Incorporation of Rb cations into Cu2FeSnS4 thin films

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May 19, 2017 - Alkali element Rb was incorporated into CFTS thin ... is uniform in the films after sulfurization, Rb ions incorporation in CFTS thin films could ...
Materials Letters 202 (2017) 36–38

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Incorporation of Rb cations into Cu2FeSnS4 thin films improves structure and morphology Shuo Wang a,b, Ruixin Ma a,c,⇑, Chengyan Wang a,⇑, Shina Li a, Hua Wang b a

School of Metallurgy and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China c Beijing Key Laboratory of Special Melting and Preparation of High-End Metal Materials, Beijing 100083, PR China b

a r t i c l e

i n f o

Article history: Received 1 May 2017 Received in revised form 18 May 2017 Accepted 19 May 2017 Available online 19 May 2017 Keywords: Cu2FeSnS4 Crystal growth Thin film Sulfurization

a b s t r a c t Quaternary chalcogenide Cu2FeSnS4 (CFTS) thin film, a potential material for application as absorber layer in thin film solar cells (TFSCs), dye-sensitized solar cell and photocatalytic, are successfully synthesized by using a convenient blade-coating method. Alkali element Rb was incorporated into CFTS thin films in order to further improve the structure and surface morphology. X-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy were used to characterize the phase purity, morphology and composition of CFTS particles and thin films. It showed that the diffusion of Rb ions is uniform in the films after sulfurization, Rb ions incorporation in CFTS thin films could improve the grain size and surface morphology. All the thin films became smooth and closely packed surface, the thickness of the incorporated films is around 1 lm. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction In research arena of photovoltaics, achievement of high power conversion efficiency has not diluted the urge of fabricating devices with newer materials, which should be composed of earthabundant, atoxic and inexpensive elements. Cu2ZnSnS4 (CZTS), the typical quaternary chalcogenide, is widely used in thin film solar cell and the power conversion efficiencies (PCE) of Cu2ZnSn(S1 xSex)4 thin film solar cells have been reported to be as high as 12.6% [1]. Recently, incorporating Fe, Co, Ni, and Mn magnetic ions into the Cu2SnS3 lattice may lead to magnetism and form the multifunctional Cu2XSnS4 (Co2+, Fe2+, Ni2+, Mn2+) compounds, their electronic properties and crystal structures are analogous to those of the parent CIGS, while their constituent elements are naturally copious and nontoxic [2]. Cu2FeSnS4 has been considered to be a promising candidate in a variety of light harvesting devices for commercial application, such as thin film solar cell [3], dye-sensitized solar cell (DSSC) [4] and photocatalytic [5]. Following the DFT calculation of the similar crystals (where Fe is replaced by Zn) principal role in the optoelectronic features and their applications play Sn-S polyhedral and replacement of Zn by Fe may only change the polarizability [6].

⇑ Corresponding authors at: School of Metallurgy and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China (R. Ma). E-mail addresses: [email protected] (R. Ma), [email protected] (C. Wang). http://dx.doi.org/10.1016/j.matlet.2017.05.079 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.

However, the reported experimental data for CFTS thin film solar cell, achieved by successive ionic layer adsorption and reaction (SILAR) method and sputtering method, are only about 1.37% [3] and 0.1% [7]. This huge gap between the theoretical and experimental PCE can be attributed to a number of factors, such as the complicated nature of the bulk, surface morphology, intrinsic defects and difficulty in producing thin film without impurity phase [8]. Improving the surface morphology and intrinsic defects is a very effective way to increase the PCE of CFTS solar cells. The oxidation-stable Rb+ can be embedded into a cation cascade to improve the surface morphology with excellent material property [9]. It is well known that the grain size is one of the important factors affecting the surface morphology and defects, increasing the grain size could increase the carrier diffusion length and reduce the carrier recombination at grain boundaries, which contributes to improving the conversion efficiency of polycrystalline solar cells. Rb-ion incorporation has received increasing attention in the improvement of the solar cells.

