Atomic Layer Deposited Corrosion Protection: A Path to Stable and

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Atomic Layer Deposited Corrosion Protection: A Path to Stable and Efficient Photoelectrochemical Cells Andrew G. Scheuermann, and Paul C McIntyre J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00631 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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The Journal of Physical Chemistry Letters

Atomic Layer Deposited Corrosion Protection: A Path to Stable and Efficient Photoelectrochemical Cells Andrew G. Scheuermann, Paul C. McIntyre* Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA. Corresponding Author *Email: [email protected]

ABSTRACT

A fundamental challenge in developing photoelectrochemical cells for the renewable production of solar chemicals and fuels is the simultaneous requirement of efficient light absorption and robust stability under corrosive conditions. Schemes for corrosion protection of semiconductor photoelectrodes such as silicon using deposited layers were proposed and attempted for several decades, but increased operational lifetimes were either insufficient or the resulting penalties for device efficiency were prohibitive. In recent years, advances in atomic layer deposition (ALD) of thin coatings have made novel materials engineering possible, leading to substantial and simultaneous improvements in stability and efficiency of photoelectrochemical cells. The selflimiting, layer-by-layer growth of ALD makes thin films with low pinhole densities possible, and

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may also provide a path to defect control that can generalize this protection technology to a large set of materials necessary to fully realize photoelectrochemical cell technology for artificial photosynthesis.

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KEYWORDS Protection layers, artificial photosynthesis, water splitting, solar fuels, metalinsulator-semiconductor (MIS), atomic layer deposition (ALD) TEXT Artificial photosynthesis and a sustainable energy economy One of the signature challenges of the 21st Century is to develop a sustainable energy economy that both fuels the engine of global economic progress while minimizing impacts on the global ecosystem. Much progress has been made in developing wind, solar, and other alternative sources of electricity. However, less than 1/3 of global carbon emissions come from electricity generation directly. The majority of the remaining arises from human transportation and the production, transportation, and consumption of goods.1 This provides motivation for artificial photosynthesis to generate both solar fuels and solar chemicals as a crucial component of an overall sustainable energy economy.


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There are myriad scientific and engineering approaches being investigated for efficient artificial photosynthesis. These range from biochemical and genetic engineering of algae and plant species2-4 to completely inorganic approaches combining solar energy harvesting with electrochemical technology. The latter approach and new developments in protection technology are the subject of this perspective. Approaches to make stable materials more efficient Inorganic approaches to artificial photosynthesis have focused on simple photoelectrochemical reactions such as the splitting of water to produce hydrogen and oxygen gas. More complex synthesis processes include CO2 reduction to make precursors for hydrocarbons and other valuable chemicals.5, 6 Water splitting in a photoelectrochemical cell (PEC) was first demonstrated by Fujishima and Honda in 1972 making use of a pH gradient and TiO2, a wide band gap metal oxide absorber that can convert solar energy to electrons and holes delivered at a useful potential without corroding in aqueous media.7 Many reports studied the long term stability of TiO2 anodes following this influential report investigating the thermodynamics and kinetics of dissolution. As shown in the titania-water Pourbaix diagram8 in Figure 1, TiO2 is the thermodynamically favored Ti-O phase over the entire range of pH for both the proton reduction and water oxidation potentials relevant to water splitting. Dissolution kinetics are slow under these operating conditions, although they do increase at more reducing potential and in more acidic pH.9-11 Experiments that did manage to show significant degradation required bandgap excitation and strong fields as well as the presence of acid to affect faster dissolution given the thermodynamic stability.11 For ultrathin layers of any material, however, even slow dissolution, especially combined with dynamic electrolyte flow, can be problematic and must be considered with regards to the long term operation of the cell. Overall, TiO2 is considered one of the most ideal materials to achieve very long term electrochemical stability during such processes. Stability aside, however, the large band gap of TiO2 means that it is an inefficient absorber of solar light. So even if stability is solved, solar absorption is not. These two issues, the electrochemical stability of materials like TiO2 and large band gaps leading to poor absorption, are related, as the band gap represents the stabilization energy of chemical bonding in the solid. Electrochemically stable TiO2 can absorb < 10% of the solar spectrum compared to 80% absorbed by silicon, the most commonly used solar cell material. The right pane of Figure 1 shows how an increasing band gap (or decreasing wavelength) corresponds to an increasingly smaller absorbed fraction of the solar irradiance.


