Tandem Si Micropillar Array Photocathodes with Conformal Copper Oxide and a Protection Layer by Pulsed Laser Deposition

Tandem Si Micropillar Array Photocathodes with Conformal Copper

Oxide and a Protection Layer by Pulsed Laser Deposition

Pramod Patil Kunturu,

Christos Zachariadis,

Lukasz Witczak,

Minh D. Nguyen,

Guus Rijnders,

and Jurriaan Huskens

*

†_{Molecular Nanofabrication Group, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, and} ‡_Inorganic Materials Science, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, Enschede 7500 AE, The Netherlands

*

ABSTRACT: This work demonstrates the influence of high-quality protection layers on Si−Cu₂O micropillar arrays created by pulsed laser deposition (PLD), with the goal to overcome photodegradation and achieve long-term operation during photoelectrochemical (PEC) water splitting. Sequentially, we assessed planar and micropillar device designs with various design parameters and their influence on PEC hydrogen evolution reaction. On the planar device substrates, a Cu2Ofilm thickness of 600 nm and a Cu2O/CuO heterojunction layer with a 5:1

thickness ratio between Cu₂O to CuO were found to be optimal. The planar Si/ Cu2O/CuO heterostructure showed a higher PV performance (Jsc= 20 mA/cm2)

as compared to the planar Si/Cu₂O device, but micropillar devices did not show this improvement. Multifunctional overlayers of ZnO (25 nm) and TiO2(100

nm) were employed by PLD on Si/Cu₂O planar and micropillar arrays to provide a hole-selective passivation layer that acts against photocorrosion. A micropillar

Si/ITO-Au/Cu₂O/ZnO/TiO₂/Pt stack was compared to a planar device. Under optimized conditions, the Si/Cu₂O photocathode with Pt as a HER catalyst displayed a photocurrent of 7.5 mA cm−2at 0 V vs RHE and an onset potential of 0.85 V vs RHE, with a stable operation for 75 h.

KEYWORDS: Si micropillar array, tandem photocathode, copper oxide, pulse laser deposition, hydrogen evolution reaction

■

Photoelectrochemical water splitting is an attractive way to produce clean hydrogen fuel by economic and convenient utilization of solar energy.1−4 The concept of photo-electrochemical (PEC) tandem device configurations that combine dual semiconducting photoabsorbers carries great potential for robust, inexpensive, and efficient unassisted solar water splitting.5−8In such a solar-to-fuel (S2F) device, sunlight is absorbed by a pair of semiconductor photoelectrodes to split water into hydrogen and oxygen.9,10Commonly, a bias voltage is needed from an external energy source for effective overall water splitting.11However, a practical S2F device should be a stand-alone cell with no need for an external voltage supply.12 To gain sufficient photovoltage, many researchers have used a combination of two or more semiconductors in a tandem configuration.13−15

Doscher et al. and Cheng et al. have reported that the maximum achievable solar-to-fuel efficiencies for the produc-tion of H2 are achieved using a pair of light absorbers with

bandgaps of approximately 1.8 and 1.0 eV.16,17Copper oxides (Cu2O and CuO) are interesting top absorber photocathode

materials,18−21 with narrow bandgap energies of 2.1 and 1.6 eV, which allow Cu2O and CuO to absorb a wide range of the

solar spectrum.22,23The combination of Cu2O with Si, which

has a bandgap of 1.1 eV, is therefore a logical choice for a tandem configuration. However, copper oxides present a

mismatch between light absorption and carrier diffusion length, and, as a result, their photovoltage is limited, which restricts their use as effective photocathodes for unassisted water splitting.18,24

To solve the photovoltage problem, an approach is to build a photovoltaic (PV)-photoelectrochemical (PEC) tandem con-figuration by combining a PEC device and series-connect it with a buried-junction PV cell.25−28Recent development in PV applications inspired the fabrication of 3D-structured Si micropillar arrays.29−31 A micropillar-based Si/Cu₂O tandem device design offers many advantages including: (i) the series addition of the photovoltage in both components of the tandem to split water without external bias,32,33(ii) orthogonal absorption of visible light and photogenerated charge carrier separation and transport,34,35 (iii) increased surface area for catalytic reactions, (iv) decreased usage of photoactive material relative to planar device designs, depending on pillar pitch and diameter,36and (v) the opportunity to spatially decouple light absorption and catalytic activity.29

To consider Si and Cu2O as effective photocathode

materials in the tandem device, however, surface passivation is needed to prevent photodegradation.19,37 Han et al. Received: August 12, 2019

Accepted: October 16, 2019

Published: October 16, 2019

Research Article

www.acsami.org

Cite This:ACS Appl. Mater. Interfaces 2019, 11, 41402−41414

This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes.

Downloaded via UNIV TWENTE on December 3, 2019 at 08:29:53 (UTC).

presented a simple method to prepare a Cu₂O/CuO heterostructure to protect the Cu2O photocathode from

photocorrosion in the PEC water splitting reaction.38 The as-prepared Cu₂O/CuO composite showed improved stability for the hydrogen evolution reaction by increasing the coverage of CuO on the Cu2Ofilm. However, CuO has limited stability

in aqueous electrolytes as suggested by the Pourbaix diagram.39 Many studies have reported CuO as a photocathode for the PEC water reduction reaction, although it is uncertain how much of the photocurrent leads to H2evolution.

40−42 In other studies, attempts to address this issue and to achieve long stability were focused on modifying the semiconductor surface with various n-type oxides (hole-selective layers) and cocatalyst layers like n-Ga₂O₃,43 AZO,18 CdS,39 NiO,44 TiO₂,45 ITO,46 Pt,47 NiSi,30 etc. Many researchers investigated semiconductor metal oxide materials, such as ZnO,48 WO3,49 Cu(x)O,50,51 SnO(x),52 and V2O5,53

either as nanoparticles or as continuousfilms, to create sensors (e.g., for humidity, ultraviolet, and biological analytes), and their sensing properties were found to be dependent on size, shape, defects, and dopants.54 Furthermore, the effect of

various dopants (Mg, Al, Ga, Fe, Ni)55−60 in the electron-selective oxide layer to increase the donor density to effectively prevent any resultant charge recombination additionally improves the photovoltage.61,62 Various deposition systems and techniques (i.e., atomic layer deposition (ALD),63 pulse laser deposition,64 chemical vapor deposition,65 spray pyrolysis,66 sputtering,67 anodic oxidation,68 thermal oxida-tion,69and electrochemical deposition70) have been employed for passivating semiconductor photoabsorbers, each with their own advantages and limitation.71−73 Pulse laser deposition (PLD) is an emerging technique for industrialized use,74 and can achieve large-area uniform coating on 3D structures. As a result of passivation, the semiconductor can be protected from the harsh acidic/basic electrolyte environment often needed for long-term operation.

