A Simple Aqueous Precursor Solution Processing of Earth-Abundant

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A Simple Aqueous Precursor Solution Processing of Earthabundant CuSnS Absorbers for Thin-film Solar Cells 2

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Mahesh P. Suryawanshi, Uma V. Ghorpade, Seung Wook Shin, Sachin Apparao Pawar, Inyoung Kim, Changwoo Hong, Minhao Wu, Pramod S Patil, Annasaheb V. Moholkar, and Jin Hyeok Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02167 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 24, 2016

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ACS Applied Materials & Interfaces

A Simple Aqueous Precursor Solution Processing of Earthabundant Cu2SnS3 Absorbers for Thin-film Solar Cells Mahesh P. Suryawanshia, Uma V. Ghorpadea, Seung Wook Shinb, Sachin A. Pawara, In Young Kimc, Chang Woo Honga, Minhao Wua, Pramod S. Patild, Annasaheb V. Moholkard, Jin Hyeok Kima* a

Department of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 300, Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea

b

Department of Chemical Engineering and Materials Science, University of Minnesota, Amundson Hall, 421 Washington Ave. SE, Minneapolis, MN 55455-0132, USA c

Gwangju Institute of Science and Technology, Cheomdangwagi-ro, Buk-gu, Gwangju, 500-712, South Korea

d

Thin Film Nanomaterials Laboratory, Department of Physics, Shivaji University, Kolhapur-416004, (MH), India

KEYWORDS. Cu2SnS3 (CTS), thin films solar cells (TFSCs), aqueous precursor solution processing, diode analysis, device stability ABSTRACT: A simple and eco-friendly method of solution processing of Cu2SnS3 (CTS) absorbers using an aqueous precursor solution is presented. The precursor solution was prepared by mixing metal salts into a mixture of water and ethanol (5:1) with monoethanolamine as an additive at room temperature. Nearly carbon-free CTS films were formed by multi-spin coating the precursor solution and heat treating in air followed by rapid thermal annealing in S vapor atmosphere at various temperatures. Exploring the role of the annealing temperature in the phase, composition and morphological evolution is essential for obtaining highly efficient CTS-based thin film solar cells (TFSCs). Investigations of CTS absorber layers annealed at various temperatures revealed that the annealing temperature plays an important role in further improving device properties and efficiency. A substantial improvement in device efficiency occurred only at the critical annealing temperature, which produces a compact and void-free microstructure with large grains and high crystallinity as a pure-phase absorber layer. Finally, at an annealing temperature of 600 °C, the CTS thin film exhibited structural, compositional and microstructural isotropy, yielding a reproducible power conversion efficiency of 1.80 %. Interestingly, CTS TFSCs exhibited good stability when stored in an air atmosphere without encapsulation at room temperature for 3 months, whereas the performance degraded slightly when subjected to accelerated aging at 80 °C for 100 hours under normal laboratory conditions.

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INTRODUCTION

Recently, the semiconductor compound CuxSnSy has received tremendous attention as a promising candidate for next-generation thin-film solar cells (TFSCs) because of its 4 -1 high absorption coefficient (>10 cm ) and tunable optical band gap energy (in the range 0.93-1.35 eV), which is strongly dependent on their crystal structure, for efficient solar 1,2 energy harvesting. Cu2SnS3 (CTS) has been suggested to be the most suitable candidate for TFSCs among the different stable phases in the Cu-Sn-S system, such as Cu3SnS4, Cu4SnS4, Cu2Sn3S7, Cu5Sn2S7 and Cu10Sn2S13, within the 3-8 temperature range from 400 to 650 °C. The CTS compound exhibits some unique advantages such as its 1) wide chemical potential stable range, 2) low-cost, 3) lack of toxicity and 4) earth-abundant constituent elements, which lower the overall fabrication costs and simplify the manufacturing 9-11 process.

Similar to thin films of CuIn(S,Se)2 (CISSe), CuIn1-xGaxSe2 (CIGS) and Cu2ZnSnS4 (CZTS), CTS thin films can be prepared by both vacuum-based physical and non-vacuum3-7, 12-15, 16-19 based solution processes. Thus far, most highly efficient CTS TFSCs have been fabricated using vacuumbased physical methods such as co-evaporation and 12-15 sputtering. However, the large capital investment required for the necessary vacuum equipment associated with these vacuum-based deposition processes is considered 20 a major hurdle for the widespread use of CTS TFSCs. By contrast, non-vacuum-based solution processes for CTS TFSC fabrication may offer cost and scalability advantages because they do not require expensive vacuum equipment at any stage, they are less energy-intensive, and because they offer high throughput and large-area processing relative to 21 the vacuum-based deposition methods. In this regard, a variety of solution methods have been 3 used to fabricate CTS thin films, including spray pyrolysis,

