Catalyst-Free Growth of Zinc Oxide Nanorod Arrays on Sputtered

Feb 8, 2011 - We report on the growth and characterization of zinc oxide nanorod arrays on indium-free transparent conducting oxide substrates, in par...
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Catalyst-Free Growth of Zinc Oxide Nanorod Arrays on Sputtered Aluminum-Doped Zinc Oxide for Photovoltaic Applications Jonas Conradt,*,† Janos Sartor,† Cornelius Thiele,† Florian Maier-Flaig,† Johannes Fallert,† Heinz Kalt,† Reinhard Schneider,‡ Mohammad Fotouhi,‡ Peter Pfundstein,‡ Volker Zibat,‡ and Dagmar Gerthsen‡ † ‡

Institut f€ur Angewandte Physik, Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany Laboratorium f€ur Elektronenmikroskopie, Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe, Germany

bS Supporting Information ABSTRACT: Incorporating tailored nanostructures into solar cells is a promising way to improve their photovoltaic conversion efficiency. For such solar cells, sputtered indium-doped tin oxide on glass is the standard substrate. As the global resources of indium are very limited, there is the challenge to move to other substrates and thus to synthesize nanostructures on these. We report on the growth and characterization of zinc oxide nanorod arrays on indium-free transparent conducting oxide substrates, in particular on sputtered aluminum-doped ZnO. We present a catalyst-free vapor-transport growth method at 510 °C, which allows for the growth of highly crystalline, well-aligned ZnO nanorods arrays with a tunable length, ranging from hundreds of nanometers to several micrometers. The nanorods exhibit an excellent crystal quality, as shown by photoluminescence measurements and high-resolution transmission electron microscopy. Optical transmission spectra show a sufficient transparency of the substrates, qualifying them for photovoltaic or optoelectronic applications. First solid-state dye-sensitized solar cells were built using nanorod arrays sensitized with a ruthenium-based dye and infiltrated with an organic hole conductor.

’ INTRODUCTION Current research on zinc oxide (ZnO) mainly focuses on its suitability for various nanodevices, e.g., gas or biomedical sensors, 1-3 optoelectronic devices, 4-6 and photovoltaic applications.7,8 For most of these applications, the ability of ZnO to form nanostructures of various morphologies in a selfassembling process is a key issue. Tailored ZnO nanostructures grown on transparent conductive oxide (TCO) substrates are of particular interest for photovoltaic applications, including excitonic solar cells (XSC)8 like dye-sensitized (DSC)9,10 and hybrid solar cells (HSC).11,12 Depending on the solar cell concept, different geometries are required in order to provide an optimized electrode morphology. For photovoltaic applications, high-temperature, carbothermal methods known to result in excellent crystalline quality of the nanostructures13 can be excluded as the TCO substrate is irreversibly damaged by the thermal stress. However, hydrothermal or chemical low-temperature processes generally result in nanostructures with a lower crystal quality as reported by Greene et al.14 Further, many nanostructures reported in the literature are grown on indiumdoped tin oxide (ITO) substrates.9,15,16 Here, economic problems are pending due to the limited availability of the resource indium. Obviously, there is a need for a solar cell technology based on alternative TCOs, such as aluminum-doped ZnO (AZO) allowing the growth of high-quality ZnO nanostructures. In this work, we present the structural and optical properties of ZnO nanorod arrays which are grown using a seed-free, vapor-solid r 2011 American Chemical Society

(VS) type process at a lowered temperature of 510 °C. Using this method, growth is possible on alternative substrates, such as AZO, without the risk of damaging the substrate. In particular, AZO is a promising TCO as it is indium-free and can be produced in large scale. It is shown that the nanorods can be tailored to the desired length by a simple variation of the growth parameters. This qualifies such nanorod arrays as electrodes both in hybrid solar cells requiring a nanorod length of several hundred nanometers and in dyesensitized solar cells demanding a nanorod length of several micrometers. Photoluminescence measurements indicate excellent crystalline quality and the incorporation of the dopant material. High-resolution transmission electron microscopy (HRTEM) confirms the excellent crystalline quality. Optical transmission spectra show a high transmission of the nanorod arrays, which is important for photovoltaic devices with light coupled in through the substrate. Exemplarily, a solid-state DSC was built, using ZnO nanorod arrays sensitized with a ruthenium-based dye and infiltrated with spiroOMeTAD as hole conductor.

