Facile Synthesis of Colloidal CuO Nanocrystals for Light-Harvesting Applications

Lim, Yee-Fun; Choi, Joshua J.; Hanrath, Tobias

doi:https://doi.org/10.1155/2012/393160

Journal of Nanomaterials

On this page

Abstract Introduction Experimental Results and Discussion Conclusions Acknowledgments References Copyright Related Articles

Special Issue

Nanocrystals for Electronic and Optoelectronic Applications

View this Special Issue

Research Article | Open Access

Volume 2012 | Article ID 393160 | https://doi.org/10.1155/2012/393160

Facile Synthesis of Colloidal CuO Nanocrystals for Light-Harvesting Applications

Yee-Fun Lim,¹Joshua J. Choi,¹and Tobias Hanrath¹

Academic Editor: Fan Zhang

Received25 May 2011

Accepted14 Jul 2011

Published14 Sept 2011

Abstract

CuO is an earth-abundant, nontoxic, and low band-gap material; hence it is an attractive candidate for application in solar cells. In this paper, a synthesis of CuO nanocrystals by a facile alcohothermal route is reported. The nanocrystals are dispersible in a solvent mixture of methanol and chloroform, thus enabling the processing of CuO by solution. A bilayer solar cell comprising of CuO nanocrystals and phenyl-C61-butyric acid methyl ester (PCBM) achieved a power conversion efficiency of 0.04%, indicating the potential of this material for light-harvesting applications.

1. Introduction

Solution-processable solar cell technologies can enable the realization of low-cost and high-throughput photovoltaic production [1]. While much work has previously focused on organic semiconductors [1], colloidal inorganic semiconductor nanocrystals (NCs) are starting to attract attention for photovoltaic applications [2]. NCs have the advantages of being solution-processable, capable of absorbing a large fraction of the solar spectrum, and tunable band-gap due to quantum-confinement effects [2]. While early NC solar cell work was based on CdSe [3] and CdTe [4], impressive results have been achieved with CuInSe₂ [5], PbSe [6–10], and PbS [11–15] NCs in recent years. In particular, Sargent and coworkers have reported a PbS NC/TiO₂ bi-layer solar cell with a power conversion efficiency (PCE) as high as 5.1% [15].

The above-mentioned materials may be good test-beds for studies of NCs as photovoltaic materials but may not be feasible candidates for wide-spread deployment due to toxicity and availability—Cd and Pb are toxic heavy metals, while In and Te are among the least abundant elements in the Earth’s crust [16]. It is thus desirable to explore other alternative solar cell materials. Copper- and iron-based semiconductors have emerged as attractive materials from an analysis by Wadia et al. based on abundance and cost [17]. For instance, Wu et al. [18] recently reported a Cu₂S NC-based solar cell with a promising PCE of 1.6%. Copper (I) oxide (Cu₂O) and copper (II) oxide (CuO) are also attractive candidates for light-harvesting applications due to their band gap energies of 1.4 eV (indirect) for CuO [19] and 2.0 eV (direct) for Cu₂O [20] that are quite close to the ideal band-gap for a single junction photovoltaic cell estimated from detailed balance [21]. Cu₂O has been investigated as a solar cell material for several decades [22], with recent reports of PCE up to 2.0% [23–25]. CuO has been employed in photo-electrochemical cells [26, 27] and as a cathode for dye-sensitized solar cells [28]. The use of CuO as the active layer in solid state solar cells has, to the best of our knowledge, not yet been investigated, and is the focus of this work. From a detailed balance analysis, CuO with a band gap of 1.4 eV can reach an ultimate solar cell efficiency of almost 30%, significantly higher than the 20% limit for Cu₂O (2.0 eV band gap) [21].

Copper oxide NC syntheses via various routes have been reported in the literature [29–44]. Relatively few of these reports, however, discuss the dispersibility of copper oxide NCs in common solvents [41–44], which is critical for solution-based processing of NC thin film absorber layers. In particular, Yuhas and Yang [43] and Hung et al. [44] reported solar cells based on films spin-coated from Cu₂O NC solutions, with PCE from 0.05% to 0.14%. In this paper, the synthesis of colloidal CuO NCs by a facile alcohothermal method is reported. The NCs are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and UV-visible absorption spectroscopy. Finally, as a proof of concept, a bilayer solar cell-based on CuO and phenyl-C₆₁-butyric acid methyl ester (PCBM) is demonstrated.

