LBIC investigations of the lifetime degradation by extended defects in multicrystalline solar silicon Markus Rinio 1, Hans Joachim Möller 1 and Martina Werner 2, 1 Institute for Experimental Physics, TU Freiberg, Silbermannstr. 1, 09596 Freiberg, Germany, Tel. +49-3731-39-2896, email: rinio@physik.tu-freiberg.de, moeller@physik.tu-freiberg.de 2 Max Planck Institute for Microstructure Physics, Weinberg 2, 06120 Halle, Germany Keywords: LBIC, Reflection measurement, Solar cell, EFG, RGS, Recombination, Dislocation Abstract. A calibrated measurement of the short circuit current and the surface reflection coefficient can be directly converted into the internal quantum efficiency (IQE) of a solar cell. The IQE at a wavelength of 833 nm were measured on ingot, EFG and RGS silicon solar cells with a spatial resolution of 6 µm. Ingot solar cells were found to be predominantly influenced by a homogeneous distribution of recombination centers. However, if the dislocation densities exceeded a certain limit the IQE was reduced by recombination at dislocations. This limit varied in different parts of the wafer. EFG solar cells only showed a lifetime reduction by dislocations whereas the investigated solar cells made of RGS silicon were dominated by recombination at grain boundaries. The RGS silicon was further investigated by TEM- measurements, which showed that the extended defects were highly decorated with SiO 2 - and SiCprecipitates. 1 Introduction Topographical measurement techniques are suitable to detect areas of efficiency losses in multicrystalline solar cells. Combining different mapping methods it is possible to find correlations with material parameters which can lead to an understanding of the processes responsible for the reduction in efficiency. The efficiency of a multicrystalline solar cell is mainly limited by defects at which recombination of minority carriers takes place. The aim of the work presented here is to identify the regions within multicrystalline solar cells where the dominating recombination processes occur. Secondly we are interested in the quantitative determination of current losses due to different types of defects. Areas with low minority carrier lifetime are made visible by the light beam induced current (LBIC) mapping technique. Most of the published LBIC investigations are without calibration. In addition, they are not corrected for reflectivity losses. This is not sufficient for quantitative investigations. In this paper we present an advanced LBIC technique consisting of a calibrated measurement of current, light intensity and reflection coefficient. To correlate the LBIC maps with small defect structures such as dislocation clusters or grains with sizes of about 100 µm the spatial resolution of our LBIC system was improved up to 6 µm. Solar Cells made of ingot, EFG and RGS silicon were investigated. Ingot silicon is produced by directional solidification in a quartz crucible. The grains have sizes of a few millimeters. Oxygen concentrations between 4 10 17 and 10 10 17 were measured [1-6]. The edge-defined film-fed growth (EFG) method is based on pulling a thin ribbon out of the molt in vertical direction. EFG ribbons contain a high fraction of narrow twins, which extend across the whole wafer. Dislocation densities are in the range of 10 5-10 7 cm -2. As grown EFG wafers have oxygen concentrations of about 1 10 16 and contain 5 to above 10 10 17 carbon atoms/cm 3 [7,8].
2 Experimental procedure In our setup the sample is illuminated by a focussed diode laser beam with a minimum spot size of about 6 µm. The reflection coefficient of the sample is measured using the upper and lower reflection detectors, which consist of two calibrated solar cells. The incident light beam travels through a hole in the lower detector and then hits the sample. At the surface of the sample some light is reflected into side directions where it is absorbed by the lower detector. Another part of reflected light travels back through the hole. This light component is measured by the upper reflection detector. Both reflection detectors together are able to detect reflected light within the complete solid angle from 0 to about 80 against the vertical direction. A third solar cell is used to measure the power of the light illuminating the sample. light intensity detector solar cell sample objective lower reflection detektor laser 833 nm XYZ stage beam splitter In the RGS technique a thin film of liquid silicon is spread on a substrate using a shaping die. RGS films are produced much faster due to the fact that the directions of pulling and solidification are perpendicular to each other. Typical grain sizes are in the range of 200-500 µm. The dislocation densities are ranging from 10 5 to 10 7 cm -2. RGS silicon contains about 2 10 18 oxygen atoms per cm 3 [1-6]. lock-inamplifier 3 upper reflection detektor lock-inamplifier 2 lock-inamplifier 1 The spot size of the laser beam can be adjusted by measuring LBIC line scans over a sharp edge which partly covers the sample. The spot diameters are determined by fitting the theoretically calculated line scans to the measured ones. The measured short circuit current I SC, the power P L of the incident light and the reflection coefficient R are converted into the local internal quantum efficiency (IQE): 1 I SC / e IQE : = (1) 1 R P / L ( h c / λ) (e: elementary charge, h: Planck's constant, c: velocity of the light, λ: wavelength of the incident light) I SC strongly depends on the amount of absorbed light. This influence is corrected by the first factor in Eq. 1. The second factor is called external quantum efficiency (EQE). The system described here