Contrasting the Material Chemistry of Cu2ZnSnSe4 and Cu2ZnSnS(4– x )Sex

Earth‐abundant sustainable inorganic thin‐film solar cells, independent of precious elements, pivot on a marginal material phase space targeting specific compounds. Advanced materials characterization efforts are necessary to expose the roles of microstructure, chemistry, and interfaces. Herein, the earth‐abundant solar cell device, Cu2ZnSnS(4– x )Sex, is reported, which shows a high abundance of secondary phases compared to similarly grown Cu2ZnSnSe4.

Demand for new power plants relying on solar technology keeps rising as investments increase, and declining production costs are mounting a competitive edge over other energy hosts. In order to further support solar technology as a competitive and secure energy resource and reach grid parity with fossil fuels, the elements of the cell must be chosen to refl ect current and future mineral supplies, as well as address environmental sustainability concerns. Avoiding the use of scarce, precious, and potentially toxic minerals, such as gallium, indium, and cadmium should be considered. [ 1 ] Given the natural abundance of precursor materials and low toxicity of the fi nal inorganic phase, Cu 2 ZnSnS 4 (CZTS) based solar cells are potential candidates for future sustainable energy production. However, additional materials-level research to support the use of these earth-abundant materials within a solar cell is necessary to overcome current technological barriers. These barriers are mostly material derived and include a constrained material phase-space where CZTS is a line compound within that framework, and there is an expectation to share the same tailored band-structures and device effi ciencies (>15%) as other competitive photovoltaics. [2][3][4] The technical approach to advance the fi eld of earth-abundant photovoltaics, therefore, hinges on the incorporation of several fundamental material insights and studies into the atomic structure, chemistry, and generated defects across multiple length scales and growth techniques. [5][6][7] In light of the inherent challenges described with studying alternative photovoltaics absorber materials such as I 2 -II-IV-VI 4 CZTS, CZTSe, and CZT(S,Se) semiconductors, these same materials have attracted broad interest as a result of the sputtered molybdenum (Mo) back contact, an e-beam evaporated NaF precursor (150 Å), a co-evaporated CZTSe or CZTSSe absorber layer (1-3 µm), a CdS buffer layer (500 Å), a ZnO bilayer (0.2 µm), an MgF 2 antirefl ection coating layer, and Ni/ Al grids. Further details on the chalcogenide deposition and annealing processes and fi nal device structure can be found in the literature. [ 20 ] Both CZTSe and CZTSSe fi nal fi lms composition had target average Cu/Zn ratio of 2 terminating Zn-poor, consistent with a Cu rich absorber material. Each of the devices photovoltaic performances were evaluated with current-density versus voltage ( JV ) plots. JV curves were collected under simulated AM 1.5G illumination (light irradiation of 100 mW cm −2 ), and device performances are reported in Table 1 . A 5.6% device conversion effi ciency was measured for CZTSSe and further confi rmed using quantum effi ciency measurements. The open circuit voltage ( V oc = 0.316) was lower than expected based on band gap expansion predicted for the amount of S present. [ 21 ] Similarly grown CZTSe demonstrated a fi nal power conversion effi ciency of 9%. The average composition of the two devices is Cu 1.91 Zn 0.99 Sn 1.01 Se 4.09 and Cu 1.92 Zn 0.99 Sn 1.02 S 3.15 Se 0.85 . Overall, comparing the JV measurements between the grown CZTSe and CZTSSe leads to nearly an order of magnitude difference.
CZTSe and CZTSSe devices were fi rst studied with aberration corrected STEM imaging. Figure 1 is a comparison of the material grain-to-grain chemistry between CZTSe and CZTSSe devices. Starting from the top of the image, Figure 1 a is a STEM high-angle annular dark fi eld (HAADF) image of the CZTSe device cross-section. This image reveals the device structure, consisting of the transparent conducting ZnO window layer from the top of the device, followed by the n-type junction layer. Below the CdS buffer is the CZTSe absorber layer, followed by the molybdenum back contact electrode. The same solar cell structure and layering is similarly shown in the STEM HAADF image, Figure 1 b, for the CZTSSe device.
