Tailoring Photoelectrochemical Performance and Stability of Cu(In,Ga

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Tailoring photoelectrochemical performance and stability of Cu(In,Ga)Se photocathode via TiO-coupled buffer layers 2

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Bon Hyeong Koo, Sung-Wook Nam, Richard Haight, Suncheul Kim, Seungtaeg Oh, Minhyung Cho, Jihun Oh, Jeong Yong Lee, Byung Tae Ahn, and Byungha Shin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15168 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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ACS Applied Materials & Interfaces

Tailoring Photoelectrochemical Performance and Stability of Cu(In,Ga)Se2 Photocathode via TiO2coupled Buffer Layers Bonhyeong Koo,1 Sung-Wook Nam,*,2,3 Richard Haight,4 Suncheul Kim,1 Seungtaeg Oh,5 , Minhyung Cho,5 Jihun Oh,5 Jeong Yong Lee,1,2 Byung Tae Ahn,1 Byungha Shin*,1 1

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), Daejeon 34141, Republic of Korea 2

Center for Nanomaterials and Chemical Reactions (CNCR), Institute for Basic Science (IBS),

Daejeon 34141, Republic of Korea 3

Department of Molecular Medicine, School of Medicine, Kyungpook National University,

Daegu 41404, Republic of Korea 4

IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA

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Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced

Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

ACS Paragon Plus Environment

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KEYWORDS Solar water splitting, Photoelectrochemical hydrogen evolution, Cu(In,Ga)Se2 photocathode, stability, overlayer

ABSTRACT We report on photoelectrochemical (PEC) performance and stability of Cu(In,Ga)Se2 (CIGS)-based photocathodes for photocatalytic hydrogen evolution from water. Various functional overlayers, such as CdS, TiO2, ZnxSnyOz, and a combination of the aforementioned, were applied on the CIGS to improve the performance and stability. We identified that the insertion of TiO2 overlayer on p-CIGS/n-buffer layers significantly improves the PEC performance. A multilayered photocathode consisting of CIGS/CdS/TiO2/Pt exhibited best current-potential characteristics among the tested demonstrating power-saved efficiency of 2.63 %. However, repeated linear sweep voltammetry resulted in degradation of performance. In this regard, we focused on the PEC durability issues through in-depth chemical characterization that revealed the degradation was attributed to atomic redistribution of elements constituting the photocathode, namely in-diffusion of Pt catalysts, out-diffusion of elements from the CIGS, and removal of the metal-oxide layers; the best-performing CIGS/CdS/TiO2/Pt photocathode retained its initial performance until the TiO2 overlayer was removed. It was also found that the durability of CIGS photocathodes with a TiO2-coated metal-oxide buffer layer such as ZnxSnyOz was better than those with a TiO2-coated CdS and the degradation mechanism was different, suggesting that the stability of a CIGS-based photocathode can be improved by careful design of the structure.


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1. Introduction Photoelectrochemical (PEC) water splitting is one of the most attractive ways of converting solar energy to a storable form of chemical fuels such as hydrogen and hydrocarbons.1-3 In choosing semiconductor photoelectrodes for an efficient PEC conversion of solar-to-hydrogen (STH), good absorption characteristics, efficient separation and transportation of photogenerated carriers to the solid/electrolyte interface (SEI), and long-term stability are key factors to consider.4 Concerning a photocathode—where hydrogen reduction takes place—various semiconductors such as transition metal-oxide, Si, or, III–V compounds have been used in a cell demonstrating a STH conversion efficiency over 10 %.4 Transition metal-oxide based photoelectrodes—such as Cu2O,5 NiO,6 CuBi2O4,7 CuNbO3,8 or CaFe2O49—have been studied due to their ease of fabrication and chemical robustness against photo-corrosion in an aqueous electrolyte. In spite of the overwhelming amount of investigations and efforts to search for highefficiency PEC cells, the durability of many PEC cells has yet been fully understood, thereby being a major concern for practical use or commercialization of the devices. Recently, the p-type copper chalcogenide such as CuInS2,10-13 Cu(In,Ga)Se2 (CIGS),13-19 and CuGaSe213,20,21 have emerged as a promising candidate for an efficient hydrogen evolving photocathode. Especially, a CIGS absorber, which has been proven to produce high performance thin solar cells with the record power conversion efficiency over 20 %,22 is expected to make a good PEC photocathode because it not only possesses excellent absorption characteristics and carrier transport but also the position of its conduction band minimum (CBM) is above hydrogen evolution potential [E0 = 0 V vs. RHE (reversible hydrogen electrode)].4 However, like many other semiconductor materials for PEC application, chemical stability of bare CIGS surfaces without any protection layer under some PEC conditions is a concern.23 In order to enhance


