Solution-processed trigonal Cu2BaSnS4 thin film solar cells

Subscriber access provided by NAGOYA UNIV

Article

Solution-processed trigonal Cu2BaSnS4 thin film solar cells Zhu Chen, Kaiwen Sun, Zhenghua Su, Fangyang Liu, Ding Tang, Hanrui Xiao, Lei Shi, Liangxing Jiang, Xiaojing Hao, and Yanqing Lai ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00514 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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 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 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.

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 23 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 Energy Materials

Solution-processed trigonal Cu2BaSnS4 thin film solar cells Zhu Chen,a† Kaiwen Sun,b† Zhenghua Su,c Fangyang Liu,*a,b Ding Tang,d Hanrui Xiao,a Lei Shi,b Liangxing Jiang,a Xiaojing Hao,*b and Yanqing Lai,* a a School of metallurgy and Environment, Central South University, Changsha 410083, China b School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia c College of physics and energy, Shen Zhen University, Shenzhen 518000, China d College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China † These authors contribute equally. KEYWORDS: Cu2BaSnS4, trigonal, Solution-processed, thin film, solar cell，band tailing Corresponding Author: [email protected] (F.Liu); [email protected] (X.Hao); [email protected] (Y.Lai)

ABSTRACT: Recently, Cu2BaSnS4 (CBTS) thin film has emerged as a promising candidate for single- or multiple-junction photovoltaic (PV) due to its excellent optical and electrical properties, and earth-abundant, non-toxic constituents. In this study, a molecular solution based non-vacuum process has been employed to prepare CBTS thin films for solar cells. The obtained CBTS films show trigonal structure with band gap of 2.01 eV and p-type conductivity. The solar cell device with configuration of Mo-coated glass/CBTS/CdS/i-ZnO/ITO has achieved a power conversion efficiency (PCE) of 1.72% for CBTS with Ba/Sn atomic ratio of 1.30. An abrupt band gap cutoff in external quantum efficiency (EQE) data coupled with the very small offset (only 10 meV) in band gap between EQE and photoluminescence (PL) measurements reveals that band tailing is not the limiting factor in CBTS.

1. Introduction Photovoltaic (PV) solar cells are the most promising technology for clean and renewable electricity generation. Conventional CdTe and Cu(In,Ga)(S,Se)2 (CIGS) thin-film solar cells have demonstrated viability for clean renewable energy generation. Both of them have demonstrated high power conversion efficiencies (PCE) ACS Paragon Plus Environment

ACS Applied Energy Materials 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 2 of 23

over 22% after decades development1-2. Although CdTe and CIGS PV solar cell technologies are being commercialized, they face some uncertain factors with respect to large-scale production due to the limitation of materials supply for In, Ga and Te. The emergence of the kesterite Cu2ZnSnS4 (CZTS) opens an alternative path to earth-abundant, low-cost and environmentally-friendly PV

3-7

. Decades-long efforts

on device optimization for CZTS-based solar cells have led to remarkable progress in device performance, from the first device of 0.66% PCE 8 to the current record device of 12.6% PCE 9. However, the record efficiency of CZTS-based solar cells is still significantly lower than its theoretical efficiency limit. The low efficiency for kesterite solar cells is mainly due to the large open-circuit voltage (Voc) deficit defined as Eg/q-Voc where the Eg is the band gap of the absorber and q is the electron charge

10-13

. For CZTS solar cells, the Voc deficit can be greater than 650 mV

comparing to a typical value of 500 mV for CIGS cells

11-12

14

. Generally, the Voc is

mainly limited by band tailing which might be related with antisite disorder and the resulting potential fluctuations

15-16

. For kesterite CZTS-based materials, the small

mismatch among Cu1+(0.91Å), Sn4+ (0.83Å) and Zn2+ (0.88Å) ions and the similar structure environment for Cu and Zn, leading to easy formation of cation antisite defects which unfortunately are not shallow defects

17-20

. As a result, recent research

has focused on the minimization of cationic disordering in CZTS-related materials 21-24

. Long-term annealing of CZTS-based materials below a nominal temperature has

been explored to overcome these fundamental disordering issues

25-28

. However, the

disorder and substantial band tailing issues have not been effectively addressed. In addition, devising zinc-blende-related structures with more substantial ionic size mismatch among constituents

21-23, 29-31

has been employed to solve the disorder

problem, but the limitations from such as toxicity of element Cd, n-type semiconductor property of Ag-based composition and multiple-charge states for transition metals (M=Mn, Fe and Co) hinder its further development. Recently, Cu2BaSnS4 (CBTS) materials with a non-centrosymmetric trigonal structure (space group P31) composed of only earth-abundant and non-toxic metals 32 have been proposed with a distinguished feature of the large ionic size mismatch of ACS Paragon Plus Environment

Page 3 of 23 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


Ba2+ (1.49Å) with Cu1+ (0.91Å) and Sn4+ (0.83Å). Different from CZTS, the structure of Cu2BaSnS4 (CBTS) is a three-dimensional (3D) polyanionic [Cu2SnS4]2- network made of corner-sharing CuS4 and SnS4 tetrahedra and the polyanionic framework exhibits channels in which the large Ba2+ cation site is a distorted S8 square antiprism 33

. The ionic size mismatch, together with coordination discrimination may prevent

the formation of detrimental antisite disorder and associated band tailing

34-37

.