2. Experimental In a typical process, 4 mmol CuCl, 2 mmol FeSO4, 2 mmol SnCl4, and 10 mmol thiourea were ground in agate mortar. All reagents were of analytical grade and used without further purification purchased from Aladdin. The obtained mixture with 40 ml DMF was transferred into a 50 mL Teflon-lined stainless autoclave,

S. Wang et al. / Materials Letters 202 (2017) 36–38

sealed, maintained at 240 °C for 24 h. The obtained precipitates were centrifuged with deionized water and absolute ethanol. Then mixing 2 g CFTS powders, 5 ml Oleylamine, 1 g Ethyl cellulose ethoce and 8 ml Terpinol were mixed stirred as the organic solvent. The ink was cast into CFTS films by a doctor-blade method onto Mo-sputtered soda-lime glass substrates (15  15  2 mm3). Following the fabrication of the CFTS films, the films were annealed in an Ar atmosphere in a quartz tube furnace at 350 °C for 40 min to remove the organic substance. Then rubidium chloride (RbCl, 99.0%) dissolved in deionized water is synthesized to incorporate doped material into CFTS. The CFTS films has been soaked in the prepared RbCl solution for a time of 10 min, The molarity of each element-chloride was varied in the range 0.1– 0.3 M with a step of 0.1 M. Then the films were sulfurized to form CFTS films by annealing under S vapor at 520 °C for 30 min followed by cooling in air. The heating rate was 10 °C/min. X-ray diffraction (XRD) patterns of the samples coated onto Mocoated substrates were obtained using a Rigaku X-ray diffractometer at 40 kV and 40 mA using Cu-K radiation (k = 0.15405 nm). Raman spectroscopy measurements were conducted at room temperature in the quasi-back scattering geometry by using the 532 nm line of an Ar laser as the excitation. The field emission scanning electron microscopy (FESEM) images of the experimental samples were obtained on a ZEISS SUPRA55 using a 15 kV acceleration voltage. Energy dispersive spectrometry (EDS, Thermo-NS7, manufacturer) was used to determine the elemental composition. Absorption spectra of the samples were obtained using a UV2550 from Shimadzu Corporation.

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spectrum was used to show that Rb was incorporated in CFTS thin films successfully. Raman spectroscopy measurements were preformed on the samples To probe the possible secondary phases in the films. Raman spectrum in Fig. 2, which is in agreement with published reports, showed a prominent peak at 283, 318, 338 and 372 cm 1 [10].

Fig. 2. Raman spectrum of CFTS NCs and CFTS thin films treated with different concentration (0, 0.1 M, 0.2 M, 0.3 M).

3. Results and discussion XRD patterns of all samples are shown in Fig. 1. The XRD patterns of all samples exhibit major peaks corresponding to diffraction peaks of the stannite structure of CFTS (JCPDS Card Number 44-1476). The patterns showed that the Cu2-XS was discovered in CFTS NCs, However, XRD data alone are unable to accurately distinguish the presence of CuSx, CuFeS2 and Cu2SnS3 phases due to the overlapping diffraction peaks with CFTS in the XRD patterns [7]. No distinct peaks of secondary phases were observed in the XRD pattern of thin films apart from the peaks of Mo (JCPDS Card Number 42-1120). The diffraction peaks from (1 1 2), (2 2 0) and (3 1 2) planes were observed clearly in all samples indicating the formation of well crystallized CFTS phase. Furthermore no secondary phase with Rb-ion peak has been observed in the pattern, Raman

Fig. 1. X-ray diffraction of CFTS NCs and thin films treated with different concentration (0, 0.1 M, 0.2 M, 0.3 M).

Fig. 3. SEM images and the cross-sectional images of CFTS films with different concentration: (a) and (b) 0 M, (c) and (d) 0.1 M, (e) and (f) 0.2 M, (g) and (h) 0.3 M.

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S. Wang et al. / Materials Letters 202 (2017) 36–38

Table 1 Compositions of the samples determined by EDS. Sample

Cu (at.%)

Fe (at.%)

Sn (at.%)

S (at.%)

Rb (at.%)

