NANO LETTERS
Platinum Nanoparticle Decorated Silicon Nanowires for Efficient Solar Energy Conversion
2009 Vol. 9, No. 11 3704-3709
Kui-Qing Peng,*,† Xin Wang,† Xiao-Ling Wu,† and Shuit-Tong Lee*,‡ Key Laboratory of Beam Technology and Material Modification of Ministry of Education and College of Nuclear Science and Technology, Department of Physics, Beijing Normal UniVersity, Beijing 100875, China, and Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, SAR, Hong Kong Received June 1, 2009; Revised Manuscript Received September 21, 2009
ABSTRACT High-density aligned n-type silicon nanowire (SiNW) arrays decorated with discrete 5-10 nm platinum nanoparticles (PtNPs) have been fabricated by aqueous electroless Si etching followed by an electroless platinum deposition process. Coating of PtNPs on SiNW sidewalls yielded a substantial enhancement in photoconversion efficiency and an apparent energy conversion efficiency of up to 8.14% for the PtNP-decorated SiNW-based photoelectrochemical solar cell using a liquid electrolyte containing Br-/Br2 redox couple. The results demonstrate PtNP-decorated SiNWs to be a promising hybrid system for solar energy conversion.
The enormous potential of solar energy utilization is widely recognized due to environmental and shortage issues of fossil fuels,1-3 and great effort is being devoted to developing solar energy conversion systems. Recently, solar energy conversion systems based on nanostructured elements have attracted considerable interest, because nanostructured materials, such as nanocrystals or quantum dots,4,5 nanorods,6,7 nanotubes,8-11 and nanowires,12-31 can significantly improve charge collection efficiency. Of particular interest are one-dimensional (1D) semiconductor nanowires which offer the unique advantages of large active surface areas for high optical absorption across a broad spectrum and for efficient charge collection. Among various solar energy conversion systems, photoelectrochemical (PEC) solar cells have emerged as a promising approach for solar energy conversion. The semiconductor/liquid junction cells offer potential cost advantages over their solid-state counterparts. In particular, recently PEC and dye-sensitized solar cells based on 1D semiconductor nanostructures12,27-32 have attracted intense attention, as they may rival traditional planar solid-state p-n junction devices and nanocrystalline dye-sensitized solar cells. For example, arrays of ZnO nanowires have been widely used to facilitate optical absorption and collection of a majority of carriers in ZnO-based dye-sensitized solar cells.12,27-31 As silicon is the leading material used in today’s photovoltaic (PV) industry, * To whom correspondence should be addressed,
[email protected] and
[email protected]. † Beijing Normal University. ‡ City University of Hong Kong. 10.1021/nl901734e CCC: $40.75 Published on Web 10/06/2009
2009 American Chemical Society
1D silicon nanowire (SiNW) solar cells have naturally become the focus of PV research since SiNW solar cell design may be readily compatible with the existing silicon industry and processing technology.12-19 Lieber et al. have reported coaxial core/shell SiNWs as building blocks for photovoltaic devices, and single coaxial SiNW solar cells with short-circuit current density (Jsc) and power conversion efficiencies reaching 23.9 mA/cm2 and to 3.4% under 1 solar equivalent illumination.13 They also fabricated axial modulation-doped p-i-n and tandem p-i-n+-p+-i-n silicon nanowire (SiNW) photovoltaic elements.17 Peng et al. reported that highly oriented SiNW arrays prepared by the electroless etching method can significantly suppress light reflection across a broad spectrum and serve as antireflection coating for Si solar cell applications.33,34 Yang et al. showed that high-density Si/TiO2 coaxial core-shell nanowire arrays exhibited 2.5 times greater photocurrent than planar Si/TiO2 structure and, thus, are a promising structure for solar water splitting.35 Sivakov et al. fabricated SiNW-based solar cells on glass substrates by electroless etching, yielding a maximum power conversion efficiency of 4.4%.36 Recent studies of PEC solar cells have shown SiNWs to be rather photoactive, revealing the high potential of SiNWs for inexpensive PEC cells.37-39 Nevertheless, the as-prepared large surface and large junction areas of nanowires generally suffer from severe junction recombination and poor carrier collection, which present a major barrier to achieving efficient solar cells. Consequently, the efficiencies of reported nanowire-
Figure 1. (a, c) Schematic of PtNP-decorated n-SiNW arrays prepared by aqueous electroless etching of silicon and electroless metal deposition. (b) Cross-sectional SEM of as-prepared Si nanowire array; dendritic Ag can be observed inside the SiNWs array. (d) Lowmagnification TEM image of PtNP-decorated n-SiNWs showing PtNPs on the wire surface. (e) High-resolution TEM image of PtNPdecorated SiNW, revealing a PtNP diameter of ∼5 nm. (f) Aside from excellent light absorption and efficient radial charge-carrier collection, SiNWs have direct 1D electronic pathways allowing for efficient 1D charge-carrier transport along the length of every wire. The red dots denote PtNPs on a n-SiNW surface and the green layer denotes the space charge layer formed owing to the conformal contact between silicon and electrolyte.
