Synthesis of Cu₂ZnSnS₄ film by air-stable molecular-precursor ink for constructing thin film solar cells

Abstract

Cu₂ZnSnS₄ (CZTS) precursor ink routes have been demonstrated to open up new possible ways to fabricate low-cost and high-efficiency CZTS film solar cells, and a prerequisite is to investigate and develop precursor inks. Herein, we report the synthesis of air-stable molecular precursor ink for the construction of CZTS film solar cells. The precursor ink was prepared by dissolving Cu(acac)₂, Zn(acac)₂, and SnC₂O₄ and sulfur powder in pyridine, which yielded a viscous black solution. Subsequently, the precursor ink was spin-coated on a Mo-coated glass substrate, and the resulting film was heated in the air to burn off organics, producing an oxide film. The oxide film was subjected to the sulfurization process, in which the effects of sulfurization conditions on composition were investigated. CZTS film was obtained with Cu/(Zn + Sn) = 0.87 and Zn/Sn = 1.09, which is a composition that is close to the previously proposed composition for high-efficiency CZTS film solar cells. This CZTS thin film was used to construct a solar cell, and it exhibited an initial power conversion efficiency of 2.30%. Further improvement of the efficiency can be expected by the optimization of the ink composition, the composition/morphology/thickness/phase of the CZTS layer, and the device structure.

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1. Introduction

The quest and demand for clean and economical energy sources have increased widespread interest in the development of solar cells.¹ Currently, different kinds of solar cells have been well investigated and developed, including silicon-based solar cells,² dye-sensitized solar cells (DSSC),^3–5 inorganic–organic hybrid solar cells^6–8 and semiconductor thin film solar cells.^9–13 Among semiconductor thin film solar cells, Cu₂ZnSnS₄ (CZTS) film solar cells have recently received widespread attention due to the suitable direct band gap (E_g = 1.45–1.50 eV) and earth-abundant environment-friendly elements of CZTS. To fabricate CZTS films, different approaches have been reported, such as thermal evaporation^14,15 and pulsed laser deposition.¹⁶ For example, Shin et al.¹⁵ developed a vacuum thermal evaporation method to construct CZTS film solar cells with a power conversion efficiency of up to 8.4%. However, those techniques require a complex control system or high vacuum condition, which are too costly and would hinder large-scale production. Thus, the further development of simple and low-cost synthesis techniques has attracted much attention for the preparation of CZTS film.

Currently, the precursor-ink printing technology has been proposed as a simple and effective synthesis method for semiconductor thin films because the inks can be printed, spin-coated, or dip-coated on the substrates, which could enable continuous roll-to-roll processing under mild conditions on nearly any type of surface. Various types of semiconductor thin films based on precursor inks including PbSe,¹⁷ Cu(In,Ga)Se₂(CIGS),^18–20 CuInS₂ (ref. 12 and 21) and CuIn(S,Se)₂ (ref. 22 and 23) have been successfully synthesized.

For CZTS film, generally two kinds of precursor inks have been well demonstrated. The first kind is CZTS nanoparticle-based inks,^24–28 which are based on the synthesis of CZTS nanoparticles and use nanocrystal dispersions as the inks. For example, by high temperature solution-based synthesis in high-boiling solvents (such as oleylamine), hydrophobic CZTS nanoparticles have been prepared, and then nanoparticle-based organic dispersions can be used as the inks.^24–26 By further tuning the composition of CZTS nanoparticles, developing a robust film coating method, and selenizing CZTS film, Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells with a conversion efficiency of up to 7.2% have been constructed.²⁷ The disadvantage of these hydrophobic CZTS nanoparticles capped with oleylamine is that they can only be dispersed in an organic solvent (such as toluene) that is environmentally unfriendly. To address this issue, we prepared hydrophilic CZTS nanocrystals using a modified solvothermal method with ethylene glycol as the solvent, and then ethanol-based ink containing CZTS nanocrystals was used to print flexible, low-cost, and environmentally friendly CZTS film solar cells with an initial power conversion efficiency of 1.94%.²⁸ However, it should be noted that nanoparticle-based ink routes may also have some limitations such as time-consuming, labor-consuming and high production cost. Furthermore, the presence of surface ligands on CZTS nanocrystals is inevitable in nanoparticle-based inks, which results in unnecessary impurities in CZTS film.