According to advanced theoretical studies on trigonal CBTS based compounds, similar to the high-performance CIGSSe, the dominant point defect in this CBTS system is the copper vacancy (VCu) as a shallow acceptor, with other acceptor and 35, 37

donor defects having higher formation energies Cu2BaSnS4

chalcogenide

material

. The earth-abundant trigonal

therefore

holds

promise

photoelectrochemical cell (PEC) and PV solar energy conversion

for

efficient

33, 38-39

. The first

prototype solar cell based on CBTS with 1.6% power conversion efficiency has been demonstrated by Shin et al. 34. A device based on CBTS absorber with slightly Cu poor composition was fabricated by Ge et al., yielding a PCE of 1.6% as well

40

.

Furthermore, Ge et al. employed oxygenated CdS to substitute CdS as a buffer layer, which greatly enhanced the open-circuit voltage (Voc) to 1.1 V and demonstrated a remarkable PCE of 2.03% 36. So far, the reported highest efficiency for CBTS-based solar cells is 5.2%, demonstrated by Shin et al., by using a high-quality Se-incorporated Cu2BaSnS4-xSex (x=3, CBTSSe)

41

. For PEC, the best CBTS device

has delivered a saturated photocurrent stabilized at 7.8 mA cm-2 under one sun illumination, which is comparable to the best-performing planar Cu2O PEC cells 39. The CBTS-based materials have been prepared by vacuum and non-vacuum based processes, such as solid-state 42, co-sputtering 45

33-34, 36, 38-41, 43-44

and ink-based solution

processes. However, all the reported efficient CBTS-based solar cell devices 33-34, 36,

40-41

and photoelectrochemical cells

33, 38-39, 43

were fabricated by vacuum-based

sputtering technology, and no work on efficient CBTS solar cells by non-vacuum based technology has been reported. Compared with the vacuum-based processes, non-vacuum based processes are low-cost and high-throughput, which has been used to achieve the high PCE of CZTSSe solar cells

9

and CuIn(Se,S)2 solar cells46.

ACS Paragon Plus Environment


Recently, Carrie L et al

45

Page 4 of 23

reported an ink-based solution process to prepare CBTS

materials and its alloys with thiol-amine solvent mixture, which only demonstrated partial characterizations on CBTS-based materials without characterization for optoelectronic

applications.

In

this

work,

we

firstly

demonstrated

molecular-solution-processed CBTS thin film solar cells. The thermal decomposition behavior of CBTS precursor was investigated. The phase, morphology, optical and electric properties of CBTS thin films were characterized. The highest efficiency of 1.72%

was

achieved

with

the

device

configuration

of

Mo-coated

substrate/CBTS/CdS/i-ZnO/ITO.

2. Experimental details 2.1 CBTS precursor solution preparation. The process for the preparation of Cu2BaSnS4 thin film from the mixed solution of Cu-Sn-S-contained solution and Ba-contained solution was as follows. First, 1.545 g Cu(CH3COO)2·H2O (AR), 0.902 g SnCl2·2H2O (AR) and 2.43584 g SC(NH2)2 (AR) were mixed and dissolved in 10 mL 2-methoxyethanol (AR) and stirring at 50 oC for 1h to get a transparent yellow solution A with Cu/Sn atomic ratio of 1.935 (marked as CTS). Meanwhile, Ba(CH3COO)2 (AR) was dissolved in 5 mL 2-methoxyethanol (AR) with proper lactic acid as addition agent while stirring at 70 oC for 30 minutes to get a colourless transparent solution B (marked as Ba). Solution A and solution B were then mixed together while stirring at 40 oC for 15 minutes to get a transparent light yellow precursor solution, as shown in Fig. S1. The atomic ratio of Ba/Sn was adjusted to be 1.15, 1.30 and 1.45 by controlling the Ba(CH3COO)2 concentration in B solution. Several drops of monoethanolamine were added into the mixed precursor solution to avoid cracks during spin coating process. All chemical reagents used above were purchased from Sinopharm Chemical Reagent Co., Ltd without further treatment.

2.2 CBTS thin films preparation. The obtained CBTS precursor solution was spin coated on molybdenum-coated soda-lime glass (SLG) substrates at 3000 rpm for 20 s, followed by air-annealing at 270 oC on a hot plate for 5 minutes for drying. This spin


