Real-Time Electron Nanoscopy of Photovoltaic Absorber Formation from Kesterite Nanoparticles

Cu2ZnSnS4 nanocrystals are annealed in a Se-rich atmosphere inside a transmission electron microscope. During the heating phase, a complete S-Se exchange reaction occurs while the cation sublattice and morphology of the nanocrystals are preserved. At the annealing temperature, growth of large Cu2ZnSnSe4 grains with increased cation ordering is observed in real-time. This yields an annealing protocol which is transferred to an industrially-similar solar cell fabrication process resulting in a 33% increase in the device open circuit voltage. The approach can be applied to improve the performance of any photovoltaic technology that requires annealing because of the criticality of the process step.

T he current energy challenge facing our society is exemplified in Shell's Sky scenario which projects that global electricity generation by 2070 will be nearly 5 times greater than today's level. 1 This increased capacity will be driven, in part, by the proliferation of new and distributed applications in healthcare, transportation, construction, aerospace, and consumer devices. The growth in these applications defines a new era for photovoltaics because of the demand for innovative and sustainable power sources that can be easily integrated with high value products. This creates a significant opportunity for inorganic thin film photovoltaics because of their stability and compatibility with a variety of flexible substrates including foils, plastics, and ultrathin glass. 2 The most commercialized thin film technologies are CdTe and CuInGa(S,Se) 2 (CIGS) and have efficiencies of >20%. 3 Another chalcogenide material that is closely related to CIGS, but with an increased Earth abundance of the constituent elements, is Cu 2 ZnSnS 4 (CZTS). However, CZTS is not yet commercialized because of the challenge associated with achieving sufficiently high open circuit voltage V oc , under illumination, which is essential for high efficiency. This issue is generally associated with disorder in the kesterite crystal lattice resulting in the formation of performance-limiting defect levels. 4,5 Nevertheless, both CZTS and CIGS have the added benefit of solution-based fabrication via nanoparticle inks 6,7 which are compatible with high volume, high value manufacturing. There is therefore an urgent need to understand the formation of the light absorbing layer and associated defects, prepared from nanoparticle inks.
A critically important step to achieve high performance in the fabrication of all thin film photovoltaic absorbers, including CZTS, is an annealing step performed at high temperature. For industrially manufactured devices, the solar absorber layers are annealed in a large-scale furnace with a specific gas atmosphere. This promotes the formation of an absorber with a granular morphology in which the grains are as large as possible to maximize the absorption of photons and grain boundaries are minimized 8,9 or passivated to reduce non-radiative recombination of photogenerated charge carriers. 10,11 Recently, this process has been studied using in situ X-ray diffraction; 12−14 however, all these studies were performed on bulk films prepared from stacked nanocrystals. Consequently, the information obtained is averaged over length scales that are much larger than the characteristic dimensions of the nanocrystal itself. This may introduce significant variations in sample properties, such as surface area to volume ratio and reaction rate.
In this work, we thermally anneal CZTS nanoparticle inks in a Se-rich atmosphere in an environmental cell nanoreactor rather than a conventional large-scale furnace and use in situ transmission electron microscopy (TEM) to study the formation of Cu 2 ZnSnSe 4 light absorbing grains directly at the characteristic scale of CZTS nanoparticles and in real-time. On the basis of insights from these results, we develop an annealing protocol which promotes cation ordering. Moreover, we apply this protocol to an industrially similar solar cell fabrication process to create solar cell devices with substantially increased V oc . Significantly, the approach is applicable to any photovoltaic technology that requires thermal annealing to achieve high performance.
A CZTS nanoparticle ink was fabricated by a hot injection method 15 and then applied to a Si-based MEMS chip via spincoating for in situ characterization inside a TEM. A second blank chip was sealed face down to the spin-coated chip to form a nanoreactor with electron transparent windows ( Figure  S1) to allow in situ investigation of thermal annealing of kesterite nanoparticles inside the TEM. A thin film of Se was thermally evaporated on top of the nanoparticles to provide a reactive Se atmosphere during annealing. Before placing the sample inside the TEM, the nanoreactor was sealed in N 2 at 1 atm. To dynamically monitor the chemical and crystal structure changes of the nanoparticles during annealing, a series of selected-area electron diffraction (SAED) patterns were recorded at temperatures from 26 to 700°C (see Figure  S2). To discriminate partially overlapping diffraction rings, the diffraction patterns were converted into normalized radial profiles using ImageJ software. 16 From these data, the signal profiles, positions, and intensities were extracted using a multipeak fit as shown in Figure 1a (peak details are summarized in Table S1).