Here, we demonstrate the use of pulsed laser deposition (PLD), by which ZnO and TiO2 overlayers can be grown

directly and homogeneously on Si/Cu₂O microstructures with an accurate film thickness and composition. Recently, we showed that a tandem of Si micropillars with copper oxide can provide efficient water-splitting photocathodes.75 We here

Figure 1.SEM images of bare Si micropillars after the DRIE process, with different heights and pitches: (a) 10 μm, 8 μm; (b) 20 μm, 8 μm; (c) 30 μm, 8 μm; and (d) 40 μm, 10 μm, respectively.

report the fabrication of tandem Si/Cu₂O micropillar array devices with passivation layers of ZnO and TiO₂with a precise thickness deposited by PLD. This study exhibits that the PLD approach can be effective and scalable for designing efficient and stable silicon-based tandem photocathodes for solar water splitting. In addition, we explore design aspects for these micropillar-based Si/Cu2O PEC tandem devices. Specifically,

we investigate the influence of geometrical variations to a micropillar array PEC device design on light absorption and onset potential (Voc). An ITO-Au (85 nm) transparent

conductivefilm is applied for coupling the top photoabsorber on the engineered light-trapping micropillars. In addition, we fabricated a Cu2O/CuO heterojunction on Si planar and

micropillar arrays to investigate the effect of the Cu₂O/CuO composite in PV and PEC performances. A micropillar Si/ ITO-Au/Cu2O/ZnO/TiO2/Pt stack is compared to a planar

device. Furthermore, we evaluate the roles of the n-ZnO (25 nm) and TiO2 (100 nm) thin films as hole-selective and

passivation layers.

■

Micropillar Array Fabrication and Interlayer Deposi-tion. The substrates with vertically ordered micropillar arrays with radial pn-junctions (p-type base and n+-type emitter) were fabricated using deep-reactive ion etching (DRIE) (Figure S1). The light absorption ability in the micropillar structure is considerably improved not only by the increased surface area of the photoelectrode but also by the manifold light scattering within the micropillar structure (Scheme 1),31,76 which is a tandem device that contains two photoabsorbers. The top cell presents a large band gap material that absorbs high energy photons, while the bottom cell has a small band gap material that harvests the low energy photons. Many device and material aspects need to be tuned carefully to achieve high efficiency, such as the bandgaps and material thicknesses, but also reflection and parasitic

absorption. Furthermore, the transparent conductive oxide (TCO) layer on the substrate can achieve an effective charge separation for a relatively thick film. In general, TCO-semiconductor Schottky contacts can lead to high recombina-tion rates and therefore usually produce low Voc values.

However, when a PV junction is decorated with a TCO layer, highV_ocvalues can still be obtained.77

Upon following the fabrication and DRIE process of the micropillars (Figure S1), a few samples were fractured and side views were taken by scanning electron microscopy (SEM) (Figure 1). The base and tip diameters of these micropillars within the arrays are approximately identical, emphasizing the highly anisotropic nature of the DRIE process. A slight tapering occurred at about∼20% from the top of the pillars. This effect was more pronounced for the 40 μm pillars (12 min etching), where the diameter decreased approximately 100− 200 nm. In addition, the SEM images (Figure 1) show the entire side surface of the micropillars to be scalloped due to the cyclic steps in the etching process, involving gas pulses of SF6

(etching step) and C₄F₈(passivation step), providing scallops with an overall height of∼340 nm, for each etching cycle.

The first step to an efficient Si−Cu₂O tandem device is to find the ideal interlayer for the Cu2O deposition on top of the

Si micropillar array. Such a layer needs to be a transparent so that sunlight can pass through it and can reach the underlying Si photoabsorber. In addition, the interlayer must be conductive to provide an ohmic contact for the growth of Cu₂O film and allow charge carrier exchange between the Si and Cu2O. Here, we investigated different interlayers such as

gold (Au), copper (Cu), molybdenum (Mo), platinum (Pt), indium tin oxide (ITO), and ITO/Au deposited using a magnetron sputtering system.

The growth of the Cu2O film on different interlayers was

characterized by scanning electron microscopy (SEM) as illustrated in Figure 2. By using Cu, Au, and Pt interlayers, Cu₂O growth was compact and homogeneous on the Si substrate. Yet, due to the low conductivity of the Mo interlayer,

Figure 2.Top-view SEM images of the electrodeposited Cu2O on a thin interlayer of (a) Cu, (b) Pt, (c) Au, and (d) Mo.

Cu₂O grew into large cuboctahedral crystals without covering the Si surface in a conformal manner. However, none of these metallic interlayers is suited for the tandem device due to their low optical transparency. Thus, a thin layer of the conducting ITO (80 nm) with a thin layer of Au (nominally 5 nm) was deposited to enhance the conductivity without compromising in transparency. The short sputtering time, which leads to small Au nanoparticles instead of a dense homogeneousfilm, plays an important role in the electrodeposition of Cu2O,

leading to specifically oriented crystal growth on the substrate.78Furthermore, the combined ITO-Au nanoparticle film (i.e., the ITO/Au layer) is sufficiently transparent and conductive to deposit Cu₂O effectively.79,80

Optimization of Cu2O Film Thickness. The second step

in the overall fabrication process is the deposition of Cu2O on

the conductive Si/ITO-Au planar and micropillar array substrates. Conformal, compact, and semitransparent Cu2O

films were prepared by chronoamperometric (CA) electro-deposition. To obtain a high-performance Cu₂O film, we deposited the material with different thicknesses (250−1300 nm) by changing the electrodeposition time (200−1300 s) at a constant cathodic potential (−0.8 V), while the temperature of the bath was maintained at 50°C. It is important to notice that we did not elongate the deposition process for micropillar arrays with different lengths (10−40 μm) and for different pitches (8−14 μm), seeFigure 3, because the highest growth rate is reached at the applied potential and temperature of the bath solution.81−83

Because the ITO-Au interlayer is conducting, and the system is mass transport-limited during the electrodeposition, a conformal layer of Cu2O is deposited on the micropillars. It

was observed that Cu2O forms columnar crystals in a uniform

manner on Si/ITO-Au planar substrates (Figure S2) and at the bottom side of the Si/ITO-Au micropillar array substrates (Figure S3) as well as in the sidewall scalloped area. This uniformity occurred despite the fact that the ITO/Au interlayer was thinner on the scalloped sides of the micropillars when compared to the bottom side of the Si substrate, which was caused by a shadowing effect by the scallops during the interlayer deposition by the directional magnetron sputtering process.

The SEM images and X-ray diffraction (XRD) character-ization revealed that the prepared films have columnar

triangular cubes or truncated cubic crystal growth with (111) orientation for shorter and longer deposition times, respec-tively (Figure 4). Paracchino et al. reported that the columnar

film with cubes exposing the triangular (111) faces parallel to the substrate showed a higher photocurrent.19 The crystal structure of the deposited layers was confirmed by X-ray diffraction (XRD).Figure 4shows the XRD pattern of Cu2O

showing (110), (111), and (200) peaks at 30°, 36.9°, and 42.7°, respectively, on p-Si/ITO/Au micropillar array. For the ITO/Au interlayer, an ITO peak was found at 31° and for the Au (200) peak was found at 38.6°. For the silicon substrate, two peaks were observed, (200) and (400) at 33.3° and 61.3°, respectively. The intensity of the ITO peak diminished with increasing Cu₂Ofilm thickness, as well as the Au peak, which was completely covered in a broad peak of the Cu2O with

higher intensity. Finally, a Cu₂O (220) peak was found in close proximity with the p-Si (400) peak at 61.3°, upon thicker Cu₂O deposition, which occurs from the difference in the surface energy of the cubic crystal facets.

