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Formation of Cu2ZnSnSe4 through direct selenization of metal oxides
Accepted Manuscript Formation of Cu2ZnSnSe4 through direct selenization of metal oxides Jian Mao, Shu Zhang, Xiaoli Peng, Jianfei Zhang, Haitao Zhang, Longyan Gu, Yong Xiang PII:
S0042-207X(15)00026-3
DOI:
10.1016/j.vacuum.2015.01.015
Reference:
VAC 6528
To appear in:
Vacuum
Received Date: 30 May 2014 Revised Date:
3 January 2015
Accepted Date: 16 January 2015
Please cite this article as: Mao J, Zhang S, Peng X, Zhang J, Zhang H, Gu L, Xiang Y, Formation of Cu2ZnSnSe4 through direct selenization of metal oxides, Vaccum (2015), doi: 10.1016/ j.vacuum.2015.01.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Formation of Cu2ZnSnSe4 through direct selenization of metal oxides
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Jian Mao1, Shu Zhang*, Xiaoli Peng, Jianfei Zhang, Haitao Zhang, Longyan Gu, and Yong Xiang*
*Corresponding
authors:
[email protected]
1
Z.),
[email protected]
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+86-028-61831556; Fax: +86-028-61831080
(S.
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School of Energy Science and Engineering, University of Electronic Science and Technology of China, State Key Laboratory of Electronic Thin Films & Integrated Devices, 2006 Xiyuan Avenue, West High-Tech Zone, Chengdu, Sichuan 611731, China (Y.
X.);
Tel:
Present address: Department of Electrical and Electronic Engineering, The University of Hong Kong,
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Pokfulam Road, Hong Kong, China
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Abstract This article describes a fabrication approach to Cu2ZnSnSe4 thin films through the direct selenization of The resulting
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precursor films of the metal oxide mixture with elemental selenium at 550 °C for 40 min.
films exhibit a well compacted morphology composed of crystallites of a few micrometers and having reliable photoelectric response in photo-electrochemical determination, suggesting the potential of To investigate the formation pathway of
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photovoltaic applications of the films fabricated by this method.
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the Cu2ZnSnSe4 film in this direct selenization method, selenization reactions were carried out with individual Cu, Zn, and Sn oxides under the same conditions, and the results showed that the formation rate of Cu2ZnSnSe4 from a mixture of metal oxides is comparable with that of Cu2Se formation, but faster than those of ZnSe and SnSe2 formation.
The facilitated selenization kinetics to form Cu2ZnSnSe4 may be
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attributed to the oxidation state change of Cu(II) to Cu(I) and Sn(II) to Sn(IV) and the formation of cubic ZnO phase during the preparation of the mixture of metal oxides.
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thin film.
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KEYWORDS: Cu2ZnSnSe4; formation pathway; metal oxide; selenization; solar cell material;
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1. Introduction Kesterite-structured Cu2ZnSn(SxSe1-x)4 (CZTSSe) samples have attracted considerable attention for thin
elements [1-4].
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film solar cell applications due to their unique electronic and optical properties and abundant constituent To date, state-of-the-art CZTSSe solar cells have been demonstrated with highest power
conversion efficiency of 12.6% [5] by Mitzi et al. through the hydrazine-based deposition process.
To
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further improve the efficiency of the cells and meet the requirement for practical applications, it still
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remains challenging to develop fabrication methods that can offer control over the composition, morphology and uniformity of CZTSSe thin films.
In the CZTSSe absorber layer, it was well demonstrated that the high Se content can not only enhance grain growth [6], but also, facilitate n-type and p-type doping [7], both of which then benefit the film These effects were verified in the champion Cu2ZnSnSe4 (CZTSe) cell with
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quality for solar cells.
efficiency as high as 10.1% [8], much higher than the record cell of its full sulfide analogues (Cu2ZnSnS4, 8.4% efficiency) [9].
Introduction of Se in CZTSSe material can be achieved by using excess Se ligand in
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a precursor [10] or gaseous Se [8,11], leading to the formation of partially [10,11] or fully [8] Another route to form selenides is the use of metal oxides as the
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Se-substituted CZTSSe materials. starting materials.
For instance, it was reported that CuInSe2 thin films suitable for photovoltaic
application can be fabricated through selenization of metal oxides by H2Se [12] or gaseous Se [13] at elevated temperature.
The direct selenization approach appears to be a prototype for the formation of thin
films of multiple selenide compounds, in which the replacement of selenium to oxygen is likely to play as the driving force for mass transfer in the solid-state reaction.
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Similar reaction, however, has not been
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demonstrated with Cu2ZnSnSe4 compound yet.
Therefore, herein we report our study on the fabrication
of CZTSe thin films through direct selenization of metal oxides.
The resulting CZTSe thin film shows
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compacted film morphology, grain size in micrometers, and suitable photoelectrochemistry properties, suggesting that this material is potentially applicable for photovoltaic devices.
The reaction pathway of
this formation process has also been investigated through the selenization of individual binary metal oxides
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at the same condition.
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2. Experimental Section 2.1. Chemicals
Copper (II) acetate monohydrate (> 99.0%, Tianjin Fu Chen Chemical Factory), zinc (II) acetate dehydrate (> 99.0%, Kelong Chemnet), tin (II) sulfate (> 99.0%, Kelong Chemnet), concentrated sulfuric
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acid (98%, Kelong Chemnet), sodium hydroxide (> 99.0%, Kelong Chemnet), selenium (> 99.9%, Kelong Chemnet) were used without purification.