measures the IQE only at the wavelength of 833 nm, which corresponds to an absorption length of 15 µm in silicon. This means that mainly the bulk recombination is detected, but almost no effects due to the emitter layer. The measured IQE is also not influenced by the optical reflection coefficient of the back contact. With the calibrated LBIC measurement one can also identify uniformly distributed defects which are not individually resolved in the map, because the IQE would be below 100 % in this case. In addition it is possible to compare different types of solar cells quantitatively. After taking LBIC pictures of the solar cells, the samples were polished and etched for 4 minutes with Secco etch, a mixture of 9 g K 2 Cr 2 O 7, 200 ml H 2 O and 400 ml HF (40 %). Spatial resolved dislocation densities were measured using an image processing system, which is able to distinguish dislocation etch pits from other objects, such as grain boundaries. Fig. 1 Schematic drawing of the LBIC system
Fig. 2 External quantum efficiency map of a section of an ingot solar cell. The contact fingers are visible as horizontal lines. The large variations between different grains are mainly due to the different light absorption which can be seen in Figures 3 and 4. Fig. 3 Reflection coefficient map. The different reflectivity and absorption of the grains are due to their different texturization which depend on the surface orientation of the crystal. The light vertical strip on the right side is caused by the antireflection layer which changes in the vicinity of the contact bus. Fig. 4 Internal quantum efficiency map (scaled with the contrast of Fig. 2). The large variations vanished.
Fig. 5 LBIC map from a multicrystalline solar cell made from ingot silicon. The dark curved lines correspond to electrically active grain boundaries. The streaks are caused by dislocation clusters. 3 Experimental results and discussion 3.1 Reflection correction Figures 2, 3 and 4 show the results of an LBIC measurement of a multicrystalline solar cell made of ingot silicon. The strongly absorbing grain in the upper middle part produces higher currents than the neighboring grains. After correction by 1/(1-R) this difference vanishes. Since the reflection coefficient ranges from 3 % to 10 % it is clear that it has a strong influence on the short circuit current and cannot be neglected. 3.2 Ingot silicon Fig. 5 shows an LBIC map taken from a part of a multicrystalline solar cell. In the regions marked A and B spatial resolved dislocation densities were determined after polishing and etching of the cell. The results are presented in Fig. 6. Both regions show a correlation between the internal quantum efficiency and the dislocation density. In region B one can conclude that dislocations play the dominant role as long as their density exceeds 10 5 cm -2. The fact, that the internal quantum efficiency remains constant at a level of 88 % for dislocation densities below 7 10 4 cm -2 indicates that there exist other homogeneously distributed recombination centers within this cell. The dislocations in region A exhibit a significantly different behavior to those in part B. Dislocation densities up to 10 6 cm -2 seem to have no considerable effect on the minority carrier lifetime, which means that again homogeneously distributed recombination centers play the dominant role below a certain density limit of dislocations. internal quantum efficiency [ %] 92 88 84 80 76 72 region B 10 4 10 5 10 6 etch pit density [cm -2 ] region A Fig. 6 Internal quantum efficiency at λ=833nm versus dislocation etch pit density in two neighbored regions of the solar cell. Etch pit densities above 2 10 6 cm -2 are a little underestimated because overlapping etch pits could not be counted correctly in this case.
Fig. 7 Internal quantum efficiency of an EFG solar cell. The eight areas of reduced current under the contact busses result from the back contacts without back surface field. Fig. 8 Internal quantum efficiency measured with a spatial resolution of 12.5 µm in section 5 marked in the map shown in Fig. 7. The left part is replaced by the optical image of the polished and subsequently etched surface. In the left picture the light regions result from high etch pit densities. The dark line at the bottom of the IQE map is due to a random grain boundary.
Fig. 9 IQE map of a section of an RGS solar cell measured with a spatial resolution of 6 µm. The vertical line is due to a contact finger. 3.3 EFG silicon The results of LBIC measurements on a EFG solar cell are shown in Fig. 7. The solar cell contains grains with uniformly reduced quantum efficiency, i. e. in the dark stripe which crosses the marked sections 1 and 2. In the magnified image in Fig. 8 one can see that the regions with reduced IQE correlate with high densities of etch pits. The removal of a thin surface layer by polishing and subsequent etching showed that these etch pits appear at almost the same positions again. Therefore we conclude that they belong to dislocations. The dark line in the lower part of the IQE map was caused by a random grain boundary. The twin grain boundaries visible in Fig. 8 have no visible recombination activity. This IQE map also emphasizes that local internal quantum efficiencies of 97 % at 833nm wavelength are possible even in multicrystalline solar cells. 3.4 RGS silicon Large internal quantum efficiencies were also found locally on RGS solar cells. Between the dark lines in Fig. 9 where the recombination is high the IQE reached values of about 96 %. Investigations carried out on another sample indicated that the dark lines in the IQE map mainly correspond to grain boundaries.
