Photovoltaic and Photoelectrochemical Solar Energy Conversion with

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Feature Article 2

Photovoltaic and Photoelectrochemical Solar Energy Conversion with CuO René Wick, and S. David Tilley J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08397 • Publication Date (Web): 28 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015

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The Journal of Physical Chemistry

Photovoltaic and Photoelectrochemical Solar Energy Conversion with Cu2 O René Wick and S. David Tilley* Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland

ABSTRACT

The amount of solar power striking the earth’s surface is vastly superior to humanity’s present day energy needs, and can easily meet our increasing power demands as the world’s population grows. In order to make solar power cost competitive with fossil fuels, the conversion devices must be made as cheaply as possible, which necessitates the use of abundant raw materials and low energy intensity fabrication processes. Cuprous oxide (Cu 2 O) is a promising material with the capacity for low cost, large scale solar energy conversion due to the abundant nature of copper and oxygen, suitable bandgap for absorption of visible light, as well as effective, low energy intensity fabrication processes such as electrodeposition. For photoelectrochemical (PEC) water splitting, protective overlayers have been developed that greatly extend the durability of hydrogen-evolving Cu 2 O-based materials. Recent developments in the advancement of protective overlayers for stabilizing photoabsorber materials for water splitting is discussed and

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it is concluded that the use of protective overlayers is a viable strategy for practical water splitting devices.

1. Introduction The idea to convert abundant sunlight into a valuable asset such as electrical power or a chemical fuel can be traced back to the discovery of the photovoltaic effect by Becquerel in 1839,1 and has captured the imagination of scientists ever since. In 2012, the average consumption rate of energy worldwide amounted to 18 TW (18 x 1012 W),2 and this is predicted to increase to 43 TW by 2100.3 Renewable energies are being sought to meet this increased demand due to the consequences of a warmer climate, the finite nature of fossil fuels, and the increasing cost associated with extracting the remaining fossil fuel reserves.4 Of the different renewable energies, solar energy is by far the most abundant, accounting for more than 99% of the total possible power from all renewable resources.5 The average amount of solar power striking the whole of earth’s surface–including cloud cover and nighttime–is about 95,000 TW,6 more than five thousand times higher than our current energy needs.2 The great challenge is to find a cost-effective method to harness just a small fraction of this vast supply of power. One of the main issues with solar energy is that sunlight is not very concentrated, and this necessitates large area devices to capture a significant amount of power. For example, to produce the predicted 43 TW needed by 2100 with solar energy alone (using the global average), one would need 10% efficient devices with a total active area of approximately 2.3 million km2, six times greater than the size of Germany. Although this is a very large number, it amounts to just a fraction of the Sahara desert (~24%), and with the increased solar insolation of the Sahara versus the global average, the area required would be substantially less. There are of course many 2 ACS Paragon Plus Environment

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suitable locations around the world where solar conversion facilities can be constructed, but this calculation shows that the amount of solar power that is available can easily meet our growing energy needs. In order for a technology to be implemented on such a large scale, however, cheap and highly abundant materials must be used to construct the devices. Moreover, the fabrication costs of the devices, including processing of the raw materials, must be kept as low as possible. Cu 2 O is a promising candidate for low cost solar energy conversion, with estimated copper reserves of 690 million tons worldwide (compared with 66,000 tons for all platinum group metals combined).7 Wadia et al. have shown that copper-based photovoltaic materials (including Cu 2 O) are among the least expensive in terms of raw materials cost.8 As such, copper-based materials have the potential to reach the terawatt scale, provided that the solar conversion devices can be fabricated in a cost-effective manner. An important challenge associated with photovoltaic (PV) electricity generation is energy storage. Low-cost energy storage is a critical factor towards scaling up intermittent renewable energy sources such as solar, due to fluctuations in energy supply versus demand, both on the daily as well as seasonal time scales. A very promising method to store energy is in the form of chemical bonds,9 which is the model adopted by nature in photosynthesis. Photoelectrochemical (PEC) water splitting addresses the issue of energy storage by converting light energy directly into a chemical fuel. Using a semiconductor material, such as Cu 2 O, absorbed light generates energetic electron–hole pairs (i.e. a photovoltage) that can be used to drive the water splitting reactions. The overall water splitting reaction consists of two half reactions, for which the standard reduction potentials (E°) are: 2 H + + 2 e− → H 2

0 Ered = 0 V vs. SHE

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(1) 3


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O2 + 4 e− + 4 H + → 2 H2 O

0 Ered = 1.23 V vs. SHE

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(2)

Combination of the two half reactions (following inversion of equation 2) gives the overall water splitting reaction: 2 H2 O → 2 H2 + O2

𝐸𝐸 0 = –1.23 V

(3)

which gives the thermodynamic minimum voltage that must be applied in order to electrolyze water (although no net current flow occurs exactly at this potential). Applying a higher voltage (or photovoltage, in the case of a PEC cell) provides the driving force to achieve net current flow, and in practice, electrolyzers operate at much higher potentials (1.8–2.0 V)10 due to the sluggish reaction kinetics at the electrodes, particularly on the oxygen side. Photovoltaic-driven electrolysis may play an important role in the short term for reducing our carbon intensity, but this will require economic incentives as the hydrogen produced in this way cannot compete on a cost basis with hydrogen produced from fossil fuels.11 PEC cells, which combine photon absorption and the water splitting catalysis in the same material, have the possibility to produce hydrogen even more cheaply by eliminating the costly electrolyzer. In the final analysis, it is the cost of the hydrogen produced that will be the determining factor for the technology that is ultimately implemented. The focus of this Feature Article is on recent progress using Cu 2 O as a photocathode in PEC water splitting. A major challenge for Cu 2 O in PEC applications is its low stability in aqueous media. However, recent research has shown that protective overlayers are highly effective, which has enabled their successful use in water splitting devices. A brief overview of recent research carried out using Cu 2 O as a PV material is also presented, as the underlying principles in PEC 4 ACS Paragon Plus Environment

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and PV are quite similar, and important parallels can be drawn between the two fields. In light of the results discussed herein, important research directions for the future are outlined. 2. Structure and Properties of Cu 2 O Cu 2 O is one of the stable phases of the copper oxides, the other two being Cu 4 O 3 and CuO. It is distributed over the entire planet12 and has been mined since ancient times due to its high copper content (88.8%). The crystal structure of Cu 2 O in its cubic Bravais lattice is shown in Figure 1, with the space group

(224) and a lattice constant of 4.2696±0.0010 Å.13 Oxygen

atoms occupy the body center and edges of the cubic unit cell, each tetragonally coordinated to four copper atoms, which in turn are linearly coordinated to two oxygen atoms.

