Novel Micropixelation Strategy to Stabilize Semiconductor

Aug 15, 2012 - National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden ... used for hydrogen production in photoelectrochemical water splitt...
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Novel Micropixelation Strategy to Stabilize Semiconductor Photoelectrodes for Solar Water Splitting Systems Fatima Toor,*,† Todd G. Deutsch, Joel W. Pankow, William Nemeth, Arthur J. Nozik, and Howard M. Branz National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: We demonstrate a novel micropixelation strategy to stabilize the p−i hydrogenated amorphous silicon (a-Si:H) photocathodes used for hydrogen production in photoelectrochemical water splitting. The main mechanism of corrosion of planar electrodes involves reduction of the underlying SnO2 contact layer by electrolyte that penetrates through pinholes in the a-Si:H. We photolithographically isolate square pixels (100 μm × 100 μm) of a-Si:H by etching narrow channels in the a-Si:H and filling with protective a-SiNx. Under illumination and bias, we observe improved durability of the micropixelated photocathodes compared to planar electrodes. Extended dark potentiostatic testing also exhibits this slowing and isolation of corrosion by the micropixelated electrode. Implementation of this micropixelation strategy is a key toward creating a water-splitting system based on micrometer-scale Si p−n junction pixels. Panels of these corrosion-resistant pixels could be connected in series to produce photovoltages sufficient to split-water while avoiding photocorrosion. Micropixelation could also improve stability in other photoelectrochemical solar fuel production systems.

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INTRODUCTION

To effectively split H2O into H2 and O2 by a scheme similar to the TI system requires ∼2 V, which could be obtained from two a-Si:H junctions or four Si junctions in series. Instead of building up this voltage from single-crystal Si microspheres, we would expect to use inexpensive thin-film Si single or multijunction,4 or film crystal Si,14 junctions on inexpensive substrates. To reproduce the corrosion fault-tolerance of the TI system, one would create micropixels by micrometer-scale patterning and processing of planar Si p−n junctions. In this work, we present initial results on this novel micropixelation strategy as applied to p−i hydrogenated amorphous silicon (a-Si:H) photocathodes.4 While tested on a-Si:H, this micropixelation strategy should be applicable to film crystal Si or other photoelectrochemical materials that exhibit a similar lateral corrosive failure mechanism.15−18 The micropixelation strategy does not require expensive materials, could be achieved with inexpensive nanoimprint lithography, and is compatible with the low-cost requirements of practical lowconcentrating solar fuel generation schemes.19−21 Of course, considerable further improvement of the proof-of-concept described here would be needed for practical applications. For example, we have used a semiconductor-aqueous electrolyte interface to carry out the cathodic electrode reaction for the

Stabilization of the semiconductor electrodes in photoelectrolysis cells against various unwanted corrosion and photocorrosion reactions is a ubiquitous1−3 but difficult problem that presently inhibits the development of durable low-cost photoelectrodes for solar fuel production. The main anticorrosion techniques that have been previously described are judicious choice of anode and cathode materials,4,5 overcoating of the semiconductor surface with low-corrosion catalytic metals or metal oxides,6−9 and control of the electrolyte pH and chemistry.10−12 The approach to photoelectrode stabilization described here is based upon a successful photoelectrochemical system for splitting liquid HBr into H2 and Br2 developed at the Texas Instruments (TI) Corporation.13 The TI system provided about 1 V from two series-connected 1.2 m2 half-panels coated with a random array of 250-μm single crystal core−shell Si microspheres produced by spraying molten Si through an atomizing nozzle and solidifying the droplets. Each half-panel therefore had nearly 100 million p−n (or n−p) microspheres. A 20-nm Au layer was deposited atop each microsphere to protect against corrosion and to catalyze the HBr oxidation and reduction reactions. Because the system contained myriad junctions operating in parallel, corrosive or shorting failure of individual microsphere junctions had little effect on the HBr splitting efficiency. © 2012 American Chemical Society

Received: April 8, 2012 Revised: August 10, 2012 Published: August 15, 2012 19262

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Figure 1. (a) Processing steps, (b) resulting schematic (not to scale), and (c) top-view scanning electron microscope (SEM) image for the micropixelated p−i a-Si:H photocathodes.