In the low temperature range, 26−200°C, the experimental data are in good agreement with tetragonal CZTS (PDF 026-0575). Additionally, as the temperature increases, the diffraction peaks become sharper and more intense indicating increased crystallinity. 17,18 As the temperature approaches the melting point of Se (225°C), the Se layer becomes thinner (as shown in Figure S3), initiating the S−Se exchange reaction. As shown in Figure 1a, a shoulder in the signal belonging to the (112) reflection of Cu 2 ZnSn(S 1−x Se x ) 4 (CZTSSe) appears at 200°C, indicating that the incorporation of Se into the CZTS lattice has already started. When the temperature reaches 300°C , Figure 1a indicates that the diffraction pattern can be indexed to a growing CZTSSe phase and the parent CZTS phase. This is schematically illustrated by the change in the parent lattice from Figure 1b to Figure 1c whereby S atoms in CZTS nanoparticles are replaced by larger Se atoms, resulting in increased interplanar lattice separation.

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Letter By 450°C ( Figure 1a) the diffraction signal matches that of tetragonal CZTSe (PDF 052-0868) indicating that the S−Se exchange reaction is complete and the sample is fully selenized, as shown schematically in Figure 1d. For an additional view of this change, Figure 1e compares the SAED diffraction rings at the extremities of the temperature range in Figure 1a. Notably, the relative intensity of the signal at 450°C has decreased, implying that sample crystallinity has degraded during the S− Se exchange process. This is due to an increased density of crystallographic boundaries within an individual CZTS nanoparticle during the S−Se exchange process which yields smaller crystalline domains and decreased crystallinity in CZTSe nanoparticles (see Figure S4 for detailed explanation).
It is generally accepted that for CZTS, Cu and Zn atoms are highly disordered 19,20 resulting in antisite defects; however, their impact on solar cell performance has been found to be somewhat limited. 4 Sn-related disorder on the other hand has been found to be more detrimental 5 and has recently been observed with atomic-scale resolution in Ag 2 ZnSnSe 4 . 21 Further randomization of the cations resulted in CZTS nanocrystals with cubic structure at the nanocrystal surface. 22 These led to Sn-related deep level antisite defects which ultimately reduce solar cell performance but also damage the tetragonal symmetry of fully ordered kesterite CZTS. As a result, tetragonal diffraction reflections such as (101), (103), and (211) are absent in the temperature 26−450°C.
When the annealing temperature reaches 500°C, the continuous diffraction rings disappear and discrete diffraction spots are visible in the SAED image in Figure 1f, implying significant crystal growth. The spots stem from reflections of pure CZTSe as emphasized by the colored curves in Figure 1f. Compared with the previous patterns, more diffraction planes can be observed due to the better crystallinity developed in this stage. Interestingly, previously absent diffraction spots from (211) reflections are now weakly visible and highlighted by the white curve. This implies increased cation ordering within the CZTSe crystal. We suggest that initial Sn-related disordering in the parent nanoparticles is significantly reduced during the recrystallization and grain growth process. This hypothesis is supported by two pieces of additional experimental evidence. First, the CZTSe grains approach stoichiometry compared to the as deposited CZTS nanoparticles (seen in Table S2). This follows from a zinc loss during the selenization process, which implies that less Zn is available to occupy Sn sites (the 2b Wyckoff positions 19 ). Second, our deep level transient spectroscopy (DLTS) data 23 shows that the deep level Zn Sn antisite defect has been passivated in the large grained CZTSe thin film. Increased Sn ordering would allow the CZTSe to possess an ordered tetragonal unit cell, resulting in the rise of the (211) reflection after selenization.
At 600°C (see Figure S2) the diffraction spots decay significantly, and the pattern can be indexed to cubic CuSe 2 (PDF 01-071-0047). This is likely to be due to the decomposition of CZTSe at high temperature. 24,25 When the temperature is further increased to 700°C, no diffraction pattern is observed (seen in Figure S2), implying that the sample has completely decomposed and transited to an amorphous state.