Figure 1. Crystal structure of Cu 2 O. Oxygen atoms are depicted in red, and copper atoms are depicted in orange. The complete band structure of Cu 2 O and a detailed view of the upper valence bands and lower conductions bands near the Γ-Point are shown in Figure 2. While the valence bands are mostly built up from Cu 3d10 orbitals, the lowest conduction bands show mostly s-like character 5 ACS Paragon Plus Environment


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and are built from the Cu 4s orbitals.14 Interestingly, the lowest absorption transitions (Γ 8 + → Γ 6 + and Γ 7 + → Γ 6 +), while direct in nature, are parity forbidden. In contrast to this, the next lowest transitions (Γ 8 + → Γ 8 − and Γ 7 + → Γ 8 −) are parity allowed and lead to much higher absorption coefficients at their corresponding wavelengths. Thus, although Cu 2 O is a direct semiconductor, the forbidden nature of the lowest energy direct transition means that it behaves more like an indirect semiconductor. The implications of this fact are discussed at the end of this section.

Figure 2. (a) Calculated complete band structure of Cu 2 O. (b) Detailed view of the bandgap highlighted in red in (a). Adapted with permission from ref. 13. Cuprous oxide is a natural p-type semiconductor. It was originally hypothesized that the p-type nature was due to copper vacancies that act as electron acceptors,15 and this theory is now widely accepted and supported by experimental16,17 as well as theoretical18,19 studies. However, there is 6 ACS Paragon Plus Environment

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still controversy as to the exact nature of these copper vacancies. In early deep-level transient spectroscopy (DLTS) studies by Papadimitriou, the hole traps were attributed to CuO islands.20 More recent DLTS studies identified two trap states in Cu 2 O, attributed to single and double Cu vacancies.21 Wright et al.19 as well as Scanlon et al.18 came to the finding that electric conductivity mainly arises from either single Cu vacancies (one Cu missing, leaving two threefold coordinated oxygens) or split Cu vacancies (neighboring Cu atom moves towards Cu vacancy and coordinates four oxygens), but they disagree on which of the possibilities is thermodynamically more stable. The formation energies of the other intrinsic acceptors, oxygen interstitials, in either tetragonal or octagonal coordination, are much higher than for Cu vacancies. Meyer et al. roughly calculated the binding energies of donors and acceptors towards Cu 2 O based on the effective mass theory.13 The binding energies of approximately 266 meV for donors and 156 meV for acceptors imply that it is more difficult to obtain n-type doping since most of the donors will not be ionized at room temperature and very high doping levels are needed to outweigh the intrinsic p-type conduction of the Cu vacancies. Nevertheless, examples of n-type Cu 2 O or p-n homojunctions have been reported by Fernando et al.,22 McShane et al.,23,24 Han et al.25 and others.26–28 Many of these groups achieved n-type doping by varying the electrochemical deposition conditions such as pH or Cu2+ ion concentration. The debate on the origin of the n-type conductivity is still ongoing, and Cu interstitials as well as O vacancies are proposed candidates. On the other hand, Scanlon and Watson29 rule out all intrinsic defects as the source of n-type conductivity, based on their HSE calculations. They furthermore suggest, based on previous studies by Nian et al.30 and Han et al.31, that formation of an inversion layer or Cl− doping leads to n-type properties. 7 ACS Paragon Plus Environment


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Two of the fundamental characteristics of semiconducting materials used in photovoltaic or photoelectrochemical applications are light penetration depth and carrier collection length. The light penetration depth of a material is the inverse of its attenuation coefficient (α-1) and measures about 2.2 µm for Cu 2 O at λ = 600 nm, near the band edge where absorption is the weakest.32 The carrier collection length depends on the mobility of the charge carriers, which is highly dependent on the synthesis technique used. Generally speaking, techniques leading to a larger Cu 2 O grain size (e.g. high temperature thermal oxidation of Cu sheets, high temperature sputtering) give higher carrier mobilities, from 50 cm2V-1s-1 up to >100 cm2V-1s-1.33,34 For electrodeposition, a low cost and low energy intensity fabrication method, the Cu 2 O grain sizes are smaller, and carrier mobilities hardly exceed 5 cm2V-1s-1.32,35,36 From the mobility, the charge carrier collection length L can be calculated using Equation 4:

L=�

𝑘𝑘 𝑇𝑇 𝑒𝑒

𝜇𝜇 𝜏𝜏

(4)

where k is the Boltzmann constant, T the temperature, e the elementary charge, µ the mobility and τ the charge carrier lifetime.37 In the case of high mobility Cu 2 O devices (>50 cm2V-1s-1) the resulting carrier collection lengths are in the range of 1–10 µm,38,39 which coincides well with the light penetration depth. In this scenario, nearly all photons with energy greater than the bandgap can be absorbed and the photogenerated carriers can be extracted, enabling high efficiencies. Devices using Cu 2 O with a lower mobility (such as with electrodeposited films) lead to charge carrier lengths of far below 1 µm,32,40 which means that the longer wavelength photons absorbed deep within the film are not able to be collected, lowering the efficiency of the device. The consequence of this phenomenon is shown clearly in Figure 3, which depicts the incident photon-to-current efficiency (IPCE) of 8 ACS Paragon Plus Environment

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an electrodeposited Cu 2 O photocathode for water splitting. Photons with wavelengths ~500–600 nm (~2.0–2.5 eV) are weakly absorbed due to the parity forbidden nature of the absorption (as discussed above). Thus, these carriers cannot be efficiently collected, as evidenced by the lower IPCE values in this wavelength range. One strategy for addressing the issue of low charge mobility is by nanostructuring the material, discussed further in the next section.