2. Experimental

2.1. Synthesis of CuO NCs

0.29 g of copper (II) acetate (Sigma Aldrich) was added to 30 mL of reagent alcohol (Sigma Aldrich) under vigorous stirring. 1 mL of deionized (DI) water was added, and the mixture was heated to 75°C. In a separate container, 1.3 mL of 25% tetramethylammonium hydroxide (TMAH) in methanol (Sigma Aldrich) was added to 10 mL of reagent alcohol. After 15 minutes of stirring, when the copper acetate has fully dissolved, the TMAH solution was gradually added over 5 minutes in regular intervals. The reaction was allowed to proceed at 75°C for 60 minutes, and the resultant product was collected by precipitation with hexane and then centrifuging at 3750 rpm for 5 minutes.

2.2. NC Characterization

TEM samples were prepared by drop-casting very dilute NC suspensions in methanol onto carbon grids (Electron Microscopy Sciences), and images were taken using an FEI T12 Spirit TEM. X-ray diffraction was performed on a Bruker General Area Detector Diffraction System (GADDS). A Thermo Scientific Nicolet iS10 FT-IR spectrometer was used to perform FTIR spectroscopy on NC samples drop-casted from methanol. These NCs have been washed and centrifuged three times with reagent alcohol to get rid of unreacted precursors. UV-visible absorption spectroscopy was performed using a Shimadzu UV-3101PC UV/Vis/Near-IR Spectrophotometer. All film thickness measurements were done using a Tencor P10 Profilometer.

2.3. Solar Cell Fabrication and Testing

Solar cells were fabricated on prepatterned indium tin oxide (ITO) coated glass substrates (Kintec, Hong Kong), which were cleaned by sonication in a mild detergent, rinsed in deionized water, dried in a nitrogen stream, and treated with a 10-minute UV-ozone exposure. CuO NCs were dispersed in a solvent mixture of 2 : 1 chloroform and methanol at a concentration of ~10 mg mL⁻¹, and then spin-coated on top of the ITO at 2000 rpm to give a film ~40 nm thick. A cell with a thicker CuO layer (~70 nm) was also fabricated by performing the spin-coating step 3 times. This is possible because the underlying layer is not completely dissolved during the spin-coating of subsequent layers. PCBM solution (20 mg mL⁻¹ in chloroform) was then spin-coated on top of the CuO at 2000 rpm to give a film ~120 nm thick. Finally, 4 Å of CsF and 400 Å of Al were thermally evaporated under high vacuum (~10⁻⁶ Torr) to form the cathode for the devices. A shadow mask was used in the evaporation to define a device-active area of 3 mm². Control PCBM-only solar cells (without CuO) were also fabricated.

Device current-voltage curves were obtained with a Keithley 236 source-measurement-unit (SMU) in the dark and as well as under AM 1.5 100 mW cm⁻² illumination from a Solar Light 16S-002 solar simulator. Light output power was calibrated using a Newport 818P-010-12 thermopile high power detector, which has a flat response over a broad spectral range. EQE measurements were performed using a Newport 1000 W xenon lamp coupled to an Oriel Cornerstone 260 1/4 m monochromator as the light source, a Keithley 236 SMU to measure short circuit current, and a Newport 918D-UV3-OD3 low power detector to monitor the light intensity.

3. Results and Discussion

The CuO NC synthesis is modified from a previously reported method for the synthesis of soluble zinc oxide NCs [9, 45]. Copper (II) acetate, Cu(OAc)₂, was dissolved in reagent alcohol and then reacted with tetramethylammonium hydroxide (TMAH) to form copper (II) hydroxide. CuO was then formed through heating:

Figure 1(a) shows TEM image of CuO NCs, in which it can be seen that they have a rather large size distribution of around 4-5 nm. The relatively broad size distribution of the CuO NCs is likely the result of the nucleation and growth dynamics of the reaction. Unlike the “hot-injection” synthesis [46], the simplified alcohothermal method lacks a single well-defined nucleation event leading to broader NC diameter distributions. X-ray diffractograms (Figure 1(b)) of the NCs are consistent with the literature values for CuO (JPCD no. 05-0661). It was observed that the addition of a small amount of deionized (DI) water (3% by volume) during the CuO synthesis produced NCs with more well-defined and narrower XRD peaks as compared to NCs synthesized without water. In previously reported syntheses of ZnO nanoparticles with zinc acetate in alcohol [47, 48], it was observed that the role of water was to increase the concentration of Zn²⁺ ions in the solution, since zinc acetate is more soluble in water than alcohol [47, 48]. Thus, it seems reasonable to arrive at a similar conclusion for the CuO synthesis, that the role of water is to promote the forward reaction due to a higher concentration of Cu²⁺ ions in the solution. Fits of the XRD peaks to the Scherrer equation for the NCs synthesized with water gave a CuO NC size of nm, which is consistent with the average NC size discerned from the TEM image.

(a)

(b)

Importantly, the CuO NCs can be dispersed in a 2 : 1 solvent mixture of chloroform and methanol (see inset in Figure 1(a)). To understand the dispersion behavior, the NC surface chemistry and surface bound species were investigated using infrared spectroscopy. Figure 2 shows an FTIR spectrum of drop-casted CuO NCs. The peak at 1560 cm⁻¹ is rather difficult to assign since both the water H-O-H scissoring vibration and the carboxylate anion asymmetrical stretching fall within this range [49], but both should be represented since they are both present in the reaction. The data thus suggests that acetate (accounting for both the carboxylate and CH peaks), hydroxide, and water molecules from the synthesis are adsorbed onto the NC surfaces. The presence of both polar and organic groups explains why the CuO NCs can be readily dispersed in the chloroform and methanol mixture.

Figure 3(a) shows the absorption spectra of CuO (80 nm) and PCBM (120 nm) films spin-casted from dispersions in their respective solvents. The CuO film absorption data was used to generate Tauc plots to determine the energy gap of the NCs (Figures 3(b) and 3(c)), which indicate a direct energy-gap at 3.07 eV and an indirect energy-gap at 1.40 eV. The latter value is similar to the value for the indirect bulk band-gap reported in the literature [19]. While there have been reports of quantum confinement observed in CuO nanostructures [33, 35], the authors did not perform an indirect energy-gap fit to their data; hence it is difficult to make meaningful comparisons.

(a)

(b)

(c)

As a proof of concept of the potential applications of the CuO NCs, a bilayer solar cell comprising of CuO and a fullerene derivative, phenyl-C₆₁-butyric acid methyl ester (PCBM), was fabricated. A single layer of CuO is unsuitable as the active layer, since the NC film contains cracks and pinholes and will result in a short-circuited device. This is similar to what has been reported about PbSe NC films, which require multiple sequential depositions of NCs and cross-linking to fill up the cracks [6]. PCBM is chosen since it is the acceptor of choice in solution-processable polymer solar cells [50], and its good film forming property allows it to cover up cracks in the CuO film. The literature value energy levels of the conduction and valence bands of CuO [19] and the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of PCBM [51] are such that CuO and PCBM form a type II semiconductor heterojunction; hence they are suitable as a donor and acceptor pair. Figure 4(a) shows a schematic of the device stack with indium tin oxide (ITO) forming the anode and CsF/Al as the cathode, as well as the literature values for the semiconductor energy levels [19, 51, 52]. Assignment of bulk CuO energy levels is supported by the small difference in NC optical band-gap relative to the literature (bulk) values [19].

(a)

(b)

(c)

Figure 4(b) shows the device current density voltage () curves both in the dark and light (under AM 1.5 100 mW cm⁻² illumination) of a bi-layer cell with a ~40 nm thick CuO layer. The device is characterized by a of 0.44 V, of 0.24 mA cm⁻², fill factor (FF) of 0.38, and a power conversion efficiency (PCE) of 0.040%. As a control, a PCBM-only cell (without CuO) was fabricated and tested (also shown in Figure 4(b)). It shows a much lower of 0.03 mA cm⁻² and PCE of 0.011%, although it gave a higher of 0.78 V and slightly higher FF of 0.41. Meanwhile, a bi-layer cell with a thicker CuO (~70 nm) layer is characterized by poorer (0.15 mA cm⁻²), FF (0.35), and PCE (0.024%) compared to the cell with thinner CuO (refer to Table 1 for a summary of device results). These results lead to the conclusion that efficient charge collection is only taking place in the thin CuO layer near the interface, and additional CuO layer thickness does not contribute to the photocurrent while possibly adding to series resistance and recombination as indicated by the poorer FF. Improvements in the charge collection from CuO NC absorber layers will therefore require better charge transport characteristics within the NC layer. One possible route to improving the charge collection is to use semiconductor nanowires (such as n-type ZnO or TiO₂) as the acceptor material, and the CuO NCs are then infiltrated into the gaps of the nanowires. Such device architectures have the advantage of separating the light absorption and charge collection pathways so that both processes can be efficient [8].

