Role of Precursor Reactivity in Crystallization of Solution-Processed

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The Role of Precursor Reactivity in Crystallization of SolutionProcessed Semiconductors: The Case of Cu2ZnSnS4 Chengyang Jiang, Wenyong Liu, and Dmitri V. Talapin Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 10 Jun 2014 Downloaded from http://pubs.acs.org on June 22, 2014

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Chemistry of Materials

The Role of Precursor Reactivity in Crystallization of SolutionProcessed Semiconductors: The Case of Cu2ZnSnS4 Chengyang Jiang,1 Wenyong Liu,1 and Dmitri V. Talapin1,2* 1 2

Department of Chemistry and James Franck Institute, the University of Chicago, Chicago, Illinois 60637, USA Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA

Supporting Information Placeholder ABSTRACT: We study the formation of Cu2ZnSnS4 (CZTS) films from various liquid-phase precursors. Our experimental data point to the significant role that reactivities of precursor components play in the quality of the final material. While reactive molecular precursors favor formation of CZTS under milder conditions, the formation of large crystalline domains requires using less reactive nanostructured precursors. We explain this effect using kinetics of nucleation and growth. We have also demonstrated a strategy to effectively enhance grain growth of CZTS using solid-state phase transition as the driving force for nanocrystal sintering. We hope this contribution will provide a useful guide toward the rational design of liquid-phase precursors for inorganic semiconductors for electronic and optoelectronic applications.

INTRODUCTION Deposition of inorganic semiconducting materials via nonvacuum techniques has attracted much interest in recent years.1 In particular, direct solution-based deposition of light-absorbing layers in inorganic solar cells provides a low-cost, high-material utilization and scalable alternative to vacuum-based deposition.2,3 Relatively mild processing conditions associated with solution-based techniques also expand the list of compatible substrates. Such an approach has led to the recent success of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells.4 In this case, a solution process using new hydrazinium chalcogenidometallate precursors has set the power conversion efficiency (PCE) above 11%,5 while the highest PCE of Cu2ZnSnS4 (CZTS) solar cells processed with a vacuum-based approach is about 8.4%.6 Besides the aforementioned chalcogenidometallate precursors, CZTSSe thin films have been fabricated from solutions of constituent metal salts and molecular chalcogen species,7-10 colloidal solutions containing CZTS nanoparticles11 or mixtures of binary nanocrystals (NCs) with different capping ligands,12,13 and several other liquid-phase precursors.14,15 Careful selection of precursors and reaction parameters is the key prerequisite to obtaining semiconductors with competitive electronic and optical properties.16,17 The complex elemental composition of the CZTS phase implies complicated phase diagrams and reaction pathways.18 While there are numerous reports on using various solution-based techniques such as spin-coating,13,14,19 doctor blade-coating,11 and spray-coating7,12 for CZTSSe thin films, few studies have dealt with understanding the chemical and morphological transformations of soluble precursors into high quality CZTS films with20,21 or without selenization22-24. Moreover, we are also not aware of prior comparative studies of different precursors that elucidate the role of precursor chemistry in achieving highly crystalline CZTS films, while such back-to-back comparison should reveal important information for rational design and improvement of soluble precursor for crystalline tin-film semiconductor phases.

This lack of mechanistic understanding is unfortunately common for solution-processed semiconductors. In this contribution, we seek to demonstrate how the chemical composition of CZTS precursors affects the morphology of the obtained CZTS phase. The transformation of a soluble precursor into a crystalline semiconductor film involves chemical reactions that generate the CZTS phase, followed by sintering and growth of CZTS grains. First, we will discuss how the kinetics of precursor decomposition affects the structural perfection of CZTS films. In the second part, we will focus on the kinetics of CZTS grain growth in the absence of selenium vapor. Our target is to identify important parameters that allow the rational design of soluble precursors for high quality semiconductor films. Although this study focuses on CZTS, our general findings are applicable to different chalcogenide semiconductors.

