1 Low-Cost, Efficient and Durable H2 Production by

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Energy, Environmental, and Catalysis Applications 2

Low-Cost, Efficient and Durable H Production by Photoelectrochemical Water Splitting with CuGaSe Photocathodes 3

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Christopher P. Muzzillo, W. Ellis Klein, Zhen Li, Alexander Daniel DeAngelis, Kimberly Horsley, Kai Zhu, and Nicolas Gaillard ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01447 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Low-Cost, Efficient and Durable H2 Production by Photoelectrochemical Water Splitting with CuGa3Se5 Photocathodes

Christopher P. Muzzillo,1,* W. Ellis Klein,1 Zhen Li,1 Alexander Daniel DeAngelis,2 Kimberly Horsley,2 Kai Zhu1,* and Nicolas Gaillard.2 1

National Renewable Energy Laboratory, 15013 Denver W Pkwy, Golden, CO 80401

2

Hawaii Natural Energy Institute, University of Hawaii, 1680 East-West Rd POST 109,

Honolulu, HI 96822 *Corresponding

authors:

(C.P.M.)

[email protected];

(K.Z.)

[email protected]

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Photoelectrochemical (PEC) water-splitting is an elegant method of converting sunlight and water into H2 fuel. To be commercially advantageous, PEC devices must become cheaper, more efficient and much more durable. This work examines low-cost polycrystalline chalcopyrite films, which are successful as photovoltaic absorbers, for application as PEC absorbers. In particular, Cu-Ga-Se films with wide band gaps can be employed as top cell photocathodes in tandem devices as a realistic route to high efficiencies. In this report, we demonstrate that decreasing Cu/Ga composition from 0.66 to 0.31 in Cu-Ga-Se films increased the band gap from 1.67 to 1.86 eV and decreased saturated photocurrent density from 18 to 8 mA/cm2 as measured by chopped-light current-voltage (CLIV) measurements in 0.5 M sulfuric acid electrolyte. Buffer and catalyst surface treatments were not applied to the Cu-Ga-Se films, and they exhibited promising stability, evidenced by unchanged CLIV after 9 months of storage in air. Finally, films with Cu/Ga = 0.36 (approximately stoichiometric CuGa3Se5) and 1.86 eV band gaps had exceptional durability, and continuously split water for 17 days (~12 mA/cm2 at -1 V vs. RHE). This is equivalent to ~17200 C/cm2, which is a world record for any polycrystalline PEC absorber. These results indicate that CuGa3Se5 films are prime candidates for cheaply achieving efficient and durable PEC water splitting.

Keywords: photoelectrochemical; water splitting; chalcopyrite; polycrystalline; hydrogen


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1. Introduction Photoelectrochemical (PEC) water splitting is an attractive means for converting sunlight and water into hydrogen gas, where H2 has the highest specific energy of any non-nuclear fuel.1 In this way, PEC water splitting spontaneously converts the abundant and free solar energy resource into chemical energy, or fuel, which can be stored, transported and ultimately used when the sun is not shining. Although research on this technology has gained momentum over the past four decades, it has not been commercialized because cheap PEC devices that operate efficiently and durably do not yet exist.2 On the other hand, photovoltaic (PV) cells are fully mature, and utilize sunlight-generated and electric field-separated electron-hole pairs in a manner very similar to PEC cells. It is therefore logical to adapt cheaply fabricated thin film solar cell materials from PV to PEC applications.3 Unassisted PEC water splitting that has high enough efficiency for cost-competitive hydrogen production is more easily achieved by partitioning the material requirements between two absorbers in a tandem configuration.2 Thin film PV absorbers are commonly p-type, so they can serve as PEC photocathodes that evolve H2 when biased against a counter electrode, which evolves O2. To achieve unassisted PEC water splitting, this bias can be supplied by a PV cell “driver” located under the photocathode, or by stacking the photocathode with a PEC photoanode. Chalcopyrite-based thin films have demonstrated excellent PV performance on laboratory (22.6% power conversion efficiency)4 and module scales (19.2% aperture area efficiency on 841 cm2),5 and have band gaps spanning a large portion of the solar spectrum (1.0 eV for CuInSe2 to 2.4 eV for CuGaS2). This has motivated the study of chalcopyrites for PEC water splitting, and impressive results have already been attained:



