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Understanding the key factors of enhancing phase and compositional controllability for 6% efficient pure-sulfide Cu2ZnSnS4 solar cells prepared from quaternary wurtzite nanocrystals Xu Liu, Jialiang Huang, Fangzhou Zhou, Fangyang Liu, Kaiwen Sun, Chang Yan, John A. Stride, and Xiaojing Hao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04620 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016

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Chemistry of Materials

Understanding the key factors of enhancing phase and compositional controllability for 6% efficient pure-sulfide Cu2ZnSnS4 solar cells prepared from quaternary wurtzite nanocrystals Xu Liu, † Jialiang Huang, † Fangzhou Zhou, † Fangyang Liu, † Kaiwen Sun, † Chang Yan, † John A. Stride, ‡ and Xiaojing Hao*† † School of Photovoltaic and Renewable Energy Engineering, ‡ School of Chemistry, University of New South Wales, Sydney, NSW 2052 ABSTRACT: Pure-sulfide Cu2ZnSnS4 thin film solar cells were fabricated employing a facile and low-cost preparation procedure by depositing wurtzite CZTS nanocrystals, followed by high temperature sulfurization. The exploration of previously reported devices with >4.8% efficiency has demonstrated the potential application of wurtzite nanocrystals in optoelectronic devices. Moreover, TEM-EDS characterization revealed the presence of compositional fluctuations within the fine-grained sub-layer that may limit the performance of the final devices. In order to reduce the fine-grained sub-layer and further improve the crystalline quality of the active large-grained layer of our solar cells, the Na doping method and the sulfurization process were both systematically studied in this work. The crystal phase, morphology, elemental composition, and photovoltaic performance were characterized. These results indicate that, for the wurtzite material system, (1) tuning the Na amount is necessary, yet insufficient to ensure the good performance of solar cells and (2) the introduction of SnS powder in the sulfurization treatment provides leverage with which to improve the microstructure and compositional distribution of the final absorber. By employing this leverage to optimize our CZTS absorbers prepared from quaternary wurtzite nanocrystals, the performances of solar cells has been increased to 6.0% in the absence of an antireflection coating.

■ INTRODUCTION Cu2ZnSnS4 (CZTS) and Cu2ZnSnSxSe4-x (CZTSSe) thin film solar cells are potential alternatives to Cu2(In,Ga)Se4 (CIGS) and CdTe, as acknowledged by the increasing number of studies reported in this field in the last few years. 1 While the current record (12.7%) of CZTSSe solar cells was obtained from non-vacuum deposition method based on toxic and flammable hydrazine, the CZTS solar cells with 9.2% efficiency has also been achieved by vacuum-based technology recently and the Cu2Zn1-xCdxSnS4 (CZCTS) solar cells with 9.24% efficiency based on cation substitution process has been evidenced as well. 2-4 Compared with Se-containing CZTSSe and Cd-containing CZCTS materials, sulfur-containing CZTS is more attractive due to its eco-friendly and cheap constitute. Based on the theoretical calculation, CZTS may also be a promising candidate for the middle cell in a 3-cell tandem stack, because of its optimum band gap of about 1.5 eV.5 Among various material synthesis methods, nanocrystal-based approaches are more feasible and versatile in designing CZTS precursor materials and modifying intrinsic properties of CZTS by doping resulting in reduced defects density and impurity phases in the final film. 1, 5 Moreover, thin film solar cells have a major potential for cost reduction, and the thin film semiconductors prepared from nanocrystal inks can further reduce the cost because of its high

production rates and yields.5-7 To date, CZTSSe thin film solar cells from selenizing kesterite CZTS nanocrystals have demonstrated the efficiencies of nearly 10%. When it comes to the CZTS solar cells from sulfurizing kesterite CZTS nanocrystal, the demonstrated efficiency are in the very low range of 2%. 1, 5, 8 The major reason for this poor device performance is believed to be correlated with the difficulties in forming compact large-grains (in micron scale) comparable to those achieved by selenizing CZTS nanocrystals or sulfurizing metal sulfide precursors. 1, 5, 9 Within this framework, by the sulfurization treatment of meta-stable wurtzite CZTS nanocrystals in several minutes, we have recently overcome the grain-growth challenge of CZTS thin film from nanocrystals by demonstrating large-grained CZTS film 5 Compared to thermodynamically stable kesterite phase CZTS nanocrystals, the driving energy for the significant grain-growth of metastable wurtzite CZTS nanocrystals upon sulfurization treatment may result from two aspects: (1) the high surface area of nanocrystals leading to a reduction in total energy; (2) free energy difference between the metastable wurtzite phase and the stable kesterite phase.5 The sulfurization annealing atmosphere, addition of NaF layer, and Cu/Sn ratios in precursor solutions all significantly influence the qualities of sulfurized CZTS absorbers.5 Finally, the solar cells with the best efficiency of 4.83% (JSC = 17.4

