ZnO Nanopillar Solar

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A Simple Two-Step Electrodeposition of Cu2O/ZnO Nanopillar Solar Cells Jingbiao Cui* Department of Physics and Astronomy, UniVersity of Arkansas at Little Rock, Little Rock, Arkansas 72204

Ursula J. Gibson Thayer School of Engineering, Dartmouth College, HanoVer, New Hampshire 03755 ReceiVed: January 15, 2010; ReVised Manuscript ReceiVed: February 23, 2010

Nanopillar heterojunctions composed of n-type ZnO nanowire arrays embedded in p-type Cu2O thin films were fabricated by using a simple two-step electrodeposition method. Structural, optical, and electrical properties of the nanopillar junctions were investigated. Nanopillar radial junction arrays have the potential for improved performance in solar cells due to increased junction area and improved charge carrier collection. Improved efficiency in the Cu2O/ZnO nanopillar junctions was experimentally observed compared to planar thin film junctions prepared under similar conditions. This study demonstrates that electrodeposition, which is easily adapted to other chemical systems, is a promising technique for large-scale fabrication of low-cost nanopillar solar cells. 1. Introduction Solar energy is considered the cleanest and least limited energy source despite the dominance of fossil fuels through the last century. Silicon-based solar cells currently dominate the solar energy market due to well-developed fabrication techniques and relatively high energy conversion efficiencies.1,2 To further improve the energy conversion efficiency and lower the fabrication cost, solar cells based on other inorganic and organic materials have been extensively investigated in the last 20 years.3-6 The development of nanowire arrays in the past decade has opened the door for fabricating highly efficient nanopillar solar cells. Several device structures have been proposed, including nanopillar arrays with axial and radial junctions (core/ shell structures)7-9 and nanopillar collectors embedded in absorbing thin films.10 All-inorganic nanopillar arrays with various semiconductors such as Si, GaN, CdS, and GaAs have been experimentally demonstrated. An overall efficiency of about 3.4% has been reported in Si nanopillar radial junctions.11 These novel device structures are potentially useful for advanced solar cell applications. Use of inorganic nanopillars as charge collectors and organic materials as absorbers has been widely studied in dye-sensitized solar cells.5,12,13 However, there are very few reports on embedded nanowires in inorganic thin films for solar cells, likely due to the complicated fabrication process. Very recently, nanopillar solar cells with efficiencies up to 6% were demonstrated with CdS nanowire arrays embedded in CdTe thin films.10 Fan and colleagues used CdS nanowires with exposed length comparable to the distance between nanowires, making it possible to deposit CdTe into the gaps by vapor deposition.10 However, longer nanowires with small separations are desirable for fabricating embedded nanopillar solar cells due to their larger junction area and high efficiency. Electrodepositon represents a promising technique for fabricating embedded nanopillar solar cells. * To whom correspondence should be addressed. Phone: (001) 501-569 8962. Fax: (001) 501-569 3314. E-mail: [email protected].

Although Cu2O/ZnO thin film solar cells have been extensively studied in recent years,14-18 to our knowledge, true radial nanopillar cells have not been reported so far in this system. Hsueh et al. reported on deposition of Cu2O onto ZnO nanowires for solar cells.19 Their method resulted in axial rather than radial junctions because only the tops of the ZnO nanowires were in contact with the Cu2O thin films, which were deposited by sputtering. In this work, we report on an economic approach to fabricate ZnO nanopillars embedded in Cu2O thin films using simple two-step electrodeposition. This new approach is a lowtemperature, low-cost, and template-free process with potential for large-scale production, and these materials have reduced environmental hazards compared to Cd-based cells. The Cu2O thin films with an optimal band gap of 2.1 eV and theoretical energy conversion efficiency of 20% (ref 20) absorb the light and the ZnO nanopillars act as charge collectors. Planar junctions comprised of Cu2O and ZnO thin films were prepared under similar conditions for comparison. The radial nanopillar junctions showed improved energy conversion efficiency which was attributed to the increased junction area. While much effort is needed to optimize device overall performance of Cu2O/ZnO nanopillar junctions, we demonstrate that electrodeposition is suitable for fabricating low-cost embedded nanopillar solar cells. 2. Experimental Details 2.1. Growth of ZnO. Both ZnO planar thin films and nanopillar arrays were prepared by using a low-temperature electrodeposition process.21,22 Commercially available indium tin oxide (ITO) coated glass substrates with sheet resistance of 10 Ω were purchased from Structure Probe, Inc. (SPI Supplies). The substrates were sonicated in acetone, rinsed in alcohol and deionized water, and blown dry with nitrogen gas. ZnO thin films were directly electrodeposited on the substrates, using 0.025 M zinc nitrate aqueous solution at a temperature of 70 °C. An electric potential of -2.5 V was applied to the substrates relative to a gold counterelectrode placed in the solution, which resulted in a growth rate of 2.5 µm per hour. The detailed growth conditions for ZnO nanopillars can be found in the literature.22 Briefly, ZnO nanopillar arrays were