2.2. Thin film fabrication and selenization
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The synthesis procedure of Cu2ZnSnSe4 thin films began by dissolving 0.887 g Cu(OAc)2⋅H2O (4.4
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mmol), 0.665 g Zn(OAc)2⋅2H2O (3.0 mmol), and 0.542 g SnSO4 (2.5 mmol) in 50 ml deionized water with 5 ml concentrated sulfuric acid, followed by co-precipitating at pH to 7-8 adjusted with sodium hydroxide solution (0.5 mol/L).
The resulting yellow co-precipitations were filtered, rinsed with deionized water for
three times, and dried at 120 °C for 2 h.
The dried co-precipitations were ball-milled for 1 h in a fast
vibrating ball mill (Nanjing NanDa Instrument Plant, QM-3A) to get homogeneous metal oxides precursors, which were spin-coated onto a 1 cm2 soda lime glass substrate at a spin rate of 2000 rpm.
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The as-coated
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precursor films were loaded into a graphite box in a tubular furnace that was vacuumed and purged with argon gas and selenized at 550 °C for 40 min in excess selenium carried by argon gas at a flow rate of
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20-40 ml/min under ambient pressure. In order to investigate the formation pathway of Cu2ZnSnSe4 from metal oxides, we performed the selenization of Cu, Zn, Sn metal oxides individually, using the similar procedure as above.
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The Cu2ZnSnSe4 thin films for photoelectrochemical measurement were prepared by spray coating the
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ink of metal oxides on fluorine-doped tin oxide (FTO) glass with a heating temperature of 200 °C by using a hot plate, and the film thickness was controlled to about 400 nm. 2.3. Characterization.
The crystallographic structures of pre-selenized and post-selenized layers metal oxides were
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characterized by both X-ray diffraction (XRD, Bruker D8 ADVANCE) with a Cu K-alpha (1.5418 Å) source at 40 kV voltage and 40 mA current by 0.1 s/step (detection angle ranging from 10° to 90°), and Raman spectrum was obtained using Raman spectroscopy (Renishaw inVia) under excitation of a 532 nm The morphology and composition were analyzed by scanning electron microscope (SEM, Hitachi
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laser.
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S-4800 and Zeiss-EVO LS-15) equipped with energy dispersive X-ray spectrometry (EDS, Bruker Nano XFlash Detector 5010).
2.4. Photoelectrochemical measurements All electrochemical experiments were carried out on an electrochemical workstation (CHI660D, CHI
Instruments, Shanghai).
A three-electrode configuration was used with Cu2ZnSnSe4/FTO as
photo-electrodes, a platinum wire as the counter-electrode, and saturated Ag/AgCl as the reference
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electrode.
The photo-electrochemical measurements were performed in 0.1 mol/L Na2SO4 aqueous
solution (with pH = 0 adjusted by adding H2SO4) and illuminated with a 40 W incandescent lamp.
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3. Results and Discussion The fabrication route to the target Cu2ZnSnSe4 film is shown in Figure 1.
The inks were prepared
based on a homogeneous mixture of metal oxides that was formed through co-precipitating a homogeneous
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acidic solution of Cu(OAc)2, Zn(OAc)2 and SnSO4 using NaOH, followed by dehydration and ball milling.
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The spin-coated thin films using the ink were selenized at 550 °C for 40 min. precursor films and selenized layers were measured by EDS, as shown in Table 1.
The compositions of
The dramatic decrease
of oxygen content from 64.56% in precursor film to 3.63% in selenized film and increase of selenium content from zero in precursor film to 50.46% in selenized film suggest that the oxygen atoms were Further study to adjust the selenization condition for
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replaced by selenium atoms at elevated temperature.
full selenization of the oxides is under investigation.
The atomic ratios of Zn/Sn and Cu/(Zn + Sn) in
selenized layer are 1.13 and 0.83, respectively, which are in close agreement with those in the starting This copper-poor and zinc-rich composition of selenized film is desirable for
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material (1.20 and 0.80).
This result also suggests that the current selenization route
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high efficiency Cu2ZnSnSe4 solar cells [8, 9].
can be used for the synthesis of CZTSe with tunable metal ratios by simply changing the ratios of starting metal salts.
In the characterization of the selenized thin film, XRD pattern in Figure 2a exhibits strong peaks that
are characteristic of highly crystalline Cu2ZnSnSe4 (JCPDS 52-0868).
Although the peaks at 2θ =
28.336°, 35.290°, and 42.935° corresponding to (103), (202), and (213) planes of Cu2ZnSnSe4 in this study
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were absent in JCPDS 52-0868 reported by Matsushita et al. [14], these diffraction peaks were observed by Zoppi et al. [15].
Furthermore, as the strongest X-ray diffraction intensity, the selenized film exhibits a
series [10].
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preferred (112) orientation, which has also been observed in the CZTSSe thin film of the champion cell Although the XRD peaks in Figure 2a are all characteristic of CZTSe, the existence of ZnSe
and Cu2SnSe3 phases cannot be ruled out because the overlap of their characteristic diffraction peaks with Therefore, Raman spectrometry (Figure 2b) was also used to further identify the
phase status of the as-selenized materials in this study.