Figure 3. Incident photon-to-current efficiency (IPCE) of an electrodeposited Cu 2 O photocathode for water splitting. Data taken from ref. 41. 3. Cu 2 O Solar Cells Cu 2 O is the most popular material used in all-oxide photovoltaics.42 Its 2.0–2.2 eV bandgap13 allows for a maximal theoretical efficiency of ~23%, whereas the optimal band gap for a single junction device is 1.34 eV (maximum 32.9% efficiency).42 There are three main architectures for solid-state solar cells: Schottky junctions (a rectifying semiconductor-metal junction), homojunctions, and heterojunctions. All three of these have been targeted for Cu 2 O-based solar cells. Figure 4 compares the energy band diagrams of Cu 2 O in these different architectures. 9 ACS Paragon Plus Environment


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Figure 4. Schematic energy band diagrams of Cu 2 O-based PV cells at open circuit under illumination. Valence and conduction band positions (E VB and E CB ) are shown. Gradients of hole and electron quasi-Fermi levels (E Fn and E Fp ) indicate that substantial recombination occurs at front and back contacts (FC and BC). (a) Cu 2 O Schottky junction, (b) ZnO/Cu 2 O heterojunction cell, (c) a p−n Cu 2 O homojunction solar cell. Adapted with permission from ref. 42. Copyright 2012 American Chemical Society. In the 1920’s, almost one century after Becquerel discovered the photovoltaic effect,1 Cu 2 O was one of the first materials to be used to fabricate solar cells, decades before the era of siliconbased solar cells. Searching for new rectifier materials, Grondahl found that Cu 2 O made by thermally oxidizing copper discs showed a response to light.43,44 These early examples and the majority of Cu 2 O solar cells in the subsequent decades were Schottky-diodes with poor efficiency. Olsen et al. explored a variety of metals for pairing with Cu 2 O, such as Yb, Mg, Mn, Al, and Cr, and found that the Schottky barrier height was 0.7–0.9 eV, independent of the metal that was used. They concluded that these Schottky-junction solar cells with low-work-function metals were essentially Cu/Cu 2 O cells due to reduction of the copper oxide and subsequent 10 ACS Paragon Plus Environment

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interdiffusion, and efficiencies hardly exceeded 1%. Homojunction Cu 2 O photovoltaic cells are difficult to prepare due to the lack of reliable methods to obtain high quality n-type Cu 2 O (as described in the previous section). For this reason, there is a paucity of publications on Cu 2 O solar cells in the second half of the 20th century. It was only in the early 2000’s, when attention was turned to heterojunction cells using Cu 2 O and ZnO (or doped ZnO), that a resurgence of research was seen in the literature.45–48 A great deal of progress has been made in recent years, with greatly improved efficiencies, and a clearer understanding of the challenges and limitations of heterojunction solar cells. In general, the main challenges one is confronted with when working with heterojunctions are improper band alignment (affecting photovoltage), lattice mismatch between the crystal structures of the different materials, or other defects at the interface that produce states that promote recombination and loss of performance. Figure 5 gives an overview of the progress in efficiency over the last few years. The highest efficiencies to date have been obtained by Minami et al.34 By using pulsed laser deposition (PLD) to deposit a layer of a wide bandgap n-type semiconductor on thermally oxidized copper sheets, the authors were able to decrease the number of defect levels at the interface and therefore reach high fill factors (FF) and photocurrents. Starting from an AZO/Cu 2 O type cell with 2.19% efficiency, the efficiency was nearly doubled by introducing an undoped ZnO layer to give AZO/ZnO/Cu 2 O.33 Then, by replacing ZnO with Ga 2 O 3 49 or AlGaO x 34 the efficiency was increased to 5.38% and 6.12%, respectively. These improvements were mainly attributed to better band alignment and higher electron lifetime due to the reduction of defects.



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Figure 5. Improvement in the efficiency of Cu 2 O solar cells in recent years. The colors are used to distinguish vacuum versus non-vacuum fabrication methods, and also homojunction and nanostructured device architectures. The reference number is given next to each point. The approach of Lee et al. involved the implementation of a very thin buffer layer between ntype semiconductor AZO and p-type Cu 2 O. A buffer layer of 5 nm amorphous zinc tin oxide (aZTO) deposited by ALD largely improved the efficiency of these AZO/a-ZTO/Cu 2 O cells due to reduced recombination at the interface.50 Inspired by Minami,49 Lee et al. prepared an AZO/Ga 2 O 3 /Cu 2 O cell with a 10 nm Ga 2 O 3 buffer layer deposited by ALD on Cu 2 O. This device gave a record breaking 1.20 V open circuit voltage and an efficiency of 3.97%.51 The authors state that the unsatisfying fill factor of 45% is due to the low carrier collection efficiency associated with the high series resistance in Cu 2 O and Ga 2 O 3 .


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One reason that low energy intensity fabrication methods (not depending on high vacuumbased techniques like PLD or sputtering) hardly exceed 1% efficiency is the challenge to obtain high quality heterojunction interfaces.45,52,53 Ievskaya et al. showed that atmospheric pressure ALD is able to produce good quality ZnMgO/Cu 2 O interfaces giving cells with 2.12% efficiency.54 This represents a further milestone towards highly efficient low cost solar cells. For electrodeposition, a cheap and straightforward technique that is amenable to scale-up, additional considerations have to be made concerning charge carrier collection length. Typically, electrodeposition leads to a much smaller grain size than thermally oxidized Cu 2 O.55 Paracchino et al. calculated an electron collection distance of 53–88 nm for electrodeposited Cu 2 O, while longer wavelength photons (energy close to the bandgap) have an absorption depth of 2.2 µm (λ = 600 nm).32 As such, large recombination losses are to be expected for these longer wavelengths. Cui et al.56 as well as Musselman et al.57 addressed the substantial discrepancy between carrier collection length and light penetration depth by fabricating nanostructured nZnO/p-Cu 2 O devices, yielding devices with 0.88% and 0.47% efficiency, respectively. To date, the best performing electrodeposited ZnO/Cu 2 O nanostructured solar cell was reported by Chen et al.58 with an efficiency of 1.52%, which slightly outperforms the most efficient electrodeposited bilayer ZnO/Cu 2 O cell (1.43%).59 Figure 6 schematically displays the bilayer as well as the nanostructured architecture and shows the increased electron collection length in the latter case.



