CopperâZincâTinâSulfide Thin Films via Annealing of Ultrasonic

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Copper Zinc Tin Sulfide Thin Films via Annealing of Ultrasonic Spray Deposited Nanocrystal Coatings Bryce A. Williams, Nancy D. Trejo, Albert Wu, Collin S. Holgate, Lorraine F. Francis, and Eray S. Aydil ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017

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ACS Applied Materials & Interfaces

Copper Zinc Tin Sulfide Thin Films via Annealing of Ultrasonic Spray Deposited Nanocrystal Coatings Bryce A. Williams, Nancy D. Trejo, Albert Wu, Collin S. Holgate, Lorraine F. Francis*, and Eray S. Aydil* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States. KEYWORDS: copper zinc tin sulfide, grain growth, nanocrystal, thin-film solar cell, ultrasonic spray coating

ABSTRACT

Thin polycrystalline films of the solar absorber copper zinc tin sulfide (CZTS) were formed by annealing coatings deposited on molybdenum-coated soda lime glass via ultrasonic spraying of aerosol droplets from colloidal CZTS nanocrystal dispersions. Production of uniform continuous nanocrystal coatings with ultrasonic spraying requires that the evaporation time is longer than the aerosol flight time from the spray nozzle to the substrate such that the aerosol droplets still have low enough viscosity to smooth the impact craters that form on the coating surface. In this work, evaporation was slowed by adding a high boiling point cosolvent, cyclohexanone, to toluene as the dispersing liquid. We analyzed, quantitatively, the effects of the solvent composition on the aerosol and coating drying dynamics using an aerosol evaporation model. Annealing coatings in

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sulfur vapor converts them into polycrystalline films with micrometer size grains but the grains form continuous films only when Na is present during annealing to enhance grain growth. Continuous films are easier to form when the average nanocrystal size is 15 nm: using larger nanocrystals (e.g., 20 nm) sacrifices film continuity.

1.#INTRODUCTION# Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is a promising material for next generation thin film solar cells. It comprises earth-abundant non-toxic elements and, when alloyed with Se to make Cu2ZnSn(Sx,Se(1-x))4 (CZTSSe), its bandgap covers the broad ideal range for photovoltaic devices (1-1.5 eV).1-3 CZTS solar absorber layers may be produced from a variety of precursor films formed by sputtering,4,5 spray pyrolysis,6 electrodeposition,7,8 and casting coatings from solutions containing molecular precursors2,9 or colloidal dispersions containing nanocrystals.10-13 These precursor films and coatings are annealed at 550-600 oC in sulfur (or selenium) vapor to form polycrystalline films. In principle, this scheme can be scaled to continuous low-cost production using roll-to-roll manufacturing.10,14 However, nanocrystal coatings for solar absorber applications have traditionally been formed through processes such as dip-coating,15,16 dropcasting,17-20 and spin-coating,21,22 which are difficult to scale up to large production volumes. Spray coating, on the other hand, is well-suited for roll-to-roll production and for this reason a number of studies on spray coating substrates with nanocrystals of CZTS and other solar cell materials, such as copper indium gallium diselenide (CIGS), have been reported.23-26 In some of these studies, it was shown that heated substrates or inert atmospheres are necessary for successful coating formation, and this can possibly complicate process scale-up.25,26 Successful