External quantum efficiency (EQE) spectra of the devices are shown in Figure 4(c). The bi-layer cell shows a superior EQE to the PCBM-only cell, and the improved EQE at higher wavelengths beyond 550 nm can be attributed to the contribution from the CuO by taking into account the absorption spectra of both materials (Figure 3(a)). Since is the integration of the EQE across the solar spectrum, the higher observed in the bi-layer cell is clearly due to its superior EQE.

4. Conclusions

A synthesis of CuO NCs by a facile alcohothermal route has been presented. The CuO NCs are soluble in a chloroform/methanol mixture, thus enabling the processing of these materials by solution. A bi-layer CuO and PCBM solar cell achieved a power conversion efficiency of 0.04%, which is 4 times higher than the control PCBM-only cell, indicating the potential of these CuO NCs for light-harvesting applications.

Acknowledgments

This work was funded by the KAUST-Cornell Center for Energy and Sustainability. The authors thank Dr. Jacek Jasieniak (CSIRO, Australia) for inspiration for the copper oxide synthesis and helpful comments on the paper, William Baumgardner and John Grazul for assistance with TEM imaging, and Dr. Maura Weathers for assistance with XRD. Device fabrication and testing was performed partly in the laboratory of Professor George Malliaras, while nanocrystal characterization was done using equipment in the Cornell Center for Materials Research (CCMR), Cornell Center for Nanoscale Systems (CNS), Cornell Nanoscale Science & Technology Facility (CNF), and the KAUST-Cornell Center for Energy and Sustainability. Y. F. Lim acknowledges a research fellowship from A*STAR, Singapore. J. J. Choi acknowledges support from NSF IGERT fellowship.