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Figure 6. Comparison of ZnO/Cu 2 O bilayer and nanostructured architectures. Charge collection is achieved by increasing the charge collection length (L C ) while decreasing the optical depth (LOD ) due to light scattering at the ZnO nanowires. Reproduced with permission from ref. 57. For Cu 2 O homojunctions, the efficiencies are typically poor, hardly exceeding 0.1% with few exceptions.25,28 McShane and Choi reported a homojunction cell fabricated by electrodeposition with an efficiency of 1.06%.60 Zhu and Panzer very recently reported Zn-doped Cu 2 O showing n-type conductivity and prepared a n-Zn:Cu 2 O/p-Cu 2 O homojunction cell with 0.42% efficiency.61 Hsu et al. also reported a Cu 2 O homojunction solar cell with 0.42% efficiency, although the authors give no explanation as to the origin of the n-type conductivity of the Cu 2 O.62 To increase further the efficiency of homojunction cells, it is critical to gain a profound understanding of the defects that give rise to n-type conductivity.29


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Table 1. Summary of Cu 2 O solar cell performance in recent years.

Cell Type

Deposition

PCE

V OC

method

(%)

(V)

FF

J SC (mA/cm2)

Reference

Vacuum-based fabrication techniques AZO/ZnO/Cu 2 O

PLD

3.83

0.69

0.55

10.1

Minami 201163

AZO/ZnO/Cu 2 O

PLD

4.12

0.72

0.60

9.69

Nishi 201333

AZO/Ga 2 O 3 /Cu 2 O

PLD

5.38

0.80

0.67

9.99

Minami 201349

AZO/AlGaO/Cu 2 O

PLD

6.12

0.84

0.66

10.95

Minami 201534

AZO/a-ZTO/Cu 2 O

ALD

2.51

0.54

0.63

7.34

Lee 201350

MgF 2 /AZO/Ga 2 O 3 /Cu 2 O

ALD

3.97

1.20

0.45

7.37

Lee 201451

Fabricated with non-vacuum techniques SnO 2 /ZnO/Cu 2 O

ED

1.28

0.59

0.58

3.8

Izaki 200764

FTO/ZnO/Cu 2 O

ED

1.43

0.54

0.60

4.47

Fujimoto 201359

ITO/ZnO/Cu 2 O

a-ALD

1.46

0.49

0.40

7.5

Ievskaya 201454

ITO/MgZnO/Cu 2 O

a-ALD

2.20

0.61

0.49

7.55

Ievskaya 201454

Nanostructured devices ZnO nanowire/Cu 2 O

ED

0.47

0.28

0.39

4.40

Musselman 201057

ZnO nanopillar/Cu 2 O

ED

0.88

0.29

0.36

8.20

Cui 201056

Sn 0.86 Co 0.14 O 2 nanorod/Cu 2 O

HT

1.20

2.33

0.36

1.43

Shiu 201165

ZnO nanorods/Cu 2 O

HT

0.86

0.36

0.31

7.80

Jia 201266

Ti 2 O nanorods/Cu 2 O

HT/ED

1.25

0.36

0.39

8.91

Luo 201267

ZnO nanorods/Cu 2 O

ED

1.52

0.42

0.37

9.89

Chen 201558

Homojunction devices ITO/n-Cu 2 O/p-Cu 2 O

ED

0.102

0.32

0.35

1.23

Han 200925

FTO/n-Cu 2 O/p-Cu 2 O

ED

0.104

0.12

0.23

3.97

Wei 201228

ITO/n-Cu 2 O/p-Cu 2 O

ED

1.06

0.62

0.42

4.07

McShane 201260

n-Cu 2 O/p-Cu 2 O

ED

0.42

0.42

0.38

2.68

Hsu, 201562

Zn:Cu 2 O/p-Cu 2 O

Co-ED

0.42

0.50

0.42

2.02

Zhu, 201561

Table footnotes: Abbreviations: PCE – power conversion efficiency; V oc – open circuit voltage; FF – fill factor; J sc – short circuit current density; AZO – aluminum-doped zinc oxide; FTO – fluorine-doped tin oxide; ITO – indium-doped tin oxide; PLD – pulsed laser deposition; ALD – atomic layer deposition; a-ALD – atmospheric pressure ALD; HT – hydrothermal; ED – electrodeposition. 15 ACS Paragon Plus Environment


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Table 1 gives an overview of the figures of merit for different Cu 2 O solar cell architectures reported in the last few years. It is evident that the heterojunction class of solar cells has shown the most progress. The heterojunction architecture has also been employed with great success in the field of photoelectrochemical water splitting with Cu 2 O, where the conformal n-type layer acts as a barrier to corrosion of the cuprous oxide by water. The following section discusses the progress that was been made with generating efficient and stable copper oxide photocathodes for hydrogen evolution from water. 4. Photoelectrochemistry with Cu 2 O Photoelectrochemical water splitting entails the direct conversion of solar energy into chemical fuel, which addresses the issue of energy storage associated with solar electricity generation. As stated before, the technology that will ultimately be adopted for solar fuel generation (PV+electrolysis versus PEC water splitting) will depend on the cost of the hydrogen produced. The cost of the hydrogen produced, in turn, depends greatly on the efficiency and stability of the system. The solar-to-hydrogen (STH) efficiency of a solar water splitting system is defined as 𝐽𝐽𝑝𝑝ℎ𝑜𝑜𝑜𝑜𝑜𝑜 �𝑚𝑚𝑚𝑚/𝑐𝑐𝑐𝑐2 �×1.23 𝑉𝑉× 𝜂𝜂𝐹𝐹

𝑆𝑆𝑆𝑆𝑆𝑆 = �

𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 [𝑚𝑚𝑚𝑚/𝑐𝑐𝑐𝑐2 ]

�

(5)

where J photo is the photocurrent density per geometric area and η F is the faradaic efficiency for hydrogen evolution. Equation 5 is only valid when oxygen evolution occurs at the counter electrode, and not when sacrificial agents (e.g. methanol) are used. As discussed previously, the value of 1.23 V is the reversible voltage for water splitting (equation 3) and represents the maximum electrical energy that can be obtained by reacting hydrogen and oxygen to form liquid 16 ACS Paragon Plus Environment