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those of CZTSe [16]
The evident Raman peaks at 170, 192, 233, and
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243 cm-1 are in accord with those of Cu2ZnSnSe4 reported in previous literature [17,18].
The Raman
result, together with the XRD data, unambiguously confirms the phase purity of the CZTSe synthesized by this selenization route.
In comparison to traditional hot-injection method [19] and ball-milling method
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[20], the current CZTSe film formation route uses easily available ink of metal oxides as the starting material, making it a potentially cost-efficient method. Figure 3 illustrates the morphology of as-prepared CZTSe layer determined by SEM.
Non-unifomity
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of the as-prepared CZTSe film can be seen from Figure 3a: grain distribution with large grains (on the order
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of a few micrometers) on the surface and relative small grains (on the order of several hundred nanometers) in internal region.
The magnified image of CZTSe thin film (Figure 3b) shows that the selenized film is
compact and the crystals are in the size of micrometer order. that the selenization undergoes a complex process.
The grain distribution of the film indicates
Non-uniform mass transfer may be an important
problem in this process, because oxygen atoms are removed from the oxides and selenium atoms are added in the lattice structure. Probably, non-uniform distribution of gaseous Se diffusing from the surface to
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inside limits the selenization rate and mass transfer, degrading the uniform grain growth.
Further
modification of the selenization conditions to improve grain growth of the film remains to be investigated.
performed.
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To determine the photo-electronic properties of the selenized thin film, photo-electrochemical study was Figure 4 shows the linear sweep voltammogram of an as-prepared CZTSe film on FTO glass
in a 0.1 mol/L Na2SO4 aqueous solution with pH = 0 adjusted by adding H2SO4.
The CZTSe film
When the electrode potential is more positive than 0.4 V (vs. Ag/AgCl), the
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changes from 0 to -0.8 V.
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electrode exhibits a current increasing and dropping during illumination/dark conditions when the potential
current showed a continued increase with enhancement of the potential, indicating the increase with redox current at the interface between CZTSe film and the solution.
In the photo-electrochemical process, a
constant potential of -0.2 V was applied to the film with the light turned on and off sequentially, and the
photo-responsive property.
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film has a stable photocurrent response during the cycling (insert of Figure 4), exhibiting reliable A rapid decrease appeared after switching on the light, showing that the
photo-generated electrons in p-type CZTSe film were immediately involved in the recombination reaction.
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These results suggest that the CZTSe films prepared through direct selenization of metal oxides may be
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suitable for photovoltaic application.
To investigate the CZTSe formation pathway in this direct selenizaition route, selenization of
individually synthesized copper oxide, zinc oxide, and tin oxide were performed on the same conditions. XRD patters of these films before and after selenization are compared and shown in Figure 5, and accordingly, Table 2 lists the related selenization rates of these reactions to form binary selenide compared with that to form CZTSe.
In Figure 5a, all of the peaks in copper oxide precursor can be indexed to the
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CuO (JCPDS 45-0937) phase, which was fully converted into Cu2Se (JCPDS 65-2982) after selenization at 550 °C for 40 min, with a comparable selenization rate with that in the formation of CZTSe, as shown in It should be noted that in this selenization process, Cu(II) was reduced to Cu(I) by
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entry 1 of Table 2.
elemental Se, and the thermodynamically more stable Cu2Se phase was formed probably through a CuSe intermediate, as has been well demonstrated [21].
On the other hand, it can be observed in Figure 5b that
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ZnO (JCPDS 36-1451) and ZnSe (JCPDS 65-9602) both existed in the post-selenized thin film of zinc
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oxide after the same period of selenization, indicating that fully selenization of zinc oxide has slower reaction kinetics than that of selenization of oxide mixtures to form CZTSe (equation of entry 2 in Table 2). Figure 5c shows that not the predicted SnO but Sn6O4(OH)4 (JCPDS 46-1486) resulted from 2 h and 120 °C dryness of the precipitate prepared by precipitating dissolved SnSO4 in concentrated sulfuric acid After selenization of the resulting Sn6O4(OH)4 for 40 min at 550 °C, the mixture
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with sodium hydroxide.
of SnO2 (JCPDS 41-1445) and SnSe2 (JCPDS 23-0602) was obtained, suggesting a slower selenization of tin selenide than that of CZTSe formation in this study (entry 3 in Table 2).
The faster formation kinetics
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for CZTSe than those of formation of individual ZnSe and SnSe2 at the same selenization condition
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suggests that there are some synergetic effects during selenization of the oxide mixture. For comparison, Figure 5d illustrates the XRD result of a precursor thin film of a mixture of binary
oxides of Cu, Zn, and Sn, showing that it consists of Cu2O (JCPDS 65-3288), ZnO (JCPDS 65-0523), and SnO2 (JCPDS 29-1484).
These metal oxide phases are different from those formed individually, which
may be the key reason for the synergetic effect in the formation of CZTSe.
During the synthesis of the
oxide mixture, the oxidation states of copper and tin have changed from Cu(II) and Sn(II) in the starting
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reagents to Cu(I) and Sn(IV) in the oxide mixture, corresponding to the oxidation states of the two metals in Cu2ZnSnS4 compound. This oxidation state change of Cu and Sn may facilitate the formation of
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Cu2SnSe3, an important intermediate involved in the formation of CZTSe [22], and then Cu2SnSe3 reacts with ZnSe formed from the selenization of ZnO, as shown in the stepwise reactions in entry 4 of Table 2. Instead of the step-wise pathway, another possible reaction pathway may be that the mixture of Cu2O, ZnO,
Furthermore, the ZnO formed in this oxide mixer is different phase to
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Zn and Sn to form CZTS [22,23].