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electrodeposition, chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR) 7 16 method, nanoparticle deposition and direct solution 17-19 coating. Most of these synthetic processes usually involve deposition of a film of the CTS precursor, which is then annealed in a S atmosphere at elevated temperatures (500600 °C) to produce a highly crystalline pure CTS phase with larger grains. Among these various solution processes, direct solution coating is a particularly promising solution-based approach for preparing absorber layers for TFSCs. A pioneering work 22 on a direct solution coating approach by the Mitzi group, who used hydrazine as a solvent for fabricating CIGS thin films, described numerous potential advantages. These advantages include i) a simple, versatile, low-cost and scalable mass processing; ii) a good control over composition throughout the film thickness; iii) the potential to remove/reduce organic impurities; and iv) avoiding the additional steps associated with nanoparticle deposition such 22-24 as synthesis, purification, ligand exchange and assembly. Thus far, direct solution coating using hydrazine as a solvent has been utilized to fabricate CIS-, CIGS- and CZTS-based absorbers with conversion efficiencies of approximately 12.2, 15.2 and 12.7 %, respectively; these results demonstrate the potential of this approach in photovoltaic (PV) 25-27 applications. Despite the higher efficiencies, a highly toxic and flammable nature of the hydrazine solvent requires various handling precautions during preparation of both the ink and thin films, which impedes its use in practical applications. Therefore, simple and green sol-gel chemistry methodologies to fabricate absorber layers for TFSCs, such as 28-30 approaches based on metal salts/thiourea and metal 31 oxides, are currently under active investigation. However, several challenges associated with these non-toxic solventbased approaches remain unsolved, such as difficulty in dissolving elemental metal powders and their corresponding chalcogenides in these solvents and the formation of a double-layered structure, usually with a large-grain top layer and a fine-grain bottom layer with a large compositional deviation along the depth direction and the formation of 31, 32 carbon residue in the absorber layer. Therefore, in this regard, an alternative aqueous solution approach that can overcome these remaining issues associated with the nontoxic solvent-based approaches could be a promising strategy to fabricate a high-quality absorber for practical applications. Table 1 summarizes the major reports of CTS absorbers fabricated by direct coating solution processing. However, many of these reports are limited to only the preparation and characterization of pure-phase CTS absorbers. Here, we demonstrate direct solution coating of CTS TFSCs using a simple, low-cost, aqueous precursor solution using green chemistry. In particular, a stable Cu-Sn aqueous precursor solution was prepared by mixing metal salts into a mixture of water and ethanol (5:1) through solution chemistry using an amine additive, which is quite similar to 30,33 the approach used to fabricate CIS TFSCs. However, dissolving metal salts into a water/ethanol mixture through solution chemistry using an amine additive is an innovative step that has not been previously explored for CTS absorbers.

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This stable aqueous precursor solution was spin-coated onto molybdenum (Mo) coated substrates, air heated to form a CTS precursor and then further annealed in an S vapor atmosphere to convert the precursor into highly crystalline, pure-phase, and nearly carbon-free dense CTS absorbers with large grains. We also systematically investigated the phase evolution pathway from amorphous to pure CTS phase in conjunction with compositional and morphological changes upon chalcogenization as a function of annealing temperature. Using this aqueous precursor solution, our initial device performance for CTS TFSCs derived from a CTS absorber annealed at 600 °C resulted in a first notable power conversion efficiency (PCE) of 1.80 % (for an active area of 2 0.30 cm ). A detailed diode analysis was performed to elucidate the origin of the lower performance of the present CTS devices; possible strategies for further improvement are proposed.

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EXPERIMENTAL DETAILS

Materials. Copper(II) acetate (Cu(OAc)2) (98%), tin(II) chloride (SnCl2) (98%), ethanol and monoethanolamine (MEA) were all purchased from Sigma-Aldrich, Korea. Preparation of Aqueous Precursor Solution and Precursor Films. The aqueous precursor solution was prepared by dissolving Cu(OAc)2 (1.8 mM) and SnCl2 (0.8 mM) in a mixture of deionized water/ethanol (5:1) (i. e. 10:2 ml) with a small amount of MEA (1 ml) as an additive. The Cu-Sn aqueous precursor solution was stirred constantly at room temperature for 30 min to yield a dark-blue, transparent solution. The aqueous precursor solution was spin-coated at 2500 rpm for 30 s onto Mo-coated glass substrates. The film was dried at 150 °C for 2 min on a hot plate in air. The spin-coating and drying processes were repeated 12 times to obtain CTS precursor films with a thickness of approximately 1.4 µm. An additional oxidation step at an elevated temperature was not necessary in this case because the precursor films were prepared using a binder-free process with an aqueous precursor solution. Formation of CTS Films by Rapid Thermal Annealing. The amorphous precursor films were converted into highly crystalline prominent phase CTS by rapid thermal annealing (RTA) in an S vapor atmosphere for 10 min. The phase evolution of the precursor films from amorphous to highly crystalline prominent phase CTS with a dense microstructure was investigated as a function of annealing temperature in the range of 300 to 600 °C (Figure S1). During RTA process, the precursor films were placed in a graphite box with a 3 volume of 12.3 cm , and approximately 30 mg of S was placed near the precursor films. After attaining a sufficient base vacuum, Ar gas was flowed into the box until a working pressure of ~ 56 Torr was achieved. The RTA system was closed during the annealing process. A ramp rate of approximately 9.67 °C/s and a dwell time of 10 min were used, and the samples were further cooled to room temperature under the same flowing Ar gas. The S vapor pressure produced inside the graphite box during RTA processing was calculated according to the ideal gas law, PV = nRT, where P is pressusre, V is volume for reaction area, n ≅ 1, R is gas constant, and T is temperature. Pressures of approximately 650, 763, 877, 934 and 990 Torr were achieved