and 12 mW/cm2. The 400-nm-thick intrinsic a-Si:H layer was deposited at 250 °C with SiH4 flow rate of 16 sccm, at 0.4 Torr total chamber pressure and 12 mW/cm2. The p−i a-Si:H layers were then micropixelated using photolithography. An S1818 positive photoresist was used to pattern 100-μm square micropixels, with 10-μm wide trenches between the micropixels. Next, sulfur hexafluoride (SF6) based reactive ion etching (RIE) was performed for 90 s to etch the trenches, using 30 W RF power and 15 sccm SF6 flow rate at 145 mTorr. Then the S1818 was removed using three separate rinses of acetone, isopropanol, and deionized water, and then, the sample was blown dry in ultrahigh-purity nitrogen. A 1 min clean in 10% hydrofluoric acid was performed, and the sample, including the exposed micropixel sidewalls, was then overcoated with a 200-nm-thick protective layer of PECVD silicon nitride (SiNx). The SiNx was deposited at 250 °C with flow rates of 4 sccm SiH4 and 24 sccm ammonia (NH3), at 0.5 Torr

reduction of H2O; we did not incorporate any transparent and protective catalyst layer on the electrode surface. The following sections describe the fabrication methodology for the micropixelated p−i a-Si:H photocathodes, the various test conditions and the corrosion reduction results.

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EXPERIMENTAL SECTION Micropixelation of p−i a-Si:H Photocathodes. Figure 1 shows (a) the processing steps, (b) a schematic of the resulting photocathode, and (c) a plan view SEM image of the 100-μm wide micropixelated, 408-nm thick, p(8 nm)−i(400 nm) a-Si:H photocathodes. The a-Si:H was synthesized using plasma enhanced chemical vapor deposition (PECVD) on textured fluorine-doped tin oxide (SnO2:F) coated Asahi U-Type glass. The 8-nm-thick p-type a-Si:H layer was deposited at 250 °C with flow rates of 2 sccm 2.6% diborane (B2H6) in H2 and 20 sccm silane (SiH4) gases, at 0.6 Torr total chamber pressure 19263

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total chamber pressure and 12 mW/cm2. Next, another S1818 based photolithography patterning was performed to remove the SiNx from only the surface of the a-Si:H micropixels, leaving a protective SiNx layer at the pixel edges and trenches. Another 1 min 10% HF etch was conducted to remove SiNx, and any native oxide from a-Si:H micropixel tops just before photoelectrochemical testing. As an additional protective layer, the 2μm-thick S1818 was left on top of the SiNx that covered the trenches. Both planar and 100-μm micropixelated p−i a-Si:H photocathodes were processed into electrodes by contacting the underlying SnO2:F with silver paint and a copper wire. The edges of the electrodes, including the Ag paint and Cu wire, were coated with insulating epoxy to avoid shorting during electrochemical measurements. The exposed surface area was measured for each photocathode by photographing the electrode using a scanner (Hewlett-Packard) at a known magnification and analyzing the digital image using ImageJ software. Electrochemical and Photoelectrochemical Testing in Corrosive Electrolyte. We tested the electrodes both in the dark and under illumination in a pH 10 carbonate buffer with 0.5 M of potassium sulfate (K2SO4) and 2 g/L Zonyl FSN-100 fluorosurfactant. We used this pH 10 electrolyte because alkaline solutions promote Si corrosion22 and enable rapid comparison of corrosion on the different electrodes. The K2SO4 was added to improve the conductivity of the electrolyte and the surfactant was added to ensure rapid bubble removal so the electrolyte remained in contact with the electrode surface. The counter electrode was platinum black, a Pt foil coated with rough electrochemically deposited Pt with high effective electrode surface area. The reference electrode was Ag/AgCl with 3 M KCl filling solution, at 0.206 V vs NHE. Three different types of testing were conducted: long duration (i) dark and (ii) light 2-electrode potentiostatic durability testing at −3 V versus a Pt counter electrode, and also (iii) a 3-electrode chopped-light cyclic voltammogram vs Ag/AgCl from −2 V to open-circuit potential. The electrodes were not removed from the cell and exposed to air during the entire 24-h durability testing; only the connections to the working, counter, and reference electrodes were switched during the measurement. The reference electrode was removed from the test cell and stored in 3 M NaCl during the time it was not in use in order to maintain its reference potential. The light measurements were taken with a tungsten lamp with the intensity calibrated to the AM1.5 global spectrum using a GaInP2 reference cell with a band gap of 1.8 eV which is similar to the ∼1.7 eV band gap of a-Si:H.