’ EXPERIMENTAL METHODS The ZnO nanorod arrays were grown using a vapor-phase transport method in a horizontal tube furnace.17 The substrates, polycrystalline AZO (12 Ω/sq, provided by the Zentrum f€ur Sonnenenergie- and Wasserstoff-Forschung (ZSW), Received: September 17, 2010 Published: February 08, 2011 3539

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Figure 1. SEM images of samples grown on AZO at different positions in the tube furnace. The nanorod length steadily increases with decreasing distance to the oxygen inlet. Estimated average nanorod length: (a) 270 nm, (b) 490 nm, and (c) 1.0 μm. The inset shows the hexagonal shape of a nanorod tip (scale bar 200 nm). Images (d) and (e) show ZnO nanostructures grown on ITO.

Baden-W€urttemberg, Germany) or ITO (20 Ω/sq, Praezisions Glas & Optik GmbH, Germany) were thoroughly cleaned with acetone and isopropanol in an ultrasonic bath and placed into the tube near the oxygen inlet. A ceramic boat filled with pure zinc powder (Alfa Aesar 99.99%, 100 mesh) is used as the source, which is positioned in the tube close to the nitrogen inlet. The tube is evacuated to 5  10-4 mbar, flooded with nitrogen to 100 mbar, and then heated to 510 °C to start the evaporation of the zinc. The carrier gas flow (N2), transporting the zinc vapor toward the samples, is set to 9-30 mL/min. When the final temperature (510 °C) is reached, oxygen is supplied by an O2/N2 gas mixture (O2 20%, N2 80%, flow 1 mL/min) directed via an extra inlet near the substrate position in order to allow oxidation of the zinc. The pressure within the tube is constantly kept at 100 mbar. Adjusting the flow rates changes the local oxygen concentration and thereby governs the growth condition at the sample position. After approximately 15 min, the heating is switched off and the samples are cooled down to room temperature. The grown nanostructures were characterized by a LEO 1530 Gemini scanning electron microscope and an FEI Titan 80-300 high-resolution transmission electron microscope. For the latter, samples were prepared by mechanically scraping the nanorods from the substrate by a scalpel and dispersing them in high-purity ethanol. The nanorod/ethanol dispersion is ultrasonicated to minimize clustering of the nanorods. A small drop of the dispersion is then deposited on a circular copper grid covered by a thin holey carbon film. In order to simulate the oxygen gradient and gas flows inside the tube furnace, COMSOL Multiphysics (v3.5a) simulations were performed, coupling the nonisothermal flow, convection, and diffusion equations and using a 3D tube furnace geometry (see the Supporting Information). Photoluminescence measurements were performed at 11 K in a cryostat. The 325 nm line of a HeCd laser was used for excitation under an angle of incidence of 45° . The luminescence signal was dispersed by a spectrometer (1200 line/mm grating) and detected by a charged-coupled device (Andor iDus, LOT Oriel Europe).

Solid-state DSCs were built using the following procedure. For the sensitization, ZnO nanorod arrays were placed for 60 min at 65 °C in a 0.5 mM solution of cis-diisothiocyanato-bis(2,20 bipyridyl-4,40 -dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719, Solaronix SA, Switzerland) dissolved in pure ethanol. After the substrate was rinsed in ethanol, the hole conductor was spin-coated on to the sample (30 s wait, then 3000 rpm for 30 s). The hole transporting material used was 2,20 ,7,70 -tetrakis(N,N-dimethoxyphenyamine)-9,90 -spirobifluorene (spiro-OMeTAD), dissolved in chlorobenzene (0.15 M) with 0.12 M tert-butylpyridine and 9 mM Li salt. Finally, a 50 nm Au top electrode is thermally evaporated onto the samples, defining the active area of the cell (0.1 cm2). Current-voltage characteristics under illumination were recorded using a Keitley Sourcemeter 2400, a LOT Oriel solar simulator with a 150 W Xe lamp, and an AM1.5G filter.