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Figure 1: Left - Titanium-water Pourbaix diagram showing the thermodynamically stable phases of titanium at a given pH and potential condition. TiO2 is one of the most ideal materials for corrosion stability under oxidizing conditions. Right – TiO2 has a band gap over 3 eV and, as a result, absorbs < 10% of the total solar irradiance. Other common materials are compared, where silicon can absorb up to 80% by comparison. In the decades after Fujishima and Honda’s initial work, much progress was made on photoelectrochemical cells, expanding the range of absorber materials studied. The cathode in water splitting cells is typically more stable than the anode, as reductive products of the exposed cathode tend to be more difficult to form than oxidative products of the anode. Many works have, therefore, characterized p-type Si,12-14 GaP,15 InP,16 and GaInP17-19 among other group IV and III-V compound semiconductor photocathodes. II-VI semiconductors have also been widely used on the cathode side, particularly for splitting HBr or HI to generate hydrogen, rather than water.20-25 While stable for short periods, many photocathode materials, such as p-Si, ultimately exhibit corrosion during extended operation. For water splitting on the most efficient photoanode materials, however, even short term stability is unusual. In order to avoid oxidative corrosion, a common approach for water splitting directly on exposed photoanodes has been to employ relatively wide band gap oxides that, with sufficiently large dopant concentrations, could achieve lower resistivities and more useful band edge energies for charge extraction.26 Beginning with the use of TiO2 in 1972, materials with slightly smaller band gaps have been investigated intensively, including WO3 and Fe2O3.27 The overarching goal of such research has been to make electrochemically stable materials more efficient. Cation dopants such as Cr, Mn, Ni, Fe as well as non-metal dopants such as C, N, and S have been explored for enhancing the visible light absorption of TiO2.28-30 A similar approach, involving doping, has also been pursued for various other metal oxide photoanodes.31-34 Because this stream of research typically assumes that a semiconductor-electrolyte junction will be a key feature of the cells, moving the conduction band to more negative potentials becomes a priority. Stability with respect to oxidation is correlated with the valence band edge energy, whereas efficient light absorption is a function of the overall band gap. Strongly polarizing cation dopants that form covalent M-O bonds promote visible light absorption and move the conduction band edge to more negative potentials, whereas N3- and S2- anions tend to move the valence band edge to more positive potentials.35, 36 Research along this line continues, including nano-structuring of hematite to


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increase its light absorption without altering the material’s inherent stability under water oxidation conditions.37 In parallel, another stream of research has focused on making already efficient light absorbers more stable. Approaches to make efficient materials more stable Research advances in PECs during the 1970’s and 80’s were, in part, motivated by the 1973 Oil Crisis. Photovoltaic (PV) technology also flourished at the same time, and progressed in parallel with PEC research, with similar challenges and advances. Differences arise, however, from the fact that solar energy is harvested to do electrical work in one case and electrochemical work in the other. Maximum power output, rather than electrochemical stability combined with surface reactivity for oxygen and proton production, is the desired characteristic of photovoltaics. PEC structures that also incorporate a catalyst to enhance the water oxidation rate have a catalystsurface oxide–semiconductor structure. This is analogous in many ways to the metal-insulatorsemiconductor (MIS) structure studied intensely in the PV community as an alternative to the pn junction solar cell. In 1979, Green and coworkers developed a silicon solar cell which achieved a 655 mV open circuit voltage and 17.6% efficiency38 building on their work with MIS solar cells. 39 They moved from MIS cells, specifically, to study and explain a whole class of “Conductor-Insulator-Semiconductor (CIS)” solar cells. In CIS solar cells an ultrathin insulator provides passivation of interface traps that would otherwise increase recombination while also being thin enough to allow unimpeded tunnel conduction between the solar light absorbing semiconductor and the overlying conductor layer.40, 41 In these cells, the built-in voltage is set by the work function difference between the semiconductor and the conductor. They also display a characteristic inversion layer at the semiconductor surface which screens defect states at the semiconductor/insulator interface and facilitates lateral minority carrier transport.42 This gives CIS solar cells properties similar to those of a pn junction cell, but without using costly doping processes. In some cases, the CIS photovoltaic cells even exceeded the performance of pn junction cells. In order to minimize resistive losses associated with the insulator, however, layers of less than 3 nm were used to ensure facile minority carrier tunnelling.40 While metal-insulator-semiconductor structures were investigated for application in solar cells, a parallel effort was underway for electrodes in photoelectrochemical cells. The overarching goal was to take already efficient absorber materials, like silicon or gallium arsenide, and make them stable in corrosive environments. Both the metal layer and insulator layer can potentially impart corrosion protection because they are typically either stable under water splitting conditions or, in the case of a catalyst metal, form a stable conductive metal oxide. Design rules for simultaneous efficiency and corrosion resistance are fundamentally different in MIS Schottky junction photoanodes as compared to the liquid-semiconductor junctions. In general, moving from liquid-semiconductor junctions (referred to as Type 0 in Figure 2 below), to metal-insulator-semiconductor Schottky junctions (Type 1), to semiconductor homoor hetero-junctions (Type 2), to completely separate PV and electrolyzer components (Type 3) increases the complexity of fabrication and the number of components on one hand, but it further decouples operation of the various physical elements, allowing for more degrees of freedom in their optimization. These trends are illustrated in Figure 2. Applying insights from MIS solar cells to electrochemical systems, new intermediates were created between the Type 0