The second kind is metal-compound-precursor solutions, which are prepared by dispersing or dissolving the metal-compound (such as metal chloride or sulfide, metal: Cu, Sn, Zn) in a certain solvent. For example, by dispersing Cu₂S/SnSe/ZnSe/S/Se in hydrazine solutions, hydrazine-based particle-containing slurries were obtained and used as the ink for the preparation of CZTS or CZTSSe film, and the resulting solar cells exhibited the very attractive power efficiency of 9.3–11.1%,^29,30 where 11.1% is a world-record efficiency for CZTS or CZTSSe film solar cells.³⁰ In addition, a hydrazine-based true solution containing molecular precursors was also prepared and used as the ink to prepare CZTS or CZTSSe film,^31,32 and the resulting solar cells exhibited a power efficiency of 8.08%.³¹ Although hydrazine can be used as a solvent for precursor ink, it is a highly toxic and very unstable compound that requires extreme caution during handling and storage. By replacing hydrazine with ethanol^33,34 or pyridine³⁵ as the solvent, several kinds of metal-compound-precursor solutions have also been explored, but the conversion efficiencies of the resulting solar cells are low compared with that of solar cells prepared from hydrazine-based particle-containing slurries.^29,30 Therefore, it is still necessary to develop precursor ink for the construction of CZTS film solar cells.

It should be noted that Cui et al. has reported an easily decomposable vulcanized polymeric ink for the preparation of uniform, dense, large-grained and contaminate-free CuInS₂ film, and the resulting CuInS₂ solar cell has achieved an initial power efficiency of 2.15%.¹² Notably, it is a comparatively simple, safe, low-cost, and general molecular precursor-based solution approach. In the present study, we extend this method to fabricate air-stable molecular-precursor ink for constructing CZTS thin film solar cell, and the solar cell exhibits an initial power conversion efficiency of 2.30%.

2. Experimental section

2.1 Preparation of precursor ink

All of the chemicals were analytical grade and used as received from Sinopharm Chemical Reagent Co. (China). The precursor ink was prepared by adding 12 ml of pyridine to a mixture containing Cu(acac)₂ (acac = acetylacetonate) (4.607 g, 17.6 mmol), Zn(acac)₂ (3.163 g, 12 mmol), SnC₂O₄ (2.067 g, 10 mmol), and sulfur powder (1.283 g, 40 mmol), and the solution was magnetically stirred for 24 hours, forming a homogeneous and black pyridine-based precursor ink. The atomic ratio of Cu/(Zn + Sn) and Zn/Sn in the precursor ink was 0.8 and 1.2, respectively.

2.2 Fabrication of CZTS film and construction of solar cells

Molybdenum (Mo) film with a thickness of approximately 500 nm was deposited on a glass substrate by radio-frequency (RF) magnetron sputtering (Magnetron Sputtering Equipment JPGF-400B-G, CAS Shenyang Scientific Instruments Development Center Co., LTD, China). The as-prepared precursor ink was spin-coated onto a Mo-coated glass substrate. The coating process was repeated 3 times using the same ink. The resulting precursor film was subjected to annealing in a Muffle furnace at 370 °C in air with the stepwise heating process carried out as follows: the temperature rose from 30 to 200 °C in 60 min, and after 100 min, it increased to 370 °C, at which it was held for 30 min. The stepwise heating process was usually used for preventing cracks caused by an excessive heating rate. After this oxidization process, organics were burned off and an oxide film was obtained. To convert the oxide film to a CZTS film, it was placed inside a horizontal tubular quartz reactor that was 1000 mm in length and 55 mm in diameter. Approximately 5 cm in front of the CZTS film, a ceramic sample boat with a S source (3 g S, or 3 g S and 3 g SnS) was also placed. Subsequently, N₂ (99.99%) gas was introduced with a flow rate of 400 scc m⁻¹ (standard cubic centimeter per minute), and the quartz tube was heated to 525 °C at a rate of 10 °C min⁻¹. At 525 °C, the film was held for 3 hours in the presence or absence of flowing N₂.