Figure 2. Stereomicroscopic images of (a) planar and (b) micropixelated p−i a-Si:H photocathodes after dark potentiostatic testing at −3 V for 1.25 h. Bright areas are Sn produced by the reduction of SnO2:F exposed by a-Si corrosion and associated liftoff; darker areas remained coated with a-Si:H, S1818, and SiNx. (c) Dark −3 V potentiostatic test current versus time for the planar (red dashed) and micropixelated (solid black) a-Si:H photocathodes.

suggests that after corrosion the electrodes are metallic (likely Sn) in some areas and bare glass in others. In comparison, areas covered with a-Si:H had a more uniform lateral resistance of ∼2 MΩ. Figure 3 shows a schematic of the corrosion mechanism and the slowing of the lateral corrosion by micropixelation. During the dark testing of the planar sample, the underlying SnO2:F was reduced to Sn, presumably by electrolyte penetrating initially through pinholes in the planar a-Si:H, as shown schematically in Figure 3a and b. According to the Pourbaix diagram23 of SnO2, exposed SnO2 will be reduced to Sn when the potential is more negative than −1.4 V vs NHE at pH 10. The SnO2 likely occupies less volume as it is reduced, providing exposed surface for electrolyte contact and promoting the peeling and removal of the a-Si:H layer. In the micropixelated sample shown schematically in Figure 3c, the dark current remains lower than on the planar electrode because current must pass either through a-Si:H, through pinholes, or through a small subset of corroded pixels with Sn exposed. When pinholes arise, the spread of corrosion past the pixel of origin is difficult, as shown schematically in Figure 3d. Any corrosion to neighboring pixels would have to pass by a durable SiNx barrier that adheres strongly to SnO2:F and therefore slows the arrival of fresh electrolyte to the corroding SnO2. Illuminated Potentiostatic Corrosion. Figure 4 shows interference microscope images of a-Si:H photocathodes tested under AM1.5G light at a −3 V potentiostatic condition. Figure 4a shows that after 5 h of testing of planar p−i a-Si:H, corrosion of a-Si:H had started as circular pits that likely formed from electrolyte penetrating pinholes in the as-deposited a-Si:H. The photocorrosion is evidently slower than the dark corrosion shown (after only 1.25 h) in Figure 2a. Under illuminated conditions, p−i a-Si:H will inject photogenerated holes into the SnO2, thereby providing some anodic protection to points of contact among the electrolyte, a-Si:H and SnO2. However, Figure 4b shows that after a 24-h test these pits eventually do spread to remove almost all of the a-Si:H from the surface, as confirmed by surface analysis by auger electron spectroscopy (AES) described below. In contrast, Figure 4c shows that

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RESULTS AND DISCUSSION Dark Potentiostatic Corrosion. Figure 2 shows that during dark testing at −3 V potentiostatic bias, micropixelation can isolate corrosion. After 1.25 h, the planar a-Si:H electrode was completely corroded, as seen in Figure 2a. In contrast, Figure 2b shows that during the same 1.25-h test, the pixelated electrode corroded only in isolated regions; pixelation slows the lateral corrosion of the electrodes. Figure 2c compares dramatic changes in the dark potentiostatic curves of the planar a-Si:H during the corrosion to the constant small current in the micropixelated electrode. We measured the resistance between the copper wire and several different locations on the planar electrode’s surface and found that some areas had extremely low resistance while others were too high to measure. This 19264

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Figure 3. Schematic of corrosion (a) starting through a pinhole in the planar a-Si:H, (b) spreading, with a-Si:H delamination, in a planar sample, (c) starting through a pinhole in the micropixelated a-Si:H, and (d) the corrosion being blocked by a SiNx-filled channel in a micropixelated sample. Reduction of SnO2 to Sn causes failure of a-Si:H adhesion.

etched semiconductor sidewalls. Of course, some pixels suffered corrosion through pinholes in the a-Si:H, but this corrosion was usually isolated from neighboring pixels by the strong adhesion and high durability of SiNx, with even better corrosion containment than in the dark potentiostatic experiments shown in Figure 2. Figure 4c suggests that improving the anticorrosion properties of SiNx or incorporating a different layer protecting the sidewalls could enable even greater resistance to photocathode corrosion. Figure 5 shows AES spectra of untested and tested planar a-Si:H electrodes. AES probes the composition of only the top 5−10 nm of surface material. The untested surface indicates silicon containing high oxygen content, which suggests a native oxide on an a-Si:H surface. After 24 h illuminated potentiostatic test at −3 V, the Si AES signal is not observed and the remaining surface is either reduced metallic Sn or the original stoichiometric SnOx; since AES is semiquantitative, we cannot distinguish these possibilities. It is clear, however, that the a-Si:H layer has disappeared completely during test, consistent with the interference microscope image of Figure 4b. Figure 6 shows the three-electrode cyclic voltammograms (CV) of planar and micropixelated p−i a-Si:H photocathodes. The CV measurements of Figures 6a and b were taken before and after 24 h of illuminated 2-electrode potentiostatic (−3 V vs Pt) testing, respectively. The current density of the micropixelated photocathode was calculated from the measured current by subtracting 10% of the measured area to account for the loss of a-Si:H during pixelation. The CV curves of planar and micropixel p−i a-Si:H photocathodes show very similar initial performance in Figure 6a, which indicates that the micropatterning itself does not affect the performance of the photocathodes. After the 24-h test, Figure 6b shows there is a large dark current density but a small additional photocurrent increase under illumination for both the micropatterned and planar electrodes. This result is due to the corrosion of a-Si:H and exposure of the underlying SnO2 or Sn, as seen in the AES. After 24 h of corrosion, current flow will pass mainly through the bare conducting surface seen in Figures 4b and c, rather than through any a-Si:H that may remain on the micropixelated electrodes.