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spray coating at ambient conditions without any additional drying steps is desirable for low-cost production of CZTS solar cells from nanocrystal dispersions. Regardless of the method used to form the CZTS nanocrystal coatings, they must be annealed into dense large-grained films to achieve high solar cell efficiencies.27 A suitable microstructure is a dense monolayer of grains with lateral dimensions approximately the same as the film thickness.27 Annealing is traditionally done in high-temperature (> 550 °C) furnaces in S or Se vapor.28 Introducing small amounts of alkali-metals (e.g., Na and K) during this annealing step has been shown to enhance CZTS grain growth and improve film morphology.29-33 However, incorporation of alkali metals into the CZTS film remains largely uncontrolled, because their concentration in the film is determined by their diffusion rate from the soda lime glass (SLG) substrate during annealing: this diffusion rate depends sensitively on the concentration of these impurities in the SLG substrate as well as the microstructure of the Mo electrode deposited on the SLG.30,34 To provide more consistent and reproducible microstructure development during annealing, various methods have been developed to deliberately and controllably incorporate alkali metals into the films.29,31,35,36 For example, Johnson et al. 31 dried aqueous NaOH solutions in controlled concentrations inside quartz tubes used for sulfidizing sputtered copper-zinc-tin metal alloy films to form CZTS. They found that Na, from NaOH dried on the walls of the ampules, could be transported to the films via the vapor phase during sulfidation and Na incorporation in this way improved film morphology: specifically, they showed that the average grain size could be controllably tuned by varying the amount of NaOH added to the ampules. Herein, we study the formation of polycrystalline CZTS films by annealing, in sulfur, CZTS nanocrystal layers deposited via ultrasonic spray coating. Specifically, we focus on the key factors that affect the nanocrystal coating thickness uniformity during ultrasonic spray deposition


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and the effects of Na addition and sulfur pressure on the microstructure evolution during annealing of these coatings.

2.#EXPERIMENTAL#SECTION# 2.1 CZTS Nanocrystal Synthesis. Metal diethyldithiocarbamate (dedc) precursors used for synthesis of the CZTS nanocrystals were prepared following the procedure described by Chernomordik et al.19 Oleic acid (technical grade, 90%), oleylamine (technical grade, 70%), and toluene (HPLC grade, 99%) were purchased from Sigma Aldrich. Cyclohexanone was purchased from Fisher Scientific (Certified ACS, 99.99 %). Reagent alcohol (histological grade, 90% ethyl alcohol, 5% methyl alcohol, 5% butyl alcohol) and acetone (HPLC grade, 99.5%) were purchased from Fisher Scientific. Isopropyl alcohol (histological grade, 99.5%) was purchased from Macron Fine Chemicals. Molybdenum-coated soda lime glass (Mo-coated SLG) was purchased from DASSTECH. CZTS nanocrystals were synthesized also as described by Chernomordik et al. but with some modifications.19 Cu(dedc)2 (108 mg), Zn(dedc)2 (54.0 mg), and Sn(dedc)4 (106.8 mg) were dissolved in 10 ml of oleic acid within a 15 ml round bottom flask. Additionally, 20 ml of oleylamine was poured into two 100 ml round bottom flasks (10 ml oleylamine in each). The three round bottom flasks were attached to separate valves on a Schlenk line. The flasks were placed on heating mantles and fitted with thermocouples to allow temperature control of their contents. All flasks were heated to 60 °C and degassed to 50 Torr before purging with pure nitrogen. The degas-purge cycle was repeated three times. The flasks remained under nitrogen flow for the remainder of the reaction. The oleic acid-precursor solution was heated to 140 °C to ensure dissolution of the precursors. The two oleylamine flasks were heated to 290, 300, or 320