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unassisted solar-to-hydrogen efficiencies of 3.7% with a CuGaSe2-based coplanar device6 and 10.5% with a CuIn0.7Ga0.3Se2-based coplanar device,7 and separately 20 d of continuous water splitting8 (see Chen et al.9 for a recent review). While films with Cu/(Ga+In) compositions > 0.8 have been most successful in PV applications, substantially lower Cu compositions (Cu/Ga ≤ 0.5) have been investigated for PEC photocathodes as a means to obtain a more favorable valence band energy (reduced with respect to vacuum), relative to stoichiometric CuGaSe2,10 as well as a (wider) band gap more suited to a tandem top cell. The first CuGa3Se5/Pt photocathodes achieved 5.0 mA/cm2 at 0 V vs. reversible hydrogen electrode (RHE; 3-electrode measurement).10 This was improved to 8.0 mA/cm2 with a CuGa3Se5/ZnS/Pt device,11 and then to 9.3 mA/cm2 with a CuGaSe2 + CuGa3Se5 two-phase mixture and a CdS/Pt surface.12 Finally, the best PEC performance/durability combination13 for any polycrystalline device was achieved with a CuGa3Se5 interlayer: 20 d of water splitting with ~7.5 mA/cm2 at 0 V vs. RHE using (Ag,Cu)GaSe2/CuGa3Se5/CdS/Pt.8 Despite having the best polycrystalline performance/durability ever measured, the above-mentioned reports comprise the literature on CuGa3Se5 water splitting research. The goal of this work was to adapt chalcopyrites from their success as narrowgap, single-junction PV absorbers to wide-gap, tandem-top cell absorbers in the lessexplored field of PEC water splitting. The CuGa3Se5 compound was targeted due to its reduced Cu content (relative to Cu(In,Ga)Se2 (CIGS) PV absorbers), which lowers the valence band to more favorably align with the water oxidation potential,14 and is additionally expected to enhance durability by reducing the dissolution rate of Cu.15


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2. Experimental Aluminosilicate glass with high Na and K content from Schott AG was used as a substrate. Mo back contacts (0.5 - 1.0 µm) were direct current sputtered and transferred to the cluster tool co-evaporation system without air exposure. Single stage (flat flux profiles) co-evaporation of Cu-Ga-Se was performed with excess Se on a rotating 6 x 6 in2 substrate held near 620 °C. Cu-Ga-Se films were 2 - 3 µm thick (by Dektak 8 profilometer), and had Cu/Ga compositions of 0.31 to 0.66 (by X-ray fluorescence). Evaporation rates were roughly 0.9, 2.6, and 25 Å/s for Cu, Ga, and Se, respectively (Se/cation molar flux ratio of ~9). Past experiments have shown that growth temperatures up to 620 °C did not require changing the composition set-point, serving as indirect evidence that negligible Ga loss occurred. On the other hand, increasing growth temperature to 700 °C resulted in significantly higher Cu/Ga compositions in the final films, which is attributed to Ga loss in the form of volatile Ga2Se.16 Films with Cu/Ga of 0.31 had 10.9(1)% Cu, 33.5(2)% Ga, and 55.6(5)% Se, while films with Cu/Ga of 0.66 had 18.9(2)% Cu, 28.5(2)% Ga, and 52.6(5)% Se. This corresponds to growth along the Cu2Se-Ga2Se3 tie-line,17 so the ternary composition is fully specified by the Cu/Ga parameter, which is used throughout the rest of this work. Phase constitution was examined by X-ray diffraction (XRD) with automatic alignment, a Cu anode and Ni filter (1.5418 Å Kα radiation). Grazing incidence XRD (GIXRD) was additionally performed at an incidence angle of 0.5°. Standard powder diffraction file XRD patterns for CuGaSe2 (35-1100) and Mo (42-1120) were used, while a pattern was calculated for CuGa3Se5 based on its published structure.18 Microstructure was revealed by scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS) was collected at a 15 kV



