Suppressed Formation of Conductive Phases in One-Pot

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Suppressed Formation of Conductive Phases in One-Pot Electrodeposited CuInSe2 by Tuning Se Concentration in Aqueous Electrolyte Byung-Seok Lee,†,‡,⊥ Sung-Yul L. Park,†,‡,⊥ Jang Mi Lee,†,‡ Jeung-Hyun Jeong,† Jin Young Kim,† Choong-Heui Chung,§ and Doh-Kwon Lee*,†,‡,∥ †

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea Nanomaterials Science and Engineering, Korea University of Science and Technology, Daejeon 305-350, Korea § Department of Material Sciences and Engineering, Hanbat National University, Daejeon 305-719, Korea ∥ Green School, Korea University, Seoul 136-713, Korea ‡

S Supporting Information *

ABSTRACT: The single-bath electrochemical deposition of CuInSe2 often leads to short-circuit behavior of the resulting solar cells due to the high shunt conductance. In this study, in an attempt to resolve this problem, the influence of the Se precursor concentration (CSe) on electrodeposited CuInSe2 films and solar cell devices is examined in the CSe range of 4.8 to 12.0 mM in selenite-based aqueous solutions containing Cu and In chlorides along with sulfamic acid (H3NSO3) and potassium hydrogen phthalate (C8H5KO4) additives. As CSe increases, the CuInSe2 layers become porous, and the grain growth of the CuInSe2 phase is restricted, while the parasitic shunting problem was markedly alleviated, as unambiguously demonstrated by measurements of the local current distribution. Due to these ambivalent influences, an optimal value of CSe that achieves the best quality of the films for high-efficiency solar cells is identified. Thus, the device prepared with 5.2 mM Se exhibits a power-conversion efficiency exceeding 10% with greatly improved device parameters, such as the shunt conductance and the reverse saturation current. The rationale of the present approach along with the physicochemical origin of its conspicuous impact on the resulting devices is discussed in conjunction with the electro-crystallization mechanism of the CuInSe2 compound. KEYWORDS: CuInSe2, thin-film solar cells, electrodeposition, single-bath, conductive phases coevaporation and sputtering.3−6 Recently, as part of the effort to reduce the fabrication cost, nonvacuum processes for CISebased (including CISe and CIGSe) films have garnered much interest as an alternative to vacuum techniques due to their cost-effectiveness and compatibility with scaled-up production techniques.7,8 Among nonvacuum processes, an electrochemical deposition (electrodeposition) technique is able to provide precursor films with the high density, potentially leading to annealed CISe-

1. INTRODUCTION Since the first demonstration of chalcopyrite CuInSe2 (CISe) as a solar cell absorber and its bandgap tailoring toward the maximum utilization of solar irradiation by Ga-doping,1,2 CuIn1−xGaxSe2 (CIGSe) has attracted much attention as a promising semiconductor for high-efficiency thin-film solar cells due to its superior optoelectronic properties, including its high absorption coefficients and tunable bandgap energies as well as its long-term stability. Thus far, the CIGSe solar cell has attained a power-conversion efficiency (PCE) of 22.3% on a laboratory scale,3 overtaking the PCEs of polycrystalline Si solar cells. CIGSe solar cells with such high PCEs have been fabricated with high vacuum-based techniques such as © 2016 American Chemical Society

Received: June 13, 2016 Accepted: September 1, 2016 Published: September 1, 2016 24585

DOI: 10.1021/acsami.6b07065 ACS Appl. Mater. Interfaces 2016, 8, 24585−24593

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ACS Applied Materials & Interfaces based films with a crystal quality comparable to those provided by vacuum processes.9 On this account, many research groups have utilized the electrodeposition method to fabricate CISebased thin films.8−14 There are two categories of electrodeposition processes. The first is an one-step electrodeposition process which uses a single bath containing all of the constituent elements.10 The second is a multistep electrodeposition process using multiple baths to deposit stacks of metallic or binary chalcogenide layers.8,11−13 The highest PCE reported for a CIGSe solar cell prepared by stack electrodeposition is 17.3% (0.5 cm2 area),13 which is slightly higher than those achieved by other nonvacuum techniques (e.g., solution or nanoparticle ink deposition methods). However, although one-step electrodeposition is promising in light of the simplicity of its process, the highest PCE of CIGSe solar cells prepared by one-step electrodeposition without an additional vacuum step is still 12.4%,10 mainly because controlling the film compositions with one-step electrodeposition is not as straightforward compared to the use of stack electrodeposition or other nonvacuum methods. This difficulty arises from the fact that each element in the electrolytic solution has a different standard reduction potential (vs standard hydrogen electrode, SHE) in a broad range, i.e., 0.74, 0.34, −0.34, and −0.55 V for Se, Cu, In, and Ga, respectively.15 The much higher reduction potentials of Cu and Se as compared to those of In and Ga lead to the formation of highly conductive Cu−Se binary phases during the deposition process, which can have a detrimental effect on photovoltaic (PV) devices by forming shunt paths in the absorber layer. Therefore, to adjust the final film compositions, an NREL research group employed a physical vapor deposition step after one-pot electrodeposition, achieving a PEC of 15.4%.14 However, the additional vacuum step may undermine the advantages of nonvacuum processes. Meanwhile, several research groups have studied the electrodeposition mechanisms of prototype-chalcopyrite CuInSe2,16−21 suggesting that the incipient Cu deposition is followed by the deposition of Se, leading to the formation of Cu−Se binary phases, with the incorporation of In beginning with the formation of the CuInSe2 phase when the Se-to-Cu concentration ratio increases. This implies that the timely supply of Se is likely critical to expedite the incorporation of In before the formation of a large amount of the Cu−Se phases. On the basis of this assumption, in this work, the one-step electrodeposition of CISe was carried out with various Se precursor concentrations (CSe) in aqueous electrolyte in order to resolve the problem of forming conductive phases. Ren et al. previously investigated the effect of the Se-to-Cu concentration ratio on the morphology and composition of the single-bath electrodeposited CISe films.20 However, its influence on the solar cell efficiency was not fully elucidated due to the relatively low overall device performance. Herein, we systematically investigated the solar cell performance of the electrodeposited CISe films in conjunction with their microstructure, chemical composition, crystallinity, and local conductivity as a function of CSe in the electrolyte.