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and SnO2 are selenized to form CZTSe (entry 5 of Table 2), similar to the reaction of binary sulfides of Cu,
the oxide formed individually (JCPDS 65-0523 vs. JCPDS 36-1451).
The former cubic ZnO phase in the
oxide mixture is proposed to have a faster selenization rate towards the formation of CZTSe than that of the latter hexagonal ZnO phase synthesized individually [22]. Similar reaction kinetics may be involved in
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the current synthesis, and be the driving force that leads to the overall faster selenization rate to produce CZTSe compared with the reaction to form individual ZnSe at the same condition.
4. Conclusions
Using elemental
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We have developed a non-vacuum route for the fabrication of Cu2ZnSnSe4 thin films.
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Se as the selenium source, direct selenization of precursor films containing a mixture of the three metal oxides, prepared by spin-coating the nano-ink, leads to the formation of compacted Cu2ZnSnSe4 thin films with grain size of a few micrometers.
The prepared Cu2ZnSnSe4 thin film has favorable copper-poor and
zinc-rich composition that can be conveniently controlled in the preparation of metal oxides, and the film exhibits a photocurrent in the photo-electrochemical study.
The study of the formation pathway of
CZTSe shows that selenization rate of metal oxide mixture is comparable with that of CuO, but faster than
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those of ZnO and tin oxide at the same conditions. The possible reasons for facilitated formation of Cu2ZnSnSe4 from mixture of metal oxides in this route are: (1) oxidation state change of Cu and Sn to This fabrication method
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produce Cu2O and SnO; (2) formation of cubic ZnO, instead of hexagonal ZnO.
has potential for cost-efficient photovoltaic applications, which is in progress in our laboratory.
Acknowledgement
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The authors gratefully acknowledge the National Science Funds of China (Contract No. 51102038), the
Century Excellent Talents in University.
References
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Fundamental Research Funds for the Central Universities (ZYGX2012J156), and the Program for New
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[12] Kapur VK, Bansal A, Le P, Asensio OI. Non-vacuum processing of CuIn1-xGaxSe2 solar cells on rigid and flexible substrates using nanoparticle precursor inks. Thin Solid Films 2003;53:431-2.
[13] Kaelin M, Rudmann D, Kurdesau F, Meyer T, Zogg H, Tiwari AN. CIS and CIGS layers from selenized nanoparticle precursors. Thin Solid Films 2003;58:431-2.
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Figure Captions
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Figure 1 Fabrication procedure of Cu2ZnSnSe4 thin films through selenization of metal oxides.
Figure 2 (a) XRD patterns of selenized layer of metal oxides compared with the reference diffraction peaks of Cu2ZnSnSe4 (red vertical lines, from JCPDS 52-0868); (b) Raman spectrum for the selenized layer.
magnification.
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Figure 3. SEM images of the surface of as-selenized Cu2ZnSnSe4 thin film: (a) low magnification, (b) high
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Figure 4. Linear sweep voltammogram of an as-selenized CZTSe film on FTO glass with light chopping in
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a 0.1 mol/L Na2SO4 aqueous solution with pH = 0 adjusted by adding H2SO4; insert is the current-time response curve of the same film at -0.2 V.
Figure 5. XRD patterns of pre-selenization (the dark line) and post-selenization (the red line) layers of (a) copper oxide, (b) zinc oxide, (c) tin oxide by using the same procedure; and (d) XRD patterns of precursor film of oxide mixture, there is an uncategorized peak at 2θ = 43.467°.
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Table 1. The composition of starting reagent, precursor film and selenized oxides Zn (%)
Sn (%)
Se (%)
O (%)
Zn/Sn
Cu/(Zn + Sn)
Starting reagent a
44.44
30.30
25.26
−
−
1.20
0.8
Precursor film b
13.86
11.70
9.88
−
64.56
1.18
0.64
Selenized layer b
20.86
13.30
11.74
50.46
3.63
1.13
a
0.83
This composition is calculated based on the amounts of Cu(OAc)2⋅H2O, Zn(OAc)2⋅2H2O, and SnSO4
starting reagents.
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Compositions determined by EDS.
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b
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Cu (%)
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Table 2. Plausible reactions and relative reaction rate involved in the formation of binary metal oxides and Cu2ZnSnSe4 through selenization of oxide(s) Reaction rate related to the
Entry
Reactiona,b
1
2CuO + 2Se → Cu2Se + SeO2
2
2ZnO + 3Se → ZnSe + SeO2
3
Sn6O4(OH)4 + 6Se → 3SnO2 + 3SnSe2 + 2H2O
4
2Cu2O + 2SnO2 + 9Se → 2Cu2SnSe3 + 3SeO2
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slower
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formation of CZTSe
2Cu2SnSe3 + 2ZnO + 3Se → 2Cu2ZnSnSe4 + SeO2
comparable
Cu2O + ZnO + SnO2 + 6Se → Cu2ZnSnSe4 + 2SeO2
comparable
For clarity, the oxide products in these selenization reactions have been all simplified to be SeO2.
b
SeO2 formed in these reactions is evaporated due to its relatively lower sublimation temperature (340
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°C) comparing with the selenization temperature (550 °C).