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mA cm-2, VOC = 524.8 mV, FF = 52.8%) were demonstrated without anti-reflection coatings.5 Our preliminary trials on using the metastable wurtzite CZTS nanocrystals demonstrated a CZTS cell performance level which is comparable to those by pure-sulfide solar cells achieved from other fabrication routes such as sol-gel and/or sputter methods. 10, 11 However, despite this initial breakthrough in using wurtzite CZTS nanocrystal precursor, our CZTS absorber layers still obviously suffered from the unexpected bi-layer structures (large-grained top layer and fine-grained bottom layer).5 Different from the largegrained top layer with high crystalline quality and compositional homogeneity, the porous fine-grained bottom layer encounters intense composition fluctuations and numbers of grain boundaries which can result in undesirable recombination and potential disturbance.5, 12, 13 In order to reduce the fine-grained sub-layer and further improve the crystalline quality of the active largegrain layer of our solar cells, we systematically studied the amount of Na-containing compound and the sulfurization treatment in this work. We observed that, for the CZTS absorber sulfurized from wurtzite nanocrystals, sufficient Na-containing amount is necessary to ensure the reduction of the fine-grained sub-layer and the photovoltaic performances of solar cells (including JSC, VOC, and FF) would be significantly improved after removing the fine-grained sub-layer. However, simply tuning the Na content is insufficient to ensure complete removal of a fine-grained layer. The introduction of SnS powder in the annealing atmosphere also offers a strong influence on the microstructure and composition of the final absorber, and the electronic properties of solar cells (including JSC and FF) would be further improved. Moreover, along with the formation of well-crystallized CZTS grains, a thin layer mainly consisting of carbon and MoS2 appears at the interface between the back contact and the absorber. Finally, the laboratory-scale 6.0% efficient pure-sulfide CZTS device without an anti-reflection coating prepared from quaternary wurtzite nanocrystals is obtained, which appears to be one of the highest pure-sulfide CZTS solar cells obtained to date with a solution-based or nanocrystal-based process.

■ EXPERIMENTAL SECTION Chemicals and Materials Cu(OAc)2·H2O (copper(II) acetate monohydrate, >98%), Zn(OAc)2·2H2O (zinc(II) acetate dehydrate, >99%), Sn(OAc)2 (tin(II) acetate, 99%), TOPO (trioctylphosphine oxide, 99%), ODE (1-octadecene, 90% tech), 1-DDT (1dodecanethiol, 98%), t-DDT (tert-dodecylmercaptan, 98.5%), TOA (trioctylamine, 98%), NaCl (sodium chloride >99%), and DMSO (dimethyl sulfoxide, >99.7%) were purchased from Aldrich. All chemicals were used as received without any further purification. Synthesis of wurtzite CZTS nanocrystals The experiment details regarding the nanocrystal synthesis can be found in our previous work (see ref. 5). In this work, we use copper(II) acetate monohydrate to replace copper(I) acetate due to its better dissolution property. In