Page 5 of 23 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


coated step was repeated 8 times to get a CBTS precursor film with desired thickness. The prepared precursor films were then sulfurized at 560-600 oC in a sulfur/N2 atmosphere for 40 minutes to form the CBTS absorbers. 2.3 CBTS solar cell device fabrication. The CBTS-based devices with the configuration of Mo-coated substrate/CBTS/CdS/i-ZnO/ITO were fabricated by a chemical bath process deposited 70 nm CdS buffer layer on CBTS absorber, followed by 50 nm intrinsic ZnO by RF magnetron sputtered, and 240 nm ITO window layer by DC magnetron sputtered sequentially. Finally, Al was thermally evaporated on the ITO layer to form top contact fingers via a shadow mask. Each device had a total area of approximately 0.2 cm2 defined by the mechanical scribing. 2.4 Characterization of materials and devices. The characterization of surface and cross-sectional morphologies of thin films was performed by FEI Quanta-200 and NOVA NanoSEM 230. The X-ray diffraction (XRD) patterns and Raman spectra were collected by using Rigaku-TTR III X and Renishaw inVia (633 nm excitation), respectively. Energy dispersive spectrometer was employed to check the element composition and its disbution by EDAX-GENSIS60S. X-Ray Photoelectron Spectroscopy (XPS, ESCALAB220i) was used to identify the elements and perform the related valence state analysis. Current density-voltage (J-V) characterization for the CBTS devices were performed using Xe-based light source solar simulator (Newport, 91160 and KEITHLEY 2400) to provide simulated AM 1.5G illumination which was calibrated with a standard Si reference cell. Photoluminescence (PL) spectra were measured using a 1/4 m monochromator (CornerstoneTM 260) equipped with a silicon charge-coupled device (CCD) camera. The continuous wave (CW) laser (405 nm, 50 mW) was used as the excitation source and the luminescence was detected by the CCD. The electrical properties were measured by the Hall effect measurement (HMS-3000/0.55T) using the van der pauw method at 300K. The external quantum efficiency (EQE) was measured using a chopped monochromator beam and lock-in amplifier, with calibration into the NIR by Si and Ge diodes measurements (QEX10 spectral response system from PV measurements, Inc.) 3. Results and discussion ACS Paragon Plus Environment


Fig. 1. Thermogravimetric analysis of the CBTS precursor powder. To get the decomposition data and then determine a suitable air-annealing temperature for CBTS, the CBTS precursor solution was dried at 80 oC for 48 hours and the obtained powder was subjected to thermogravimetric analysis (TGA). The TGA data in Fig. 1 shows the first weight loss regime before around 210 oC, representing the loss of 2-methoxyethanol solvent and other organic additives. The weight loss range from 210 oC to 300 oC is due to the thermal decomposition of metal-thiourea-oxygen complexes

47

. Metal sulfides and few metal oxides are often

formed during this thermal decomposition process, which can be converted into sulfides if treated by high temperature post-sulfurization. Above 300 oC, the weight loss is relatively slower due to the oxidation of sulfides. Therefore, the air-annealing temperature was set at 270 oC to achieve the complete thermal decomposition of the metal-thiourea-oxygen complexes without excessive oxidation during drying. The CBTS thin films on Mo-coated soda-lime glass (SLG) substrates were obtained via a post-sulfurisation treatment on the precursor films. In order to verify the valence states of Cu, Ba, Sn and S in the CBTS thin films, X-ray photoelectron spectra (XPS) measurements were performed and the results are presented in Fig. 2. The XPS peaks were calibrated by XPS line of C1s at 284.6 eV. The peak of Cu 2p is split into two


Page 6 of 23

Page 7 of 23 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


separate features at 932.9 (2p3/2) and 952.7 eV (2p1/2), with a splitting width of 19.8 eV, which is in good accordance with the value of Cu1+ 7. However, there is also a sub-peak in higher binding energy, which may correspond to Cu-related impurity phases, like CuxS.48 The peaks of Ba 3d appeared at 780.2 and 795.5 eV with a splitting value of 15.3 eV, correspond to Ba2+ 49. The peaks at binding energies of 486.4 and 494.8 eV of Sn 3d can be assigned to Sn4+ with a peak splitting of 8.4 eV 47. The S 2p peak is located at 161.8 eV, which is consistent with S2- 47. Moreover, from the XPS results of the precursor film in Fig. S2, it can be seen that all of the binding energies of Cu (Cu 2p3/2 and Cu 2p1/2), Ba (Ba 3d5/2 and Ba3/2), Sn (Sn 3d5/2 and Sn3/2) and S(2p) correspond well with the valence states of Cu+, Ba2+, Sn4+ and S2-, respectively. No other valence states, such as Cu2+ and Sn2+, can be found in precursor, suggesting no Cu2+ and Sn2+ related impurity phases present in CBTS precursor.

Fig. 2. X-ray photoelectron spectra (XPS) of Cu 2p (a), Ba 3d (b), Sn 3d (c), S 2p (d) of the sulfurized CBTS film.



Page 8 of 23

Fig. 3. X-ray diffraction (XRD) patterns (a), Raman spectra (b), absorption coefficients (c), and band gap estimation (d) of CBTS films with different Ba/Sn atomic ratios. Fig. 3a shows the X-ray diffraction patterns of CBTS films with different Ba/Sn atomic ratios of 1.15, 1.30 and 1.45. The peak of Mo film remains centered at 40.5° and multiple diffraction reflections from the trigonal Cu2BaSnS4 (P31, PDF30-0124) can be clearly observed, showing visibly intense diffraction peaks from (104) and (110) reflections, similar to the previous report by Shin et al. 34. However, the XRD patterns also reveal several visible peaks labelled with star marks which cannot be assigned to any known phases, which has been also found by other group