CFTS NCs 0 0.1 0.2 0.3

28.42 27.03 26.36 27.47 27.75

10.83 12.29 11.87 10.61 10.82

12.03 12.47 12.79 11.83 11.37

48.72 48.21 48.07 48.37 47.45

0 0 0.91 1.72 2.61

These peaks are contributed to CFTS, the small peak at 217 cm 1 is contributed to SnS2. The peak refers to the strongest asymmetric vibration of a pure anion mode of sulfur atom around tin and also confirms phase purity of the material. Since Raman scattering is a surface sensitive technique, it is expected that changes in Raman peak positions was caused mainly by the anion composition variation. Furthermore, full-width-at-half-maximum (FWHM) of peaks was the same for the most intense CFTS peak (318 cm 1) of samples. It indicated the absence of significant chemical disorder variation compared to the CFTS with no incorporation. The SEM images of the surface morphology and the crosssectional image of all the samples are showed in Fig. 3. The surface defects and intrinsic defects are the main factors to influence the material properties. The high concentration of VCu, CuFe and FeCu intrinsic defects, their compensation and the local deviations in the distribution of these defects introduce potential fluctuations in the band structure. These fluctuations would lead to a decrease in the effective band gap energy Eg and a large Voc deficit in solar cells [8]. In Fig. 3, the surface images of the four samples exhibited different density and crystalline texture. The results showed that large grain size and well-defined boundaries in the films were achieved with the increasing of solution concentration. A closely packed absorber film can effectively absorb the solar spectrum and reduce recombination at the back contact. The thickness of the film was found to be only 0.8 lm in (b) whereas the thickness of the films was about 1 lm in (d), (f) and (h). The composition (atomic weight %) of CFTS thin-films was determined by EDS spectrum and the results were summarized in Table 1. The Rb atomic weights in the samples were observed to be 0.91%, 1.72% and 2.62% on the top of the films, and 0.81%, 1.51% and 2.31% at the bottom, it indicated that the content of Rb-ion incorporation in CFTS thin films was acceptable and the diffusion of Rb ions was uniform. The compositions of thin films are almost the same with each other indicating that the RbCl concentration has little impact on the composition of thin films during the grain growth of CFTS. 4. Conclusion In conclusion, CFTS thin films were fabricated by using a solvothermal synthesis and blade-coating process followed by

sulfurization. The composition of the powder was Cu-rich and Fe-poor. The effects of Rb elements on the properties of CFTSbased solar cells have been studied. These doped ions were incorporated in the CFTS compounds by soaking the CFTS thin films in a solution containing these elements prior to sulfurization annealing. The FESEM images of the films showed that the diffusion of Rb ions was uniform after sulfurization. And the incorporation of Rb element was found to improve the grain size and the quality of the films Acknowledgments This work was supported by National Nature Science Foundation of China (No. 51674026) and Fundamental Research Funds for the Central Universities (FRF-BD-15-004A) in 2015. References [1] W. Wang, Mark T. Winkler, T. Gokmen, Device characteristics of CZTSSe thinfilm solar cells with 12.6% efficiency, Adv. Energy Mater. 4 (7) (2014) 403–410. [2] F. Ozel, Earth-abundant quaternary semiconductor Cu2MSnS4 (M = Fe Co, Ni and Mn) nanofibers: fabrication, characterization and band gap arrangement, J. Alloys Compd. 657 (2016) 157–162. [3] A.J.P. Soumyo Chatterjee, A solution approach to p-type Cu2FeSnS4 thin-films and pn-junction solar cells: Role of electron selective materials on their performance, Solar Energy Mater. Solar Cells 160 (2017) 233–240. [4] K. Mokurala, S. Mallick, Effect of annealing atmosphere on quaternary chalcogenide-based counter electrodes in dye-sensitized solar cell performance: synthesis of Cu2FeSnS4 and Cu2CdSnS4 nanoparticles by thermal decomposition process, RSC Adv. 7 (25) (2017) 15139–15148. [5] M.K. Gonce et al., Dye-sensitized Cu2 XSnS4 (X=Zn, Ni, Fe Co, and Mn) nanofibers for efficient photocatalytic hydrogen evolution, ChemSusChem 9 (6) (2016) 600–605. [6] A.H. Reshak, K. Nouneh, I.V. Kityk, Structural, electronic and optical properties in earth-abundant photovoltaic absorber of Cu2ZnSnS4 and Cu2ZnSnSe4 from DFT, Int. J. Electrochem. Sci. 9 (2014) 955–974. [7] X. Meng et al., Heating rate tuning in structure, morphology and electricity properties of Cu2FeSnS4 thin films prepared by sulfurization of metallic precursors, J. Alloys Compd. 680 (2016) 446–451. [8] X. Liu et al., The current status and future prospects of kesterite solar cells: a brief review, Prog. Photovolt. 24 (6) (2016) 879–898. [9] M. Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science (2016). [10] D.B. Khadka, J. Kim, Structural, optical and electrical properties of Cu2FeSnX4 (X=S, Se) thin films prepared by chemical spray pyrolysis, J. Alloys Compd. 638 (2015) 103–108.