Evolution of Morphology and Composition during Annealing and

Sep 27, 2017 - Cu2ZnSn(S,Se)4 absorbers deposited from a nontoxic, DMSO-based molecular ink have yielded the most efficient, hydrazine-free Cu2ZnSn(S,...
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Evolution of Morphology and Composition during Annealing and Selenization in Solution-Processed Cu2ZnSn(S,Se)4 James A. Clark, Alexander R. Uhl, Trevor R. Martin, and Hugh W. Hillhouse Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03313 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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Chemistry of Materials

Evolution of Morphology and Composition during Annealing and Selenization in Solution-Processed Cu2ZnSn(S,Se)4 James A. Clark †, Alexander R. Uhl†, Trevor R. Martin‡, and Hugh W. Hillhouse† † Department of Chemical Engineering, University of Washington, Seattle, Washington 98195-1750 ‡ Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States Abstract: Cu2ZnSn(S,Se)4 absorbers deposited from a nontoxic, DMSO-based molecular ink have yielded the most efficient, hydrazine-free Cu2ZnSn(S,Se)4 photovoltaic (PV) device made through solution-processing. Although this chemistry has been widely adopted, absorber morphologies with a device-limiting fine-grained bottom layer are often reported. Here we demonstrate how the annealing profile of coatings from this ink critically affect absorber morphologies. Calibrated glow discharge optical emission spectroscopy (GDOES) is used to determine depth-dependent elemental ratios and atomic concentrations of impurities before and after selenization. An annealing temperature of 400 ° C is shown to exhibit the most pronounced fine-grained bottom layer, which contains 10 at% Carbon (C) and 5 at% Nitrogen (N). Raman analysis of the mechanically-exfoliated, fine-grained layer reveals that it contains amorphous carbon nitride, which is attributed to the polymerization of thiourea decomposition products during annealing. An annealing temperature of 300 °C avoids this polymerization and allows C & N to escape during selenization, while an annealing temperature of 500 °C vaporizes C & N compounds before selenization. The annealing profile of 500 C 1.5 min/layer removes nearly all but ~0.25 at% of C & N, which impedes the formation of a fine grain layer and allows for a PV device efficiency of 10.7%. It is also shown how lithium doping enhances sodium transport from the soda-lime glass substrate.

1. Introduction Cu2ZnSn(S,Se)4 (CZTSSe) based thin-film solar cells are an exciting prospect for inexpensive photovoltaic (PV) electricity generation due to their high absorption coefficient, earth-abundant constituents, and tunable bandgap of 1.0-1.5 eV, which is optimal for the solar spectrum.1 Of the deposition techniques available for CZTSSe, solution-processed absorbers are particularly interesting because of the possibility of low capital expenditure (CAPEX) required to build a manufacturing facility, which is one of the most significant barriers to growth of the PV industry2. Hydrazine-based solution processes have led to CZTSSe devices with a power conversion efficiency (PCE) of 12.6%, which is the most efficient device using this absorber material produced from any deposition technique.3 One of the key reasons for the success of this process is that hydrazine is able to directly solvate metal chalcogenides but then decompose and leave cleanly upon annealing without leaving behind impurities in the film. However, hydrazine is explosive and highly toxic, which limits its practical potential as a low CAPEX option to scale CZTSSe module production.

A number of other CZTSSe solution processes utilizing less hazardous constituents have been explored. The first reports of solution-processed CZTSSe absorbers used either solutions of Cu, Zn and Sn salts and thiourea in methoxy-ethanol4 or CZTS nanoparticle (NP) inks,5, 6 both of which led to devices with PCEs near 1%. PCEs were quickly increased to 7.2% with CZTS NP inks,7 however devices made from this process have not surpassed 9.3% to date.8 The methoxy-ethanol process has improved device PCE as well,9 with a recent result reporting a device PCE of 10.1%.10 Su et al. adopted the methoxyethanol chemistry to fabricate Cd-alloyed kesterite devices which lead to devices a PCE up to 9.2%.11 Other notable solution processes which have led to high PCE kesterite devices include: spray-coated colloidal waterand-ethanol-based inks (PCE 10.8%)12, 13 and solutions of Cu, Zn, Ge, and Sn salts and thiourea in DMF (PCE 14 11.0%). In this work, we focus on a DMSO-based solution of Cu, Zn, and Sn salts and thiourea (hereafter referred to as DMSO molecular ink) to deposit CZTSSe absorber layers, first described by Ki and Hillhouse.15 Xin et al. later revealed the importance of redox reactions in this ink.16 In a subsequent report, Xin et al. showed that lithium-