40

. These

non-identifiable XRD peaks might come from the diffractions of the unknown secondary phases due to the narrow chemical region of CBTS37. The CBTS film with Ba/Sn atomic ratio of 1.15 contains the most impurity phase. With the Ba/Sn atomic ratio of 1.30, CBTS film shows the best crystallinity from smallest FWHM (as shown in Table. S1) and the least impurity phases from weakest XRD peaks of unknown secondary phases. The XRD diffraction patterns (Fig. S3) of CBTS films with a Ba/Sn ACS Paragon Plus Environment

Page 9 of 23 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


atomic ratio of 1.30 sulfurized at different temperatures also match well with the trigonal CBTS, though some minor secondary phases can still be observed in all the obtained films. Overall, CBTS film with a Ba/Sn atomic ratio of 1.30 and sulfurized at 580 oC demonstrates the best crystallinity and least unknown secondary phases. Raman scattering measurement has been performed to further confirm the phases of CBTS films and the results are shown in Fig. 3b. All the films prepared with different Ba/Sn atomic ratios show four visible vibrational peaks with the dominant one at 341 cm-1, which corresponds well to trigonal CBTS 38, 40. However, no secondary phase as shown in XRD patterns can be detected in Raman spectra. Therefore, the identification and evolution of the secondary phases in CBTS materials system still needs further investigation. Films obtained from different sulfurization temperatures show similar Raman features as discussed above (see as Fig. S4). Fig. 3c shows the absorption coefficients of CBTS films with different Ba/Sn atomic ratios. It can be observed that the absorption coefficients (α) of CBTS films with Ba/Sn atomic ratios of 1.15, 1.30 and 1.45 reach 104 cm-1 at the region of photon energy larger than about 2 eV. The fundamental bandgaps of all atomic ratios of Ba/Sn are determined to be 2.01 eV from the plot of (αhv)2 versus energy for a direct semiconductor 33 in Fig. 3d. The optical properties of the films prepared from different sulfurization temperatures are shown in Fig. S5 and Fig. S6, and it is found that all films show the bandgaps of about 2.01 eV, independent of sulfurization temperature. This bandgap value suggests that the CBTS film is suitable as a top-cell absorber for the tandem solar cells 50. Fig. 4 shows the top-view scanning electron microscope (SEM) images of sulfurized CBTS films with different atomic ratios of Ba/Sn. It is observed that all samples consist of large-grain with size larger than 1µm. The film with Ba/Sn atomic ratio of 1.15 shows uncompact microstructure with some fine grains between large grains. On the contrary, the films with Ba/Sn atomic ratio of 1.30 and 1.45 show compact and uniform morphology. The corresponding cross-sectional SEM are shown in Fig. S7.The film with Ba/Sn atomic ratio of 1.30 shows dense and continuous grains which span the entire film thickness. The quality of the film with Ba/Sn atomic



Fig. 4. Top-view scanning electron microscopic (SEM) images of CBTS with Ba/Sn atomic ratios of 1.15(a), 1.30(b) and 1.45(c). ratio of 1.15 is slightly inferior due to smaller grains and some voids. The film with Ba/Sn atomic ratio of 1.45 shows obvious hierarchical structure with large grains stacking on top of fine grains, which is unbeneficial to the photocarrier transportation and increase the bulk recombination. The effect of the sulfurization temperature on film morphology was also investigated and shown in Fig. S8. For sulfurization temperature of 580 oC, the obtained film shows the most homogeneous morphology. Combining XRD and SEM results, 580 oC can be considered as the most suitable sulfurization temperature for CBTS film fabrication in our current experimental system. In order to check composition homogeneity of the prepared CBTS film, EDS elemental mapping measurements were conducted for film with Ba/Sn atomic ratio of 1.30 and from sulfurization of 580 oC. The top-view and cross-sectional elements mapping images were shown in Fig. S9 and Fig. S10, respectively. Obviously, the ACS Paragon Plus Environment

Page 10 of 23

Page 11 of 23 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


four elements Cu, Ba, Sn, and S show uniform distribution in both cases. A homogeneous elemental distribution is believed to be favorable to the CBTS thin film used in solar cell. To understand the electrical properties of CBTS material, the Hall measurements were performed at room temperature on film deposited on glass with identical process as on Mo-coated glass. As shown in Table. S2, the film with Ba/Sn atomic ratio of 1.30 and from sulfurization of 580 oC exhibits p-type semiconductor behavior, carrier density of 2.236×1014 cm-3 and high mobility of 7.223 cm2 V-1S-1, with carrier density similar to that for a typical CdTe absorber 51, but lower than that reported for CIGSSe and CZTS (~1016 cm-3). The films with Ba/Sn atomic ratios of 1.15 and 1.45 show lower carrier densities and mobilities due to the voids, fine grains and more secondary phase existing in the films.