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> A cost effective fabrication method for Cu2ZnSnSe4 thin films has been developed. > Elemental selenium is used to directly selenize metal oxides into Cu2ZnSnSe4. > The synthesized Cu2ZnSnSe4 material has suitable properties for photovoltaic applications. > The metal oxide phases formed in mixture is likely to facilitate the formation of Cu2ZnSnSe4.
S0042-207X(15)00026-3
DOI:
10.1016/j.vacuum.2015.01.015
Reference:
VAC 6528
To appear in:
Vacuum
Received Date: 30 May 2014 Revised Date:
3 January 2015
Accepted Date: 16 January 2015
Please cite this article as: Mao J, Zhang S, Peng X, Zhang J, Zhang H, Gu L, Xiang Y, Formation of Cu2ZnSnSe4 through direct selenization of metal oxides, Vaccum (2015), doi: 10.1016/ j.vacuum.2015.01.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Formation of Cu2ZnSnSe4 through direct selenization of metal oxides
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Jian Mao1, Shu Zhang*, Xiaoli Peng, Jianfei Zhang, Haitao Zhang, Longyan Gu, and Yong Xiang*
*Corresponding
authors:
[email protected]
1
Z.),
[email protected]
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+86-028-61831556; Fax: +86-028-61831080
(S.
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School of Energy Science and Engineering, University of Electronic Science and Technology of China, State Key Laboratory of Electronic Thin Films & Integrated Devices, 2006 Xiyuan Avenue, West High-Tech Zone, Chengdu, Sichuan 611731, China (Y.
X.);
Tel:
Present address: Department of Electrical and Electronic Engineering, The University of Hong Kong,
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Pokfulam Road, Hong Kong, China
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Abstract This article describes a fabrication approach to Cu2ZnSnSe4 thin films through the direct selenization of The resulting
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precursor films of the metal oxide mixture with elemental selenium at 550 °C for 40 min.
films exhibit a well compacted morphology composed of crystallites of a few micrometers and having reliable photoelectric response in photo-electrochemical determination, suggesting the potential of To investigate the formation pathway of
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photovoltaic applications of the films fabricated by this method.
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the Cu2ZnSnSe4 film in this direct selenization method, selenization reactions were carried out with individual Cu, Zn, and Sn oxides under the same conditions, and the results showed that the formation rate of Cu2ZnSnSe4 from a mixture of metal oxides is comparable with that of Cu2Se formation, but faster than those of ZnSe and SnSe2 formation.
The facilitated selenization kinetics to form Cu2ZnSnSe4 may be
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attributed to the oxidation state change of Cu(II) to Cu(I) and Sn(II) to Sn(IV) and the formation of cubic ZnO phase during the preparation of the mixture of metal oxides.
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thin film.
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KEYWORDS: Cu2ZnSnSe4; formation pathway; metal oxide; selenization; solar cell material;
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1. Introduction Kesterite-structured Cu2ZnSn(SxSe1-x)4 (CZTSSe) samples have attracted considerable attention for thin
elements [1-4].
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film solar cell applications due to their unique electronic and optical properties and abundant constituent To date, state-of-the-art CZTSSe solar cells have been demonstrated with highest power
conversion efficiency of 12.6% [5] by Mitzi et al. through the hydrazine-based deposition process.
To
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further improve the efficiency of the cells and meet the requirement for practical applications, it still
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remains challenging to develop fabrication methods that can offer control over the composition, morphology and uniformity of CZTSSe thin films.
In the CZTSSe absorber layer, it was well demonstrated that the high Se content can not only enhance grain growth [6], but also, facilitate n-type and p-type doping [7], both of which then benefit the film These effects were verified in the champion Cu2ZnSnSe4 (CZTSe) cell with
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quality for solar cells.
efficiency as high as 10.1% [8], much higher than the record cell of its full sulfide analogues (Cu2ZnSnS4, 8.4% efficiency) [9].
Introduction of Se in CZTSSe material can be achieved by using excess Se ligand in
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a precursor [10] or gaseous Se [8,11], leading to the formation of partially [10,11] or fully [8] Another route to form selenides is the use of metal oxides as the
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Se-substituted CZTSSe materials. starting materials.
For instance, it was reported that CuInSe2 thin films suitable for photovoltaic
application can be fabricated through selenization of metal oxides by H2Se [12] or gaseous Se [13] at elevated temperature.
The direct selenization approach appears to be a prototype for the formation of thin
films of multiple selenide compounds, in which the replacement of selenium to oxygen is likely to play as the driving force for mass transfer in the solid-state reaction.
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Similar reaction, however, has not been
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demonstrated with Cu2ZnSnSe4 compound yet.
Therefore, herein we report our study on the fabrication
of CZTSe thin films through direct selenization of metal oxides.
The resulting CZTSe thin film shows
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compacted film morphology, grain size in micrometers, and suitable photoelectrochemistry properties, suggesting that this material is potentially applicable for photovoltaic devices.
The reaction pathway of
this formation process has also been investigated through the selenization of individual binary metal oxides
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at the same condition.
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2. Experimental Section 2.1. Chemicals
Copper (II) acetate monohydrate (> 99.0%, Tianjin Fu Chen Chemical Factory), zinc (II) acetate dehydrate (> 99.0%, Kelong Chemnet), tin (II) sulfate (> 99.0%, Kelong Chemnet), concentrated sulfuric
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acid (98%, Kelong Chemnet), sodium hydroxide (> 99.0%, Kelong Chemnet), selenium (> 99.9%, Kelong Chemnet) were used without purification.