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water in a fuel cell.68 However, when driving the water splitting reaction, the photoabsorbers in the system must provide a voltage much greater than this (>1.6 V) in order to achieve a reasonable photocurrent density due to kinetic overpotentials. Thus, in contrast to PV electrical efficiency, the STH efficiency already includes the losses associated with energy storage (in this case, in the bond of the hydrogen molecules). It is widely accepted that a practical solar water splitting device based on thin film technology should be at least 10% efficient–and preferably much more–in order to effectively compete with hydrogen produced from fossil fuels.69 Ten percent STH efficiency corresponds to overall water splitting photocurrents of 8.1 mA/cm2 (Equation 5) and stoichiometric generation of hydrogen and oxygen. It should be noted that although the photovoltage does not appear in the STH efficiency calculation, the photoelectrode material(s) must generate a photovoltage significantly greater than 1.23V in order to drive the overall water splitting reaction at a practical rate (i.e. >8 mA/cm2). The theoretical maximum STH efficiency for any material can be calculated by considering the solar photon flux70 and the bandgap of the material. For Cu 2 O, if all solar photons with energy greater than the bandgap (2.0 eV) were absorbed and used for the water splitting reaction, the resulting photocurrent density under one sun illumination conditions would be about 15 mA/cm2, corresponding to a maximum theoretical STH efficiency of about 18% (Equation 5). A general scheme for one type of PEC cell is shown in Figure 7. In this idealized case, the band positions of the semiconducting photocathode straddle the thermodynamic water splitting redox potentials of the aqueous electrolyte. Absorption of light generates a photovoltage (indicated in the Figure by the dashed lines, representing the splitting of the quasi-Fermi levels)



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that in this case is sufficient to drive the overall water splitting reaction. Hydrogen is evolved at the surface of the photocathode, and oxygen is evolved at the counter electrode.

Figure 7. General scheme of one type of PEC cell employing a photocathode. A tool for quantitative calculation of the band positions of water splitting photoelectrodes can be found in ref. 71. As of yet, no single known material meets the criteria for such a water splitting system: ideally placed band positions with respect to the water splitting potentials, photovoltage great enough to drive the reactions (including overpotentials), and stability in an aqueous environment. However, a single material that has all of these characteristics is not, in fact, needed. Numerous studies have shown that systems involving two (or more) photoabsorbing materials that harvest different parts of the solar spectrum are vastly more efficient than systems that utilize a single material.72,73 These so-called “tandem systems” relax the constraints on any one material, as they 18 ACS Paragon Plus Environment

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must supply only part of the overall voltage and need drive only one of the water splitting reactions, which greatly expands the number of viable materials that may be used in a PEC system. In fact, the 2.0 eV bandgap of Cu 2 O is nearly ideal for a top cell in a tandem PEC water splitting device.72

Figure 8. Band positions of Cu 2 O with respect to the normal hydrogen electrode (NHE). The potential of the corrosion reactions lie within the bandgap (red), and also between the thermodynamic water splitting potentials, indicating a thermodynamic preference for corrosion over water splitting when illuminated. Figure 8 shows the band positions of Cu 2 O with respect to the thermodynamic redox potentials of water reduction and water oxidation.74 At first glance, one might be tempted to think that this material is suitable for overall water splitting. It was reported in 1998 that illumination of powdered Cu 2 O suspended in aqueous buffer resulted in overall water splitting with no sign of 19 ACS Paragon Plus Environment


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deactivation for greater than 1900 hours.75 However, the same authors later that same year reported that the water splitting reaction continued long after the light was turned off, a phenomenon that was attributed to a mechanical effect on the catalyst as a result of the vigorous stirring of the PEC cell.76 Indeed, the valence band is not deep enough to provide enough driving force (overpotential) to carry out water oxidation at a reasonable rate. Photoelectrons in the conduction band do, however, have sufficient potential to drive water reduction, and even to drive more kinetically demanding reactions such as CO 2 reduction. Copper oxide is therefore an excellent candidate for use as a photocathode in water splitting and other fuel generation systems. Although oxide materials typically exhibit high stability in aqueous environments, copper oxide is one of the exceptions that undergoes photocorrosion due to the fact that the redox potentials for the oxidation and reduction potentials of Cu 2 O lie within the bandgap (Figure 8). In the presence of water, photoexcited electrons preferentially reduce the copper oxide lattice to form elemental Cu, and holes on the surface preferentially oxidize copper to yield cupric oxide (CuO). Studies on single crystals of Cu 2 O demonstrated the photocathodic instability of this material.77,78 However, there have been several literature reports that claim cathodic stability under illumination for electrodeposited polycrystalline samples.79,80 This phenomenon has been rationalized by proposing that it is only those crystal facets that terminate with O2− that are susceptible to proton assisted reduction of the Cu 2 O.77 Indeed, the single crystals investigated by Takegushi et al. involved facets that terminated with O2−.78 A thorough electrodeposition study that investigated different synthesis conditions that yielded different exposed facets of Cu 2 O to the electrolyte did observe a difference in the rate at which the films photocorroded, although all films suffered degradation at extended time scales.32 In any case, the fact that the thermodynamic 20 ACS Paragon Plus Environment

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potentials for oxidation and reduction of the Cu 2 O lie within the bandgap and are thermodynamically more favorable than the corresponding water splitting reactions makes longterm stability a key concern. One method for stabilizing electrodes involves coating them with a conformal film of a wide bandgap (transparent) semiconductor that can conduct the photogenerated charge carriers yet block water and protons from facilitating corrosion reactions. This strategy was investigated several decades ago using CVD deposited TiO 2 (where conformality was a concern),81,82 and has only recently seen a resurgence of interest as a strategy to stabilize materials for practical water splitting cells. It should be noted that when conformal overlayers are used to protect a photoactive material, one has shifted from a PEC system that uses the semiconductor liquid junction (SCLJ) to generate the photovoltage (as in Figure 7) to one that uses a so-called “buried junction” to generate the photovoltage.83 In the latter case, the protective overlayer forms an Ohmic contact with the electrolyte (or catalyst on the surface), and it is the internal interface of the photoactive material with the overlayer that establishes the photovoltage (similar to Figure 4b). One advantage of the buried junction photoelectrode is that the band edge position of the photoactive material is decoupled from the thermodynamic water splitting potentials, which means that the losses associated with non-ideal band positioning are eliminated. This phenomenon is evident from the fact that buried homojunction silicon electrodes can be used both for water reduction as a photocathode84 and for water oxidation as a photoanode,85 covering a range of potential much greater than the 1.1 eV bandgap would suggest.