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°C for synthesis of CZTS nanocrystals with 10, 15, or 20 nm diameters, respectively. When the oleylamine reached the desired reaction temperature (290, 300 or 320 °C), 5 ml of the oleic acidprecursor solution was injected into each of the oleylamine-containing flasks. The reacting solutions were kept at the injection temperature (290, 300 or 320 °C) for 10 minutes after injection. After 10 minutes, the heating mantles were removed from beneath the flasks and the solutions were allowed to cool to room temperature, which took approximately 15 minutes. After cooling, the solutions were washed with three cycles of successive reagent alcohol dilution, sedimentation by centrifuge, and redispersion in toluene. After the final washing cycle, the nanocrystals were dispersed in 1 ml of toluene with 10-4 volume fraction oleic acid or a solution of 0.3 ml cyclohexanone and 0.7 ml of toluene containing 10-4 volume fraction of oleic acid. The dispersions were sonicated for 30 minutes prior to coating. 2.2 CZTS Nanocrystal Coating. A 120 kHz Sono-tek Accumist spray nozzle set to constant power (3.4 W) was used for ultrasonic spray coating. A schematic of the spray coating setup and a digital photograph of the coating apparatus are shown in Supporting Information Figure S1. The nozzle was attached to a metal arm above a stationary coating stage and was positioned such that the tip was 10 cm above the substrate. The arm could be translated along the x-axis at a variable speed but this speed was maintained constant at 2.5 mm/s for this work. A syringe pump was used to deliver the CZTS dispersion at a constant flow rate of 0.025 ml/min through 1.5 feet of Teflon tubing (0.038 in inner diameter). A nitrogen gas line was attached to the nozzle to provide focusing flow at 4.5 psig. These coating parameters produced an aerosol spray with an lateral length of approximately 2 cm. During coating, several 6 mm × 10 mm Mo-coated SLG substrates were aligned below the spray nozzle. Mo-coated SLG was chosen as the substrate because it is used almost exclusively for making solar cells and is the most relevant choice. After


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coating the desired number of substrates, the translation motor and syringe pump were stopped and the nozzle returned back to the starting position. To produce thicker coatings, multiple coating passes were conducted. The period between coating passes was approximately 70 seconds with 12±3 second variations depending on the number of substrates coated in each pass. 2.3 Annealing of the CZTS Nanocrystal Coatings. CZTS nanocrystal coatings were annealed in sulfur vapor as described elsewhere.18-20,24 Briefly, substrates covered with CZTS nanocrystal coatings were placed in quartz tubes along with predetermined mass of solid sulfur (3 to 6 mg) to produce the desired S pressure in the tube during annealing. The tubes were evacuated to ~10-6 Torr, typically overnight, and flame-sealed to form ampules. The annealed films appeared nonuniform if the ampules were placed into a preheated furnace at 600 °C (see Figure S2 in Supporting Information.) This was attributed likely to the fast temperature transient the tube and the substrate experience when placed into the hot furnace and is similar to that observed by Chernomordik et al. during selenization of CZTS nanocrystal coatings.20 Instead, the ampules were placed in the furnace at room temperature and the furnace was heated slowly. Specifically, the temperature was ramped to 175 °C for 30 minutes, then to 275 °C for 30 minutes, and finally to 600 °C for 75 minutes. Following, the furnace was turned off and allowed to cool naturally to room temperature. In some cases, a known concentration and volume of NaOH solution (and thus known total moles) was dried in the ampules to coat the interior walls with NaOH before annealing. The procedure was modified from that reported by Johnson et al.31 The volume of the solution was kept constant at 1 ml and the NaOH solution concentration was varied to change the total amount of NaOH added to the ampule. The total amount of NaOH added to the tubes was either 0.1 or 1 µmol. The tubes were dried in an oven at 115 °C for a minimum of 4 hours to ensure complete water evaporation. During drying, the tubes were placed


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horizontal with a slight offset angle (≈5°), as illustrated in Figure S3a in Supporting Information. This resulted in a NaOH-rich region inside the tube, which covered approximately one-half of the tube circumference and extended approximately 2.5 cm from the end of the 10 cm long tube. After drying, the substrates with the CZTS nanocrystal coating were placed in the ampules (one per ampule), which were then sealed as described previously. During annealing, the coating on the substrate was oriented within the ampule such that the NaOH-rich area was situated directly above the nanocrystal coating, as illustrated in Supporting Information Figure S3, to ensure consistent and reproducible Na exposure. 2.4 Characterization. Coatings produced via ultrasonic spray coating were imaged before and after annealing using a JEOL 6500 field-emission scanning electron microscope (SEM). The thickness of the coatings prior to annealing and the abnormal CZTS grain coverage area after annealing were determined from the SEM images using particle analysis functions of the ImageJ (NIH) image analysis software37 obtained through the Fiji distribution.38 A Thermo-Noran Vantage energy dispersive X-ray spectrometer was used for energy dispersive x-ray spectroscopy (EDS). X-ray diffraction (XRD) was conducted on a Bruker D8 Discover system equipped a Co Kα source and a VÅNTEC-500 2D area detector. The Co Kα XRD patterns were converted to the Cu Kα patterns using the analysis software JADE (Materials Data Incorporated). A Witec Alpha300R confocal Raman microscope with a 532 nm excitation source was used for Raman spectroscopy. The microscope was connected to a DV401 CCD detector, and a UHTS300 spectrometer. Spectra were collected with an 1800 lines/mm grating that resulted in a spectral resolution of 0.02 cm-1.


