Defect-Induced Breakdown in Multicrystalline Silicon Solar Cells

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 9, SEPTEMBER 2010

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Defect-Induced Breakdown in Multicrystalline Silicon Solar Cells Otwin Breitenstein, Jan Bauer, Jan-Martin Wagner, Nikolai Zakharov, Horst Blumtritt, Andriy Lotnyk, Martin Kasemann, Wolfram Kwapil, and Wilhelm Warta

Abstract—We have identified at least five different kinds of local breakdown according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under a reverse bias. These are 1) early prebreakdown (negative TC, low slope), 2) edge breakdown (positive TC, low slope, no EL), 3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550-nm defect luminescence), 4) strong defectinduced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and 5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at a high reverse bias. The defects leading to the etch pits are investigated in detail. In addition to the local breakdown sites, there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication. It has been found recently that it is caused by metal-containing precipitates lying in grain boundaries.

the other cells may reverse bias this cell. The mechanisms behind local junction breakdown, particularly their relation to material defects, are not yet well understood. However, this understanding becomes increasingly important since the trend in solar cell industry goes toward enabling the production of silicon solar cells from high-impurity [upgraded metallurgical grade (UMG)] feedstock. There are clear indications that high impurity concentrations lead to breakdown at even lower voltages [1], [2]. This paper was extended to the contribution given at the 34th IEEE Photovoltaic Specialists Conference [3] by adding new high-resolution transmission electron microscopy (TEM) results investigating the physical nature of the line defects being responsible for the observed etch pits. One new author was added (N. Zakharov) who has contributed to these new results.

Index Terms—Avalanche breakdown, electric breakdown, photovoltaic cells, semiconductor defects.

II. E XPERIMENTAL

I. I NTRODUCTION

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OCAL junction breakdown at low reverse biases has become an important technological problem, particularly in multicrystalline silicon solar cells. Theoretically, cells with a base doping concentration of 1 ∗ 1016 cm−3 should break down at about −50-V reverse bias by avalanche breakdown, but in reality, breakdown often begins already below −5 V. The appearance of local hot spots at breakdown sites may lead to permanent damage of modules under certain operating conditions of these cells. If one cell of a string is shadowed, Manuscript received January 7, 2010; revised May 26, 2010; accepted June 4, 2010. Date of current version August 20, 2010. This work was supported by the German cluster “SolarFocus” under Project BMU 327650. This paper was presented at the 34th IEEE Photovoltaic Specialists Conference, Philadelphia, PA, June 7–12, 2009. The review of this paper was arranged by Editor S. Ringel. O. Breitenstein, N. Zakharov, and H. Blumtritt are with the Max Planck Institute of Microstructure Physics, 06120 Halle, Germany (e-mail: breiten@ mpi-halle.mpg.de). J. Bauer was with the Max Planck Institute of Microstructure Physics, 06120 Halle, Germany. He is now with CaliSolar Inc., 12489 Berlin, Germany. J.-M. Wagner was with the Max Planck Institute of Microstructure Physics, 06120 Halle, Germany. He is now with Christian Albrechts University of Kiel, 24143 Kiel, Germany. A. Lotnyk was with the Max Planck Institute of Microstructure Physics, 06120 Halle, Germany. He is now with the Faculty of Engineering, Christian Albrechts University of Kiel, 24143 Kiel, Germany. M. Kasemann, W. Kwapil, and W. Warta are with the Fraunhofer Institute of Solar Energy Systems, 79110 Freiburg, Germany. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2010.2053866

The results presented in this paper are based on various physical methods for investigating breakdown phenomena, most of them being imaging techniques. They were applied to a set of standard industrial acidic-etched multicrystalline cells made from closely neighbored wafers from the same brick, which were free of ohmic shunts. The material was cast by the vertical gradient freeze method from standard solar grade feedstock (not a UMG material). In addition to temperature-dependent current–voltage I–V characteristic measurements, mostly dark lock-in thermography (DLIT) under a reverse bias has been used for localizing the breakdown sites. Since DLIT images can quantitatively be scaled in units of a local current density, the results of DLIT images taken under different reverse biases and temperatures allow the creation of images of the local temperature coefficient (TC) and the steepness (slope) of the reverse current (TC-DLIT and slope-DLIT [4]). In these techniques, the derivative of the local current density with respect to temperature or voltage is approximately obtained as a difference quotient, which is then normalized to the average local current density. Thereby, the resulting TC or slope images show the values of the relative TC or slope at the breakdown sites in units of “% current change per K” (or per V, respectively), independent of the absolute value of the breakdown current. Since for this procedure absolute values of reverse current and reverse voltage are used, a positive TC (slope) means an increase in absolute current strength for increasing temperature (stronger reverse bias). These magnitudes are appropriate to distinguish different types of breakdown from each other. The local avalanche multiplication factor (MF) was imaged by a special variant