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chemical stability and charge separation, a CIGS photocathode is often configured to couple with an n-type buffer layer. Although the PEC performance of those photoelectrodes was greatly enhanced compared to the pristine CIGS, the degradation of photocurrent density over continued reaction was observed, which has been attributed to photo-corrosion of the coating layers without rigorous investigation of chemical changes leading to the degradation.19 Here, we introduced TiO2 coupled-CIGS PEC cell structures to enhance the efficiency of the water-splitting performance and, at the same time, to promote the durability by protecting the cells from aqueous electrolyte. TiO2 has been applied to various PEC photoelectrodes including Si and III-V extending their stability.24,25 We identified that the insertion of TiO2 layer consistently improved the PEC performance, yet the performance-degradation tendency was different depending on n-type buffer layers such as CdS and ZnO. By investigating the degradation behaviors, we found out that the durability issues strongly depended on the n-type buffer layers chemistry and their electrochemical reactions in the aqueous environments. We believe that this study provides useful guidelines for choosing suitable functional overlayer(s) for CIGS photocathodes for efficient and stable PEC water splitting.

2. Results and discussion Figure 1a presents a schematic of PEC hydrogen evolution through Pt catalysts on TiO2-coated CIGS/CdS buried heterojunction. Photoexcited electrons and holes are forced to separate by the internal electric field established by the junction between p-CIGS and n-CdS and electrons are drifted to SEI while holes are transported to the Mo back contact. As shown in the TEM images in Figure 1b, we identified uniformly distributed Pt nanoparticles with the presence of TiO2 layers on the CdS/CIGS layers (magnified view of the region 1). Additionally, high-resolution


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image in Figure 1c exhibits the formation of the high-quality interface between the CIGS and the CdS. The strains developed during CdS growth are detected as diffraction-spot elongations as shown in Fast-Fourier-Transformation (FFT) images in the inset of Figure 1c (see also Figure S1). The uniform CIGS/CdS interface is likely to be an ideal resource to lead an efficient p-n junction property. It is observed that several hundred nm-sized voids were formed near the bottom Mo/CIGS interface in an otherwise compact polycrystalline CIGS film. It has been suggested that the formation of the voids was due to the Kirkendall effect driven by the differences in the diffusivity of outward-diffusing indium, gallium, and/or copper, and that of sluggishly in-diffusing selenium during the low temperature selenization.26,27 However, the presence of the voids did not appear to seriously affect the operation of CIGS as a light absorbing electrode, as confirmed by good device performance (photo conversion efficiency ~14.7 %) of solar cells based on a CIGS absorber prepared by the same batch (see Figure S2 in supporting information). A MoSe2 layer with an average thickness of 40 nm was formed during the selenization, which has been known to create good ohmic contact to the CIGS and this should be beneficial for the PEC performances.28,29 One of the main focuses of our PEC study is to determine the durability of CIGS photocathode stack consisting of different buffer layers. The durability of a PEC cell is most commonly studied by chronoamperometry measurement such as that shown in Figure S3.30,31 However, the chronoamperometry measurements are often influenced by external effects such as sticking of hydrogen gas bubbles onto the electrode surface which hinder us to study the intrinsic materials changes (degradation) in the photocathode. Therefore, we adopted repeated LSV scans over a relatively wide range of bias, from -0.7 VRHE to 0.7 VRHE, as a means of testing durability.32,33


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We repeated LSV scans as many as 30 times and monitored the changes of the measurements, which has not been reported in previous studies of CIGS photocathodes. To study the roles of TiO2 layers on the PEC performance, we compared multiple scans of LSV under AM 1.5G illumination for both CIGS/CdS/Pt and CIGS/CdS/TiO2/Pt photocathodes, as shown in Figures 1d and 1e, respectively. To obtain efficient hydrogen-evolution-reactions (HER), the loading amount of the Pt catalysts was optimized by comparing the photocurrent density at 0 VRHE from the CIGS/CdS/Pt photocathodes with varying thickness of the Pt (0.5 nm, 1 nm, 1.5 nm, and 2 nm); the Pt thickness of 1.5 nm resulted in the highest current density (see Figure S4 in supporting information). Hence, all CIGS-based photocathodes in this work were loaded with 1.5 nm-thick Pt catalysts. In Figure 1d, the photocurrent density at 0 VRHE in CIGS/CdS/Pt photocathode was measured as about -8.45 mA/cm2, which is much larger than a CIGS/Pt (Figure S5 in supporting information) due to the coupling with a CdS layer, the most commonly used a n-type counterpart for the p-type CIGS light absorber in thin film solar cell application; all of PEC measurement parameters are listed in the Table 1. Further increase in the PEC performance was achieved by the addition of the TiO2 overlayer as shown in Figure 1e; the photocurrent density at 0 VRHE increased to -17.8 mA/cm2 and onset potential shifted to a more positive value of 0.54 VRHE compared to the CIGS/CdS photocathode. The enhancement of PEC performance with the TiO2 layer can be explained by the band structure alignment described in Figure 1f (The band diagram was constructed by UPS measurement, details of which are given in Supporting Information, Figures S6 and S7). In particular, the valence band maximum (VBM) of CdS and TiO2 is more positively positioned than that of CIGS in electrochemical energy scale (i.e., VBM of CdS and TiO2 is below that of CIGS in the diagram), which results in a potential barrier blocking