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3.#RESULTS#AND#DISCUSSION# 3.1 Spray Coating CZTS Nanocrystals: Effect of Cosolvent on Coating Morphology. Figure 1 shows CZTS nanocrystal coatings produced via ultrasonic spray coating from dispersions in toluene (Figures 1a and 1c), and from dispersions in a solution of 30 vol% cyclohexanone and toluene (Figures 1b and 1d). The average nanocrystal diameter in these dispersions was 10 nm. When toluene alone is used, we find large circular regions (> 10 µm radius) that are noticeably thinner than the rest of the coating (Figure 1a and 1c). Cross sectional images clearly show up to an order of magnitude variation in coating thickness decreasing from ~2 µm to < 200 nm, in the circular depressions. This nonuniformity would be problematic for solar cells because of the likelihood of noncontinuous films forming after annealing.

Figure 1. SEM images of CZTS nanocrystal coatings formed using ultrasonic spray deposition. Mo-coated SLG substrates were coated in 3 passes at a stage speed of 2.54 mm/s and liquid flow rate of 0.025 ml/min from 5 wt% nanocrystal dispersions in (a,c) toluene and (b,d) 70 vol% toluene/30 vol% cyclohexanone. The dispersions contained 10-4 volume fraction oleic acid. The average nanocrystal diameter in the dispersion was 10 nm.


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We attribute the circular nonuniformities in Figures 1a and 1c to depressions formed upon impingement of aerosol droplets on a coating with low liquid fraction such that the high viscosity of the coating at the impingement site prevents these depressions from leveling by flow. Reducing the evaporation rate to increase the fraction of droplets with higher liquid fraction should help reduce these nonuniformities. Consistent with this hypothesis, dispersing the nanocrystals in a solution of 30 vol% cyclohexanone in toluene with the same CZTS nanocrystal loading (≈90 mg/ml) improves the coating uniformity significantly (Figures 1b and 1d). Crosssectional images reveal a more uniform coating thickness (1.0±0.2 µm) compared to coatings formed from toluene dispersions. We attribute this dramatic improvement in coating uniformity with the addition of cyclohexanone to the lowering of the evaporation rate during the droplet’s flight from the nozzle to the substrate. (The normal boiling points and vapor pressures at 20 oC of hexane and cyclohexanone are 69 oC and 157 oC, and 150 Torr and 5 Torr, respectively; See also the discussion in the Supporting Information.) Since the liquid content at the time of impingement is the critical factor in coating formation, it is important to analyze, quantitatively, the effects of the solvent composition on the aerosol and coating drying dynamics. We do this using an aerosol evaporation model developed by Ravindran and Davis39 and simulate the aerosol droplet radius as a function of time assuming that the droplet is a well-mixed, isothermal, ideal liquid solution (with no nanocrystals) and evaporation is limited by diffusion into a stagnant gas phase that is dilute in solvent vapors. Details and equations used to simulate the evaporation are provided in the Supporting Information. We use this model to help understand and describe the mechanism behind the morphological differences observed in Figures 1a and 1b.


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Figure 2. Normalized aerosol droplet radius as a function of time for toluene droplets, depicted in blue, and for binary liquid droplets comprising cyclohexanone (30 vol%) and toluene, depicted in red. For both liquids, the solid line represents the temporal evolution of the mean droplet size during drying. In both cases, the mean initial droplet radius is 8 μm. The shaded regions around the blue and red curves depict drying time distribution for aerosol droplets with initial radius 8 μm ±3 μm.