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Fig. 1. (a) Reverse-bias current–voltage I–V characteristic of a typical cell at two temperatures. (b) I–V characteristics of different separated regions (see text).

of illuminated lock-in thermography (MF-ILIT [4]), which is based on imaging the local pulsed heat dissipation at a constant reverse bias for pulsed weak homogeneous illumination. At a low reverse bias, this image is essentially homogeneous and displays the laterally varying photocurrent. If the reverse bias is increased above the avalanche limit, in avalanche sites, the photocurrent is locally increased. By relating the pulsed photocurrent under the avalanche condition to that at a low reverse bias, a quantitative image of the avalanche MF is obtained [4]. In another set of experiments, the local lifetime and the presence of recombination-active defects in the base material was detected by forward-bias electroluminescence (EL) imaging at 1100 nm, and defect (dislocation)-induced luminescence was imaged by forward-bias EL imaging at 1550 nm. Highresolution imaging of breakdown sites was performed by EL imaging under a reverse bias. The avalanche sites were also microscopically imaged by reverse-bias lock-in electron beaminduced current in comparison with secondary electron imaging in the scanning electron microscope. By applying TEM at the dominant breakdown sites, the origin of avalanche breakdown has been identified as avalanche breakdown caused by field enhancement at etch pits. The crystallographic defects leading to the etch pits were investigated by high-resolution TEM using a JEOL 4010 microscope. III. R ESULTS A. I–V Characteristics The general breakdown behavior of a solar cell is obtained from its reverse-bias I–V characteristic, as shown in Fig. 1(a) for two different temperatures. For the cell type under investigation, typically, there is a crossover at about 13 V; at a lower reverse bias, the current shows a positive TC, and at a higher bias, it shows a negative TC. The latter is typical for avalanche breakdown, which, however, for a plane junction is expected to appear only beyond −60 V. A positive TC is expected for internal field emission breakdown, which for a monocrystalline material is expected only for doping concentrations of orders of magnitude higher than 1016 cm−3 . Since our cells are made from a multicrystalline material, it can be expected that this early breakdown is connected with crystal and surface defects. The transition from a positive to a negative TC points to the fact that different breakdown mechanisms exist in different

Fig. 2. (a) Forward-bias 1100-nm EL image. DLIT images of the leakage current measured at (b) −5 V, (c) −12 V, and (d) −15 V. The maximum scaling limits [corresponding to the scaling bar in (b)] are indicated.

bias regions. Fig. 1(b) shows the room temperature I–V characteristics of different separated regions containing different breakdown sites (see the next section and Section III-F). B. 1100-nm EL and Bias-Dependent DLIT For distinguishing different breakdown mechanisms and relating them to grown-in crystal defects, bias-dependent DLIT investigations have been made and compared with forward-bias EL images [5]. Since these EL images were obtained by using a thermoelectrically cooled charge-coupled device camera, they display the luminescence due to electron–hole recombination at a wavelength of about 1100 nm. This EL signal diminishes in the presence of grown-in recombination-active crystal defects such as dislocations and grain boundaries [6]. Fig. 2 shows a set of DLIT images made at room temperature at different biases, in comparison with a 1100-nm EL image of the same sample. According to their very different signal height, the DLIT images are differently scaled. First weak breakdown sites appear at −5 V. They are mostly located at the edge and do not correlate with grown-in recombination-active defects. Some of them are also found in the cell area. In the following, we will call these sites in the area “early prebreakdown” sites [labeled “I” in Fig. 2(b)] and the sites at the edges “edge breakdown” sites. At higher reverse biases [Fig. 2(c) and (d)], the breakdown sites increasingly correlate with the grown-in defects visible in the EL image [Fig. 2(a)]. Thereby, defects with a darker EL contrast seem to cause stronger breakdown sites. Such a site is labeled “II” in Fig. 2(c). At −15 V [Fig. 2(d)], new breakdown sites appear, showing a very steep current increase with voltage (hard breakdown) in positions where nearly no recombinationactive defects are visible in EL (see, e.g., circle). Such a site is labeled “III” in [Fig. 2(d)]. One of the cells containing all these typical breakdown sites was cut into small pieces so that each piece contained only

BREITENSTEIN et al.: DEFECT-INDUCED BREAKDOWN IN MULTICRYSTALLINE SILICON SOLAR CELLS

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Fig. 4. (a) Slope image at −12.25 V, scaled from 0 to 100%/V. (b) Slope image at −13.5 V, scaled from 0 to 200%/V.