Notably in Figure 1a, the position of the parent CZTS diffraction peaks remains constant over a relatively wide temperature range as indicated by the vertical green line. As a result, the CZTS lattice constants (i.e., a ∼ 0.55 nm and c ∼ 1.10 nm) are stable in this range as shown in Figure 2a,b. Therefore, although consumed by the growing CZTSSe phase, the parent CZTS phase is structurally stable during the S−Se exchange process. In contrast, however, the CZTSSe lattice constant a increased from ∼0.57 to ∼0.58 nm, and c increased

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Letter from ∼1.13 to ∼1.17 nm in the range 300−450°C as shown in Figure 2a,b.
To further investigate the unit cell evolution introduced by the chalcogen atom exchange, we calculated the evolution of the lattice constants a and c as a function of Se concentration x for ordered tetragonal, i.e., kesterite Cu 2 ZnSn(S 1−x Se x ) 4 as shown in Figure 2c (see SI for the detailed computational design). From Vegard's law, the gradient of a linear fit to the evolution of the lattice constants with composition yields the lattice mismatch between CZTS and CZTSe. 26 Differences between the gradients obtained for a and c indicate an uneven unit cell expansion driven by cation disorder, especially along the c-axis. 27 As a result, an increased c/a ratio (values given in inset to Figure 2c) is expected for disordered CZTS nanoparticles compared with the model.
As expected, the parent CZTS phase has no significant unit cell volume change while the S−Se exchange reaction introduces a unit cell volume expansion up to ∼20% for the growing CZTSSe phase as shown in Figure 2d. The TEM frame at 450°C (captured in Figure 2e) shows monodispersed CZTSe nanoparticles after S−Se exchange with an average particle size of 15.9 nm. By comparison, the initial CZTS nanoparticles at 26°C have an average size of 13.3 nm (for ex situ TEM micrograph, see Figure S5). This unit cell expansion can be fully explained by the replacement of S by Se, which is confirmed by the comparison with the calculations shown in Figure 2c. Despite this volume expansion, the shape and characteristic length scale of the initial CZTS nanoparticles is preserved during the phase transition as neighboring nanoparticles did not coalesce or merge into larger grains.  (d) PL spectra of slow-cooled (SC) and fast-cooled (FC) CZTSSe thin films at 6 K and the same excitation intensity. The dashed line is an asymmetric double sigmoidal curve fit used to evaluate the peak of each PL band. The feature apparent around 0.9 eV is due to light absorption by water vapor. The inset shows the V oc of the solar cells made from SC and FC absorbers, respectively. □ is the average value. × symbols represent the minimum and maximum positions. The three horizontal lines of each box stand for the 25%, 50%, and 75% of the reading distribution. The whisker range is determined by the standard deviation of the 15 devices.

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Letter Above 450°C, we observed the formation of large-sized CZTSe grains in real-time in the TEM. The dynamics of CZTSe grain growth in the temperature range from 480 to 500°C are recorded in Video 1. The snapshot frames captured are shown in Figure 3a−d, and as shown in Figure 3a, the nanoparticle indicated by the red arrow is measured to have a lateral size of ∼20 nm, which is slightly larger than the average particle size of CZTSe. As the temperature increases, the nanoparticle diameter increases to ∼50 nm at 496°C, together with the nanoparticles enclosed by the white dashed line becoming smaller and blurred. When the temperature reaches 498°C in less than 1 min, the lateral size of CZTSe grains has been enlarged three times while the nanoparticles enclosed by the dashed line disappear (see in Figure 3d and Video 1). This behavior is consistent with Ostwald ripening 28 schematically illustrated in Figure 3e where CZTSe large grains with limited orientations grow bigger by consuming small grains with different crystallographic orientations. This model accounts for the emergence of discrete diffraction spots observed in Figure  1f. Moreover, the rise of the weak (211) reflection of the tetragonal structure around this temperature implies that the initially disordered Sn sites on (002) and (001) planes can be significantly reduced during the recrystallization and grain growth process. The increased structural order associated with the tetragonal symmetry is shown schematically in Figure 3f.