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semiconductor-electrolyte configuration and Type 3 completely separate configuration. In particular, the insertion of even an ultrathin metal layer between the semiconductor and electrolyte causes the Fermi level in each layer and the band bending to be set, at zero bias, by equilibration of electrons in the metal and the semiconductor absorber, rather than by the electrolyte. This has the effect of allowing any electrolyte to be used without compromising the photovoltaic efficiency of the device. Likewise, the band edge positions of the semiconductor with respect to the redox energy levels in the electrolyte no longer matter because the metal forms an Ohmic contact to the electrolyte. On the other hand, if the metal film does not completely block the electrolyte, the semiconductor band edges will matter, particularly for the stability of the underlying semiconductor and/or insulator. Likewise, Type 2 semiconductor homo- or hetero-junctions decouple the underlying semiconductor substrate’s Fermi level from that of the metal, removing a constraint on the acceptable range of catalyst work functions. This allows a very wide range of catalysts to be used as long as the buried semiconductor junction contains a degenerately-doped layer at the interface with the catalyst. In this fashion, successive added layers increase the total bill of materials and device complexity, but also decrease stability-efficiency trade-offs.

Figure 2: Different photoelectrode types with an increasing bill of materials and complexity from left to right. The increased number of components simultaneously allows for the decoupling of physical properties from individual components resulting in more design degrees of freedom.

Thin metal films, particularly gold and platinum, were investigated early on as protection layers for photoanodes.43 The layers needed to be impermeable to offer sufficient protection, but this was difficult to achieve without making the metal too thick for efficient light transmission. Similarly, ultrathin oxide insulators were studied but, like metals, it was difficult to fabricate impermeable, pinhole-free films. Light absorption was not a problem for thicker insulating films, but electrical resistance was. When tunnelling is the dominant mechanism of current transport across them, which is often the case, they become prohibitively resistive at thicknesses greater than ~3 nm.44 Organic conductors were also explored in great detail as protection


