Aging Precursor Solution in High Humidity ... - ACS Publications

Subscriber access provided by GAZI UNIV

Article

Aging precursor solution in high humidity remarkably promoted grain growth in Cu2ZnSnS4 films Zhongjie Guan, Wenjun Luo, Yao Xu, Qiuchen Tao, Xin Wen, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11397 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

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

ACS Applied Materials & Interfaces

Aging precursor solution in high humidity remarkably promoted grain growth in Cu2ZnSnS4 films Zhongjie Guan,a Wenjun Luo,*bc Yao Xu,c Qiuchen Tao,c Xin Wenc and Zhigang Zou*c a

College of Engineering and Applied Science, Nanjing University, Nanjing 210093, P. R. China b Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China c Eco-materials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, P. R. China *To whom correspondence should be addressed, E-mail: [email protected], [email protected]

Abstract Earth-abundant Cu2ZnSnS4 (CZTS) is a promising material for thin film solar cell or solar water splitting cell. Generally, large grain size and vertical penetration are highly desirable microstructures to high-efficiency solar conversion devices. Up to date, some kinds of vacuum methods have been used to prepare large grain-sized CZTS, which are expensive and limit their applications in a large scale. It is still a key challenge to prepare large-grained and vertical-penetration CZTS by a low-cost solution method. In this study, we obtained vertical-penetration CZTS thin film with 1.3 micron grain sizes by a faclie solution method. Different from previous studies, precursor solution was aged in high-humidity air before it was used to prepare CZTS films. The grain size prepared with aging precursor solution was one of the largest among the samples prepared by a solution method after sulfurizing. Moreover, the large-grained CZTS films were used as photocathodes for solar water splitting, which exhibited much higher photocurrent than those of the samples without aging. To the best of our knowledge, this is the first demonstration to promote grain growth in CZTS by aging precursor solution in high-humidity air. This aging method can offer reference to prepare other high-performance films. Keywords: Cu2ZnSnS4; grain size; aging; high-humidity; photocathode;

ACS Paragon Plus Environment


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

1.

Introduction Thin film solar cells and solar water spliting cells are promising technologies to

utilize solar energy in a large scale.1-3 In these decives, grain boundary recombination is one of key challenges to high conversion efficiency.4-6 It is very important to prepare thin films with large grain sizes (micron scale), which can reduce grain boundaries’ recombination and improve performance.7,8 Recently, Cu2ZnSnS4 (CZTS) has attracted considerable attention as a promising light-capture material for a film solar cell or a solar water splitting cell due to its optimal band gap (about 1.5 eV), abundance and non-toxicity.9-12 Nowadays, one most common method to promote grain growth is doping Na, K, Bi, or Sb into CZTS films.13-17 The precise mechanism of the metal element enhanced the CZTS grain growth remains a challenge. A likely reason is that the metal elements in CZTS film form Na, K, Bi or Sb-containing impurity phases, which act as fluxing agents to promote CZTS grain growth. However, most of preparation methods for large-grained CZTS film require high vacuum condition and are expensive, which limit their application in a large scale.6,13 A nanocrystal-ink method and some solution-based processes were explored to fabricate large-grained CZTS. However, undesirable bi-layer structures were usually observed in these methods.18-22 Especially for a nanocrystal-ink method, the films were usually consisted of a large-grain top layer and a fine-particle bottom layer due to amount of carbon residuals.18,19 Very recently, a large-grained and vertical-penetration CZTS film was obtained by a solution method, with excess Sn ratio in precursor solution.23 However, in this solution method, large-grained and vertical-penetration CZTS film was only obtained by annealing in a closed reactor in nitrogen, not sulfur, which resulted in amount of impurities and sulfur deficiencies. Therefore, it is still a key challenge to prepare large-grained and vertical-penetration CZTS with appropriate composition by a low-cost solution method. In this study, we obtained a vertical-penetration CZTS thin film with micron-scale grain sizes by a facile solution method.24 Different from previous studies, precursor solution was firstly aged in high-humidity air, which promoted grain growth of Cu2ZnSnS4 films and led to much larger grain size than that of a sample


