, D.; Afonso, C. N.; Beer, N.; Barnes, J.-P.; Petford-Long, A. K.; Hole, D. E.; Townsend, P. D. Phys. Rev. B 2005, 71, 125420. (22) Tan, B.; Toman, E.; Li, Y.; Wu, Y. J. Am. Chem. Soc. 2007, 129, 4162–4163. (23) Zhang, Y.; Zhang, H.; Wang, Y.; Zhang, W. F. J. Phys. Chem. C 2008, 112, 8553–8557. (24) Wei, X.; Xu, G.; Ren, Z.; Xu, C.; Shen, G.; Han, G. J. Am. Ceram. Soc. 2008, 91, 3795–3799. (25) Huang, S. Y.; Schlichthorl, G.; Nozik, A. J.; Gratzel, M.; Frank, A. J. J. Phys. Chem. B

1997, 101, 2576–2582. (26) Chen, J.; Lu, L.; Wang, W. J. Phys. Chem. C 2012, 116, 10841–10847. (27) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. 14


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(28) Nevins, J. S.; Coughlin, K. M.; Watson, D. F. ACS Appl. Mater. Interfaces 2011, 3, 4242–4253. (29) Fedorov, V. A.; Ganshin, V. A.; Korkishko, Y. N. Phys. Status Solidi A 1991, 126, K5–K7. (30) Ohring, M. Materials Science of Thin Films: Deposition and Structure; Academic Press: San Diego, CA, 2002. (31) Chakrapani, V.; Baker, D.; Kamat, P. V. J. Am. Chem. Soc. 2011, 133, 9607–9615. (32) Lee, H. J.; Yum, J.-H.; Leventis, H. C.; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. I.; Gratzel, M.; Nazeeruddin, M. K. J. Phys. Chem. C 2008, 112, 11600–11608. (33) Alpuche-Aviles, M. A.; Wu, Y. J. Am. Chem. Soc. 2009, 131, 3216–3224. (34) Jasieniak, J.; Califano, M.; Watkins, S. E. ACS Nano 2011, 5, 5888–5902. (35) Baskoutas, S.; Terzis, A. F. J. Appl. Phys. 2006, 99, 013708–4. (36) Lee, H.; Wang, M.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Gratzel, M.; Nazeeruddin, M. K. Nano Lett. 2009, 9, 4221–4227. (37) Lee, H.; Leventis, H. C.; Moon, S.-J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nuesch, F.; Geiger, T.; Zakeeruddin, S. M.; Gratzel, M.; Nazeeruddin, M. K. Adv. Funct. Mater. 2009, 19, 2735–2742. (38) Xu, J.; Yang, X.; Wang, H.; Chen, X.; Luan, C.; Xu, Z.; Lu, Z.; Roy, V. A. L.; Zhang, W.; Lee, C.-S. Nano Lett. 2011, 11, 4138–4143. (39) Lee, H. J.; Bang, J.; Park, J.; Kim, S.; Park, S.-M. Chem. Mater. 2010, 22, 5636–5643. (40) Gonzalez-Pedro, V.; Xu, X.; Mora-Sero, I.; Bisquert, J. ACS Nano 2010, 4, 5783–5790. (41) Micic, O. I.; Curtis, C. J.; Jones, K. M.; Sprague, J. R.; Nozik, A. J. J. Phys. Chem. 1994, 15


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98, 4966–4969. (42) Algar, W. R.; Krull, U. J. ChemPhysChem 2007, 8, 561–568. (43) Zhang, Y.; Mi, L.; Wang, P.-N.; Ma, J.; Chen, J.-Y. J. Lumin. 2008, 128, 1948–1951. (44) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183–3188. (45) Guijarro, N.; Lana-Villarreal, T.; Mora-Sero, I.; Bisquert, J.; Gomez, R. J. Phys. Chem. C