Over the same area scanned, to expose differences in bonding, periodicity, mass, and chemistry at these grain boundaries, STEM-based chemical imaging was performed combining both EELS and energy dispersive X-ray spectroscopy (EDS). For each of the elements, we compare and contrast the chemical morphology between the CZTSe and CZTSSe devices for Cd ( Figure 1  The material chemistry similarities between the two devices include the presence of secondary phases. In both cases, a secondary phase made of zinc and selenium is visible at the absorber surface based of the Zn ( Figure 1 f) and Se ( Figure 1 h) maps as expected from the Zn-rich termination during growth. This layer is not uniform over the length of the heterojunction with CdS in either case. In those same regions of zinc selenide, we also note that tin is depleted (Figure 1 g) because it shares the same cation site as zinc in both CZTSe and CZTSSe. This same behavior has also been noted in several reports in the literature and is not unique to rapid quenched thin-fi lm compounds. [ 4,6,7,19 ] CZTS also shows similar secondary phases in the literature. [ 22,23 ] For thin-fi lm solar cells, the presence of secondary phases, especially at and in vicinity of the junction region presents a series of implications on the probability that minority carriers   (electrons in p-type CZTSe and CZTSSe) can diffuse to the edge of the depletion region. This, in turn, reduces carrier separation effi ciency. On the other hand, at the holes from the n-type semiconductor and those photogenerated within the absorber layer can become "trapped" at these potential wells, reducing the overall carrier effi ciency due to the presence of antibonding with the valence band maximum. The presence of a Zn-rich layer is estimated as ≈10 nm, where the size of Wannier-Mott excitons in semiconductors is on the order of a few nanometers. [ 24 ] Comparing the relative sizes of the composition inhomogeneity suggests that the recombination of electrons and holes is dominant within both CZTSe and CZTSSe. Despite presumably identical growth conditions between CZTSe and CZTSSe, there are discerning differences in material chemistry. Differences between CZTSe and CZTSSe include Cd enrichment at grain boundaries in CZTSSe, as well as nonuniform sulfur, copper, zinc, tin, selenium, and oxygen stoichiometry. Sulfur content presumably from the chemical bath deposition of CdS migrates into the grain interiors of CZTSe, but at a lower atomic percent compared to CZTSSe. The formation of Cd enrichment resolved at grain boundaries in Figure 1 c for CZTSSe is signifi cant. We speculate the presence of CdS at the grain interiors is due to a lack of grain to grain uniformity. Further, we hypothesize that this is leading to interior grain diffusion, where Cd 2+ is presumably acting as a grain boundary stabilizer. Under this presumption, CdS would readily diffuse into the CZTSSe due to the presence of defect content along the grain boundaries, forming extended vertical heterojunctions. Beyond Cd and S, elements such as Cu (Figure 1  also share the differences from grain to grain. Based on the measurable variances between the two devices, we presume the lower than expected device performance is related to the lack of consistent stoichiometry. These results further suggest a closer atomic structure comparison between CZTSe and CZTSSe. CZTSe and CZTSSe were further studied to compare and resolve any differences in atomic ordering between the two devices. The comparison to CZTSe starts with a high-resolution micrograph shown in Figure 2 a of CZTSe, where a specifi c grain was oriented to the [001] beam direction resolved using selected area electron diffraction (SAED), shown in Figure 2 b. To resolve any differences in lattice ordering along the [100] CZTSe beam direction, a selected region of the sample shown in Figure 2 c was analyzed using column-column STEMbased EELS. Figure 2 d is the atomic column-by-column CZTSe chemistry taken for Cu-L , Zn-L , Sn-L , and Se-K using STEM-EELS. Based on these collected images, we resolve no clear differences outside of the expected lattice between the Cu/Zn and Se atomic columns. We must however note that the Se column may also contain a low concentration of sulfur based on the chemical image presented in Figure 1 d due to CdS diffusion but to a very minimal amount compared to the CZTSSe cell.