2.2. Thin film fabrication and selenization
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The synthesis procedure of Cu2ZnSnSe4 thin films began by dissolving 0.887 g Cu(OAc)2⋅H2O (4.4
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mmol), 0.665 g Zn(OAc)2⋅2H2O (3.0 mmol), and 0.542 g SnSO4 (2.5 mmol) in 50 ml deionized water with 5 ml concentrated sulfuric acid, followed by co-precipitating at pH to 7-8 adjusted with sodium hydroxide solution (0.5 mol/L).
The resulting yellow co-precipitations were filtered, rinsed with deionized water for
three times, and dried at 120 °C for 2 h.
The dried co-precipitations were ball-milled for 1 h in a fast
vibrating ball mill (Nanjing NanDa Instrument Plant, QM-3A) to get homogeneous metal oxides precursors, which were spin-coated onto a 1 cm2 soda lime glass substrate at a spin rate of 2000 rpm.
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The as-coated
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precursor films were loaded into a graphite box in a tubular furnace that was vacuumed and purged with argon gas and selenized at 550 °C for 40 min in excess selenium carried by argon gas at a flow rate of
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20-40 ml/min under ambient pressure. In order to investigate the formation pathway of Cu2ZnSnSe4 from metal oxides, we performed the selenization of Cu, Zn, Sn metal oxides individually, using the similar procedure as above.
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The Cu2ZnSnSe4 thin films for photoelectrochemical measurement were prepared by spray coating the
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ink of metal oxides on fluorine-doped tin oxide (FTO) glass with a heating temperature of 200 °C by using a hot plate, and the film thickness was controlled to about 400 nm. 2.3. Characterization.
The crystallographic structures of pre-selenized and post-selenized layers metal oxides were
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characterized by both X-ray diffraction (XRD, Bruker D8 ADVANCE) with a Cu K-alpha (1.5418 Å) source at 40 kV voltage and 40 mA current by 0.1 s/step (detection angle ranging from 10° to 90°), and Raman spectrum was obtained using Raman spectroscopy (Renishaw inVia) under excitation of a 532 nm The morphology and composition were analyzed by scanning electron microscope (SEM, Hitachi
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laser.
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S-4800 and Zeiss-EVO LS-15) equipped with energy dispersive X-ray spectrometry (EDS, Bruker Nano XFlash Detector 5010).
2.4. Photoelectrochemical measurements All electrochemical experiments were carried out on an electrochemical workstation (CHI660D, CHI
Instruments, Shanghai).
A three-electrode configuration was used with Cu2ZnSnSe4/FTO as
photo-electrodes, a platinum wire as the counter-electrode, and saturated Ag/AgCl as the reference
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electrode.
The photo-electrochemical measurements were performed in 0.1 mol/L Na2SO4 aqueous
solution (with pH = 0 adjusted by adding H2SO4) and illuminated with a 40 W incandescent lamp.
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3. Results and Discussion The fabrication route to the target Cu2ZnSnSe4 film is shown in Figure 1.
The inks were prepared
based on a homogeneous mixture of metal oxides that was formed through co-precipitating a homogeneous
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acidic solution of Cu(OAc)2, Zn(OAc)2 and SnSO4 using NaOH, followed by dehydration and ball milling.
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The spin-coated thin films using the ink were selenized at 550 °C for 40 min. precursor films and selenized layers were measured by EDS, as shown in Table 1.
The compositions of
The dramatic decrease
of oxygen content from 64.56% in precursor film to 3.63% in selenized film and increase of selenium content from zero in precursor film to 50.46% in selenized film suggest that the oxygen atoms were Further study to adjust the selenization condition for
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replaced by selenium atoms at elevated temperature.
full selenization of the oxides is under investigation.
The atomic ratios of Zn/Sn and Cu/(Zn + Sn) in
selenized layer are 1.13 and 0.83, respectively, which are in close agreement with those in the starting This copper-poor and zinc-rich composition of selenized film is desirable for
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material (1.20 and 0.80).
This result also suggests that the current selenization route
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high efficiency Cu2ZnSnSe4 solar cells [8, 9].
can be used for the synthesis of CZTSe with tunable metal ratios by simply changing the ratios of starting metal salts.
In the characterization of the selenized thin film, XRD pattern in Figure 2a exhibits strong peaks that
are characteristic of highly crystalline Cu2ZnSnSe4 (JCPDS 52-0868).
Although the peaks at 2θ =
28.336°, 35.290°, and 42.935° corresponding to (103), (202), and (213) planes of Cu2ZnSnSe4 in this study
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were absent in JCPDS 52-0868 reported by Matsushita et al. [14], these diffraction peaks were observed by Zoppi et al. [15].
Furthermore, as the strongest X-ray diffraction intensity, the selenized film exhibits a
series [10].
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preferred (112) orientation, which has also been observed in the CZTSSe thin film of the champion cell Although the XRD peaks in Figure 2a are all characteristic of CZTSe, the existence of ZnSe
and Cu2SnSe3 phases cannot be ruled out because the overlap of their characteristic diffraction peaks with Therefore, Raman spectrometry (Figure 2b) was also used to further identify the
phase status of the as-selenized materials in this study.
