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J. P. Rakotoniaina, O. Breitenstein, M. Langenkamp, M. Werner, and G.Hahn*
Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany Phone +49-345-5582-760, Fax +49-345-5511-223, E-mail pati@mpi-halle.de
*Universität Konstanz, Fakultät für Physik, D-78457 Konstanz, Germany
$EVWUDFWRibbon growth on substrate (RGS) is a novel technology to produce silicon for solar cells directly in thin sheets.
This provides some advantages like the fast and low cost crystallization process without any losses due to the wafer cutting.
However, this fast crystallization process involves a rather high density of structural defects like dislocations and an increased concentration of oxygen and carbon (> 10 18 cm-3). Although the efficiency of solar cells made on RGS can reach 12%, there are unusual properties of the RGS solar cells. A high short circuit current Jsc in the range of 30mA/cm2 has been obtained, which is usually observed only for monocrystalline silicon solar cells. Furthermore, the fill factor and the open circuit voltage Voc are lower than expected. In addition, the dark I-V characteristic of some RGS solar cells shows ideality factors larger than two, which can not be explained with the classic theory of pn junction. In this paper we show that these unusual properties are caused by the existence of a 3dimensional network of inversion channels which are correlated to dislocations decorated with precipitates.
Keywords: Solar cells - 1, dislocations - 2, secco etching - 3, EBIC - 4 1. INTRODUCTION
In the silicon Photovoltaics (PV) industry wafer cost covers 60% of manufacturing cost [1]. In order to reduce the Wp cost, wafer cost must be lowered. One possibility to produce low cost wafers of bulk silicon is the fabrication of crystalline silicon ribbons.
Ribbon Growth on Substrate (RGS) [2] is one of the 4 most actual Ribbon technologies [3]. In the RGS technology liquid silicon is directly solidified into wafers. Furthermore the crystallization velocity and the casting velocity are decoupled so that a high throughput of silicon wafer is possible (one wafer per second). An efficiency of 11-12.5
% could be reached on lab scale [4, 5]. This efficiency is above the actual value of amorphous solar cells. The fast crystalization speed causes a high density of structural defects. This effect combined with a high oxygen and carbon concentration affects the electrical properties of the solar cells. A low minority carrier diffusion length LD of 20 µm is observed [5]. Despite this short diffusion length for the as-grown material a high short circuit current Jsc in the range of 30 mA/cm2 has been obtained, which is usually only observed in good monocrystalline silicon solar cells.
Furthermore, the fill factor and the open circuit voltage Voc
of RGS cells are often lower than expected. In addition, the dark I-V characteristic of RGS solar cells shows ideality factors larger than two, which cannot be explained within the classical theory of pn junctions. It was suggested that these unusual properties of RGS solar cells are due to a 3- dimensional network of inversion channels, possibly caused by a network of decorated dislocations [4]. In the present contribution we will show the correlation between the inversion channels and the decorated dislocations.
In chapter 2 the basics of the RGS process and solar cells based on RGS are shown. To prove the existence of the inversions channels, EBIC measurements using special sample geometries were performed (chapter 3). Thereafter Secco etching has been performed on the same sample and EBIC results are compared to the etching results. (chapter 4). In chapter 5, TEM investigations are shown to image the precipitate-covered dislocations. On the basis of the experimental results a physical model has been developed by Breitenstein et al. [11] to interprete the unusual behavior
of the RGS solar cells. The basics of this model and results of I-V characteristic simulations are presented in chapter 6 . 2. THE RGS PROCESS AND RGS SOLAR CELLS
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Figure 1: the Ribbon Growth on Substrate (RGS) process.
Liquid silicon inside a casting frame is solidified when wetting the surface of a high temperature substrate.
The scheme of the RGS solar cells is presented in Figure1.
The liquid silicon is in a casting frame. The crystallization occurs vertically so that it is decoupled from the horizontal pulling direction. This leads to a high production rate of 1 wafer per second. Moreover the substrate can be reused.