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at temperatures of 300, 400, 500, 550 and 600 °C, respectively. Solar Cell Device Fabrication. A solar cell device was fabricated according to the standard Mo/CTS/CdS/i-ZnO/n34 ZnO/Al configuration. A ~ 65-nm-thick CdS buffer layer was prepared on the CTS absorber using chemical bath deposition (CBD), and i-ZnO (~ 75 nm)/Al-doped ZnO (AZO) (~ 500 nm) were deposited by radio-frequency (RF) magnetron sputtering. Finally, the devices were finished by depositing an Al grid (~ 500 nm) as a current collector using direct-current (DC) sputtering. The final devices (2.5 cm × 2.5 cm) were mechanically scribed into cells with an active 2 34 area of 0.30 cm . Characterization. Fourier transform infrared spectroscopy (FT-IR, PerkinElmer, Frontier-89063) confirmed the formation of an amine complex in the aqueous precursor solution. The thermal behavior of the aqueous precursor solution was examined under an air atmosphere by thermogravimetry coupled with differential scanning calorimetry (SDT Q600, TA Instruments). The phase evolution and crystal structure of the CTS films were analyzed by high-resolution X-ray diffraction (XRD, X'pert PRO, Philips, Eindhoven, Netherlands). Samples were o scanned over the range 20-80 ; the X-ray source was operated at 40 kV and 30 mA. The phases of the CTS films were further confirmed by Raman spectra recorded using a Raman microscope (LabRam HR8000 UV, Horiba JobinYvon, France) equipped with a laser with a wavelength of 532 nm. Quantitative analysis of the elemental compositions of the annealed CTS films was conducted using X-ray fluorescence (XRF) spectroscopy (ZSX Primus II RIGAKU Corp.). The morphological improvement in the precursor films after thermal processing was studied using fieldemission scanning electron microscopy (FE-SEM, Model Hitachi S 4800, Japan) on a microscope equipped with EDS and located at the Korean Basic Science Institute (KBSI), Gwangju, South Korea. The chemical binding energy of the annealed films was examined using high-resolution X-ray photoelectron spectroscopy (HR-XPS, VG Multi lab 2000, Thermo VG Scientific, UK) at room temperature. The PCE and external quantum efficiency (EQE) of the CTS TFSCs were characterized using a class AAA solar simulator (Sol31, Oriel, USA) and an incident PCE measurement unit (PV measurement, Inc., USA), respectively. The solar simulator was calibrated using an OKL-HSSR-1500N high-speed, high2 precision spectroradiometer. The solar irradiance (100 W/m ) was measured during the J-V measurements using a Daystar solar digital meter equipped with a polycrystalline silicon PV cell as a sensor. Light was illuminated from the side with the Al front contact. No intentional temperature control or aperture was applied during the measurements.

■ RESULTS AND DISCUSSION Recent reports have indicated that, in the pure solution, the elements are mixed at the molecular level, which enables precise control of the stoichiometry because of molecular35 level homogeneity. This control, in turn, provides spatial uniformity in the precursor and annealed films and thereby improves the solar cell performance. In this work, a simple

and eco-friendly aqueous precursor solution processing method that involves a mixture of water and ethanol as a solvent and MEA as an additive was used to dissolve metal salts to form a homogenous precursor solution. Water and ethanol were used as stable, non-toxic, and low-cost solvents that easily dissolve most inorganic salts and have effective o boiling points of 100 and 78.5 C, respectively. The use of ethanol along with water favors the complete dissolution of SnCl2 and enables stable precursor solution processing. Furthermore, MEA, which is inexpensive and non-toxic and has both hydrophilic (-OH) and hydrophobic (-NH2) groups, can improve the solubility of metal salts in the water-ethanol solvent mixture. Thus, the precursor solutions are less prone to precipitation, which leads to stable shelf lives of several 30,33 months. In addition, the use of ethanol and MEA may reduce or eliminate carbon residue after soft- and postannealing, which is commonly observed in solution31,32 processed thin films. In our precursor solution system, when Cu(OAc)2 and 2+ SnCl2 are mixed with the water-ethanol solvent mixture, Cu 2+ + 4+ reacts with Sn to form Cu and Sn . These products are a 2+ 2+ consequence of the redox reaction between Cu and Sn , in 2+ + 2+ 4+ which Cu is reduced to Cu and Sn is oxidized to Sn . Note that the colors of the precursor solution after Cu(OAc)2 and SnCl2 were mixed were faint blue and milky white, as shown in Figure S2a and b, respectively. After MEA was added to the mixture, the -NH2 group of MEA coordinated with metal ions; this coordination resulted in a metal-amine complex solution whose formation was confirmed by the change in color of the precursor solution from milky-white to 30 The precursor solution was deep-blue (Figure S2c). analyzed by FT-IR to further confirm the formation of the amine complex; the spectrum is shown in Figure S3. TGA was used to reveal the decomposition behavior of the aqueous precursor solution during annealing in air (Figure 1). The TGA curve of the formulated aqueous precursor solution shows a sharp 80 % weight loss before 100 °C because of solvent evaporation. The weight loss continues at a lower rate up to 250 °C, which is attributed to the decomposition of 36-38 anions. This behavior indicates that, in the case of the annealing treatment in air after successive spin coating of each layer of the precursor film, 300 °C is sufficient to remove the solvent and other volatile moieties from the precursor film. The spin-coated precursor films preheated in air were amorphous phase. The XRD pattern (Figure 2c) of the precursor film does not reveal the appearance of any peaks related to the mixed oxide compounds of Cu and Sn, which can be also confirmed from the Raman spectrum (Figure S4). The microstructure of the precursor film was also investigated by FE-SEM; the micrographs are shown in Figure 2a and b. A uniform and continuous precursor film with a thickness of 1.4 µm was obtained after 12 spincoating/drying cycles. Furthermore, an EDX elemental linescan analysis (Figure 2d) of the precursor film revealed a nearly carbon-free precursor film. The precursor films were converted into highly crystalline CTS phase when annealed in an S vapor atmosphere using the RTA process. Figure 3 depicts the XRD patterns of CTS