Similarly, we performed high-resolution STEM imaging to discern differences in atomic contrast for CZTSSe. We oriented CZTSSe to the [111] CZTSSe zone axis shown in Figure 3 a and confi rmed our orientation using SAED in Figure 3 b. To distinguish differences in atomic contrast, a closer look at the simultaneous HAADF and annular bright fi eld (ABF) imaging shown in Figure 3 c,d, respectively, reveals the Cu/Sn and S/Se atomic columns. A closer look at the HAADF (Figure 3 c) and ABF (Figure 3 d) image reveals not only the differences in intensity associated with Cu/Zn and S/Se columns, but resolves differences in scattering potential (i.e., ionic size). To confi rm differences in atomic column intensity and ionic size resolved in the HAADF STEM images, Figure 3 e is the acquired STEM-EELS column-column map for CZTSSe. The expected atomic lattice is confi rmed based on the Cu/Zn and S/Se column-column chemistry. Based on both the structural and chemical imaging, we do not observe atomic scattering differences due to lattice ordering, but rather cannot distinguish. STEM thereby cannot  defi nitively disassociate an ordered structure for either CZTSe or CZTSSe. [ 9,25 ] Aware of the differences in grain-to-grain stoichiometry in CZTSSe, we turn our attention to report on the electronic structure associated with the device using valence EELS. Across a series of grains and boundaries, shown in Figure 4 a, we performed STEM EELS linescans. Figure 4 b is a horizontal linescan across multiple grains and boundaries. This linescan progressively changes across the CZTSSe grains. Certainly the differences in stoichiometry are giving rise to differences. Specifi cally, we observe the presence of the 0.7 eV energy-loss feature and the shape of the 1.25 eV peak from grain to grain. The CZTSSe feature at roughly 0.7 eV is certainly below the expected band gap for this material (1.0-1.5 eV). This spectral profi le therefore identifi es the presence of a mid-gap state between the conduction band minimum and valence band maximum. [ 26 ] In terms of the overall device, valence EELS specifi cally confi rms the presence of a spatially varying electrostatic potential from grain to grain that causes differences in the electronic transitions between the presumed donor and acceptor levels. This is a series of important results that consequentially alter the fi nal electronic and device properties due to differences in band-gap alignment between the CdS junction and the absorber layer.
To follow, we performed VEELS over length of the CZTSSe device cross-section. We performed three separate vertical VEELS linescans starting from the ZnO layer, continuing to CdS buffer layer and ending in CZTSSe chalcogenide. Each of these linescans reports on the trending differences in the near-IR to valence region associated with the material. In the fi rst vertical linescan, Figure 4 c resolves the near valence electronic structure, where each point is separated by 1 nm and displayed as an offset scatter plot. Figure 4 d is a similarly performed linescan over the CdS and CZTSSe junction, ending in a separate grain, respectively.
Unlike the grain-to-grain linescan, comparing these three separate linescans, Figure 4 c,d reveals a trending difference to a higher band-gap offset as function of probe position. In Figure 4 c the CZTSSe grain probed is consistent with a signifi cant amount of oxygen, as well as the depletion of copper and selenium. The grain chemistry is vastly different than the other two grains. The second STEM-VEELS linescan shown in Figure 4 d resolves a clear band gap offset at 1.18 eV with no lower optical features, dissimilar to the previous two linescans. These points clearly identify differences in band-offsets that depend on the CZTSSe grain chemistry, especially in the vicinity of the CdS heterojunction. The presence of nonstoichiometric CZTSSe not only showcases the departure from uniform device chemistry but highlights the crucial role it plays in the performance of the device, in particular its effect on the valence electronic structure.