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doping of the DMSO molecular ink significantly improves device PCE.17 Doping the ink with 2.5% LiCl/metals lead to a device PCE of 11.8%, which is currently the highest PCE of a hydrazine-free, solution-processed CZTSSe device. All of the above solution routes are generally much safer than hydrazine; however, many of these processes can lead to multilayer absorber morphologies after selenization.7, 8, 10, 13, 16, 18 Absorbers generally exhibit largegrained upper layer and a fine-grained under-layer, which is rich in impurities. Depending on the ink chemistry, this fine-grained layer can have a number of different impurities and physical properties. The most well-studied impurity-rich layer (IRL) of the above solution processes is the fine-grained layer produced from the CZTS nanoparticle process.8, 19, 20 Martin et al. showed that this IRL is composed of graphitic carbon produced from the pyrolysis of the coordinating ligands during annealing and that the degree of crystallization of the IRL can significantly affect CZTSSe grain growth.19 Hages et al. examined the effect of various selenization conditions on grain growth and bi-layer formation in CZTS nanoparticles.8 Wu et al. showed that the bilayer morphology could be avoided for the methoxy-ethanol based ink process by performing a sulfurization prior to selenization.10 CZTSSe absorbers fabricated from the DMSO molecular ink process are often reported with a variety of multilayer morphologies.21-30 Moreover, devices in most of these studies suffer from low device PCEs (275 °C in multiple studies,21, 22, 24, 29 which risks the oxidization of the metals to some extent and otherwise affect decomposition pathways. In this study, we performed TGA of the molecular ink under constant N2 flow from 25-600 °C to track the stages of degradation in an inert environment as shown in Figure 1. From 25150 °C, 66% of the mass had been lost which corresponds to the mass percent of DMSO and H2O in the ink as shown in the blue shaded region of the mass balance in Table 2 150 °C is also below the onset of the formation of light-absorbing metal sulfide species as shown in the inset of Figure 1. This suggests that the DMSO and water-ofhydration in the precursors cleanly volatilize from the film without decomposing. From 150°C - 400 °C, the primary source of mass loss can be attributed to the decomposition and volatilization of the thiourea, chloride, and acetate species, which is indicated by the pink shading. Note that from 260°C to 280°C, there was a notable increase in the rate of mass loss, indicating that this is a critical temperature for impurity removal. In the absence of a significant sulfur partial pressure, Scragg et al. have demonstrated that CZTS decomposes into solid Cu2S and ZnS and volatile SnS and Sx.35 The driving force for this decomposition reaction and the rate of mass loss of SnS and Sx increase with increasing temperature. The onset of SnS and Sx loss is indicated by the transition to yellow shading in Figure 1. Near 580 °C the mass percent was approximately equal to 14.5%, which corresponds the mass percent of CZTS as shown in Table 2. However, given that some portion of the mass loss can be attributed to Sx and SnS vaporization, this suggests that some impurity elements remained in the film even at these highest temperatures.

Figure 2. Near-resonant Raman spectra of precursor films annealed for different temperatures (a) and times (b), collected with 785 nm excitation. Gridlines are indicative of major peaks attributed to the indicated metal sulfides. Films are composed of increasing amounts of CZTS with increasing anneal time and temperature.

The TGA ramp of 1 °C/min up to 600 °C leads to a much higher thermal exposure than the brief anneals between

spin-coated layers. During TGA, the material spends 300

Figure 3. XRD patterns for precursor films, (a) and (b), and CZTSSe films after selenization (c) and (d) annealed at different conditions after spin-coating. Films are increasingly more crystalline with increasing annealing temperature (a) and increasing annealing time at 500 °C (b). After selenization, a peak shift toward higher two theta angle with increasing annealing temperature at 1.5 min/layer (c) and with increasing annealing time at 500 °C (d) is apparent which indicates increasing S/(S+Se).