’ RESULTS AND DISCUSSION The grown nanostructures were investigated by a scanning electron microscope (SEM). The nanorods grow almost perpendicular to the substrates, show a narrow length distribution on a centimeter lateral scale, and exhibit a hexagonal morphology (see inset to Figure 1c), indicating a growth along the c-axis [0001]. The series of Figures 1 a-c shows the dependence of the nanorod morphology on the position relative to the oxygen inlet within the tube. The length of the nanorods increases from sample to sample with decreasing distance (several tenths of a centimeter) to the oxygen inlet. The nanorod dimension ranges from 200 nm to several micrometers in length and from 40 nm to 1.2 mm in diameter. In order to understand this finding, the growth process is discussed in more detail in the following. During the preheating of the tube furnace, the nitrogen carrier gas flow is switched on, unlike the oxygen flow. When the source material has reached a certain temperature, zinc vapor is produced which is transported through the tube by the carrier gas flow. A thin zinc seed layer may form on the substrates by this vapor. No significant differences between the seed layers on the 3540

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Figure 2. Photoluminescence spectra of short and long ZnO nanorods grown on AZO (dashed red and solid blue line) and on ZnO nanorods grown on ITO (dotted green line), revealing several pronounced peaks which can be assigned to the free excitons (FXA), neutral donor bound excitons (D0X), and surface bound excitons.

substrates placed at different positions in the tube are expected, as at this point the artificial air flow is not yet started and the tube furnace is uniformly heated. When the whole furnace has reached its final temperature, the artificial air flow is started, providing oxygen by the central inlet. The gradual increase of the nanorod dimension toward this inlet can thus basically be explained by the local amount of oxygen available at the different positions during the growth process. The local oxygen concentration is therefore the essential parameter controlling the growth. Taking advantage of this effect, the nanorod length can also be controlled by the carrier gas flow rate. If this flow is increased, the oxygen concentration at a fixed position decreases simultaneously resulting in shorter rods. The nanorod length can thus be controlled by either position or carrier gas flow rate. Simulations of the gas flows with COMSOL Multiphysics using a 3D tube furnace geometry confirm this gradient in oxygen concentration on the same scale as seen in the experiment (see the Supporting Information). In order to grow ZnO nanorods with a homogeneous length on larger areas, a more sophisticated tube furnace with multiple oxygen inlets is required. The wide range of tunable nanorod lengths makes them suitable as electrodes in hybrid solar cells as well as in dyesensitized solar cells. Short nanorods in the range of 100-500 nm are of particular interest for hybrid solar cells.11 Excitons generated by incident photons in the organic layer require an interface nearby in order to dissociate into free carriers. This interface, providing energetically more favorable states for the electrons and thereby allowing to overcome the strong binding energy of the excitons, is given by the nanostructured ZnO electrode. The small exciton diffusion length (commonly around 10 nm18) in the organic semiconductors surrounding the nanorods therefore requires a dense array of short nanorods. In addition, the ZnO nanorods improve the electron extraction due to their higher electron mobility compared to the organic material. For bulk-heterojunction organic solar cells, this interface is provided by acceptor molecules mixed into the absorbing donor polymer. In this case, the bottleneck is not the dissociation at a nearby interface; it is rather the extraction of carriers from the organic material before their recombination. In this case, the