Page 2 of 19

Page 3 of 19

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


without aging. The grain size prepared with aging precursor solution is one of the largest among the samples prepared by a solution method after sulfurizing. To the best of our knowledge, this is the first demonstration to promote grain growth in CZTS by controlling aging humidity of precursor solution. The mechanism of aging promoting CZTS grain growth was also investigated in details. A Sn segregation phase formed during aging, which evaporated and played as a fluxing agent to promote CZTS grain growth when the films were sulfurized at high temperature. Moreover, the large-grained CZTS films were used as photocathodes for solar water splitting, which exhibited much higher photocurrent than those of the samples without aging. 2. Experimental 2.1 Preparation of CZTS thin films In a typical experiment,24 precursor solution was prepared by dissolving the thiourea (CH4N2S, 1.5224 g), stannous chloride hydrate (SnCl2•2H2O, 0.4513 g), copper acetate hydrate (Cu(CH3COO)2 •H2O, 0.7187 g) and zinc chloride (ZnCl2, 0.3271 g) into 20 ml 2-methoxyethanol (C3H8O2) step-by-step. Precursor solution was aged in air with 60% humidity at 20 °C for 12 hours. Thiourea firstly function as a complex agent to react with metal ions and form thiourea-metal complexes. Another role is to provide the sulfur source when the CZTS precursor thin films were calcined in air. Thin films were prepared on Mo-coated soda lime glass by spin-coating method with 3000 rpm for 30 s, and then calcined at 400 °C for 5 minutes in air. The coating process was repeated six times to obtain an optimum thickness with about 1.3 µm. After calcined in air, the crystalline quality of the CZTS nanocrystalline thin films were still very poor, which indicated a low performance. In order to further improve the crystallinity and performance of the samples, the CZTS nanocrystalline thin films were sulfurized at 580 °C for 60 minutes in sulfur vapor with a continuous nitrogen flow (100 mL min-1) as carrier gas. In order to investigate effect of aging precursor solution in a high-humidity air, a reference sample without aging was also prepared by similar process. To exclude the thickness effect on the performance, we prepared the samples with the same thickness, but different layers. The transparent precursor solution became milky and more viscous after aging. Therefore, the coating process



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

was repeated six times for the sample with aging, while the reference sample without aging was coated for twenty-eight layers to obtain the same thickness. 2.2 Surface coating of CdS and Pt on CZTS thin films Deposition of CdS layer was carried out in a chemical bath solution at 65 °C for 7 min, which consisted of 2.5 mL thiourea (1.5 mol/L), 6.5 mL NH3·H2O (28-30%), 5 mL CdSO4 (0.015 mol/L) and 36 mL distilled water as previous reports.25,26 The CdS coated CZTS electrodes were heated at 175 °C for 60 minutes in a protective atmosphere. Pt co-catalyst was electrodeposited on CdS/CZTS in a H2PtCl6 solution (0.1 mmol/L) under visible light (λ>420 nm) irradiation. The electrodeposition potential was -0.3 V vs. SCE. 2.3 Characterization of the samples Crystal structures of the samples were determined using an X-ray diffractometer (XRD, Rigaku, Ultima III) with Cu-Kα rays. The elemental compositions and morphologies of the samples were examined with a SEM (Nova NanoSEM 230, FEI Co). Raman spectra were recorded by a Jobin Yvon HR800 Raman scattering system with excitation wavelengths of 514 and 325 nm. 2.4 Photoelectrochemical measurements Photoelectrochemical properties were measured in a Na2HPO4 aqueous solution (0.2 mol/L, pH = 10) under N2 purging. A prepared CZTS photocathode, a Pt slice and a saturated calomel electrode (SCE) were used as a working electrode, a counter and a reference electrode, respectively. A potential scan rate of 10 mV s-1 was used to measure photocurrent. Potentials in a RHE scale were calculated by the following formula: VRHE = VSCE + 0.0592pH + 0.242V. An AM 1.5G simulated sunlight (Oriel 92251A-1000, 100 mW cm-2) was used as a light source for photocurrent and stability measurement. The light intensity was calibrated by a standard Si solar cell. Incident photon-to-current efficiency (IPCE) was evaluated by a Xe lamp with monochromatic filters. The monochromatic light intensities were measured by a photometer (Newport, 840-C, USA). The Faradaic efficiencies of H2 and O2 were carried out in a sealed cell using a three-electrode system. Amount of evolved H2 and O2 gases were analyzed by chromatography (GC-8A, Shimadzu).


Page 4 of 19

Page 5 of 19

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


3.