Fig. 3. (a) Forward-bias EL image at 1550 nm. TC-DLIT images taken at (b) −13 V [blanked (see text), scaled from −3 to +3%/K], (c) −15 V (blanked, scaled from −3 to +3%/K), and (d) −10 V (unblanked, scaled from −10 to +10%/K). The color scale in (b) also holds for (c) and (d).

one type of breakdown site. The I–V characteristics of these pieces were measured with the results shown in Fig. 1(b). While the early prebreakdown site “I” shows an almost linear characteristic, that of the defect-induced breakdown “II” starts conducting at about −6 V and shows a moderate slope, whereas the new breakdown site “III” starts conducting at −13 V and then steeply increases its current. These findings have recently been supported also by bias-dependent DLIT and EL investigations [2], [7]. C. 1550-nm EL and TC-DLIT In Fig. 2, it was shown that only some part of the recombination-active defects lead to stronger breakdown sites. To obtain further information about the underlying defects, the defect-induced luminescence was investigated. Fig. 3(a) shows the forward-bias EL image at 1550 nm taken by using an InGaAs camera with a corresponding interference filter. It does not image the defects causing nonradiative recombination, as the 1100-nm image does, but rather luminescence from defectinduced bands in the band gap [6]. The most recombinationactive regions [showing the strongest 1100-nm EL contrast and the strongest DLIT signal at −12 V in Fig. 2(a) and (c)] are highlighted in Fig. 3(a). The most intensive 1550-nm radiation is generated not in these regions but in other defect regions, which show a weaker 1100-nm EL contrast and a weaker −12-V DLIT signal [6]. An interpretation of this behavior will be given in the discussion. Since different breakdown mechanisms show characteristic TCs, the TC was imaged under different biases. The TC measured at −13 V [Fig. 3(b)] shows that nearly all breakdown sites (particularly those highlighted in the 1550-nm EL image) have a negative or a close-to-zero TC at a temperature of around 25 ◦ C. The edge breakdown sites, on the other hand, show a

positive TC. Note that the normalized TC is imaged; hence, for an isolated breakdown site, the magnitude of the local current does not influence the results. The local TC is determined by the breakdown site that most strongly influences the temperature signal in this position. Thereby, the contribution of an adjacent weak breakdown site may be obscured. For effectively canceling the strong noise between the breakdown sites, which arises due to the normalization to the average DLIT signal, all regions with an average DLIT signal below a certain value have been blanked to zero (red) in Fig. 3(b) and (c). This TC-DLIT image, which is measured at −14 V of the same cell, shows significantly more regions of a negative TC. They mostly appear in regions where new breakdown sites appear at a higher reverse bias. In Fig. 3(b) and (c), many breakdown regions are surrounded by a bright edge. It turns out that this is due to the influence of the region between the breakdown sites, which has been blanked out in Fig. 3(b) and (c). Indeed, in the TC-DLIT image [Fig. 3(d)], where these regions are not blanked to zero, they show a strongly positive TC of up to +10%/K [note the different scaling ranges of Fig. 3(b) and (c) versus Fig. 3(d)]. For obtaining a sufficiently good signal-tonoise ratio also in these regions, 3 ∗ 3 pixel binning has been applied to the data used for Fig. 3(d). This result resolves the contradiction between the positive TC of the overall reverse current [cf. Fig. 1(a)] and the negative TC of the localized breakdown sites [8]. D. Slope-DLIT In addition to the TC, the slope of the local I–V characteristic can also be used to discern regions of different breakdown types. In Fig. 4, two slope-DLIT images obtained by using two different pairs of images measured at different reverse biases are compared. Similar to TC imaging, the magnitude of the current does not influence the result of slope-DLIT for isolated breakdown sites. For adjacent breakdown sites, it images the slope of the site that influences the temperature signal in a certain position most strongly, and weaker breakdown sites may be obscured. In Fig. 4(a), measurements taken at −11.5 and −13 V were used; hence, this slope refers to an average bias of −12.25 V, and for Fig. 4(a), measurements taken at −13 and −14 V were used, corresponding to an average bias of −13.5 V. Comparing Fig. 4(a) and (b) with Fig. 2(c) and (d), the new breakdown sites appear in both cases above −13 V. Note that Fig. 4(a) and (b) are differently scaled. While

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Fig. 5. MF-ILIT images scaled from MF = 1−3 showing the MF at (a) −14 V and (b) −15 V.