based solar cells remain substantively lower than the 8-10% required for practical applications; thus further efficiency improvement is much needed. As decoration of discrete semiconductor or metallic nanoparticles on carbon nanotube40,41 and ZnO nanowire42 sidewalls can yield a significant enhancement in photoconversion efficiencies, thus similar enhancement may be anticipated for SiNW and SiNW-based PVs. In this work, we prepared platinum nanoparticle (PtNP)decorated SiNW arrays and evaluated their potential application for solar energy conversion. PtNP-decorated SiNW arrays yielded significant enhancement in solar energy conversion efficiency up to 8.1%, which is the highest ever reported for SiNW solar cells, demonstrating them to be a promising hybrid system for efficient solar energy conversion. The PtNP-Si junctions and the excellent catalytic activity of PtNPs for the electrode processes are considered responsible for enhancing the solar energy conversion efficiency of PtNP-decorated SiNW arrays. A schematic of a PtNP-decorated SiNW electrode configuration is shown in Figure 1. Highly oriented SiNWs arrays were fabricated directly on Si wafer by metal-catalyzed electroless etching of Si etching (Figure 1a),43-45 with the wire lengths controlled by the etching time. Figure 1b is the typical cross-sectional scanning electron microscope image of the SiNWs array prepared on an n-type 2-4 Ω·cm Si(100) wafer, showing large-area aligned SiNWs perpendicular to the Si surface. The diameter distribution of the nanowires is Nano Lett., Vol. 9, No. 11, 2009
in the range of 20-300 nm, and the axial wire length is about 17 µm. Silver dendrites were observed on and within the SiNW arrays, while silver particles were at the SiNW/Si interface. The prepared SiNW arrays were washed in a concentrated nitric acid bath for at least 1 h to remove all Ag from the nanowire surfaces. After the arrays were chemically etched in 50:1 HF solution (containing one part of 49% HF and 50 parts of deionized water) to remove the oxide layer, the n-SiNW samples were immediately immersed into a solution containing 50:1 HF solution and H2PtCl6 (0.001 M) to decorate SiNW surface with PtNPs via electroless metal deposition (EMD) (Figure 1c). The PtNP-decorated n-SiNWs were characterized using a transmission electron microscope (TEM). Both electron diffraction (ED) patterns and HRTEM images confirmed the highquality single-crystal characteristics of the nanowires. Figure 1d shows the low-magnification TEM image of NP-decorated SiNWs, revealing that most of the PtNPs are primarily located at the wire tips. The mean diameter of PtNPs is typically 5-10 nm, and distribution of NPs on the SiNW surface is microscopically discontinuous, as shown in Figure 1e. SEM characterization of Pt deposition on SiNW arrays showed that PtNPs are primarily located at the tips of the nanowires for deposition less than 10 min. PtNPs would appear on other parts of nanowires for longer time deposition, and for prolonged Pt deposition, PtNPs would cover the whole body of nanowires. The small particle diameter is the 3705
Figure 2. Photocurrent densities (I) versus voltage (V) curves under simulated solar illumination for (a) a PtNP-decorated n-SiNW array electrode, (b) a PtNP-decorated planar n-Si electrode, (c) the naked n-SiNW array electrode, and (d) the naked planar n-Si electrode.