Results and discussion

3.1 Increased grain size of CZTS via aging precursor solution in high-humidity air A transparent precursor solution was obtained by dissolving thiourea, copper, zinc, and tin containing compounds into 2-methoxyethanol step-by-step, as shown in Figure 1 (a). After exposed in 60% humidity air at 20 °C for 12 hours, the solution became milky (Figure 1 (b)). A control experiment was also carried out by aging precursor solution in low-humidity (30%) air (Figure S1 (b)). No obvious change was observed in the solution, which suggested that aging effect depended on humidity of air. The aging effect possibly came from hydrolysis reaction of metal ions in the solution. The solutions before and after aging were spin-coated on Mo-coated soda lime glass to prepare CZTS films. The solution after aging can uniformly covered the Mo-coated substrate by a spin-coating method (Figure S2). The CZTS thin films were obtained after calcined in air and sulfur vapor, respectively. Figure 2 shows surface and cross-sectional SEM images of the CZTS films prepared with and without aging precursor solutions. The thicknesses of two samples are similar. The sample with aging indicates much larger grain size (about 1.3 µm) than that of the sample (about 0.5 µm) without aging. More importantly, the CZTS grains with aging vertically penetrate the entire film, which is a highly desirable microstructure for an efficient solar conversion device. Grain boundaries are remarkably reduced by aging the precursor solution in high-humidity air. In previous studies, bi-layer structure CZTS films with about 1 micron grain sizes are usually observed in a solution method.20-22 Very recently, a CZTS film with about 2 micron grain sizes was obtained by a solution method.23 The large-grained and vertical-penetration CZTS film was only achieved by annealing in nitrogen without sulfur vapor.23 If the sample was annealed in sulfur vapor, CZTS with only about 200 nm grain size was obtained. Calcining in nitrogen at high temperature led to amount of impurities and sulfur deficiencies in the films, which indicated poor performance. Therefore, the samples were annealed in sulfur vapor in our experiments. The grain size with aging in this study is one of the largest among the samples by a solution method after sulfurizing.



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

Figure 1. Photograph of the precursor solution before (a) and after aging in air with 60% humidity at 20 °C for 12 hours (b).

Figure 2. Surface and cross-sectional SEM images of the CZTS thin films prepared with (a and c) and without aging precursor solutions (b and d). The phases of the CZTS samples prepared with and without aging precursor solution were also measured by XRD and Raman, and the results are shown in Figure S3. In Figure S3 (a), no obvious peaks were identified as secondary phases. According to our and others’ previous studies,27,28 only XRD data is not enough to determine the CZTS purity due to similar crystal structures between some binary and ternary chalcogenides, such as ZnS and Cu2SnS3, with CZTS. Therefore, visible light and UV Raman spectra of the samples were used to characterize the samples, which were shown in Figure S3 (b) and (c), respectively. From visible light Raman spectra in Figure S3 (b), three peaks at 287 cm-1, 339 cm-1, and 370 cm-1 are assigned to CZTS,27-29 and no other impurities' peaks are observed. However, from UV Raman in Figure S3 (c), three peaks at 351 cm-1, 700 cm-1, and 1051 cm-1 are assigned to ZnS phase.30 The results suggest that some ZnS impurities exist on the surface of CZTS


Page 6 of 19

Page 7 of 19

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


thin films. ZnS impurities are also usually observed in CZTS by different methods.31-33 The chemical compositions of the samples with and without aging were further characterized by EDX and the results are shown in Table S1. Two samples both exhibit Cu-poor and Zn-rich compositions, which is close to an optimum ratio in efficient CZTS-based solar conversion devices.34,35 Moreover, Sn element loss was also observed in the two CZTS films after sulfurized at a high temperature due to high vapor pressure of SnSx.36,37 In a word, the two samples with and without aging indicate similar crystal structures and compositions, but different grain sizes. 3.2 Mechanism of increased grain size of CZTS by aging Table 1 Atomic ratios in the transparent precursor solution without aging and suspended matter after aging precursor solution by EDS Samples

Atomic ratio Sn/Cu

Sn/Zn

without aging

0.57

0.83

suspended matter

1.79

3.03

In order to investigate the mechanism of increased grain size of CZTS by aging, suspended matter in the aged precursor solution was collected by centrifuging and analyzed by EDS (Table 1). In the transparent precursor solution without aging, the ratios of Sn/Cu and Sn/Zn are 0.57 and 0.83, respectively. However, after aging, the ratios of Sn/Cu and Sn/Zn in the suspended particles remarkably increase to 1.79 and 3.03, respectively. These results suggest that Sn segregate from the aged precursor solution by a hydrolysis reaction. After the thin films were calcined at 400 °C for 5 minutes in air, crystalline CZTS was obtained even without sulfuration at 580 °C (Figure S4). The non-sulfurized samples were characterized with Raman spectra and the results are shown in Figure 3. Two peaks at 335 cm-1 and 288 cm-1 are assigned to CZTS, which are in good agreement with XRD data in Figure S4. However, an additional broad peak at 312 cm-1 is observed in the non-sulfurized sample with aging, which is attributed to SnS2 phase.38 These results are also further confirmed by EDS (Table S2). Higher Sn ratio was observed in the non-sulfurized CZTS thin film with



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

aging. The SnS2 impurities possibly came from decomposition of Sn segregation at 400 °C in air as mentioned above.

