Available online at ScienceDirect. Energy Procedia 92 (2016 ) 10 15

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1 Available online at ScienceDirect Energy Procedia 92 (16 ) 15 6th International Conference on Silicon Photovoltaics, SiliconPV 16 Local solar cell efficiency analysis performed by injectiondependent PL imaging (ELBA) and voltage-dependent lockin thermography (Local I-V) Otwin Breitenstein a, Felix Frühauf a, Jan Bauer a, Florian Schindler b, Bernhard Michl b a Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 61, Germany b Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, Freiburg 791, Germany Abstract In this contribution two methods for performing local efficiency analysis of solar cells are compared with each other by applying them to a solar cell and a neighboring wafer. The first method called "ELBA" is based on injection-dependent photoluminescence (PL) imaging of a passivated wafer. The second method called "Local I-V" is based on voltage-dependent dark lock-in thermography (DLIT) on a solar cell. The results of both methods with respect to the influence of the bulk on the local solar cell parameters are comparable with each other. However, since only "Local I-V" is investigating a finished solar cell, it may image also local ohmic shunts, inhomogeneous front- and backside and depletion region recombination, and R s effects, whereas ELBA is suited for assessing bulk-related efficiency losses in detail, which are not concealed by the aforementioned cellrelated loss mechanisms in this method. 16 The The Authors. Authors. Published Published by Elsevier by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ( Peer review by the scientific conference committee of SiliconPV 16 under responsibility of PSE AG. Peer review by the scientific conference committee of SiliconPV 16 under responsibility of PSE AG. Keywords: Local efficiency analysis; PL imaging; lock-in thermography; ELBA; Local I-V 1. Introduction In multicrystalline silicon solar cells the carrier lifetime is very inhomogeneous. Here low lifetime regions limit the performance of the whole cell by (1) limiting the short circuit current and (2) increasing the dark current and thus reducing the fill factor (FF) and open circuit voltage (V oc ). Several methods have been developed to study and evaluate this effect. Two of them are Efficiency Limiting Bulk recombination Analysis (ELBA [1]) and voltagedependent dark lock-in thermography (Local I-V [2]). ELBA is an injection-dependent PL image evaluation method, which is performed on passivated wafers that have experienced the same thermal processes as solar cells. Local I-V, The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer review by the scientific conference committee of SiliconPV 16 under responsibility of PSE AG. doi:.16/j.egypro

2 Otwin Breitenstein et al. / Energy Procedia 92 ( 16 ) on the other hand, is performed on complete solar cells and thus also implies the influence of the emitter and the metallization. In this work both methods are compared quantitatively on an industrial solar cell and a parallel wafer exposed to the same thermal history. The differences can be well explained by regarding the different regions of the device where both methods gain information for the analysis. 2. Experimental The Local I-V measurements have been made on a multicrystalline silicon solar cell of PERC technology (laserfired contacts on the rear, 19 μm thick 1 cm p-type bulk, acidic texture, 9 /sqr P-doped emitter), the cell data are shown in Table I. The wafer used for the ELBA analysis was a parallel wafer, which was exposed to the same diffusion and annealing history as the cell. Then the emitter was etched away and the wafer was passivated by a stack of aluminum oxide and silicon nitride on both sides. The PL measurements used for the ELBA analysis were performed by using a nm short-pass filter. The used LIT system was an InfraTec PV-LIT system, which enables local IR emissivity correction. The EL measurements for measuring the local diode voltage in the dark were performed by using a nm long-pass filter ELBA Efficiency limiting bulk recombination analysis (ELBA) [1] allows for a prediction of solar cell efficiency limited by injection-dependent bulk recombination for a given cell concept. The local bulk recombination is assessed by injection-dependent lifetime imaging [2] of a surface-passivated lifetime sample that has been subjected to the same thermal process steps as the solar cell under investigation. This ensures that the bulk recombination in the lifetime sample corresponds to that in the finished solar cell. Photon blurring in the PL images was reduced here by the use of a -nm short-pass filter. We have checked that deconvolution of the photon scattering by a measurement of the point spread function after [4] does not lead to significant changes in the mean lifetime of the different regions anymore. In combination with local PC1D cell simulations accounting for the characteristics of the chosen cell concept, high-resolution images of V oc, J sc, FF, and the efficiency limited by bulk recombination are calculated. Global values for the whole wafer are calculated considering balancing mechanisms between the areas on the wafer in the model of independent diodes. The PC1D cell model was adjusted to a monocrystalline solar cell of the same cell batch. It has to be noted that the ELBA results do not rely in any way on measurements of the final multicrystalline cell, but only on measurements of the bulk lifetime on a wafer. Therefore, the results are real independent predictions of the bulk limited efficiency. Furthermore, the wafer based ELBA simulation allows a very detailed investigation of the material s efficiency limits, which would be not possible by a cell based method: An arbitrary cell concept can be applied, an arbitrary cell thickness can be investigated, even the impact of base doping can be estimated, and by a combination with measurements of defect concentrations, the efficiency limit due to specific defects in the wafer can be assessed Local I-V "Local I-V" is a dark lock-in thermography (DLIT) image evaluation method, which exploits the proportionality of the DLIT signal and the locally dissipated power density [3]. Four DLIT images are evaluated to fit each pixel to a 2-diode model, leading to images of J 1, J 2, n 2, and G p = 1/R p. The local series resistance R s is regarded by entering an electroluminescence- (EL)-based local diode voltage image taken at the highest forward bias (here 6 mv). The EL images, taken for wavelengths above nm here, were corrected for light scattering in the detector according to [4], and the local diode voltage was artificially blurred (thermal blurring) for having the same spatial resolution as the DLIT-based J images. Using the DLIT-based local J 1 data, for a given global value of J sc, local J sc expectation data are calculated according to [5]. Once this and the local 2-diode parameters are known, the method calculates efficiency-related images like the local V oc, the local FF, and the local efficiency [6], which may be compared with the corresponding ELBA data. All these images are calculated, like the ELBA images, assuming electrically isolated pixels, but considering lateral balancing currents in the model of independent diodes. Therefore these local efficiency data are expectation values of the corresponding efficiency parameters, which would hold for