Fig. 4(a) displays mostly sites of recombination-active lattice defects according to Fig. 2(a) with a moderate slope of about 70%/V, in Fig. 4(b), the breakdown sites newly appearing above −13 V are visible with a very high slope above 200%/V. Therefore, these sites are the dominating breakdown sites at a high reverse bias. E. Avalanche MF Imaging An avalanche breakdown site can uniquely be identified by demonstrating multiplication of the photo-induced minority carrier flow [9]. By evaluating ILIT images measured at pulsed homogeneous weak illumination under two different constant reverse biases, images of the local avalanche MF can be obtained [4]. Breakdown sites that are not influenced by the lightinduced photocurrent are not displayed in this image. Only if the (essentially homogeneous) photocurrent is amplified in certain positions, these positions appear bright in the MF-ILIT image. As long as the higher reverse bias is below −13 V, the MF-ILIT image shows a nearly homogeneous value of 1 across the whole image; hence, no measurable carrier multiplication occurs. Fig. 5(a) shows an MF-ILIT image of a cell used in this paper, for which ILIT images at −13 V (below avalanche) and −14 V were taken. This image displays the MF at −14 V. While the signal in most of the area is unity, in some regions, weak multiplication is visible. These are mostly the positions of breakdown sites newly appearing above −13 V. In Fig. 5(b), for which images taken at −13 and −15 V were used, displaying the MF at −15 V, the local MF exceeds MF = 3 in some positions. Here, a weak amount of carrier multiplication (M > 1) is visible in most regions, particularly in the regions containing recombination-active defects.

Fig. 6. (a) Reverse-bias EL image at −14 V. (b) Bias-dependent EL intensities in different positions.

the dark with the naked eye or in a light microscope. The origin of this luminescence is most probably bremsstrahlung; hence, it is generated by the acceleration of carriers in the breakdown sites [12], [13]. Fig. 6(a) shows a reverse-bias EL image of an adjacent cell of that used for Fig. 2 taken at −14 V. Here, equivalent breakdown sites marked “I”, “II”, and “III,” as shown in Fig. 2, are clearly visible. The bias dependence of the luminescence intensity of these sites is shown in Fig. 6(b). It is visible that the early prebreakdown site “I” indeed starts to emit luminescence already at −5 V, the defectinduced breakdown “II” starts at about −11 V, and the steep breakdown “III” starts at −13 V. We do not believe that the light intensity is strictly proportional to the flowing current I, since it may nonlinearly depend on I and probably depends on many other parameters, particularly on the local temperature. This inevitable temperature increase may be the reason why the luminescence in position “I” starts only at −5 V, then saturates, and even reduces with increasing reverse bias. Nevertheless, this measurement may be used to distinguish different breakdown mechanisms from each other by comparing the different bias dependences of their EL signal. The result in Fig. 6(b) may be compared with directly measured I–V characteristics shown in Fig. 1(b). In these directly measured characteristics, the early prebreakdown “I” shows a nearly linear characteristic instead of the onset at −5 V, but the onsets of the defectinduced breakdown “II” and the steep breakdown “III” roughly correspond to those in Fig. 6(b). It cannot be excluded that the small pieces used for Fig. 1(b) still contain different breakdown sites, such as, e.g., edge breakdown due to the newly cut edges. G. Electron Microscopy Results

F. Reverse-Bias EL Imaging Most breakdown sites emit bright light when reverse biased [6], [10]. Since reverse-bias EL imaging can detect breakdown light with a very high spatial resolution [11], it is very useful for the exact determination of the location of the breakdown sites. Investigations have shown that the spectrum of breakdown light strongly differs from forwardbias luminescence (radiative recombination). While forwardbias EL is spectrally centered around 1100 nm, reverse-bias EL shows a broad emission band extending into the visible range. Indeed, this reverse-bias luminescence can be seen in