Figure 3. Visible light Raman spectra (514 nm) of the non-sulfurized CZTS thin films prepared with and without aging precursor solutions. In order to understand the aging effect on the grain size of CZTS, morphologies evolution of the samples sulfurized at 580 °C for different times is indicated in Figure 4. When the samples were calcined at 580 °C, some large CZTS grains formed on the top layers of the two films. With increasing sulfuration time, large grains gradually extend from the top to the bottom layers of the films. However, for the same sulfuration time, the grain sizes of the samples with aging are much larger than those of the samples without aging. After 60 min sulfuration at 580 °C, there are amount of grain boundaries in the sample without aging, while large grains vertically penetrate the entire CZTS film with aging and much less grain boundaries exist. Moreover, even though the sample without aging was sulfurized at 580 °C for a longer time to 80 min or 120 min, the grain sizes still did not increase obviously (Figure S5). The result suggests that aging precursor solution in high-humidity air is necessary for a large-grained CZTS thin film. Figure 5 shows XRD patterns of the CZTS thin films sulfurized at 580 °C with different times. From Figure 5 (a), only CZTS, no other impurities, was detected in the sample without aging after different sulfuration times. With increasing time, the XRD peaks become stronger, which suggests that the crystal size of CZTS become larger. Though Raman peaks of SnS2 were observed in the non-sulfurized sample after


Page 8 of 19

Page 9 of 19

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


aging, no XRD peaks were not detected in the samples after 0 min sulfuration at 580 °C due to poor crystallinity of SnS2. However, strong diffraction peak of SnS2 was observed in the aged sample after sulfurized at 580 °C for 10 min, and the peaks disappear after sulfurized for 60 min. The results suggest that SnS2 intermediate products evaporate at a high temperature due to its high vapor pressure, which play a key role as a fluxing agent and promote the grain growth of CZTS. According to above results, we propose a mechanism to understand promoting grain growth by aging precursor solution (Figure 6). Sn segregation formed when the precursor solution was aged in high-humidity air. After the samples were calcined at 400 °C in air, SnS2 intermediate phase formed. During sulfuration at 580°C, SnS2 plays as a fluxing agent to promote CZTS grain growth, which were also observed in previous studies.23,39,40 After sulfurized, we obtained vertical-penetration CZTS thin film with micron-scale grain sizes by aging precursor solution in a high-humidity air.

Figure 4. Cross-sectional SEM images of the CZTS thin films sulfurized at 580 °C for different times. When sulfuration temperature reaches 580 °C, the sample is taken out by quenching to room temperature, which is assigned to 0 min sulfuration. (a)-(c) samples prepared without aging the precursor solution, (d)-(f) samples prepared with aging the precursor solution.



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

Figure 5. XRD patterns of the CZTS thin films sulfurized at 580 °C for different times. (a) CZTS samples prepared without aging the precursor solution; (b) CZTS samples prepared with aging the precursor solution.

Figure 6. Mechanism of increased grain size of CZTS by aging precursor solution in high-humidity air. 3.3 Enhanced photoelectrochemical performance in a large grain-sized CZTS photocathode with aging In order to indicate advantages of the large grain-sized CZTS films after aging, we compared the photoelectrochemical properties of the as-grown samples. Figure S6 shows photocurrents of the CZTS photocathodes prepared with and without aging precursor solutions. The sample with aging yields about 3 times higher photocurrent than that of the sample without aging. Since the thicknesses of the two samples are close, reducing grain boundaries’ recombination plays a key role for higher performance in the sample with aging. In order to further improve the photoelectrochemical performance of CZTS photocathodes, an n-type CdS layer were


Page 10 of 19

Page 11 of 19

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


usually employed to load on the surface of CZTS, which could form p-n heterojunction and facilitate charge separation.41,42 Moreover, Pt was also used as a co-catalyst to promote a hydrogen evolution reaction.43 Figure 7 indicates morphologies of the Pt/CdS/CZTS photocathodes with and without aging precursor solutions. A compact CdS layer and Pt nano-particles are uniformly covered on the CZTS films.