From the detailed characterization in this study, we report that the lack of stoichiometry in CZTSSe compared to CZTSe leads to altered band-gap offsets and the presence of undesired electro-optical features. We speculate that the lack of stoichiometry in CZTSSe drastically affects the performance of the fi nal photovoltaic device. In the present case, we fi nd the buildup of Cd decorates grain boundaries in CZTSSe, where the lack of stoichiometry can signifi cantly accelerate the accumulation of elements at open-surfaces and act as a stabilizing agent. This is beyond the void structure observed in previous literature reports and extends well below the junction region. [ 27 ] More importantly, we have found in this study that this particular growth process and presumably co-evaporation of SnS, lead to a variation from the expected grain morphology and device performance. We have demonstrated that the importance of grain chemistry can play a detrimental role on the measured device properties and ultimate performance of the solar cell. These points further reiterate the need for in situ studies to track and control the material chemistry of this otherwise complex quaternary polycrystalline chalcogenide material during growth.  A systematic experimental investigation of the structure, chemistry, and electronic structure of polycrystalline CZTSe and CZTSSe was investigated. The goal was to report on the complex relationship of material chemistry and its effects for a set of quaternary polycrystalline inorganic thin-fi lm photovoltaics by high-resolution analytical electron microscopy.
Our results demonstrate that polycrystalline CZTSSe shows higher amount of secondary phases and nonstoichiometry compared to similarly grown CZTSe. Detailed materials characterization enabled us to profi le both CZTSe and CZTSSe following growth, accounting for changes in material chemistry and electronic structure. We fi nd that grain interiors and their interfaces are centers for differences in secondary compound formation. The grain interiors lead to presumably destabilized interface formation energies that were compensated by the presence of cadmium cations. We suggest that the lack of consistent stoichiometry in CZTSSe destabilizes cell and leads to a lower than expected performance of the cell.
In conclusion, our results support the idea that the junction and lack of stoichiometry in polycrystalline CZTSe-related solar cells is otherwise related to the fi nal device performance of the cell. This study indicates that a heightened attention to resultant   grain structure and chemistry is warranted to tailor the responses and effi ciency of future sustainable earth-abundant photovoltaics.

Experimental Section
Microscopy Sample Preparation : Electron transparent samples were obtained utilizing the standard focus ion beam (FIB) lift-out technique for the same areas. A fi nal thinning was performed using a 5 kV accelerating voltage and a beam current of 12 pA to remove material redeposited during the TEM FIB lift-out process and reduce the damage from the initial 30 kV ion milling, followed by low energy cleaning at 600 eV and 170 °C, for ±10°, in a Fischione nanomill FIB instrument. Care was taken to minimize the ion beam interaction with the face of the sample throughout the TEM sample preparation. The initial standard lift-out was done using a typical sample size of about 10 µm × 30 µm and a thickness of ≈2 µm prior to further thinning to electron transparency. The electron transparent region observed in TEM was only about 8 µm × 5 µm.
Analytical Microscopy : Analytical transmission electron microscopy was performed on the probe-corrected JEOL ARM 200F, FEI ChemiSTEM, and FEI Titan S. High-resolution atomic contrast imaging was performed on the JEOL ARM operated in STEM mode at 200 kV with 20 mrad semiconvergence angle and equipped with a Gatan Enfi nium ER electron energy loss image fi lter, high solid angle 50 mm 2 X-ray detector, and the latest precession electron diffraction system from AppFive located at the LeRoy Eyring Center for Solid State Science at Arizona State University. The FEI ChemiSTEM operated at 200 kV located at Sandia National laboratory performed electron dispersive X-ray spectral (EDS) chemical imaging using four simultaneous solid-state EDS detectors to acquire the Cu-L , Sn-K , Zn-L , Se-K , Zn -K, Zn -L , Cd-K, and S-L edges with the best achievable spatial and energy resolution for the microscope. The acquisition time to resolve the EDS was performed over a series of consecutive subsecond exposures over a period of 1 h. The EDS data was then processed using hyperspectral component image analysis, where the signifi cant components are matched to the individual phases within the material. [ 28 ] In brief, the composite hyperspectral images resolve the individual major component spectra, and resolve each of those as a composite image. The probe-corrected FEI Titan S located at Oak Ridge National Laboratory was also utilized to perform complementary high-resolution imaging at 300 kV operated in STEM mode with a 22.4 mrad convergence angle.