Figure 3.SEM images of pn-Si/ITO-Au/Cu2O micropillar arrays with different heights and pitches: (a) 10 μm, 12 μm; (b) 20 μm, 14 μm; (c) 30

μm, 10 μm; (d) 40 μm, 12 μm; (e) 20 μm, 8 μm; (f) 20 μm, 10 μm; (g) 20 μm, 12 μm; and (h) 20 μm, 14 μm, respectively.

Figure 4. XRD patterns for (a) the Si Substrate, (b) Si/ITO-Au interlayer, (c) Si/ITO-Au/Cu2O (250 nm thick film), and (d) Si/

Preparation of Cu2O/CuO Heterojunction. After the

electrodeposition, the Cu2O film was subsequently converted

into a Cu2O/CuO heterojunctionfilm by a thermal annealing

process at 350°C in air. To optimize the film thickness ratio between Cu₂O and CuO, we followed two different strategies. First, electrodeposited Cu₂O samples were prepared with different film thicknesses by variation of the deposition time on the Si/ITO-Au substrate, followed by annealing all samples at a

constant annealing time of 1 h at 400 °C. Second, samples were prepared with the same initial Cu2O thickness, followed

by annealing the samples at different annealing times (0.5−3 h) at 400°C.

SEM images were taken to provide the thicknesses of both copper oxides on the Si/ITO-Au substrates (Figure 5). The morphology of the sample changed after thermal oxidation in air, from a continuous film before thermal oxidation to a

Figure 5.SEM images of Si/ITO-Au/Cu2O/CuOfilms prepared by electrodeposition of Cu2O at different deposition times of (a) 350 s, (b) 700 s,

and (c) 1000 s, followed by thermal oxidation for 1 h at 400°C, or by electrodeposition of Cu2O for 700 s, followed by thermal oxidation for (d)

0.5 h, (e) 1 h, and (f) 3 h at 400°C.

Figure 6.(a) Copper oxide (Cu2O and CuO)film thicknesses as a function of annealing time. (b) XRD patterns of Si/ITO-Au/Cu2O/CuO

photocathodes after thermal oxidation at 400°C for different annealing times. (c) XRD patterns of Si/ITO-Au/Cu2O/CuO photocathodes at

different electrodeposition times after annealing for 30 min at 400 °C.

bilayer-structured film after thermal oxidation. This observa-tion implies that the outer layer of the Cu2O film was

transformed into CuO, and a Cu2O/CuO heterojunction was

formed. When samples with different initial thicknesses of Cu₂O were oxidized in air using the same annealing time (Figure 5a−c), the thickness of the newly formed CuO film on top of Cu₂O layer was the same. In the second case, when the thermal oxidation process time was increased from 0.5 to 3 h (Figure 5d−f), the thickness of the outer CuO layer increased. By consuming the Cu2O layer underneath the CuO, the

thickness of the inner Cu2O layer was reduced. This trend is

clearly indicated in Figure 6a. Diao et al. reported that the whole Cu2O layer can be oxidized to CuO, leading to a

continuous porousfilm.21These results confirm that the Cu2O

to CuO thickness ratio can be controlled accurately by varying the deposition time of the initial Cu₂Ofilm thickness and the subsequent thermal oxidation time.

The XRD patterns of the Cu2O/CuO heterostructure film

on Si/ITO-Au substrates, in which the diffraction peaks of both cubic Cu2O and monoclinic CuO appear (Figure 6b,c),

provide direct evidence for the formation of the Cu2O/CuO

bilayer heterostructure. The emerging XRD diffraction peaks at 31.1°, 36.5°, and 39.2° could be indexed to the (110), (002), and (200) planes of CuO, respectively. In addition, from the XRD plots at increasing annealing times (Figure 6b), the intensity of the CuO [002] peaks increases, while the Cu2O

[111] peak intensities decrease. In contrast, when the initial Cu2O film thickness increased by the subsequent oxidation

time is kept constant (Figure 6c), the spectrum hardly changes. Photovoltaic (JV) Measurements on Si Micropillar Arrays with a Radial pn-Junction. Micropillar array dimensions (height and pitch) have an influence on important functional parameters such as surface area, light absorption, and charge separation, which finally regulate the device efficiency. In addition, the relationships between the fabrication process and the characteristics of device perform-ance are tough to predict. For instperform-ance, it is known that with increasing micropillar height, the total junction area in the solar cell increases, and the reflectivity of sunlight probably decreases.35,76The effect of micropillar dimensions (i.e., height and pitch) with integrating radial pn-junction was here

Figure 7.JV measurements of (a−d) pn-Si micropillar array substrates without Cu2O, for varying pillar heights and pitches as indicated, and (e)

investigated by examining the resulting photoelectrical performance.

Figure 7a−d shows the JV characteristics of samples with different pillar heights and pitch from 10 to 40 μm and 8 to 14 μm, respectively. Samples with pillars performed better in both current density (JSC) and open circuit voltage (VOC) as

compared to a flat sample, which indicates that the pillar structures significantly improve the properties such as local surface area and light absorption properties in solar cells. The values of thefill factor (FF) and efficiency (η), along with their respective J_SC and V_OC, are shown in Table S1. A minor increase of VOCis observed, from 450 mV for theflat sample to

≥550 mV for micropillars.

The FF determines the maximum power point of a solar cell and can be calculated from J_SCand V_OCwitheq 1. Using thefill factor (FF), short-circuit current density (JSC), and

open-circuit voltage (V_OC), the efficiency can be calculated usingeq 2. FF J V J V mp mp SC OC = (1) V J P FF OC SC in η = (2)

where Jmp is the current density, Vmp is the voltage at the

maximum power point, and P_in is the input power, which is 100 mW/cm2(AM 1.5).

When viewing the dependence on pillar height (Figure 7e), JSCvalues increase from theflat substrate up to 20 μm height,

but drop again for≥30 μm pillar heights. The latter decrease is due to charge carrier recombination by an increasing number of defect states with increasing micropillar height.76,84 Most likely, these defects have been introduced by the DRIE method

used to make the pillars. The working principle of the applied Bosch process, which is an alternating process of etching (SF6)

and passivation (C₄F₈) steps, has led to the scallop structures,85 which further increases the number of defect states. We conclude that for these pillar arrays, optimal performance is achieved with a 4μm diameter, 12 μm pitch, and pillar height of 20μm.