Fig.5. Light current density-voltage (J-V) curves (a), and external quantum efficiency (EQE) spectra of CBTS thin-film solar cells (b), the bandgap derived from the EQE data of CBTS films (c), photoluminescence spectra for CBTS devices obtained with 405nm laser excitation at room temperature (d). ACS Paragon Plus Environment


Devices with configuration of glass/Mo/CBTS/CdS/i-ZnO/ITO were fabricated using CBTS absorbers with three atomic ratios of Ba/Sn. Almost no PCE was observed for device using CBTS absorber with Ba/Sn atomic ratio of 1.45 due to severe leakage current caused by poor film quality (as shown in Fig. S11). Fig.5 (a) shows the light J-V curves of CBTS solar cells using absorbers with Ba/Sn at 1.15 and 1.30, and the corresponding device parameters are given in the inset. For Ba/Sn atomic ratio of 1.30, the CBTS device generates the better performance, showing open-circuit voltage (Voc), short-circuit density (Jsc), fill factor (FF) and PCE of 697.8mV, 5.25mAcm-2, 46.9% and 1.72%, respectively. This is the first report on efficient CBTS-based thin-film solar cells from a non-vacuum solution based process. The PCE of 1.72% is comparable to those from co-sputtering process reported by Shin et al. (2.03%)

36

and Ge et al. (1.62%)

34

. The performance of device using

CBTS with Ba/Sn atomic ratio of 1.15 is lower in all electrical parameters than that with Ba/Sn atomic ratio of 1.30. This may be related to more secondary phases in CBTS absorber with Ba/Sn atomic ratio of 1.15 as discussed in XRD part. Fig. 5b displays the EQE of the CBTS solar cell devices using absorber with Ba/Sn atomic ratios of 1.15 and 1.30. The CBTS device with Ba/Sn atomic ratio of 1.30 shows higher EQE value almost in the whole measurement region, meaning better carrier collection efficiency. The integrated short-circuit current densities extracted from the EQE is 5.28 and 4.49 mA/cm2 for devices with Ba/Sn of atomic ratios 1.30 and 1.15, respectively, in good accordance with the values measured by J-V curves. A sharp decline in EQE data between 590 and 610 nm region corresponds to the band gap cutoff for the CBTS absorber and the 2.04 eV bandgap derived from the EQE data (see as Fig. 5c) is consistent with the bandgap determined from UV-vis measurements and the previous report

42

. The relatively sharp cut-off in EQE suggests that band

tailing may not be a substantial issue for CBTS. In addition to UV-vis absorption measurements, photoluminescence (PL) measurements were performed on CBTS devices to examine radiative recombination and confirm optical band gap of the CBTS films. Fig. 5d shows the PL spectra of CBTS with two atomic ratios of 1.15 and 1.30. The CBTS device with Ba/Sn atomic ACS Paragon Plus Environment

Page 12 of 23

Page 13 of 23 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


ratio of 1.30 shows stronger PL peak than that for device with Ba/Sn atomic ratio of 1.15, echoes well with its higher efficiency. For both samples, the PL peaks emitted at 604nm (corresponding to 2.05eV) are in good accordance with the bandgaps derived from the UV-vis spectroscopy measurement and EQE data with very small offset of 10 meV. The negligible bandgap difference between PL and EQE of CBTS, significantly lower than the over 100 meV from CZTS-based solar cells, further confirms very low level of band tailing in trigonal CBTS.

4. Conclusion In this work, we demonstrate the first efficient Cu2BaSnS4 thin film device by molecular solution non-vacuum method. The solution-synthesized CBTS film shows large-grained (>1µm) microstructure without pinhole defects employing only earth-abundant metals. The Cu2BaSnS4 film presents a trigonal structure with space group P31 but comprises of unknown secondary phases due to off-stoichiometric composition. This film exhibits a fundamental band gap of ~2.01 eV, high absorption coefficient of ~104 cm-1, Hall mobility of 7.223 cm2V-1S-1 and p-type conductivity. PL measurements and a sharp cut-offs in the EQE data suggest that the dominant defects in CBTS produce shallow energy levels. With a typical device configuration of Mo-coated glass/CBTS/CdS/i-ZnO/ITO, we demonstrated 1.72% efficient CBTS solar cell device by employing CBTS absorber with a Ba/Sn atomic ratio of 1.30 and sulfurized at 580 oC. Compared with CZTS, the negligible bandgap difference between PL and EQE of CBTS (about 10 meV) reveals very low level of band tailing in trigonal CBTS. These trial results suggest that earth-abundant CBTS is a promising top-cell absorber for low-cost and efficient tandem PEC water-splitting and PV devices. Our work provides a new processing method to further investigate in the future.

Supporting Information Illustration of the formation CBTS precursor solution; XPS, top-view and cross-sectional EDS mapping data for CBTS film; XRD ,Raman, absorption ACS Paragon Plus Environment


coefficient, bandgap and top-view SEM data for CBTS films sulfurized at temperatures; FWHM, cross-sectional SEM and Hall effect data for CBTS films with different Ba/Sn atomic ratio; Dark/Light J-V curve for CBTS solar cell with Ba/Sn=1.45.

Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51720105014 and 51774341) and Fundamental Research Funds of Central South University (Grant No. 2017zzts444).