The optimized CZTS film was used to construct film solar cells with the structure of glass/Mo/CZTS/CdS/i-ZnO/Ni–Al. The n-type junction partner CdS (approximately 50 nm) was deposited on the CZTS film by the chemical bath deposition (CBD) technique, described as follows: first, distilled water (30 ml), CdCl₂·5H₂O aqueous solution (5 ml, 0.015 M), NH₄OH solution (6 ml, 25–28 wt%), and NH₂CSNH₂ solution (10 ml, 0.375 M) were mixed and then magnetically stirred for several minutes, resulting in a homogenous bath solution. Then the bath solution together with the vertically immersed CZTS film sample was transferred to a water-heated vessel that was kept at 80 °C and stirred during the deposition process. Five min later, the CZTS film sample was taken out and washed with deionized water and absolute ethanol, successively, several times. Additionally, the high resistivity i-ZnO (approximately 50 nm) film, transparent conductive indium tin oxide (ITO) layer (approximately 100 nm), and dual layer top contact Ni (50 nm)/Al (3 μm) were deposited in turn by the DC magnetron sputtering process, as described in detail in our previous report.^10,28 Thereby, CZTS film solar cells with the structure of glass/Mo/CZTS/CdS/i-ZnO/Ni–Al were finally constructed.

2.3 Characterization and photoelectrical measurement

The morphology and size of the samples were investigated using a scanning electron microscopy (SEM, Hitachi S-4800). During the SEM measurement, an energy dispersive spectroscopy (EDS) line scan was also performed to locate the distribution of composition. The X-ray diffraction (XRD) measurements were carried out on a Bruker D4 X-ray diffractometer using a Cu Kα radiation source (λ = 1.5418 Å). The optical measurement was directly performed on an UV-Vis-NIR spectrophotometer (Lambda 950). For the metal content analysis, CZTS powder was scraped from the film and then dissolved in HNO₃ solution, and the amounts of elemental Cu, Zn, and Sn were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman Prodigy).

The photocurrent density–voltage curve of CZTS film solar cell was recorded under standard AM 1.5 solar illumination with an intensity of 300 mW cm⁻² using a computerized Keithley Model 2400 SourceMeter unit. A 300 W xenon lamp (Newport Oriel) served as the light source.

3. Results and discussion

In the precursor ink presented here, Cu(acac)₂, Zn(acac)₂, SnC₂O₄, and sulfur powder were dissolved in pyridine, forming a black and viscous solution. The high viscosity of the precursor ink results from the formation of the polymer created from the reaction between sulfur and acac (known as vulcanization³⁶), which can promote uniform dispersion of metal atoms in the solution and is a material with behavior similar to that described in a previous report on CuInS₂ ink.¹² It should be noted that the precursor ink without sulfur is not viscous. Compared with using the nonviscous ink, it is easy and convenient to prepare a thick film on glass by spin-coating using viscous ink. In addition, in the present precursor viscous ink, the atomic ratios of Cu/(Zn + Sn) and Zn/Sn were 0.8 and 1.2, respectively, which is strictly consistent with the proposed proportions of Cu/(Zn + Sn) and Zn/Sn according to previous studies.^27–29,37 Based on this viscous precursor ink, we fabricated CZTS film solar cells, and the fabrication process involved four steps, as demonstrated in Fig. 1. This process can be extended to a large-scale roll-to-roll process for the construction of CZTS film solar cells.

Fig. 1 Schematic illustration of the fabrication process of glass/Mo/CZTS/CdS/i-ZnO/ITO/Ni–Al solar cells based on air-stable molecular-precursor ink.