The fast crystallization process leads to a high dislocation density of 105-107 cm-2.There is also a high amount of impurities (e.g C and O in the range of 1018 cm-3).Further details on the RGS process are given elsewhere [3]. Prior to the solar cell process the wafers were mechanically planarized and V-textured using a dicing saw [3]. The formation of new thermal donors from the high oxygen content is avoided by a high temperature annealing step (>
1000° C). The solar cells were produced by a phosphorous diffusion followed by aluminum gettering, hydrogen passivation and contact formation [3]. The best solar cells produced by this technique have short circuit currents densities in the range of Jsc = 34 mA/cm² under standard illumination, normally only observed on good monocrystalline silicon solar cells with much higher minority carrier diffusion lengths. Spectrally resolved LBIC (Light Beam Induced Current) measurements show that the high short circuit current is mainly caused by the red part of the spectrum, thus resulting from carriers generated deep in the solar cell.
Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2-1tyc6q27l9tdd1
Erschienen in: Seventeenth European Photovoltaic Solar Energy Conference : proceedings of the international conference held in Munich, Germany, 22 - 26, October 2001 ; Vol. 2 / McNelis, Bernard et al. (Hrsg.). - München :
WIP-Renewable Energies, 2001. - S. 1444-1447. - ISBN 3-936338-07-8
1444
Contrary to the high ISC the open circuit voltage (VOC) and the fill factor are lower than expected. Both factors are related to the behavior of the diode under forward bias.
This is caused by macroscopically homogeneous defects in the material rather than by single localized shunts as can be shown using highly sensitive lock-in thermography under forward bias [7].
Further evidence for an unusual behavior of these samples is given by the measurement of the capacitance at different frequencies. A difference of the capacitance at 100 kHz compared to the capacitance at 100 Hz (C- dispersion) can be measured and correlated to the solar cell parameters [6]. A clear correlation was found between this capacitance difference, indicating a slow charge carrier exchange mechanism, and a high Isc and a low FF and Voc
of the cells.
3. EBIC INVESTIGATIONS
The model of the inversion channel is the following:
channels are formed along extended defects like grain boundaries and dislocations. The channels exhibit n-type conduction and thus form an extension of the n-emitter into the p-bulk.The inversion channel structure forms a three-dimensional extension of the n-doped solar cells emitter into the bulk. This enhances the collection probability of electrons (minority carriers in p type) and can explain the high short circuit current despite the short diffusion length.
Figure 2: EBIC setup of the bevelled sample geometry. The electron beam irradiates the bevelled surface. Each position on the surface corresponds to a certain distance between the generation volume and the pn-junction.
In order to investigate these 3d-inversion channels experimentally, Electron Beam Induced Current (EBIC) was chosen. Since we are interested in the charge collection properties in the bulk, the sample were bevelled under a small angle from the backside and EBIC investigations were performed on this surface (Figure 2). The choice of the bevelling from the backside is to observe any conducting path from the generation volume towards the pn junction. Moreover each position on the investigated surface represents a well-defined distance between the generation volume and the pn junction.
Fig 3a (left) shows one result of the EBIC investigation on the bevelled surface. Fig 3b is the secondary electron (SE) image of the same area. On the left hand side of Figure 3a the thin wedge of the sample is visible and on the right hand side we see the full cell thickness with the residual back contact. Owing to the mechanically grooved emitter structure the left wedge shows a jagged shape.
Figure 3: EBIC image of the bevelled surface at 5kV (Figure 3a. left) and the SE image (Figure 3b. right).
Bright EBIC contrasts indicating charge collection region are clearly visible across the whole investigated area. The collection of minority carriers is inhomogeneous with respect to the grain structure (reduced collection near the grain boundaries) and becomes increasingly inhomogeneous in the thicker region. However the charge collection efficiency at the individual bright spots is nearly independent from the actual distance to the pn-junction. It could be proven that this signal is definitively not caused by beam-induced surface inversion, as it is often observed in EBIC measurements on bare p-type silicon surfaces.