References

C. J. Brabec and J. R. Durrant, “Solution-processed organic solar cells,” MRS Bulletin, vol. 33, no. 7, pp. 670–675, 2008.
View at: Google Scholar
J. Tang and E. H. Sargent, “Infrared colloidal quantum dots for photovoltaics: fundamentals and recent progress,” Advanced Materials, vol. 23, no. 1, pp. 12–29, 2011.
View at: Publisher Site | Google Scholar
W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, “Hybrid nanorod-polymer solar cells,” Science, vol. 295, no. 5564, pp. 2425–2427, 2002.
View at: Publisher Site | Google Scholar
I. Gur, N. A. Fromer, M. L. Geier, and A. P. Alivisatos, “Materials science: air-stable all-inorganic nanocrystal solar cells processed from solution,” Science, vol. 310, no. 5747, pp. 462–465, 2005.
View at: Publisher Site | Google Scholar
Q. Guo, S. J. Kim, M. Kar et al., “Development of CulnSe₂ nanocrystal and nanoring inks for low-cost solar cells,” Nano Letters, vol. 8, no. 9, pp. 2982–2987, 2008.
View at: Publisher Site | Google Scholar
J. M. Luther, M. Law, M. C. Beard et al., “Schottky solar cells based on colloidal nanocrystal films,” Nano Letters, vol. 8, no. 10, pp. 3488–3492, 2008.
View at: Publisher Site | Google Scholar
K. S. Leschkies, T. J. Beatty, M. S. Kang, D. J. Norris, and E. S. Aydil, “Solar cells based on junctions between colloidal Pbse nanocrystals and thin ZnO films,” ACS Nano, vol. 3, no. 11, pp. 3638–3648, 2009.
View at: Publisher Site | Google Scholar
K. S. Leschkies, A. G. Jacobs, D. J. Norris, and E. S. Aydil, “Nanowire-quantum-dot solar cells and the influence of nanowire length on the charge collection efficiency,” Applied Physics Letters, vol. 95, no. 19, Article ID 193103, 2009.
View at: Publisher Site | Google Scholar
J. J. Choi, Y. F. Lim, M. B. Santiago-Berrios et al., “PbSe Nanocrystal Excitonic Solar Cells,” Nano Letters, vol. 9, no. 11, pp. 3749–3755, 2009.
View at: Publisher Site | Google Scholar
C. Y. Kuo, M. S. Su, Y. C. Hsu, H. N. Lin, and K. H. Wei, “An organic hole transport layer enhances the performance of colloidal PbSe quantum dot photovoltaic devices,” Advanced Functional Materials, vol. 20, no. 20, pp. 3555–3560, 2010.
View at: Publisher Site | Google Scholar
J. M. Luther, J. Gao, M. T. Lloyd, O. E. Semonin, M. C. Beard, and A. J. Nozik, “Stability assessment on a 3% bilayer PbS/ZnO quantum dot heterojunction solar cell,” Advanced Materials, vol. 22, no. 33, pp. 3704–3707, 2010.
View at: Publisher Site | Google Scholar
J. J. Choi, W. Wenger, R. Hoffman et al., “Solution-processed nanocrystal quantum dot tandem solar cells,” Advanced Materials, vol. 23, no. 28, pp. 3144–3148, 2011.
View at: Publisher Site | Google Scholar
S. W. Tsang, H. Fu, J. Ouyang et al., “Self-organized phase segregation between inorganic nanocrystals and PC₆₁BM for hybrid high-efficiency bulk heterojunction photovoltaic cells,” Applied Physics Letters, vol. 96, no. 24, Article ID 243104, 2010.
View at: Publisher Site | Google Scholar
R. Debnath, J. Tang, D. A. Barkhouse et al., “Ambient-processed colloidal quantum dot solar cells via individual pre-encapsulation of nanoparticles,” Journal of the American Chemical Society, vol. 132, no. 17, pp. 5952–5953, 2010.
View at: Publisher Site | Google Scholar
A. G. Pattantyus-Abraham, I. J. Kramer, A. R. Barkhouse et al., “Depleted-heterojunction colloidal quantum dot solar cells,” ACS Nano, vol. 4, no. 6, pp. 3374–3380, 2010.
View at: Publisher Site | Google Scholar
P. H. Stauffer and J. W. Hendley II, “Rare Earth Elements—Critical Resources for High Technology,” USGS Fact Sheet 087-02, 2011, http://pubs.usgs.gov/fs/2002/fs087-02.
View at: Google Scholar
C. Wadia, A. P. Alivisatos, and D. M. Kammen, “Materials availability expands the opportunity for large-scale photovoltaics deployment,” Environmental Science and Technology, vol. 43, no. 6, pp. 2072–2077, 2009.
View at: Publisher Site | Google Scholar
Y. Wu, C. Wadia, W. Ma, B. Sadtler, and A. P. Alivisatos, “Synthesis and photovoltaic application of copper(1) sulfide nanocrystals,” Nano Letters, vol. 8, no. 8, pp. 2551–2555, 2008.