The first step is to create a precursor film on a Mo-coated glass substrate by spin-coating of the as-prepared viscous precursor ink, and the coating process is repeated 3 times. The morphologies of the surface and cross-section of the precursor film were investigated by SEM images (Fig. 2a and b). Part of the surface of the precursor film became black during SEM measurement (Fig. 2a) due to the presence of organic compounds with poor conductivity. Cross-sectional morphology reveals that the precursor film is uniform and has a thickness of approximately 10 μm (Fig. 2b). This substantial thickness should result from the high viscosity of the precursor ink and the presence of organic and inorganic compounds in the precursor film.

Fig. 2 Typical surface and cross-section morphologies of precursor film (a and b), oxide film (c and d), and CZTS film (e and f). All insets are the corresponding magnification of SEM images.

The second step is to convert the precursor film into an oxide film by heating it in a Muffle furnace at 370 °C. The heat treatment will burn off organics, producing an oxide film with a mixture of CuO, ZnO and SnO₂ on a glass substrate. The surface morphology (Fig. 2c) reveals that the oxide film is composed of grains with diameters of approximately 500 nm, and there are many nanopores among these grains. In addition, a cross-sectional SEM image (Fig. 2d) further confirms that the entire film consists of large grains, and these grains are aggregates from numerous oxide nanoparticles with diameters of 50–100 nm during the heating process, as demonstrated by high-magnification SEM images (the inset of Fig. 2d). Furthermore, the oxide film becomes significantly thinner with an average thickness of approximately 3 μm, which indicates the decomposition of organics at high temperature (370 °C). It should be noted that the thickness (approximately 3 μm) of the oxide film is still sufficient for the conversion to CZTS film. The above results also indicate that it is easy and convenient to prepare thick (approximately 3 μm) oxide film by repeatedly spin-coating 3 times using the present viscous ink (the oxide film growth rate is approximately 1 μm per time), compared with the film from other precursor inks.^35,38,39 For example, the rate is approximately 0.18 μm per time,³⁸ since the approximately 1.4 μm-thick oxide film was obtained by repeating eight spin-casting/sintering cycles using an ethanol-based precursor ink containing metal oxides (Cu₂O, ZnO, SnO) in the absence of sulfur.

The third step is to sulfurize the oxide film to CZTS film by heating it in a furnace with different sulfurization conditions (S with flowing N₂, S and SnS with flowing N₂, and S and SnS without flowing N₂) at 525 °C for 3 hours. The difference in S sources has no obvious effects on surface and cross-sectional morphologies of CZTS. Fig. 2e and f present typical surface and cross-sectional morphologies of CZTS film that was prepared by sulfurizing the oxide film with S/SnS as the S source in the absence of flowing N₂ gas. The surface morphology reveals that CZTS film is composed of many densely packed grains with diameters of 100–400 nm, and there are few nanopores in the compact CZTS film (Fig. 2e). Cross-sectional morphology (Fig. 2f) further confirms that the entire film was constructed from densely packed grains, and the thickness of the CZTS film was approximately 3 μm. These facts indicate that the sulfurization process results in the compaction of the film.

Subsequently, the effect of sulfurization conditions on the phase of the CZTS film was investigated. To facilitate the analysis, CZTS films were prepared on ordinary glass substrates without the Mo layer, except that CZTS film on Mo-coated glass was sulfurized with S/SnS as the S source in the absence of flowing N₂. Fig. 3 shows the XRD patterns of CZTS films that were prepared with different sulfurization parameters (S with flowing N₂, S and SnS with flowing N₂, and S and SnS without flowing N₂). The XRD patterns reveal that all CZTS films have high crystallinity. The diffraction peaks at 28.53, 32.99, 47.33, 56.18 and 76.44° are assigned to the (112), (200), (220), (312) and (332) planes, which are in good agreement with the literature values for kesterite structure of CZTS (JCPDS card, no. 26-0575). It should be noted that CZTS film on Mo-coated glass exhibits two additional diffraction peaks at 40.5° and 73.6° (marked by triangles), which should be attributed to the presence of the Mo layer.