Fig. 10 TEM image of a RGS sample which was not processed into a solar cell RGS samples were also examined by TEM. Spherical amorphous SiO 2 -precepitates with sizes up to 50 nm and densities between about 10 10 and 10 13 cm -3 and also SiC-precepitates were found in the solar cells. Fig. 10 shows that in RGS silicon dislocations and grain boundaries are highly decorated with precepitates. 3.5 Comparison of ingot, EFG and RGS solar cells Tab. 1 shows IQE-values that were obtained by averaging over different areas excluding the contact grid. These values indicate, that the short circuit current of the ingot solar cell is mainly reduced by homogeneously distributed recombination centers whereas the EFG and RGS solar cells are mainly influenced by local enrichments of extended defects. The recombination within the grains of the multicrystalline ingot cell may occur at finely distributed microdefects like oxygen precipitates together with metallic impurities. Also back surface recombination may contribute to the reduction of the IQE. In EFG silicon the localized defects were shown to be dislocations. For RGS silicon the results indicate that grain boundaries are more dominant than dislocations. Tab. 1 Comparison of internal quantum efficiencies measured on three different solar cells at a wavelength of 833 nm. ingot cell EFG cell RGS cell average IQE in the whole cell 89 % 93 % 85 % average IQE in good grains 92 % 98 % 96 % IQE loss from recombination at local enrichments of 3 % 5 % 11 % extended defects total IQE loss 11 % 7 % 15 % We have to emphasize that the internal quantum efficiencies measured at a wavelength of 833 nm are connected to recombination of minority carriers within the p-doped layer of the solar cell. There exist, however, other causes of a reduction of the efficiency. For instance shunting paths through the pn junction due to dislocations and grain boundaries [9-11] as well as recombination within the emitter or light absorption at the back contact are not detected by the LBIC system. On the other hand, it is known that a lower short circuit
current, reduced by recombination processes, also affects the open circuit voltage. Taking this into account one can expect that the efficiency loss due to recombination within the bulk is larger than the loss of IQE measured here. 4 Conclusions A calibrated LBIC technique was used to investigate regions of enhanced recombination in three different types of multicrystalline solar cells. The results show that the short circuit current of the ingot solar cells are mainly reduced by homogeneously distributed recombination centers that may consist of finely distributed precipitates in association with metallic impurities. The limit of dislocation densities, at which this recombination rate is exceeded by the dislocations, depends on their electrical activity. This density limit was found to be different in different regions of the cell. In EFG solar cells the short circuit current is mainly reduced by dislocations. LBIC measurements on RGS solar cells indicate that highly decorated grain boundaries play the dominant role in this samples. The TEM investigations show that this material is extremely enriched with SiO 2 -precepitates, which partly decorate extended defects and may act as recombination centers. SiO 2 -precepitates also act as gettering centers for metallic impurities. [3] H. J. Möller, Gettering and Defect Engineering in Semiconductor Technology, (eds. H. Richter, M. Kittler, C. Claeys, Transtech Publ., Zurich 1995), Solid State Phenomena 47-48, p.127 [4] H. J. Möller, D. Yang, S. Riedel, M. Werner, D. Wolf, The Role of Impurities and Defects in Silicon Device Processing, NREL Report SP-413-21550, USA, (1996) 34 [5] M. Werner, H. J. Möller, E. Wolf, MRS Symposia Proceedings (1997), in press [6] H. J. Möller, Proc. 7th Conf. on The role of Impurities and Defects in Silicon Device Processing, Vail, USA, (1997) p18 [7] B. Pivac, M. Aminotti, A. Borghesi, A. Sassella and J. P. Kalejs, J. Appl. Phys. 71 (8), (1992) 3785-3787 [8] J. P. Kalejs, Journal of Crystal Growth 128 (1993) 298-303, North-Holland [9] H. El Ghitani. and M. Pasquinelli, J. Phys. III France 3 (1993) 1931-1939. [10] H. El Ghitani, M. Pasquinelli, S. Martinuzzi, J. Phys. III France 3 (1993) 1941-1946. [11] J. J. Simon, E. Yakimov, M. Pasquinelli, J. Phys. III France 5 (1995) 1327-1336. 5 Acknowledgements This work was supported by the BMBF, Federal Republic of Germany, under Contact Numbers 0329743D7 and 0328661D. 6 References [1] J. Park, M. Döscher., H. J. Möller, Proc. 12th E.C. Photovoltaic Solar Energy Conference (H.S. Stephens & Associates, Bedford, UK, 1994) 996 [2] H. J. Möller, M. Ghosh, S. Riedel, M. Rinio, D. Yang, Proc. 13th E.C. Photovoltaic Solar Energy Conference (H.S. Stephens & Associates, Bedford, UK, 1995) 1390