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a representative synthesis, Cu(OAc)2·H2O, Zn(OAc)2·2H2O, and Sn(OAc)2 were dissolved into a mixture of TOA and ODE and then evacuated at room temperature for 1h. The Cu/Zn/Sn ratio in the metal precursors was off-stoichiometric (1.38/1.05/1.00). This precursor solution was then heated to 140 ⁰C and held at 140 ⁰C for 10 min under a N2 atmosphere, where a mixture of 1-DDT and t-DDT was injected into the flask. The reaction was allowed to progress at 250 ⁰C for 1 h with continuous stirring. The as-prepared nanocrystals were then washed for several times by 2-propanol and centrifuged to yield a dark centrifuged precipitate. Preparation of CZTS thin films Previously reported devices with > 4.8% efficiency adopted the physical vapor deposition of sputtered NaF layer as the Na-source to ensure the sufficient grain growth. 5 In order to make the fabrication process more facile, quantitative, and versatile, in this work, we use chemical solution method of incorporating extrinsic NaCl salts directly into the nanocrystal ink and the corresponding films. The NaCl salt is dissolved into DMSO to form 0.015 M NaClDMSO solution, and the wurtzite CZTS nanocrystals are subsequently dispersed to form inks for the sequence experiment. The inks are stable at room temperature in air for over 2 weeks. The precursor thin films are deposited by spinning precursor inks on Mo-coated soda lime glass (SLG), followed by annealing on a hotplate at 300⁰C for 3 min in air. This coating step is repeated until achieving the desired thickness. The precursor films are then transferred into a RTP furnace and annealed at 580 ⁰C in a combined sulfur and SnS atmosphere (evaporation of 0.2 g solid sulfur pellets and 0.05 g solid SnS powder) for 10 min. Solar cell device fabrication The sulfurized films as described above are first submerged in DI water for a few minutes and then immediately put into NH4OH/CdSO4 solution for chemical bath deposition (CBD) of 50-60 nm CdS with details shown elsewhere.5, 10 A 100 nm i-ZnO layer and a 200 nm ITO layer were then sequentially sputtered on top of the CdS layer.5, 10 The final devices were scribed into small areas of about 0.1 cm2, with a small dap of silver paint as the front contact. Characterization X-ray diffraction (XRD) measurement was performed by the PANalytical's Empyrean thin film Xpert materials research diffraction system at a voltage of 45 kV and a current of 40 mA with Cu Ka radiation (λ = 15.4 Å). Raman measurement was conducted by an inVia Renishaw spectrometer coupled with a microscope at 325 and 514 nm at room temperature. The scanning electron microscopy (SEM) images and the corresponding elemental analysis were taken by using the FEI Nova NanoSEM system with a Bruker Silicon Drift Energy Dispersive X-ray microanalysis system. The microstructure and elemental distribution in each layer of the film were measured by JEOL JEM-ARM200F (200kV) aberration-corrected scanning transmission electron microscope (STEM) equipped with energy dispersive X-ray spectroscopy (EDAX) system. The device performance was obtained by light current–voltage


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Figure 1. (a) XRD of the precursor thin film on the glass; (b) XRD of the sulfurized thin film on the glass; (c) Raman spectra of the sulfurized thin film with 514 nm excitation; (d) Raman spectra of the sulfurized thin film with 325 nm excitation. All these films are made from the 0.015 M NaCl-DMSO nanocrystal ink.

(I–V) measurements under AM1.5 (1000 W m-2) irradiation and external quantum efficiency (EQE) measurements (QEX10 spectral response system from PV measurements, Inc.).

■ RESULT AND DISCUSSION Figure 1a is the XRD of the precursor thin film deposited from the 0.015 M NaCl-DMSO nanocrystal ink. All of the reflections can be attributed to the wurtzite CZTS structure.5, 14-16 The (200) plane of kesterite CZTS (at around 33°) is not detected in the XRD pattern, suggesting the high purity of wurtzite phase.5, 17 Figure 1b is the XRD pattern of the sulfurized thin film made from the 0.015 M NaCl-DMSO nanocrystal ink. All of the reflections match well with the standard kesterite CZTS and wurtzite CZTS are not present in Figure 1b, suggesting a thorough phase change from the wurtzite- to the kesterite-CZTS upon sulfurizaion. In order to further examine the phases within the absorber, Raman measurements were done at two

different excitation wavelengths (514 and 325 nm) where the 325 nm excitation wavelength is applied to detect the presence of the secondary phases like ZnS. Figure 1c is the Raman spectrum of the sulfurized thin film made from the 0.015 M NaCl-DMSO nanocrystal ink at 514 nm excitation wavelength. The strongest Raman peak centered at 338 cm-1 (Raman A mode) is associated with the breathing mode of S atoms surrounded by metal atoms in the kesterite CZTS structure. 17 The peaks of CuxS (474 cm-1) and Cu2SnS3 (305 and 355 cm-1) secondary phases are weakly observed.18 Figure 1d is the Raman spectrum of the sulfurized thin film at 325 nm excitation wavelength. No Raman peaks of ZnS (275 and 350 cm -1) secondary phase are detected as well. 18 It is known that the penetration depth of 325 nm laser in CZTS films is less than 10 nm. Therefore, we cannot exclude the presence of ZnS phase in the whole layer.5 Figure 2a is a top-view SEM image of the sulfurized CZTS thin film deposited from the 0.015 M NaCl-DMSO nanocrystal ink. This CZTS absorber layer displayed continuous densely-packed large grains with di