Figure 4: High magnification EBIC at 5 kV (detail of Figure 3). Resolved bright spot indicates charge collection regions. In this position the sample thickness is above 150 µm.
In Figure 4, a selected area of Fig 3 can be seen in higher magnification. The investigated position corresponds to a sample thickness of about 200 µm. We can see regions with a high collection probability, even at distances exceeding several times the minority carrier diffusion length LD. The fine structure of islands consists of single bright micron-sized spots indicating one-dimensional type of defects. Hence, the charge collection is most probably due to dislocations.
4. EBIC COMPARED TO DEFECT ETCHING
Are dislocations really responsible for the bright EBIC spots? In order to answer this question Secco etching [8]
was performed and the SE images of the etched sample are compared to the EBIC images. In this experiment another sample geometry was chosen. The samples were cuted and polished perpendicular to the pn-junction. EBIC measurements were performed at 5kV. One result is presented in Figure 5a. We can obvserve the V-texture; the pn-junction appears bright. There are also several bright spots (like in Figure 4) related to regions with high LD
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collection probability. Thereafter Secco etching has been performed for three minutes on the same sample.
Figure 5 (left Figure 5a) EBIC at 5 kV of a RGS cells cut perpendicular to the pn-junction. The regions of high collection probability are visible as bright spots. The pn junction appears as a bright line. Figure 5b (right) is the SE image of the etched sample. Dislocations are visible as etch pits. The position of bright spots in Figure 5a corresponds to etch pits in Figure 5b .
The SE image of the etched sample is presented in Figure 5b. Large and small etch pits can be seen. The funnel shape of the large etch pit indicates clearly the one- dimensional structure of the defects, being mostly probably dislocations. There is a good agreement between the position of the bright spots in Figure 5a and the etch pits in Figure 5b. A small fraction of etch pits are not seen in the EBIC measurement, they might be not electrically active or not electrically connected to the emitter.
5. TEM INVESTIGATION
Concerning the origin of the electrical activity of the dislocations we first assumed that the dislocations density would be the main factor. Thus, we compared the dislocation density of two solar cells, one with a high Jsc
(more bright spots in EBIC) and the other one with a moderate Jsc (without bright spots ). The obtained result is that the dislocation density in these two samples was in the same order (5*10-6 cm-2) so that the electrical activity does not depend on the dislocation density.
Figure 6: TEM picture of the precipitate-decorated dislocation in a region with high collection probability (plan view).
Another reason for the electrical activity of the dislocations might be the impurity decoration. Thus TEM investigations were performed in the region with bright
spots to check the existence of decorated dislocations. As mentioned at the beginning the RGS material is characterised by a high content of oxygen and carbon in the range of 1018 cm-3. Furthermore, Gottschalk has demonstrated by chemical etching experiments that there are densily packed precipitates around dislocations [9].One result of the TEM investigation is presented in Figure 6. In this dark field image we observe a decorated dislocation . The spiral structure is an indication of passive climbing, which is often combined with the growth of SiO2- precipitates. This demonstrates the decoration of the dislocation by Silicon oxide. Moreover SIMS measurements [10] indicated also that closely packed precipitates along dislocation lines are present in areas of high current collection probability.
6. THE PHYSICAL MODEL
On the basis of these experimental results Breitenstein et al. [11] developed a physical model to interprete this behavior. We will give only the basic of this model here, details can be found elsewhere [11].
Figure 7: Scheme of a precipitate-coated dislocation.
Formation of the inversion channel caused by the SiO2 charge (Qox) and the inteface charge (Di)
This model is based on the electrical activity of dislocation covered by precipitates of SiO2 and/or SiC. Figure7 shows a band diagram of the precipitate-coated dislocation.
The precipitate-coated dislocation is considered as a cylinder with a constant radius rox. The interface between the precipitate cylinder and the silicon is characterised by a fixed oxide charge Qox and a spatially homogeneous interface charge density Di. The barrier height of the depletion channel ΦB depends on Qox and Di. For a sufficiently high value of Qox inversion may occur. In this model it is not decisive whether the precipitates consist from SiO2 or from SiC.