View at: Publisher Site | Google Scholar
F. P. Koffyberg and F. A. Benko, “A photoelectrochemical determination of the position of the conduction and valence band edges of p-type CuO,” Journal of Applied Physics, vol. 53, no. 2, pp. 1173–1177, 1982.
View at: Publisher Site | Google Scholar
G. Nagasubramanian, A. S. Gioda, and A. J. Bard, “Semiconductor electrodes—37. Photoelectrochemical behavior of p-type Cu/sub 2/O in acetonitrile solutions,” Journal of the Electrochemical Society, vol. 128, no. 10, pp. 2158–2164, 1981.
View at: Google Scholar
W. Shockley and H. J. Queisser, “Detailed balance limit of efficiency of p-n junction solar cells,” Journal of Applied Physics, vol. 32, no. 3, pp. 510–519, 1961.
View at: Publisher Site | Google Scholar
B. P. Rai, “Cu₂O solar cells: a review,” Solar Cells, vol. 25, no. 3, pp. 265–272, 1988.
View at: Google Scholar
T. Minami, H. Tanaka, T. Shimakawa, T. Miyata, and H. Sato, “High-efficiency oxide heterojunction solar cells using Cu₂O sheets,” Japanese Journal of Applied Physics, vol. 43, no. 7A, pp. L917–L919, 2004.
View at: Publisher Site | Google Scholar
T. Minami, T. Miyata, K. Ihara, Y. Minamino, and S. Tsukada, “Effect of ZnO film deposition methods on the photovoltaic properties of ZnO-Cu₂O heterojunction devices,” Thin Solid Films, vol. 494, no. 1-2, pp. 47–52, 2006.
View at: Publisher Site | Google Scholar
A. Mittiga, E. Salza, F. Sarto, M. Tucci, and R. Vasanthi, “Heterojunction solar cell with 2% efficiency based on a Cu₂O substrate,” Applied Physics Letters, vol. 88, no. 16, Article ID 163502, 2006.
View at: Publisher Site | Google Scholar
K. H. Yoon, W. J. Choi, and D. H. Kang, “Photoelectrochemical properties of copper oxide thin films coated on an n-Si substrate,” Thin Solid Films, vol. 372, no. 1, pp. 250–256, 2000.
View at: Publisher Site | Google Scholar
Y. S. Chaudhary, A. Agrawal, R. Shrivastav, V. R. Satsangi, and S. Dass, “A study on the photoelectrochemical properties of copper oxide thin films,” International Journal of Hydrogen Energy, vol. 29, no. 2, pp. 131–134, 2004.
View at: Publisher Site | Google Scholar
S. Anandan, X. Wen, and S. Yang, “Room temperature growth of CuO nanorod arrays on copper and their application as a cathode in dye-sensitized solar cells,” Materials Chemistry and Physics, vol. 93, no. 1, pp. 35–40, 2005.
View at: Publisher Site | Google Scholar
C. H. Kuo and M. H. Huang, “Morphologically controlled synthesis of Cu₂O nanocrystals and their properties,” Nano Today, vol. 5, no. 2, pp. 106–116, 2010.
View at: Publisher Site | Google Scholar
M. Cao, C. Hu, Y. Wang, Y. Guo, C. Guo, and E. Wang, “A controllable synthetic route to Cu, Cu₂O, and CuO nanotubes and nanorods,” Chemical Communications, vol. 9, no. 15, pp. 1884–1885, 2003.
View at: Google Scholar
W. Wang, Z. Liu, Y. Liu, C. Xu, C. Zheng, and G. Wang, “A simple wet-chemical synthesis and characterization of CuO nanorods,” Applied Physics A: Materials Science and Processing, vol. 76, no. 3, pp. 417–420, 2003.
View at: Publisher Site | Google Scholar
Y. Chang and H. C. Zeng, “Controlled synthesis and self-assembly of single-crystalline CuO nanorods and nanoribbons,” Crystal Growth and Design, vol. 4, no. 2, pp. 397–402, 2004.
View at: Publisher Site | Google Scholar
J. Zhu, H. Chen, H. Liu, X. Yang, L. Lu, and X. Wang, “Needle-shaped nanocrystalline CuO prepared by liquid hydrolysis of Cu(OAc)₂,” Materials Science and Engineering A, vol. 384, no. 1-2, pp. 172–176, 2004.
View at: Publisher Site | Google Scholar
Y. Yu, F. P. Du, J. C. Yu, Y. Y. Zhuang, and P. K. Wong, “One-dimensional shape-controlled preparation of porous Cu₂O nano-whiskers by using CTAB as a template,” Journal of Solid State Chemistry, vol. 177, no. 12, pp. 4640–4647, 2004.
View at: Publisher Site | Google Scholar
H. Fan, L. Yang, W. Hua et al., “Controlled synthesis of monodispersed CuO nanocrystals,” Nanotechnology, vol. 15, no. 1, pp. 37–42, 2004.
View at: Publisher Site | Google Scholar
C. Lu, L. Qi, J. Yang et al., “One-pot synthesis of octahedral Cu₂O nanocages via a catalytic solution route,” Advanced Materials, vol. 17, no. 21, pp. 2562–2567, 2005.