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diffusion of holes into the electrolyte, while still favors the flow of the electrons to the electrolyte. Our results are also supported by the work of Zhao and co-workers, which suggested that the TiO2 coating layer should enhance charge carrier transfer and suppress recombination.11 One of the important parameters describing PEC performance is OCP, which is shown in the inset of Figures 1d and 1e. The difference between OCP values under light and dark, that is a photovoltage produced in the light absorber, is proportional to the band bending of the photoelectrode. Therefore, the larger the magnitude of OCP means a higher band bending of the photoelectrode near the electrolyte and hence an easier transport of the photogenerated minority carriers (electrons in our case) to the electrolyte. The CIGS/Pt photocathode showed an OCP of approximately 0.016 V (Figure S5b), while for the CIGS/CdS/Pt photocathode it improved to ~0.11 V (Figure 1d). This enhancement must originate from the band bending near the CIGS/CdS heterojunction. An initial OCP of CIGS/CdS/TiO2/Pt photocathode was 0.16 V and decreased to 0.1 V (Figure 1e). These values are similar to that of the CIGS/CdS/Pt photocathode. The EF of the TiO2 is close to the HER level in the dark, therefore it is not likely that an additional band bending is introduced when compared to the CIGS/CdS/Pt photocathode.34 In addition, the OCPs of Figures 1d and 1e exhibit initial transient behavior in the form of a spike immediately after the light is on or off. It was proposed that this transient behavior of OCPs was from the fast recombination of charge carriers created by illumination.35 It is interesting to note that the total transient time until the OCP reached the saturation was shorter in the light phase than the dark phase. In both samples, the photocurrents gradually degraded with the increasing number of LSV scans. To understand the cause of the degradation in the PEC performance, we performed chemical analysis after the LSV was scanned for 30 times. The detailed descriptions of chemical


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analysis of the CIGS/CdS/Pt photocathodes before and after LSV cycles are presented in the supporting information (see Figures S8-S10). In short, the initial transient appears to be closely correlated with gradual out-diffusion of indium from the CIGS through the CdS into the SEI and loss of the Pt catalysts from the surface, as suggested by the chemical analysis. The out-diffusion of elements from the CIGS must have slowed down during the later scans because there was no noticeable change in the XPS intensity of both Cd 3d and In 3d after the 30 scans (supporting information, Figure S10), which is consistent with the stabilization of LSV curves (Figure 1d). Interestingly, during this diffusion process, oxidation states of elements on the surface did not change as revealed by XPS. Therefore, photo-corrosion of the top surface is not likely responsible for the observed degradation in the LSV curves. Figure 2a presents bright field TEM image of the CIGS/CdS/TiO2/Pt that experienced 30 LSV scans, focusing on the interfacial regions. The green arrow in the inset of Figure 2a indicates where EDX line scan was performed. Figures 2b and 2c compare the EDX line scans both before and after the 30 cycles of LSV scanning. Notably, both TEM image and EDX profile shows that the TiO2 layer on top of CdS completely disappeared after 30 cycles of LSV, which is also confirmed by SIMS elemental depth profiles (supporting information Figure S11). EDX elemental mapping shown in Figures 2d and 2e also confirms the disappearance of the TiO2 layer while the other elements have almost same distributions. The out-diffusion of the In had already occurred in the as-prepared CIGS/CdS/TiO2/Pt as was the case for the CIGS/CdS/Pt photocathodes, however, the diffusion was contained below the CdS/TiO2 interface; the TiO2 overlayer appears to act as an effective diffusion barrier for In. XPS data in Figures 2f and 2g shows that the peak of Ti completely disappeared while that of Cd became pronounced, implying that the CdS layer was entirely exposed to the surface after 30 cycles of LSV scanning