PV Performance of Copper Oxide-Covered Micro-pillar Arrays. In a tandem device, the overall photovoltage is generated by the absorption of two photons instead of one. Each of these photons creates a pair of charge carriers (e−and h+_{) in the total photoabsorber stack. First, it is important to}

find the proper thickness of the top photoabsorber so that part of the sunlight can pass through it and illuminate the bottom photoabsorber to get the total photovoltage. Second, we fabricated Si micropillar substrates with varying heights, pitches, and copper oxide thicknesses to find the maximum current density output resulting from these aspects. A Cu₂O film thickness of ∼650 nm appears most favorable with respect to the resulting short circuit current density (J_sc), measured in an aqueous PEC setup (Figure S6b), as shown inFigure 8a. It is well-known that a thicker layer limits the charge separation efficiency and a thinner layer does not absorb all incoming light. Si micropillars covered with optimized Cu₂O film thickness of ∼650 nm were fabricated with varying pillar heights, from a planar substrate to 40 μm height, and the highest Jscwas produced at 20μm height (Figure 8b). Finally,

the micropillar pitches were varied between 8 and 14μm, and 12μm pitch gave the highest J_sc; seeFigure 8c.

In case of Si/Cu2O/CuO heterostructure fabrication, we

deposited 650 nm thick Cu₂O on a planar substrate followed by thermal oxidation process at 400 °C while varying the annealing time as shown inFigure 6a. As a result, Cu₂O/CuO heterojunction layer with a different thickness ratio between

Figure 8.Plots of the current density as a function of different parameters of the Si/ITO-Au/Cu(x)O PEC tandem samples with (a−c) Cu2Ofilm

and (d−f) Cu2O/CuO heterostructurefilm as top photoabsorbers: (a,d) varying the Cu2Ofilm and Cu2O/CuOfilm thickness on planar substrates,

(b,e) varying the micropillar height, for micropillar samples covered with a Cu2O film (650 nm) or Cu2O (630 nm)/CuO (120 nm)

heterostructurefilm (with 5:1 thickness ratio between Cu2O/CuO) at a pitch of 12μm, and (c,f) varying the micropillar pitch, for micropillar

samples covered with a Cu2Ofilm (650 nm) or Cu2O (630 nm)/CuO (120 nm) heterostructurefilm (with 5:1 thickness ratio between Cu2O/

CuO) at a pillar height of 20μm.

Cu2O and CuOfilm was formed on planar Si substrates. The

highest J_scoutput was observed for the Cu₂O (630 nm)/CuO (120 nm) heterostructurefilm with thickness ratio of 5:1 on planar Si substrate, respectively (Figure 8d). This is considered as an optimum Cu2O/CuO heterostructure film for flat

samples, and this was applied on Si micropillar substrates with varying micropillars height and pitch. When the optimized layer thickness of the Cu2O/CuO heterostructure was placed

on Si micropillars with varying heights and pitches, surprisingly no clear trends in the output current density were observed with respect to these parameters (Figure 8e and f). The observed behavior could possibly be due to parasitic resistances and optical losses, where the former occurs presumably between the copper oxides and between Cu2O

and silicon. In a comparison between the Cu2O and Cu2O/

CuO heterojunction systems, the former all perform better at the optimized pillar parameters (Figure 8), while only at the low-performance sides of these parameters are comparable performances observed. Therefore, we conclude that Si micropillar arrays with a pitch of 12μm, a height of 20 μm, and a (nonoxidized) Cu₂O layer thickness of 650 nm are optimal, and wefixed these for further study.

The JV curves of planar and micropillar Si substrates were measured without and with different overlayers on top (Figure

9; JV-measurement setup shown inFigure S6a), by measuring dry and contacting only the Si cell on top and bottom. After the ITO layer (∼80 nm) was deposited, the current density increased for both the planar and the micropillar Si solar cells due to its antireflection property. After a Cu2O film of ∼650

nm thickness was electrodeposited over the Si/ITO-Au substrates, both the VOC as well as the JSC values dropped

significantly, due to the parasitic light absorption by Cu2O and

the consequently reduced light intensity on the Si. For a planar solar cell, the JSCdecreased by 7 mA/cm2 due to the Cu2O

layer, while for a Si micropillar solar cell the J_SCdecreased with 22 mA/cm2_{, making the output practically comparable. This}

indicates that the Cu2O layer absorbs more sunlight when

fabricated over the micropillar array.

Interestingly, when a Cu2O (630 nm)/CuO (120 nm)

heterojunction layer was deposited on the planar and micropillar substrates, the drop of the VOC and JSC values

was nearly similar as compared to the Cu₂O overlayer, but the fill factor (FF) decreased considerably (Figure 9). The low slope close to VOC in the JV curves shows that the device

behavior is dominated by a series resistance, which is attributed to a poor conductance between the top (Cu2O/CuO) and

bottom (Si) photoabsorbers. We found voids formed between the Si surface and the Cu2O/CuO layer after thermal oxidation

Figure 9.JV characteristics of (a) planar and (b) micropillar Si substrates covered with different overlayers; a bare substrate, with ITO, with Cu2O

and Cu2O/CuOfilms over the ITO-Au layer.

Figure 10.HR-SEM cross-section images of PLD-coated ZnO (20 nm) and TiO2(100 nm) on top of the Si/ITO-Au/Cu2O (a) planar and (b)

micropillar devices. False-colored on eachfigures indicates conformal layers (ITO/Au layer, yellow; Cu2Ofilm, light red; n-ZnO, blue; and TiO2,

of the preceding Si/Cu2O device. Interestingly, this void

formation was more extensive in case of the micropillar arrays (Figure S4b), potentially because of the larger surface area and/or the presence of different curvatures in the micropillar samples. Apparently these voids explain the increased series resistance observed inFigure 9. Possibly, the thermal annealing process, which was carried out to prepare the Cu2O/CuO

heterojunction, led to cracks at the Cu₂O−Si interface (see Figure S4).

Pulsed Laser Deposition of Protection Layers. For a high-performance Cu2O-based PEC electrode, the quality of

the buried p−n junction, the protection layer, and the cocatalyst are the most important parameters. The p−n junction must be connecting and uniform, the protection layer must be conformal and pinhole-free, and the catalyst nanoparticle islands must be robust and adhered strongly on the electrode surface. A protection layer is needed to prevent direct contact of the Cu₂O film with the electrolyte to avoid photocorrosion, and thus to maximize the performance of the device. Here, we explored the use of pulsed laser deposition (PLD) as a method to apply a conformal and high-quality protection overlayer on micropillar array structures.86

Previously, Luo et al. reported a high photocurrent density by fabricating a Cu₂O nanowire-structured photoelectrode.87 Yet, the challenging part is to increase the photovoltage for unassisted water-splitting devices. The photovoltage is depend-ent on the difference between the quasi-Fermi level of the electrons in the n-type oxide layer and the holes in Cu₂O under illumination in a heterojunction device. Therefore, choosing a suitable n-type oxide layer is important for producing a high photovoltage.43 We explored the use of

zinc oxide (n-ZnO), an n-type material, combined with titanium oxide (TiO₂) as a protective layer to improve the stability and performance of a Cu2O-based photocathode.