3 12 Otwin Breitenstein et al. / Energy Procedia 92 ( 16 ) 15 a homogeneous solar cell having the properties of the pixel in position (x,y). The 'Local I-V 2' software performing these evaluations is available [7]. 3. Results Figure 1 shows lifetime images resulting from PL images of the investigated wafer for various illumination intensities and resulting ELBA-based results of the expected cell, all scaling ranges are indicated. We see significant and locally different changes in the lifetime over the whole range of illumination intensities from.4 to 1.54 suns. Fig. 2 (a) shows results of a PC1D analysis of this cell type, which is used to predict local V oc and J sc data from the lifetime images. Fig. 2 (b) shows some examples of carrier-dependent lifetimes from three typical positions marked by the corresponding symbols in Fig. 1 (c) and from the whole wafer. The predicted local cell data in Fig. 1 (g) and (h) were calculated by assuming a homogeneous series resistance of 1.5 cm 2. (a) (.4 suns) to 5 μs (b) (.1 suns) to 5 μs (c) (1 sun) to 25 μs (a) (1.54 suns) to 25 μs (e) ELBA V oc 56 to 66 mv (f) ELBA J sc (g) ELBA FF 32 to 38 ma/cm 2 6 to 83 % max (h) ELBA 13 to % min 2 cm Fig. 1. (a) - (d): Lifetime images of the wafer for various illumination intensities. The symbols in (c) mark the positions of the carrier-dependent lifetime curves in Fig. 2 (b). (e): ELBA-based V oc (expectation value image, see text), (f): J sc, (g): FF, (h): efficiency expectation image, the latter calculated under the assumption of a homogeneous R s of 1.5 cm 2. The color bar in (g) holds for all images. (a) (b) Fig. 2. (a) PC1D results for the investigated cell type, (b) carrier-dependent lifetime data from three positions marked by the corresponding symbols in Fig. 1 (c).

4 Otwin Breitenstein et al. / Energy Procedia 92 ( 16 ) Figure 3 shows the most relevant images coming from the DLIT- and EL-based Local I-V analysis. Images (e) to (h) may be directly compared with the ELBA-based images in Figure 1. Figure 4 shows local I-V characteristics of some positions in the cell obtained from evaluating the DLIT images, which have led to Figs. 3 (a) to (c). These characteristics are from the position of a J 1 -type shunt (a), from a 'good' region (b), from a J 2 -type shunt at the left edge (c) and from an ohmic shunt (d). The different current contributions (J 1, J 2, ohmic) are shown separately, the symbols mark the measured points. The positions of these local characteristics are indicated in Figs. 3 (a) to (c). (a) J 1 (b) J rec (6 mv) to 4 pa/cm 2 to 5 ma/cm 2 (c) J shunt (-1 V) (d) V d (6 mv) to ma/cm 2 58 to 6 mv ohmic shunt max J 1 shunt good region J 2 shunt 2 cm min (e) Local I-V V oc 56 to 66 mv (f) Local I-V J sc 32 to 38 ma/cm 2 (g) FF 6 to 83 % (h) Local I-V 13 to % Fig. 3. (a): DLIT-based dark saturation current density, (b): depletion region recombination current density at 6 mv, (c): ohmic shunt current density at -1 V (note the different scaling range), (d): Local I-V based local diode voltage at 6 mv, (e): V oc expectation image, (f): J sc image calculated after [4], (g): FF image (with R s based on the J dark image (a) and the V d image (d)), (h): efficiency expectation image. The color bar in (c) holds for all images. 4. Discussion The comparison between Figs. 1 and 3 shows good correlations but also distinct differences, which all may be well interpreted. The spatial resolution of the ELBA results is generally much better than that of the Local I-V results, since only the latter are thermally blurred. Note that these PL results are obtained on a wafer, hence they are not disturbed by horizontal balancing currents. Both evaluations clearly indicate the typical crystal defect regions in the material and regions of lower lifetimes at the upper and the left edge of the cell, which are due to the fact that the wafers stem from a corner brick. Hence the contamination with impurities (e.g. iron) of the material at these edges is stronger limiting the lifetime there. The Local I-V based V oc image predicts clearly lower values in the crystal defect regions than the ELBA-based one. A possible explanation is that the crystal defects not only influence the bulk lifetime but also the emitter and/or backside recombination. This would become visible in Local I-V, but not in ELBA. It was detected in [8] (a similar PERC cell process was investigated there) that the emitter recombination can be significantly higher in dislocated areas. Using this increased recombination ( cm/s for effective surface recombination of emitter and front surface) in the ELBA simulation leads to a 3mV lower V oc potential in a dislocated area. In [9] it was observed that, in recombination-active defect regions, also the front metallization contribution of J 1 increases. Due to its limited spatial resolution, DLIT cannot distinguish between regions below and between gridlines. Thus, these effects could explain the main differences between ELBA and Local I-V. The J sc images of both methods look quite similar, except that the influence of the top and left edge regions is stronger predicted in ELBA than in the J 1 -based analysis after [5]. This may be due to the fact that the parameters in [5] are fitted to regions of recombination-active grain boundaries, whereas in the edge regions of the cell presented here we have a homogeneously low lifetime. Hence, in this region other parameters may apply. The two FF images differ