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films after RTA processing in an S vapor atmosphere for 10 min at various temperatures. The phase transformation from amorphous to highly crystalline CTS phase is confirmed by the presence of several well-resolved diffraction peaks o o o o marked by an asterisk (*) at 28.43 , 32.95 , 47.24 and 56.08 . The phase formation started at a temperature greater than 300 °C. However, the presence of an extra secondary phase of SnS marked by “#” was observed in the patterns of the films sulfurized at 300 and 400 °C (JCPDS card No. 00-001-0984 (SnS)). Impurity phase of SnS was finally removed at 500 °C, resulting in prominent CTS phase. The peak marked by (∆) o at 40.5 corresponds to the Mo substrate (JCPDS Card No. 3065-7442 (Mo)). The intensity of the major diffraction peak o at 28.43 increased with increasing annealing temperature, indicating the enhanced crystallinity in the CTS films at higher annealing temperatures. This enhanced crystallinity can be better understood by comparing the crystallite size values calculated from the XRD patterns. The calculated average crystallite sizes were 35.43 nm, 52.16 nm, 65.80 nm, 78.49 nm and 90.77 nm for the films annealed at 350, 450, 500, 550 and 600 °C, respectively. However, XRD cannot clarify the formation of what kind of CTS phases such as cubic CTS (ICDD#01-089-2877), tetragonal CTS (ICDD#01089-4714), monoclinic CTS (ICDD#04-010-5719) and triclinic CTS (JCPDS#27-0198) as well as the existence of secondary phases due to overlapping of the diffraction peaks of each 39a other. Raman spectroscopy was used to more closely investigate the phase indentification and presence of secondary phases in the CTS thin films. Figure 4 shows the Raman spectra of films annealed at different temperatures in an S vapor atmosphere. The spectra of all films exhibit two major -1 Raman peaks at 290 and 351 cm , which correspond to 39a Thus, it is monoclinic CTS, according to Berg et al. reasonable to state that the CTS formed using our aqueous precursor solution approach exhibits a most stable 39b monoclinic phase. Moreover, the additional weak peaks at -1 315 and 375 cm corresponding to secondary phase of Cu2Sn3S7 were observed in all the films. The films annealed at lower temperatures of 300 and 400 °C exhibited an extra -1 weak peak at 221 cm corresponding to SnS along with the 40 presence of Cu2Sn3S7 and CTS phases. When the annealing temperature was further increased above 400 °C, the secondary SnS phase disappeared at an annealing temperature of 500 °C. No traces of other impurity phases such as SnS and Cu2-xS or ordered vacancy compounds except the presence of extra phase of Cu2Sn3S7 were detected within the limit of experimental precision for films annealed at and above 500 °C. Thus, the obtained Raman results demonstrate that prominent phase CTS formation is systematically dependent on the annealing temprature, 41,42 which is consistent with previously reported results. Further, the texture coefficient (TChkl) were calculated to determine the preferred orientation. The texture coefficient exhibits a part of crystallites that is oriented along a particu43 lar crystallographic direction. The higher value of the texture coefficient along a certain crystallographic plane, the larger quantity of crystallites along that plane orientated 43 parallel to the surface is present in the film. The obtained

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results indicated that pure CTS is highly textured along the (1 3 1)/(2 0 0) plane, as shown in Figure S5a. Furthermore, the dislocation densities and strain were calculated for the major diffraction peak using well-known equations described 44 in the literature and are presented in Figure S5b. These values were observed to decrease with increasing annealing temperature, suggesting that the annealed films were strain45 free. The change in the composition of the CTS films with increasing annealing temperature correlates well with the XRD and Raman spectroscopy results. The compositional changes as a function of annealing temperature are represented in terms of [Cu]/[Sn] and [S]/([Cu]+[Sn]) mole fractions in Figure 5. The results reveal that the sulfur replaced the oxygen even at 300 °C. Furthermore, the [S]/([Cu]+[Sn]) mole fraction gradually increased with increasing annealing temperature, which is attributed to sufficient incorporation of S during annealing at higher 46 This sufficient incorporation of S is temperatures. confirmed by the higher content of S in CTS films up to 600 °C. By contrast, the [Cu]/[Sn] mole fraction decreased from 1.85 to 1.74 with increasing annealing temperature. This result suggests that the films annealed at higher annealing temperatures had slightly higher Sn contents. To further ascertain the valance states of the Cu, Sn and S, CTS films annealed at 500, 550 and 600 °C were analyzed by XPS, as shown in Figure S6. The valance states of Cu and Sn + 4+ were determined to be Cu and Sn in annealed CTS films 47,48 according to their peak splitting values. The 2p peaks of S were observed in the range 160-164 eV, which suggests that 47,48 S was present in its sulfide state. Figure 6 shows the surface (top-view) and cross-sectional FE-SEM micrographs of CTS films annealed at different temperatures for 10 min in an S vapor atmosphere using the RTA process. A negligible change was observed in the microstructure of the films annealed at 300 and 400 °C compared to that of the precursor film. Although the presence of some pores was observed, the films annealed at 300 and 400 °C exhibited a uniform microstructure with smaller grains, which were densely packed together. When the annealing temperature was increased to 500 °C, the film underwent noticeable recrystallization, and smaller grains coalesced to form nearly spherical grains that cover the whole surface, leaving no pores at the surface of the film. The grain growth further continued, and the absorber layers adopted a dense microstructure without voids or cracks and with more faceted grains at 550 and 600 °C. The thickness is found to be 1.1, 1.02, 0.98, 0.96 and 0.93 µm for CTS films annealed at temperatures of 300, 400, 500, 550 and 600 °C, respectively as observed from the cross-sectional FE-SEM images as shown in Figure 6. Thus, the morphology evolved with increasing annealing temperature until a high-quality CTS absorber layer formed with a void-free, dense microstructure and larger grains extended from the top to the bottom of the film at a higher annealing temperature. A high-quality absorber layer with larger grains is well known to be important for achieving a high conversion efficiency by minimizing the recombination of charge carriers at grain 49-51 boundaries. Notably, the absorber layer fabricated using