RESULTS AND DISCUSSION So far the highest efficiency CZTS solar cells have been made using hydrazinium chalcogenidometallate precursors. Two major breakthroughs in this area are associated with the work of the D. B. Mitzi group at IBM19,25 and the Y. Yang group at UCLA.15,23 These groups introduced two closely related precursors that share the exact same copper, tin, and sulfur sources (i.e., Cu7S4- and Sn2S64-), while the difference is in the zinc source used. The first precursor (further referred to as “M-precursor”), contains Zn in the form of a suspension of micron-sized particles of ZnS-N2H4 complex (opaque solution in Figure 1A). This precursor contains prolate microparticles appearing in TEM as bundles of nanorods with preferential orientation (Figure 1B). Above 200 °C these microparticles decompose into wurtzite phase ZnS (w-ZnS) nanoparticles with an average Scherrer size of 9 nm (Figure S3). In contrast, the second precursor (further referred to as “Y-precursor”) contains molecular (N2H4)2Zn(N2H3COO)2 species as a Zn source23 and forms a clear solution (Figure 1A) convenient for uniform component mixing and deposition.

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phases. At 500 °C, well-crystalline CZTS forms, but the remaining ZnS is still seen (Figure 1C, cyan pattern). Raman spectra also support this observation (Figure 1D), where the presence of a peak at 374 cm-1 reveals the formation of CZTS27 (cyan vs. green spectra, see Section 2 of Supporting Information for further details). Y-precursor, in which all constituents are soluble molecular species, showed formation of CZTS at significantly lower temperatures. For Y-precursor, formation of crystalline CZTS can be demonstrated in Raman spectra mainly by the emergence of CZTS peak at 374 cm-1 because X-ray diffraction spectra of CZTS, Cu2SnS3, and ZnS overlap. When Y-precursor was annealed for 5 min at 200 °C, there was hardly any trace of the Raman peak at 374 cm-1 corresponding to crystalline CZTS (Figure 1E,F). The formation of CZTS at 300 °C was confirmed by Raman spectrum (Figure 1F, yellow spectrum) showing the transitions at 338 cm-1 and 374 cm-1. The corresponding XRD pattern (Figure 1E, yellow pattern) also shows four broad peaks, all of which can be assigned to kesterite. It can also be concluded that above 400 °C, even 5 min of annealing results in crystalline CZTS. The differences between M- and Y-precursors outlined in Figures 1C-F reflect slow CZTS formation kinetics for M-precursor. When annealed for 60 min, formation of the CZTS phase occurred at 400 °C, as indicated by both the peaks at 28.5° in XRD and at around 374 cm-1 in Raman spectrum (Figure 2A,B). Annealing at 500 °C for 60 min resulted in the completion of the solid-state reaction with no ZnS left (Figure S4). When Y-precursor was annealed for 60 min, 200 °C was already sufficient to generate the CZTS phase (Figure S5),23 while annealing at 400 °C for 60 min improved crystallinity of the film (Figure 2C,D).

Figure 1. (A) Appearance of M-precursor (left) and Y-precursor (right). (B) TEM image of ZnS-N2H4 particles in M-precursor, scale bar is 100 nm. (C, E) XRD patterns and (D, F) Raman spectra of CZTS films prepared from (C, D) M-precursor or (E, F) Y-precursor annealed at different temperatures for 5 min (* indicate characteristic CZTS Raman transitions at 288, 337, 365 and 374 cm-1).

We used X-ray diffraction (XRD) and Raman spectroscopy to monitor the reaction pathways and kinetics for both precursors upon heat treatment. It was recently proposed that the M-precursor undergoes the following reaction sequence:22 (N 2 H 5 )Cu 7S 4 + (N 2 H 5 ) 4Sn 2S6 + " ZnS - N 2 H 4 " Step  1  Cu 2SnS3 + ZnS Step  2 Cu 2 ZnSnS4

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The first step in generating Cu2SnS3 occurs below 200 °C. The second reaction involves solids since the melting points of Cu2SnS3 and ZnS are much higher than 600 °C,26 the highest temperature used in our study. The solid-state transformation involves w-ZnS, which has a diffraction pattern different from that of CZTS. Due to this distinction, we were able to use XRD to monitor the entire annealing process (Figure 1C,2A). The dominant peak in the XRD pattern of ZnS nanoparticles formed in M-precursor is the (100) peak at 26.9°, while that of kesterite CZTS is the (112) peak at 28.5°. If M-precursor is annealed for 5 min in a furnace at various temperatures (Figure 1C,D), it can be observed that even at temperatures as high as 400 °C, there is almost no peak at 28.5°. The pattern is dominated by the peak at 26.9° (Figure 1C, green pattern), suggesting the presence of w-ZnS and absence of CZTS

Figure 2. (A, C, E) XRD patterns and (B, D, F) Raman spectra of (A, B) M-precursor (C, D) Y-precursor annealed at 400 °C for 5 min or 60 min.