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accelerating voltage. PV devices were fabricated with methods optimized for narrow band gap Cu(In,Ga)Se2 absorbers: chemical bath deposition (CBD; 75 °C bath) of 50 - 80 nm CdS buffer layer, radio frequency sputtering of 100 nm intrinsic ZnO, 120 nm ZnO:Al (2 wt. % Al2O3 target) transparent conductive oxide, electron-beam evaporated top contact masked finger grids (50 nm Ni and 3 µm Al), and photolithographic 0.42 cm2 device isolation. Current density-voltage curves were measured in four-probe configuration at 1 sun on a 25 °C temperature-controlled stage (1 kW Xe arc lamp simulating air mass 1.5 after standard Si cell calibration). No surface treatment was applied to the Cu-Ga-Se films for PEC water splitting. The PEC device area was defined by epoxy (Loctite Hysol 9462) application around the perimeter and measured by scanned digital image processing. Three-electrode PEC measurements were performed in 0.5 M sulfuric acid electrolyte with 1 mM Triton X-100 surfactant, Pt counter electrode, Hg/Hg2SO4 or a saturated Ag/AgCl/NaCl reference electrode, and 1 sun illumination from a 250 W tungsten lamp calibrated to a 1.8 eV GaInP2 reference solar cell with water filter (chopped light current-voltage, or CLIV). H2 evolution was not directly measured in this work, although a previous study measured Faradaic efficiencies of 94% for devices with CuGa3Se5 layers.8 The shelf life of a PEC device was examined by measuring CLIV, storing it in air in the dark, re-measuring CLIV 4 months later, and re-measuring 5 months after that (9 months total). Incident photon-to-current efficiency (IPCE) was performed to establish absorbers’ electronic band gaps, providing complementary information to optical band gap measurements obtained from ultraviolet-visible spectroscopy. This analysis was performed in a pH 2 buffer solution with 1 mM hexaammineruthenium(III) chloride (redox mediator), Pt counter electrode, Ag/AgCl


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reference electrode, and -1.0 V bias (-0.8 V vs. RHE). The IPCE electrolyte was chosen to facilitate interfacial charge transfer and improve electronic band gap calculation reliability. On the other hand, the CLIV electrolyte was chosen to favor the hydrogen evolution reaction.19 It is worth noting that IPCE analysis performed with a supporting electrolyte cannot be used as a proxy to estimate photon-to-hydrogen conversion efficiency since the chemical reaction involving a redox mediator is not water splitting. As such, IPCE data were not used to calculate the maximum photocurrent density achieved with our absorbers. Band gaps were estimated by plotting (h⋅ν⋅ln[1-IPCE])2 at the long wavelength IPCE drop, fitting a line to the linear data, and extrapolating to IPCE of 0.20 To confirm those band gaps, ultraviolet-visible spectroscopy was performed on a CuGa3Se5 film grown on bare soda-lime glass at 620 °C. Transmittance, reflectance (using a diffuse reflectance integrating sphere), and film thickness (2.2 µm from profilometry) were used to calculate absorptivity and construct a Tauc plot to extrapolate the band gap.21 Durability measurements consisted of continuous galvanostatic testing (Princeton Applied Research VersaSTAT MC potentiostat) in a 2-electrode configuration with a Pt counter electrode, at 1 sun illumination and 8 mA/cm2. At regular intervals, a 3electrode configuration CLIV measurement was performed.

3. Results and Discussion The Cu/Ga composition of co-evaporated films was varied from 0.31 to 0.66, as measured by XRF. XRD in Figure 1 showed that films with Cu/Ga of 0.66, 0.52, and 0.36 had single-phase CuGaSe2, two-phase CuGaSe2 + CuGa3Se5, and single-phase CuGa3Se5, respectively, similar to previous reports.22-23 No Cu2Se or Ga2Se3 impurities



were detected, although Raman spectroscopy will be needed in future work to rule out their presence. SEM in Figure S1 revealed similar microstructures for films with Cu/Ga of 0.66 and 0.36, although the former sample exhibited larger grains at the surface. These basic material characterization results were all in line with previous studies.22-23 Mo

Intensity (a. u.)

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Cu/Ga = 0.66 0.52 0.36

CuGa3Se5 30

CuGaSe2 40

50

2θ (°) Figure 1. XRD data for Cu-Ga-Se films with Cu/Ga compositions of 0.66 (top black), 0.52 (middle red), and 0.36 (bottom blue). Standard diffraction pattern peak positions for CuGaSe2 (green triangles) and CuGa3Se5 (purple circles) are included for reference.