10.1021/jp1004314  2010 American Chemical Society Published on Web 03/10/2010

Electrodeposition of Cu2O/ZnO Nanopillar Solar Cells

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Figure 1. Experimental procedures for the fabrication of Cu2O/ZnO nanopillar heterostructures.

electrodeposited in an equimolar aqueous solution of 0.0063 M zinc nitrate and hexamine. The growth was performed at 95 °C with an applied potential of -2.5 V. Nanopillars with lengths from a few hundred nanometers to several micrometers were obtained, depending on the growth time. Growth of ZnO nanopillars directly on ITO glass showed a relatively low nucleation density. To enhance the nucleation, a 50 to 150 nm planar layer of ZnO was deposited on top of the ITO glass by e-beam evaporation and annealed at 450 °C in air for 60 min prior to the growth of ZnO nanopillars. 2.2. Growth of Cu2O. polycrystalline Cu2O thin films were grown at 60 °C by electrodeposition. The source materials were copper sulfate (4 g) and lactic acid (6 g) dissolved in 300 mL of deionized water. Concentrated sodium hydroxide solution was slowly added to adjust the pH value to 11-12 while stirring. A potential between -0.7 and -0.9 V was applied to the ZnO/ ITO/glass substrates resulting in a current density of 0.2 mA/ cm2. Pure Cu2O phase with a growth rate of about 30-40 nm/h was obtained under these growth conditions. Figure 1 shows the procedure for growing Cu2O/ZnO nanopillar heterojunctions. A ZnO nucleation film is deposited on ITO glass (Figure 1a) and then ZnO nanopillars are grown by electrodeposition (Figure 1b). To avoid the etching of ZnO nanopillars in the Cu2O growth solution, a negative potential is applied to the ZnO nanopillar substrate prior to immersion in the copper salt bath. It was found that the potential applied for Cu2O thin film growth ensures little etching of ZnO nanopillars even at times greater than 30 h. The Cu2O/ZnO nanopillar heterojunctions are complete when the Cu2O thin film becomes continuous and embeds the nanopillars (Figure 1c). 2.3. Material Characterization. The structure, morphology, and composition of the samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). UV-vis absorption spectra of thin film samples from 300 to 900 nm were measured by an Ocean Optics USB4000 spectrometer. Circular Au contact pads with diameters from 1 to 5 mm were thermally evaporated onto the top of Cu2O/ZnO structures and a portion of the exposed ITO substrate for electrical measurements. Electrical properties and photovoltaic effects were measured with use of a computer controlled Keithley 2400 source-measure unit. During photovoltaic characterization, a power meter was used to monitor the intensity of the white light source. 3. Results and Discussion 3.1. Cu2O/ZnO Planar Heterojunctions. To establish a baseline, Cu2O/ZnO planar heterojunctions were first deposited by using the electrodeposition process. Figure 2 shows the top view of a ZnO thin film (Figure 2a) and a Cu2O thin film (Figure 2b), and a cross section of Cu2O/ZnO bilayers (Figure 2c). Both Cu2O and ZnO thin films have polycrystalline structures. Individual layers of Cu2O, ZnO, and ITO on the glass substrate are visible in the cross-section (Figure 2c). The thicknesses of both Cu2O and ZnO thin films are around 500 nm. We confirm

Figure 2. SEM images of the surface of a ZnO thin film (a), a Cu2O thin film (b), and the cross section of a Cu2O/ZnO planar junction (c).

Figure 3. Absorption spectra of ITO coated glass, a ZnO thin film on ITO/glass, and Cu2O thin films of different thicknesses on ZnO/ITO/ glass.