View at: Publisher Site | Google Scholar
P. He, X. Shen, and H. Gao, “Size-controlled preparation of Cu₂O octahedron nanocrystals and studies on their optical absorption,” Journal of Colloid and Interface Science, vol. 284, no. 2, pp. 510–515, 2005.
View at: Publisher Site | Google Scholar
J. Zhang, J. Liu, Q. Peng, X. Wang, and Y. Li, “Nearly monodisperse Cu₂O and CuO nanospheres: Preparation and applications for sensitive gas sensors,” Chemistry of Materials, vol. 18, no. 4, pp. 867–871, 2006.
View at: Publisher Site | Google Scholar
J. Y. Ho and M. H. Huang, “Synthesis of submicrometer-sized Cu₂O crystals with morphological evolution from cubic to hexapod structures and their comparative photocatalytic activity,” Journal of Physical Chemistry C, vol. 113, no. 32, pp. 14159–14164, 2009.
View at: Publisher Site | Google Scholar
K. X. Yao, X. M. Yin, T. H. Wang, and H. C. Zeng, “Synthesis, self-assembly, disassembly, and reassembly of two types of Cu₂O nanocrystals unifaceted with {001} or {110} planes,” Journal of the American Chemical Society, vol. 132, no. 17, pp. 6131–6144, 2010.
View at: Publisher Site | Google Scholar
M. Yin, C. K. Wu, Y. Lou et al., “Copper oxide nanocrystals,” Journal of the American Chemical Society, vol. 127, no. 26, pp. 9506–9511, 2005.
View at: Publisher Site | Google Scholar
T. Kida, T. Oka, M. Nagano, Y. Ishiwata, and X. G. Zheng, “Synthesis and application of stable copper oxide nanoparticle suspensions for nanoparticulate film fabrication,” Journal of the American Ceramic Society, vol. 90, no. 1, pp. 107–110, 2007.
View at: Publisher Site | Google Scholar
B. D. Yuhas and P. Yang, “Nanowire-based all-oxide solar cells,” Journal of the American Chemical Society, vol. 131, no. 10, pp. 3756–3761, 2009.
View at: Publisher Site | Google Scholar
L. L. Hung, C. K. Tsung, W. Huang, and P. Yang, “Room-temperature formation of hollow Cu₂O nanoparticles,” Advanced Materials, vol. 22, no. 17, pp. 1910–1914, 2010.
View at: Publisher Site | Google Scholar
W. J. E. Beek, M. M. Wienk, M. Kemerink, X. Yang, and R. A. J. Janssen, “Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells,” Journal of Physical Chemistry B, vol. 109, no. 19, pp. 9505–9516, 2005.
View at: Publisher Site | Google Scholar
C. B. Murray, D. J. Norris, and M. G. Bawendi, “Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites,” Journal of the American Chemical Society, vol. 115, no. 19, pp. 8706–8715, 1993.
View at: Publisher Site | Google Scholar
E. A. Meulenkamp, “Synthesis and growth of ZnO nanoparticles,” Journal of Physical Chemistry B, vol. 102, no. 29, pp. 5566–5572, 1998.
View at: Google Scholar
M. S. Tokumoto, S. H. Pulcinelli, C. V. Santilli, and V. Briois, “Catalysis and temperature dependence on the formation of ZnO nanoparticles and of zinc acetate derivatives prepared by the sol-gel route,” Journal of Physical Chemistry B, vol. 107, no. 2, pp. 568–574, 2003.
View at: Publisher Site | Google Scholar
R. M. Silverstein and F. X. Webster, Spectrometric Identification of Organic Compounds, John Wiley & Sons, New York, NY, USA, 1998.
C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia, and S. P. Williams, “Polymer-fullerene bulk-heterojunction solar cells,” Advanced Materials, vol. 22, no. 34, pp. 3839–3856, 2010.
View at: Publisher Site | Google Scholar
Y. K. Jin, K. Lee, N. E. Coates et al., “Efficient tandem polymer solar cells fabricated by all-solution processing,” Science, vol. 317, no. 5835, pp. 222–225, 2007.
View at: Publisher Site | Google Scholar
K. Sugiyama, H. Ishii, Y. Ouchi, and K. Seki, “Dependence of indium-tin-oxide work function on surface cleaning method as studied by ultraviolet and x-ray photoemission spectroscopies,” Journal of Applied Physics, vol. 87, no. 1, pp. 295–298, 2000.
View at: Google Scholar

Copyright

Copyright © 2012 Yee-Fun Lim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

PDF Download Citation

Download other formats

Order printed copies

Views

3667

Downloads

2722

Citations