PV devices fabricated with these absorbers had poor power conversion efficiencies (≤ 3.7%; Table S1), relative to absorbers typically used for PV (Cu/Ga > 0.7).10 This is in agreement with previous studies24-25 that found poor PV performance for extremely Cu-poor Cu-Ga-Se films. In particular, widening the band gap of CuGaSe2 by moving to more Cu-poor compositions actually decreases open-circuit voltage and fill factor, making films with Cu/Ga < 0.7 unattractive for PV applications. The device architecture was not re-optimized around these absorbers, so issues like non-optimal film thicknesses and charge carrier recombination at the absorber/buffer interface could 8 ACS Paragon Plus Environment

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detract from the PV performance observed in this study. In particular, the solar cells suffered losses not present in the PEC cells due to grid shadowing, parasitic ZnO and CdS absorption, and poor CdS conduction band alignment (0.15 – 0.28 eV cliff26). The performance of the bare CuGa3Se5 PEC photocathodes was particularly good compared to previous reports,10, 12 despite the absence of both buffer layers (e.g. CdS) and catalyst layers (e.g. Pt). The source of this superior performance is the subject of ongoing investigation. Figure 2 shows the best CLIV curves at different Cu/Ga compositions. In chalcopyrite compounds such as CuGaSe2, the repulsion of Cu d and Se p orbitals is known to shift the valence band to higher energies.27 Therefore, reducing the Cu/Ga composition from 1 to 0.33 (CuGaSe2 to CuGa3Se5) reduces the valence band energy by 0.2 eV.28 Thus, the band gap is expected to vary from 1.65 eV to 1.86 eV for Cu/Ga of 1 to 0.33,28-29 which explains the trend of decreased saturated photocurrent density (JSAT) with decreased Cu/Ga composition. Throughout this work the JSAT is taken as the light current density minus the dark current density at -1 V vs. RHE. 0

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4

0.31 0.36 0.38 0.52 Cu/Ga

-8 -12 -16

0.66 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

Potential vs. RHE (V) Figure 2. CLIV data for Cu-Ga-Se photocathodes with Cu/Ga compositions of 0.66 (black), 0.52 (red), 0.38 (green), 0.36 (blue), and 0.31 (purple).



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The IPCE long wavelength cutoffs (Figure S2) followed a similar trend with the JSAT in CLIV measurements. The long wavelength drop in IPCE data was used to extrapolate effective band gaps on a logarithmic scale, where this effective band gap contains optical and electronic information. The transmittance of a CuGa3Se5 film on bare soda-lime glass was also measured (Figure S3), and the band gap calculated from the Tauc plot was in good agreement with effective values from IPCE. The effective band gap from IPCE inversely correlated with PV short-circuit current density (Table S1), showing that the PEC and PV devices operate similarly. For each absorber, the JSAT and the IPCE effective band gap were plotted against Cu/Ga in Figure 3, illustrating the tradeoff between band gap and JSAT. There was some variability in JSAT for repeated growths with compositions near stoichiometric CuGa3Se5. The compositional homogeneity range of CuGa3Se5 is not clear from the published Cu-Ga-Se phase diagram,17 and phase metastability or minor phase impurities may occur. The possibility of uncontrolled phase constitution and its effect on photocurrent should be studied in future work. Similar to the Shockley-Queisser limit for photovoltaics, a detailed balance limit for the effect of photocathode band gap on best-possible JSAT can be calculated.19 On changing the band gap from 1.67 to 1.85 eV, the maximum theoretical JSAT changes from 23.4 to 18.3 mA/cm2, while the best JSAT in Figure 3 changed from 18.4 to 10.4 mA/cm2. Thus, performance at lower Cu/Ga compositions (higher band gaps) achieves a lower fraction of the possible JSAT, an indication of enhanced recombination—an effect that will ultimately have to be weighed against the benefits at reduced Cu/Ga compositions.


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1.85 16 1.80 12 1.75 8 1.70

JSAT (|mA/cm2|)

20

Band gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 1.65

0.3

0.4

0.5

0.6

0.7

Cu/Ga composition

Figure 3. Effective band gap extracted from IPCE data (left axis) and saturated photocurrent density (JSAT at -1 V vs. RHE; right axis) for Cu-Ga-Se photocathodes as a function of Cu/Ga composition. The dashed line represents stoichiometric CuGa3Se5.