2009, 113, 4208–4214. (46) Larramona, G.; Chone,, C.; Jacob, A.; Sakakura, D.; Delatouche, B.; Pere,, D.; Cieren, X.; Nagino, M.; Bayon, R. Chem. Mater. 2006, 18, 1688–1696. (47) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265–322. (48) Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. Proc. Natl. Acad. Sci. USA 2011, 108, 29–34. (49) Robel, I.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2007, 129, 4136–4137. (50) Stockwell, D.; Yang, Y.; Huang, J.; Anfuso, C.; Huang, Z.; Lian, T. J. Phys. Chem. C 2010, 114, 6560–6566. (51) Anderson, N. A.; Lian, T. Annu. Rev. Phys. Chem. 2005, 56, 491–519. (52) Tvrdy, K. Electron Transfer Reactions in Quantum Dot Sensitized Solar Cells. Ph.D. Dissertation, University of Notre Dame: United States - Indiana, 2011. (53) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. Int. J. Photoenergy 2002, 4, 17–20. (54) Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S. V.; Rogach, A. L. J. Phys. Chem. Lett. 2011, 2, 1879–1887. (55) O’Regan, B. C.; Durrant, J. R. Acc. Chem. Res. 2009, 42, 1799–1808. (56) Gregg, B. A.; Pichot, F.; Ferrere, S.; Fields, C. L. J. Phys. Chem. B 2001, 105, 1422–1429. 16


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Figure Captions Figure 1. PLD-coated CdSe QDs on Zn2SnO4 nanowires. Low and high magnification TEM images of CdSe QD-coated Zn2SnO4 nanowires using different laser fluences: (a, e) 6.4 J/cm2; (b, f) 7.6 J/cm2; (c, g) 9.6 J/cm2; (d, h) 12.6 J/cm2.

Figure 2. Structural characterizations of PLD-coated CdSe QDs. (a) EDS spectra of Zn2SnO4 nanowires coated with CdSe QDs using different laser fluences. The insets are enlarged plots for the Cd and Se peaks. The spectra are shifted for clarification purpose. (b) Selected area electron diffraction patterns of a Zn2SnO4 nanowire coated with CdSe QDs. The green and red arrows correspond to the diffractions from CdSe QDs and Zn2SnO4 nanowire, respectively. (c) A typical HRTEM image of CdSe QDs coated on a Zn2SnO4 nanowire. The (111) planes of the CdSe QDs and (1 11) planes of Zn2SnO4 were labeled in green and red, respectively. The inset shows the angles between the CdSe (111) planes and Zn2SnO4 (1 11) planes, which were measured from similar HRTEM images.

Figure 3. PLD QD deposition in a nanowire network structure. (a) Cross-sectional SEM image of the CdSe QD-coated Zn2SnO4 nanowires on an FTO substrate. (b) EDS element mapping of the boxed area in (a). The color map range for Zn and Sn is 0 - 100 counts, while the range for Cd and Se is set to be 0 - 20 counts in order to show their distribution more clearly. (c) TEM image of a Zn2SnO4 nanowire that shows an inhomogeneous QD coating due to the shadow effect. (d) and (e) are high magnification images of the corresponding areas specified in (c).

Figure 4. Performance of QDSSCs based on PLD-coated QDs. (a) IPCE measurement results of bare and CdSe QD-coated Zn2SnO4 nanowires. The coating was done using different laser fluences. (b) Energy band alignment between a Zn2SnO4 nanowire and a 5-nm CdSe QD. The band diagram of the bulk CdSe is also included. (c) J-V characteristics of the QDSSCs fabricated 17


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using different laser fluences. Dashed line is a typical J-V in the dark. (d) JSC, VOC, FF, and η of the fabricated QDSSCs.

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Figure 1. PLD-coated CdSe QDs on Zn2SnO4 nanowires. Low and high magnification TEM images of CdSe QD-coated Zn2SnO4 nanowires using different laser fluences: (a, e) 6.4 J/cm2; (b, f) 7.6 J/cm2; (c, g) 9.6 J/cm2; (d, h) 12.6 J/cm2.