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layers,45 as were ferrocene species covalently bonded to nSi46 and later methanol silicon surface passivation.47 Ultimately these organic approaches only offered short term stability or required non-aqueous environments for longer term stability making them unsuitable for many fuel and chemical artificial photosynthesis applications. Despite these challenges, progress was made in each respective area. For MIS Type 1 cells in particular, Howe et al. of the Amoco Research Center utilized ultrathin tunnel oxide materials as protection layers in conductor-insulator-semiconductor (CIS) cells. Both native and non-native oxides were studied and were found to be capable, in some instances, of providing enhanced stability of photoanodes for solar water splitting.48-51 Fundamentally, these experiments were limited by the ability to create uniform, ultrathin layers that were pinhole-free with methods existing at that time. This was also the case for the early work of Kohl et al., coating n- and ptype Si with CVD-TiO2. Titania layers believed to be thick enough to avoid substrate corrosion through pinholes or cracks were found to inhibit charge transfer between the semiconductor and the electrolyte.52 Other early attempts at coating CdSe or CuO with TiO2 likewise failed to produce both efficient and stable structures.53-55 Throughout the 1980’s research continued on ultrathin protection layers in MIS Type 1 architectures, including RuO2 and IrSi protection of nSi achieving 7 days of stable chlorine evolution,56 RuO2 and ITO protection of nSi stable for 65 hours of chlorine evolution,57 and Pt and relatively thick and doped SiO2 on nSi stable for 100 hours of water oxidation58 although the anode’s performance decayed sharply at 120 hours.59 Another report described a thicker protection layer of 20 nm of MnO2 on nSi, achieving 650 hours of stable water oxidation in a Type 0 cell at a low current density.60 The turn on potential was not unacceptably large, but the saturation photocurrent for this protected anode was very low, ~1 mA/cm2, during water splitting. Recognizing the thickness trade-off for insulator layers to achieve corrosion protection and facile carrier conduction, Campet et al. enumerated possible alternatives for photoanode protection as shown in Figure 3.61 First, it was believed that relatively thick layers were necessary for adequate oxidation resistance and, therefore, a protective tunnel oxide was not viable in spite of the prior, encouraging, reports by Howe et al.48-51 Second, a transparent conductive oxide (TCO) with a valence band aligned for conduction was considered an ideal approach for both corrosion protection and hole conduction to the anode surface, but no good candidate materials existed at the time. Tin oxide and indium-doped tin oxide (ITO) were good candidates based on their band offsets to silicon, but, empirically, their performance was very poor. Campet proposed that a space charge region in the ITO blocked hole tunnelling into the electrolyte.61 Kraft et al. showed with sputtered ITO that electrolyte was still able to penetrate through a 230 nm thick protection layer leading to corrosion of GaAs.62 Finally, Campet proposed conduction via a defect band in a protective oxide layer to be a practical alternative to achieve hole transport from the valence band of silicon. This was demonstrated empirically with cerium doping of a SrTiO3 protection layer deposited on GaAs,61 and by sputtering a SrTiO3 layer under conditions that apparently generated point defects that pinned the Fermi level at the Ti:3d(t2g) energy, allowing for hole conduction through 700 nm of the oxide. This yielded a GaAs protected cell with 14% efficiency at a tenth of a sun. After 100 hours of stability testing, the 700 nm protection layer delaminated entirely in a sheet and it was found that electrolyte was able to penetrate through the whole structure as in the ITO case. This indicated that a thicker oxide was, by itself, an insufficient condition for corrosion protection.61-63 The same authors also investigated the


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formation of semiconductor-insulator-semiconductor junctions in silicon solar cells when the protective oxide was doped to the point it became a degenerate semiconductor. Degenerate SrTiO3 and ITO make n+ systems, so p-type Si was required for an effective built-in field.64 Switzer, alternatively, successfully explored Tl2O3, one of the few oxides for which an efficient SIS junction with nSi has been reported, achieving an 11% efficient solar cell at 0.75 sun illumination.65

Figure 3: Alternatives for photoanode protection described by Campet et al. Tunnel oxide protection of efficient photoanodes was thought to be impossible because thick layers were required for corrosion protection, giving unacceptably high electrical resistance. (Reproduced with permission from ref 61) Considering these two general approaches, making electrochemically stable semiconductors more efficient or making efficient semiconductors more stable under water splitting conditions, success in either now relies on precise nanoscale manipulation of matter to create novel structures that can break long-standing design trade-offs. For the 2nd approach of making efficient semiconductors more stable, the application of ALD to protection layer deposition that has emerged in recent years has done exactly that. Atomic Layer Deposition: A new paradigm for protection of photoelectrodes In 2011, Chen et al. first used 1-2 nm of ultrathin ALD-TiO2 coating to protect SiO2/nSi anodes while achieving facile hole transport limited by tunneling.66 These samples displayed a photovoltage of 550 mV, saturation current density of 30 mA/cm2 (essentially 100% of the expected current, accounting for reflection losses), and showed no decrease in performance over 8 hours of testing. In other words, close to ideal solar conversion efficiency was achieved at the same time as dramatically improved stability. Previous research efforts had long sought ultrathin corrosion protection layers that would permit facile hole transport, and transparency to solar light, while being sufficiently uniform in thickness to block oxidative corrosion. Atomic layer deposition (ALD) with its self-limiting, layer by layer, growth mechanism is a technique for synthesizing such layers. Moreover, its excellent uniformity over complex surface features offers the capability to protect general three dimensional structures in addition to planar photoelectrode surfaces.67