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The first example of a protected copper oxide photocathode was reported by Siripala et al.86 Films of Cu 2 O were grown on titanium foil and then 100 nm of TiO 2 was deposited by e-beam evaporation. Although the photocurrents obtained from the system were modest (2–3 years at 200–400 mA/cm2). A typical construction consists of a base metal of titanium covered 26 ACS Paragon Plus Environment

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by rutile TiO 2 , doped with 30% RuO 2 , and is used most notably for chlorine evolution.91,92 It is clear that the bonding between TiO 2 and RuO 2 yields an extremely stable electrocatalyst under highly demanding conditions. Thus, RuO x was deposited by cathodic photoelectrodeposition from an aqueous solution of KRuO 4 onto a TiO 2 -protected Cu 2 O photocathode.41 The most stable electrodes resulted from depositing an amorphous, porous, conformal layer of RuO x about 40 nm thick. Although the electrodes were slightly darkened, resulting in reduced photocurrents, 5 mA/cm2 could be easily obtained under one sun illumination. A comparison of the stability of the RuO x and Pt catalysts is shown in Figure 11a. In order to remove any potential complications from the photovoltaic junction, a dark experiment was designed to imitate the potential steps that the catalysts experience under light chopping on the photocathode. Fluorine-doped tin oxide (FTO) was coated with ZnO and TiO 2 ALD overlayers and the Pt and RuO x catalysts were deposited by electrodeposition. The potentials used in the experiment were chosen such that similar currents were obtained as in the photocathode system. As can be seen, the Pt catalyzed electrode exhibits a typical decrease in the maximum current that can be obtained over the first 10 minutes. Meanwhile, the RuO x -catalyzed electrode maintains its high current density. The photocathode stability with RuO x catalyst is shown in Figure 11b, with 95% stability maintained at ~5 mA/cm2 after 8 hours of light chopping.



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Figure 11. a) Dark potential step experiment that imitates the light chopping effect. The electrodes were prepared without Cu 2 O (only ALD layers and the catalyst are present). b) Durability of an ALD layer protected Cu 2 O photocathode with RuO x catalyst under light chopping. Adapted with permission from ref. 41. Although the RuO x -catalyzed photocathodes exhibit greatly enhanced stability over the Ptcatalyzed ones, it could still be argued that ruthenium is rare and expensive and therefore not amenable to scaling up to the terawatt scale. Hence, the use of earth abundant catalysts in PEC systems are of high interest.


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Molybdenum sulfide is a well-known earth abundant HER catalyst.93,94 An effective method was discovered for electrodeposition of this material onto the TiO 2 overlayer of Cu 2 O photocathodes that involved excitation of the TiO 2 with ultraviolet light to enable anodic current flow in the photocathode.55 This work also investigated further the effects of pH on the stability of the system. All of the previous works in these ALD-protected studies had used the slightly acidic pH 5 as the electrolyte. It has been argued that neutral or near-neutral pH is desirable from a safety and public acceptance point of view,95 but it has also been shown that a highly conductive electrolyte is required for a system operating at high efficiency.96 Thus, it was important to investigate the extremes of pH where conductivity is the highest. Greater than –5.5 mA/cm2 of photocurrent were obtained at a bias potential of 0V vs. RHE with the MoS x catalyst in pH 1.0 electrolyte.55 Moreover, the durability of the photocathode with the MoS x catalyst at this acidic pH was greatly enhanced versus a similar photocathode that used a platinum catalyst, although the electrode did eventually fail in slightly over 7 hours. It appears that the MoS x is mostly conformal by cross-sectional SEM, which helps shield the amorphous TiO 2 layer from etching in the acidic electrolyte, although clearly the electrolyte was able to penetrate the catalyst to eventually corrode the photocathode. Another work demonstrated that MoS x and NiMo both showed good stability in basic pH (1M KOH),97 which is advantageous for coupling with photoanodes or counterelectrodes featuring OER catalysts, which have the highest performance and stability in strongly basic pH. From these previous works, it was evident that a more robust TiO 2 layer would be required to further improve the durability. Amorphous TiO 2 etches at a much faster rate than polycrystalline TiO 2 in hot sulfuric acid,98 and so it was hypothesized that a crystalline TiO 2 overlayer would be more resistant to chemical etching and loss of catalyst, and would therefore yield more stable 29 ACS Paragon Plus Environment


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photocathodes. One simple method to crystallize the amorphous TiO 2 film would be to simply heat the electrode to ~400 °C. Unfortunately, once the Cu 2 O/ZnO junction is formed, the electrode suffers degradation in its electronic properties if heated to temperatures beyond 200 °C. A low temperature technique was therefore sought to improve the quality of the TiO 2 overlayer. One well-known method that can be used to generate crystalline oxides at relatively low temperatures (50 hours.


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Figure 12. Durability experiments for steam-treated Cu 2 O photocathodes featuring ALD overlayers, biased at 0 V vs. RHE in pH 5 sulfate–phosphate electrolyte under light chopping (one sun). (a) with RuO x catalyst, (b) with Pt catalyst. Adapted with permission from ref. 100. The steam-treated electrodes were characterized with a number of techniques to look for evidence of crystallization of the amorphous TiO 2 overlayer, and compared to control samples that had undergone no steam treatment. No additional peaks were observed in the XRD spectrum, indicating that if the film was crystallized then the grain sizes must be small. Raman spectroscopy, ellipsometry, and impedance spectroscopy/Mott–Schottky analysis of the steamtreated ALD TiO 2 layers were similarly inconclusive. Cross-sectional transmission electron microscopy (TEM) was used after removing an electron transparent sliver of the sample with a focused ion beam (FIB), and small crystalline domains were found in the largely amorphous layer for both the steam-treated and untreated samples. It was therefore concluded that the steam treatment does not largely crystallize the amorphous ALD TiO 2 layer. The samples were then investigated for their morphological differences with scanning electron microscopy (SEM). Indeed, the steam-treated samples had a smoothed appearance as compared 31 ACS Paragon Plus Environment