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Figure 2. (a) Top-view SEM of the CZTS thin film made from 0.015 M NaCl-DMSO solution; (b) Bright field TEM image of the same CZTS device; (c) HAADF STEM image of the same CZTS device (the inset shows the carbon-rich feature of the squared area); (d) Bright field TEM image of the magnified flake-like structure (the inset shows the interlayer distance of one individual flake); (e) EDS scan taken along the red-colour scan line in (c); (e) EDS scan taken along the scan line in the inset of (c).

ameter up to micron-size and no noticeable holes on the surface of absorber. Compared with the smooth and compact surface of the precursor thin film (see Figure SI 1), the sulfurized CZTS absorber layer has a much rougher morphology due to the growth of grains upon sulfurization treatment. The metal stoichiometry ratios of Cu/Sn and Zn/Sn in the final whole film are 1.91 and 1.34, respectively. The increase of Zn/Sn ratio in the final sulfurized CZTS absorber indicates there is Sn loss occurring during the sulfurization treatment.5 Self-regulation of Cu/Sn ratio was recently reported for kesterite CZTS films prepared from quaternary wurtzite nanocrystals.5 Similar phenomenon also exists in the fabrication of CZTS films by sulfurizing Cu-Zn-Sn alloy.19 We would like to stress that the ratios of Cu/Sn and Zn/Sn as described above correspond to the overall film rather than the ones within individual grains. As it was reported in the references 20, 21 , the overall stoichiometry ratio of the kesterite film prepared from off-stoichiometric precursor’s ratios is partially dominated by presence of some certain secondary phases, in particular ZnS phase, which is also observed by TEM-EDS mapping images in this work (see Figure SI 3). Figures 2b and 2c are the bright field and high angle annular dark field (HAADF) TEM images of completed

CZTS devices, respectively. The thickness of the precursor film is about 900 nm. The absorber obtained by annealing wurtzite CZTS nanocrystal precursor thin film consists of large grains with the diameter up to micron-sized level, suggesting that the sulfurization duration of 10 min is sufficient to allow the growth of nanocrystals into large grains. Usually, the growth of grain is regarded as a key parameter of forming high quality absorber layer. 1, 5, 9 In this work, the thickness of MoS2 layer is about 200 nm, which is much thinner than our previously reported values (about 500 nm). 5 Notably that a thin layer of about 100 nm thickness is observed between the absorber layer and the MoS2 layer. The microstructure of this thin layer exhibits unique flake-like morphology, which is particularly distinct from the reported structure of fine-grained layers. 1, 5, 22 Figure 2d is the magnified TEM image of the flake-like thin layer. This flake-like thin layer seems like a mixture consisting of crystalline nano-plates and other amorphous materials. The interlayer distance of the crystalline nano-plate is 0.62 nm which corresponds to the (002) plane of MoS2 materials. 23 In order to further clarify details of the large-grained absorber and the flake-like layer, we used the EDS scan (taken along the red-colour line in Figure 2c) to show the local compo


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Figure 3. (a) Performance of a CZTS solar cell made from the 0.015 M NaCl-DMSO nanocrystal ink; (b) EQE of the same device.