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ALD was first reported in the 1960’s by researchers in the Soviet Union who named it ‘molecular layering’.68 It was apparently independently developed in the 1970’s by Suntola and colleagues at the University of Helsinki, and given the name ‘atomic layer epitaxy’.69, 70 Initially only atomic precursors were used, hence the name ‘atomic’ applied to the technique. Subsequently, many molecular precursors were used, expanding ALD’s applicability to a vast array of materials. Today, more than half of the periodic table of elements can be deposited using ALD precursors, providing a large selection of oxides, nitrides, phosphides, sulfides, carbides, and an increasing number of elemental metals. As shown in Figure 4, in a typical ALD process, two precursors are pulsed into a reactor separately. Half reactions occur where a layer of the first precursor, in this case tetrakis(dimethylamido)titanium, forms by chemisorption at reactive sites on the substrate, until the surface is saturated. Intermediate steps of inert gas purging and/or evacuation clear the chamber of excess of the first precursor and then the second is introduced, often an oxidant such as water vapor, with a further purge/evacuation completing the deposition cycle. By continuing the process for multiple cycles, films can be deposited with sub-monolayer thickness precision and, in many cases, with film closure on the substrate surface at sub-nanometer thicknesses.71 The ALD chemistry shown schematically in Figure 4-Awas used to give the 2011 result (Figure 4-B) that made a stable tunnel oxide protected silicon photoanode. Figure 4-C shows an example from another report of conformal coverage of a 2 nm ALD-Al2O3 film over a TiO2 nanoparticle demonstrating the capability of ALD to conformally coat arbitrary three dimensional structures. The electronics industry has fueled advances in ALD where it is already an essential part of manufacturing high-k gate structures for transistors,72 and diffusion barrier liners for Cu metallization. As such, the technology is well suited to continued technological advances in corrosion protection and photoelectrochemistry.

Figure 4: Atomic layer deposition allows deposition of ultrathin, conformal pinhole-free films. A) Shown here is the TDMAT-water ALD process for deposition of TiO2 protection layers. B) TEM showing ALD used to fabricate < 2 nm thick ALD-TiO2 tunnel oxide protection layers on SiO2/nSi substrates for protection of photoanodes with an overlying Ir-based oxygen evolution


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catalyst. C) SEM of 2 nm ALD-Al2O3 film on TiO2 nanoparticle demonstrating the ability of ALD to conformally coat three dimensional structures (reprinted with permission from ref. 73. Copyright 2009, The Royal Society of Chemistry). Despite the initial promise of ALD to fabricate ultrathin metal oxide protection layers for Si photoanodes, there is interest in depositing thicker layers by this technique if the mechanism of corrosion protection requires blocking diffusion of oxidant species through the protection layer. Because the resistance contributed by a large band gap tunnel oxide increases exponentially above a certain thickness, e.g. ~ 2 nm of SiO2 for a metal/SiO2/Si MIS structure,74 it is critical to keep the thickness below this value while also maintaining conformal coverage. Following the initial demonstration of ALD-TiO2 protection of silicon photoanodes,66 Scheuermann et al. studied amorphous as-deposited ALD-TiO2 films in the thickness range of 1 - 10 nm deposited on p+Si with a ~ 1.3 nm thick chemically-derived SiO2 layer. It was discovered that the amorphous TiO2 samples exhibited an Ohmic resistance corresponding to an additional overpotential of ~21 mV/nm to reach 1 mA/cm2 photocurrent for TiO2 film thicknesses greater than ~ 2 nm.75 Without intentional doping, this ALD-TiO2 system was able to achieve the same ‘defect band’ type hole conduction described previously by Campet et al.61 To explain this unexpectedly low barrier for hole conduction through the ALD-TiO2, a conduction model was proposed whereby holes tunnel from the Si anode through the ultrathin SiO2 interlayer and then hop via trap states in the TiO2, that have a spectrum of energies centered ~1eV below the TiO2 conduction band edge.76-79 With atomic layer deposition, the thickness could be varied with high precision over the range of interest allowing for a precise measurement of the linear overpotential thickness scaling. This scaling matched the predicted electric field to mediate conduction across the TiO2/SiO2 bilayer, suggesting that this model was appropriate to describe the phenomenon.75 More recently, Hu et al. fabricated ALD-TiO2 films on Si, GaAs, and GaP with the same precursors and similar process conditions to those employed in references 63 and 72 and studied considerably thicker films, ranging from 4 to 143 nm.80 They similarly observed anomalously high hole conductivity, so high that the photocurrent onset for water splitting was effectively TiO2 thickness independent. These samples were stable for 100 hours, maintaining 90% of the original current density, but also exhibited poor photovoltages, ~ 400 mV compared to the 550 mV observed for ultrathin ALD-TiO2 protection layers on nSi.66 Hu et al. proposed a similar mechanism of hole hopping via trap states in the TiO2, although it would require at least 4 orders of magnitude greater hole conductivity of the TiO2 compared to the films prepared, using the same precursors and a similar ALD process, in reference 75. It was also reported in reference 80 that in order to make electrical contact to the highly conductive TiO2 beneath the film surface, a nickel overlayer was required. Mercury drop contacts did not produce enhanced conductivity in the as-deposited sample, but they did after the top layer of the TiO2 film was etched away. This observation suggested that a resistive surface layer was formed on the ALD-TiO2 films, although this layer had not been reported in prior publications. Subsequent research by members of the same group showed that highly conductive TiO2 could also be realized using sputtered TiO2 and by ALD of TiO2 using an alternate Ti precursor, titanium isopropoxide (TTIP). However, the reported thickness uniformity of the isopropoxide-derived films was poor, ranging from 50 nm to 150 nm. Furthermore, the reported photovoltages observed for nSi-TiO2-Ni photoanodes ranged between 150 and 350 mV both before and after annealing treatments.81 This is consistent with the formation of a semiconductor-insulator-semiconductor (SIS) junction in which the highly