References (1) Jackson, P. H., Dimitrios Wuerz, Roland Kiowski, Oliver Bauer, Andreas Friedlmeier, Theresa Magorian Powalla, Michael. Properties of Cu(In,Ga)Se2 Solar Cells with New Record Efficiencies up to 21.7%. Phys. Status Solidi RRL 2015, 9 (1), 28-31. (2) Steinmann V, B. R. E., Buonassisi T. Photovoltaics: Non-Cubic Solar Cell Materials. Nat. Photonics 2015, 9 (6), 355. (3) Englund, S. P., Valentina Primetzhofer, Daniel Ren, Yi Donzel-Gargand, Olivier Larsen, Jes K. Scragg, Jonathan Platzer Björkman, Charlotte. Characterization of TiN Back Contact Interlayers with Varied Thickness for Cu2ZnSn(S,Se)4 Thin Film Solar Cells. Thin Solid Films 2017, 639, 91-97. (4) Guo, Q., Ford, G. M., Yang, W. C., Walker, B. C., Stach, E. A., Hillhouse, H. W., Agrawal, R. Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals. J Am Chem Soc 2010, 132 (49), 17384-17386. (5) Larramona, G. B., Stéphane Jacob, Alain Choné, Christophe Muto, Takuma Cuccaro, Yan Delatouche, Bruno Moisan, Camille Péré, Daniel Dennler, Gilles. 8.6% Efficient CZTSSe Solar Cells Sprayed from Water–Ethanol CZTS Colloidal Solutions. J Phys Chem Lett 2014, 5 (21), 3763-3767. (6) Zeng, F. L., Yanqing Han, Zili Ng, Boon K. Zhang, Zhian Zhang, Hongliang Jiang, Liangxing Liu, Fangyang. Fabrication of Earth-Abundant Cu2ZnSn(S,Se)4 Light Absorbers by a Sol-Gel and Selenization Route for Thin Film Solar Cells. RSC Adv. 2015, 6 (8), 6562-6570. (7) Liu, F. S., Shanshan Zhou, Fangzhou Song, Ning Wen, Xiaoming Stride, John A. Sun, Kaiwen Yan, Chang Hao, Xiaojing. Kesterite Cu2ZnSnS4 Thin Film Solar Cells by a Facile DMF-Based Solution Coating Process. J Mater Chem C 2015, 3 (41), 10783-10792. (8) Katagiri, H. J., Kazuo Maw, Win Shwe Oishi, Koichiro Yamazaki, Makoto Araki, Hideaki Takeuchi, Akiko. Development of CZTS-Based Thin Film Solar Cells. Thin Solid Films 2009, 517 (7), 2455-2460.


Page 14 of 23

Page 15 of 23 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


(9) Wang, W. W., Mark T. Gunawan, Oki Gokmen, Tayfun Todorov, Teodor K. Zhu, Yu Mitzi, David B. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Adv Energy Mater 2014, 4 (7). (10) Gunawan, O. G., Tayfun Mitzi, David B. Suns-VOC Characteristics of High Performance Kesterite Solar Cells. J Appl Phys 2014, 116 (8), 084504. (11) Gokmen, T. G., Oki Todorov, Teodor K. Mitzi, David B. Band Tailing and Efficiency Limitation in Kesterite Solar Cells. Appl Phys Lett 2013, 103 (10), 103506. (12) Todorov, T. K. T., Jiang Bag, Santanu Gunawan, Oki Gokmen, Tayfun Zhu, Yu Mitzi, David B. Beyond 11% Efficiency: Characteristics of State-of-the-Art Cu2ZnSn(S,Se)4 Solar Cells. Adv Energy Mater 2013, 3 (1), 34-38. (13) Wang, K. G., 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 (14), 143508. (14) Niki, S. C., Miguel Repins, Ingrid Powalla, Michael Kushiya, Katsumi Ishizuka, Shogo Matsubara, Koji. CIGS Absorbers and Processes. Prog. Photovoltaics: Research and Applications 2010, 18 (6), 453-466. (15) Romero, M. J. D., H. Teeter, G. Yan, Y. F.Al-Jassim, M. M. Comparative Study of the Luminescence and Intrinsic Point Defects in the Kesterite Cu2ZnSnS4 and Chalcopyrite Cu(In,Ga)Se2 Thin Films Used in Photovoltaic Applications. Phys Rev B 2011, 84 (16). (16) Schubert, B. A. M., B. Cinque, S. Unold, T. Klenk, R. Schorr, S. Schock, H. W. Cu2ZnSnS4 Thin Film Solar Cells by Fast Coevaporation. Prog Photovoltaics 2011, 19 (1), 93-96. (17) Chen, S. Y. G., X. G. Walsh, A. Wei, S. H. Defect Physics of the Kesterite Thin-Film Solar Cell Absorber Cu2ZnSnS4. Appl Phys Lett 2010, 96 (2), 021902. (18) Walsh, A. C., S. Y. Wei, S. H. Gong, X. G. Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4. Adv Energy Mater 2012, 2 (4), 400-409. (19) Han, D. S., Y. Y. Bang, Junhyeok Zhang, Y. Y. Sun, Hong-Bo Li, Xian-Bin; Zhang, S. B. Deep Electron Traps and Origin of p-Type Conductivity in the Earth-Abundant Solar-Cell Material Cu2ZnSnS4. Phys Rev B 2013, 87 (15), 155206. (20) Yee, Y. S. M.-K., Blanka Nishi, Yoshio Bent, Stacey F. Clemens, Bruce M. Deep Recombination Centers in Cu2ZnSnSe4 Revealed by Screened-Exchange Hybrid Density Functional Theory. Phys Rev B 2015, 92 (19). (21) Su, Z. T., Joel Ming Rui Li, Xianglin Zeng, Xin Batabyal, Sudip Kumar Wong, Lydia Helena. Cation Substitution of Solution-Processed Cu2ZnSnS4Thin Film Solar Cell with over 9% Efficiency. Adv Energy Mater 2015, 5 (19), 1500682. (22) Shin, D. S., Bayrammurad Mitzi, David B. Defect Engineering in Multinary Earth-Abundant Chalcogenide Photovoltaic Materials. Adv Energy Mater 2017, 7 (11), 1602366. (23) Gershon, T. S., Kasra Gunawan, Oki Mankad, Ravin Singh, Saurabh Lee, Yun Seog Ott, John A. Kummel, Andrew Haight, Richard. Photovoltaic Device with over 5% Efficiency Based on an n-Type Ag2ZnSnSe4 Absorber. Adv Energy Mater 2016, 6 (22), 1601182. (24) Gershon, T. L., Yun Seog Antunez, Priscilla Mankad, Ravin Singh, Saurabh Bishop, Doug Gunawan, Oki Hopstaken, Marinus Haight, Richard. Photovoltaic Materials and