sitional profile. The element compositions of Cu, Zn, and Sn in the CZTS large grain layer are quite homogeneous. The homogeneous large grain layer is believed to not only contribute to the phase stability but also reduce the local fluctuation of open circuit voltage as well as carrier recombination that could ultimately lead to the poor device performance. 5, 24 Similar to those pointed out by some reported studies,20, 21 note that, in this work, the measured proportion of Cu, Zn, and Sn of the individual CZTS grains are close to stoichiometry (i.e. Cu2ZnSnS4). Specifically, the compositional ratios of Cu/Sn and Zn/Sn within an individual large grain are about 1.86 and 1.05, respectively. Figure 2f is the EDS scan taken along the line in the inset of (c) focusing on the compositional tendency within the fake-like thin layer. Interestingly, according to the analysis of EDS data, the flake-like structure between the large-grained absorber and MoS2 layer presents significantly carbon enrichment. No signals of element Zn and element Sn can be detected within this layer. The source of carbon should result from organic materials (i.e., like ligands, surfactants, metal or sulfur precursors) in the intermediate solution or in the final ink used for the deposition of the precursor thin film. Recently, a thin carbon layer introduced on Mo/SLG substrate prior to the deposition of CZTS absorbers was found to have a positive effect in the performance of solar cells by modifying the back contact and reducing the series resistance.25 Therefore, the flake-like carbon-rich layer found here might be a bonus to boost the cell performance though it needs further investigation. The formation mechanism of this carbon-rich layer can be explained by the efficient growth of CZTS nanocrystals into large grains with high crystalline quality. The continuously and densely large-grained absorber layer would push the residual carbon materials to the back contact area and form a thin layer with a flake-like morphology. The photovoltaic device current density-voltage of a typical CZTS device made from the 0.015M NaCl-DMSO nanocrystal ink without antireflection coating is shown in

Figure 3a, demonstrating open circuit voltage (VOC) of 583.6 mV, short circuit current density (JSC) of 18.3 mA cm-2, fill factor (FF) of 56.1 %, and PCE of 6.0 % under AM 1.5 illumination. To the best of our knowledge, all these electrical values are comparable to those highest reported for pure-sulfide CZTS thin film solar cells made from solution-based methods (especially from nanocrystal-based method). Figure 3b is the EQE of the same CZTS device. The band gap of the CZTS absorber estimated from the EQE data is about 1.5 eV (Figure SI 2). Referring to the curve in the visible range, the intensity of the EQE signal starts to decay in the long wavelength region (around 600 nm) suggesting the decreased collecting ability of the carriers which are generated deep in the bulk CZTS, i.e. far away from the depletion region. This lower collection rate of carriers is caused by the problem of narrow depletion region and short diffusion length of CZTS materials.5, 22 Hereafter, this 6% efficient CZTS solar cell made from the 0.015M NaCl-DMSO nanocrystal ink is referred to as sample A. Factors Impacting Device Performances To investigate the effect of the Na on the properties of CZTS absorber, the concentration of NaCl-DMSO solution was changed from 0.015 M to 0.01 M while remaining other experimental parameters the same as those of sample A. The CZTS solar cell made from the 0.01 M NaClDMSO nanocrystal ink is referred to as sample B. Figure 4a shows the cell performance as a function of the concentration of NaCl-DMSO solution. Typically, three photovoltaic parameters (JSC, VOC, and FF) present clearly lower values for sample B compared with A. Specifically, for sample B, VOC of 517.6 mV, JSC of 16.5 mA cm-2, FF of 53.9%, and PCE of 4.6% are demonstrated under AM1.5 illumination. Compared to the EQE of the sample A, the sample B demonstrates the similar curve in the short wavelength region as shown in Figure 4b. However, it shows a faster decay from the visible wavelength to the



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long wavelength region, suggesting its weak collection rate of carriers generated from absorber, especially the region far away from the p-n junction. The weaker collection capability of carriers in the long wavelength may be caused by more recombination happened within the CZTS absorber, especially the region near the back contact. Figure 4c shows the TRPL transient of the complete devices made from different NaCl-DMSO solutions. Compared to sample B, sample A yields a slightly longer minority carrier lifetime from 2.46 ns to 3.54 ns. The TRPL results match well with the deduction from the aforementioned EQE spectral response. In order to further explore the reason hindering the device performance, we studied the microstructure of sample B by using TEM-EDS characterization. Figure 5a and 5b are the bright field TEM and HAADF TEM images of sample B. The unique flakelike thin layer which is found to be a mixture of crystalline MoS2 and residual carbon as described in sample A has not been observed in Sample B. The obtained CZTS absorber layer of sample B has a typically bi-layered structure consisting two regions, i.e. a small-grained bottom layer and a large-grained top layer. The average thick-

nesses of the large-grained layer and the fine-grained layer are both about 450 nm. Moreover, the grain sizes within the fine-grained layer are normally less than 100 nm. In addition, tiny isolated voids are observed throughout the whole fine grain layer. Figure 5c is the compositional profile taken along the red-colour line shown in Figure 5b. Cu, Zn, and Sn keep quite homogeneous through the large-grained CZTS layer. The average intensity of detected Sn signal is nearly half that of Cu, but quite close to that of Zn (the compositional ratios of Cu/Sn and Zn/Sn within the large grain are about 1.96 and 1.00, respectively). In contrast, the elemental composition, especially Cu and Zn, fluctuates widely within the fine-grained layer. The loss of elemental Sn is observed as well. The signal intensity of Cu, in some regions, is even lower than that of Zn. Those regions with large compositional fluctuation would undoubtedly increase the risk of forming secondary phases within the fine-grained layer, which is known quite detrimental to the device performance. In addition, according to the EDS result, most Cu-rich regions correspond to the Zn-poor features and vice versa. The opposite trend of Cu and Zn agrees with the report on the un-