For a broad window of temperatures, CZTS is a more thermodynamically stable phase than Cu2SnS3 and ZnS,28 but the reaction (1) appears to be slow since the second step involves a solid-state transformation of a highly stable, intermediate ZnS. In contrast, Yprecursor likely forms CZTS in one step, as suggested in a recent study by the Yang group.23 Such a difference in reaction mechanism has important implications on the crystal grain structure of obtained CZTS films. For M-precursor annealed at 400 °C for 20 min, an SEM image in Figure 3A shows that rod-shaped aggregates of unreacted w-ZnS nanoparticles are ubiquitous in the sample. Though when the same precursor is annealed at 500 °C for 20 min,(this is a typical anneal-


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ing time for CIGS or CZTS layer in solar cells29), large phase-pure CZTS grains (206±52 nm, average of 150 grains) form (Figure 3C). In the case of Y-precursor, although annealing at 400 °C for 20 min is sufficient to transform precursors into crystalline CZTS, the resulting grains are small, approximately 35 nm (Figure 3B). A comparison of the film annealed at 500 °C for 20 min (Figure 3D) where grains have a larger average size (87±16 nm, average of 160 grains) reflects grain growth via sintering. Figures 3C and 3D significantly contrast in the dimensions of crystalline CZTS grains (>10-fold difference in average grain volume!) obtained from Mand Y-precursors annealed under exactly the same conditions. It should be noted that Y-precursor contains hydrozinocarboxylic acid that is absent in M-precursor. In a control experiment, we added the same amount of hydrozinocarboxylic acid to M-precursor and found no difference in terms of both the reaction kinetics and grain size compared to pristine M-precursor (Figure S6 and Section 5 of SI). This is understandable given the volatility of hydrozinocarboxylic acid at low temperatures.23,30 This observation further suggests that a difference in grain size directly originates from different reactivity of zinc precursor. The reactions of transformation of soluble precursors involve nucleation of the CZTS phase followed by the growth of nuclei. Such growth occurs until all reactants are consumed. The nucleation of the new phase generally occurs from a highly supersaturated state needed to overcome the nucleation barrier.31 In the case of highly reactive precursors, a supersaturated reaction mixture produces multiple nuclei acting as seeds for CZTS grains. Though slow superaturation in less reactive precursors results in few nucleation events. Slow destabilization of the reaction mixture is commonly used to grow large crystals whereas fast destabilization produces an amorphous or polycrystalline precipitate. Our experimental data suggest that the balance between the rates of nucleation and growth affects the final size of CZTS grains. When a reactive precursor (Y-precursor in this case) is used for the reaction, nucleation rate upon heat treatment is fast. A fast reaction produces a high concentration of nuclei in the system, while this concentration is low for less reactive precursors (M-precursor) because a slow reaction between Cu2SnS3 and ZnS does not generate high supersaturation of the reaction mixture. Given that we have the same amount of reactive and inert precursors, a higher concentration of nuclei implies that less precursor can be allocated to each nucleus for further growth, and hence the final grain size should be smaller for reactive precursors, as illustrated in Figure 3E.32 Overgrowth of CZTS nuclei in the case of M-precursor lasts significantly longer due to the low concentration of nuclei and continuous mass supply from ZnS nanoparticles, which eventually leads to large CZTS grains. This concept resembles the approach used in the synthesis of colloidal nanocrystals in which nucleation rate can be used for tuning nanocrystal size.32,33

Figure 3. (A-D) Representative SEM images of M-precursor annealed at (A) 400 °C or (C) 500 °C, and Y-precursor annealed at (B) 400 °C or (D) 500 °C, all for 20 min; (E) Scheme illustrating the relation between reactivity of soluble precursor and grain size of semiconductor phase.