KCl (99%, Sigma-Aldrich) as the supporting electrolyte in deionized (DI) water. To investigate the effect of the Se precursor content on the resulting CISe films, the concentration of SeO2 (CSe) was varied from 4.8 to 12.0 mM. Additionally, 12 mM sulfamic acid (H3NSO3, 98%, Sigma-Aldrich) and 12 mM potassium hydrogen phthalate (C8H5KO4, 99.95%, Sigma-Aldrich) were dissolved in DI water as pH buffering and/or complexing agents.22,23 The pH of the electrolyte was fixed at 2.2. Electrodeposition was carried out in a three-electrode configuration, where Mo sputtered on soda-lime glass substrates (3 × 4 cm2 in size) and a platinum plate (3 × 3 cm2) were used as the working and counter electrodes, respectively. As a reference electrode, Ag/AgCl in 3 M KCl(l) (0.210 V vs SHE at 25 °C, CH Instruments, CHI111) was employed. The CISe films were electrodeposited by applying a constant potential (Va = −0.54 V) with respect to the Ag/ AgCl electrode for 5400 s using a potentiostat (AMETEK Princeton Applied Research, PARSTAT MC), with the cathodic current being monitored as shown in Figure S1 of the Supporting Information (SI). The temperature of the electrolyte was maintained at approximately 27 °C during the electrodeposition process using a jacket beaker with a constant-temperature water circulator. 2.2. Fabrication of CuInSe2 Solar Cells. The electrodeposited CISe films were annealed (sintered) at 580 °C for 30 min (with heating and cooling rates of 10 and ca. 3 °C min−1) in a Se-containing atmosphere to enhance their crystallinity while inducing grain growth. During the annealing process, Se vapor was supplied from Se pellets (99.99%, Sigma-Aldrich) placed ca. 20 cm away from the CISe film in a single-chamber horizontal tube furnace, where Ar was used as a carrier gas at a flow rate of 100 sccm. When the CISe film was heated to 580 °C, the measured temperature of the Se was found to be 350 °C due to the temperature gradient in the furnace. Under this condition, the equilibrium Se partial pressure was estimated to be 1.6 × 10−3 atm.24 The annealed CISe films were chemically etched in a 0.1 M KCN solution for 60 s to minimize conductive CuxSe phases. Solar cells were fabricated in a conventional structure of Mo/CISe/CdS/ ZnO/ZnO:Al/Ni/Al according to the standard procedure.25,26 First, the CdS buffer layer (ca. 60 nm in thickness) was deposited on the annealed CISe films by means of chemical bath deposition for 15 min at 60 °C in a solution containing 2 mM CdSO4, 1.02 M NH4OH, and 84 mM thiourea. The intrinsic ZnO and Al-doped ZnO layers (with typical thicknesses of 50 and 500 nm, respectively) were sequentially deposited on the CdS layer via radio frequency magnetron sputtering. Finally, a Ni/Al (50 nm/500 nm) grid was deposited as a front electrode by thermal evaporation. The active cell area measuring from 0.40 to 0.46 cm2 was defined by mechanical scribing. 2.3. Characterization of Thin Films and Devices. The surface and cross-sectional morphologies of the electrodeposited and annealed films were characterized by field-emission scanning electron microscopy (FE-SEM) at an acceleration voltage of 10 kV, and their compositions were analyzed by energy-dispersive X-ray spectrometry (EDS) at an acceleration voltage of 20 kV from the CuK, InL, and SeL peaks of the overall spectra (FEI, Inspect F50). The crystal structure of the electrodeposited and annealed films was characterized by X-ray diffraction (XRD, Rigaku, D/max 2500) with Cu Kα radiation (λ = 0.15418 nm). The Raman spectra of the electrodeposited and annealed films were measured by a Renishaw inVia spectrometer equipped with a 4 mW Nd:YAG laser beam with a wavelength of 532 nm. The atomic concentration profiles of the as-annealed films were examined by Auger electron spectroscopy (AES, Scanning Auger Nanoprobe PHI-700 & LCTOFMS. Conductive atomic force microscopy (C-AFM) was carried out using a Park Systems XE-100 instrument in I-AFM mode. The current density−voltage (j−V) curves of the CISe solar cells were measured by a class-AAA solar simulator (Yamashita Denso, YSS-50S) equipped with a 1000 W xenon lamp. The light intensity was calibrated to AM 1.5G 1-sun (100 mW cm−2) using an NREL-calibrated silicon solar cell. The external quantum efficiencies (EQEs) were measured with an incident photon-to-current conversion efficiency measurement system (PV Measurements, Inc.) after calibration with NIST-calibrated silicon and germanium photodiodes in the ranges of 300 to 1100 nm and 1100 to 1400 nm, respectively. The capacitance−voltage (C−V) measurement was

2. EXPERIMENTAL SECTION 2.1. Electrochemical Deposition of CuInSe2 Precursor Layers. For the coelectrodeposition of CISe thin films, the electrolytic solution was prepared by dissolving 2.4 mM CuCl2·H2O (99%, SigmaAldrich), 9.6 mM InCl3 (99.999%, Sigma-Aldrich), and various concentrations of SeO2 (99.9%, Sigma-Aldrich) along with 0.24 M 24586


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ACS Applied Materials & Interfaces carried out to extract the carrier density of the CISe absorbers using an impedance analyzer (Solartron 1260, U.K.) at a frequency of 50 kHz in a voltage range from −1.5 to 0.5 V. The above device characteristics (j−V, EQE, C−V) were measured at ambient conditions.