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favourable formation of the Cu-Zn-S ternary phase but heterogeneous Cu2-xS-ZnS materials instead.26 The EDS mapping results of the sample B (Figure SI 4) confirm the analysis from EDS line scans. Interestingly, in this work, we find that the interface between the large-grained layer and fine-grained layer is usually strongly Cu-rich. This phenomenon has been confirmed by measurements at different sites of samples as shown in Figure SI 5. Similar to CIGS materials, a Cu-rich growth step leads to larger grains during the high temperature growth for kesterite materials.27 Therefore, this Cu-rich interface as described above may be a potential driving force for the further crystallization of the CZTS absorber. The EDS line scan in Sample B (see Figure SI 6) also shows the carbon enrichment at the interface between the large grained and the fine-grained layer where lots of voids also exhibit. In the denser region of the fine-grained layer, the overall carbon intensity was found similar to that in the top largegrained layer. Up to date, the reason for this fine-grained structure widely existing in thin films made from nanocrystals still remains unclear. Some hypothesis and/or experimental results have been therefore proposed to explain the origin of the fine-grained layer, such as the thermal decomposition of organic materials, faster element loss from some regions, the segregation of certain metal cations, the faster penetration rate of vapor into the top region of the film, and different profiles of temperature vs. time at the top and bottom of the film.5, 8, 22, 24, 28, 29 While several publications claimed that the fine-grained sub-layer does not significantly affect their device performance, some reported studies believe that numbers of surface states within the fine-grained layer are detrimental to electrical parameters of devices due to the increased recombination at grain boundaries and the decreased mobility of carriers. 1,9,14,29,30 In this work, for sample B, the possible reasons of its inferior device performance may be caused by: 1) Reduction of the thickness of the top layer, which is believed to dominate the electrooptical properties of the whole film. 2) Changes in the properties of the top layer, such as the compositional difference of the large-grained layer between Sample A and Sample B. Compared to Sample A, the large grain of Sample B is relatively Cu-richer, increasing the possibility of forming detrimental CuZn deep defect. 3) Recombination at the fine-grained layer. Compared to the previous physical vapour deposition of sputtered NaF layer, a more quantitative solution-based approach is adopted in this work to controllably study the impact of Na on the puresulfide CZTS absorber made from wurtzite nanocrystals. By comparing sample B (0.01M NaCl-DMSO solution) with sample A (0.015M NaCl-DMSO solution), a sufficient quantity of Na is observed to be one of the key factors to reduce the thickness of the notorious fine-grained layer. Studies about understanding the role of Na in CIGS-based and CZTS-based materials have revealed the importance of Na in enhancing the grain size, passivating deep defects and traps, and increasing the carrier concentration and/or mobility.31-33 It was found that certain phases with low melting points, which act as fluxing agents to improve the growth of grains, may form from Na com-