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our aqueous precursor solution was a single-layered structure with a minimal amount of carbon residue, as revealed by the EDX line scan of the CTS film annealed at 600 °C (Figure S7). This minimal carbon residue is a core advantage of our aqueous precursor solution approach compared with metal nitrate-, metal chloride-, and EC-based solution approaches, in which the absorber layers usually result in a double-layered microstructure containing a large28-32 grained top layer and a thick carbon bottom layer. Our well-crystallized, single-layer CTS microstructure with larger grains achieved by simply controlling the annealing temperature is solely attributed to the use of the water/alcohol/MEA combination to dissolve the metal salts. This result is mainly due to the monodentate characteristics and the lower dissociation energy of MEA, which easily dissociates by forming CO2 during annealing, leaving 33 no/minimal carbon residue in the CTS absorber layer. It is also observed the S-poor composition near the Mo back contact region. This behaivor may be attributed to the compact and void-free microstructure of the precursor film (Figure 2a and b) and impurity (oxyen) concentration (Figure 2d) in the precursor film as well as a short sulfurization time of 10 min. This could be also attributed to a short sulfruization time of 10 min which may insufficeint for the S vapor to be fully diffused in the Mo back contact region, leading to the graded compositional behavior (S-poor) near the Mo back contact region. Regarding the applicability of the CTS thin film as an absorber layer in solar cells, the films annealed at 500, 550 and 600 °C appear to be more promising as far as their electrical properties (Table S1) are concerned compared to those annealed at lower temperatures. These films exhibit a highly dense, void-free microstructure with larger grains, which is a primary requirement for a device-quality absorber 49-51 layer. However, a very thin MoS2 layer was observed to form at the CTS/Mo interface, which might be a consequence of the slightly high S vapor pressure during annealing process. Nevertheless, a thick layer of MoS2 introduces high series resistance and hampers device performance, whereas a very thin layer (~ 100 nm) is beneficial because it improves the adhesion and quasi-ohmic contact at the absorber/Mo 52 interface. To elucidate the effectiveness of our aqueous precursor solution approach, CTS absorber layers annealed at 500, 550 and 600 °C were incorporated into solar cells with a chemically deposited CdS buffer layer and sputtered ZnO layers. Figure 7a shows the current density-voltage (J-V) characteristics under AM 1.5G illumination of the CTS devices. The detailed device parameters are listed in Table 2. The overall device performance of the CTS-based TFSCs shows substantial variation because of the various annealing temperatures. The variation in the device parameters may be related to the microstructure, elemental losses during thermal annealing, phase formation and/or phase distribution, which could serve to offer provisional insights into how the annealing temperature affects device performance. The extracted device parameters of the CTS absorber layers annealed at various temperatures are graphically illustrated in Figure 8. The short-circuit current

density (Jsc), open circuit voltage (Voc), and fill factor (FF) substantially increased with increasing annealing temperature. As a consequence, the PCE increased and exhibited the highest value of 1.80 % for the device fabricated using the CTS absorber layer annealed at 600 °C. We observed that the CTS absorber layer annealed at 600 °C 2 showed a higher Jsc of 19.68 mA/cm , a Voc of 0.267 V, an FF of 0.34 and a PCE of 1.80 %. Large, micron-sized, densely packed grains are considered to be the morphological 49-51 features of high-efficiency TFSCs. Thus, we attributed the linear increase in the PCE value with increasing annealing temperature to the improved microstructure and grain size, which are clearly evident in Figure 6. The improvement in the PCE is solely attributed to the improved grain size, which reduces the recombination of charge carriers at the grain boundaries, resulting in better charge transport properties in 53 the annealed CTS devices. We also measured the average performance of six cells from each device fabricated using the CTS absorber layers annealed at 500, 550 and 600 °C, as shown in Figure S8. In Figure 7b, the EQE measurements provide further insights into photogenerated carrier absorption and collection in the corresponding devices. The short wavelength region is dominated by the ZnO window layer and CdS buffer layer cut-off with characteristic shoulder 54,55 peaks at approximately 400 nm and 450 nm, respectively. The decrease in the red spectral region and the longwavelength cut-off provides information about the quality of 4 CTS absorber layers. We observed the quantum efficiency of the CTS TFSCs to continuously increase with increasing annealing temperature. Among all the devices, the device fabricated using the CTS absorber layer annealed at 600 °C exhibited the highest EQE value of ~ 63 % at approximately 560 nm. The gradual decrease in the EQE response, especially at longer wavelengths, may result from the loss of deeply absorbed photons as a consquence of the short minority carrier lifetime or insufficient penetration of the 56,57 depletion region width in the absorber layers. We also compared integrated Jsc values from the EQE with Jsc values 2 obtained from the J-V curves recorded under a 100 mW/cm solar stimulator, as listed in Table 2. Notably, the observed and calculated Jsc values exhibit negligible mismatch. The band gaps of the absorber layers are estimated from the 2 linear extrapolation plot of [hν ln(1 − EQE)] versus (hν), which were observed to be 1.07, 1.13 and 1.18 eV for the CTS absorber layers annealed at 500, 550 and 600 °C, respectively. The charge carrier recombinations adversely affect the performance of solar cells and are present in all types of solar cells. Therefore, the J-V characteristics in dark conditions were further measured and analyzed to determine the loss mechanism in the CTS absorber layers. The dark J-V curves were redrawn in three consecutive plots (Figure 9) to determine the diode parameters such as shunt conductance (Gsh), series resistance (Rs), ideality factor (nd) and reverse saturation current density (Jo) using standard analysis for 58 TFSCs. The resulting values of Gsh, Rs, nd and Jo as functions of annealing temperature are listed in Table 3. In Figure 9a, the Gsh values were evaluated from the plots of dJ/dV versus V. The Rs and nd were estimated from the y-intercept and the