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minutes above 300 °C, 200 minutes about 400 °C, and 100 minutes above 500 °C. By contrast, films are only annealed at a maximum of 3 min/layer during spin coating in this study. Thus, the TGA data is a useful upper limit for impurity removal in a spin-coated thin film, since the impurities have a larger potential time window to volatilize. In the following sections, a number of annealing profiles with varying time and temperature between spin-coating layers were explored to determine the effects of these impurities on precursor films, CZTSSe absorbers, and PV device performance. 3.2 Chemical Composition, Morphology, and Crystallinity of Precursor Films. Near-resonant Raman spectra of CZTS films with various annealing conditions show a clear difference in the metal sulfide phase distribution as shown in Figure 2. A number of metal sulfides exhibit Raman bands from 250-400 cm-1 including: ZnS (350 cm-1), monoclinic Cu2SnS3 (CTS) (290 and 352 cm-1), SnS2 (314 cm-1), and CZTS (289, 339, 368, and 377 cm-1).36-40 Given that the Raman bands for these species have many peaks and shoulders which overlap, is it difficult to deconvolute the spectra in Fig. 2 to determine the exact quantity of each of these phases. However, there are clear trends in the relative peak intensities with changing annealing time and temperature. As shown in Fig. 2a, the ratio between the primary CZTS peak at 339 cm-1 and the primary peaks for ZnS and CTS near 350 cm-1 substantially increases with increasing annealing temperature. Thus, films annealed at higher temperatures are composed of more CZTS and less secondary phases. This trend continues for increasing annealing times at 500 °C as shown in Fig. 2b. For longer durations all CZTS peaks become sharper and more prominent in comparison to the broad background from 250-400 cm-1. Exclusively for the longest annealing condition (500 °C-3 min/layer), peaks appear at 190 cm-1 and 220 cm-1 which can be attributed to SnS.36, 37 The amount of CZTS formed in the precursor film is closely related to the crystallinity of the film as indicated by XRD in Figure 3a and 3b. For annealing temperatures at or below 400 °C, little to no crystallization of the metal sulfides precursor film is visible. However, at 500 °C, the film is readily crystalized, and crystallization increases with increasing annealing time. Note that due to the overlapping diffraction patterns, the crystalline phases of Cu2ZnSnS4, ZnS, and monoclinic CuSnS2 cannot be distinguished by XRD. Despite the differences in phase distribution and crystallinity, SEM of all precursor films annealed at different conditions look identical, except for the 500°C for 3 minutes/layer condition. For all except this condition, precursor films have a porous, flat morphology with features 10% (shown in the SI). Films annealed at 400 °C, with a C & N enriched fine-grained layer, consistently led to devices with a low fill factor and PCE. Films annealed at 500 °C showed a range of morphologies, but all annealing times were shown to lead to average device PCEs >7%. The annealing condition of 500 °C 1.5 min/layer lead to the highest average device PCE and a champion device PCE of 10.7%. However, we note that neither lithium17 or germanium14 doping were not used for device fabrication in this study, and we expect that combining the understandings from this study with optimized doping may provide a route to world-record CZTS devices. The results presented in this paper have shed new light on the DMSO molecular ink process and have implications for any solution-processed PV absorber which utilizes thiourea as a sulfur source. The C & N containing thiourea decomposition products must be removed from the absorber layer, while avoiding their polymerization into an a-C:H:N network to avoid potentially device limiting multi-layer morphologies. Further, given that sodium rich precursor films (from Nadoping or Li-doping) lead to absorbers free from bilayer morphologies, and that all fine-grained layers in this work appear to be enriched in Na and Se, this suggests that Na2Sex is a key fluxing agent to remove impurities during selenization of CZTSSe films.

ASSOCIATED CONTENT This material is available free of charge via the Internet at http://pubs.acs.org. GDOES calibration information, Raman data, SEM micrographs GDOES data and device performance for two additional anneal conditions,

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (HWH)

Author Contributions This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

Acknowledgments JAC, TRM, and HWH acknowledge support from the National Science Foundation (NSF) Sustainable Energy Pathway (SEP) Award CHE-1230615. TRM acknowledges support from NSF Division of Materials Research (DMR) Award 1533372 ARU acknowledges the financial support from the Swiss National Science Foundation (SNSF) under the project number P300P2_164660. The NanoTech (NTUF) user facility is acknowledged for GDOES, powder XRD, Raman, and SEM measurements. The authors thank Anna Murry, Liam Bradshaw, and Micah Glaz for help with sample generation, GD-OES measurements, and Raman measurements, respectively.

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Chemistry of Materials

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