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procedures. We examined the intermediate stages during the course of the LSV scanning process: XPS data of after the 4th scan (the second spectra in Figures 2f and 2g) shows weak Cd peak along with Ti peaks of the nearly same intensities to those of as-prepared (pristine) stage. This suggests that the TiO2 layer remained nearly intact at least up to the 4th scan with a partial surface-exposure of the CdS layer. We interpret it as a formation of local pinholes within the TiO2 overlayer. The pinholes can initiate a catastrophic delamination of TiO2 layer at a certain point of LSV cycles. Simultaneously, the embedded Pt particles inside the CdS, as shown in TEM image after 30 cycles of LSV (Figure 2a), are indicative of continuous Pt in-diffusion occurring during the repeated LSV cycles. Comparing the CIGS/CdS/Pt and CIGS/CdS/TiO2/Pt photocathodes, we noted that applying a thin TiO2 layer promotes the PEC cell efficiency. However, the durability of the TiO2 overlayer on the CIGS/CdS appears to be limited to only about 6 scans of LSV under light illumination. Therefore we applied other metal-oxide layers such as ZnO in pursuit of the better cell-efficiency and durability at the same time regarding the CIGS photocathodes. Figure 3a represents repeated LSV of the ZnO-coated CIGS photocathodes. The photocurrent density at 0 VRHE is lower than that of the CIGS/CdS/Pt photocathode and LSV curves exhibit continuous degradation with the repeated scans suggesting poor stability of the ZnO under out measurement conditions (i.e., liquid electrolyte with a pH 6.8 and the applied bias between -0.7 VRHE and 0.7 VRHE). The ZnO overlayer completely disappeared after just one LSV scan (see Figure S12 in supporting information). The difference of the OCP values under dark and light is about 0.09 V. Shown in Figure 3b is LSV curves up to 30 scans from CIGS/ZnO/TiO2/Pt photocathode. Remarkably, PEC parameters—photocurrent at 0 VRHE, onset potential, and OCP—of the ZnO-coated photocathodes were greatly improved by simply adding a TiO2 layer


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on top. The improved OCP means charge recombination is suppressed in the CIGS/ZnO/TiO2 hence keeping the separation between electron and hole quasi-Fermi levels under illumination larger. However, similar to the CIGS/CdS/TiO2/Pt, LSVs from the TiO2-coated CIGS/ZnO photocathode experienced gradual degradation after the initially stabilized period. A noteworthy observation, however, is that the duration of the initial stability was much longer than in the case of the CIGS/ZnO/TiO2/Pt (stable up to ~14 LSV scans) compared to the CIGS/CdS/TiO2/Pt (stable up to ~6 LSV scans). It, therefore, appears that the type of the layer residing below the TiO2 has a great influence on the stability of the TiO2. We extended the examination of the stability of TiO2 layer by applying it to other transition metal-oxide buffer layers such as Zn0.75Sn0.25O1.25 and SnO2 as shown in Figures 3c to 3f. The addition of TiO2 upon Zn0.75Sn0.25O1.25 and SnO2 buffer layers consistently improved the PEC performance. The role of hole-carrier blocking by the TiO2 overlayer seems to work on those buffer layer systems. Likewise, the number of initial stabilization was enhanced for the CIGS/Zn0.75Sn0.25O1.25/TiO2/Pt (stable up to ~15 LSV scans) and the CIGS/SnO2/TiO2/Pt (stable up to ~10 scans) photocathodes compared to the CIGS/CdS/TiO2/Pt photocathode. In addition to the extended initial stability of the CIGS/ZnxSnyOz/TiO2/Pt compared to CIGS/CdS/TiO2/Pt, the origin of degradation of PEC performance is markedly different as explained below. Figures 4a to 4c present Pt 4f, Ti 2p and Zn 2p XPS spectra from CIGS/ZnO/TiO2/Pt taken before, after 4 scans, and after 30 scans. Even in the as-prepared sample, substantial amount of Zn near the surface region is evident. From the peak intensities of Ti and Zn, the ratio of TiO2 to ZnO of the surface layer (~10 nm or so, the sampling depth of XPS measurement) is nearly 50:50. This Zn-enriched surface is likely to be originated from the surfactant effect where species from the substrate “float” to the surface of the growing film.36 As the LSV scans


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proceeded, the intensity of Ti 2p continuously increased while that of Zn 2p reduced likely due to fast (photo)corrosion of the Zn residing on the top TiO2 overlayer. Even after 30 scans, there was no sign of the corrosion of the TiO2 layer. The degradation of CIGS/ZnO/TiO2/Pt photocathode, therefore, seems to be caused by the fast (photo)corrosion of Zn and the loss of Pt catalysts. This is in contrast to the degradation of the CIGS/CdS/TiO2/Pt photocathode, which was caused by the mechanical failure of the TiO2, i.e., physical delamination of the TiO2, as opposed to (photo)corrosion. Figure 4d summarizes degradation behavior of CIGS/ZnO/TiO2/Pt photocathodes with respect to scan numbers. After 30 scans, top surface of the CIGS photocathode was nearly composed of TiO2 without floated Zn and Pt catalysts. To compare quantitative PEC performance of both CIGS/CdS/TiO2/Pt and CIGS/ZnO/TiO2/Pt, we extracted the PEC cell efficiencies as the LSV cycles were repeated. Figure 5a presents the 4th quadrant—where the system does electrical work by converting solar input power—of some selected LSV curves under illumination, marking the maximum power point (MPP). The socalled power-saved efficiency for water reduction half-cell reaction is determined at the MPP as follows:

| (/ )| × !"#$ () Power − saved efficiency = * %&'&!( ()/ ) +, -./0

where JPH and Vsaved are the photocurrent density and the applied potential versus RHE, respectively, at the MPP, and Ptotal is the incident light intensity (100 mW/cm2 at 1 sun).37 The power-saved efficiency curves after the 1st and 30th LSV scan for the CdS/TiO2- and the ZnO/TiO2-overlayered CIGS photocathodes are displayed in Figure 5b. The highest power-saved efficiency from our work is 2.63 % of CIGS/CdS/TiO2/Pt photocathode. However, the issue is


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again the stability of the TiO2 layer; after the 30th scan the power-saved efficiency of the CIGS/CdS/TiO2/Pt photocathode degraded to 1.52 %. Figure 5c presents the power-saved efficiency, applied-bias photon-to-current efficiency (ABPE), and half-cell solar-to-hydrogen conversion efficiency (HC-STH) of various copper chalcogenides and Si based photoelectrodes reported in the literature.11,17,20,32,33,38-41 Our best power-saved efficiency of 2.63 % from the initial LSV scan using the CIGS/CdS/TiO2/Pt photocathode is comparable to the most of the copper chalcogenide and Si photocathodes from the literature. The main results of our study are summarized in Figures 5d and 5e, which presents PEC stability of the CdS/(TiO2)- and the ZnO/(TiO2)-coated CIGS photocathodes in terms of powersaved efficiency and photocurrent density at -0.5 VRHE. We analyze the shape of the degradation transient curves to investigate the durability mechanisms, which could not be clarified by the spectroscopy analysis (Figures 2 and 4). Obviously the shape of the degradation has a difference depending on the n-type buffer layers, such as CdS and ZnO. The initial PEC performance of the CIGS/CdS/TiO2/Pt photocathode was maintained for the first 6 scans. Between the 6th scan and the 15th scan, during which the TiO2 overlayer was physically detached away, the performance progressively deteriorated. Once the TiO2 was completely removed, no further degradation was observed (after the 15th scan). Interestingly, the saturated PEC performance of the CIGS/CdS/TiO2/Pt after the removal of the TiO2 layer was still higher than that of the CIGS/CdS/Pt photocathode almost by 0.5 % even though both samples were chemically very similar at that point. One conceivable possibility is that the CIGS/CdS/TiO2/Pt sample underwent an unintentional annealing at during the ALD of the TiO2 (150 oC) which may have resulted in further intermixing between the CIGS and the CdS. The intermixing at the interface may have facilitated charge transport increasing photocurrent density even after the complete removal of


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the TiO2 overlayer from the CdS. In the case of ZnO-incorporated samples, the addition of the TiO2 protection layer significantly enhanced the cycling durability up to as many as 14 scans. The gradual decrease over the continued LSV cycles might be attributed to the chemical photocorrosion of Zn elements. Similarly, the sharp decrease of the PEC performance of CIGS/ZnO/Pt samples is strongly related to the electrochemical removal of the surface Zn along with the Pt catalysts. Our analysis shows that the degradation mechanisms of TiO2-coupled buffer layer systems are affected by the underneath layers. In order to check the resistance of the overlayers coating the CIGS photocathodes against oxidative reaction (i.e., difficulty of hole transport from the electrode to the electrolyte), CV measurement was carried out using ferri/ferro-cyanide redox couple in the dark (Figure 5f). The CIGS/Pt photocathode exhibited a very pronounced oxidation peak over the entire range of positive potentials tested here. On the other hand, CIGS photocathode coated with CdS overlayer demonstrated a significantly reduced oxidative peak area and the oxidation peak potential was also positively shifted to 1.9 VRHE. The peak height of ZnO-coated CIGS photocathode was lower than that of the CIGS/CdS/Pt photocathode, which is due to the formation of a deeper valence band offset from the ZnO than that of the CdS. Among those investigated, the CIGS/CdS/TiO2/Pt demonstrated the greatest resistance against the oxidation of the ferri/ferrocyanide redox couple; the peak height was the smallest and the oxidation peak shifted to as large as 2.03 V, meaning hole transport from the photoelectrode to the electrolyte is greatly suppressed, consistent with the large valence band offset formed by the TiO2. An ALD-TiO2 layer is therefore well-suited for the protection of the underlying CIGS/buffer heterojunction and careful selection of n-buffer layers is required to enhance the durability of the TiO2 overlayer.