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slope of a plot of dV/dJ versus (1/(J + Jsc)) (Figure 9b), respectively. Furthermore, Figure 9c shows semi-logarithmic plots of (J + Jsc - GV) versus (V - RJ), which allows us to extract Jo and nd from the y-intercept and the slope, respectively. The nd values presented in Table 3 are the average values obtained from the two previously described procedures. The estimated parameters in Table 3 shed light on the inferior performance of the present devices, in particular, the low Voc and FF values. Table 3 shows that the Gsh, nd and Jo values significantly decreased with increasing annealing temperature for CTS devices. In particular, the improved (lower) Gsh value in the CTS device annealed at 600 °C indicates that the microstructure of the CTS film was improved to larger grains without voids and cracks, as clearly shown in Figure 6. Although the nd and Jo values improved with increasing annealing temperature and are lower for the CTS device annealed at 600 °C, the obtained values are still 31, 58,59 relatively higher compared to the reported values. The higher Jo values led to poor Voc values in our devices. The higher Jo and nd values are attributable to poor junction formation between CdS and CTS and to high recombination 59 at the CdS/CTS interfaces. The higher Jo and nd values are also known to reduce the FF. In addition, the higher Gsh and Rs values in our devices also affected the FF of the devices. The Rs values were observed to decrease with increasing annealing temperature, which is attributed to enhacned crystallinity and to larger grain sizes at higher annealing 60 temperatures. However, the Rs values are still relatively 61,62 high compared with previously reported values. This results may be due to the graded compositinal ratio (S-poor) near the Mo back contact region, indicating the presence of unexpected secondary phases with small size (below 100 nm), which can not be detected within the limits of XRD and Raman characterizations. Because our results represent the initial performance of our fabricated CTS-based TFSCs without the absorber layer or the device fabrication methodology being fully optimized, the performance is relatively poor as compared to that of previously reported devices (Table 1). This poor performance is attributed to the large Rs caused by the formation of the MoS2 layer at the CTS/Mo interface and by high carrier recombination in the CTS absorber; it is also attributed to reflection losses at the front contacts and unwanted 34 absorption in the window layer. Despite the poor device performance, the stability of the highest PCE CTS TFSCs was investigated under various conditions and is summarized in Figure 10. When the devices were stored in air at room temperature without encapsulation, they were stable for at least 3 months and exhibited a slight decrease in PCE. Furthermore, the device exhibited excellent thermal stability when subjected to accelerated aging at 80 °C, maintaining 90 % of its initial value after 100 h at this temperature. In addition, the stability of both the aqueous precursor solution and the prepared CTS films was evaluated under various conditions. The precursor solution and CTS films were stable for several months when stored in a nitrogenfilled glovebox. By contrast, the precursor solution exhibited complete decomposition, leaving a whitish-blue precipitate

Page 6 of 19

within 1 week of storage in air, which is as shown in Figure S9. However, the CTS films prepared using the aqueous precursor solution were stable for at least 1 month in air, without the appearance of new phases in their XRD patterns. To the best of our knowledge, the results presented here represent the highest efficiency thus far obtained for solution-processed CTS-based TFSCs fabricated with the standard configuration of SLG/Mo/CTS/CdS/ZnO/AZO/Al (Table 1). However, the highest efficiency of 1.80 % for our CTS TFSCs is relatively low compared to the record efficiency 62 of 4.63 % for co-evaporated CTS TFSCs. The low Voc and FF values are common issues with CTS TFSCs fabricated with the aforementioned standard configuration described above. Possible reasons for the low Voc in our CTS devices are i) the poor p-n junction formed between the CTS absorber and CdS buffer layer and the interface recombination mechanism, as previously discussed, and ii) compositional deviation, which may inevitably lead to secondary phases such as Cu2Sn3S7 in 4 the CTS absorber layer. Moreover, further optimization of the precise precursor composition and annealing conditions is needed to increase the Voc and FF values, decrease the Rs and thereby enhance the device efficiency.

■ CONCLUSIONS In summary, a simple and eco-friendly aqueous precursor solution processing for a high-quality CTS absorber for TFSCs is presented. Metal salts were sequentially dissolved through solution chemistry into a mixture of water/ethanol (5:1) solvents using MEA as an additive, forming a precursor solution that was homogenous at the molecular level. The Cu-Sn aqueous molecular precursor solution enabled the formation of CTS absorbers when annealed in a S vapor atmosphere using an RTA process. The resulting absorbers were observed to be prominent CTS phase after annealing at 500 °C or higher in an S vapor atmosphere for 10 min. Notably, the annealed CTS absorbers exhibited a nearly carbon-free single-layered dense microstructure with large grains. A phase evolution from amorphous to mixed binary and ternary phases to a prominent phase occurred with increasing annealing temperature, in conjunction with compositional and microstructural changes in the CTS absorber layers. Accordingly, the annealing temperature is suggested to play a critical role in the formation of prominent phase, compositionally uniform, well-crystallized and dense CTS absorbers with large grains. Our initial CTS device derived from the CTS absorber annealed at 600 °C in a S vapor atmosphere for 10 min exhibited an active area efficiency of 1.80 %. Remarkably, the CTS TFSC exhibited good stability in air at room temperature 0 for 3 months and under damp heating at 80 C for up to 100 h. Our CTS device achieved the highest efficiency reported thus far among devices with CTS absorbers fabricated by direct solution coating processes. However, the efficiency of our CTS device is limited by its higher Gsh, Rs, and Jo, as well as by its nd, as revealed by a detailed diode analysis, which suggests possible pathways for further improvement in device performance. Further studies are in progress to precisely optimize the elemental composition and the annealing conditions to achieve a well-developed

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microstructure with micron-sized grains while simultaneously preventing the formation of a resistive MoS2 layer at the CTS/Mo interface. A simple and eco-friendly aqueous precursor solution represents the first step toward the inexpensive, safe and scalable fabrication of efficient CTS absorber layers and can be further exploited for other types of absorber layers for photovoltaic applications.

■

(5)

Li, J.; Xue, C.; Wang, Y.; Jiang, G.; Liu, W.; Zhu, C. Cu2SnS3 Solar Cells Fabricated by Chemical Bath Deposition-Annealing of SnS/Cu Stacked Layers. Sol. Energy Mater. Sol. Cells. 2016, 144, 281-288.

(6)

Avellaneda, D.; Nair, M. T. S.; Nair, P. K. Cu2SnS3 and Cu4SnS4 Thin Films Via Chemical Depostion for Photovoltaic Application. J. Electrochem. Soc. 2010, 157, D346-D352.

(7)

Su, Z.; Sun, K.; Han, Z.; Liu, F.; Lai, Y.; Li, J.; Liu, Y. Fabrication of Ternary Cu-Sn-S Sulfides by A Modified Successive Ionic Layer Adsorption and Reaction (SILAR) Method. J. Mater. Chem. 2012, 22, 16346-16352.