result of slow deposition due to the minute concentration of platinum ions in 50:1 HF solution. Energy dispersive X-ray spectroscopy (EDS) spectrum recorded from the NPs confirms that the NPs are platinum. In contrast to Pt deposition, gold nanoparticles were deposited in an almost continuous layer on SiNW surface under the same condition (Figure S1 in Supporting Information). The NP coated nanowire electrode geometry has several advantages for solar cell applications, in addition to strong suppression of optical reflection over the wide spectral bandwidth of 250-1000 nm. First, a conformal radial charge-separating junction would form around the wire structure when SiNWs are in contact with liquid electrolyte, as a result photogenerated carriers only need to travel a short distance to the charge-separating junction, thus leading to a higher carrier-collection efficiency (Figure 1f) via solid nanowire radial p-n junctions.14-16 Second, the SiNWs are conductive and can provide direct 1D electronic pathways allowing for efficient charge-carrier transport to Si wafer substrate. Third, the PtNPs would serve as catalysts for electrochemical reaction on SiNW surfaces. The photovoltaic characteristics of the PtNP-decorated n-SiNW array were investigated using a liquid electrolyte consisting of 8.6 M hydrogen bromide and 0.05 M bromine redox solution, which provided a simple conformal contact to high surface area structures. The n-SiNW electrode was attached to the modified PEC cell with a quartz window. Ohmic contact to a n-SiNW electrode was obtained with Ti/ Pd/Ag. The photoelectrochemical (PEC) measurements were performed using a Princeton 2273 electrochemical workstation. A two-electrode cell was used for the PV PEC solar cell. The n-type SiNW array worked as the photoelectrode and the Pt mesh as the counter electrode. During measurement, the n-SiNW array electrode was irradiated with an ORIEL solar simulator at a constant light intensity of 100 mW/cm2 through the quartz window. Figure 2 shows the typical photocurrent density (I) versus potential (V) curves for PEC solar cells made with various n-Si electrodes. Curve a represents the PtNP-decorated n-SiNW electrode, curve b the PtNPs-decorated planar n-Si electrode, curve c the corresponding naked n-SiNWs elec3706
trode, and curve d the naked (H-terminated) planar n-Si electrode. For a semiconductor PEC cell, the Voc is related to the redox potential of the redox couple used. The larger Voc of the naked planar n-Si electrode and naked n-SiNWs electrode than that of the corresponding PtNP-decorated Si electrodes is likely due to the galvanic oxidation of both silicon electrodes. The lower Voc of the Pt nanoparticle decorated planar device is due to the higher overpotential caused by higher increased photocurrent density at the electrode. Figure 2 shows that the formation of a SiNW array significantly increases the short-circuit photocurrent density (Isc) to 12.73 (curve c) from 1.74 mA cm-2 of planar Si electrode (curve d), while decoration of SiNWs with PtNPs further increases the photocurrent to ∼17.2 mA cm-2 (curve a). The excellent antireflection ability and higher surface area of SiNWs are responsible for the large increase in photocurrent and high optical absorption owing to multiple scattering of light46,47 in the nanowire array. Among the electrodes, the PtNP-decorated planar n-Si electrode generates the highest photocurrent of ∼20.2 mA cm-2 (curve b), which may be due to the lower series resistance. However, the Isc of PtNP-coated nanowire array is still lower than that of the PtNP-coated planar n-Si electrode, which is attributed to increased recombination losses through increased textured or defective surface area of the former. The enhanced recombination of photogenerated carriers at the silicon/ electrolyte interface would reduce the Isc. Significantly, decoration of PtNPs substantively enhanced the fill factor (FF) of the device. The PtNP-decorated n-SiNWs electrode and PtNP-decorated planar n-Si electrode yielded larger fill factors (FF) of 0.61 and 0.47, respectively. Notably, PtNPdecorated n-SiNW photoelectrodes showed power conversion efficiency up to 6.1%, which is higher than that of previously reported SiNW-based solar cells. Meanwhile, the overall power conversion efficiency of PtNP-decorated planar n-Si photoelectrode is 4.8%; particularly the FF reduces quickly during measurement and leads to severe degradation (see Figure S2 in Supporting Information), whereas the FF of PtNP-decorated n-SiNW photoelectrodes remains rather stable during measurement. Generally, the increase of PV device series resistance would lead to the current-controlled portion of the I-V curve sagging toward the origin and also a slight reduction in Isc, while the decrease of PV device shunt resistance leads to the current-controlled portion of the I-V curve sagging toward the origin and also a slight reduction in Voc. Therefore, the severe degradation of PV performance of PtNPs-decorated planar n-Si devices should result from the increasing device series resistance. The effect of Pt deposition time on SiNW PEC devices was investigated. It was found (not shown) that the solar energy conversion efficiency η was roughly proportional to the number density of Pt nanoparticles obtained via increasing deposition time (Figure 3a). The photocurrent increases quickly with increasing loading of Pt nanoparticles and tends to be stable soon. However, the η begins to gradually reduce after prolonged Pt deposition longer than 30 min. In addition, the Voc are not evidently affected by Pt deposition time. Figure 3b shows the photocurrent density (I) versus potential Nano Lett., Vol. 9, No. 11, 2009
Figure 4. Short-circuit photocurrent responses of the electrodes versus ON-OFF cycles of illumination.