Our in situ TEM experiments reveal the new insight that chalcogen atom exchange and grain growth are distinct and separate processes. Furthermore, the grain growth is accompanied by an improvement in the crystallinity of the material and cation ordering within the kesterite structure. To promote this ordering and reduce the density of defects in a CZTS thin film photovoltaic absorber, we transferred the insights obtained from the in situ TEM experiment to a photovoltaic device fabrication step that more closely resembles an industrial process. This involved creating CZTSe thin film photovoltaic absorbers in a selenization furnace (see SI for details). To distinguish this experiment from the in situ TEM, we refer to it as selenization, and a comparison of the heating profiles in each may be found in Figure 4a, where it can be seen that the heating rates between the TEM and selenization experiments are very similar.
The top-view scanning electron microscope image in Figure  4b shows a uniform thin film of CZTS nanoparticles before selenization. On the basis of the insights from the in situ TEM experiment, an annealing temperature of 500°C was maintained in the selenization furnace for ∼20 min before being cooled to room temperature. A slow-cooling (SC) temperature profile was used to maximize the cation ordering and grain growth observed in the TEM, and for comparison, a fast-cooling (FC) profile was also performed on a different sample. In both SC and FC, a representative sample morphology following the selenization experiment is shown in Figure 4c, and it can be seen that the thin film morphology has transitioned from nanoparticles to smooth, micrometersized grains.
In order to assess the impact of the relative cooling rate on the selenized thin films, a low temperature photoluminescence (PL) experiment was performed, and the data are shown in Figure 4d. Both samples show a similar broad, asymmetric PL signal indicative of a semiconductor material with large defect densities which cause electrostatic potential fluctuations in the conduction and valence bands. 29 The PL spectra in Figure 4d are shown relative to the 6 K bandgap energy, and the significant red-shift for both samples is further evidence of defect clusters within both films.
In our recent work 23,30 we have shown that donor/acceptor defect states have low thermal activation energies in contrast to the large red-shift of the PL bands from the bandgap energy of CZTSSe films, indicating the presence of deep defects. Tiwari et al. have shown such behavior results from recombination related to defect clusters that lead to a narrowing of the absorber bandgap. 31 The defect cluster [2Cu Zn + Sn Zn ], which is caused by both Cu−Zn and Sn−Zn disorder, was found to induce a large bandgap decrease of 350 meV and results in spatial bandgap fluctuations within the absorber. 32 The magnitude of the red-shift in the FC film in Figure 4d could be attributed to this defect cluster. In the SC sample, however, this red-shift is notably 60 meV smaller. From powerdependence measurements of the PL (see Figure S6) the ratio of the radiative to total defect concentrations was determined to be 0.12 and 0.10 for the SC and FC samples, respectively. Therefore, given that the SC absorber has a lower proportion of nonradiative defects, including the electrontrapping Sn Zn deep defect, we conclude that slower cooling results in increased crystallographic order.
To further assess the impact of improved crystallinity and grain growth observed in the TEM, both SC and FC thin film photovoltaic absorbers were integrated into a conventional thin film solar cell device architecture (details given in SI). The inset to Figure 4d shows an average increase in open circuit voltage (V oc ) of ∼50 meV for the SC absorbers across 15 devices with the best efficiency of ∼7% without optimization of the other device parameters (see Figure S7), demonstrating a feasible route to improving device performance in kesterite solar cells.
In summary, CZTS earth-abundant nanoparticles were annealed in a Se-rich atmosphere for the first time in a TEM. This revealed the chalcogen atom exchange and grain growth processes to be distinct and occurring within different temperature ranges. A further observation was the improved crystallinity at a temperature within a typical range used for industrial fabrication of thin film chalcogenide solar cells. The insights obtained at the nanometer scale in the TEM were subsequently exploited to increase the open circuit voltage of kesterite solar cells via reduction of cation disorder. These experiments demonstrate a new approach to improve the performance of any photovoltaic technology that requires thermal annealing in the device fabrication.
Experimental details and following additional data: sideview schematic of the gas cell for the in situ TEM study; the full SAED survey from 26 to 700°C; peak details extracted from the radial profiles in Figure 1a; TEM images captured at 26, 100, and 200°C; the composition of as-deposited CZTS nanoparticle and selenized thin film; crystal structures of CZTS; the ex situ TEM micrograph and elemental map of CZTS nanoparticles; Avrami−Erofeev nucleation and growth model in a single nanoparticle; excitation-dependent PL spectra of SC and FC absorbers; and JV curves of the best device in this research series (PDF)