corresponding to the (110) and (211) planes of the Mo substrate (JCPDS no. 42−1120). In the Raman spectrum, as shown in Figure 1d, only an intense peak at 172 cm−1 and a weak peak at 211 cm−1 were identified, which can be assigned to the main vibration mode A1 and the B2/E mode of chalcopyrite CISe, respectively,27 thus corroborating the absence of impurity phases such as cubic sphalerite CuInSe2 (|4̅2m), CuAu-ordered CuInSe2, and In-rich ordered defect compounds (ODCs).28,29 As described above, the CISe films prepared by electrodeposition employing a 4.8 mM CSe solution followed by annealing at 580 °C exhibited quite promising morphological, compositional, and structural properties for solar cell applications. Unfortunately, however, the solar cell devices fabricated with the present CISe films in the structure as schematically depicted in Figure 1e were revealed not to have a power-generating function. As shown in Figure 1f, the CISe solar cells prepared with 4.8 mM Se typically exhibited ohmic j−V curves, indicating that the present absorber films are highly conductive. In an effort to address this issue, a series of experiments were designed where the concentration of selenite ions (CSe) in the electrolyte were established as a variable. 3.2. CuInSe2 Films Prepared with Various CSe Values. The microstructures, compositions, and structural properties of the electrodeposited layers are shown in Figure 2 as a function of the Se concentration, CSe, in the electrolyte (ranging from 4.8 to 6.0 mM). The SEM images in Figure 2a−d show that the as-deposited films comprise a cauliflower-like structure with agglomerates of a few hundreds of nanometers in size, consisting of much smaller crystallites. A closer look at the surface morphology revealed that the porosity of the electrodeposited CISe layer gradually increases as CSe increases, without a significant change in the agglomerate size. Concomitantly, as shown in the cross-section (Figure 2e−h) and Figure 2i, the thickness of the as-deposited layer increases (from 1.46 to 1.86 μm) with an increase in CSe. These morphological changes in the film porosity and thickness depending on the Se concentration are more pronounced in the CSe range higher than 7.2 mM (see Figure S2). However, the films prepared with CSe values exceeding 7.2 mM were too porous and readily exfoliated from the substrate after the annealing process (Figure S3). Therefore, further investigations including device characterizations focused on the samples deposited with CSe values ranging from 4.8 to 6.0 mM. The chemical compositions of the as-deposited CISe layers are presented in Figure 2j. While there is no discernible variation in the [Cu]/[In] ratio in the CSe range below 6.0 mM, an obviously decreasing trend of the [Cu]/[In] ratio is observed in the higher CSe range (see Figure S2). However, the [Se]/([Cu] + [In]) ratio increased monotonically in the entire CSe range examined, as shown in Figure 2j (from 4.8 to 6.0 mM) and Figure S2 (up to 12 mM). It was also noted that oxygen began to be incorporated into the as-deposited layers when the CSe value exceeded 7.2 mM (Figure S2), which is presumably related to the highly porous nature of the layers prepared under these conditions. Figure 2k shows the XRD patterns of the as-deposited layers prepared using the solutions with CSe from 4.8 to 6.0 mM. All of the samples exhibited the characteristic reflections for the chalcopyrite CISe phase as denoted by the red circles beside the peaks originating from the Mo substrate at 40.5, 58.5, and 73.6°. The intensity of the Mo (110) peak at 40.5° was found to diminish gradually with an increase in CSe, which can be explained by the corresponding increase in the CISe film

3. RESULTS 3.1. CuInSe2 Films and Devices Prepared with a 4.8 mM Se Precursor. Figure 1 shows the characteristics of the

Figure 1. Characterization of sintered CuInSe2 thin films electrochemically prepared using a solution containing 4.8 mM Se: (a) Surface morphology, (b) cross-section, (c) XRD pattern, (d) Raman spectrum, (e) schematic of solar cell devices, and (f) a typical j−V curve.

CISe thin films prepared by electrodeposition in a 4.8 mM selenite-based electrolyte, followed by annealing at 580 °C for 30 min in the presence of Se vapor. As shown in the SEM images (Figure 1a,b), the sintered CISe thin films have highly dense surface and cross-sectional morphologies, leaving only a few pores, as essentially required for the fabrication of highperformance thin-film solar cells. The average film thickness was measured to be 1.26 ± 0.05 μm with an average grain size of ca. 0.8 μm. The chemical composition of the sintered CISe film was such that [Cu]/[In] = 0.89 ± 0.07 and [Se]/([Cu] + [In]) = 1.17 ± 0.10, as measured by EDS, where [k] denotes the atomic concentration of the k species. The measured overall composition, the [Cu]/[In] ratio in particular, is well within the copper-poor regime, which is generally aimed during the fabrication of CISe-related compounds by electrodeposition to alleviate the segregation of conductive Cu−Se binary phases. Figure 1c shows the XRD pattern of the sintered CISe film prepared with the 4.8 mM Se source. As indexed in Figure 1c, all of the detected diffraction peaks belonged to the α-CuInSe2 phase in the tetragonal chalcopyrite structure (|4̅2d, JCPDS card no. 87-2265), except for the small peaks at 40.5° and 73.6° 24587


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Figure 2. Characteristics of the electrodeposited CuInSe2 layers prepared with Se concentrations (CSe) of 4.8, 5.2, 5.6, and 6.0 mM: SEM images for (a−d) surface morphologies and (e−h) cross sections, (i) film thickness and grain size, (j) chemical composition, and (k) XRD pattern. Note that the dashed curves in parts (i) and (j) are for visual guidance.

Figure 3. SEM images for (a−d) surface morphologies and (e−h) cross sections, (i) film thickness and grain size, (j) chemical composition, and (k) XRD pattern of the as-annealed CuInSe2 films prepared with CSe of 4.8, 5.2, 5.6, and 6.0 mM.