Our argument based on precursor reactivity is further supported by evidence from other precursors with various reactivities for solution-processed CZTS.24,34 Furthermore, in control experiments, we have observed very fast CZTS formation in a molecular precursor containing metal salts and thiourea,8 in which CZTS grains formed after annealing at only 200 °C (Figures S7,8). However, this precursor shows small crystalline domains that do no sinter even at 500 °C (See Section 5 of SI for details). In the opposite extreme case, a mixture containing micron-size Cu2S and ZnS particles combined with (NH4)4Sn2S6 is too unreactive to form a SZTS phase upon annealing at 500 °C (Figure S10). We therefore suggest that soluble precursors with modest reactivity should be optimal for obtaining high quality semiconductor layers. The above discussion does not include post-synthesis grain growth via sintering. In the nucleation and growth steps, the transformation is driven by the difference in chemical potentials of reactants and products, while sintering is governed by elimination of grain boundaries and associated defect energy. In control experiments, we annealed a film of close-packed 14 nm CZTS nanocrystals capped with (NH4)2S ligands35 (Figure S1) and found that sintering of nanoscale CZTS grains at 500 °C is slow (Figures S1D, S2) and cannot lead to crystal domains observed for M-precursor annealed at 500 °C (Figure 3C). It is known that CZTS grain growth can be promoted by annealing in H2Se or Se vapor;11,36



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however, each presents safety (and associated cost) concerns for solar cell manufacturing. To solely analyze grain growth during sintering, we hereby circumvent the solid-state reactions presented in M- and Y-precursors by using colloidal CZTS NCs, a much simpler system in which only sintering is involved upon heat treatment. Despite a significant body of information on sintering and grain growth in metal alloys and ceramics, little is known about sintering of ligand-capped sub20 nm semiconductor NCs. We noticed that sintering of CZTS NCs required temperatures above 300 °C. We also found that CZTS NCs with different ligands and solvents show very diverse kinetics of grain growth (Figure 4A), pointing to the importance of rationally designing NC surface chemistry. For example, switching from 1-dodecanethiol (DT) to oleylamine surface ligands significantly suppressed grain growth from kesterite CZTS NCs. The inorganic (NH4)2S ligands showed efficient sintering of NCs, however, not as fast as DT-capped CZTS NCs. In all cases studied, even annealing at 500 °C for 60 min could not produce an average grain size larger than 85 nm (Figure 4A). This observation is in agreement with the behavior of Y-precursor, which first generated small CZTS crystallites (Figure 3C,D). It also suggests that slow grain growth is an intrinsic property of CZTS due to low mobility of grain boundaries in this material. Obtaining large crystal grains at mild conditions can be achieved by controlling CZTS nucleation and growth rates described in previous sections. During crystal growth, the heat of the chemical reaction helps with annealing out structural defects. As an alternative, large CZTS grains can be obtained by sintering metastable wurtzite CZTS NCs. The free energy released during the phase transition from the metastable wurtzite (w) to kesterite (k) phase can also efficiently promote grain growth. To demonstrate this point, we synthesized w-CZTS NCs capped with 1-dodecanethiol (DT) surfactants following a previous report.37 As-synthesized CZTS NCs are bullet-shaped and about 12 nm in diameter (Figure 4B); their XRD pattern (Figure 4C, pink pattern) clearly shows the wurtzite CZTS structure. After w-CZTS NCs are subject to heat treatment at various temperatures for 60 min, a clear phase transformation can be observed based on changes in the XRD patterns (Figure 4C). At 325 °C, the XRD pattern already corresponds to kesterite structure, indicative of the phase transformation between 300 °C and 325 °C. More importantly, dramatic sharpening of the XRD peaks occurred in parallel with the phase transition. The Raman spectra of w-CZTS NCs showed no other phases except CZTS before and after phase transformation (Figure S11), further suggesting that no chemical reactions are involved and observed abrupt grain growth is indeed caused by the phase transition. The phase transition greatly increases the mobility of grain boundaries, enabling rapid growth of crystalline domains. The organic ligands (e.g., DT) coating the surface of w-CZTS NCs can contaminate the CZTS layer, which was noticed in earlier works on solar cells made of sintered CZTS NCs. This problem can be addressed by using inorganic sulfide ligands.35 To explore the effect of phase transition on sintering inorganically-capped NCs, we performed ligand exchange for both kesterite and w-CZTS NCs with (NH4)2S ligands. We annealed these NC films at various temperatures while monitoring the evolution of grain growth (Figure 4D). In k-CZTS NCs, grain growth stalled at ~75 nm, too small for optimal solar cell performance. In contrast, w-CZTS NCs sintered at much lower temperatures that corresponded to the phase transition point and formed >200 nm grains (Figure 4D). Very recently,

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fast grain growth was observed in the arrays of pre-aligned w-CZTS nanorods.38 Our work shows that no pre-alignment or ordered packing of NCs is necessary for accelerated grain growth. Moreover, the use of inorganic ligands resulted in materials not contaminated with organic ligand residue.