5 14 Otwin Breitenstein et al. / Energy Procedia 92 ( 16 ) 15 most strongly. The ELBA analysis cannot take into account the J rec and J shunt images in Fig. 1, which both strongly influence the FF image of the cell. Moreover, since in the ELBA analysis a homogeneous R s was assumed, the position-dependence of R s is not considered. Thus, as expected, the ELBA-based efficiency expectation image correctly considers the influence of the bulk lifetime, but not that caused by locally inhomogeneous ohmic shunts, emitter, backside, and depletion region recombination, and R s effects. Table I shows the measured global cell parameters and that predicted by ELBA and Local I-V (a) J 1 shunt (c) J 2 shunt (b) good region (d) ohmic shunt Fig. 4. Local dark I-V characteristics: (a) J 1 shunt (position shown in Fig. 3 a), (b) good region (position shown in Fig. 3a), (c) J 2 shunt (position shown in Fig. 3 b), (d) ohmic shunt (position shown in Fig. 3 c). The symbols mark the measured points. Table I. Measured and simulated global cell parameters. efficiency parameter flasher cell data ELBA Local I-V V oc [mv] J sc [ma/cm 2 ] (fitted) FF [%] efficiency [%] Acknowledgements O. Breitenstein, F. Frühauf, and J. Bauer acknowledge the cooperation with InfraTec (Dresden) [] and the financial support by the German Federal Ministry for Commerce and Energy and by industry partners within the research cluster "SolarLIFE" (contract no D). The content is in the responsibility of the authors. References [1] B. Michl, M. Rüdiger, J.A. Giesecke, M. Hermle, W. Warta, and M.C. Schubert, "Efficiency limiting bulk recombination in multicrystalline silicon solar cells", Solar Energy Materials & Solar Cells 98, (12).

6 Otwin Breitenstein et al. / Energy Procedia 92 ( 16 ) [2] J.A. Giesecke, M.C. Schubert, B. Michl, F. Schindler, W. Warta, Minority carrier lifetime imaging of silicon wafers calibrated by quasisteady-state photoluminescence, Solar Energy Materials & Solar Cells 95, (11). [3] O. Breitenstein, "Nondestructive local analysis of current-voltage characteristics of solar cells by lock-in thermography", Solar Energy Materials & Solar Cells 95, (11). [4] D. Walter, A. Fell, E. Franklin, D. Macdonald, B. Mitchell, and T. Trupke, "The impact of silicon CCD photon spread on quantitative analyses of luminescence images", IEEE Journal of Photovoltaics 4 (1), (14). [5] O. Breitenstein, F. Fertig, and J. Bauer, "An empirical method for imaging the short circuit current density in silicon solar cells based on dark lock-in thermography", Solar Energy Materials & Solar Cells 143, 46-4 (15). [6] O. Breitenstein, "Local efficiency analysis of solar cells based on lock-in thermography", Solar Energy Materials & Solar Cells 7, (12). [7] (Febr. 16). [8] B. Michl, M. Padilla, I. Geisemeyer, S. T. Haag, F. Schindler, M. C. Schubert, and W. Warta, "Imaging techniques for quantitative silicon material and solar cell analysis", IEEE Journal of Photovoltaics 4 (6), (14). [9] S. Rißland, T.M. Pletzer, H. Windgassen, and O. Breitenstein, "Local thermographic efficiency analysis of multicrystalline and cast-mono silicon solar cells", IEEE Journal of Photovoltaics 3 (4), (13). [] (Febr. 16)