For the investigation of the physical nature of the steep breakdown appearing above −13 V, which leads to avalanche multiplication, microscopic imaging of the avalanche sites is desirable. Unfortunately, the spatial resolution of thermal imaging of the MF by MF-ILIT is limited by the inevitable lateral heat spreading in the silicon material. Therefore, local avalanche sites were imaged in the scanning electron microscope (SEM) in the electron-beam-induced current (EBIC) mode under a high reverse bias. Since under this condition, even without any irradiation, the breakdown sites generate a strong current noise signal and may overload the current amplifier,


Fig. 7. Lock-in EBIC image at (a) 0 V and (b) −15 V at low magnification. (c) Lock-in EBIC image at −15 V and (d) corresponding SE image at higher magnification [14].

beam pulsing at about 1 kHz as well as ac coupling to the current amplifier and lock-in processing of the EBIC signal have been used [14]. These investigations were made on the material of the region framed in Fig. 5(b), which showed significant avalanche multiplication. Fig. 7 shows a lock-in EBIC image at 0 V at low magnification [Fig. 7(a)] together with lock-in EBIC images taken in the region framed in Fig. 7(a) at −15 V at different magnifications [Fig. 7(b) and (c)], in comparison with a secondary electron (SE) image [Fig. 7(d)] of the region of Fig. 7(c). In the 0-V EBIC image [Fig. 7(a)], only the surface structure and a grain boundary are visible. Under −15 V [Fig. 7(b)], in one grain, bright lines appear, which in higher magnification [Fig. 7(c)] can be resolved as rows of single bright spots. These spots represent sites of locally increased EBIC. Traditionally, these sites are also called “microplasma” and are known to be due to avalanche multiplication of the beam-induced current [15]. It will be shown below that this is indeed the case here. The SE image [Fig. 7(d)] shows that, in the positions of these microplasma sites, dark spots are visible, which are proven below to be etch pits. The etch pits are even better visible in the higher resolution SEM images in Fig. 8(a) and (c). In Fig. 8(c), the region framed in Fig. 8(a) is shown with the object inclined, so that the SEM is looking vertically into the etch pits. It is visible that this pair of pits is lying within a planar defect, which is probably a stacking fault or a twin lamella. Between the two arrows in Fig. 8(c), a TEM cross-sectional specimen was cut out of the material by focused ion beam (FIB) preparation. A bright-field TEM image of this specimen is shown in Fig. 8(d). This image shows a line defect, which proves that these are indeed etch pits. A higher magnification dark-field TEM image of another tip is shown in Fig. 8(b). In addition, this image demonstrates that these pits are really line-defect-induced etch pits. Here, also other dislocations can be seen, which did not lead to etch pits. Obviously, under the acidic etching conditions used for these cells, only certain types of line defects are leading to etch pits. The high-magnification TEM image Fig. 8(b) shows that the tip radius is about 20 nm.

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Fig. 8. (a) SEM image of a region containing etch pits. (b) High-magnification dark-field TEM image of a similar etch pit [14]. (c) Enlarged view of the region marked in (a). (d) Cross-sectional bright-field TEM image of the same region.

H. Investigation of the Nature of the Line Defect As Fig. 7(c) and (d) shows that most of the etch pits being responsible for the avalanche breakdown are aligned in rows showing a clear etch line, the line defects leading to the etch pits are obviously lying within a planar defect. This is also visible in Fig. 8(c). The acidic etching solution was selected not to lead to etch pits at dislocations. Indeed, Fig. 8(b) shows that usual dislocations (e.g., at the bottom left) are not leading to these etch pits. Obviously, the line defects leading to the etch pits are no ordinary dislocations. In the following, the nature of these line defects and of the planar defect containing them are investigated in detail by high-resolution TEM. A planar TEM specimen was prepared by FIB below a region showing a linear row of etch pits, similar to those shown in Fig. 7(c) and (d). The etch pits were inclined to the surface, indicating that the line defects have an inclination of about 20◦ to the surface. Fig. 9(a) shows a lower magnification image of the specimen. The specimen orientation is close to 101, for imaging the specimen was tilted into the 112 direction. The planar defect, which was perpendicular to the surface, is lying in the 11¯1 plane and is weakly visible edge-on. The line defects leading to the etch pits are labeled “D.” These line defects are running in the 1¯10 direction; hence, they are inclined to the image plane, leading to the observed “zigzag” contrast in Fig. 9(a). Fig. 9(b) shows the planar defect between the line defects in high resolution with the specimen tilted into the 101 direction. It clearly shows that the planar defect is a 180◦ twin lamella being about 10 nm thick. Fig. 9(c) shows a high-resolution image of a line defect lying within this lamella. Here, the specimen orientation was again 112, and the twin boundaries are not visible under this imaging condition. The line defect consists of a diagonally lying lamella (between the arrows “S”), which probably represents a split dislocation with a stacking fault in between. In the strain field of this dislocation (at the upper right of the fault), there is a circular region of a modified material marked “D.”