Devices Based on the Alloyed Kesterite Absorber (AgxCu1-x)2ZnSnSe4. Adv Energy Mater 2016, 6 (10), 1502468. (25) Rey, G. R., A. Sendler, J. Weiss, T. P. Thevenin, M. Guennou, M. El Adib, B. Siebentritt, S. The Band Gap of Cu2ZnSnSe4: Effect of Order-Disorder. Appl Phys Lett 2014, 105 (11), 112106. (26) Krämmer, C. H., Christian Zimmermann, Christian Lang, Mario Schnabel, Thomas Abzieher, Tobias Ahlswede, Erik Kalt, Heinz Hetterich, Michael. Reversible Order-Disorder Related Band Gap Changes in Cu2ZnSn(S,Se)4 via Post-Annealing of Solar Cells Measured by Electroreflectance. Appl Phys Lett 2014, 105 (26), 262104. (27) Lin, X. E., Ahmed Levcenko, Sergiu Dittrich, Thomas Kavalakkatt, Jaison Kretzschmar, Steffen Unold, ThomasLux-Steiner, Martha Ch. Defect Study of Cu2ZnSn(SxSe1−x)4 Thin Film Absorbers Using Photoluminescence and Modulated Surface Photovoltage Spectroscopy. Appl Phys Lett 2015, 106 (1), 013903. (28) Scragg, J. J. S. L., Jes K. Kumar, Mukesh Persson, Clas Sendler, Jan Siebentritt, Susanne Platzer Björkman, Charlotte. Cu–Zn Disorder and Band Gap Fluctuations in Cu2ZnSn(S,Se)4: Theoretical and Experimental Investigations. Phys. Status Solidi B 2016, 253 (2), 247-254. (29) Meng, X. D., Hongmei He, Jun Sun, Lin Yang, Pingxiong Chu, Junhao. Synthesis,Structure, Optics and Electrical Properties of Cu2FeSnS4 Thin Film by Sputtering Metallic Precursor Combined with Rapid Thermal Annealing Sulfurization Process. Mater Lett 2015, 151, 61-63. (30) Cui, Y. D., Ruiping Wang, Gang Pan, Daocheng. A General Strategy for Synthesis of Quaternary Semiconductor Cu2MSnS4 (M = Co2+, Fe2+, Ni2+, Mn2+) Nanocrystals. J Mater Chem 2012, 22 (43), 23136. (31) Gillorin, A. B., A. Marie, X. Dufour, P. Chane-Ching, J. Y. Synthesis and Optical Properties of Cu2CoSnS4 Colloidal Quantum Dots. J Mater Chem 2011, 21 (15), 5615. (32) Alekseenko, V. A., Alexey. The Abundances of Chemical Elements in Urban Soils. J Geochem Explor 2014, 147, 245-249. (33) Ge, J. Y., Yue Yan, Yanfa. Earth-Abundant Orthorhombic BaCu2Sn(SexS1–x)4(x≈ 0.83) Thin Film for Solar Energy Conversion. ACS Energy Letters 2016, 1 (3), 583-588. (34) Shin, D. S., Bayrammurad Zhu, Tong Huhn, William P. Blum, Volker Mitzi, David B. BaCu2Sn(S,Se)(4): Earth-Abundant Chalcogenides for Thin-Film Photovoltaics. Chem Mater 2016, 28 (13), 4771-4780. (35) Xiao, Z. M., Weiwei Li, Jian V. Yan, Yanfa. Distant-Atom Mutation for Better Earth-Abundant Light Absorbers: A Case Study of Cu2BaSnSe4. ACS Energy Lett 2016, 2 (1), 29-35. (36) Ge, J. K., Prakash Grice, Corey R. Roland, Paul J. Yu, Yue Tan, Xinxuan Ellingson, Randy J. Collins, Robert W. Yan, Yanfa. Oxygenated CdS Buffer Layers Enabling High Open-Circuit Voltages in Earth-Abundant Cu2BaSnS4 Thin-Film Solar Cells. Adv Energy Mater 2017, 7 (6), 1601803. (37) Hong, F. L., Wenjun Meng, Weiwei Yan, Yanfa. Trigonal Cu2-II-Sn-VI4 (II = Ba, Sr and VI = S, Se) Quaternary Compounds for Earth-Abundant Photovoltaics. Phys Chem Chem Phys 2016, 18 (6), 4828-4834.