Figure 2 shows the calculated, normalized aerosol droplet radius as a function of time as it evaporates during its flight from the nozzle towards the substrate for two different dispersing liquids, toluene (blue curve) and a solution of cyclohexanone (30 vol.%) and toluene (red curve). The expected initial droplet size distribution was calculated from manufacturer provided data and are shown in Figure S4 in the Supporting Information. The plot in Figure 2 depicts the evaporation of a droplet with mean initial radius !! = 8 µm at the ultrasonic spray nozzle. A shaded band around each curve represents the evaporation of droplets with radii within !! ± !/2, where ! is the standard deviation (see Supporting information for estimation of the size distribution).


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Figure 2 immediately suggests a mechanism for the coating thickness nonuniformities shown in Figure 1 when toluene alone is used. According to Figure 2, a toluene droplet with a mean radius of 8 µm evaporates completely within 0.03 s, but at t = 0.03 s droplets that started their flight with radii one standard deviation larger than 8 µm have evaporated only to approximately 68% of their initial volume, indicating the possibility of significant variations in the liquid contents of droplets containing nanocrystals. When dispersions are in toluene, if the time of flight from the nozzle to the substrate is on the order of 10s of ms, the majority of the liquid in the aerosol droplets evaporates rapidly and consequently, they reach the substrate with low liquid content. The droplets at the upper end of the distribution, however, contain enough liquid such that when they impinge on the coating, they redisperse the already deposited nanocrystals, leading to the formation of a liquid depression (not unlike rain drops on damp sand). These regions then dry before they get a chance to flatten to form the thin, non-uniform circular regions observed in Figures 1a and 1c. The problem with toluene then is that the size distribution impinging on the substrate includes too many drops with very low liquid fraction. Figure 2 also suggests the reason for reduction in these nonuniformities when a cosolvent with lower volatility is added to toluene. Cyclohexanone is less volatile than toluene and aerosol droplet evaporation rate decreases with the addition of cyclohexanone to the dispersion: the normal boiling points of toluene and cyclohexanone are 110 °C and 155 °C, respectively. Figure 2 shows that addition of 30 vol.% cyclohexanone to toluene triples the drying time for the 8 µm droplet to 0.09 s. Importantly, the evaporation time distribution widens and includes more drops with higher liquid fraction. This increases the saturation state of the nanocrystal coating and helps reduce the cratering displayed in Figure 1. There is evidence in literature that cratering is reduced in granular beds when the granular bed is slightly wet. For example, Zhang et al.40


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analyzed the formation of impact craters formed by oil droplets impinging on a wet granular bed of soda-lime glass beads (~90 µm diameter) at different saturation levels. They reported a decrease in crater size and depth as the granular bed was slightly saturated compared to a dry granular bed.

Figure 3. SEM images of CZTS nanocrystal coatings formed by ultrasonic spray coating from a 10 wt% CZTS nanocrystal dispersion using (a) 3, (b) 6, and (c) 9 coating passes. The coating deposited during a single pass is referred to as a layer. Bose et al.41 found similar effects during ultrasonic spray coating of films from polymer solutions: more uniform polymer film formed when cyclohexanone was used as the solvent compared to toluene. The surface tensions of cyclohexanone and toluene are similar. Consequently, we conclude that the droplet evaporation rate differences between toluene and cyclohexanone and toluene mixtures is the key factor that determines the differences in coating morphology and uniformity. In both the polymer solutions and nanoparticle dispersions, the liquid content of the impinging droplets is a critical factor in the formation of a uniform coating. 3.2 Spray Coating CZTS Nanocrystals: Coating Thickness. Figures 3a-c show crosssectional SEM images from approximately the lateral center (i.e., 5 mm from either edge of the substrate) of CZTS nanocrystal coatings formed by ultrasonic spraying using 3, 6, and (c) 9 coating passes. The coating thickness increases from 2.1 µm to 3.6 µm and to 4.4 µm for the 3-, 6- and 9-pass coatings, respectively. As expected, the coating thickness increases with the


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number of passes, indicating the ability of spray coating to control coating thickness over a wide range. Interestingly, the thickness did not increase linearly with the number of layers deposited as expected, but instead the average thickness per layer decreased as more layers were deposited. More experiments are needed to verify this effect and determine its underlying cause. For the annealing experiments that follow, six pass coatings with an initial thickness of ~3.6 µm were used.