Figure 3. The effects of different amounts of PtNP loading via increasing deposition time on (a) solar energy conversion efficiency and (b) the curve of photocurrent density (I) versus potential (V).
(V) curves for PEC solar cells made with SiNW electrodes loaded with different amounts of PtNPs. Significantly, solar energy conversion efficiency of 8.14% was obtained for SiNW electrodes decorated with PtNPs deposited for 25 min. Evidently, the solar energy conversion efficiency η of SiNW devices is mainly determined by the fill factors (FF). The FF was also roughly proportional to the number density of Pt nanoparticles and gradually reduces after prolonged Pt deposition longer than 30 min. We suggest that the fill factor was mainly affected by the series resistance in the present PEC solar cell system. At the initial stage of electroless Pt deposition on the SiNW surface, the deposited Pt nanoparticles would reduce the series resistance and thus lead to the improvement of fill factors. However, prolonged Pt deposition on SiNWs would result in the formation of porous silicon layer with large resistance, which would degrade the FF and reduce the photocurrent. For comparison, the PEC solar cells made of Au nanoparticle (AuNP)-decorated n-SiNWs and Ag nanoparticle (AgNP)-decorated n-SiNWs also were similarly prepared (Figure S3 in Supporting Information). The performances of both AuNP-decorated and AgNP-decorated n-SiNWs PEC solar cells are found to be poorer than that of the PtNPdecorated cell. For example, the AuNP-decorated cell showed not only a lower photocurrent but also a smaller FF of ∼0.23, which is far smaller than the FF (0.61) of PtNP-decorated n-SiNW cells. The electrocatalytic metals, such as Pt, Au, Nano Lett., Vol. 9, No. 11, 2009
Pd, and Ru, are known as efficient photoconversion promoters for semiconductor PEC cells. Among these metals, Pt has been demonstrated to be the best promoter, which explains why silicon nanowire PEC solar cells modified with PtNPs shows higher solar energy conversion efficiency. The stability of n-Si electrodes in aqueous redox electrolyte solution is an important issue; we thus investigated the stability of the photocurrent response of various Si electrodes. Figure 4 shows the photocurrent response of various Si electrodes versus ON-OFF cycles of illumination. It reveals that the photocurrent response of various Si electrodes is sensitive to illumination and that the photocurrents of both n-SiNW and n-type planar Si electrodes decay considerably during the initial stage of illumination. Although the initial photocurrent of the planar n-Si electrode decorated with PtNPs is relatively higher, it decays much more rapidly and eventually disappears after long time illumination. In contrast, the photocurrent of PtNP-decorated n-SiNW electrodes, though initially lower, remains rather stable after the initial rapid decay. While in a liquid electrolyte the photocurrent is generally limited by the mass transport of the redox species, however the polarization effect at the n-type silicon electrode-electrolyte interface is not the decisive factor in the photocurrent decay in the present experimental system. The major difference between the planar n-Si electrode and n-SiNWs electrode is the huge difference of surface area. Compared with the planar n-Si electrode, the much higher surface area of SiNWs electrode would lead to a higher overall amount of PtNPs coating, which would increase the interface area with the salt solution. Further, the typical surface roughness48,49 of the SiNW electrode due to electroless etching would lead to higher adhesion of the deposited PtNPs to the SiNW surface. During PEC measurements, the redox electrolyte solution would oxidize the silicon surface and increase the device series resistance, which in turn led to the rapid decay of photocurrents in both SiNWs and planar electrode-based devices. The photocurrent should have become stable after the initial rapid decay. However, as the redox electrolyte solution could penetrate between silicon surface and PtNPs to induce further Si oxidation and increase in series resistance, thus the photocurrent would continue to decay. The effect of rapid increase in series resistance could 3707