ZnO nanorods are expected to improve the electron extraction from the organic material by providing a fast and direct pathway toward the electrode. However, longer nanorods with a length of several micrometers are suitable as electrodes for DSCs9 where a large dyecovered area is required for sufficient absorption of the sunlight. In addition, the ZnO nanorods provide a more direct and faster pathway for the electrons toward the electrode9 than layers of nanocrystalline particles commonly used.19-21 The presented growth method of nanorod arrays on AZO does not rely on a catalytic metal as many others do.13 Deposition of an Au layer prior to the growth does not change the nanorod morphology significantly. In addition, no Au droplets at the tips of the nanorods can be seen by SEM, which would be typical for the vapor-liquid-solid (VLS) mechanism. This indicates that even in the presence of Au the underlying growth mechanism is of a VS- and not of a VLS-type. Growth of ZnO nanostructures on ITO using a VS-type process is also possible (Figure 1d), but of less interest for photovoltaics, as indium-based substrates should be avoided. The growth on ITO was recently also reported by Chen et al.22 The orientation of the nanostructures on ITO is much more random than the one on AZO using the same growth conditions. To clarify the reason for this difference, further investigations are required. We performed photoluminescence (PL) measurements to investigate the crystalline quality of the grown ZnO nanostructures. The samples were cooled to 11 K and excited at 325 nm under an angle of incidence of 45°. Figure 2 depicts typical photoluminescence spectra of the nanostructures. A contribution to the luminescence originating from the substrate can be excluded as measurements on substrates before growth did not give a detectable signal. The recorded luminescence of the following measurements can thus exclusively be attributed to the grown nanostructures. The free exciton (FXA) line at 3.376 eV can be seen in all measurements. The presence of this line indicates a highly crystalline quality of the grown ZnO nanostructures. The free exciton line is followed by several bound exciton lines. The peak 3541

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Figure 3. HRTEM image of a single nanorod grown on AZO. Continuous crystal planes can be seen, confirming the excellent crystal quality of the ZnO nanorods.

at 3.312 eV can be assigned to surface bound excitons,23 accompanied by its phonon replica at 3.253 eV. A contribution of donor-acceptor pair transitions is indicated by the peak at 3.235 eV24 and its phonon replica at 3.164 eV. A close-up of the high-energy part of the PL spectra is shown on the right side of Figure 2. The spectral positions of the neutral donor bound exciton (D0X) lines I4, I6, and I925 are indicated. For nanostructures grown on AZO (solid and dashed lines), the I4 and I6 lines associated with hydrogen and aluminum point defects, respectively, are significant. On the other hand, for nanostructures grown on ITO (dotted line) the I9 line dominates, which is related to the presence of indium point defects. These findings indicate the diffusion of Al and respectively In from the substrate into the nanostructures (see also refs 17 and 26). Since Al and In act as donors in ZnO, their incorporation into the rods is actually favorable for the transport properties. In order to cross-check the crystalline quality of the nanorods, high-resolution transmission electron microscopy was performed on single rods. The TEM samples were prepared by mechanically scraping the nanorods from the substrate, dispersing them in ethanol, and depositing a drop of the dispersion on a thin holey carbon film. On the HRTEM images (Figure 3), continuous crystal planes can be seen, which are perpendicular to the growth direction and exhibit an interplanar distance of 5.2 Å corresponding to the lattice constant of the c-axis of ZnO.27 No defects are visible, except for a thin amorphous shell. This shell was analyzed using energy dispersive X-ray spectroscopy (EDXS). It consists of carbon, most likely from remaining ethanol of the preparation process (see Experimental Methods). The HRTEM investigation confirms the excellent crystalline quality deduced from the photoluminescence measurements. For most photovoltaic devices, the light is coupled in through the TCO substrate on which the nanostructures are grown. A good optical transmission of the substrate and a low-absorbing nanostructure grown on top is therefore a key issue. Despite this fact, many publications on nanostructures for photovoltaic applications do not comment on this requirement. The substrates, consisting of sputtered AZO on 1 mm thick float glass (Figure 4, dotted line), exhibit an average transmittance of 80% in the visible spectral range, which is comparable to other TCOs as ITO or fluorine-doped tin oxide (FTO). Corrected for the loss introduced by the substrate, the transmittance of the electrodes with short ZnO nanorods reaches 75% (Figure 4, solid line). The actual transmittance value is certainly higher, as scattered light is not detected by the setup. In comparison, the nanostructures grown on ITO show a brownish staining, using the same growth

Figure 4. Transmittance of the AZO/glass substrate (dotted line) and of the short nanorods on AZO (solid line), corrected for the loss of the AZO/glass substrate.