Figure 7. Surface and cross-sectional SEM images of Pt/CdS/CZTS photocathodes prepared with (a and c) and without (b and d) aging the precursor solutions. Figure 8 (a) shows photocurrents of the Pt/CdS/CZTS photocathodes prepared with and without aging the precursor solutions under AM 1.5G simulated sunlight (100 mW cm-2) irradiation. Thus, solar photocurrents of the Pt/CdS/CZTS photocathodes are enhanced after surface modification. A solar photocurrent of 1.2 mA cm-2 at 0 VRHE was obtained on the large-grained Pt/CdS/CZTS photocathode. After Pt/CdS coating, the photocurrent of the CZTS sample after aging is still much higher than that of the sample without aging. Figure 8 (b) shows IPCE spectra of the two Pt/CdS/CZTS photocathodes at 0 VRHE. The IPCE of the Pt/CdS/CZTS photocathode after aging is close to 6% at a range of 400-600 nm, which is about 3 times higher than that of the sample without aging. The photo-response wavelength is about 900 nm, which is in good agreement with the absorption edge of CZTS. Photostability and Faradaic efficiencies of H2 and O2 on a the large-grained Pt/CdS/CZTS photocathode were also measured and the results are indicated in



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

Figure 8(c) and (d), respectively. Photocurrent increased during the first hour testing, which possibly came from excess Pt co-catalyst peeled off. The Faradaic efficiencies of H2 and O2 are about 98.5% and 87.1%, which suggests that the photocurrent comes from proton reduction, not self-corrosion of the CZTS photocathode.

Figure 8. (a) Photocurrents of the Pt/CdS/CZTS photocathodes prepared with and without aging precursor solutions. Electrolyte: 0.2mol L-1 Na2HPO4 (pH = 10), light source: AM 1.5G simulated sunlight (100 mW cm-2). The scan rate is 10 mV s-1. (b) Incident photon to current efficiency (IPCE) spectra at 0 VRHE of the Pt/CdS/CZTS photocathodes. (c) Photocurrent-time curve of the Pt/CdS/CZTS photocathode prepared with aging the precursor solution at 0 VRHE. Light source: AM 1.5G simulated sunlight (100 mW cm-2). (d) Time courses of evoluted hydrogen and oxygen on the large-grained Pt/CdS/CZTS photocathode with aging. Light source: AM 1.5G simulated sunlight (100 mW cm-2). The theoretical number of hydrogen and oxygen molecules is denoted by e-/2 and e-/4 respectively. 4. Conclusion Aging precursor solution in a high-humidity air remarkably promoted the gain


Page 12 of 19

Page 13 of 19

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


growth of CZTS by a solution method and a vertical-penetration CZTS film with 1.3 micron grain sizes was obtained. The aging process led to Sn segregation from the precursor solution. When the sample was calcined in air, SnS2 intermediate product formed, which evaporated and played as a fluxing agent to promote CZTS grain growth during sulfuration at high temperature. Moreover, the CZTS films were used as photocathodes for solar water splitting. The large grain-sized sample with aging indicates about 3 times higher photocurrent than that of the sample without aging. After CdS and Pt coating, a solar photocurrent of 1.2 mA cm-2 at 0 VRHE was obtained on the large-grained Pt/CdS/CZTS photocathode. This aging method is novel and facile, which is going to be tried on other semiconductor films. Acknowledgements This work is supported by the National Basic Research Program of China (973 Program, 2013CB632404 and 2014CB239303), the National Natural Science Foundation of China (51272101), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Natural Science Foundation of Jiangsu Province of China (No. 15KJB150010), the Open Research Fund of Key Laboratory for Organic Electronics & Information Displays, and the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology of Nanjing University. References (1) Bailie, C. D.; Christoforo, M. G.; Mailoa, J. P.; Bowring, A. R.; Unger, E. L.;

Nguyen,W. H.; Burschka, J.; Pellet, N.; Lee, J. Z.; Grätzel, M.; Noufi, R.; Buonassisi, T.; Salleo, A.; McGehee, M. D. Semi-Transparent Perovskite Solar Cells for Tandems with Silicon and CIGS. Energy Environ. Sci. 2015, 8, 956-963. (2) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (3) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338-344. (4) Yin, W.; Wu, Y.; Wei, S.; Noufi, R.; Al-Jassim, M. M.; Yan, Y. Engineering Grain Boundaries in Cu2ZnSnSe4 for Better Cell Performance: A First-Principle Study. Adv. Energy Mater. 2014, 4, 1300712.