(8)

Fiechter, S.; Martinez, M.; Schmidt, G.; Henrion, W.; Tomm, Y.; Phase Relations and Optical Properties of Semiconducting Ternary Sulfides in the System Cu-Sn-S. J. Phys. Chem. Solids. 2003, 64, 1859-1862.

(9)

Dahman, H.; Arrabaoui, S.; Alyamani, A.; Mir, L. Ei. Structural, Morphological and optical Properties of Cu2SnS3 Thin Film Synthesized by Spin Coating Technique. Vacuum. 2014, 101, 208-211.

ASSOCIATED CONTENT

Supporting Information. The sulfurization temperature profile, the FT-IR spectrum of the formulated aqueous precursor solution, the Raman spectrum of precursor film, plots of the variation in the texture coefficient and dislocation density, variation in the crystallite size and strain as a function of annealing temperature in sulfurized CTS films, high-resolution XPS spectra of the CTS films prepared using an aqueous precursor solution after annealing at 500, 550 and 600 °C in an S vapor atmosphere for 10 min, an SEMEDX linescan of a fractured cross-section of the CTS film annealed at 600 °C for 10 min in an S vapor atmosphere, the statistics of different cells in CTS devices derived from the CTS absorber annealed at 600 °C for 10 min, a photograph of the aqueous precursor solution stored for 1 week in air, and the electrical properties of films annealed at 300, 400, 500, 550 and 600 °C in for 10 min under S vapor atmosphere

■

The Ternary Compound Cu2SnS3. Thin Solid Films. 2012, 520, 6291-6294.

AUTHOR INFORMATION

(10) Yasar, S.; Kahraman, S.; Centinkaya, S.; Bilican, I. Improved Characteristics for Chemically Grown Cu2SnS3 Promising Solar Absorbers Through the Use of TritonX100 Surfactant. J. Alloys Compd. 2015, 618, 217-221. (11)

Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

This work was supported by a Human Resources Development grant (No. 20124010203180) from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Government Ministry of Trade, Industry and Energy and is partially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2015R1A2A2A01006856).

■

REFERENCES

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Figures

Figure 1. Thermogravimetric analysis (TGA) of the aqueous precursor solution; the results indicate that a temperature of 300 °C is sufficient to remove the solvent and other moieties from the precursor film.

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Figure 2. (a) Top view and (b) cross-sectional FE-SEM micrographs; (c) XRD pattern; and (d) EDS linescan recorded along the cross-section in (b) of the as-deposited precursor film prepared using an aqueous precursor solution.

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*# CTS SnS

* *

∆

*

*

∆ Mo ∆ 600 C o

550 oC 500 oC

#

400 oC 300 oC

20

30

40

50

60

70

80

2θ (Degree)

-1

351 cm -1 375 cm

-1

290 cm

315 cm

221 cm

-1

-1

Figure 3. XRD patterns of the thin films prepared using an aqueous precursor solution after the films were annealed at various temperatures for 10 min in an S vapor atmosphere. The films annealed at 500 °C or greater exhibited pure-phase CTS, whereas the films annealed at 300 and 400 °C showed the formation of CTS along with the presence of an SnS secondary phase

Intensity (a. u.)

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

Intensity (a.u.)


600 600ooC 550 550ooC

500ooC 500 400 400ooC 300 300ooC 200

300

400

500

600

Raman shift (cm-1) Figure 4. Raman spectra of the thin films prepared using an aqueous precursor solution after the films were annealed at various temperatures for 10 min in an S vapor atmosphere. The Raman spectra confirmed the formation of pure-phase CTS for the films annealed at 500 °C or greater and the presence of an SnS secondary phase along with CTS for the films annealed at 300 and 400 °C.

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1.95

1.4

Mole fraction

[Cu]/[Sn] 1.3

1.90

[S]/([Cu]+[Sn]) 1.2

1.85 1.1 1.80

1.0 0.9

1.75

Mole fraction

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


0.8 1.70 300

350

400

450

500

550

600

0.7

o

Annealing temperature ( C) Figure 5. Chemical compositional fractions of the thin films prepared using an aqueous precursor solution, as measured by XRF as a function of the annealing temperature in an S vapor atmosphere for 10 min. The solid curves are for visual guidance.

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Figure 6. Top view (left column) and cross-sectional (right column) SEM micrographs of the thin films prepared o o using an aqueous precursor solution after the films were annealed at various temperatures (300 C (a and b), 400 C o o o (c and d), 500 C (e and f), 550 C (g and h), 600 C (i and j)) for 10 min in an S vapor atmosphere. The annealingtemperature-dependent microstructural evolution led to the formation of a well-crystallized CTS absorber with large grains at an annealing temperature of 600 °C.

14



(a)

70

EQE (%)

20

80

500 oC 550 oC 600 oC

16 12

500 oC 550 oC 600 oC

(b)

60 50 40 30

8

20 4 0 0.00

10 0.05

0.10

0.15

0.20

0.25

0

0.30

400

Voltage (V)

600

800

1000

1200

Wavelength (nm)

Figure 7. Photovoltaic properties of the CTS TFSCs fabricated using the CTS absorbers annealed at 500, 550 and 600 °C: (a) J-V characteristics under AM 1.5G illumination; (b) external quantum efficiencies (EQE) without bias under short-circuit conditions

(a)

(b)

0.38

2.5

20

0.32 0.20

2.0 15 1.5 10

PCE (%)

0.34 0.22

2

0.36 0.24

Jsc (mA/cm )

0.26

Voc (V)

3.0

25

0.40 0.28

FF

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

Current density (mA/cm2)

Page 15 of 19

1.0

0.30 0.18

5

0.5

0.28 0.16

Voc

JSC

FF 500

550

600

Annealing temperature (oC)

0.26

0

PCE 500

550

600

0.0

Annealing temperature (oC)

Figure 8. Device characteristics of the CTS TFSCs fabricated using the CTS absorbers annealed at 500, 550 and 600 °C: (a) variation of the open-circuit voltage, Voc, and fill factor, FF; (b) variation of the short-circuit current density, Jsc, and power conversion efficiency, PCE.