Figure 4. Interference microscope images of p−i a-Si:H electrodes tested under illumination at −3 V potentiostatic conditions: (a) planar electrode tested 5 h, (b) planar electrode tested 24 h, and (c) micropixelated electrode tested 24 h.

pixelated a-Si:H still remained in most areas after 24 h testing. Figure 4c strongly suggests that after 24 h testing of the pixelated electrodes the corrosion of a typical area occurred mainly by penetration of the protective SiNx coating on the 19265

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Figure 5. AES elemental spectrum from a planar a-Si:H photocathodes (a) before and (b) after being illuminated at −3 V for a 24 h potentiostatic test.

Figure 6. Three-electrode cyclic voltammogram data for representative 100 μm micropixels (black solid) and planar (red dashed) p−i a-Si:H photocathodes (a) before test and (b) after 24 h. The data was obtained with chopped light to determine both the light and dark components of the 3-electrode current density.

Figure 7. Long-term 24-h illuminated potentiostatic (−3 V vs Pt) data for planar (black) and 100 μm micropixel (red dashed) p−i a-Si:H photocathodes.

Figure 7 compares the photoelectrochemical currents observed during the long −3 V, 2-electrode, illuminated potentiostatic corrosion of planar and micropixelated p−i a-Si:H photocathodes. The current density reduction rate for the planar photocathode is faster than that for the micropixelated photocathode. Micropixels roughly double the duration of the electrochemical reactions above a long-time baseline we attribute mainly to hydrogen evolution from pure tin. There may be contributions including either photoelectrochemical H2-production by water splitting on the remaining a-Si:H or reduction of exposed SnO2. The planar and micropixellated electrodes have similar dark current at the end of illuminated testing, most likely because similar amounts of exposed Sn or SnO2 remained on each electrode surface. At the end of testing, we observe that Sn flaked completely off large areas of the planar sample, leaving bare nonconductive glass. As seen in Figure 4c, the micropixelated electrode suffered little flaking, but also had little Sn exposed due to the protective effect of the micropixellation.

is direct corrosion of the active semiconductor materials. Addition of a protective transparent and catalytic layer over the anodic and cathodic redox surfaces in a complete water-splitting cell would reduce photocorrosion of the Si-based photoelectrodes even further. The micropixelation strategy reported here creates the huge number (>104/cm2) of parallel 100 μm × 100 μm Si cubic pixels needed to create a stabilized TI-type water splitting module. The present strategy replaces the expensive and complex TI process of producing single crystal Si microspheres and processing the spheres to enable the contacting of only the cores to a common metal conducting support. By applying nanoimprint lithography and film-silicon formation techniques, a robust and inexpensive water-splitting system could be obtained. This technique of pixelation could be applied to stabilize photoelectrodes against corrosion in any photoelectrochemical solar fuel production system by isolating incipient corrosion to individual micrometer-scale pixels

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CONCLUSIONS In conclusion, we have demonstrated a novel strategy that utilizes micropatterning of semiconductor electrodes to stabilize them against electrochemical corrosion. Our preliminary results indicate a 2-fold increase in the duration of photoelectrochemical current with a micropixelated photocathode, compared to a planar photocathode. After both dark and illuminated potentiostatic testing, it is clear that a-Si:H corrosion is slowed and isolated by the micropixelation. Although the mechanism of corrosion in this test device was corrosion of the underlying SnO2, the principal of using micropixels to stabilize photoelectrodes could certainly be generalized to systems in which the principle failure mechanism

ASSOCIATED CONTENT

S Supporting Information *

Scanning electron microscope images of corroded micropixelated a-Si:H photocathodes are included to provide evidence that SiNx is more stable than a-Si:H and protects the individual micropixels. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Lux Research Inc., 234 Congress Street, Boston, MA 02110.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. John Turner, Dr. Jihun Oh, Dr. Qi Wang, and Adam Welch for helpful insights. This work was supported by the SISGR program of the DOE, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences under U.S. Department of Energy (DOE) Contract No. DEAC36-08-GO28308.

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