Fig. 3 XRD patterns of CZTS thin films that were prepared with different sulfurization parameters (S with flowing N₂, S and SnS with flowing N₂, and S and SnS without flowing N₂).

It was previously found that sulfurization parameters have obvious effects on the composition of CZTS film, and the decomposition reaction of CZTS should be prevented.⁴⁰ Herein, we also investigated the effects of sulfurization conditions on the composition of CZTS film by ICP-AES analysis (Table 1). When the oxide film was sulfurized with S powder as the S source under flowing N₂ gas at 525 °C for 3 hours, the atomic ratios of Cu/(Zn + Sn) and Zn/Sn from the resulting CZTS film are 0.86 and 3.74, respectively, where the Zn/Sn ratio (3.74) greatly deviates from that (1.2) of the precursor ink. This fact suggests the serious loss of the element Sn, which results from CZTS film decomposition and high vapor pressure of SnS at temperatures above 400 °C.⁴⁰

Table 1 Effects of sulfurization conditions on the composition of CZTS

Sulfurization conditions	Atomic ratio of composition
	Cu/(Zn + Sn)	Zn/Sn
A: S + N2	0.86	3.74
B: S + SnS + N2	1.05	0.98
C: S + SnS	0.87	1.09

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It has been revealed that sufficiently high partial pressures of SnS and S prevent the decomposition reaction of CZTS at high temperatures and introduce the missing Sn into a Sn-deficient film.⁴⁰ To prevent the loss of elemental Sn, we used SnS and S powders as the S source, and the oxide film was then sulfurized under flowing N₂ gas at 525 °C for 3 hours. For the resulting CZTS film, the atomic ratios of Cu/(Zn + Sn) and Zn/Sn were 1.05 and 0.98, respectively. The decrease of the Zn/Sn ratio from 3.74 to 0.98 suggests that the decomposition of the CZTS thin film and the loss of Sn were apparently inhibited. However, because the loss of Sn could not be completely prevented, the prepared CZTS film has a Cu-rich and Sn-poor composition, which is not desired for high-efficiency CZTS solar cells. This phenomenon probably results from reduced partial pressures of SnS and S due to flowing N₂. To further avoid the loss of elemental Sn, we then sulfurized the oxide film in a relatively closed furnace with S/SnS as the S source in the absence of flowing N₂. CZTS film with a Zn/Sn-rich and Cu-poor composition was successfully obtained, and the atomic ratios of Cu/(Zn + Sn) and Zn/Sn were 0.87 and 1.09, respectively, which are close to those (0.8 and 1.2) of the precursor ink and consistent with the proposed proportions of Cu/(Zn + Sn) and Zn/Sn according to previous studies.^27–29,37

Because CZTS film with Cu/(Zn + Sn) = 0.87 and Zn/Sn = 1.09 was considered to be the expected film, this film was further characterized by local composition analysis and optical absorption. Fig. 4 gives detailed information on the local atomic composition of CZTS film. The red arrow on the SEM image (the inset of Fig. 4) indicates the scanning path of an electron beam, and a clear presentation of the elemental distribution is given by a plot of the EDS line scan signal versus the distance along the film. Overall, the EDS line scan profile shows that the signal peak of the Cu, Zn, and Sn elements is located at the middle regions, suggesting the homogeneous metal distribution within the film and therefore the optimal formation of the CZTS film. In addition, the S signal profile shows a similar trend except for a considerable intensity increase at the interface between the CZTS film and the Mo-coated glass, probably resulting from the formation of the MoS₂ layer during the sulfurization process. The optical absorption of the CZTS film was measured by an UV-Vis-NIR spectrometer, and Fig. 5 shows a typical diffuse reflection spectrum. The film exhibits strong adsorption within a broad range between 400 and 800 nm, which is the characteristic absorption of CZTS and consistent with the black color of the film.

Fig. 4 An EDS line scan profile along the red arrow is indicated in the cross-sectional SEM image of CZTS film (upper left inset), which was obtained by sulfurizing the oxide film in a relatively closed furnace with S/SnS as the S source in the absence of flowing N₂.