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Figure 2. Structural characterizations of PLD-coated CdSe QDs. (a) EDS spectra of Zn2SnO4 nanowires coated with CdSe QDs using different laser fluences. The insets are enlarged plots for the Cd and Se peaks. The spectra are shifted for clarification purpose. (b) Selected area electron diffraction patterns of a Zn2SnO4 nanowire coated with CdSe QDs. The green and red arrows correspond to the diffractions from CdSe QDs and Zn2SnO4 nanowire, respectively. (c) A typical HRTEM image of CdSe QDs coated on a Zn2SnO4 nanowire. The (111) planes of the CdSe QDs and (1 11) planes of Zn2SnO4 were labeled in green and red, respectively. The inset shows the angles between the CdSe (111) planes and Zn2SnO4 (1 11) planes, which were measured from similar HRTEM images.

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Figure 3. PLD QD deposition in a nanowire network structure. (a) Cross-sectional SEM image of the CdSe QD-coated Zn2SnO4 nanowires on an FTO substrate. (b) EDS element mapping of the boxed area in (a). The color map range for Zn and Sn is 0 - 100 counts, while the range for Cd and Se is set to be 0 - 20 counts in order to show their distribution more clearly. (c) TEM image of a Zn2SnO4 nanowire that shows an inhomogeneous QD coating due to the shadow effect. (d) and (e) are high magnification images of the corresponding areas specified in (c).

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Figure 4. Performance of QDSSCs based on PLD-coated QDs. (a) IPCE measurement results of bare and CdSe QD-coated Zn2SnO4 nanowires. The coating was done using different laser fluences. (b) Energy band alignment between a Zn2SnO4 nanowire and a 5-nm CdSe QD. The band diagram of the bulk CdSe is also included. (c) J-V characteristics of the QDSSCs fabricated using different laser fluences. Dashed line is a typical J-V in the dark. (d) JSC, VOC, FF, and η of the fabricated QDSSCs. 22


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TOC Figure

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Figure 1. PLD-coated CdSe QDs on Zn2SnO4 nanowires. Low and high magnification TEM images of CdSe QD-coated Zn2SnO4 nanowires using different laser fluences: (a, e) 6.4 J/cm2; (b, f) 7.6 J/cm2; (c, g) 9.6 J/cm2; (d, h) 12.6 J/cm2. 358x182mm (150 x 150 DPI)


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Figure 2. Structural characterizations of PLD-coated CdSe QDs. (a) EDS spectra of Zn2SnO4 nanowires coated with CdSe QDs using different laser fluences. The insets are enlarged plots for the Cd and Se peaks. The spectra are shifted for clarification purpose. (b) Selected area electron diffraction patterns of a Zn2SnO4 nanowire coated with CdSe QDs. The green and red arrows correspond to the diffractions from CdSe QDs and Zn2SnO4 nanowire, respectively. (c) A typical HRTEM image of CdSe QDs coated on a Zn2SnO4 nanowire. The (111) planes of the CdSe QDs and (1-11) planes of Zn2SnO4 were labeled in green and red, respectively. The inset shows the angles between the CdSe (111) planes and Zn2SnO4 (11-1)planes, which were measured from similar HRTEM images. 416x492mm (150 x 150 DPI)


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Figure 3. PLD QD deposition in a nanowire network structure. (a) Cross-sectional SEM image of the CdSe QD-coated Zn2SnO4 nanowires on an FTO substrate. (b) EDS element mapping of the boxed area in (a). The color map range for Zn and Sn is 0 - 100 counts, while the range for Cd and Se is set to be 0 - 20 counts in order to show their distribution more clearly. (c) TEM image of a Zn2SnO4 nanowire that shows an inhomogeneous QD coating due to the shadow effect. (d) and (e) are high magnification images of the corresponding areas specified in (c). 395x342mm (150 x 150 DPI)


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Figure 4. Performance of QDSSCs based on PLD-coated QDs. (a) IPCE measurement results of bare and CdSe QD-coated Zn2SnO4 nanowires. The coating was done using different laser fluences. (b) Energy band alignment between a Zn2SnO4 nanowire and a 5-nm CdSe QD. The band diagram of the bulk CdSe is also included. (c) J-V characteristics of the QDSSCs fabricated using different laser fluences. Dashed line is a typical J-V in the dark. (d) JSC, VOC, FF, and η of the fabricated QDSSCs. 417x423mm (150 x 150 DPI)


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TOC Figure 426x190mm (150 x 150 DPI)


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Pulsed Laser Deposition of CdSe Quantum Dots on Zn2SnO4

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