HR-SEM images of the electrodes after protection layer deposition of 20 nm of ZnO and 100 nm of TiO2are shown in

Figure 10a,b on both planar and micropillar samples. These images clearly show the homogeneous coating of the complete stack of layers on both substrates. The difference in contrast of each layer in images shows the quality and conformity of deposition achieved by the PLD technique. The band energy-level alignment diagrams of p-Cu2O with n-ZnO and n-TiO2

are shown inFigure S5. A conduction band offset between the Cu2O and ZnO layer is about ∼1 eV, which improves the

separation of the quasi-Fermi levels in the two oxides under illumination to 0.45 eV in agreement with the 0.45 V positive shift of the onset potential, and hence the buildup of an extra photovoltage for hydrogen generation.88−90

Photoelectrochemical Performance of the Photo-cathodes. To assess the PEC performance of the devices, JE measurements of Cu₂O/ZnO/TiO₂/Pt on the Si/ITO-Au planar and micropillar array photocathodes were performed with the same Cu₂O film thickness of ∼650 nm (see Figure 11). As a proof of concept, Pt was used as the HER catalyst, and it was deposited electrochemically under dark condition using an aqueous solution of 1 mM H2PtCl6for 15 min (for

planar) and 19 min (for micropillars) with a thickness of nominally 3 nm using a procedure reported previously.18Pt is used as a catalyst because (i) it has been well studied, and (ii) it is purely as a reference case for testing the activity of the PEC cell without having to worry about the catalytic performance. PEC measurements were performed under

Figure 11.(a) PEC performance in the dark (dashed lines) and under illumination (solid lines) from simulated 1 sun illumination for planar and micropillar Cu2O/ZnO/TiO2/Pt photocathodes. (b) Current density−time behavior of pn-Si/ITO-Au/Cu2O/ZnO/TiO2/Pt photocathodes

measured at 0 V vs RHE. Here, two“dark” periods were incorporated of each 30 min. (c) IPCE measurements of both photocathode devices at 0 V vs RHE under monochromatic illumination (solid lines) and the integrated photocurrent over the solar spectrum (dotted lines). (d) Hydrogen gas produced and calculated versus time for the micropillar device.

deposition of ZnO and TiO₂.

To better understand the PEC performance of these photocathodes, we measured the incident-photon-to-current efficiency (IPCE) of both devices and used it to calculate the integrated photocurrent under AM 1.5 G (100 mW cm−2) solar irradiation (Figure 11c). The overall current densities correspond well with the values of the photocurrent plotted in Figure 11a. The IPCE clearly shows a broad plateau response through a wider spectral range for the micropillar array compared to the planar device. The production of H2gas was

confirmed by the formation of bubbles evolving, and measurements by gas chromatography for a micropillar photocathode biased at 0 V vs RHE with a near-ideal faradaic efficiency (Figure 11d; small deviations attributed to measure-ment inaccuracies, gas dissolution, and possible parasitic processes).

■

In summary, we have developed a highly photostable tandem Si/Cu2O micropillar photocathode with passivating ZnO and

TiO₂ overlayers directly on the micropillar array devices by using the pulsed laser deposition method. These conformal protectionfilms grown with precise thickness formed on both planar and microstructure devices. The Cu₂O photocathode with a thickness of ∼650 nm combined with Si micropillar arrays (20μm height and 12 μm pitch) delivered an optimal photocurrent of 7.5 mA/cm2at 0 V vs RHE, a photovoltage of 0.85 V, and stability beyond 75 h using an n-ZnO hole-selective layer, a TiO2protection layer, and a Pt HER catalyst.

In case of Cu2O/CuO heterostructure, an improved

perform-ance was gained for planar samples but not for micropillared samples. The high onset potential achieved in Si/Cu₂O micropillar sample is attributed to the radial buried pn-junction PV cell as the bottom photoabsorber. The ZnO layer forms a promising heterojunction with Cu₂O for a good separation of the photogenerated charge carriers in thefilm. As a result, high photostability is achieved by the protective layers that prevent photocorrosion of the PV−PEC tandem device. Our study presents a successful case for the use of PLD to achieve conformal coating of protective layers on high-aspect ratio micropillar arrays, which offers a new path to stable PEC water splitting devices as well as to high-efficiency solar cells.

■

*

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acsami.9b14408.

■

Wouter Vijselaar is acknowledged for discussions and his assistance with JV measurements. Henk van Wolferen is acknowledged for the FIB experiments. We thank Prof. Guido Mul for use of the solar simulator. P.P.K. is grateful to the Karnataka state government for providing a scholarship under the D Devraj Urs Videshi Vyasanga Vetana scheme.

■

(1) Kim, J. H.; Hansora, D.; Sharma, P.; Jang, J.-W.; Lee, J. S. Toward Practical Solar Hydrogen Production − An Artificial Photosynthetic Leaf-to-Farm Challenge. Chem. Soc. Rev. 2019, 48, 1908−1971.

(2) Tuller, H. L. Solar to Fuels Conversion Technologies: APerspective. MaterRenew Sustain Energy 2017, 6, 1−16.

(3) Chen, D.; Liu, Z.; Guo, Z.; Yan, W.; Xin, Y. Enhancing Light Harvesting and Charge Separation of Cu2O Photocathodes with Spatially Separated Noble-metal Cocatalysts Towards Highly Efficient Water Splitting. J. Mater. Chem. A 2018, 6, 20393−20401.

(4) Liu, Z.; Song, Q.; Zhou, M.; Guo, Z.; Kang, J.; Yan, H. Synergistic Enhancement of Charge Management and Surface Reaction Kinetics by Spatially Separated Cocatalysts and p-n Heterojunctions in Pt/CuWO4/Co3O4 Photoanode. Chem. Eng. J. 2019, 374, 554−563.

(5) Chu, S.; Li, W.; Yan, Y.; Hamann, T.; Shih, I.; Wang, D.; Mi, Z. Roadmap on Solar Water Splitting: Current Status and Future Prospects. Nano Futures 2017, 1, 022001.

(6) He, Y.; Hamann, T.; Wang, D. Thin film Photoelectrodes for Solar Water Splitting. Chem. Soc. Rev. 2019, 48, 2185−2215.

(7) Alfaifi, B. Y.; Ullah, H.; Alfaifi, S.; Tahir, A. A.; Mallick, T. K. Photoelectrochemical Solar Water Splitting: From Basic Principles to Advanced Devices.Veruscript Funct. Nanomater. 2018, 2, BDJOC3.

(8) Lan, Y.; Liu, Z.; Guo, Z.; Li, X.; Zhao, L.; Zhan, L.; Zhang, M. A ZnO/ZnFe2O4 Uniform Core−shell Heterojunction with a Tubular Structure Modified by NiOOH for Efficient Photoelectrochemical Water Splitting. Dalton Trans 2018, 47, 12181−12187.