The general trend in onset potential (taken here to be the potential at which the current density exceeds |1| mA/cm2) from all the CLIV data is evident in Figure 2: the smallest band gap film (highest Cu/Ga) had the most cathodic onset, and on moving to larger band gaps (lower Cu/Ga) the onset shifted to more anodic values. At still larger band gaps the slope of the photocurrent onset decreases (Figure 2), similar to the decrease in PV fill factor for Cu/Ga ≤ 0.39 (Table S1). The trend of more anodic onset potentials at decreased Cu/Ga from 0.66 to 0.31 was in rough agreement with a previous study on Cu-Ga-Se/Pt.10 The only reports on bare CuGa3Se5 photocathodes without buffer or catalyst surface treatments also showed relatively cathodic onset potentials, in agreement with the present study.10, 12 In both of these previous studies, the onset potential was shifted from about 0 V to 0.2 - 0.5 V vs. RHE by applying Pt catalyst to the CuGa3Se5 surface. To make our CuGa3Se5 films a viable technology for H2 production (i.e. 10% solar-to-hydrogen efficiency or better9, 30), two routes should be pursued: (1) the onset 11 ACS Paragon Plus Environment


potential should be made more anodic with surface treatment (e.g. buffer layer with catalysts), and (2) bias should be applied with a PEC photoanode or PV driver in a tandem configuration. For the latter, the Mo back contact must be replaced with a transparent conductor. The shelf life of a PEC device with Cu/Ga of 0.66 was examined by measuring CLIV initially, after 4 months and after 9 months of storage in air (Figure 4). The PEC performance was effectively unchanged by aging, or even slightly improved. This result was unexpected, as the PV properties of bare CIGS absorbers deteriorate rapidly in air, and even when stored under N2.31 This result indicates that the PEC properties of Cu-poor Cu-Ga-Se films could be recovered during operation, even though the absorber has been stored in humid air for a long period of time. It is speculated that the acidic electrolyte dissolves oxidation products, similar to the way potassium cyanide etches unwanted phases from CIGS surfaces before device fabrication.23 0 2


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-4 -8 -12

9 months

-16

4 months Initial -1.0 -0.8 -0.6 -0.4 -0.2 0.0

Potential vs. RHE (V) Figure 4. Initial (black), 4 months aged (red), and 9 months aged (green) CLIV data for a Cu-Ga-Se photocathode with Cu/Ga of 0.66.


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While the shelf life result in Figure 4 is promising, it is no substitute for continuous water splitting in reactive electrolyte. Continuous galvanostatic testing was therefore carried out at 1 sun and 8 mA/cm2 for photocathodes with Cu/Ga compositions of 0.66, 0.52 and 0.36. CLIV data for the most durable device (Cu/Ga of 0.36) are presented in Figure 5. The onset potential and JSAT both improved after 1 d, followed by negligible degradation for 10 d. The JSAT (light minus dark current) and dark current density at -1.0 V vs. RHE were plotted against time for the three photocathodes in Figure 6. For all three samples, degradation in photocurrent roughly coincided with increased dark currenta possible indication of shunting, which could be due to pinhole formation or a phase transformation producing conductive material. Relative to the nearstoichiometric CuGa3Se5, the films with higher Cu/Ga of 0.66 and 0.52 both showed faster photocurrent degradation. These films both contained CuGaSe2 (Figure 1), which may degrade faster than CuGa3Se5, as a previous study linked Cu+/Cu2+ dissolution/readsorption to the degradation of Cu(In,Ga)Se2 PEC films.15 Films with lower Cu concentrations are therefore speculated to produce Cu+ at a lower rate, and degrade slower as a result. More study will be needed to confirm this connection. On the other hand, the film with Cu/Ga of 0.36 only exhibited CuGa3Se5 by XRD, and evolved H2 continuously for 17 days at 11.7 mA/cm2, equivalent to ~17200 C/cm2the most durable PEC result for any polycrystalline absorber.13 For comparison, the previous world records13

for

polycrystalline

PEC

durability

used

more

complex

(Ag,Cu)GaSe2/CuGa3Se5/CdS/Pt and CIGS/CdS/Ti/Mo/Pt architectures and achieved 20 days at ~7.5 mA/cm2 (13000 C/cm2)8 and 10 days at ~19.5 mA/cm2 (16900 C/cm2),3 respectively.