Fig. 5 UV-Vis-NIR spectrum of CZTS film that was obtained by sulfurizing the oxide film in a relatively closed furnace with S/SnS as the S source in the absence of flowing N₂.

The last step is to use the CZTS film with Cu/(Zn + Sn) = 0.87 and Zn/Sn = 1.09 to construct CZTS film solar cells with the structure of SLG/Mo/CZTS/CdS/i-ZnO/Ni–Al. The CdS buffer layer (approximately 50 nm) was directly deposited on a CZTS absorber layer by the classic chemical bath deposition (CBD) method. The ZnO layer with a thickness of approximately 50 nm, an ITO layer with a thickness of approximately 100 nm, and a dual top contact layer of Ni (50 nm)/Al (3 μm) were successively deposited by DC magnetron sputtering. The photocurrent density–voltage characteristic of the as-fabricated CZTS solar cells was investigated, as demonstrated in Fig. 6. The solar cell exhibits a short-circuit current density (J_sc) of 10.1 mA cm⁻², open-circuit voltage (V_oc) of 0.424 V and fill factor (FF) of 0.537, yielding a power energy conversion efficiency (η) of 2.30%, which is comparable to CuInS₂ solar cells prepared by a similar method.¹² However, this efficiency is low compared with the efficiencies (7.2–11.1%) of the CZTSSe solar cells,^27,29,30 which probably results from small particle size, grain boundaries, pinholes, and unoptimized band gap of CZTS film. There are several possible solutions to these issues, including (1) by selenizing CZTS film in Se vapor, the majority of sulfur in CZTS can be replaced by Se, resulting in the growth of grain, the reduction/disappearance of pinholes, and the narrowing of band gap;²⁷ (2) by incorporating beneficial metal ions such as Li, Na, K and even nonalkali metal or combinations onto the CZTS nanocrystal surface, the carrier concentration can be increased and the minority carrier lifetime of the CZTS film can be elongated;⁴¹ (3) by optimizing film-printing techniques and the annealing process, the film thickness and quality can be improved.

Fig. 6 J–V characteristics of the SLG/Mo/CZTS/CdS/i-ZnO/Ni–Al solar cells.

4. Conclusions

In summary, air-stable molecular CZTS precursor ink has been prepared by dissolving Cu(acac)₂, Zn(acac)₂, SnC₂O₄, and sulfur powder in pyridine, which yields a viscous black solution. After spin-coating the ink on a Mo-coated glass substrate and subsequent oxidization and sulfurization processes, CZTS films are obtained. Sulfurization parameters have strong effects on the composition of CZTS film, and the expected CZTS film with Cu/(Zn + Sn) = 0.87 and Zn/Sn = 1.09 was obtained by sulfurizing the oxide film in a relatively closed furnace with S/SnS as the S source in the absence of flowing N₂. Based on this CZTS film, solar cells with the structure of SLG/Mo/CZTS/CdS/i-ZnO/Ni–Al have been constructed with an initial power conversion efficiency of 2.30%. Further enhancements should focus on the optimization of ink composition, the composition/morphology/thickness/phase of the CZTS layer, and the device structure, as well as the entire fabrication process, to achieve more efficient CZTS thin film solar cells. More importantly, this work provides some insight into the design of new CZTS precursor ink and the construction of CZTS film solar cells.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 21107013, 21171035, and 51272299), High-Tech Research and Development Program of China (Grant no. 2012AA030309), Program for Changjiang Scholars and Innovative Research Team in University (Grant no. IRT1221 and T2011079), Specialized Research Fund for the Doctoral Program of Higher Education (Grant no. 20110075120012), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, project of the Shanghai Committee of Science and Technology (13JC1400300), Innovation Program of Shanghai Municipal Education Commission (Grant no. 13ZZ053), Shanghai Leading Academic Discipline Project (Grant no. B603), the Program of Introducing Talents of Discipline to Universities (Grant no. 111-2-04), the Fundamental Research Funds for the Central Universities, and the DHU Distinguished Young Professor Program.

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