pounds.34 Recently, the correlation between Na and Zn out of the CZTS film bulk has been also reported.31 As grains grow, excess Zn not incorporated into the CZTS material should be expelled and precipitate as ZnS, which is known as solute drag process likely impeding grain boundary motion. Na appears to help the re-distribution of the Zn within the bulk film when grain growth is enhanced.31 This surfactant behavior of Na could be due to the formation of a liquid Na2S or Na-Zn-S phase at increased annealing temperatures, which is believed to facilitate the mass transport along the grain boundaries.31 To investigate the effect of the annealing atmosphere on the properties of CZTS absorber, the SnS powder was removed before the sulfurization treatment while maintaining all other experimental parameters the same as those of sample A. Figure 6a shows the photovoltaic response of a corresponding CZTS device under AM 1.5 illumination, demonstrating VOC of 584.1 mV, JSC of 17.3 mA cm-2, FF of 48.4%, and PCE of 4.9%. This 4.9% efficient device without SnS in the sulfurization treatment is referred to as sample C. According to the I-V curves shown in Figure 6a, the sample C without SnS sulfurization treatment leads to a nearly same VOC value, whereas apparently decreased JSC and FF values. In Figure 6b, compared to sample A, the EQE spectral response of sample C shows a similar curve in the long-wavelength part, whereas a clear decrease in the short-wavelength region. Figure 7a is the top-view SEM of sample C, suggesting the smaller average grain size than that of sample A. Moreover, valleys and/or holes are quite obvious in sample C, indicating the correlation between the annealing environment for the growth of particles and film morphology, which could deteriorate the final device performance via rougher surface and/or potential shunting sites. In this case, the deterioration of the absorber surface may cause the non-uniform coverage of CdS layer resulting in the thicker CdS layer in some regions and the decreased EQE response in the region with short wavelength. Recently, the systematic study of the CdS buffer layer from the view of optical design was reported.35 It is known that a nonnegligible fraction of the solar spectrum will be absorbed in the CdS layer as CdS is a direct-gap material with a large absorption coefficient for photon energies above 2.4 eV (λ < 520 nm.35 Therefore, in order to improve the device performance by maximizing JSC, the CdS should be made as thin as possible when its coverage is good.35 Figure SI 7 is the EDS mapping results of sample C. The clear agglomeration phenomenon of CdS particles is observed at the p-n junction interface, which is in good agreement with the observed surface morphology and the corresponding analysis. Therefore, one of possible reasons about the lower JSC of Sample C may be caused by the deteriorated CdS coverage. Figure 7b and 7c are the bright field TEM and HAADF TEM images of sample C. Compared to sample A and sample B, sample C presents a unique bi-layered microstructure, particularly in the porous bottom part of the absorber. No clear carbon-rich



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Figure 7. (a) Top-view SEM of sample C, scale bar 1μm; (b) Bright field TEM image of the same CZTS device; (c) HAADF STEM image of the same CZTS device; (d) EDS scan taken along the red-colour scan in (c); (e) EDS scan taken along the blue-colour scan in (c).

layer as described in sample A is found here. For sample C, the top layer is still the large-grained morphology, whereas the bottom layer exhibits certain grain growth feature of fine grains. The sizes of some bottom grains have reached several hundred nanometres. Figure 7d and 7e is the compositional profiles taken along the red-

colour line and blue-colour line shown in Figure 7c, respectively. The compositional ratios of Cu/Sn and Zn/Sn in the large-grained layer of sample C are about 2.0 and 1.24, which are both higher than the ones in sample A, indicating more Sn loss may occur in sample C. The redline scan presents the average signal intensities of ele-


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ment Cu, Zn, and Sn within the bottom large grain keep nearly the same levels with those in the top large-grained layer, suggesting the successful grain growth of particles in this region. However, the nearby region around the blue line shows the typical fine-grained characteristic. Element Cu and Zn encounter intense fluctuation in the bottom layer present the adverse sequence as described above. Moreover, the signal of element Sn significantly drops in the bottom layer, aggravating the possibility of forming secondary phases, such as ZnS (see Figure SI 7). The EDS mapping images of sample C shown in Figure SI 6 confirm the results of EDS line scan. It has already reported that, for CZTS materials, the presence of ZnS phase are directly related to the short-circuit current and the series resistance of devices.36 This secondary phase mainly leads to a degradation of the series resistance due to its high band gap and low conductivity.36 In the semilog plotted dark I-V curve shown in Figure SI 8, the bending magnitude towards the right high voltage at high current levels can be used as an indication of the magnitude of series resistance (RS).37 The higher bending magnitude, the higher RS.37 By fitting the dark I-V curve, focusing on the high voltage range (V>500mV), the RS of Sample A and Sample C have been determined to be 6 Ωcm2 and 14 Ωcm2, respectively. In addition, Sn loss has been reported to occur in the annealing process through the route of the decomposition of kesterite phase followed by subsequent evaporation of SnSx.38, 39 Therefore, Sn compounds (either SnSx or Sn metal) introduced in the annealing process (i.e. combined sulfur and SnSx atmosphere) has been proposed as an effective countermeasure to this loss.40 Very recently, high performance CZTSSe solar cells was obtained by fine-tuning the Sn content. This results in the control of some key properties of active absorber, particularly the grain size and Sn-related deep defects.21 In fact, Sn content seems determines the balance among the deep midgap defect density, film morphology as well as the presence of secondary phases, which significantly affect the device performance.21 Although our as-synthesized wurtzite CZTS nanocrystals were already Sn-rich, extrinsic Sn is still necessary to ensure the well-crystalized morphology and homogeneous composition which can promote the good coverage of CdS layer and decrease the amount of ZnS secondary phase. In addition, compared with Sample A, large voids forming at the back contact region are observed, which may also decrease the JSC value due to incomplete connection with back contact.