Figure 4. Plan view SEM images of coatings comprised of 15 nm CZTS nanocrystals after annealing at 600 ◦C for 75 minutes with (a-c) no NaOH, (d-f) 0.1 μmol NaOH, and (g-i) 1 μmol NaOH, coated on the quartz tube in (a, d, g) 150 Torr (3 mg S), (b, e, h) 225 Torr (4.5 mg S), and (c, f, j) 300 Torr (6 mg S) of sulfur vapor.


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3.3 Annealing: Effects of Na and S Vapor Pressure. Thermal annealing of CZTS coatings in presence of S or Se vapor converts CZTS nanocrystal coatings into large-grained thin films, the desired film morphology for efficient solar cells. Film microstructure development during annealing has been studied in detail and shown to be affected by numerous variables including the nanocrystal size,42 the coating density,24 the sulfur pressure,18 the carbon content,42-44 and the substrate.18,31,45 CZTS grain growth rate increased when nanocrystal coatings were formed on soda lime glass (SLG) instead of on quartz. This increase was shown to be due to the presence of Na and K in SLG, which diffuse into the coating during annealing and enhance grain growth through a mechanism whose details are not well understood.18,30,31,34 Inspired by these earlier findings, we investigated the effect of Na addition and sulfur pressure on microstructure development of spray-coated CZTS nanocrystal coatings during annealing. Figure 4 shows SEM images of coatings spray deposited from dispersions of 15 nm CZTS nanocrystals after they were annealed at 600 °C for 1.25 hours with 3 mg (150 Torr), 4.5 mg (225 Torr), and 6 mg (300 Torr), sulfur (columns of SEMs in Figure 4) with no NaOH, 0.1 µmol NaOH, and 1 µmol NaOH added to the annealing ampule (rows of SEMs in Figure 4). Crosssectional SEM images, Raman spectra and XRD from these coatings are shown in Figures S5 and S6 in the Supporting Information. Prior to annealing, the CZTS nanocrystals had an average nanocrystal size of 15 nm as determined by Scherrer’s equation from the FWHM of the (112) XRD peak at 2θ = 28.5°. All coatings exhibited abnormal grain growth with large CZTS grains (~2 µm) forming on top of the CZTS nanocrystals as observed previously.18,42-44 Recently, we have shown that abnormal grain growth can be promoted by high carbon concentration inherent in coatings comprised of small (< 25 nm) CZTS nanocrystals with carbon containing surface


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ligands: small nanocrystals have high surface area and proportionately more ligands are trapped in the coatings as compared to larger nanocrystals (> 25 nm).42 Figures 4 and 5 show that, without Na, increasing the sulfur pressure from 150 Torr to 225 and 300 Torr, the abnormal abnormal grain aerial coverage increases from 36% to 75% and 92%, respectively. At highest sulfur pressures, the abnormal grains are no longer isolated and grain boundaries begin to form between most of the neighboring grains though some gaps still remain between polycrystalline islands. With the addition of 0.1 µmol and 1 µmol of Na to the ampule, the growth rate of abnormal grains increases significantly compared to the coatings annealed without Na. The aerial coverage of abnormal grains approaches 100% (Figure 5) even at the lowest sulfur pressure (150 Torr) for both 0.1 µmol and 1 µmol of Na addition. In fact, for these coatings comprised of 15 nm CZTS nanocrystals, when Na is included in the ampule, the sulfur pressure has little effect on the film morphology or abnormal grain area coverage. The grain growth enhancement due to presence of Na eclipses the effects of sulfur pressure on grain growth and film morphology. This is clearly the case when the coatings are annealed for one hour but differences in microstructure due to sulfur pressure may emerge at shorter annealing times. Importantly, the addition of 0.1-1 µmol of NaOH to the ampule converts the CZTS nanocrystal coatings into continuous polycrystalline films. In some of the cross-sectional SEM images of these films shown in Figure 4, a thin fine-grain layer is visible beneath the abnormal grains. (Cross-sectional images shown in Supporting Information Figure S5). EDS analysis of this fine-grain layer in films annealed in 300 Torr sulfur and without Na (Figure 5Sc) showed that it contains ~30 at.% carbon and ~44 at.% S (we caution that Mo peak interferes with S). The Cu, Zn, and Sn concentrations in this layer are 12, 7, and 7 at.%, respectively. The ratios of these concentrations approximately match the expected