be clearly observed in Figure S2 in Supporting Information. The rapid increase of series resistance leads to the rapid degradation of fill factors and also the reduction in Isc for PtNP-decorated planar n-Si devices. In contrast, the overall higher amount of PtNPs on SiNWs electrodes and also the higher adhesion of PtNPs to SiNWs surface due to increased surface roughness of SiNWs would greatly counteract the oxidizing effect of electrolyte and thus stabilize the device series resistance, thereby resulting in stable photocurrent. The open circuit potential versus elapsed time for PtNPdecorated n-SiNWs and PtNP-decorated planar n-Si electrode was also obtained (Figure S4 in Supporting Information). Note the electrodes had been in the dark for the first 60 s when the electrodes were illuminated. A large photopotential response was observed when the n-Si electrode is decorated with PtNPs, while the naked n-Si electrode showed a smaller photopotential response under illumination. In the case of PtNP-decorated n-SiNWs, the photopotential upon illumination was about -500 mV, while it was -350 mV for PtNPdecorated planar n-Si. These results indicate that surface reactivity of the n-Si electrodes is significantly modified by PtNPs. From the responses of photocurrent and photopotential to illumination, it can be concluded that the PtNPdecorated SiNWs showed greatly enhanced photoactivity and thus promising potential as a hybrid system for solar energy conversion. Figure S5 in Supporting Information shows the schematic structure of a PtNP-decorated n-SiNW electrode in hydrogen bromide and bromine redox solution. Under illumination, the sparsely distributed 5-10 nm PtNPs on the n-SiNW surface would serve as catalysts for the electrode process and as gates for the charge carrier transport.50,51 Meanwhile, in the electrolyte containing the Br-/Br2 redox couple, the naked parts of the n-SiNW surface would be oxidized and passivated by the formation of a thin silicon oxide layer. Thus, surface recombination at the bare Si surface would be slow. In addition, the thin SiOx layer may also induce in the silicon underneath an inversion layer, which could improve the collection efficiency for photoexcited minority carriers.52 Although the excess minority carriers could transfer between silicon and electrolyte across the thin passivating silicon oxide layer via tunneling, the majority of photocurrent would flow directly through the Pt-Si junctions. The PtNPs produced via electroless metal deposition form direct semiconductor-metal junctions with SiNWs with no interlayer impurities. The direct Pt-Si junction would enhance the charge carrier transfer rate from n-SiNW to PtNPs and lead to the improvement of the photovoltaic performance. Further, since PtNPs would work as a catalyst for the electrode processes and has stronger catalytic activity than other metals such as gold, PtNP-decorated n-SiNWs PEC solar cells generally show higher solar energy conversion efficiency. In conclusion, we have investigated the photoelectrochemical properties of SiNWs and planar Si decorated with metal nanoparticles. PtNP-decorated SiNWs photoelectrode generated significantly larger photocurrents and larger fill factors than uncoated SiNWs electrode, yielding a high power 3708
conversion efficiency up to 8.1%, substantively higher than previously reported values for SiNW-based solar cells. The much improved performance demonstrates the superior ability of metal nanoparticles to modify the electronic properties of SiNWs, and the high potential of PtNPsdecorated SiNWs as a hybrid system for solar energy conversion. Further improvement of SiNW PEC solar cells may be envisioned via optimization of the materials properties of PtNPs (or other metal NPs) and alternative solutions/ materials to present redox electrolyte. Acknowledgment. This work is supported by the National 973 project of the Major State Research Development Program of China (Grant No 2006CB933000), National Natural Science Foundation of China (Grant No 50702010), National Excellent Doctoral Dissertations of China (Grant No 200743), the Program for NCET-08-0060, Beijing Nova Program 2008B24, Research Grants Council of Hong Kong SAR-CRF Grant (Grant No. CityU5/CRF/08), and Joint research project of RGC and NSFC (Grant No. N_CityU108/ 08). Supporting Information Available: Description of fabrication of PtNP-decorated n-type SiNW arrays and photoelectrochemical measurements and figures showing TEM image of AuNP-decorated n-SiNW, photocurrent density vs voltage curves, open-circuit potential responses of the electrodes vs on-off cycles of illumination, and illustration of PtNP-decorated n-SiNW array electrode. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
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