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were strongly textured along the [112] crystallographic direction. The (112) plane is the most closely packed atomic plane in the chalcopyrite structure. It has also been argued that CISe-related chalcopyrite compounds with Cu-poor compositions tend to grow along the [112] direction, as the (112) polar surfaces are stabilized possibly due to the presence of Cu 33,34 vacancies (VCu ′ ) and In antisite defects on Cu sites (In•• Cu). On this basis, we speculate that the β-In2Se3 secondary phase distributed in the as-deposited layers played a role during the recrystallization process in developing the strongly oriented CISe structure along the (112) direction. Finally, we point out that the intensity of the CISe (112) peak decreased with an increase in CSe from 5.2 mM, which is indicative of worse crystallinity and/or a smaller grain size as compared to the 4.8 and 5.2 mM samples, consistent with the microstructures observed in Figure 3a−h. In passing, it should be noted that the surface Raman analyses of the as-deposited and annealed films detected no other secondary phases. This is discussed in detail below. 3.3. CuInSe2 Solar Cell Performance. Figure 4a−b shows the representative j−V curves and EQE spectra of the solar cells

thickness and the consequent attenuation of the X-rays used to detect the underlying Mo substrate (Figure 2e−h). One may recognize that an additional peak can be identified at ca. 25°, as denoted by the black diamond on the left shoulder of the CISe (112) peak at 26.7°, which can be assigned to the (110) plane of the β-In2Se3 phase.30,31 The XRD results can thus be summarized such that the present single-bath electrodeposition method produces the crystalline CISe compound in the chalcopyrite structure with a binary impurity phase (In2Se3) included. The CISe (112) peaks were further analyzed using the Scherrer equation to estimate the primary crystallite size, yielding a value of ca. 8 nm regardless of the Se concentration, as depicted in Figure 2i. The error caused by the β-In2Se3 shoulder peak was negligible because its intensity is less than half of the maximum intensity of the CISe (112) peaks (see Figure S4). The annealing (sintering) process induced recrystallization and grain growth of the electrodeposited layers as shown in Figure 3. Figure 3a−h shows that the cauliflower-like agglomerates composed of ca. 8 nm crystallites were transformed into the polycrystalline structure with much larger grains by annealing. It is also notable that the trends of the morphological variations found in the as-deposited layers prepared with different values of CSe, such as the film thickness and porosity, remained to a certain degree in the annealed films. Namely, the annealed films become thicker and more porous as CSe increases. As can be compared in Figures 2i and 3i, the sintered films were generally thinner than the asdeposited layers, indicating that densification and/or a possible In2Se(g) loss32 occurred during the sintering process. The increase in the film thickness (1.26 to 1.44 μm) with increasing CSe was less pronounced as compared to the as-deposited layers. It is apparent in the SEM images and in Figure 3i that the grain size of the annealed films becomes smaller, from 0.8 to 0.2 μm, as the CSe value increases, leaving more pores and cracks in the films prepared with higher CSe. These inferior microstructural features of the 5.6 and 6.0 mM samples are possibly due in part to the low density of the green bodies (the as-deposited layers). Figure 3j shows the chemical composition of the as-annealed films. As in the as-deposited layers, no clear trend of the variation was recognized in the [Cu]/[In] ratio with an increase in the CSe value in the range below 6.0 mM, while the [Cu]/ [In] ratio decreased considerably in the higher CSe regime (Figure S3). In contrast, the clearly increasing trend of the [Se]/([Cu] + [In]) ratio of the as-deposited layers (Figure 2j) disappeared in the annealed films, as denoted by the blue symbols in Figure 3j. Specifically, the Se content was apparently reduced approximately by the amount of excess-stoichiometry via the evaporation of Se from the films during the annealing process. Hence, considering that the as-deposited layers prepared with higher CSe values contained higher amounts of Se (Figure 2j), it can be inferred that more Se evaporated from the films prepared with higher CSe values, possibly contributing in part to the more porous microstructures of the 5.6 and 6.0 mM samples. Upon annealing, the XRD patterns also changed significantly, as shown in Figure 3k. It is apparent that the β-In2Se3 shoulder peak completely disappeared after the annealing process. The CISe (112) peaks were greatly intensified and narrowed by annealing, signifying better crystallinity and/or a larger grain size of the annealed films as compared to the as-deposited layers. It may be worth noting that the annealed CISe films

Figure 4. j−V characteristics under simulated AM 1.5G illumination at 100 mW cm−2 as well as in the dark (a, c) and EQE spectra (b, d) of the representative (a, b) and the best (c, d) CuInSe2 solar cells prepared by electrodeposition with various selenium concentrations (CSe). The inset in part (b) represents [hν·ln(1 − EQE)]2 vs hν curves near the band edge regime.

with CISe films prepared with CSe values ranging from 4.8 to 6.0 mM, of which the power-conversion efficiencies (PCEs) are comparable with the average PCE values for each CSe condition. The detailed PV parameters are listed in Table 1. The j−V and EQE curves of the best-performing cell, which were attained from the 5.2 mM CSe condition, are also presented in Figure 4c−d. As described above, the solar cell prepared with the 4.8 mM CSe exhibited the short-circuit behavior despite the largest grain size and the densest microstructure of the CISe films in the CSe range examined in this work. In contrast, only a slight shift of CSe to 5.2 mM resulted in a dramatic enhancement in the PV performance, with the PCE reaching 10.01% (Figure 4c), although the corresponding CISe layer was composed of smaller grains relative to those in the 4.8 mM sample (Figure 3). A further increase in CSe from 5.2 to 6.0 mM turned out to 24589


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Table 1. Typical Photovoltaic and Diode Parameters, Evaluated from j−V Curves (Figure 4a), EQE Spectra (Figure 4b), and Diode Analysis (Figure S6), for the CuInSe2 Solar Cells Prepared by Electrodeposition with Various Selenium Concentrations (CSe)

a

CSe (mM)

VOC (V)

jSC (mA cm−2)

FF

η (%)

Eg (eV)

Gsh (mS cm−2)

Rs (Ω cm2)

n

j0 (mA cm−2)

4.8 5.2 5.6 6.0

0 0.375 0.317 0.295

0 37.8 (36.4)a 36.5 (34.9) 37.2 (34.3)

0 0.587 0.518 0.494

0 8.32 6.00 5.41

1.00 ± 0.06 1.00 ± 0.05 1.00 ± 0.04

0.8 ± 0.1 3.4 ± 0.2 5.0 ± 0.2

0.81 ± 0.01 0.79 ± 0.02 0.87 ± 0.03

1.70 ± 0.03 1.84 ± 0.01 1.82 ± 0.03

(4.0 ± 0.1) × 10−3 (3.2 ± 0.1) × 10−2 (5.2 ± 0.2) × 10−2

The numeral in parentheses represents the integrated current density estimated from the EQE curve by using the AM 1.5G spectra.