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stoichiometry of CZTS. This fine-grain layer (200 – 300 nm thick) is similar to those seen in Cu2ZnSnSe4 films formed via selenization of CZTS nanocrystal coatings. 20,46 In films annealed with Na added to the ampule, the fine grain layer was completely covered with abnormal grains and could not be analyzed by EDS.

Figure 5. Abnormal grain coverage as a function of sulfur pressure for coatings comprised of 15 nm and 20 nm diameter CZTS nanocrystals after annealing with no NaOH, 0.1 μmol NaOH, and 1 μmol NaOH in the ampule. The coatings comprised of 20 nm nanocrystals do not exhibit any abnormal grain growth when there is no Na present in the ampoule and if plotted would be a line at zero coverage.

Figure 6 shows plan view SEM images of coatings comprised of 20 nm CZTS nanocrystals after they were annealed at 600 °C for 1.25 hr with 3 mg, (150 Torr) 4.5 mg (225 Torr), and 6 mg (300 Torr) sulfur (columns of SEMs in Figure 6) with no NaOH, 0.1 µmol NaOH, and 1 µmol NaOH added to the annealing ampule (rows of SEMs in Figure 6). Cross-sectional images, Raman spectra and XRD patterns of these coatings are shown in Figures S7 and S8 in the


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Supporting Information. There is little to no discernable grain growth regardless of the sulfur pressure in the range 150-300 Torr when Na is not added to the ampules (Figure 6 a-c). This is expected based on our previous work where we found that higher sulfur pressures (e.g., 500 Torr) were required to induce grain growth when coatings are composed of large (> 20 nm) CZTS nanocrystals with low concentrations of alkali metals present during growth.18,24 Here, it appears that the relatively thick (~ 1 µm) Mo layer prevents significant diffusion of Na, resulting in negligible grain growth. When Na is added to the ampule, however, abnormal grain growth is observed on top of the nanocrystals at all sulfur pressures though the coverage is significantly lower than on the films formed using 15 nm nanocrystals (Figure 5). Figure 5 compares the abnormal grain coverage on films made from 20 nm nanocrystals with coverage on films made from 15 nm nanocrystals. The abnormal grain coverage on coatings comprised of 20 nm nanocrystals increases with sulfur pressure for both Na amounts but the abnormal grain coverage with 1 µmol of Na is higher at all sulfur pressures than with 0.1 µmol of Na. Importantly, the abnormal grain coverage is noticeably lower on films made from 20 nm nanocrystals than on films made from 15 nm nanocrystals. For example, while abnormal grain coverage is nearly 100% for films made from coatings comprising 15 nm nanocrystals, the coverage is significantly lower (40-60%) for films made from coatings comprising 20 nm nanocrystals, at low sulfur pressure (150 Torr). Even when sulfur pressure is increased to 300 Torr, the abnormal grain coverage rises only to ~80%. It is remarkable that only 5 nm difference in the average size of the nanocrystals can have such a drastic effect on the microstructure of the annealed films. The cross-sectional SEM of the film made from 20 nm nanocrystals with 1µmol of Na (Figure S7) show that the abnormal grains have completely consumed the underlying nanocrystal coating. EDS of the gaps between abnormal grains of the films in Figure 6i reveals that these regions are


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composed entirely of Mo and S with no evidence of CZTS nanocrystals or a carbon containing fine-grained layer. In contrast, we find carbon and CZTS, indicative of the presence of a finegrained layer, in the gaps between the abnormal grains in Figures 6d-f. It appears that lowering the Na added to the ampule to 0.1 µmol leads to the formation of a fine grain layer at all sulfur pressures.

