ANALYSIS
OF
SHUNTS IN MULTICRYSTALLINE SILICON SOLAR CELLS USING MICROPROBE X-RAY FLUORESCENCE TECHNIQUET.Buonassisi, O.F.Vyvenko, A.A.lstratov,
E.
R.Weber University of California, Berkeley, CA 94720RSchindler
Fraunhofer IS€, Freiburg, Germany G.Hahn
University of Konstanz, Konstanz, Germany
Experimental procedures and sample preparation
Experiments .were conducted at Beamline 10.3.1 of the Advanced Light Source (synchrotron facility) at the Lawrence Berkeley National Laboratory. This beamline was originally developed for high-resolution x-ray fluorescence microprobe (p-XRF) studies. An intense x-ray beam from the synchrotron with approximately 10” photonsfs is incident on the sample, focused to an optimum spot size of (1 -2)xf 1 3 2 . 5 ) pm2 by a pair of elliptically bent rnultilayer mirrors in Kirkpatrick-Baez configuration. The emanating x-ray fluorescence is detected by a Si:Li detector with a resolution of about 180 eV. The sampling depth of this technique is determined by the escape depth of the fluorescence x-rays of interest from the silicon matrix, typically between 10 and 100 microns depending on the element [ 11. In order to correlate elemental distribution maps obtained by p-XRF with minority carrier lifetime maps, x- ray beam induced current maps (XBIC, [2]) were measured in-situ simultaneously with p-XRF.
The advantage of XBIC is that it provides a direct and unambiguous correlation of the chemical nature and recombination activity of the defects. Additionally, XBIC proved to be an extremely helpful tool for finding the areas of interest on the solar cells. This can be achieved by taking a fast overview XBIC map using short accumulation times and large steps in x-y direction. Both p-XKF and XBIC were performed on fully processed solar cells.
pre-characterized with thermography of the forward biased cells [3] in order to find location of the shunts. The size of the BaySix cell was 45x45 mm, the size of the RGS cell was 20x20 mm.
In our experiments, we used two types of fully processed solar cells: BaySix and RGS,
Experimental resuits: analysis of the BaySix solar cell
XBIC mapping of large areas of the cell, performed with a relatively large step (50- 100 microns), enables one to find the area of the shunt by correlating XBIC maps with the
therrnography and LBIC maps. Since the size of the “hot spot” on the thermography maps is typically around 0.5-2 mm, the task of correlating the XBIC, thermography, and LBIC maps with this accuracy was fairly straightforward. Figure 1 demonstrates a good correlation between XBIC and LBIC images of large area scans of a BaySix solar cell. Then, the area of the shunt was rescanned with a smaller step and a longer accumulation time in order to achieve the optimum XRF sensitivity.
Erschienen in: 12th Workshop on Crystalline Silicon Solar Cell Materials and Proceses : Extended Abstracts and Papers ; Beaver Run Resort, Breckenridge, Colorado, August 11-14,
2002 / Sopori, B. L. et al. (Hrsg.). - Golden, Colorado : NREL, 2002. - S. 258-262
XRTC LBIC
Figure 1 - Comparison of XBIC (left) and LBlC (right) maps of a 9x7mm region of a pre-characterized with thermography and LBlC BaySix solar cell. XBIC step size is 100 pm. The circles on the LBlC picture corresponds to areas where shunts was detected by the thermography measurements. Horizontal lines are contact grids; the distance between them is 2.125 mm.
The regions in the center of the map highlighted by the circles in Fig. 1 are of particular interest, as two separate shunts were found there by the thermography measurements. Detailed p-
XRF
analyses were performed on these regions of interest. The p X R F map of the area which corresponds to the lower circled area in Fig. 1 is shown in Fig. 2 along with the XBIC map which was obtained during the same scan. The map of the shunt area circled in the upper half of Fig. 1 is shown in Fig. 3. In Fig. 2, a cluster of silver particles with the diameter of up to 10-20 pm was found within the area of the shunt. An examination of p-XRF and XBIC images in the Figure 2 revealed that a dark XBIC contrast spot could be found for every silver particles visible onXRF-
map. An optical microscope image of this area did not reveal any surface contamination,
suggesting that these precipitates are located within the bulk of the material, possibly close to the surface given the strong fluorescence signal. This suggests that these metal particles may be responsible for the shunt.