to offer a strong leverage in the improvement of the final absorber. Along with the formation of high quality grains, an interesting carbon-rich MoS2 thin layer was observed between the CZTS absorber layer and back contact, which might improve the electrical properties by reducing series resistance. In addition, in this work, the devices with the fine-grained CZTS layer and/or incomplete growth of bottom fine-grained layer presented inferior performance, which may be caused by reduction of the thickness of the top layer, changes in the properties of the top layer, formation of numerous grain boundaries and big voids, reduced lifetime of carriers, deteriorated surface morphology, and/or intense compositional fluctuations. Finally, we demonstrated CZTS solar cells with the best efficiency of 6.0% without anti-reflection coatings. Future studies would focus on boosting the efficiency of pure-sulfide CZTS solar cells by understanding the evolution progress from wurtzite nanocrystals to kesterite large grains.

■ ASSOCIATED CONTENT Supporting Information. Additional characterization, inclusive of SEM of the precursor thin film, pattern of the estimated band gap, EDS line-scan, and EDS mapping images, the photograph of device, and more photovoltaic parameters of solar cells. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT This research was supported under the Australian Government through the Australian Renewable Energy Agency (grant number 1-USO028) and Australian Research Council’s Discovery funding scheme (grant number DE160101100). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Electron Microscope Unit, University of New South Wales (UNSW). The authors acknowledge use of the facilities in the Electron Microscopy Centre at University of Wollongong and the assistance and discussion of David Mitchell and Gilberto Casillas Garcia.

■ CONCLUSION In summary, tuning the Na content in the wurtzite CZTS nanocrystal ink and introducing SnS powder in the sulfurization treatment for fabricating our pure-sulfide CZTS absorber thin films have a clear and significant effect on our photovoltaic devices, in particular in terms of the grain size, the surface morphology, the fine-grained layer, and compositional distribution. Na content is necessary yet insufficient to ensure the good performance of solar cells prepared from wurtzite nanocrystals. Extrinsic treatment with Sn-containing compounds was also found

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35.Winkler, M. T.; Wang, W.; Gunawan, O.; Hovel, H. J.; Todorov, T. K.; Mitzi, D. B., Optical designs that improve the efficiency of Cu2ZnSn(S,Se)4 solar cells. Energ Environ Sci 2014, 7, 10291036. 36.Fairbrother, A.; Garcia-Hemme, E.; Izquierdo-Roca, V.; Fontane, X.; Pulgarin-Agudelo, F. A.; Vigil-Galan, O.; PerezRodriguez, A.; Saucedo, E., Development of a Selective Chemical Etch To Improve the Conversion Efficiency of Zn-Rich Cu2ZnSnS4 Solar Cells. J Am Chem Soc 2012, 134, 8018-8021. 37.King, D. L.; Hansen, B. R.; Kratochvil, J. A.; Quintana, M. A., Dark current-voltage measurements on photovoltaic modules as a diagnostic or manufacturing tool. Conference Record of the Twenty Sixth Ieee Photovoltaic Specialists Conference - 1997 1997, 1125-1128. 38. Scragg, J. J.; Ericson, T.; Kubart, T.; Edoff, M.; PlatzerBjorkman, C., Chemical Insights into the Instability of Cu2ZnSnS4 Films during Annealing. Chem Mater 2011, 23, 46254633. 39. Redinger, A.; Berg, D. M.; Dale, P. J.; Siebentritt, S., The Consequences of Kesterite Equilibria for Efficient Solar Cells. J Am Chem Soc 2011, 133, 3320-3323. 40.Chen, S. Y.; Walsh, A.; Gong, X. G.; Wei, S. H., Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth-Abundant Solar Cell Absorbers. Adv Mater 2013, 25, 15221539.



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