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2

0

-4 Initial 1d 3d 4d 8d 10 d

-8

-12

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

Potential vs. RHE (V) Figure 5. CLIV data taken for a Cu-Ga-Se film with Cu/Ga of 0.36 at various times throughout continuous galvanostatic testing: initial (black), after 1 d (red), 3 d (orange), 4 d (green), 8 d (blue), and 10 d (purple).

-1.0 V vs. RHE Photocurrent Dark 2 2 (|mA/cm |) (|mA/cm |)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



(a)

16 12 8 4

Cu/Ga 0.66 0.52 0.36

0 8

(b)

4 0 0

2

4

6

8

10

12

14

16

18

Time (d)

Figure 6. (a) Photocurrent (light minus dark current density) and (b) dark current density from CLIV data at -1 V vs. RHE as a function of continuous galvanostatic testing time for Cu-Ga-Se films with Cu/Ga compositions of 0.66 (black up triangles), 0.52 (red squares), and 0.36 (blue down triangles).


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In order to compare the present results with other studies, the water splitting durability was also examined by plotting light current at 0 V vs. RHE against time (Figure 7). The film with Cu/Ga of 0.36 in Figure 7 had a light current density that averaged to 4.4 mA/cm2 over 17 d, equivalent to ~6500 C/cm2. The present water splitting performance/durability result is important, especially under the harsher -1.0 V vs. RHE condition, as the present work did not employ a catalyst or buffer layer, and used a wider band gap absorber material (1.86 eV), which is better suited to tandem device implementation for commercially viable solar-to-hydrogen efficiencies. After durability testing, the degradation mechanism was investigated by characterizing the best photocathodes with Cu/Ga compositions of 0.52 and 0.36. The overall film compositions by XRF were unchanged (Table S2), and GIXRD revealed no crystallographic changes (Figures S4 and S5). Etch pits were previously found to degrade photocathodes with greater Cu contents.15 Etch pits were not observed on the present Cupoor films, but after degradation the surfaces appeared to be etched (finer, more rounded grains), and nodules appeared on the surface (Figures 8 and 9) that had relatively Cu-rich compositions by EDS (Figure S6 and Table S3). The Cu-rich nodules should be more conductive32 and could act as shunt paths if they penetrate the films. The observed morphology changes (and lack of structural and compositional changes) are in line with the previously proposed degradation mechanism of Cu+ dissolution and Cu2+ readsorption.15 That model also explains why decreasing Cu/Ga improves durability, so future work should seek a barrier to Cu transport that can slow the degradation.



0 V vs. RHE Light current (|mA/cm2|)

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Cu/Ga 0.66 0.52 0.36

8

4

0 0

2

4

6

8

10 12 14 16 18

Time (d) Figure 7. Light current density from CLIV data at 0 V vs. RHE as a function of continuous galvanostatic testing time for Cu-Ga-Se films with compositions of 0.66 (black up triangles), 0.52 (red squares), and 0.36 (blue down triangles).

Figure 8. Plan view SEM micrographs of photocathodes with Cu/Ga of 0.52 before, (a) and (b), and after PEC degradation, (c) and (d).


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Figure 9. Plan view SEM micrographs of photocathodes with Cu/Ga of 0.36 before, (a) and (b), and after PEC degradation, (c) and (d).

4. Conclusions The prospect of adapting chalcopyrite-based thin film PV absorbers to PEC water splitting devices was explored. In particular, the suitability of Cu-Ga-Se absorber films for H2 evolution was examined at Cu/Ga compositions of 0.31 to 0.66. These compositions had poor PV performance, but exhibited PEC performance that was superior to previous reports, despite the absence of buffer or catalyst surface layers. Decreasing Cu/Ga composition increased the effective band gap (determined from IPCE cutoff), resulting in a decrease in saturated photocurrent density, as expected. The photocathodes showed excellent shelf-life, where PEC performance was unchanged after 9 months of storage in air. More importantly, a bare CuGa3Se5 film with a band gap of 1.86 eV exhibited remarkable operando durability, achieving 17 d of continuous water splitting (~12 mA/cm2 at -1 V vs. RHE). This is equivalent to ~17200 C/cm2, which is a world record for any polycrystalline PEC absorber. The results also suggest that Cu 17 ACS Paragon Plus Environment


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content plays a strong role in degradation, since the greatest durability was found for the film with the lowest Cu/Ga composition. Increasing dark current coincided with degradation, so the formation of shunts or pinholes are possible degradation mechanisms. The degraded photocathodes had altered morphology in line with the previously proposed mechanism of Cu+ dissolution and Cu2+ re-adsorption. The present work demonstrates that CuGa3Se5 is a promising wide band gap candidate for top cell photocathodes in tandem water splitting devices. Further work to improve surface energetics and reduce Cu dissolution will be critical to achieving low-cost, efficient and durable PEC H2 production.