Figure 5. Current-voltage characteristics of a solid-state DSC incorporating ZnO nanorods grown on AZO. The solar cell has a fill factor of FF = 57.5%, an open circuit voltage of VOC = 0.47 V, and an external quantum efficiency of η = 0.14%. The layer sequence of the cell is AZO | ZnO nanorods | N719 | spiro-OMeTAD | Au (see inset). The red curve is recorded under illumination (100 mW/cm2), and the blue, dashed curve without illumination.

process and conditions as for AZO. This staining results in a transmittance of the substrates below 40% and therefore disqualifies them for photovoltaic applications. Finally, a solid-state DSC was built using an AZO substrate covered with ZnO nanorods. Solid-state DSCs exhibit a solid, nonvolatile hole-transporting material and thus an advantage over DSCs, which commonly rely on a liquid electrolyte based on a volatile solvent. In general, solid-state DSCs do not require an extensive encapsulation, but suffer from a lower photovoltaic conversion efficiency. The nanorod array in these cells was sensitized with a ruthenium-based dye (cis-diisothiocyanato-bis(2,20 -bipyridyl4,4 0 -dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719)) and afterward infiltrated with spiro-OMeTAD as the hole conductor. Finally, a 50 nm Au top electrode was deposited by thermal evaporation. The current-voltage characteristics of the 3542

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The Journal of Physical Chemistry C cells were measured under AM1.5G condition (100 mW/cm2); see Figure 5. An open circuit voltage (VOC) of 0.47 V, a fill factor (FF) of 57.5%, and an external quantum efficiency (EQE) of η = 0.14% were calculated. To our knowledge, no solar cell with VS-type grown nanorods arrays and using this layout (ZnO nanostructure | dye | spiroOMeTAD | metal electrode) has been reported yet. Only devices using hydrothermally grown ZnO nanorods can be found in the literature: Plank et al.28 report an EQE of η = 0.072% (FF = 42%) for ZnO nanowires and up to η = 0.33% for a core (ZnO)-shell (MgO) system. In 2009, they achieved a conversion efficiency of η = 0.25% (FF = 44%) for nanowires directly covered with an organic dye (D149),29 which is a more optimized dye for the sensitization of ZnO.30,31 Keeping in mind that the presented solar cells have not yet been optimized in terms of the ZnO nanorod length, thickness of the hole-conducting layer (spin-coating parameters), and sensitization, and that the fill factor of the cells is good, it is expected that the EQE can be significantly improved for this system.

’ CONCLUSIONS We successfully synthesized ZnO nanostructures with a tunable length on sputtered aluminum-doped ZnO/glass substrate and on indium-doped tin oxide using a low-temperature vaporsolid method. This method neither damages the TCO substrate nor requires any catalytic metal layer. Photoluminescence measurements indicate the successful incorporation of Al/In dopants. In particular, the nanorods grown on AZO exhibit an excellent crystal quality, which is confirmed by HRTEM images. The achieved optical transmission of the ZnO nanorods on AZO is sufficient for electrodes in photovoltaic or optoelectronic applications. Solid-state dye-sensitized solar cells based on the investigated ZnO nanorod arrays exhibiting a reasonable external quantum efficiency are presented. ’ ASSOCIATED CONTENT

bS

Supporting Information. COMSOL simulations: drawing of the 3D-model (Figure S1), table of the boundary conditions (Table S2), cross section of the tube furnace showing the simulated oxygen concentration (Figure S3), sketch visualizing the gas flow near the oxygen inlet (Figure S4). 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 has been supported by the DFG Research Center for Functional Nanostructures (CFN) Karlsruhe (project F1.1 and F1.2) and a grant from the Ministry of Science, Research and the Arts of Baden-W€urttemberg (Grant No. Az. 7713.14-300). J. C. acknowledges financial support from the Karlsruhe School of Optics and Photonics (KSOP). Furthermore, we acknowledge the Zentrum f€ur Sonnenenergie- und Wasserstoff-Forschung Baden-W€urttemberg (ZSW) and the Light Technology Institute (LTI) at Karlsruhe Institute of Technology (KIT) for the supply of AZO substrates, and Peter Marek (Institute of Nanotechnology, KIT) for the preparation of the spiro-OMeTAD.

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