(9) Seger, B.; Castelli, I. E.; Vesborg, P. C. K.; Jacobsen, K. W.; Hansen, O.; Chorkendorff, I. 2-Photon Tandem Device for Water Splitting: Comparing Photocathode First versus Photoanode First Designs. Energy Environ. Sci. 2014, 7, 2397−2413.

(10) Zhang, K.; Ma, M.; Li, P.; Wang, D. H.; Park, J. H. Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis. Adv. Energy Mater. 2016, 6, 1600602.

(11) Prévot, M. S.; Sivula, K. Photoelectrochemical Tandem Cells for Solar Water Splitting. J. Phys. Chem. C 2013, 117, 17879−17893. (12) Brillet, J.; Yum, J.-H.; Cornuz, M.; Hisatomi, T.; Solarska, R.; Augustynski, J.; Graetzel, M.; Sivula, K. Highly Efficient Water Splitting by a Dual-absorber Tandem Cell. Nat. Photonics 2012, 6, 824.

(13) Qiu, Y.; Liu, W.; Chen, W.; Chen, W.; Zhou, G.; Hsu, P.-C.; Zhang, R.; Liang, Z.; Fan, S.; Zhang, Y.; Cui, Y. Efficient Solar-driven Water Splitting by Nanocone BiVO4-Perovskite Tandem Cells. Sci.

(14) Vilanova, A.; Lopes, T.; Spenke, C.; Wullenkord, M.; Mendes, A. Optimized Photoelectrochemical Tandem Cell for Solar Water Splitting.Energy Storage Materials 2018, 13, 175−188.

(15) Kim, J. H.; Jo, Y.; Kim, J. H.; Jang, J. W.; Kang, H. J.; Lee, Y. H.; Kim, D. S.; Jun, Y.; Lee, J. S. Wireless Solar Water Splitting Device with Robust Cobalt-Catalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf. ACS Nano 2015, 9, 11820−11829.

(16) Döscher, H.; Geisz, J. F.; Deutsch, T. G.; Turner, J. A. Sunlight Absorption in Water − Efficiency and Design Implications for Photoelectrochemical Devices. Energy Environ. Sci. 2014, 7, 2951− 2956.

(17) Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett. 2018, 3, 1795−1800. (18) Paracchino, A.; Laporte, V.; Sivula, K.; Grätzel, M.; Thimsen, E. Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nat. Mater. 2011, 10, 456.

(19) Paracchino, A.; Brauer, J. C.; Moser, J.-E.; Thimsen, E.; Graetzel, M. Synthesis and Characterization of High-Photoactivity Electrodeposited Cu2O Solar Absorber by Photoelectrochemistry and Ultrafast Spectroscopy.J. Phys. Chem. C 2012, 116, 7341−7350.

(20) Zhang, Z.; Dua, R.; Zhang, L.; Zhu, H.; Zhang, H.; Wang, P. Carbon-Layer-Protected Cuprous Oxide Nanowire Arrays for Efficient Water Reduction.ACS Nano 2013, 7, 1709−1717.

(21) Yang, Y.; Xu, D.; Wu, Q.; Diao, P. Cu2O/CuO Bilayered Composite as a High-Efficiency Photocathode for Photoelectrochem-ical Hydrogen Evolution Reaction.Sci. Rep. 2016, 6, 35158.

(22) Huang, Q.; Kang, F.; Liu, H.; Li, Q.; Xiao, X. Highly Aligned Cu2O/CuO/TiO2 Core/shell Nanowire Arrays as Photocathodes for Water Photoelectrolysis. J. Mater. Chem. A 2013, 1, 2418−2425.

(23) Guo, X.; Diao, P.; Xu, D.; Huang, S.; Yang, Y.; Jin, T.; Wu, Q.; Xiang, M.; Zhang, M. CuO/Pd Composite Photocathodes for Photoelectrochemical Hydrogen Evolution Reaction. Int. J. Hydrogen Energy 2014, 39, 7686−7696.

(24) de Jongh, P. E.; Vanmaekelbergh, D.; Kelly, J. J. Cu2O: Electrodeposition and Characterization. Chem. Mater. 1999, 11, 3512−3517.

(25) Cox, C. R.; Lee, J. Z.; Nocera, D. G.; Buonassisi, T. Ten-percent Solar-to-Fuel Conversion with Nonprecious Materials. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 14057−14061.

(26) Xia, Y.; Pu, X.; Liu, J.; Liang, J.; Liu, P.; Li, X.; Yu, X. CuO Nanoleaves Enhance The c-Si Solar Cell Efficiency. J. Mater. Chem. A 2014, 2, 6796−6800.

(27) Gao, F.; Liu, X.-J.; Zhang, J.-S.; Song, M.-Z.; Li, N. Photovoltaic Properties of The p-CuO/n-Si Heterojunction Prepared Through Reactive Magnetron Sputtering.J. Appl. Phys. 2012, 111, 084507.

(28) Ghadimkhani, G.; de Tacconi, N. R.; Chanmanee, W.; Janaky, C.; Rajeshwar, K. Efficient Solar Photoelectrosynthesis of Methanol from Carbon dioxide Using Hybrid CuO−Cu2O Semiconductor NanorodAarrays. Chem. Commun. 2013, 49, 1297−1299.

(29) Vijselaar, W.; Westerik, P.; Veerbeek, J.; Tiggelaar, R. M.; Berenschot, E.; Tas, N. R.; Gardeniers, H.; Huskens, J. Spatial Decoupling of Light Absorption and Catalytic Activity of Ni−Mo-loaded High-aspect-ratio Silicon Microwire Photocathodes. Nat. Energy 2018, 3, 185−192.

(30) Vijselaar, W.; Tiggelaar, R. M.; Gardeniers, H.; Huskens, J. Efficient and Stable Silicon Microwire Photocathodes with a Nickel Silicide Interlayer for Operation in Strongly Alkaline Solutions. ACS Energy Lett. 2018, 3, 1086−1092.

(31) Elbersen, R.; Vijselaar, W.; Tiggelaar, R. M.; Gardeniers, H.; Huskens, J. Fabrication and Doping Methods for Silicon Nano- and Micropillar Arrays for Solar-Cell Applications: A Review. Adv. Mater. 2015, 27, 6781−6796.

(32) Rocheleau, R. E.; Miller, E. L. Photoelectrochemical Production of Hydrogen: Engineering Loss Analysis. Int. J. Hydrogen Energy 1997, 22, 771−782.

(33) Bolton, J. R.; Strickler, S. J.; Connolly, J. S. Limiting and Realizable Efficiencies of Solar Photolysis of Water. Nature 1985, 316, 495−500.

(34) Strandwitz, N. C.; Turner-Evans, D. B.; Tamboli, A. C.; Chen, C. T.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical Behavior of Planar and Microwire-Array Si|GaP Electrodes. Adv. Energy Mater. 2012, 2, 1109−1116.