Upon closer inspection of the area of interest using p-XRF scans with longer
accumulation times, it was found that the small silver precipitates also contain a small amount of titanium. Since the contact grid contains a significant amount of both silver and titanium, we conclude that the formation of shunts may be caused by the process of the contact strips deposition of the contact strips. Interestingly, these clusters of impurities occur not under the contact strips, but approximately at the midpoint between two contact strips, the closest of which is located nearly a millimeter away from the center of the cluster. Traces of iron could also be found at the Ag-Ti precipitates, which may further contribute to the lowering ofthe lifetime in this region of the cell.
Similar procedure has been applied for the investigation of another shunt on the same celi, circled in the upper part of the XBIC image of Figure 1. The experimentaily obtained p-
XRF
maps are presented in Fig. 3. Similarly to the first shunt, both iron and titanium were found.Thermography map Silver (La)
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Figure 2 - Comparison of thermography (upper left), XBIC (bottom left), p-XRF Ag-La (top right), and p- XRF Ti-Ka (bottom right) maps of a fufly processed Baysix solar cell mapped in the area of the lower shunt region shown in Fig. 1. The XRFIXBIC step size is 14 pm. The dotted structure in the thermography map is the result of the low resolution of this thermography scan, and not a physical effect.
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Figure 3 - p X R F Fe-Ka ( left) and Ti-Ka (right) maps of a fully processed Baysix solar cell of the upper shunt region shown in the Figure 1. The step size is 3 prn. In this case a large precipitate consisting mainly of iron as well as numerous titanium spots were found in the area of a bright thermography contrast .
Experimental results: analysis of RGS solar cell
For the analysis of RGS material, the same experimental procedure was used. The area of a shunt was mapped using a combination of p-XRF and XBIC (Figure 4). The areas of shunts had a strong XBIC contrast. p-XRF mapping revealed several large clusters of titanium within the shunt location. Additionally, these precipitates contained small amount of silver, iron, and copper. All four of these metals could be reliably identified in X-ray fluorescence spectra by their KCX and Kp lines, see Fig. 5.
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figure 4. XRF (left) and XBIC maps taken around an area of a shunt in RGS material. Bright XBIC contrast is believed due to an enhanced minority carriers collection due to presence of channels of inversed conductivity type [4]. The position of the largest precipitate coincides with the location of the location of the current collecting channel.
2 3 4 5 6 7 8 9 10
Photon energy, eV
Figure 5. XRF spectrum taken from the largest "Ti"-precipitate shown in the Figure 5. Besides titanium,
XRF mapping of BaySix a on samples revealed higher than average metal concentration at th Specifically, our investigations revealed the presence of silver, titanium, iron, er at the location of the shunt, this suggesting that shunts are process-induced defects formed during deposition of the contact strips. Hence, our data indicate that (a) the shunts we measured contain, or could possibly even be determined by metal contamination, and (b) this metal contamination was process-induced.
Unfortunately, the spatial resolution of thermography, which is the only technique capable of reliably detecting and mapping shunt locations, is much worse than the resolution of our X-ray microprobe technique. Therefore, we can only conclude that the metal concentration at the shunt location is higher than average, but cannot unambiguous1 rove that these metals clusters cause the formation of the
nfirmation that Ag and Ti cluster
information on the depth of these precipitates from the sample surface. Indeed, the metal clusters should be within the depth of the p-n junction in order to affect its properties. The lack of any indications of these precipitates in an optical microscope suggest that these metal clusters are located inside of the solar cell, while the strong XRF signal indicates that these metal clusters are close to the wafer surface. However, so far we could not accurately measure the depth of these clusters from the wafer surface. At the present time, we are developing an experimental procedure which would enable us to obtain information not only on the spatial location of the precipitate, but also on its depth from the surface.
se the shunts would be the
Acknowledgements
The authors would like to acknowledge 0. Breitenstein of the Max-Planck-Institute of Microstructure Physics Halle for the thermography measurements on the RGS material, in addition to J. Isenberg and E. Schaffer of Fraunhofer ISE for the thermography and LBIC
measurements on BaySix material, respectively. This study yas made possible with the financial support from NREL, subcontract No. ATT-2-3 1605-03, AG-Solar project of the government of
orthrhein-W estfal
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4. 0. Breitenstein, M. Langenkamp, and J. P. Rakotoniaina, SoZid State Phenom. 78-79,29 and E. R. Weber, J.AppZ.Phys. 91,3614 (2002).
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