Figure 4. (A) Grain growth progression of kesterite CZTS NCs with various ligands: 1-dodecanethiol (DT), oleylamine (OAm) and ammonium sulfide. (B) TEM image of as-synthesized w-CZTS NCs; (C) XRD patterns of DT-capped w-CZTS NCs annealed at various termpatures for 60 min. (D) In situ XRD monitoring the grain growth of kCZTS and w-CZTS NCs, both capped with (NH4)2S and annealed with 3 °C/min ramping rate.

CONCLUSIONS We hope this study reveals new strategies for the rational design of soluble precursors for solution-processed CZTS and other inorganic semiconductors. We have shown the relations between reactivity of soluble precursors and quality of obtained semiconductor film. In the case of molecular precursors, the ratio between nucleation and growth rates should be small enough to prevent conversion of precursor into a material with average grain size that is too small to provide good mobility and carrier diffusion length. Slow sintering kinetics inhibits obtaining large CZTS grains from nanocrystalline particles, unless these particles belong to a metastable (e.g., wurtzite) phase, in which sintering is dramatically accelerated in the vicinity of the wurtzite to kesterite phase transition. The use of inorganic ligands allows converting metastable colloidal NCs into a semiconductor film with large crystal domains not contaminated by residual surface ligands. This approach should be very promising in fabricating inorganic semiconductor thin films at low annealing temperatures.

METHODS Chemicals and instrumentation. Copper(II) acetylacetonate (Cu(acac)2, 98+%, Strem); zinc acetate (Zn(OAc)2, 99.99%, Aldrich); tin(II) chloride (99.99+%, Aldrich); sulfur (S powder, 99.998%, Aldrich); oleylamine (OAm, 70%, Aldrich); ammonium sulfide ((NH4)2S, 40-48 wt% solution in water, Aldrich); selenium (Se powder, 100mesh, 99.99%, Aldrich); zinc (Zn powder, 99.9%, Strem), 1-dodecanethiol (DT, 98%, Aldrich); hexane, ethanol, Nmethylformamide (NMF, 99%), acetonitrile (CH3CN), hydrazine


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(N2H4, 98%), dimethylsulfoxide (DMSO, 99.9+%) were purchased from Aldrich. Wide-angle powder X-ray diffraction (XRD) patterns were collected with a Bruker D8 powder X-ray diffractometer. Samples were typically prepared in the following way: CZTS precursor solutions were first drop-cast on a glass slide. Depending on the polarity of the solvent used, glass substrates may require treatment with O2 plasma. Samples were then dried at 60 °C under ambient pressure or vacuum. Lastly, samples were placed into a furnace under dry N2 atmosphere inside an MBraun glovebox and annealed at different temperatures for a controlled amount of time. Raman spectroscopy spectra were measured with a Renishaw inVia Raman microscope. Samples were prepared in the same way as for XRD. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) data were obtained using a TA Instruments SDT Q600 thermogravimetric analyzer at a heating rate of 5 ºC/min. Dynamic light scattering (DLS) information on size-distribution and ζ-potential was gathered with a Malvern Nano-ZS Zetasizer; measurements were performed using diluted solutions. Transmission electron microscopy (TEM) samples were prepared by drop-casting diluted colloidal solutions onto copper grids and drying the grids in a vacuum oven if high boiling point solvents were used; images were taken using a FEI Tecnai F30 electron microscope. Fourier transform infrared spectroscopy (FTIR) samples were prepared by drop-casting and drying concentrated solutions on the surface of KBr IR cards (or single crystal substrates for samples that would subsequently be annealed); vacuum may be applied to dry samples with high-boiling point solvents; spectra were acquired with a Nicole t Nexus-670 FTIR spectrometer. Scanning electron microscopy (SEM) samples were prepared either by spin-coating precursor solutions (5-10 cycles) on silicon substrates, with short low-temperature annealing between each cycle and major annealing after the last cycle, or for cross-sectional images, samples were prepared by drop-casting precursor solutions on silicon substrates, drying out solvents and annealing at desired temperature, then redispersing products in acetone and dropcasting the suspension on silicon substrates to expose the interior of the films.