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

(5) Li, J.; Mitzi, D. B.; Shenoy, V. B. Structure and Electronic Properties of Grain Boundaries in Earth-Abundant Photovoltaic Absorber Cu2ZnSnSe4. ACS Nano 2011, 5, 8613-8619. (6) Gershon, T.; Shin, B.; Bojarczuk, N.; Hopstaken, M.; Mitzi, D. B.; Guha, S. The Role of Sodium as a Surfactant and Suppressor of Non-Radiative Recombination at Internal Surfaces in Cu2ZnSnS4. Adv. Energy Mater. 2015, 5, 1400849. (7) Lee, Y. S.; Gershon, T.; Gunawan, O.; Todorov, T. K.; Gokmen, T.; Virgus, Y.; Guha, S. Cu2ZnSnSe4 Thin-Film Solar Cells by Thermal Coevaporation with 11.6% Efficiency and Improved Minority Carrier Diffusion Length. Adv. Energy Mater. 2014, 1401372. (8) Shin, B.; Gunawan, O.; Zhu, Y.; Bojarczuk, N. A.; Chey, S. J.; Guha, S. Thin Film Solar Cell with 8.4% Power Conversion Efficiency Using an Earth-Abundant Cu2ZnSnS4 Absorber. Prog. Photovolt: Res. Appl. 2013, 21, 72-76. (9) Katagiri, H.; Sasaguchi, N.; Hando, S.; Hoshino, S.; Ohashi, J.; Yokota, T. Preparation and Evaluation of Cu2ZnSnS4 Thin Films by Sulfurization of E-B Evaporated Precursors. Sol. Energy Mater. Sol. Cells 1997, 49, 407-414. (10) Ki, W.; Hillhouse, H. W. Earth-Abundant Element Photovoltaics Directly from Soluble Precursors with High Yield Using a Non-toxic Solvent. Adv. Energy Mater. 2011, 1, 732-735. (11) Wen, X.; Luo, W.; Zou, Z. Photocurrent Improvement on Nanocrystalline Cu2ZnSnS4 Photocathodes by Introducing Porous Structure. J. Mater. Chem. A 2013, 1, 15479-15485. (12) Jiang, F.; Gunawan; Harada, T.; Kuang, Y.; Minegishi, T.; Domen, K.; Ikeda, S. Pt/In2S3/CdS/Cu2ZnSnS4 Thin Film as an Efficient and Stable Photocathode for Water Reduction Under Sunlight Radiation. J. Am. Chem. Soc. 2015, 137, 13691-13697. (13) Johnson, M.; Baryshev, S. V.; Thimsen, E.; Manno, M.; Zhang, X.; Veryovkin, I. V.; Leighton, C.; Aydil, E. S. Alkali-Metal-Enhanced Grain Growth in Cu2ZnSnS4 Thin Films. Energy Environ. Sci. 2014, 7, 1931-1938. (14) Prabhakar, T.; Jampana, N. Effect of Sodium Diffusion on the Structural and Electrical Properties of Cu2ZnSnS4 thin films. Sol. Energy Mater. Sol. Cells 2011, 95,


