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Dennis S. Pruzan, Anna E. Caruso, Yijin Liu, Yu Lin, Carolyn Beall, Ingrid Repins, Michael F. Toney, and Michael A. Scarpulla* composition fluctuations and phase segregation. One example is the formation of second phases[2] (there exist at least 11 in the quaternary phase diagram[3]), which may result in bandgap fluctuations[4] and recombination-active interfaces.[5] The formation of secondary phases has been shown to be highly dependent on the growth temperature of the film, with higher phase purity CZTS(e) generally corresponding to films grown at higher temperatures.[5] However, due to the high vapor pressure of the SnS(e) phase, elevated temperatures in the absence of a SnS(e) or at least S(e) overpressure leads to decomposition of the CZTS(e) phase into ZnS(e) and Cu2S(e) binary phases,[6] which can lead to significant reductions in device performance. The bulk material is known to exhibit a rich native defect chemistry,[7,8] orderdisorder transitions,[9] and possibly spinodal decomposition.[10–12] Higher temperatures and fast cooling rates have been shown to induce compositional disorder via completely random distributions of Cu and Zn atoms on their respective lattice sites,[9] inducing significant bandgap fluctuations which through the detailed balance analysis results in a lowering of the potential open-circuit voltage (VOC) of devices.[13] This is primarily due to the shallow defect behavior of CuZn/ZnCu antisite pairs.[14] Additionally, in thin polycrystalline films these composition and phase complications may be compounded by differences in composition within single grains, between different grains, along grain boundaries[15] as well as through the thickness of the film. As composition fluctuations are coupled to changes in bandgap[16], doping, and defects, understanding composition and phase on length scales from atomic to macroscopic is paramount for advancing CZTSSe thin film photovoltaics. Transmission X-ray microscopy tomography (TXMT) is a powerful nondestructive imaging technique that yields X-ray absorption contrast in 3D with spatial resolution in principle down to the tens of nanometers[17] as determined by the outermost width of the Fresnel zoneplate used as the objective lens in the experiment. Furthermore, when coupled with a tunable X-ray source such as a synchrotron, element-specific reconstructions can be generated with a combination of resolution and sampling volume paralled only by Nanoscale Secondary Ion Mass
The origins of open-circuit voltage deficits in Cu2ZnSnS(e)4-based solar cells have been an intense topic of research over the past few years as device efficiencies have never approached those of CuInGaSe2 based cells despite the materials sharing similar crystal and electronic structures. In this work, we use transmission X-ray microscopy tomography to investigate the length scales over which elemental fluctuations occur. We find and show evidence of micron-scale Cu to Zn anti-correlations over a previously inaccessible combination of resolution and sample size that is consistent with the length scale of grains in this material. This result yields further insight into the causes of the large opencircuit voltage deficits regularly seen in these devices as well as the challenges of achieving compositional homogeneity in this material.
1. Introduction Cu2ZnSn(S,Se)4 (CZTSSe) is an attractive material for thin film photovoltaic absorber layers because its elements would not face the same production bottlenecks predicted for In and Te containing materials for production scaled to 100 GWp/yr.[1] Whereas, Cu(In,Ga)Se2 (CIGSe) is an alloy of two ternaries, CZTS and CZTSe are quaternary compounds and are thus particularly susceptible to property variation engendered by Prof. M. A. Scarpulla, D. S. Pruzan Department of Materials Science and Engineering, University of Utah, 50 Central Campus Drive, Salt Lake City, UT 84112, USA E-mail:
[email protected] A. E. Caruso, Prof. M. A. Scarpulla Department of Electrical and Computer Engineering, University of Utah, 50 Central Campus Drive, Salt Lake City, UT 84112, USA Dr. Y. Liu, Dr. M. F. Toney Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Dr. Y. Lin Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Dr. C. Beall, Dr. I. Repins National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401, USA
DOI: 10.1002/solr.201600024
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Cu2ZnSnSe4 Photovoltaic Absorber Layers Evaluated by Transmission X-Ray Microscopy Tomography: Composition Fluctuations on the Length Scale of Grains
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Spectrometry (NanoSIMS),[18] which in contrast is a destructive imaging technique. With resolution in the hundreds of nm and sampling volumes in the thousands of mm3,[19] synchrotronbased TXMT and NanoSIMS bridge a critical gap in composition mapping techniques between the 1 nm resolution over 1 mm3 volumes offered by atom probe tomography (APT), the atomic-scale resolution over 0.1–1 mm3 offered by transmission electron microscopy (TEM), and the >1 mm resolution over macroscopic sampling volumes available from scanning electron microscope (SEM) based energy dispersive spectroscopy (EDS). While SEM imaging is certainly capable of resolving micronsized grains in CZTSe, SEM-based EDS is not capable of assessing composition of single grains with high statistical certainty because of the electron interaction volume exceeding the grain size at the 8–13 keV accelerating voltages required to excite the Cu, Zn, and Se K edges. On the other hand, while TEM-based EELS or EDS would certainly be capable of assessing composition fluctuations within single grains and between adjacent ones, the required electron-transparent lamella sample preparation is known to be prone to artifacts, especially Cu diffusion,[20] and it is a practical