that Cu2O/ZnO planar heterojunctions can be fabricated by the two-step electrodeposition process, consistent with previous publications.15,16 Our materials had somewhat different electrical properties, as described below. Optical absorption measurements, to identify band edges, were performed on Cu2O/ZnO planar junctions rather than nanopillar structures due to optical scattering from the nanopillar arrays. Figure 3 shows the absorption spectra of ITO glass, a ZnO thin film, and a Cu2O/ZnO bilayer. The ITO glass has little absorption from near-UV to the near-infrared region. The deposition of 500 nm ZnO onto ITO results in an obvious absorption below 380 nm, which is due to the band edge of ZnO. No absorption features were seen in the visible region. However, the absorption spectrum was significantly altered after subsequent deposition of Cu2O for 3 h. The appearance of an absorption band around 550 nm is attributed to the formation of nanocrystalline Cu2O. The short growth time resulted in the formation of isolated Cu2O nanocrystals as confirmed by SEM (data not shown). A continuous Cu2O film of 500 nm thickness was obtained after growth for 14 h, resulting in an absorption edge around 600 nm. The band gap of Cu2O is 2.1 eV (590 nm).23 Therefore the strong absorption below 600 nm is attributed to band edge absorption of the Cu2O thin film. 3.2. Cu2O/ZnO Nanopillar Heterojunctions. Figure 4 shows SEM images of ZnO nanopillar arrays and the electrodeposited Cu2O/ZnO nanopillar heterostructures. Top-down and cross-section views shown in Figure 4a,b indicate that the average length and diameter of the ZnO nanopillars are respectively around 2 µm and 100 nm. Subsequent growth of Cu2O for 14 h with use of the nanopillar arrays as a substrate resulted in formation of isolated Cu2O crystals, even though this growth time was sufficient to form a continuous film on a

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Figure 4. SEM images of ZnO nanopillars and Cu2O/ZnO nanopillar heterostructures. (a) Top view of ZnO nanopillars; (b) cross section of ZnO nanopillars; (c, d) Cu2O/ZnO nanopillar heterostructures after growth of Cu2O for 14 (c) and 28 h (d). Circled areas highlight the partially embedded ZnO nanopillars in the Cu2O crystals. (e) Cross section of the sample shown in panel d. Note that the spaces among ZnO nanopillars are completely filled with Cu2O. The rectangular region highlights imprints left when ZnO nanopillars were pulled out during sample fracture.

planar ZnO film. Nucleation is inhibited on the nanopillar array substrate, as evidenced by the larger crystallite size, and the growth rate of Cu2O may be reduced inside the arrays due to the diffusion-limited availability of source materials. Note that the Cu2O crystals grew among the nanopillars as highlighted in the circled areas of Figure 4c. ZnO nanopillars are encased by the Cu2O crystals, forming nanowire radial junctions. The surface morphology of the sample after growth of Cu2O for 28 h is shown in Figure 4d. This top view image indicates that a continuous film of Cu2O is obtained. To determine whether the gaps among ZnO nanopillars were completely filled with Cu2O beneath the top layer, a cross-sectional image of the sample was taken and is shown in Figure 4e. It can be seen that Cu2O thin films completely fill the spaces among ZnO nanopillars. Some of the nanopillars were pulled out of the Cu2O crystals during sample preparation for SEM measurements, which left nanopillar imprint marks as highlighted in the rectangular region. However, some of the nanopillars survived the fracture process and remained partially embedded inside the Cu2O thin films. These observations indicate unambiguously that the electrodeposited Cu2O fills the narrow spaces between the ZnO nanopillars to form Cu2O/ZnO radial nanopillar heterojunctions. This is an advantage of electrodeposition over vapor-phase techniques which are typically incapable of filling the deep interpillar voids. The composition of the electrodeposited Cu2O/ZnO nanopillar heterostructures was analyzed by EDX measurements. The atomic ratio of Cu to O in our Cu2O thin films was measured to be 2.0:1.0, consistent with Cu2O formation. The ratio of Zn to O was found to be 48:52 in the ZnO thin films. Figure 5 shows the EDX spectra taken on three typical samples of pure ZnO nanopillars, ZnO nanopillar with Cu2O grown for 14 h, and that with Cu2O grown for 28 h. Only a narrow energy range is shown here in order to see the details of Cu (0.95 keV) and Zn (0.99 keV) signals. As the growth time of Cu2O increases, the relative intensity of the Cu signal increases while that of Zn decreases. After growing Cu2O for 14 h, the Zn signal still dominates in the spectrum. After growing Cu2O for 28 h, however, the Cu signal is the dominant peak in the energy window. The growth of Cu2O among ZnO nanopillars was also investigated by XRD measurements. Figure 6 shows XRD

Cui and Gibson

Figure 5. EDX spectra of Cu2O/ZnO nanopillar heterostructures: (a) ZnO nanopillars; (b) Cu2O/ZnO nanopillar heterostructures with Cu2O growth for 14 h; and (c) Cu2O/ZnO nanopillar heterostructures with Cu2O growth for 28 h.

Figure 6. X-ray diffraction patterns of ZnO nanopillars (a) and Cu2O/ ZnO nanopillar heterostructures (b). The stars in part b label the diffraction peaks of ZnO.