Supporting Information Plan view and cross-sectional SEM micrographs of glass/Mo/Cu-Ga-Se films with Cu/Ga compositions of 0.66 and 0.36 (Figure S1); Champion PV performance efficiency, opencircuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) for Cu-Ga-Se absorbers with varied Cu/Ga compositions (Table S1); IPCE data (at -0.8 V vs. RHE) for Cu-Ga-Se photocathodes with Cu/Ga compositions of 0.66, 0.52, 0.38, 0.36, and 0.31 (Figure S2); Transmittance of a CuGa3Se5 film with Cu/Ga of 0.37 and Tauc plot data and linear extrapolation (Figure S3); Overall film compositions before and after PEC degradation by XRF (Table S2); XRD and GIXRD with a 0.5° incidence angle for the original and PEC degraded film with Cu/Ga of 0.52 (Figure S4) and 0.36 (Figure S5); EDS spectra of the photocathode with Cu/Ga of 0.52 after PEC degradation for film and nodule locations (indicated on inset secondary electron micrograph; Figure S6); EDS compositions of photocathodes with Cu/Ga of 0.52 and 0.36 after PEC degradation for


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locations on the films and nodules that appeared on the films’ surfaces after degradation (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment This work was supported by the U.S. Department of Energy under Contract No. DEAC36-08GO28308 with Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory, and administered by the University of Hawaii under Contract No. DE-EE0006670. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy. The authors would like to thank Miguel Contreras, Jeff Carapella, Carolyn Beall, Stephen Glynn, Karen Bowers, James L. Young, Vincenzo LaSalvia, and Lorelle Mansfield for assistance with experiments.

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Table of Contents Image -1.0 V vs. RHE Photocurrent Dark 2 2 (|mA/cm |) (|mA/cm |)

0

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

16

-4

12

0.31 0.36 0.38 0.52 Cu/Ga

-8 -12 -16

0.66 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

Potential vs. RHE (V)

8 4

Cu/Ga 0.66 0.52 0.36

0 8

(b)

4 0 0

2

4

6

8

10

12

14

16

18

Time (d)



Figure 1. XRD data for Cu-Ga-Se films with Cu/Ga compositions of 0.66 (top black), 0.52 (middle red), and 0.36 (bottom blue). Standard diffraction pattern peak positions for CuGaSe2 (green triangles) and CuGa3Se5 (purple circles) are included for reference. 82x72mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 3. Effective band gap extracted from IPCE data (left axis) and saturated photocurrent density (JSAT at -1 V vs. RHE; right axis) for Cu-Ga-Se photocathodes as a function of Cu/Ga composition. The dashed line represents stoichiometric CuGa3Se5. 83x62mm (300 x 300 DPI)



Figure 4. Initial (black), 4 months aged (red), and 9 months aged (green) CLIV data for a Cu-Ga-Se photocathode with Cu/Ga of 0.66. 83x64mm (300 x 300 DPI)


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Figure 5. CLIV data taken for a Cu-Ga-Se film with Cu/Ga of 0.36 at various times throughout continuous galvanostatic testing: initial (black), after 1 d (red), 3 d (orange), 4 d (green), 8 d (blue), and 10 d (purple). 83x64mm (300 x 300 DPI)



Figure 6. (a) Photocurrent (light minus dark current density) and (b) dark current density from CLIV data at -1 V vs. RHE as a function of continuous galvanostatic testing time for Cu-Ga-Se films with Cu/Ga compositions of 0.66 (black up triangles), 0.52 (red squares), and 0.36 (blue down triangles). 83x66mm (300 x 300 DPI)


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Figure 7. Light current density from CLIV data at 0 V vs. RHE as a function of continuous galvanostatic testing time for Cu-Ga-Se films with compositions of 0.66 (black up triangles), 0.52 (red squares), and 0.36 (blue down triangles). 83x64mm (300 x 300 DPI)




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