found to have ca. 4 and 8 times larger Gsh and j0 values, respectively, than those of the 5.2 mM devices. The Gsh and j0 values increase further by ca. 50 to 60% as CSe increases to 6.0 mM. This result indicates that the CISe film prepared with 5.2 mM C Se has the smallest shunt current and lowest recombination rate, which is in accordance with the morphological features described in association with Figure 3. Specifically, the porous microstructure of the 5.6 and 6.0 mM films likely provides a larger number of parasitic shunt paths and/or recombination sites. Their lower crystallinity may induce additional recombination, possibly due to the larger number of lattice defects. In summary, the degraded VOC and FF of the 5.6 and 6.0 mM devices can be ascribed to the higher shunt and, more decisively, to the higher recombination currents than those of the 5.2 mM device, which, in turn, are likely caused by the porous microstructure and poor crystallinity of the films. The carrier density profiles of CISe devices along the distance from the junction were calculated from the C−V measurements, as shown in Figure S7. The measured carrier densities were 3.4 × 1015, 4.5 × 1015, and 4.2 × 1015 cm−3 for 5.2, 5.6, and 6.0 mM devices, respectively. The difference in the carrier density among three samples, particularly between 5.2 and 5.6 mM samples is not as significant as that found in the Gsh values, thus implying that the substantial difference in Gsh is not caused by the carrier density, but rather originates from other property of the films, likely their distinguishable porosity (Figure 3b−c). On the other hand, the origin of the dramatic difference between the PV performances of the 4.8 and 5.2 mM films does not appear to be immediately obvious. One may suspect any secondary phases, which are highly conductive, to induce the short-circuit behavior of the 4.8 mM devices. However, as shown in Figure 6, no significant difference was identified in the surface Raman spectra of the films prepared with 4.8 and 5.2 mM CSe. Both samples exhibited only the A1 mode (at 172 cm−1) and B2/E modes (at 205, 223, or 210 cm−1), which correspond to the chalcopyrite CISe phase,35 in the asdeposited state (Figure 6a) as well as after being annealed (Figure 6b). The most frequently observed Cu-rich secondary phase in the literature,36 i.e., CuxSe was not detected at approximately 260 cm−1 by the present Raman analysis. Figure 6c shows the Raman spectra measured on the Mo/CISe interface after being lifted-off from the Mo substrate. The Raman scatterings at 125, 172, 200, and 225 cm−1 can be assigned to α-CISe,27,35 while the peak at 240 cm−1 for 4.8 mM sample is attributed to MoSe2 or elemental Mo phases.37,38 As is the case with the film surface, no peaks corresponding to CuxSe or ODCs were detected near the Mo/CISe interface. Nevertheless, its existence in a very small amount may not be completely ruled out. Figure 7 shows the surface morphology (a, b) and local current (c, d) maps obtained over an identical area (10 × 10

have an adverse effect on the solar cell properties, particularly on the open-circuit voltage (VOC) and fill factor (FF). The short-circuit current densities (jSC) of the 5.6 and 6.0 mM devices are only slightly smaller than that of the 5.2 mM device, as also indicated by a comparison of the EQE spectra (Figure 4b). The bandgap energies of the CISe films, as assessed from the EQE curves, are all 1.00 eV irrespective of the CSe values ranging from 5.2 to 6.0 mM. Figure 5 represents the statistics of

Figure 5. Statistics of photovoltaic parameters, (a) VOC, (b) jSC, (c) FF, and (d) η, of all the CuInSe2 solar cells as a function of selenium concentration, CSe (see Figure S5 and Table S1 for all of the individual j−V curves and the numerical values for the mean parameters along with their standard deviations).

the PV parameters as a function of CSe, which, to enable a statistical comparison, were calculated from 14 to 18 devices for each condition (see Figure S5 and Table S1 for details). It is clearly shown in Figure 5 that the decreases in PCEs of the 5.6 and 6.0 mM devices in comparison with the 5.2 mM device are attributed to the deterioration of VOC and FF.

4. DISCUSSION In order to gain more insight into the differences in the PV performances of the devices prepared with CSe values ranging from 5.2 to 6.0 mM, their dark j−V curves were analyzed (Figure S6), resulting in the diode parameters, i.e., the shunt conductance (Gsh), series resistance (Rs), ideality factor (n), and saturation current density (j0), as listed in Table 1. Among the extracted diode parameters, the most conspicuous variation can be found in Gsh and j0, while the variation in the Rs and n values is not as clear. The cell prepared with 5.6 mM CSe was 24590


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resistance values of the 5.2 mM devices were more than 3 orders of magnitude higher than those of the 4.8 mM devices, indicating that the 4.8 mM samples contain a lot more amount of conductive phase(s) after the identical annealing, KCN etching, and device fabrication processes to 5.2 mM samples. Figure S10 shows the relative atomic concentration across the thickness of the sintered films prepared with 4.8 and 5.2 mM CSe, as measured by an AES depth profile analysis. There was no noticeable difference in the atomic distribution of the major constituent elements near the surface and in the bulk regions of both films, apart from the fact that In exists at a slightly higher level in the 5.2 mM sample. Despite the large uncertainty associated with the EDS-determined [Cu]/[In] ratios of the annealed films (Figure 3j), seemingly due to the high sensitivity of the film composition to the humidity level, the average [Cu]/[In] value of the 5.2 mM samples is also slightly lower than that of the 4.8 mM samples. The chalcopyrite CISe, while nominally represented as CuInSe2, is known to accommodate a large number of native point defects. In particular, due to the exceptionally low defect ′ , a large extent of cation nonformation energy of VCu molecularity, i.e., [Cu]/[In] is allowed within the single phase regime (e.g., around 0.9 in this work).39,40 The V′Cu acceptors, however, tend to form a defect complex with In•• Cu donors due to the negative formation energy, rendering 39 In this way, electrically neutral defects, (2V′Cu − In•• Cu). despite the high Cu-deficiency, the effective carrier concentration and its polarity are presumably determined by the uncompensated defect species of VCu ′ and In•• Cu when the cationto-anion nonstoichiometry, i.e., [Se]/([Cu] + [In]) is fixed, consequently endowing the CISe semiconductors with either pand n-type characteristics.40 The formation of native defect species and their complex formation in a complicated manner, while not fully understood under various electrochemical and chemical potential conditions, can eventually lead to a wide range of carrier concentrations in Cu-deficient CISefrom the optimal values for high-efficiency devices (ca. 1016 cm−3) to the values possibly causing the shunting behavior (1020 cm−3).41 In a single-bath, potentiostatic electrodeposition process of CISe, the formation of the ternary compound is mediated by the instantaneous nucleation of Cu−Se phases and the subsequent assimilation of In via its underpotential reduction, which is enabled by the free energy gain associated with the compound formation.18−20,42 Specifically, elemental deposition proceeds in the order of Cu, Se, and In, as evidenced by the atomic concentration evolution in the deposits during the early stage of the deposition process (Figure S11). It can be seen in Figure S11 that the reduction of In to complete the formation of the ternary compound appears to be facilitated when the [Se]/ [Cu] ratio exceeds approximately unity, in agreement with a previous observation.18 From this, it is inferred that prompt Se deposition may be critical for the subsequent incorporation of In, thus alleviating the formation of conductive Cu−Se phases in the deposits. The detailed change in the electrodeposition mechanism due to the increase in the Se precursor concentration is yet to be cleared. However, from the fact that there is practically no difference in the Raman signals for CuxSe between 4.8 and 5.2 mM samples (Figure 6) and that the entire surface of 4.8 mM sample were found to be much more conductive than 5.2 mM sample, one may speculate that the atomic scale distribution of Cu and In, not the distribution of submicron-sized precipitates of CuxSe phases, were possibly altered in such a way that the formation of highly conductive