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conductive (degenerate semiconductor) TiO2 layer screens the influence of the Ni catalyst work function, thus achieving less band bending in the nSi compared to that of a corresponding MIS junction. Prior literature indicates that n+-type degenerately-doped metal oxides exhibit small Schottky barriers to n-type semiconductors. Aside from possible negative effects on photovoltage, two other recent reports also investigated the ultrahigh conductivity phenomenon of TiO2 and the proposed uniqueness of nickel in causing this effect. In one report, a Ti metal layer was inserted between the TiO2 and a p+n Si junction.82 If the p+ region was highly doped, a very conductive structure could be realized with Pt rather than a Ni top contact. This enhanced conductance may, however, rely on a different mechanism utilizing conduction band states instead of trap states within the TiO2 forbidden gap. Furthermore, adding a low-work function Ti interlayer is also expected to produce a low photovoltage for Type 1 Schottky junction nSi photoanode. In another recent report, our group coated equivalent ALD-TiO2/SiO2/p+Si anodes with 2 nm thick iridium or nickel oxygen evolution catalysts to test the influence of catalyst material on anode electrical resistance. In this case, the nickel did not produce a conductance enhancement but, instead, gave significantly greater bulk resistivity than did an Ir OER coating.83 Summarizing these reported results (Figure 5) demonstrates that the bulk conductivity of ALDTiO2 protection layers is a complex function of materials chosen for each layer in the electrode stack and the detailed conditions under which they are synthesized. It is evident that the trivial application of a nickel OER catalyst alone will not guarantee a highly conductive structure.


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Figure 5: Reported results from different laboratories on the electrical resistance of silicon anodes with TiO2 protective coatings and various catalyst layers (a) Reference 77: very high bulk hole conductivity observed across a wide range of TiO2 thickness with a TDMAT/H2O TiO2 ALD process; (b) Reference 78: TDMAT/H2O ALD, TTIP/H2O ALD, and sputtered TiO2 can all produce highly hole-conductive protection layers; (c) Reference 79: Pt catalyst on 100 nm TiO2 with Ti layer interposed between the TiO2 and np+Si substrate; (d) Reference 80: Ni and Ir OER coatings on otherwise identical sets of TDMAT/H2O ALD-TiO2 anodes showing increased bulk resistance for the Ni catalyst case. For either the water oxidation or reduction half-cell in a full water splitting cell, the photovoltage is the value most closely related to the ultimate cell efficiency, particularly photovoltage measured at practical operating current density such as 10 mA/cm2.84 In a recent report, we have described design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes.85 The main conclusions for that work are illustrated in Figure 6 below. The recognition that degenerately doped n-type protection layers lead to poor photovoltage for n-type semiconductors due to reduced band bending,80, 81 means that less heavily doped protection layers are necessary.85 However, a different kind of photovoltage loss is then observed. This loss occurs due to the need to accumulate charge across a more insulating protection layer, to mediate hole conduction across the layers. It increasingly affects these structures as the insulator becomes thicker or more defect free. This sets up a direct trade-off between built-in field for an ideal MIS structure and charge accumulation photovoltage loss as a result of thicker and less


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defective insulator (I) layers that may be necessary for long-term corrosion protection. In reference,85 it was demonstrated that conduction across a protection layer can be decoupled from charge accumulation in the silicon absorber by using a Type 2 p+n buried junction, thus removing the insulator-dependent loss and achieving much higher photovoltages. A value of 630 mV was achieved with an 8 nm thick ALD-TiO2 film.