Page 14 of 19

Page 15 of 19

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


1001-1004. (15) Carrete, A.; Shavel, A.; Fontané, X.; Montserrat, J.; Fan, J.; Ibáñez, Maria; Saucedo, E.; Pérez-Rodríguez, A.; Cabot, A. Antimony-Based Ligand Exchange to Promote Crystallization in Spray-Deposited Cu2ZnSnSe4 Solar Cells. J. Am. Chem. Soc. 2013, 135, 15982-15985. (16) Guo, H.; Cui, Y.; Tian, Q.; Gao, S.; Wang, G.; Pan, D. Significantly Enhancing Grain Growth in Cu2ZnSn(S,Se)4 Absorber Layers by Insetting Sb2S3, CuSbS2, and NaSb5S8 Thin Films. Cryst. Growth Des. 2015, 15, 771-777. (17) Tong, Z.; Zhang, K.; Sun, K.; Yan, C.; Liu, F.; Jiang, L.; Lai, Y.; Hao, Xi.; Li, J. Modification of Absorber Quality and Mo-Back Contact by a Thin Bi Intermediate Layer for Kesterite Cu2ZnSnS4 Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 144, 537-543. (18) Chernomordik, B. D.; Béland, A. E.; Deng, D. D.; Francis, L. F.; Aydií, E. S. Microstructure Evolution and Crystal Growth in Cu2ZnSnS4 Thin Films Formed by Annealing Colloidal Nanocrystal Coatings. Chem. Mater. 2014, 26, 3191-3201. (19) Liu, X.; Zhou, F.; Song, N.; Huang, J.; Yan, C.; Liu, F.; Sun, K.; Stride, J. A.; Hao, X.; Green, M. A. Exploring the Application of Metastable Wurtzite Nanocrystals in Pure-Sulfide Cu2ZnSnS4 Solar Cells by Forming Nearly Micron-Sized Large Grains. J. Mater. Chem. A 2015, 3, 23185-23193. (20) Zhang, K.; Su, Z.; Zhao, L.; Yan, C.; Liu, F.; Cui, H.; Hao, X.; Liu, Y. Improving the Conversion Efficiency of Cu2ZnSnS4 Solar Cell by Low Pressure Sulfurization. Appl. Phys. Lett. 2014, 104, 141101. (21) Maeda, K.; Tanaka, K.; Fukui, Y.; Uchiki, H. Influence of H2S Concentration on the Properties of Cu2ZnSnS4 Thin Films and Solar Cells Prepared by Sol-Gel Sulfurization. Sol. Energy Mater. Sol. Cells 2011, 95, 2855-2860. (22) Tunuguntla, V.; Chen, W.; Shih, P.; Shown, I.; Lin, Y.; Hwang, J.; Lee, C.; Chen, L.; Chen, K. A Nontoxic Solvent Based Sol-Gel Cu2ZnSnS4 Thin Film for High Efficiency and Scalable Low-Cost Photovoltaic Cell. J. Mater. Chem. A 2015, 3, 15324-15330. (23) Sutter-Fella, C. M.; Uhl, A. R.; Romanyuk, Y. E.; Tiwari, A. N. Large-Grained



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

Cu2ZnSnS4 Layers Sintered From Sn-Rich Solution-Deposited Precursors. Phys. Status Solidi A 2015, 212, 121-125. (24) Su, Z.; Sun, K.; Han, Z.; Cui, H.; Liu, F.; Lai, Y.; Li, J.; Hao, X.; Liu, Y.; Green, M. A. Fabrication of Cu2ZnSnS4 Solar Cells with 5.1% Efficiency via Thermal Decomposition and Reaction Using a Non-Toxic Sol-Gel Route. J. Mater. Chem. A 2014, 2, 500-509. (25) Contreras, M. A.; Romero, M. J.; To, B.; Hasoon, F.; Noufi, R.; Ward, S.; Ramanathan, K. Optimization of CBD CdS Process in High-Efficiency Cu(In,Ga)Se2Based Solar Sells. Thin Solid Films 2002, 403-404, 204-211. (26) Guan, Z.; Luo, W.; Feng, J.; Tao, Q.; Xu, Y.; Wen, X.; Fu, G.; Zou, Z. Selective Etching of Metastable Phase Induced an Efficient CuIn0.7Ga0.3S2 Nano-photocathode for Solar Water Splitting. J. Mater. Chem. A 2015, 3,7840-7848. (27) Guan, Z.; Luo, W.; Zou, Z. Formation Mechanism of ZnS Impurities and Their Effect on

Photoelectrochemical Properties on

a Cu2ZnSnS4 Photocathode.

CrystEngComm 2014, 16, 2929-2936. (28) Fontané, X.; Calvo-Barrio, L.; Izquierdo-Roca, V.; Saucedo, E.; Pérez-Rodriguez, A. In-depth Resolved Raman Scattering Analysis for the Identification of Secondary Phases: Characterization of Cu2ZnSnS4 Layers for Solar Cell Applications. Appl. Phys. Lett. 2011, 98, 181905. (29) Khare, A.; Himmetoglu, B.; Johnson, M.; Norris, D. J.; Cococcioni, M. Calculation of the Lattice Dynamics and Raman Spectra of Copper Zinc Tin Chalcogenides and Comparison to Experiments. J. Appl. Phys. 2012, 111, 083707. (30) Fairbrother, A.; García-Hemme, E.; Izquierdo-Roca, V.; Fontané, X.; Pulgarín-Agudelo, F. A.; Vigil-Galán, O.; Pérez-Rodríguez, A.; Saucedo, E. Development of a Selective Chemical Etch to Improve the Conversion Efficiency of Zn-Rich Cu2ZnSnS4 Solar Cells. J. Am. Chem. Soc. 2012, 134, 8018-8021. (31) Fairbrother, A.; Fontané, X.; Izquierdo-Roca, V.; Espíndola-Rodríguez, M.; López-Marino, S.; Placidi, M.; Calvo-Barrio, L.; Pérez-Rodríguez, A.; Saucedo. E. On the Formation Mechanisms of Zn-Rich Cu2ZnSnS4 Films Prepared by Sulfurization of Metallic Stacks. Sol. Energy Mater. Sol. Cells 2013,112, 97-105.