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3. Conclusion The comparative study of the TiO2-coupled buffer structures enables us to design the PEC structures using CIGS to meet both conditions of cell performance and stabilities. In our study, the TiO2 layer leads a significant improvement of PEC performance regardless of the n-type buffer layers, which could be explained by the band diagram. The hole blocking mechanism is likely to be a main reason of the performance improvements. However, to meet the condition of the stability improvements, the interfacial bonding natures between TiO2 and the underneath ntype buffer layer are critical. In this regard, the addition of the oxide semiconducting layer could be a solution by enhancing the adhesion of TiO2 layer. At the same time, the benefits of TiO2 overlayer including hole blocking behavior lead to better PEC performance.

4. Experimental Section Fabrication of Cu(In,Ga)Se2 thin films. CIGS thin films were grown on a Mo-coated sodalime substrate using a co-sputtering system. First, elemental forms of In and Ga were codeposited on the Mo layer followed by a deposition of Cu, and this was repeated for 10 times to achieve the desired film thickness, 1.5 – 1.7 µm. During the deposition, the substrate temperature was kept at 350 oC with a constant Ar working pressure of 3 mTorr. Afterward, the entire films were annealed at 400 oC under selenium atmosphere. After the selenization, sulfurization process was carried out to create band gap opening at the CIGS surface. The final composition of the bulk CIGS was determined to be Cu0.9(In0.7Ga0.3)Se2, which corresponds to a bandgap of approximately 1.1 eV. All of the CIGS absorbers used in this work were grown at the same batch to eliminate possible run-to-run fluctuations.


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Growth of buffer layers. CdS was deposited by the chemical bath deposition. The temperature of an outer water circulator of the bath was preheated to 90 °C. Cd precursor solution (0.125 g of CdSO4 dissolved in 200 mL deionized (DI) water) and S precursor solution (1.52 g of Thiourea in 100 mL DI water) were then added to an inner bath along with 40 mL of 28 wt% of ammonia solution. CIGS was kept in the bath for 6.5 minutes after the temperature of the CdS solution reached 45 °C, which resulted in the deposition of a 50 nm-thick conformal CdS film. The CIGS/CdS electrode was then rinsed with DI water and dried with N2 blow. The deposition of ZnO buffer layer was carried out using Atomic Layer Deposition (ALD). The substrate temperature was held at 150 °C, which was optimized to grow dense films and to prohibit severe intermixing between the CIGS and the buffer layer. The final thickness of these buffer layers was fixed to 50 nm. The details of the deposition condition can be found in Ref. 42. TiO2 layers were deposited by ALD with tetrakis-dimethyl-amido titanium (TDMAT) precursor and H2O as an oxygen source. The substrate temperature was 150 °C. The number of cycles was varied from 120 cycles for 20 nm-thick TiO2. Decoration of Pt electrocatalyst. After the formation of the buffer layer, the samples were immediately transferred into a DC sputtering chamber with a base pressure of mid-10-6 Torr. Pt electrocatalysts in the form of nanoparticles with the size of 2 - 5 nm were uniformly decorated on the topmost layer by sputtering under a working Ar pressure of 5 mTorr at room temperature. PEC measurements. PEC measurements were carried out using a three-electrode configuration. Ag/AgCl (3 M KCl) was used as a reference electrode and a Pt coil served as a counter electrode. A Mo back contact was wired using Ag paste and the entire electrode was sealed by an epoxy (Hysol 9460) except an active area (0.15~0.2 cm2). The exact dimension of which was determined by an imaging software, Image J.43 A phosphate buffer consisting of 0.1 M Na2SO4,


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0.25 M NaH2PO4, and 0.25 M Na2HPO4 with the addition of 1 M NaOH to adjust the pH to 6.8 was used as a working electrolyte. A quartz cell with flat window with the volume of 250 mL was used for plentiful supply of a fresh aqueous solution to the exposed active area of the CIGS working electrode. A photoresponse of CIGS photocathode was measured using a 300 W Xe arc lamp (Oriel Instruments, Model 6528) with an AM 1.5G filter (Newport 81094) and a water filter (Newport 61945)—which was calibrated using a Si photodiode (Oriel Instrument, Model 91150V) before PEC measurements—to simulate AM 1.5 G illumination (100 mW/cm2). A potentiostat (BioLogic SP-150) was used to measure open circuit potential (OCP), linear sweep voltammetry (LSV), chronoamperometry, and cyclic voltammetry (CV). The OCP values described in this study were the average value of the center point of a light phase and the end point of a dark phase. The scan rate of LSV and chronoamperometry was always 20 mV/s and the scan direction of LSV was from negative to positive potential versus RHE. The CV was carried out using the identical configuration except the working solution was replaced with a ferri/ferrocyanide redox couple (1 M KCl, 10 mM K3Fe(CN)6, and 10 mM K4Fe(CN)6). The scan rate for CV was 20 mV/s and the direction of sweep was negative to positive potential, that was, from oxidation to reduction. Chemical and structural characterization. X-ray photoelectron spectroscopy (XPS) measurements were done with a Thermo Scientific Sigma Probe with a base pressure of 1 x 10-9 Torr. Al Kα (1486.6 eV) was used as an X-ray source and a pass energy of the fixed analyzer transmission was set to 100 eV and 40 eV for a survey scan and a high resolution scan, respectively. For a high resolution scan, the spectrum was recorded at every 0.05 eV and averaged over 10 scans. The sampling area was 400 µm2. An adventitious C 1s (284.5 eV) peak