Page 16 of 23

Page 17 of 23 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


(38) Ge, J. Y., Yue Yan, Yanfa. Earth-Abundant Trigonal BaCu2Sn(SexS1-x)4(x=0-0.55) Thin Films with Tunable Band Gaps for Solar Water Splitting. J Mater Chem A 2016, 4 (48), 18885-18891. (39) Ge, J. R., Paul J. Koirala, Prakash Meng, Weiwei Young, James L. Petersen, Reese Deutsch, Todd G. Teeter, Glenn Ellingson, Randy J. Collins, Robert W. Yant, Yanfa. Employing Overlayers to Improve the Performance of Cu2BaSnS4 Thin Film based Photoelectrochemical Water Reduction Devices. Chem Mater 2017, 29 (3), 916-920. (40) Ge, J. Y., Yanfa. Synthesis and Characterization of Photoelectrochemical and Photovoltaic Cu2BaSnS4 Thin Films and Solar Cells. J Mater Chem C 2017, 5 (26), 6406-6419. (41) Shin, D. Z., Tong Huang, Xuan Gunawan, Oki Blum, Volker Mitzi, David B. Earth-Abundant Chalcogenide Photovoltaic Devices with over 5% Efficiency Based on a Cu2BaSn(S,Se)(4) Absorber. Adv Mater 2017, 29 (24). (42) Teske C, V. O. Präparative und Röntgenographische Untersuchung am System Cu2−xAgxBaSnS4. Z Anorg Allg Chem 1976, 426 (3), 281-287. (43) Shin, D. N., Edgard Zhou, Yihao Glass, Jeffrey T. Mitzi, David B. Synthesis and Characterization of an Earth-Abundant Cu2BaSn(S,Se)4 Chalcogenide for Photoelectrochemical Cell Application. J Phys Chem Lett 2016, 7 (22), 4554-4561. (44) Ge, J. G., Corey R. Yan, Yanfa. Cu-Based Quaternary Chalcogenide Cu2BaSnS4 Thin Films Acting as Hole Transport Layers in Inverted Perovskite CH3NH3PbI3 Solar Cells. J. Mater. Chem. A 2017, 5 (6), 2920-2928. (45) McCarthy, C. L. B., Richard L. Solution Deposited Cu2BaSnS4-xSex from a Thiol-Amine Solvent Mixture. Chem Mater 2017. (46) Luo, P. L., Z.; Ding, Y.; Cheng, J. A Novel Non-Vacuum Process for the Preparation of CuIn (Se,S)2 Thin-Film Solar Cells from Air-Stable, Eco-Friendly, Metal Salts Based Solution Ink. J Power Sources 2015, 274, 22-28. (47) Su, Z. S., kaiwen Han, Zili Cui, Hongtao Liu, Fangyang Lai, Yanqing Li, Jie Hao, Xiaojing Liu, Yexiang Green, Martin. Fabrication of Cu2ZnSnS4 Solar Cells with 5.1% Efficiency Via Thermal Decomposition and Reaction of Non-toxic Sol-gel Route. J Mater Chem A 2013, 2 (2), 500-509. (48) Zhang Y, H. J., Liao C. Investigation of the Role of Sodium in Cu2ZnSnS4 Films and the Resulting Phase Evolution during Sulfurization. Crystengcomm 2016, 18 (47), 9026-9032. (49) Schmitz, P. J. Characterization of the Surface of BaSO4 Powder by XPS. Surf Sci Spectra 2001, 8 (3), 195-199. (50) Nishiwaki, S. S., S.Walk, P.Ch. Lux-Steiner, M. A Stacked Chalcopyrite Thin-Film Tandem Solar Cell with 1.2 V Open-Circuit Voltage. Prog. Photovoltaics: Research and Applications 2003, 11 (4), 243-248. (51) Sites, J. P., Jun. Strategies to Increase CdTe Solar-Cell Voltage. Thin Solid Films 2007, 515 (15), 6099-6102.




Page 18 of 23

Page 19 of 23 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


Fig. 1. Thermogravimetric analysis of the CBTS precursor powder. 206x143mm (300 x 300 DPI)



Fig. 2. X-ray photoelectron spectra (XPS) of Cu 2p (a), Ba 3d (b), Sn 3d (c), S 2p (d) of the sulfurized CBTS film.


Page 20 of 23

Page 21 of 23 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


Fig. 3. X-ray diffraction (XRD) patterns (a), Raman spectra (b), absorption coefficients (c), and band gap estimation (d) of CBTS films with different Ba/Sn atomic ratios



Fig. 4. Top-view scanning electron microscopic (SEM) images of CBTS with Ba/Sn atomic ratios of 1.15(a), 1.30(b) and 1.45(c).


Page 22 of 23

Page 23 of 23 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


Fig.5. Light current density-voltage (J-V) curves (a), and external quantum efficiency (EQE) spectra of CBTS thin-film solar cells (b), the bandgap derived from the EQE data of CBTS films (c), photoluminescence spectra for CBTS devices obtained with 405nm laser excitation at room temperature (d).


Solution-processed trigonal Cu2BaSnS4 thin film solar cells

Recommend Documents