Figure 6. Raman spectra of (a) as-deposited and (b) annealed CuInSe2 films prepared with Se concentration (CSe) of 4.8 and 5.2 mM, and (c) the corresponding Mo/CuInSe2 interfaces lifted off from the completed devices.

Figure 7. Conductive AFM images of annealed CuInSe2 films prepared with CSe of 4.8 mM (a, c) and 5.2 mM (b, d) showing morphology (a, b) and current (c, d) maps.

μm2) using C-AFM for the annealed CISe films prepared with 4.8 and 5.2 mM CSe. Figure 7c−d clearly indicates that the CISe film with 4.8 mM CSe is highly conductive, in contrast to the 5.2 mM film. Thus, the short-circuit behavior of the 4.8 mM device can be unambiguously attributed to this highly conductive nature of the corresponding CISe film. Moreover, the facts that these conductive domains were found over nearly the entire surface, not limited to the grain boundaries, and that the secondary phases were not detected by surface Raman spectroscopy, imply that the observed high conductivity is possibly an intrinsic property of the CISe film prepared with 4.8 mM CSe. It is also noted that the macroscopic resistance measured by a 4-probe technique was consistent with the local observation under C-AFM, as shown in Figure S9. The 24591


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ACS Applied Materials & Interfaces phases was suppressed in the resulting annealed CISe films prepared with 5.2 mM Se precursor.

program, and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) (20153030013060).

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5. SUMMARY AND CONCLUSIONS We have shown that the judicious control of the aqueous electrolyte composition can modify significantly the one-pot electrodeposited CISe films. Although the CISe films prepared using a solution with CSe of 4.8 mM were found to have the largest grains, the resulting solar cells exhibited the short-circuit behavior. This shunting problem, however, was dramatically resolved by employing a higher selenium concentration. However, using a CSe value higher than 5.2 mM also degraded the microstructural properties of CISe films, resulting in a smaller grain size, higher porosity, and lower crystallinity. Due to these ambivalent effects, the highest solar cell efficiency was achieved at an optimal CSe value; the devices prepared with 4.8, 5.2, 5.6, and 6.0 mM CSe had average PCEs of 0.1, 8.2, 5.7, and 5.1%, respectively, exhibiting the highest PCE of 10.01% at 5.2 mM CSe. The local current distribution measurement employing C-AFM has revealed that the shunting behavior of the CISe solar cells prepared with 4.8 mM Se is attributed to the highly conductive nature of the corresponding CISe films, which does not appear to be caused by the segregation of impurity phases. On the basis of these C-AFM and Raman analyses together with the compositional evolution of the electrodeposited layers, it is suggested that the facilitated Se deposition using a solution with higher CSe possibly modifies the atomic scale distribution of Cu and In and/or the point defect structure of the CISe films. Overall, the effects of a slight change in the electrolyte solution on the electrodeposited CISe films and their physicochemical origins all deserve further investigation.