(35) Elbersen, R.; Tiggelaar, R. M.; Milbrat, A.; Mul, G.; Gardeniers, H.; Huskens, J. Controlled Doping Methods for Radial p/n Junctions in Silicon. Adv. Energy Mater. 2015, 5, 1401745.

(36) Kayes, B. M.; Atwater, H. A.; Lewis, N. S. Comparison of The Device Physics Principles of Planar and Radial p-n Junction Nanorod Solar Cells. J. Appl. Phys. 2005, 97, 1−11.

(37) Oh, S.; Kim, J. B.; Song, J. T.; Oh, J.; Kim, S.-H. Atomic Layer Deposited Molybdenum disulfide on Si Photocathodes for Highly Efficient Photoelectrochemical Water Reduction Reaction.J. Mater. Chem. A 2017, 5, 3304−3310.

(38) Han, J.; Zong, X.; Zhou, X.; Li, C. Cu2O/CuO Photocathode with Improved Stability for Photoelectrochemical Water Reduction. RSC Adv. 2015, 5, 10790−10794.

(39) Septina, W.; Prabhakar, R. R.; Wick, R.; Moehl, T.; Tilley, S. D. Stabilized Solar Hydrogen Production with CuO/CdS Heterojunc-tion Thin Film Photocathodes. Chem. Mater. 2017, 29, 1735−1743.

(40) Cots, A.; Bonete, P.; Gómez, R. Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron. ACS Appl. Mater. Interfaces 2018, 10, 26348−26356.

(41) Shaislamov, U.; Krishnamoorthy, K.; Kim, S. J.; Abidov, A.; Allabergenov, B.; Kim, S.; Choi, S.; Suresh, R.; Ahmed, W. M.; Lee, H.-J. Highly Stable Hierarchical p-CuO/ZnO Nanorod/Nanobranch Photoelectrode for Efficient Solar Energy Conversion. Int. J. Hydrogen Energy 2016, 41, 2253−2262.

(42) Emin, S.; Abdi, F. F.; Fanetti, M.; Peng, W.; Smith, W.; Sivula, K.; Dam, B.; Valant, M. A Novel Approach for the Preparation of Textured CuO Thin films from Electrodeposited CuCl and CuBr.J. Electroanal. Chem. 2014, 717−718, 243−249.

(43) Pan, L.; Kim, J. H.; Mayer, M. T.; Son, M.-K.; Ummadisingu, A.; Lee, J. S.; Hagfeldt, A.; Luo, J.; Grätzel, M. Boosting the Performance of Cu2O Photocathodes for Unassisted Solar Water Splitting Devices. Nat. Catalysis 2018, 1, 412−420.

(44) Sun, K.; Saadi, F. H.; Lichterman, M. F.; Hale, W. G.; Wang, H.-P.; Zhou, X.; Plymale, N. T.; Omelchenko, S. T.; He, J.-H.; Papadantonakis, K. M.; Brunschwig, B. S.; Lewis, N. S. Stable Solar-driven Oxidation of Water by Semiconducting Photoanodes Protected by Transparent Catalytic Nickel Oxide Films.Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 3612−3617.

(45) Xing, Z.; Ren, F.; Wu, H.; Wu, L.; Wang, X.; Wang, J.; Wan, D.; Zhang, G.; Jiang, C. Enhanced PEC Performance of Nanoporous Si Photoelectrodes by Covering HfO2 and TiO2 Passivation Layers. Sci. Rep. 2017, 7, 1−10.

(46) Yamane, S.; Kato, N.; Kojima, S.; Imanishi, A.; Ogawa, S.; Yoshida, N.; Nonomura, S.; Nakato, Y. Efficient Solar Water Splitting with a Composite “n-Si/p-CuI/n-i-p a-Si/n-p GaP/RuO2” Semi-conductor Electrode. J. Phys. Chem. C 2009, 113, 14575−14581.

(47) Maier, C. U.; Specht, M.; Bilger, G. Hydrogen Evolution on Platinum-Coated p-Silicon Photocathodes. Int. J. Hydrogen Energy 1996, 21, 859−864.

(48) Gupta, S. K.; Joshi, A.; Kaur, M. Development of Gas Sensors Using ZnO Nanostructures. J. Chem. Sci. 2010, 122, 57−62.

(49) Wang, C.; Li, X.; Feng, C.; Sun, Y.; Lu, G. Nanosheets Assembled Hierarchical Flower-like WO3 Nanostructures: Synthesis, Characterization, and Their Gas Sensing Properties. Sens. Actuators, B 2015, 210, 75−81.

(50) Gao, P.; Liu, D. Facile Synthesis of Copper Oxide Nanostructures and Their Application in Non-Enzymatic Hydrogen Peroxide Sensing. Sens. Actuators, B 2015, 208, 346−354.

(51) Wan, X.; Wang, J.; Zhu, L.; Tang, J. Gas Sensing Properties of Cu2O and its Particle Size and Morphology-Dependent Gas-Detection Sensitivity. J. Mater. Chem. A 2014, 2, 13641−13647.

2013, 33, 157−165.

(57) Lim, J. H.; Lee, S. M.; Kim, H.-S.; Kim, H. Y.; Park, J.; Jung, S.-B.; Park, G. C.; Kim, J.; Joo, J. Synergistic Effect of Indium and Gallium co-doping on Growth Behavior and Physical Properties of Hydrothermally Grown ZnO Nanorods. Sci. Rep. 2017, 7, 41992.

(58) Yang, S.; Ge, C.; Liu, Z.; Fang, Y.; Li, Z.; Kuang, D.; Su, C. Novel Ga-doped, Self-Supported, Independent Aligned ZnO Nano-rods: One-pot Hydrothermal Synthesis and Structurally Enhanced Photocatalytic Performance. RSC Adv. 2011, 1, 1691−1694.

(59) Srinivasulu, T.; Saritha, K.; Reddy, K. T. R. Synthesis and Characterization of Fe-doped ZnO Thin films Deposited by Chemical Spray Pyrolysis.Mod. Electron. Mater. 2017, 3, 76−85.

(60) Das, S. C.; Green, R. J.; Podder, J.; Regier, T. Z.; Chang, G. S.; Moewes, A. Band Gap Tuning in ZnO Through Ni Doping via Spray Pyrolysis. J. Phys. Chem. C 2013, 117, 12745−12753.

(61) Liu, X.; Chen, Z.; Cao, M. NiFe Layered Double Hydroxide on Nitrogen Doped TiO2 Nanotube Arrays toward Efficient Oxygen Evolution.ACS Appl. Energy Mater. 2019, 2, 5960−5967.

(62) Wang, L.; Wang, W.; Chen, Y.; Yao, L.; Zhao, X.; Shi, H.; Cao, M.; Liang, Y. Heterogeneous p−n Junction CdS/Cu2O Nanorod Arrays: Synthesis and Superior Visible-Light-Driven Photoelectro-chemical Performance for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2018, 10, 11652−11662.