Page 16 of 19

Page 17 of 19

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


(32) Vigil-Galán, O.; Espíndola-Rodríguez, M.; Courel, M.; Fontané, X.; Sylla, D.; Izquierdo-Roca, V.; Fairbrother, A.; Saucedo, E.; Pérez-Rodríguez, A. Secondary Phases Dependence on Composition Ratio in Sprayed Cu2ZnSnS4 Thin Films and Its Impact on the High Power Conversion Efficiency. Sol. Energy Mater. Sol. Cells 2013, 117, 246-250. (33) Wang, J.; Zhang, P.; Song, X.; Gao, L. Surfactant-Free Hydrothermal Synthesis of Cu2ZnSnS4 (CZTS) Nanocrystals with Photocatalytic Properties. RSC Adv. 2014, 4, 27805-27810. (34) Hsu, C.; Duan, H.; Yang, W.; Zhou, H.; Yang, Y. Benign Solutions and Innovative Sequential Annealing Processes for High Performance Cu2ZnSn(Se,S)4 Photovoltaics. Adv. Energy Mater. 2014, 4, 1301287. (35) Wang, K.; Gunawan, O.; Todorov, T.; Shin, B.; Chey, S. J.; Bojarczuk, N. A.; Mitzi, D.; Guha, S. Thermally Evaporated Cu2ZnSnS4 Solar Cells. Appl. Phys. Lett. 2010, 97, 143508. (36) Redinger, A.; Berg, D. M.; Dale, P. J.; Siebentritt, S. The Consequences of Kesterite Equilibria for Efficient Solar Cells. J. Am. Chem. Soc. 2011, 133, 3320-3323. (37) Scragg, J. J.; Ericson, T.; Kubart, T.; Edoff, M.; Platzer-Björkman, C. Chemical Insights into the Instability of Cu2ZnSnS4 Films During Annealing. Chem. Mater. 2011, 23, 4625-4633. (38) Price, L. S.; Parkin, I. P.; Hardy, A. M. E.; Clark, R. J. H. Atmospheric Pressure Chemical Vapor Deposition of Tin Sulfides (SnS, Sn2S3, and SnS2) on Glass. Chem. Mater. 1999, 11, 1792-1799. (39) Schurr, R.; Hölzing, A.; Jost, S.; Hock, R.; Voβ, T.; Schulze, J.; Kirbs, A.; Ennaoui, A.; Lux-Steiner, M.; Weber, A.; Kötschau, I.; Schock, H.-W. The Crystallisation of Cu2ZnSnS4 Thin Film Solar Cell Absorbers from Co-Electroplated Cu-Zn-Sn Precursors. Thin Solid Films 2009, 517, 2465-2468. (40) Malerba, C.; Leonor, C.; Ricardo, A.; Valentini, M.; Biccari, F.; Müller, M.; Rebuffi, L.; Esposito, E.; Mangiapane, P.; Scardi, P.; Mittiga, A. Stoichiometry Effect on Cu2ZnSnS4 Thin Films Morphological and Optical Properties. J. Renew. Sust.



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

Energy 2014, 6, 011404. (41) Moriya, M.; Minegishi, T.; Kumagai, H.; Katayama, M.; Kubota, J.; Domen, K. Stable Hydrogen Evolution from CdS-Modified CuGaSe2 Photoelectrode under visible-Light Irradiation. J. Am. Chem. Soc. 2013, 135, 3733-3735. (42) Rovelli, L.; Tilley, S. D.; Sivula, K. Optimization and Stabilization of Electrodeposited Cu2ZnSnS4 Photocathodes for Solar Water Reduction. ACS Appl. Mater. Interfaces 2013, 5, 8018-8024. (43) Jacobsson, T. J.; Platzer-Björkman, C.; Edoff, M.; Edvinsson, T. CuInxGa1-xSe2 as an Efficient Photocathode for Solar Hydrogen Generation. Int. J. Hydrogen Energy 2013, 38, 15027-15035.


Page 18 of 19

Page 19 of 19

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


Table of Contents (TOC) graphic Aging precursor solution in high humidity air lead to much larger CZTS grain size than that of the sample without aging.


Aging Precursor Solution in High Humidity ... - ACS Publications

Recommend Documents