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(1) Wagner, S.; Shay, J. L.; Migliorato, P.; Kasper, H. M. CuInSe2/ CdS Heterojunction Photovoltaic Detectors. Appl. Phys. Lett. 1974, 25, 434−435. (2) Wei, S.−H.; Zhang, S. B.; Zunger, A. Effects of Ga Addition to CuInSe2 on Its Electronic, Structural, and Defect Properties. Appl. Phys. Lett. 1998, 72, 3199−3201. (3) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 47). Prog. Photovoltaics 2016, 24, 3−11. (4) Chirila, A.; Reinhard, P.; Pianezzi, F.; Bloesch, P.; Uhl, A. R.; Fella, C.; Kranz, L.; Keller, D.; Gretener, C.; Hagendorfer, H.; Jaeger, D.; Erni, R.; Nishiwaki, S.; Buecheler, S.; Tiwari, A. N. PotassiumInduced Surface Modification of Cu(In,Ga)Se2 Thin Films for HighEfficiency Solar Cells. Nat. Mater. 2013, 12, 1107−1111. (5) Kemell, M.; Ritala, M.; Leskela, M. Thin Film Deposition Methods for CuInSe2 Solar Cells. Crit. Rev. Solid State Mater. Sci. 2005, 30, 1−31. (6) Jackson, P.; Hariskos, D.; Wuerz, R.; Kiowski, O.; Bauer, A.; Friedlmeier, T. M.; Powalla, M. Properties of Cu(In,Ga)Se2 Solar Cells with New Record Efficiencies up to 21.7%. Phys. Status Solidi RRL 2015, 9, 28−31. (7) Hibberd, C. J.; Chassaing, E.; Liu, W.; Mitzi, D. B.; Lincot, D.; Tiwari, A. N. Non-Vacuum Methods for Formation of Cu(In,Ga) (Se,S)2 Thin Film Photovoltaic Absorbers. Prog. Photovoltaics 2010, 18, 434−452. (8) Romanyuk, Y. E.; Hagendorfer, H.; Stücheli, P.; Fuchs, P.; Uhl, A. R.; Sutter-Fella, C. M.; Werner, M.; Haass, S.; Stückelberger, J.; Broussillou, C.; Grand, P.−P.; Bermudez, V.; Tiwari, A. N. All Solution-Processed Chalcogenide Solar Cells − from Single Functional Layers Towards a 13.8% Efficient CIGS Devices. Adv. Funct. Mater. 2015, 25, 12−27. (9) Lincot, D.; Guillemoles, J. F.; Taunier, S.; Guimard, D.; SicxKurdi, J.; Chaumont, A.; Roussel, O.; Ramdani, O.; Hubert, C.; Fauvarque, J. P.; Bodereau, N.; Parissi, L.; Panheleux, P.; Fanouiller, P.; Naghavi, N.; Grand, P. P.; Benfarah, M.; Mogensen, P.; Kerrec, O. Chalcopyrite Thin Film Solar Cells by Electrodeposition. Sol. Energy 2004, 77, 725−737. (10) Duchatelet, A.; Sidali, T.; Loones, N.; Savidand, G.; Chassaing, E.; Lincot, D. 12.4% Efficient Cu(In,Ga)Se2 Solar Cell Prepared from One Step Electrodeposited Cu−In−Ga Oxide Precursor Layer. Sol. Energy Mater. Sol. Cells 2013, 119, 241−245. (11) Fischer, J.; Larsen, J. K.; Guillot, J.; Aida, Y.; Eisenbarth, T.; Regesch, D.; Depredurand, V.; Fevre, N.; Siebentritt, S.; Dale, P. J. Composition Dependent Characterization of Copper Indium Diselenide Thin Film Solar Cells Synthesized from Electrodeposited Binary Selenide Precursor Stacks. Sol. Energy Mater. Sol. Cells 2014, 126, 88−95. (12) Bhattacharya, R. N. CIGS-Based Solar Cells Prepared from Electrodeposited Stacked Cu/In/Ga Layers. Sol. Energy Mater. Sol. Cells 2013, 113, 96−99. (13) Oliva, F.; Kretzschmar, S.; Colombara, D.; Tombolato, S.; Ruiz, C. M.; Redinger, A.; Saucedo, E.; Broussillou, C.; de Monsabert, T. G.; Unold, T.; Dale, P. J.; Izquierdo-Roca, V.; Pérez-Rodríguez, A. Optical Methodology for Process Monitoring of Chalcopyrite Photovoltaic Technologies: Application to Low Cost Cu(In,Ga)(S,Se)2 Electrodeposition Based Processes. Sol. Energy Mater. Sol. Cells 2016, DOI: 10.1016/j.solmat.2015.12.036. (14) Bhattacharya, R. N.; Batchelor, W.; Hiltner, J. F.; Sites, J. R. Thin-Film CuIn1−xGaxSe2 Photovoltaic Cells from Solution-Based Precursor Layers. Appl. Phys. Lett. 1999, 75, 1431−1433. (15) Lide, D. R. CRC Handbook of Chemistry and Physics, 87th ed.; CRC Press, Taylor and Francis Group: Boca Raton, FL, 2006; pp 823−8-33.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07065. Characterization of as-deposited and annealed films prepared with Se precursor concentrations of 7.2, 9.6, and 12.0 mM, fragmentary enlarged view of XRD patterns for the as-deposited layers, j−V curves of all the devices used to evaluate the average PV parameters and their numerical values, diode analysis on CISe solar cells, AES depth profiles for the as-annealed films, and atomic concentration evolution during electrodeposition (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +82-2-958-6710. Fax: +82-2-958-6649. E-mail: dklee@ kist.re.kr (D.-K.L.). Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the internal program of Korea Institute of Science and Technology (Project No. 2E26510), by the KIST-UNIST partnership program, by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) for University-Institute cooperation 24592