Figure 6: Band diagrams illustrating the difference between protection layers in a Type 1 metalinsulator-semiconductor (MIS) junction and a Type 2 buried p+n junction with respect to the ability for holes to be extracted from the junction before recombining. In order to mediate leakage conduction through the limited pathways available, holes must be accumulated in the nSi region of the Type 1 junction establishing a field across the protection layer leading to a voltage loss. In the Type 2 configuration, this hole accumulation is built in, effectively eliminating the photovoltage loss. For the Type 1 nSi Schottky junction structure, however, only moderately conductive and thin TiO2 protections layers promote high photovoltages. Figure 7-A summarizes the photovoltage vs. thickness trends and water oxidation cyclic voltammograms showing the increasing disparity between Type 1 nSi and Type 2 p+nSi structures for thicker ALD-TiO2 (Figure 7-B).85 In recent work, forming gas (H2/N2) anneals after OER catalyst deposition were found to improve the performance of ALD-TiO2 protected Type 1 nSi photoanodes. It was shown that the TiO2 crystallizes to the anatase and then to the rutile phase with increasing temperature in the range 300⁰C - 600⁰C. Larger film thicknesses promote crystallization at lower temperatures. At 450⁰C, the oxide thickness-dependent photovoltage loss of the TiO2-coated photoanodes was observed to decrease by more than a factor of two, consistent with the increase in TiO2 dielectric constant after crystallization. Furthermore, forming gas anneal also passivates


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oxide/semiconductor interface traps, thus reducing recombination and further increasing the photovoltage. Figure 7-C shows results from a 2 nm Ir / 1.2 nm TiO2 / SiO2 / nSi anode annealed at 450⁰C that has a photovoltage over 600 mV and a turn-on potential of 0.92 vs NHE in acid, at the time of this Perspective, the best such performance for a Type 1 nSi photoanode achieved to date.83

Figure 7: (A) Reference 82: The photovoltage loss trends as measured in ferri/ferrocyanide (FFC) electrolyte with TiO2 (green) and SiO2 (blue) are juxtaposed against the near constant photovoltage with respect to oxide thickness achieved for the p+nSi buried junction. (B) Reference 85: Water oxidation cyclic voltammograms in acid showing the disparity between the p+nSi and nSi results. (C) Reference 83: Type 1 TiO2-protected nSi Schottky junction prepared with a post- catalyst deposition forming gas anneal at 450⁰C. Summary of advances and future challenges in ALD-protected water splitting photoelectrodes Considering these results in sum, it is possible to distinguish the key factors that affect the electrode performance of water splitting cells and ongoing efforts in each area to advance the state-of-the-art: In silicon electrodes, a salient failure mechanism from the first report of ALD protection was further oxidation of the silicon leading to a growth of thicker, insulating SiO2 layer, which was inhibited by the presence of an overlying TiO2 protection layer.66 Later results showed a substantial performance advantage occurs by thinning the SiO2 interface layer between the Si and the TiO2.74 Ongoing work in this area is looking at methods, such as remote oxygen scavenging to reliably thin the interlayer SiO2 that can simultaneously achieve photovoltage and conductivity enhancements for a given TiO2 thickness.86 Assuming the SiO2 thickness is stabilized and perhaps thinned to < 1 nm, prior literature reports indicate that a major factor controlling the resistance of metal oxide protected photoanodes is the interaction between the metal and the metal oxide.80-83 Recent reports have shown that the complicated interaction can vary by metal type, affecting fill factor and built-in field.83, 87 This interaction may cause extrinsic doping of the TiO2 whether 1) by locally reducing the TiO2 to create an effectively degenerate n-type material, 2) by doping the metal oxide layer with ionic