15


ACS Applied Materials & Interfaces 50

30

(a)

500 oC 550 oC 600 oC

20 15 10

Gsh

5

500 oC 550 oC 600 oC

(b) dV/dJ (Ω.cm2)

2

dJ/dV (mS/cm )

25

0

40

30

Y-intercept (Rs)

20

Slope (nd)

10

-5 -10 -1.0

-0.5

0.0

0.5

0

1.0

0

5

J + Jsc - GV (mA/cm2)

Voltage (V) 500 oC 550 oC 600 oC

(c) 0.01

15

20

25

30

+ Jsc)-1 (mA/cm2)

Slope (nd)

1E-3

1E-4

10

(J

Y-intercept (Jo)

1E-5 0.0

0.2

0.4

0.6

0.8

1.0

V - RJ (V) Figure 9. Diode analysis of the CTS TFSCs fabricated using the CTS absorbers annealed at 500, 550 and 600 °C: (a) -1 plots of dJ/dV vs V for Gsh evaluation; (b) plots of dV/dJ vs (J + Jsc) for the determination of Rs and nd; (c) semilogarithmic plots of (J + Jsc – GV) vs (V-RJ) for the determination of nd and Jo. 2.0

2.0

(a)

(b) PCE (%)

1.8

PCE (%)

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

Page 16 of 19

1.6

1.4

1.8

1.6

1.4

o

80 C (air ambient)

air ambient (room temperature) 1.2

0

10

20

30

40

50

60

70

80

90

1.2

0

10

Time (day)

20

30

40

50

60

70

80

90 100

Time (hour)

Figure 10. Stability of the highest-efficiency CTS TFSCs employing the CTS absorbers annealed at 600 °C: (a) when 0 stored in ambient air at room temperature without encapsulation; (b) when subjected to accelerated aging at 80 C in ambient air.

16


Page 17 of 19

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Tables Table 1 Results of the representative reports in the literature on direct solution coating of CTS thin films Precursor

Cu(OAc)2,

Solvent

Methanol

SnCl2,

Annealing

Device

conditions

configuration

o

300 C for 10

Reported properties

Ref.

-----

Eg = 1.34 eV

9

-----

p-type, electrical activation

10

min in air

thiourea o

Cu(OAc)2,

2-

550 C for 2 h

SnCl2,

methoxyet

in S vapor

thiourea

hanol,

energy = 54-250 meV.

diethanola mine Cu(OAc)2,

Methanol

SnCl2,

o

150-300 C in

-----

Eg = 1.10 to 1.34 eV, Rs = 500 to 320

the interval of

11

Ω

o

thiourea

50 C for 2 h in air

CuCl2,

Methanol

SnCl2,

o

200 C for 10

-----

Eg = 1.12 eV, p-type, σ = 0.5 S/cm, 18

min in air

1

Methanol

SnCl2,

o

200 C for 4 h

2

17

-

p = 5.7 x 10 cm , μ = 0.55 cm .V

thiourea, CuCl2,

-3

SLG/ITO/ZnO/Cu2SnS3/graphite

-1

-1

.S , TEP = + 175 μV.K

Eg = 1.12 eV, Voc = 0.816 V, Jsc = 6.14

18

2

in air

mA/cm , FF = 0.42, PCE = 2.10 %

thiourea, Cu, Sn, S

Hydrazine

powders

o

600 C for 20

Mo/Cu2SnS3/CdS/i-ZnO/AZO/Au

Eg = 0.88 eV, p-type, σ = 0.66 -1

-1

18

19

-3

min on a hot

Ω .cm , p = 4.8 x 10 cm , μ =

plate in excess

0.86 cm .V .S ,

S

Voc = 0.199, Jsc = 14.20, FF = 0.27,

2

-1

-1

PCE = 0.78 % Cu(OAc)2,

Aqueous

Rapid

SnCl2

precursor

thermal

solution

annealing at

Mo/Cu2SnS3/CdS/i-ZnO/AZO/Al

2

Voc = 0.267 V, Jsc = 19.68 mA/cm ,

This

FF = 0.34, PCE = 1.80 %

Work

o

600 C for 10 min in a S vapor atmosphere where, CuCl2 = copper(II) chloride, Cu(OAc)2 = copper(II) acetate, SnCl2 = tin(II) chloride, Eg = band gap energy, σ = conductivity, p = carrier concentration, μ = mobility, TEP = thermoelectric power, Rs = sheet resistance, Voc = open circuit voltage, Jsc = short-circuit current density, FF = fill factor, PCE = power conversion efficiency

17



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Page 18 of 19

Table 2 Photovoltaic parameters obtained from J-V and EQE measurements for the solar cells fabricated using the CTS absorbers annealed at 500, 550 and 600 °C for 10 min in an S vapor atmosphere

Sample

Voc (V)

Jsc

FF 2

(mA/cm )

PCE

Data from EQE

(%)

Eg

Jsc 2

(mA/cm )

(eV)

o

0.214

10.79

0.28

0.64

10.23

1.07

o

0.242

16.40

0.33

1.31

15.78

1.13

o

0.267

19.68

0.34

1.80

19.11

1.18

500 C 550 C 600 C

Table 3 The extracted diode parameters such as shunt conductance Gsh, series resistance Rs, ideality factor nd; and reverse saturation current density Jo from Figures 9 for solar cells fabricated using the CTS absorbers annealed at 500, 550 and 600 °C for 10 min in an S vapor atmosphere

Sample

Rs

Gsh 2

nd 2

Jo 2

(mS/cm )

(Ω.cm )

o

4.52

9.22

2.13

5.46 x 10

o

2.42

4.62

1.59

4.09 x 10

o

1.44

1.47

1.21

7.19 x 10

500 C 550 C 600 C

(mA/cm ) -5 -5

-6

18


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Table of contents (TOC)

19


A Simple Aqueous Precursor Solution Processing of Earth-Abundant

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