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those of CZTSe [16]
The evident Raman peaks at 170, 192, 233, and
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243 cm-1 are in accord with those of Cu2ZnSnSe4 reported in previous literature [17,18].
The Raman
result, together with the XRD data, unambiguously confirms the phase purity of the CZTSe synthesized by this selenization route.
In comparison to traditional hot-injection method [19] and ball-milling method
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[20], the current CZTSe film formation route uses easily available ink of metal oxides as the starting material, making it a potentially cost-efficient method. Figure 3 illustrates the morphology of as-prepared CZTSe layer determined by SEM.
Non-unifomity
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of the as-prepared CZTSe film can be seen from Figure 3a: grain distribution with large grains (on the order
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of a few micrometers) on the surface and relative small grains (on the order of several hundred nanometers) in internal region.
The magnified image of CZTSe thin film (Figure 3b) shows that the selenized film is
compact and the crystals are in the size of micrometer order. that the selenization undergoes a complex process.
The grain distribution of the film indicates
Non-uniform mass transfer may be an important
problem in this process, because oxygen atoms are removed from the oxides and selenium atoms are added in the lattice structure. Probably, non-uniform distribution of gaseous Se diffusing from the surface to
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inside limits the selenization rate and mass transfer, degrading the uniform grain growth.
Further
modification of the selenization conditions to improve grain growth of the film remains to be investigated.
performed.
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To determine the photo-electronic properties of the selenized thin film, photo-electrochemical study was Figure 4 shows the linear sweep voltammogram of an as-prepared CZTSe film on FTO glass
in a 0.1 mol/L Na2SO4 aqueous solution with pH = 0 adjusted by adding H2SO4.
The CZTSe film
When the electrode potential is more positive than 0.4 V (vs. Ag/AgCl), the
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changes from 0 to -0.8 V.
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electrode exhibits a current increasing and dropping during illumination/dark conditions when the potential
current showed a continued increase with enhancement of the potential, indicating the increase with redox current at the interface between CZTSe film and the solution.
In the photo-electrochemical process, a
constant potential of -0.2 V was applied to the film with the light turned on and off sequentially, and the
photo-responsive property.
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film has a stable photocurrent response during the cycling (insert of Figure 4), exhibiting reliable A rapid decrease appeared after switching on the light, showing that the
photo-generated electrons in p-type CZTSe film were immediately involved in the recombination reaction.
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These results suggest that the CZTSe films prepared through direct selenization of metal oxides may be
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suitable for photovoltaic application.
To investigate the CZTSe formation pathway in this direct selenizaition route, selenization of
individually synthesized copper oxide, zinc oxide, and tin oxide were performed on the same conditions. XRD patters of these films before and after selenization are compared and shown in Figure 5, and accordingly, Table 2 lists the related selenization rates of these reactions to form binary selenide compared with that to form CZTSe.
In Figure 5a, all of the peaks in copper oxide precursor can be indexed to the
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CuO (JCPDS 45-0937) phase, which was fully converted into Cu2Se (JCPDS 65-2982) after selenization at 550 °C for 40 min, with a comparable selenization rate with that in the formation of CZTSe, as shown in It should be noted that in this selenization process, Cu(II) was reduced to Cu(I) by
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entry 1 of Table 2.
elemental Se, and the thermodynamically more stable Cu2Se phase was formed probably through a CuSe intermediate, as has been well demonstrated [21].
On the other hand, it can be observed in Figure 5b that
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ZnO (JCPDS 36-1451) and ZnSe (JCPDS 65-9602) both existed in the post-selenized thin film of zinc
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oxide after the same period of selenization, indicating that fully selenization of zinc oxide has slower reaction kinetics than that of selenization of oxide mixtures to form CZTSe (equation of entry 2 in Table 2). Figure 5c shows that not the predicted SnO but Sn6O4(OH)4 (JCPDS 46-1486) resulted from 2 h and 120 °C dryness of the precipitate prepared by precipitating dissolved SnSO4 in concentrated sulfuric acid After selenization of the resulting Sn6O4(OH)4 for 40 min at 550 °C, the mixture
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with sodium hydroxide.
of SnO2 (JCPDS 41-1445) and SnSe2 (JCPDS 23-0602) was obtained, suggesting a slower selenization of tin selenide than that of CZTSe formation in this study (entry 3 in Table 2).
The faster formation kinetics
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for CZTSe than those of formation of individual ZnSe and SnSe2 at the same selenization condition
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suggests that there are some synergetic effects during selenization of the oxide mixture. For comparison, Figure 5d illustrates the XRD result of a precursor thin film of a mixture of binary
oxides of Cu, Zn, and Sn, showing that it consists of Cu2O (JCPDS 65-3288), ZnO (JCPDS 65-0523), and SnO2 (JCPDS 29-1484).
These metal oxide phases are different from those formed individually, which
may be the key reason for the synergetic effect in the formation of CZTSe.
During the synthesis of the
oxide mixture, the oxidation states of copper and tin have changed from Cu(II) and Sn(II) in the starting
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reagents to Cu(I) and Sn(IV) in the oxide mixture, corresponding to the oxidation states of the two metals in Cu2ZnSnS4 compound. This oxidation state change of Cu and Sn may facilitate the formation of
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Cu2SnSe3, an important intermediate involved in the formation of CZTSe [22], and then Cu2SnSe3 reacts with ZnSe formed from the selenization of ZnO, as shown in the stepwise reactions in entry 4 of Table 2. Instead of the step-wise pathway, another possible reaction pathway may be that the mixture of Cu2O, ZnO,
Furthermore, the ZnO formed in this oxide mixer is different phase to
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Zn and Sn to form CZTS [22,23].