Figure 7. Dark current-voltage (I-V) curve of Cu2O/ZnO nanopillar heterojunctions. The inset depicts the device structure for electrical characterization.

patterns taken on both pure ZnO nanopillars and Cu2O/ZnO nanopillar heterostructures. A very strong ZnO (002) peak was observed in the pure ZnO nanopillars, indicating that oriented nanopillars were obtained. After deposition of Cu2O for 28 h, various diffraction peaks from Cu2O were observed in addition to those from ZnO nanopillars. The lattice constant of the electrodeposited Cu2O is consistent with the reported value in the literature.24 The electrical properties and photovoltaic effects of the electrodeposited Cu2O/ZnO nanopillar heterostructures were investigated at room temperature. Figure 7 shows a typical dark current-voltage (I-V) curve of the heterostructures. The inset of Figure 7 shows the Cu2O/ZnO nanopillar device structure for electrical characterization. The nonlinear I-V curve is typical

Electrodeposition of Cu2O/ZnO Nanopillar Solar Cells

Figure 8. Current density versus voltage of Cu2O/ZnO nanopillar solar cells. The light intensity was 100 mW/cm2.

of p-n junctions. The p-n heterojunctions are formed at the interfaces of n-type ZnO nanopillars and p-type Cu2O because ZnO nanopillars grown without intentional doping generally show n-type semiconducting properties while Cu2O grows as a p-type semiconductor. Photovoltaic effects of Cu2O/ZnO nanopillar heterostructures were measured during illumination through the ITO glass substrate. Data from Cu2O/ZnO planar thin film junctions are included in Figure 8 for comparison. The light intensity was maintained at 100 mW/cm2 for both measurements. Typical solar cell behavior was observed in the I-V curves. The short circuit current density of the nanopillar junctions was 8.2 mA/cm2, which is almost two times larger than that obtained from the planar junctions (4.3 mA/cm2). However, the open circuit voltage was 0.29 V, somewhat less than the 0.33 V observed for the thin film junctions. Since the open circuit voltage depends on the band alignment of p- and n-type semiconductors, the different values observed suggest a shift associated with the ZnO nanowire configuration, possibly due to the dominance of different exposed crystal faces in the nanowires. The increased short circuit current of nanopillar junctions is likely due to the increased p-n junction area in the nanopillar structures. The fill factor (maximum output power divided by open circuit voltage and short circuit current) is similar for both devices, i.e., 0.36 for the nanopillar junctions and 0.38 for the planar junctions. Due to the large short circuit current of the nanopillar cell, its energy conversion efficiency was found to be 0.88%, as compared to 0.55% for the planar cell. Open circuit voltages up to 0.595 V have been reported for Cu2O/ZnO planar junctions,18 significantly higher than observed in this study. Mittiga and colleagues used high temperature grown Cu2O doped with Cl for conductivity enhancement.18 This intentional doping helped improve the electrical properties of Cu2O films, which result in a large open circuit voltage in the corresponding solar cells. The relatively small open circuit voltage in our nanopillar junctions may result from the low dopant concentration in Cu2O since no intentional doping was performed. Other reasons may include the quality of the junction interfaces, which have not yet been optimized. Improvement of the open circuit voltage may be achieved by alterations in the growth conditions as well as surface treatment of ZnO prior to Cu2O growth. Much effort is still needed to increase the energy conversion efficiency of Cu2O/ZnO nanopillar solar cells. It may include the following: (1) Improvement of material quality. Both Cu2O and ZnO are naturally grown semiconductors; however, intentional doping will help improve their electrical properties for charge collection. (2) Increases in p-n junction area by increasing the density or length of ZnO nanopillars. Note that