impossibility to sample large volumes of CZTSe using TEM. In this paper, we use TXMT with resolution in the hundreds of nanometers over volumes of >100 mm3 containing hundreds of grains to investigate the composition uniformity of CZTSe solar cell absorber layers with Zn/Sn ¼ 1.0 (Cu/(ZnþSn) 0.9) and Zn/Sn ¼ 1.4 (Cu/(ZnþSn) 0.85) grown by coevaporation and processed into devices with efficiencies of 5.7 and 8.2%, respectively. While grains and grain boundaries could not be resolved, we find anti-correlation between Zn and Cu composition in adjacent volumes on the micron scale, matching or exceeding the grain size in the films. To our knowledge, this is a previously unreported type and length scale of cationic composition fluctuation in CZTS(e) thin film photovoltaic absorber layers enabled by the unique capabilities of TXMT. As the Cu/Zn ratio is related to bandgap, doping, and defect-assisted recombination,[21] this observation helps to establish the sources of disorder responsible for some of the VOC losses seen in CZTS(e) based devices. Evidence has previously been presented for anionic compositional fluctuations of the S and Se ions on the microngrain scale in CZTSSe films,[22] which are the only other reports to our knowledge of compositional fluctuations on a similar length scale in this material. Cationic compositional fluctuations were recently found in single crystalline CZTS with 8% to 6.5%.[30] As the Zn/Sn ¼ 1.4 sample exhibited efficiencies 8.2%, it is very unlikely that this film contains significant ZnSe near the front of the device, but rather should contain ZnSe near the back where it has been shown not to substantially affect the electrical characteristics of devices.[2,31] However, it has been noted that a Zn-rich “inversion” layer toward the surface, where, Figure 3. Intensity maps of Cu, Zn, and Se from a lateral slice taken toward the front of the Zn is thought to replace Cu, results in the absorber layer. Yellow corresponds to higher intensity and blue corresponds to lower intensity in highest efficiencies grown by this each of these maps, but each of the images is independently scaled to bring out contrast.
These raw 3D composition fields can be further analyzed and processed to gain information on the characteristic length of compositional fluctuations both through the depth of the film and laterally as well as compositional depth profiles through the thickness of the samples. Animations S3–S6 in the supporting section show relative Cu and Zn elemental compositions in slices stepping depth wise through the full absorber layers for both samples. Extracted depth profiles for the full thickness of the films are discussed and subsequently presented in the supporting material. We attempted to investigate intra-voxel correlations between Cu, Zn, and Se in order to determine any characteristic length scales of composition fluctuations. Large micron-scale fluctuations are clearly apparent in the 10 10 mm2 slices of the reconstruction volume as shown in Figure 3. In both samples, anticorrelation is clearly observed between Cu and Zn as well as a weaker positive correlation between Cu and Se (especially in the Zn/Sn ¼ 1.4 sample) and these correlations are shown quantitatively through the absorber layer in Figure 4. These compositional fluctuations occur over domains up to a few microns, which is many times the estimated resolution and thus is significant. The slices in Figure 3 were taken from the front half of the absorber (closer to the ZnO layers) and this distinction is made as some differences were seen in the compositional fluctuations between the front and back regions of the absorber as shown in Figure 4. Figure 4 shows 2D histograms[28] of voxel count as functions of Zn, Cu, and Se composition within 500 nm 10 mm 10 mm sub-volumes of the total analysis volume and Figure 5 shows histograms of Cu/Zn versus Cu/Se ratios over the same volumes. In addition, the means and standard deviations of all the histograms show in Figure 4 and 5 are given in the supporting material. The sub-volumes used in Figure 4 and 5 were chosen to be closer to the center of the absorber layer than to either the front or back in order to avoid any complications from roughness and/or blurring at either interface or thickness variation which could bring the CdS/ZnO layers within the volume at the front interface. First, we note the wider
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FULL PAPER Figure 4. 2D histograms showing the distribution of elemental correlations of Cu to Zn and Cu to Se for each of the two samples from both the front half and back half of the absorber layer. Correlation coefficients (r) were fit analytically to a bivariate normal distribution.
method,[27,31] which is in part attributed to lower surface recombination due to increasing Zn content.[32] Therefore, the growth conditions of the films promote this Zn-rich “cap” with films initially deposited slightly Cu-rich toward the back of the films to promote grain growth and then the Cu-flux is turned off during the second half of the deposition to finish the devices Znrich.[27] In turn, it seems as though this Zn-rich layer toward the front of the devices is what induces the grain-scale segregation between Cu-rich and Zn-rich regions in both samples, although this effect is enhanced in the Zn/Sn ¼ 1.4 sample with an average Cu/Zn ratio 20% lower toward the front of the device versus the stoichiometric sample (1.37 vs. 1.70). It is possible, however, that the “Zn-rich inverted layer” is a nearly continuous solid solution of ZnSe and CZTSe that has been shown to form in other CZTSe samples characterized by APT,[11] where ZnSe forms over domains 20 nm or less in length, which would be below the resolution of our technique. In contrast, a previous publication from our collaborator[12] showed cation fluctuations in CZTS crystals using scanning TEM (STEM)