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to the untreated samples, and this effect was apparent in the cross-sectional TEM images as well. This morphological change had a dramatic impact on the morphology of the RuO x catalyst that was next deposited by photoelectrodeposition. It was proposed that a more uniform thickness of the TiO 2 layer enabled a more uniform deposition of the catalyst, thereby promoting hydrogen evolution over the whole surface and minimizing electron trapping in the overlayer. The durability of Cu 2 O photocathodes, and indeed any photoelectrode, is a highly important aspect of PEC water splitting, and it has been shown how focusing on different aspects of the protective overlayer can result in significant improvements to the long-term stability. However, another very important aspect is the photovoltage that can be delivered by the photoabsorbing layers. Theoretically, the maximum photovoltage that a semiconductor can furnish is a few hundred millivolts less than the bandgap,101,102 and in practice, the obtained open-circuit voltages (V OC ) are usually less. Since at least 1.6 V of photovoltage is needed to split water at a practical rate (≥8.1 mA/cm2) due to the overpotentials for both the OER and HER reaction, each partner of the tandem PEC cell72 should provide a photovoltage that is at least half of its bandgap. Hence, one should expect Cu 2 O with its 2.0 eV bandgap to furnish at least 1.0 V of photovoltage to the overall water splitting reaction. Although not as straightforward to measure as in a PV cell, an estimate of the photovoltage in a PEC electrode is given by the onset potential versus the thermodynamic water splitting potentials (the overpotentials for the HER are much smaller than the OER, so the estimate is better for photocathodes). In the ALD-coated Cu 2 O materials discussed thus far, the onset potentials range from +0.3 V vs. RHE to +0.6 V vs. RHE, depending on surface treatments, the catalyst used, and the pH of the electrolyte. Neglecting the overpotential for the HER, these onset potentials translate into photovoltages of ~0.3–0.6 V, which corresponds well to the V OC obtained in Cu 2 O/ZnO solar cells. Just as has been shown 32 ACS Paragon Plus Environment

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with Cu 2 O-based PV (section 3), a very effective way to increase the photovoltage is by tuning the junction material. One example of tuning the junction potential in a Cu 2 O PEC system was demonstrated by Dai et al., who showed that ZnS forms a high quality interface with Cu 2 O, giving an earlier onset potential as a result of the increased photovoltage.103 By replacing ZnO with ZnS, they improved the onset potential to +0.72 V vs. RHE in pH 7 electrolyte, increasing the obtainable photovoltage to greater than 700 mV. Li et al. employed a Ga 2 O 3 buffer layer, resulting in a Cu 2 O photocathode with a very early onset potential of +1 V vs. RHE, 104 achieving similar photovoltage to that obtained using this junction in a PV device.51 It is important to recognize that the Cu 2 O/Ga 2 O 3 junction generates a photovoltage that is >50% of the bandgap of Cu 2 O, and is therefore a highly promising material combination for a practical tandem cell. In addition to using TiO 2 as a protective layer, one can use a catalytic material that serves the dual role of protective layer and HER catalyst. Lin et al. demonstrated this approach by coating Cu 2 O nanowires with a layer of NiO x , which acts as both a protective layer (no H 2 was observed in its absence, indicating corrosion of the Cu 2 O) and HER catalyst, achieving high photocurrents at strong electrical bias.105 It was subsequently shown by Yang et al. that an additional HER catalyst improves the performance of Cu 2 O/NiO photocathodes.106 The presence of nanoparticle catalysts on a Cu 2 O cathode (in the absence of protective overlayers) can significantly suppress the corrosion reaction. Dubale et al. fabricated a copperbased photocathode by thermal oxidation of a thin film of copper metal to yield a mixture of Cu 2 O and CuO. Nickel nanoparticles were spin-coated onto the surface as HER catalyst, and this improved the stability greatly although some corrosion was still observed (reported Faradaic 33 ACS Paragon Plus Environment


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efficiency of 84% for hydrogen generation).107 Addition of a discontinuous HER co-catalyst directly to the Cu 2 O surface can significantly bias the branching ratio between hydrogen evolution and semiconductor corrosion, although it is uncertain that corrosion can be suppressed to the degree necessary to enable years of hydrogen production, which will be required for a practical device.88 Thus, a conformal protective layer is very likely a necessity for Cu 2 O. A summary of the different electrodes discussed in this section is given in Table 2. Table 2. Summary of the performance metrics for Cu 2 O-based photocathodes in a 3-electrode configuration.

Photocathode Architecture

Deposition methoda

J at 0 V vs RHE (mA/cm2)b

V onset vs RHE

Durabilityc

Ref

FTO/Au/Cu 2 O/AZO/TiO 2 /Pt (pH 5)

ED/ALD

7.6

+0.35

2 min

Paracchino 201174

FTO/Au/Cu 2 O/AZO/TiO 2 /Pt (pH 5) FTO/Au/Cu 2 O/AZO/TiO 2 /RuO x (pH 5) FTO/Au/Cu 2 O/AZO/TiO 2 /RuO x (pH 5) FTO/Au/Cu 2 O/AZO/TiO 2 /Pt (pH 5) FTO/Au/Cu 2 O/AZO/TiO 2 /MoS x (pH 4) Au/Cu 2 O/ZnS/TiO 2 /Pt (pH 7)

ED/ALD

4.5

+0.35

30 min

Paracchino 201289

ED/ALD

5.0

+0.50

4h

Tilley 201441

ED/ALD

5.2

+0.55

25 h

Azevedo 2014100

ED/ALD

5.6

+0.64

10 h

Azevedo 2014100

ED/ALD

4.8

+0.45

10 h

Morales-Guio 201455

TO/TE/ALD

2.0

+0.72

(30 h)f

Dai 2014103

Cu/Cu 2 O/Ga 2 O 3 /TiO 2 /Pt (pH 4.3)

ChO/ALD

(3.5)d

+0.96

3h

Li 2015104

Cu/Cu 2 O/NiO x (pH 6)

ChO/SC

(5.0)e

+0.45

20 ming

Lin 2012105

FTO/Cu 2 O/NiO/Cu 2 MoS 4 (pH 5)

ED/SC

1.3

+0.45

3 minh

Yang 2014106

FTO/Cu 2 O/CuO/Ni (pH 5)

ED/TO

4.3

+0.61

20 mini

Dubale 2015107

Table footnotes: a: Fabrication method for Cu 2 O / Deposition method for the overlayers. b: Current density under simulated solar AM 1.5 G illumination (100 mW cm–2). c: Durability is defined as the time required for the photocurrent to decrease by 10% from its initial value under continuous illumination at a bias voltage of 0 V vs. RHE, with quantitative Faradaic efficiency for H 2 generation (within error). d: 500 W Xe lamp. e: LED light source (425–660 nm; 26 mW 34 ACS Paragon Plus Environment