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and SnO2 are selenized to form CZTSe (entry 5 of Table 2), similar to the reaction of binary sulfides of Cu,
the oxide formed individually (JCPDS 65-0523 vs. JCPDS 36-1451).
The former cubic ZnO phase in the
oxide mixture is proposed to have a faster selenization rate towards the formation of CZTSe than that of the latter hexagonal ZnO phase synthesized individually [22]. Similar reaction kinetics may be involved in
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the current synthesis, and be the driving force that leads to the overall faster selenization rate to produce CZTSe compared with the reaction to form individual ZnSe at the same condition.
4. Conclusions
Using elemental
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We have developed a non-vacuum route for the fabrication of Cu2ZnSnSe4 thin films.
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Se as the selenium source, direct selenization of precursor films containing a mixture of the three metal oxides, prepared by spin-coating the nano-ink, leads to the formation of compacted Cu2ZnSnSe4 thin films with grain size of a few micrometers.
The prepared Cu2ZnSnSe4 thin film has favorable copper-poor and
zinc-rich composition that can be conveniently controlled in the preparation of metal oxides, and the film exhibits a photocurrent in the photo-electrochemical study.
The study of the formation pathway of
CZTSe shows that selenization rate of metal oxide mixture is comparable with that of CuO, but faster than
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those of ZnO and tin oxide at the same conditions. The possible reasons for facilitated formation of Cu2ZnSnSe4 from mixture of metal oxides in this route are: (1) oxidation state change of Cu and Sn to This fabrication method
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produce Cu2O and SnO; (2) formation of cubic ZnO, instead of hexagonal ZnO.
has potential for cost-efficient photovoltaic applications, which is in progress in our laboratory.
Acknowledgement
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The authors gratefully acknowledge the National Science Funds of China (Contract No. 51102038), the
Century Excellent Talents in University.
References
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Fundamental Research Funds for the Central Universities (ZYGX2012J156), and the Program for New
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Figure Captions
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Figure 1 Fabrication procedure of Cu2ZnSnSe4 thin films through selenization of metal oxides.
Figure 2 (a) XRD patterns of selenized layer of metal oxides compared with the reference diffraction peaks of Cu2ZnSnSe4 (red vertical lines, from JCPDS 52-0868); (b) Raman spectrum for the selenized layer.
magnification.
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Figure 3. SEM images of the surface of as-selenized Cu2ZnSnSe4 thin film: (a) low magnification, (b) high
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Figure 4. Linear sweep voltammogram of an as-selenized CZTSe film on FTO glass with light chopping in
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a 0.1 mol/L Na2SO4 aqueous solution with pH = 0 adjusted by adding H2SO4; insert is the current-time response curve of the same film at -0.2 V.
Figure 5. XRD patterns of pre-selenization (the dark line) and post-selenization (the red line) layers of (a) copper oxide, (b) zinc oxide, (c) tin oxide by using the same procedure; and (d) XRD patterns of precursor film of oxide mixture, there is an uncategorized peak at 2θ = 43.467°.
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Table 1. The composition of starting reagent, precursor film and selenized oxides Zn (%)
Sn (%)
Se (%)
O (%)
Zn/Sn
Cu/(Zn + Sn)
Starting reagent a
44.44
30.30
25.26
−
−
1.20
0.8
Precursor film b
13.86
11.70
9.88
−
64.56
1.18
0.64
Selenized layer b
20.86
13.30
11.74
50.46
3.63
1.13
a
0.83
This composition is calculated based on the amounts of Cu(OAc)2⋅H2O, Zn(OAc)2⋅2H2O, and SnSO4
starting reagents.
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Compositions determined by EDS.
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b
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Cu (%)
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Table 2. Plausible reactions and relative reaction rate involved in the formation of binary metal oxides and Cu2ZnSnSe4 through selenization of oxide(s) Reaction rate related to the
Entry
Reactiona,b
1
2CuO + 2Se → Cu2Se + SeO2
2
2ZnO + 3Se → ZnSe + SeO2
3
Sn6O4(OH)4 + 6Se → 3SnO2 + 3SnSe2 + 2H2O
4
2Cu2O + 2SnO2 + 9Se → 2Cu2SnSe3 + 3SeO2
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slower
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formation of CZTSe
2Cu2SnSe3 + 2ZnO + 3Se → 2Cu2ZnSnSe4 + SeO2
comparable
Cu2O + ZnO + SnO2 + 6Se → Cu2ZnSnSe4 + 2SeO2
comparable
For clarity, the oxide products in these selenization reactions have been all simplified to be SeO2.
b
SeO2 formed in these reactions is evaporated due to its relatively lower sublimation temperature (340
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°C) comparing with the selenization temperature (550 °C).
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> A cost effective fabrication method for Cu2ZnSnSe4 thin films has been developed. > Elemental selenium is used to directly selenize metal oxides into Cu2ZnSnSe4. > The synthesized Cu2ZnSnSe4 material has suitable properties for photovoltaic applications. > The metal oxide phases formed in mixture is likely to facilitate the formation of Cu2ZnSnSe4.