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ACS Applied Materials & Interfaces (16) Thouin, L.; Massaccesi, S.; Sanchez, S.; Vedel, J. Formation of Copper Indium Diselenide by Electrodeposition. J. Electroanal. Chem. 1994, 374, 81−88. (17) Thouin, L.; Vedel, J. Electrodeposition and Characterization of CuInSe2 Thin Films. J. Electrochem. Soc. 1995, 142, 2996−3000. (18) Roussel, O.; Ramdani, O.; Chassaing, E.; Grand, P.-P.; Lamirand, M.; Kerrec, O.; Guillemoles, J.-F.; Lincot, D. First Stages of CuInSe2 Electrodeposition from Cu(II)−In(III)−Se(IV) Acidic Solutions on Polycrystalline Mo Films. J. Electrochem. Soc. 2008, 155, D141−D147. (19) Chassaing, E.; Ramdani, O.; Grand, P.-P.; Guillemoles, J.-F.; Lincot, D. New Insights in the Electrodeposition Mechanism of CuInSe2 Thin Films for Solar Cell Applications. Phys. Status Solidi C 2008, 5, 3445−3448. (20) Ren, T.; Yu, R.; Zhong, M.; Shi, J.; Li, C. Microstructure Evolution of CuInSe2 Thin Films Prepared by Single-Bath Electrodeposition. Sol. Energy Mater. Sol. Cells 2011, 95, 510−520. (21) Huang, H.-C.; Lin, C.-S.; Chen, F.-J.; Li, W.-C. Direct Observation of the Electrocrystallization of Compound CuInSe2 during the Early Stages of Deposition. Electrochim. Acta 2013, 97, 244−252. (22) Bhattacharta, R. N.; Fernandez, A. M. CuIn1−xGaxSe2-Based Photovoltaic Cells from Electrodeposited Precursor Films. Sol. Energy Mater. Sol. Cells 2003, 76, 331−337. (23) Calixto, M. E.; Dobson, K. D.; McCandless, B. E.; Birkmire, R. W. Controlling Growth Chemistry and Morphology of Single-Bath Electrodeposited Cu(In,Ga)Se2 Thin Films for Photovoltaic Application. J. Electrochem. Soc. 2006, 153, G521−G528. (24) Brooks, L. S. The Vapor Pressures of Tellurium and Selenium. J. Am. Chem. Soc. 1952, 74, 227−229. (25) Lim, Y. S.; Kwon, H.-S.; Jeong, J.; Kim, J. Y.; Kim, H.; Ko, M. J.; Jeong, U.; Lee, D.-K. Colloidal Solution-Processed CuInSe2 Solar Cells with Significantly Improved Efficiency up to 9% by Morphological Improvement. ACS Appl. Mater. Interfaces 2014, 6, 259−267. (26) Lee, B.-S.; Hwang, Y.; Pham, H. N.; Kim, J. Y.; Song, M. H.; Lee, D.-K. High-Density Cu−In Intermetallic Nanocrystal Layers: Towards High-Efficiency Printable CuInSe2 Solar Cells. J. Mater. Chem. A 2015, 3, 15889−15896. (27) Rincon, C.; Ramirez, F. J. Lattice Vibrations of CuInSe2 and CuGaSe2 by Raman Microspectrometry. J. Appl. Phys. 1992, 72, 4321− 4324. (28) Witte, W.; Kniese, R.; Powalla, M. Raman Investigations of Cu(In,Ga)Se2 Thin Films with Various Copper Contents. Thin Solid Films 2008, 517, 867−869. (29) Xu, C.-M.; Xu, X.-L.; Xu, L.; Yang, X.-J.; Zuo, J.; Kong, N.; Huang, W.-H.; Liu, H.-T. Composition Dependence of the Raman A1 Mode and Additional Mode in Tetragonal Cu−In−Se Thin Films. Semicond. Sci. Technol. 2004, 19, 1201−1206. (30) Kampmann, A.; Cowache, P.; Lincot, D.; Vedel, J. Junction Formation Studies of One-Step Electrodeposited CuInSe2 on CdS. J. Electrochem. Soc. 1999, 146, 150−155. (31) Lee, H.; Lee, W.; Kim, J. Y.; Ko, M. J.; Kim, K.; Seo, K.; Lee, D.K.; Kim, H. Highly Dense and Crystalline CuInSe2 Thin Films Prepared by Single Bath Electrochemical Deposition. Electrochim. Acta 2013, 87, 450−456. (32) Kushiya, K.; Shimizu, A.; Yamada, A.; Konagai, M. Development of High-Efficiency CuInxGa1‑xSe2 Thin-Film Solar Cells by Selenization with Elemental Se Vapor in Vacuum. Jpn. J. Appl. Phys. 1995, 34, 54−60. (33) Zhang, S. B.; Wei, S.-H. Reconstruction and Energetics of the Polar (112) and (1̅1̅2̅) versus the Nonpolar (220) Surfaces of CuInSe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 0814021−081402-4. (34) Siebentritt, S.; Papathanasiou, N.; Albert, J.; Lux-Steiner, M. Ch. Stability of Surfaces in the Chalcopyrite System. Appl. Phys. Lett. 2006, 88, 151919-1−151919-3. (35) Ruiz, C. M.; Fontané, X.; Fairbrother, A.; Izquierdo-Roca, V.; Broussillou, C.; Bodnar, S.; Pérez-Rodríguez, A.; Bermúdez, V. Impact

of Electronic Defects on the Raman Spectra from Electrodeposited Cu(In,Ga)Se2 Solar Cells: Application for Non-Destructive Defect Assessment. Appl. Phys. Lett. 2013, 102, 091106-1−091106-4. (36) Izquierdo-Roca, V.; Saucedo, E.; Ruiz, C. M.; Fontané, X.; Calvo-Barrio, L.; Á lvarez-Garcia, J.; Grand, P.-P.; Jaime-Ferrer, J. S.; Pérez-Rodríguez, A.; Morante, J. R.; Bermudez, V. Raman Scattering and Structural Analysis of Electrodeposited CuInSe2 and S-Rich Quaternary CuIn(S,Se)2 Semiconductors for Solar Cells. Phys. Status Solidi A 2009, 206, 1001−1004. (37) Redinger, A.; Hönes, K.; Fontané, X.; Izquierdo-Roca, V.; Saucedo, E.; Valle, N.; Pérez-Rodríguez, A.; Siebentritt, S. Detection of a ZnSe Secondary Phase in Coevaporated Cu2ZnSnSe4 Thin Films. Appl. Phys. Lett. 2011, 98, 101907-1−101907-3. (38) Saucedo, E.; Izquierdo-Roca, V.; Ruiz, C. M.; Parissi, L.; BrousSillou, C.; Grand, P. P.; Jaime-Ferrer, J. S.; Perez-Rodriguez, A.; Morante, J. R.; Bermudez, V. Key Role of Cu−Se Binary Phases in Electrodeposited CuInSe2 Precursors on Final Distribution of Cu−S Phases in CuIn(S,Se)2 Absorbers. Thin Solid Films 2009, 517, 2268− 2271. (39) Zhang, S. B.; Wei, S.-H.; Zunger, A.; Katayama-Yoshida, H. Defect Physics of the CuInSe2 Chalcopyrite Semiconductor. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 9642−9656. (40) Siebentritt, S.; Igalson, M.; Persson, C.; Lany, S. The Electronic Structure of Chalcopyrites-Bands, Point Defects and Grain Boundaries. Prog. Photovoltaics 2010, 18, 390−410. (41) Noufi, R.; Axton, R.; Herrington, C.; Deb, S. K. Electronic Properties versus Composition of Thin Films of CuInSe2. Appl. Phys. Lett. 1984, 45, 668−670. (42) Saucedo, E.; Ruiz, C. M.; Chassaing, E.; Jaime-Ferrer, J. S.; Grand, P. P.; Savidand, G.; Bermudez, V. Phase Evolution during CuInSe2 Electrodeposition on Polycrystalline Mo. Thin Solid Films 2010, 518, 3674−3679.

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