In order to estimate the influence of the charged dislocations on the I-V characteristic of the cell, the model in Figure 7 is applied. The assumption is that in the dislocation position the local barrier height for electron injection from the n+ emitter into the p base is lowered by Φb. Thus, the current density within this circular inversion channel around the dislocation line jch is the usual diffusion current density, multiplied by a Boltzmann factor exp(Φb/kT):
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Figure 8: Measured and simulated I-V characteristic of a typical RGS solar cell. Parameters used for the simulation:
rox = 10 nm, A = 4 cm2, Na = 1016 cm-3, Qox = 8*1012 cm-2, Di = 1013 cm-2eV-1, Nd = 5*105 cm-2, Dn = 35 cm2/s, Ld = 10 µm, T = 300 K
In Figure 8, the measured IV characteristics of a typical RGS solar cells is compared with a simulation (dashed line) using the described model above. A good agreement between the experimental and the simulated curves can be seen. Hence, the large ideality factor of 3.1 of the measured curve can be fitted by this theory.
7. CONCLUSIONS
Using EBIC investigations of RGS solar cells with two special geometries we could show that the large short circuit current density Jsc in these solar cells is due to a 3- dimensional arrangement of inversion channels, which are electrically connected with the emitter. Correlation between dislocation-induced etch pits and single inversion channels has proven that these channels are in the positions of dislocations. TEM images have shown that these dislocations are coated with precipitates. Due to a dense coating with SiO2 and SiC precipitate particles, dislocations in RGS material may act as line-shaped inversion channels. A quantitative simulation of the diffusion current into these inversion channels has been performed, regarding the influence of charges trapped at the interface states. This charge trapping leads to a gradual decrease of the potential barrier height around the dislocations with increasing forward bias, finally leading to an exponential I-V characteristic with an ideality factor well above two, as it has been measured in RGS solar cells.
The additional current injection into the inversion channels around the precipitate-covered dislocations is the reason of the low open circuit voltage and of the low fill factor of RGS solar cells.
ACKNOWLEDGEMENTS
This work was partly supported by the BMWi “Kosi”
project (contract No. 0329858D).
REFERENCES
[1] T.M. Bruton et al., Multi-megawatt Upscalling of Silicon and Thin Film Solar Cell and Module Manufacturing, (Music FM), publishable final European Community project report.
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[3 G. Hahn, P. Geiger, A. Hauser, Silicon Ribbons- State of Art And Results from UKN Research, 11th Workshop on crystalline Silicon Solar Cell Materials and Processes, Colorado (2001),pp 85-92
[4] C. Häßler, H.-U. Höfs, S. Thurm, Proc. 2nd World Conference on Photovoltaic Solar Energy Conversion, Vienna (1998), pp. 1886-1889
[5] G. Hahn, C. Häßler, M. Langenkamp, Proc. 10th Workshop on crystalline silicon solar cells, Copper Mountain, USA (2000), pp. 208-211
[6] C. Häßler, H.-U. Höfs, S.Thurm, O. Breitenstein, M.
Langenkamp, 16th European Photovoltaic Solar Energy Conference, Glasgow 2000, Proc. in print [7] O. Breitenstein, M. Langenkamp, O. Lang, and A.
Schirrmacher, Solar Energy Materials and Solar Cells (2000), pp. 55-62
[8] F. Secco d´Aragona, J. Elec. Soc. (1972), p. 948 [9] H. Gottschalk, phys. stat. sol. (b) (2000), p.353 [10] G Hahn, D. Sontag, C. Haessler, Current Collecting
Channels in RGS Silicon Solar Cells – Are they Useful?, EMRS 2001 Spring Meeting Strasbourg, to be published in Sol. En. Mat. and Solar Cells.
[11] O. Breitenstein, M. Langenkamp, J.P. Rakotoniaina, Solid State Phenomena (2001) 29.
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forward bias [V]
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