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was utilized for the calibration of XPS spectra. Deconvolution of peaks was done using a commercial XPS software (XPSPEAK4.1). A time-of-flight-secondary ion mass spectrometry (TOF-SIMS, ION-TOF GmbH) was used for depth-profiling of elements. A raster size of 300 x 300 µm2 was sputtered by Cs+ ions with 0.25 keV energy. For the detection of positive ions, Bi+ ions with 30 keV were used as a primary ion. Cs-corrected scanning transmission electron microscopy (STEM, JEOL JEM-ARM200F) was utilized to verify the atomic distribution of photocathode cross-section using energy-dispersive X-ray spectroscopy (EDX, Bruker Quantax 400). Ga in CIGS thin film was excluded from the analysis, because samples were fabricated by focused ion beam using Ga primary ion beam. Additionally, sulfur was excluded from mapping data, because sulfur came from two different sources, i.e., CdS and CIGS for band gap opening. Those sulfur atoms are likely to intermixing with each other.


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Figure 1. (a) Schematic illustration of the hydrogen evolution from Cu(In,Ga)Se2/CdS/TiO2/Pt photocathode (illustration is not to scale). (b) Cross-sectional STEM and TEM images of an asprepared Mo/MoSe2/CIGS/CdS/TiO2/Pt photoelectrode. Region 1 and 2 show an enlarged view of CIGS/CdS/TiO2/Pt and Mo/MoSe2/CIGS interfaces, respectively. (c) The magnified HR-TEM image showing CIGS/CdS interface. Insets in the image are fast Fourier transformation images of the CdS (top left) and the CIGS (bottom right) layers. Atomic d-spacing of CIGS (112) planes (d112) and CdS (111) planes (d111) were extracted from the line profile of the lattice plane. Multiple LSV scans of (d) CIGS/CdS/Pt (up to 30 scans) and (e) CIGS/CdS/TiO2/Pt (up to 30 scans) photocathodes under AM 1.5 G illumination. Several scans were selected for an ease of comparison. Dark LSV curves are presented as a dotted line in each figure. Black arrow indicates the sweep direction—from negative to positive potential versus RHE. Insets present open circuit potential measurement under chopped illumination. (f) Band diagram of


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CIGS/CdS/TiO2/electrolyte heterojunction at equilibrium where the Fermi level (EF) of the photocathode and hydrogen evolution reaction (HER) level are aligned.


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Figure 2. (a) TEM image of CIGS/CdS/TiO2/Pt photocathode after 30 scans showing the CdS layer with Pt embedded as a result of in-diffusion of Pt catalysts on the surface. Line scans of elemental compositions in (b) as-prepared and (c) after 30 scans. Profiles correspond to atomic compositions in yellow arrow of figure 1(b) and green arrow of inset in figure 2(a). EDX elemental mapping of Pt, Ti, O, Cd, Cu, In, and Se for (d) as-prepared and (e) after 30 scans. XPS core level spectra of (f) Ti 2p and (g) Cd 3d for as-prepared, after 4 scans, and after 30 scans of LSV. Arrow in Cd 3d spectra indicates N 1s. N 1s may exist as residual contaminant from ALD process using TDMAT precursor. Numerical values of Ti 2p3/2 and Cd 3d5/2 peak areas are presented for comparison.


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Figure 3. Multiple LSV scans of (a) CIGS/ZnO/Pt (20 scans), (b) CIGS/Zn0.75Sn0.25O1.25/ Pt (20 scans), (c) CIGS/SnO2/Pt (20 scans), (d) CIGS/ZnO/TiO2/Pt (30 scans), (e) CIGS/Zn0.75Sn0.25O1.25/TiO2/Pt (30 scans), and (f) CIGS/SnO2/TiO2/Pt (30 scans) photocathodes under AM 1.5G illumination. LSV curves under dark are presented as a dotted line. Insets in figures are OCP measurement.