Figure 6. Plan view SEM images of coatings comprised of 20 nm CZTS nanocrystals after annealing at 600 ◦C for 75 minutes with (a-c) no NaOH, (d-f) 0.1 μmol NaOH, and (g-i) 1 μmol NaOH, coated on the quartz tube in (a, d, g) 150 Torr (3 mg S), (b, e, h) 225 Torr (4.5 mg S), and (c, f, j) 300 Torr (6 mg S) of sulfur vapor.


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When Na is not added to the ampule during annealing, the coatings comprised of 15 nm CZTS nanocrystals (Figures 4a-c) exhibit abnormal grain growth while the coatings comprised of 20 nm CZTS crystals do not show such growth. We attribute this difference to the decrease in abnormal grain driving force in coatings comprised of 20 nm CZTS nanocrystals compared to the driving force in coatings comprised of 15 nm CZTS nanocrystals. This decrease in driving force has two components both rooted in the smaller surface-to-volume ratio of the larger nanocrystals. First, the driving force due to the surface energy of the crystals is lower for larger nanocrystals. Second, the larger nanocrystals have less carbon, which was shown to increase grain growth. When the coatings are annealed with 0.1 or 1 μmol of Na added to the ampule (15 nm - Figures 4c-d; 20 nm - Figures 6c-d), bilayer films with abnormal grains growing on the nanocrystal coating is obtained with coatings comprised of 15 nm and 20 nm nanocrystals. However, grain growth rate and grain sizes are larger on coatings comprised of 15 nm CZTS nanocrystals than on coatings comprised of 20 nm nanocrystals. The additional Na supplied aids abnormal grain growth with both nanocrystal sizes but the grain growth enhancement is larger for films comprised of 15 nm CZTS nanocrystals.

4.#SUMMARY#AND#CONCLUSIONS# We studied the formation of polycrystalline CZTS films by annealing, in sulfur, CZTS nanocrystal layers deposited via ultrasonic spray coating. The CZTS nanocrystal layers are deposited by spraying nanocrystal dispersions in toluene or in mixtures of toluene and cyclohexanone (30 vol%), a cosolvent. Using a cosolvent reduced the coating surface roughness and improved the large-scale coating uniformity. These improvements were linked to the effects of the cyclohexanone addition on both the aerosol droplet evaporation rates and nanocrystal


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coating saturation during spray coating. Moreover, we investigated the effects of Na addition and sulfur pressure on the microstructure evolution during annealing of these ultrasonic spray-coated CZTS nanocrystal layers. Without Na addition, coatings comprised of small nanocrystals (15 nm) formed large abnormal grains on top of the coating upon annealing in S, while no substantial grain growth was observed when the coating comprised larger nanocrystals (20 nm). When Na was added to the annealing ampoule, grain growth rates increased and microstructures suitable for solar cells could be formed from small and larger nanocrystals. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxx/acsami.xxxxxxx. Schematic and photograph of the spray coating apparatus and Na addition procedure, photograph of annealed films, additional discussion of coating morphology, evaporation model description and equations, aerosol droplet size distributions, additional SEMs, X-ray diffraction and Raman spectra. (PDF)

AUTHOR INFORMATION Corresponding Author * Email [email protected] (E.S.A.). * Email [email protected] (L.F.F.).


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Award Number DMR-1420013. The authors thank Sono-Tek for the donation of the ultrasonic spray coater and Wieslaw Suszynski for assistance in setting up the coating apparatus. REFERENCES

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