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components from the metal, or 3) by formation of shunting metallic filaments that span some or all of the thickness of the metal oxide layer. It is quite possible that the high photocurrents reported by different groups or with different processing methods rely upon different mechanisms to achieve their reported resistances to hole transport through TiO2. Future work is needed to move beyond descriptive models and to elucidate the underlying mechanisms that control the conductivity. The junction type, whether Type 1 nSi or Type 2 p+nSi has a strong effect on device performance that is related both to the oxide thicknesses and the TiO2 conductivity.84, 85 For highly conductive n-type TiO2, only the Type 2 p+nSi structure can produce a high efficiency cell because the degenerate semiconducting oxide screens the effect of high work function surface catalyst layers and thereby eliminates the built-in field in the Type 1 MIS structures. Therefore, for a Type 1 nSi Schottky junction structure, moderately resistive (and, therefore, thin) TiO2 is preferred to maximize photovoltage. In addition to the direct impact of higher electrical resistivity of the TiO2 on the effective overpotential for water splitting, a more resistive protection layer turns the electrode stack into a leaky capacitor that incurs photovoltage loss that is proportional to the oxide thickness, another motivation for keeping protection layers thin in the Type 1 photoanodes. This loss can be removed by using a Type 2 p+nSi structure.85 Regarding the simpler Type 1 nSi structure, however, the best results to date have been obtained by using moderately resistive TiO2 and annealing after catalyst deposition in forming gas to minimize the density of interface traps and increase the dielectric constant, both of which enhance photovoltage.83 Finally, the long-term efficiency of these devices is related to the stability of a given high photovoltage and conductivity, and is of course the principal reason to pursue ALD-grown protection layers in photoelectrochemical cells. This Perspective follows the evolution of research on protection layers and the challenges associated with making them fully protecting while also maximizing the efficiency of the junction for light absorption and charge separation at a high built-in potential. Systematic study of failure mechanisms, and characterization techniques to give accelerated insights for very-long-term stability are important areas of further work. The possibility of pinholes that could lead to corrosion and undercutting of the protective film is an area of ongoing concern. Atomic layer deposition has enabled the use of thinner layers than ever before, with a deposition mechanism noted for synthesis of films that are conformal to complex topography and that can be pinhole free at small thicknesses, but remaining questions surround the ability to limit pinholes statistically with very large scale films in total area. Research on non-ALD protection layers of Si photocathodes illustrates that passivating SiO2 can block individual pinholes from triggering catastrophic failure in acid, but not in alkaline solutions where undercutting does occur.88 Other failure mechanisms such as covering of the catalyst active area by solution impurities and slow kinetic dissolution of the catalyst are also concerns.88, 89 While more in depth study is needed to elucidate the mechanisms of failure and how to ameliorate them, great progress has already been made in just the last five years suggesting that the pace of advances may continue. In the first report on ALD-TiO2 protection in 2011, 8 hours of stability were demonstrated.63 Since then, stability has been pushed to days, and even months with one report demonstrating 2,200 hours of stability for ALD-TiO2 coated Si nanowire arrays, also demonstrating the ability of ALD to protect high-surface area semiconductor structures.90


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While the latter half of this perspective has focused largely on ALD-TiO2 layers on Si electrodes because of fundamental breakthroughs in understanding their efficiency, there have been an increasing number of reports of using ALD-TiO2 to protect other interesting absorbers (GaAs and GaP,80 CdTe,91 and BiVO492), and of other metal oxide protection layers synthesized by ALD, including MnOx,93 Ta2O3,94 and Al2O3.74 Beyond ALD, there are many promising protection layer approaches beyond the scope of this Perspective, including use of 2D materials such as graphene protection layers,95-97 and transition metal dichalcogenides.98-100 MoS2, which has promise in stabilizing photoelectrochemical systems, can also be deposited by ALD producing wafer-scale uniform layers with excellent control over thickness.101 There has also been a resurgence of interest in thin metal protection layers including recent reports of Ni102 and Ir/IrOx103 catalyst protection layers deposited on silicon photoanodes. A recent review gives a more complete summary of thin film protection strategies beyond ALD.104 Finally, there have been an increasing number of reports on ALD of metals, showing the ability to uniformly coat 3D structures, such as Pt nanoparticles on a Si nanowire array.105 Given the importance of the catalyst for efficiency and cost of photoelectrochemical device, atomic layer deposition has great potential for optimizing this critical component of water splitting devices as well.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors thank all members of the McIntyre group, Chidsey group, and US-Ireland RENEW collaboration, specifically members of the Hurley, Pemble, and Mills groups, for their work that has contributed to major developments reviewed here. This work was partially supported by the Stanford Global Climate and Energy Project and National Science Foundation program CBET1336844. A.S. graciously acknowledges financial support from a Stanford Graduate Fellowship and a National Science Foundation Graduate Fellowship.


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Atomic Layer Deposited Corrosion Protection: A Path to Stable and

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