sublimed, the solution is diluted by half before 2 mmol Zn powder (130.8 mg) was added to 2 mL of this solution under vigorous stirring to eventually form a semi-transparent viscous solution. Cu precursor and Sn precursor were prepared in the same manner as for M-precursor. The three precursors were then mixed together in portion with 2 equivalent of 1M S in N2H4, forming a pale-yellow, transparent solution. Synthesis of CZTS nanocrystals. Kesterite phase CZTS NCs capped with (NH4)2S (further referred to as k-CZTS/S2- NCs) were prepared in the following way: Cu2ZnSnS4 NCs were first synthesized via colloidal chemistry route.41 In brief, 2.4 mmol Cu(acac)2 (633.1 mg), 2.0 mmol Zn(OAc)2 (367.0 mg), 1.2 mmol SnCl2 (227.5 mg) and 8.0 mmol S (256.5 mg) were mixed with 40 mL degassed OAm. The mixture was degassed before rapidly heated up to 280°C and kept for 60 min. After cooling down, 30 mL ethanol was added to the mixture and NC product was collected by centrifugation at 9000 rpm for 5 min. The product was further purified by dissolving with 7.5 mL toluene, flocculating with 15 mL ethanol and centrifuging at 9000 rpm for 10 min. This procedure is repeated once. Finally, the product was dissolved in 8 mL toluene and centrifuged at 3000 rpm for 1 min to remove large and poorlycapped NCs. The supernatant was stored for characterization and further treatment. To replace their surface ligands with S2-, 100 mg Cu2ZnSnS4 NCs dispersed in 6 mL hexane was mixed with 5.7 mL NMF and 0.3 mL (NH4)2S (aq.). The two-phase mixture was stirred vigorously for several hours before the colorless upper phase (hexane) was discarded and the black bottom phase (NMF) was washed 3 times with hexane. Next, 24 mL CH3CN was added to the NMF colloidal solution for flocculation, and the mixture was centrifuged at 9000 rpm for 10 min. The precipitate was redispersed in 1mL NMF, forming a S2--capped Cu2ZnSnS4 NC solution at a concentration of 100 mg/mL. To switch the solvent from NMF to N2H4, (NH4)2Scapped NCs were first precipitated out by adding 4 times the volume of CH3CN to the NMF colloidal solution followed by ultracentrifugation. Precipitated NCs were redispersed in N2H4 forming a colloidal suspension. To replace OAm molecules on the surfaces of as-synthesized CZTS NCs with DT molecules, NCs were first dried then dispersed in DT for overnight stirring, after which ethanol was added and the mixture was centrifuged to precipitate NCs out. The precipitates were redispersed in toluene.

Synthesis of CZTS precursors. M-precursor was prepared according to a previous report,25 with a slight modification. To prepare Zn precursor, 4 mL hydrazine was added to the mixture of 1 mmol Zn powder (65.4 mg) and 2 mmol S powder (64.1 mg). The suspension was vigorously stirred until it became white, indicating the formation of ZnS-N2H4 micron-sized particles. Cu precursor was prepared by stirring 1 mmol Cu2S (159.2 mg) and 2 mmol S (64.1 mg) in 4 mL N2H4 for 5 days before filtering the solution with 0.2 μm PTFE filter.39 Sn precursor was prepared by sequentially dissolving 3 mmol S (96.2 mg) and 1 mmol Sn (118.7 mg) in 4 mL N2H4 under vigorously stirring for 3 days.40 The Zn precursor was then mixed with Cu precursor and Sn precursor in proportion to form stoichiometric CZTS.

Wurtzite CZTS colloidal NCs were synthesized based on a previous recipe.37 Ligand exchange using (NH4)2S and redissolution into N2H4 were performed in the same manner as described above for kesterite CZTS NCs.

Y-precursor was prepared according to a recent report on a new type of hydrazine-based molecular precursor for CZTS.15 To prepare Zn precursor, dry ice was first mixed with N2H4 to produce saturated hydrozinocarboxylic acid solution. After excess dry ice

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ASSOCIATED CONTENT Supporting Information Additional experimental details, discussions, and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

Notes The authors declare no competing financial interests.



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ACKNOWLEDGMENT This work was supported by the DOE SunShot Program under award no. DE-EE0005312 and by the Keck Foundation. D. V. T. acknowledges support from the David and Lucile Packard Foundation. The work at the Center for Nanoscale Materials (ANL) was supported by the US Department of Energy under Contract No. DE-AC0206CH11357. This work also used facilities supported by the NSF MRSEC Program under Award Number DMR-0213745.

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Role of Precursor Reactivity in Crystallization of Solution-Processed

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