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6411 an increase of nanopillar length implies a longer growth time for Cu2O in order to fully fill the spaces among nanopillars. (3) Improved homogeneity in length and separation of ZnO nanopillars over a large area. This effort will help minimize leakage current and increase fill factor. Leakage current can be a serious problem for nanopillar junctions. Uniformity among individual nanopillar junctions would increase the fill factor of the whole device. Although the overall performance of electrodeposited Cu2O/ ZnO nanopillar solar cells is not optimized yet, our study sheds light on a simple and economic approach to fabricating these novel nanopillar devices. The advantage of this approach is that it is simple, low-cost, template-free, and has potential for largescale production. Importantly, electrodeposition can effectively fill the deep narrow gaps among nanowires, which is challenging with use of vapor deposition. This unique feature makes electrodeposition better suited for fabricating nanopillar arrays embedded in thin films for solar cell applications. 4. Summary Nanopillar hereojunctions are successfully fabricated by embedding ZnO nanowires in Cu2O thin films with use of a simple two-step electrodepositon. The Cu2O with a direct band gap of 2.1 eV acts as a solar absorber while ZnO nanopillars are charge carrier collectors. Compared with planar solar cell structure prepared under similar conditions, an improvement in energy conversion efficiency was observed in the nanopillar solar cells. This improved performance is attributed to the increased p-n junction area in the latter case. Although much effort is still needed to get a better understanding and to further improve the overall device performance, our study demonstrates the promise of this process for fabricating nanopillar solar cells. Electrodeposition can be easily adapted to other chemical systems, and thus may have an important role in nanopillar solar cell development. References and Notes (1) Schropp, R. E.; Zeman, M. In Amorphous and microcrystalline silicon solar cells: modeling, materials, and deVice technology; Kluwer Academic Publishers: Norwell, MA, 1998; p 107. (2) Emery, A. G. K.; Hishikawa, Y.; Warta, W. Prog. PhotoVoltaics: Res. Appl. 2008, 16, 61–67. (3) Gra¨tzel, M. J. Photochem. Photobiol. C 2003, 4, 145–153. (4) Olson, D. C.; Piris, J.; Collins, R. T.; Shaheen, S. E.; Ginley, D. S. Thin Solid Films 2006, 496, 26–29. (5) Greene, L. E.; Law, M.; Yuhas, B. D.; Yang, P. D. J. Phys. Chem. B 2007, 111, 18451–18456. (6) Pasquier, A. D.; Mastrogiovanni, D. D. T.; Klein, L. A.; Wang, T.; Garfunkel, E. Appl. Phys. Lett. 2007, 91, 183501. (7) Kayes, B. M.; Atwater, H. A.; Lewis, N. S. J. Appl. Phys. 2005, 97, 114302. (8) Czaban, J. A.; Thompson, D. A.; LaPierre, R. R. Nano Lett. 2009, 9, 148–154. (9) Garnett, E. C.; Yang, P. D. J. Am. Chem. Soc. 2008, 130, 9224. (10) Fan, Z. Y.; Razavi, H.; Do, J. W.; Moriwaki, A.; Ergen, O.; Chueh, Y. L.; Leu, P. W.; Ho, J. C.; Takahashi, T.; Reichertz, L. A.; Neale, S.; Yu, K.; Wu, M.; Ager, J. W.; Javey, A. Nat. Mater. 2009, 8, 648–653. (11) Tian, B. Z.; Zheng, X. L.; Kempa, T. J.; Fang, Y.; Yu, N. F.; Yu, G. H.; Huang, J. L.; Lieber, C. M. Nature 2007, 885, 889. (12) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455–459. (13) Diamant, Y.; Chappel, S.; Chen, S. G.; Melamed, O.; Zaban, A. Coord. Chem. ReV. 2004, 248, 1271–1276. (14) Tanaka, H.; Shimakawa, T.; Miyata, T.; Sato, H.; Minami, T. Thin Solid Films 2004, 469-470, 80–85. (15) Jeong, S. S.; Mittiga, A.; Salza, E.; Masci, A.; Passerini, S. Electrochim. Acta 2008, 53, 2226–2231. (16) Katayama, J.; Ito, K.; Matsuoka, M.; Tamaki, J. J. Appl. Electrochem. 2004, 34, 687–692. (17) Izaki, M.; Mizuno, K.; Shinagawa, T.; Inaba, M.; Tasaka, A. J. Electrochem. Soc. 2006, 153, C668–C672.

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(18) Mittiga, A.; Salza, E.; Sarto, F.; Tucci, M.; Vasanthi, R. Appl. Phys. Lett. 2006, 88, 163502. (19) Hsueh, T. J.; Hsu, C. L.; Chang, S. J.; Guo, P. W.; Hsieh, J. H.; Chend, I. C. Scr. Mater. 2007, 57, 53–56. (20) Tanaka, H.; Shimakawa, T.; Miyata, T.; Sato, H.; Minami, T. Appl. Surf. Sci. 2005, 244, 568. (21) Izaki, M.; Omi, T. J. Electrochem. Soc. 1997, 144, 1949–1952.

Cui and Gibson (22) Cui, J. B.; Gibson, U. J. J. Phys. Chem. B 2005, 109, 22074. (23) Jones, P. M.; May, J. A.; Reitz, J. B.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 1506. (24) Poizot, P.; Hung, C. J.; Nikiforov, M. P.; Bohannan, E. W.; Switzer, J. A. Electrochem. Solid-State Lett. 2003, 6, C21–C25.

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