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Figure 4. XPS core level spectra of (a) Pt 4f, (b) Ti 2p, and (c) Zn 2p from CIGS/ZnO/TiO2/Pt photocathodes for as-prepared, after 4 scans, and after 30 scans of LSV. Numerical values of Ti 2p3/2 and Zn 2p3/2 peak areas are presented for comparison. Inset is Zn 2p spectra after the 30th scan with y-axis magnified. (d) Atomic ratio changes according to scan numbers of Pt 4f, Ti 2p, and Zn 2p from CIGS/ZnO/TiO2/Pt photocathodes.


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Figure 5. (a) Maximum power point (MPP) in LSV and (b) power-saved efficiency of 1st and 30th LSV scans of CIGS/CdS/TiO2/Pt and CIGS/ZnO/TiO2/Pt photocathodes. MPP (or maximum power-saved efficiency) is marked as the blue dots. (c) Histogram of power-saved efficiency (our work), reported applied-bias photon-to-current efficiency (ABPE; pink), and half-cell solar-tohydrogen conversion efficiency (HC-STH; pale blue) of several selected photocathodes is also displayed for comparison. Evolution of power-saved efficiency and photocurrent density (JPH) at -0.5 VRHE versus LSV scan number of (d) CIGS/CdS/(TiO2)/Pt and (e) CIGS/ZnO/(TiO2)/Pt photocathodes. (f) Cyclic voltammetry of the pristine (CIGS/Pt), single buffered (CIGS/CdS/Pt and CIGS/ZnO/Pt), and TiO2 protected (CIGS/CdS/TiO2/Pt) CIGS photocathodes in ferri/ferrocyanide redox coupled electrolyte under dark.


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Table 1. Summary of photoelectrochemical properties of CIGS photocathodes.

Photocathode

OCP [V]

JPH (0 VRHE) [mA/cm2]

VONSET [V]a)

JPH (-0.5 VRHE) [mA/cm2]

JMPP [mA/cm2]

VMPP [V]

ABPE [%]

1st

30th

1st

30th

1st

30th

1st

30th

1st

30th

1st

30th

0.04

0.14

0.1

-1.1

-0.4

-7.7

-1.5

-0.5

-0.2

0.05

0.04

0.02

0.01

CIGS/Pt

0.01

0.3

0.08

-3.1

-0.5

-8.4

-2.1

-1.4

-0.2

0.1

0.04

0.2

0.01

CdS/Pt

0.1

0.5

0.5

-8.5

-8.8

-20.7

-13.9

-3.8

-5.2

-0.2

0.19

0.8

1

0.1

0.54

0.52

-17.8

-11.6

-33.5

-21.8

-10.5

-6.3

0.3

0.24

2.6

1.5

0.1

0.3

0.16

-9.1

-1.7

-16.9

-5.16

-4.4

-0.9

0.1

0.05

0.5

0.04

ZnO/TiO2/Pt

0.2

0.4

0.4

-13.5

-3.0

-26.2

-7.9

-7.1

-1.7

0.2

0.2

1.3

0.4

ZnSnO/Ptd)

0.1

0.4

0.1

-3.6

-0.5

-6.2

-4.8

-1.7

-0.2

0.1

0.03

0.2

0.01

ZnSnO/TiO2/Pt

0.2

0.5

0.3

-14.1

-1.8

-29.5

-7.8

-7.1

-0.9

0.2

0.2

1.4

0.2

SnO2/Ptd)

0.05

0.17

0.06

-3.9

-0.13

-11.1

-2.5

-1.8

-0.06

0.06

0.03

0.1

0.01

-

0.12

-0.1

-5.4 c)

0.15

-22.7

-0.5

-2.7

-

0.06

-

0.16

-

CIGSb) c)

CdS/TiO2/Pt ZnO/Pt

d)

SnO2/TiO2/Pt a)

b)

d)

V versus RHE; Up to 5 scans; Up to 10 scans; Up to 20 scans

ASSOCIATED CONTENT SUPPORTING INFORMATION Supporting information including FFT image, I-V characteristic (with external quantum efficiency) of CIGS solar cell, chronoamperometry, LSV, OCP, a detailed description of construction of band diagram of CIGS/buffer heterostructures, fs-UPS, Tauc plot, band diagram, STEM, STEM-EDX, XPS, and TOF-SIMS.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]


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ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (Ministry of Science, ICT & Future Planning) (No. 2014R1A1A1004284), by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20163030013690), and by IBS-R004-G3.

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BRIEFS The PEC performance of CIGS/n-type buffer photocathodes is enhanced by the formation of a TiO2 overlayer, while the stability is significantly influenced by the layer underlying the TiO2.

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Tailoring Photoelectrochemical Performance and Stability of Cu(In,Ga

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