(63) Lee, S. W.; Lee, Y. S.; Heo, J.; Siah, S. C.; Chua, D.; Brandt, R. E.; Kim, S. B.; Mailoa, J. P.; Buonassisi, T.; Gordon, R. G. Improved Cu2O-Based Solar Cells Using Atomic Layer Deposition to Control the Cu Oxidation State at the p-n Junction. Adv. Energy Mater. 2014, 4 (1−7), 1301916.

(64) Lee, J.; Yoon, H.; Kim, S.; Seo, S.; Song, J.; Choi, B.-U.; Choi, S. Y.; Park, H.; Ryu, S.; Oh, J.; Lee, S. Long-term Stabilized High-density CuBi2O4/NiO Heterostructure Thin film Photocathode Grown by Pulsed Laser Deposition.Chem. Commun. 2019, 55, 12447. (65) Eisermann, S.; Kronenberger, A.; Laufer, A.; Bieber, J.; Haas, G.; Lautenschläger, S.; Homm, G.; Klar, P. J.; Meyer, B. K. Copper Oxide Thin films by Chemical Vapor Deposition: Synthesis, Characterization and Electrical Properties. Phys. Status Solidi A 2012, 209, 531−536.

(66) Pavan, M.; Rühle, S.; Ginsburg, A.; Keller, D. A.; Barad, H.-N.; Sberna, P. M.; Nunes, D.; Martins, R.; Anderson, A. Y.; Zaban, A.; Fortunato, E. TiO2/Cu2O All-oxide Heterojunction Solar Cells Produced by Spray Pyrolysis. Sol. Energy Mater. Sol. Cells 2015, 132, 549−556.

(67) Lee, S. H.; Yun, S. J.; Lim, J. W. The Characteristics of Cu2O Thin films Deposited Using RF-Magnetron Sputtering Method with Nitrogen-Ambient. ETRI Journal 2013, 35, 1156−1159.

(68) Abrantes, L. M.; Castillo, L. M.; Norman, C.; Peter, L. M. A Photoelectrochemical Study of the Anodic Oxidation of Copper in Alkaline Solution. J. Electroanal. Chem. Interfacial Electrochem. 1984, 163, 209−221.

(69) Shahbazi, P.; Kiani, A. Fabricated Cu2O Porous Foam Using Electrodeposition and Thermal Oxidation as a Photocatalyst under Visible light Toward Hydrogen Evolution from Water. Int. J. Hydrogen Energy 2016, 41, 17247−17256.

D: Appl. Phys. 2014, 47, 1−8.

(75) Vijselaar, W.; Kunturu, P. P.; Moehl, T.; Tilley, S. D.; Huskens, J. Tandem Cuprous Oxide/Silicon Microwire Hydrogen-Evolving Photocathode with Photovoltage Exceeding 1.3 V. ACS Energy Lett. 2019, 4, 2287−2294.

(76) Elbersen, R.; Vijselaar, W.; Tiggelaar, R. M.; Gardeniers, H.; Huskens, J. Effects of Pillar Height and Junction Depth on the Performance of Radially Doped Silicon Pillar Arrays for Solar Energy Applications. Adv. Energy Mater. 2016, 6, 1501728.

(77) Hu, S.; Lewis, N. S.; Ager, J. W.; Yang, J.; McKone, J. R.; Strandwitz, N. C. Thin-film Materials for the Protection of Semiconducting Photoelectrodes in Solar-Fuel Generators. J. Phys. Chem. C 2015, 119, 24201−24228.

(78) Malinský, P.; Slepička, P.; Hnatowicz, V.; Švorčík, V. Early stages of growth of gold layers sputter deposited on glass and silicon substrates. Nanoscale Res. Lett. 2012, 7, 241.

(79) Layani, M.; Kamyshny, A.; Magdassi, S. Transparent Conductors Composed of Nanomaterials. Nanoscale 2014, 6, 5581−5591.

(80) Lee, Y.-W.; Reddy, M. S. P.; Kim, B.-M.; Park, C. Surface Morphological, Structural, Electrical and Optical Properties of GaN-based Light-Emitting Diodes using Submicron-Scaled Ag Islands and ITO Thin films. Opt. Mater. 2018, 81, 109−114.

(81) Taob, L. W. a. M. Fabrication and Characterization of p-n Homojunctions in Cuprous Oxide by Electrochemical Deposition. Electrochem. Solid-State Lett. 2000, 10, 248−250.

(82) Siegfried, M. J.; Choi, K.-S. Directing the Architecture of Cuprous Oxide Crystals during Electrochemical Growth. Angew. Chem., Int. Ed. 2005, 44, 3218−3223.

(83) Choi, K.-S. Shape Control of Inorganic Materials via Electrodeposition. Dalton Trans 2008, 40, 5432−5438.

(84) Luo, L.-B.; Yang, X.-B.; Liang, F.-X.; Xu, H.; Zhao, Y.; Xie, X.; Zhang, W.-F.; Lee, S.-T. Surface Defects-Induced p-type Conduction of Silicon Nanowires. J. Phys. Chem. C 2011, 115, 18453−18458.

(85) Jansen, H. V.; de Boer, M. J.; Unnikrishnan, S.; Louwerse, M. C.; Elwenspoek, M. C. Black Silicon Method X: A Review on High Speed and Selective Plasma Etching of Silicon with Profile Control: An in-depth Comparison Between Bosch and Cryostat DRIE Processes as a Roadmap to Next Generation Equipment. J. Micromech. Microeng. 2009, 19, 033001.

(86) Krebs, H.-U.; Weisheit, M.; Faupel, J.; Süske, E.; Scharf, T.; Fuhse, C.; Störmer, M.; Sturm, K.; Seibt, M.; Kijewski, H.; Nelke, D.; Panchenko, E.; Buback, M., Pulsed Laser Deposition (PLD) -- A Versatile Thin film Technique. In Advances in Solid State Physics; Kramer, B., Ed.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2003; Vol. 43, pp 505−518.

(87) Luo, J.; Steier, L.; Son, M.-K.; Schreier, M.; Mayer, M. T.; Grätzel, M. Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Lett. 2016, 16, 1848−1857.

(88) Lahmar, H.; Azizi, A.; Schmerber, G.; Dinia, A. Effect of the Thickness of the ZnO Buffer Layer on the Properties of Electro-deposited p-Cu2O/n-ZnO/n-AZO Heterojunctions. RSC Adv. 2016, 6, 68663−68674.

(89) Wang, C.; Xu, J.; Shi, S.; Zhang, Y.; Gao, Y.; Liu, Z.; Zhang, X.; Li, L. Optimizing Performance of Cu2O/ZnO Nanorods Hetero-junction based Self-powered Photodetector with ZnO Seed Layer.J. Phys. Chem. Solids 2017, 103, 218−223.

(90) Kim, S.; Lee, Y.; Gu, A.; You, C.; Oh, K.; Lee, S.; Im, Y. Synthesis of Vertically Conformal ZnO/CuO Core−Shell Nanowire Arrays by Electrophoresis-Assisted Electroless Deposition. J. Phys. Chem. C 2014, 118, 7377−7385.