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cm–2). f: Durability measurement was carried out by fixing the photocurrent at –0.5 mA cm–2 and monitoring the potential. g: Faradaic efficiency = 32±6%. h: Faradaic efficiency = 31.2%. i: Faradaic efficiency = 84%. Abbreviations: AZO – aluminum-doped zinc oxide; FTO – fluorine-doped tin oxide; ALD – atomic layer deposition; ED – electrodeposition; TO – thermal oxidation; TE – thermal evaporation; ChO – chemical oxidation (then annealing); SC – spin coating. The vast majority of research on Cu 2 O (and other photoelectrodes) is carried out in a 3electrode configuration, where a calculation of STH efficiency is not valid. Two examples have been reported (to the best of our knowledge) for 2-electrode tandem cell measurements involving Cu 2 O photocathodes, for which a STH efficiency can be measured. Lin et al. coupled a Cu 2 O/NiO x photocathode with a WO 3 photoanode. The efficiency was less than 0.05% when the two materials were short-circuited together.105 Bornoz et al. demonstrated a bismuth vanadate– cuprous oxide tandem cell that operated at ~0.5% STH efficiency at short-circuit.108 It should be noted that these reports were a proof of principle. In both cases, Cu 2 O was used as the smaller band gap partner in the tandem cell, where in order to achieve high efficiency, Cu 2 O must be the larger band gap partner.72 Improved photovoltages of Cu 2 O will enable coupling with smaller bandgap partners going forward, enabling higher efficiency tandem cells. 5. SUMMARY AND OUTLOOK The work described herein details the strategies that have been used for improving the solar energy conversion efficiency of Cu 2 O-based materials for both PV and PEC applications. There are distinct challenges associated with PEC water splitting, but the methods for maximizing the performance of light absorbing materials remains the same in both fields. The most efficient 35 ACS Paragon Plus Environment


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Cu 2 O PV cells to date employ the p–n heterojunction strategy, and very high V OC values have been obtained with a suitable junction material.51 Likewise, the highest performance Cu 2 O PEC cells to date employ the p–n heterojunction architecture, and very early onset potentials (high photovoltage) have been reported for hydrogen evolution.104 Further improvements to the performance of Cu 2 O-based heterojunctions will arise from pairing with different n-type materials that result in well-aligned bands at the p–n junction, a low level of defects at the interface, and high conductivity to minimize resistive losses. Another strategy to minimize defects at the interface of the p–n junction is the insertion of a thin buffer layer, which has proved effective in PV systems. To improve the performance of electrodeposited Cu 2 O systems, the mobility and/or the electron lifetime must be increased in order to harvest more of the longer wavelength photons, which may happen by increasing the grain size of the electrodeposited films. Alternatively, nanostructured Cu 2 O can be employed, but improvements in the junction are necessary to realize high efficiencies. A Cu 2 O homojunction is very attractive as it is theoretically possible to have perfect band alignment and crystal match at the interface (i.e. no defects), but the implementation of this strategy has proved difficult. A more thorough understanding of the nature of n-type Cu 2 O and reliable synthesis methods will shed light on the viability of this strategy. It is clear that buried junction PEC electrodes for water splitting are essentially solar cell materials for which one of the current collectors has been replaced with the aqueous electrolyte (and catalyst for the appropriate water splitting reaction). Perhaps not surprisingly, great care must be taken to ensure that the electrolyte does not degrade the light absorber material. A substantial amount of fundamental research has been carried out in recent years on the use of protective overlayers for improving the durability of photoelectrodes in PEC cells, since this is 36 ACS Paragon Plus Environment

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known to be a key parameter for energy and cost-efficient hydrogen production.88 The work on Cu 2 O discussed in this Article highlights some salient aspects of preparing protective layers on temperature sensitive materials, such as Cu 2 O/ZnO, and this is directly applicable to other temperature sensitive photoabsorber materials. For materials that do not exhibit this temperature sensitivity such as silicon, high temperature annealing of ALD TiO 2 yields extremely robust photocathodes, with very little loss of photocurrent density over 500 hours.109 Protective layers can also be used for anodes. Although wide bandgap, transparent p-type oxides are hard to prepare, a recent report demonstrated that amorphous, n-type ALD TiO 2 could in fact be used to protect photoanodes.110 The authors proposed that the hole transfer is mediated by trap states in the amorphous material. Recently, however, it has been shown that charge transfer can occur through the conduction band in properly-doped crystalline TiO 2 .111 Hence, TiO 2 is a generic protective layer that can be used to protect both photocathodes and photoanodes. Sputtered NiO x has also proven to be a high quality protective and catalytic layer for the OER on photoanodes, with durabilities exceeding 1000 h for n-CdTe.112 6. CONCLUSION These results with improving the quality of protective layers and extended durability experiments demonstrate that this strategy is a viable option for fabricating practical devices with materials that are thermodynamically unstable in water (under illumination). This fact opens up a whole host of PV materials for use in PEC cells that were thus far ignored due to their instability in aqueous media. In addition to improving the performance of the PV materials themselves, future work in PEC water splitting will focus on ensuring an Ohmic contact between the PV material and the protective overlayer, and minimizing the resistive losses in the overlayer. Also, 37 ACS Paragon Plus Environment


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it will be important to investigate in more detail the charge transfer dynamics of electron or hole transfer to the electrolyte from the overlayer, which will likely be mediated by catalysts. In the case of heterogeneous catalysts, ensuring an Ohmic contact to the overlayer is again a necessity. For homogeneous catalysts, more fundamental work will be required in order to understand the limitations of charge transfer from an overlayer to a molecular catalyst that may arise.113

AUTHOR INFORMATION Corresponding Author *Tel. +41 44 635 46 75. Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The University of Zurich and the University Research Priority Program (URPP) LightChEC are gratefully acknowledged for financial support. Picture of David Tilley by Sara Suter.


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AUTHOR BIOGRAPHIES WITH PICTURES René Wick obtained his MSc in Chemistry from University of Zürich. From 2013–2014 he performed his master thesis focusing on molecular photocatalysts under the supervision of Prof. Roger Alberto. He then started his Ph.D. in the Molecular Approaches to Renewable Energies group led by Prof. David Tilley.

David Tilley obtained a Ph.D. in chemistry from the University of California, Berkeley in 2007. Following a postdoctoral stay at Princeton University, he moved to the EPFL in Switzerland in 2009 as a postdoctoral fellow in the group of Prof. Michael Graetzel. After serving as group leader of the water splitting group in this laboratory from 2012–2014, he was appointed as tenure-track assistant professor at the University of Zurich, Switzerland, where he continues research on materials for PEC water splitting.


Photovoltaic and Photoelectrochemical Solar Energy Conversion with

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