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Fig. 9. High-resolution TEM results of the line defect lying within a planar defect. (a) Overview image shown in the 112 direction. (b) Planar defect shown in the 101 direction. (c) Line defect shown in the 112 direction.

In this region, the lattice parameter is reduced compared with that of the matrix by 1.5%. We assume that this reduction is caused by implementation of carbon impurities. According to Vegard’s law, the carbon atom content of this region should be about 2.5%, which would be far above the carbon solubility. Thus, according to these observations, the line defects leading to the etch pits are dislocations in the [1¯ 10] direction split into two partials with a stacking fault in between, lying in a [11¯1]oriented twin lamella, which are probably heavily decorated by carbon. The Burgers vector of the line defect could not yet be determined. The incorporation of metal in this defect seems to be less probable since metal precipitates should show another atomic structure than the silicon matrix and a clear interface to it. IV. D ISCUSSION From all these investigations, the following physical picture of breakdown in acidic-etched multicrystalline silicon solar cells emerges: At low voltages (here below −10 V), some very soft prebreakdown sites appear, which are not correlated to any recombination-active crystal defects, since in neighboring cells, they appear in different positions. They show a negative TC and a low slope (not shown here). According to the curve of “I” shown in Fig. 1(b), their characteristic is essentially linear so that they are not very dangerous. At biases beyond −14 V, they are negligible. Their physical origin is not yet identified. At an intermediate reverse bias (here up to −13 V), also relatively soft breakdown with moderate slope occurs mainly at sites of recombination-active crystal defects, which are most probably dislocations or grain boundaries. These sites are the same in neighboring cells; hence, they are due to grown-in crystal de-

fects. Two groups may be distinguished: Defects with a weaker 1100-nm forward-bias EL contrast, which show a strong 1550-nm defect-induced EL signal, seem to be only weakly contaminated by impurities and also show weaker breakdown sites. More strongly contaminated defects show only weak 1550-nm luminescence, most probably due to stronger nonradiative recombination in these sites, and lead to stronger breakdown sites. It had been shown that these breakdown sites even appear at a flat surface, but then at a somewhat higher reverse bias [11]. The latter is a general finding for alkalineetched cells, which we attribute to the higher surface roughness of acidic-etched surfaces. Originally, we proposed that the physical mechanism being responsible for these defect-induced breakdown sites is a kind of breakdown implying defect states in the gap. A well-known mechanism of this type is trapassisted tunneling. However, since the gap energy reduces with increasing temperature, trap-assisted tunneling should show a positive TC of the current, just as internal field emission (Zener breakdown) does. However, we have observed nearly zero or even negative TCs in these sites. Moreover, at least at higher biases, the regions of recombination-active crystal defects also show weak avalanche multiplication. Therefore, it could also be possible that a kind of “trap-assisted avalanche” breakdown mechanism is responsible for this defect-induced breakdown. Meanwhile, it has been shown that metal precipitates (e.g., iron) may exist in defect-induced breakdown sites [16]. These precipitates are preferentially forming in grain boundaries. It has to be concluded that metal precipitates are at least one important reason for defect-induced breakdown. At higher reverse biases, here above −13 V, very hard (steep) breakdown occurs in new sites outside of recombination-active defects. Mostly in these regions and only in this bias range, strong avalanche multiplication occurs. Microscopic investigations have proven that this hard breakdown is due to avalanche effects at the tips of etch pits, many of them being arranged in lines. At these tips, the p-n junction should be bowl shaped with a radius of 300 nm, which according to the literature [17] quantitatively explains the reduction of the breakdown voltage from −60 to −13 V. The acidic etchant industrially used is selected not to produce etch pits at dislocations. However, obviously, the special type of decorated dislocations shown in Fig. 9 leads to etch pits also for this etchant. It is understandable that “classic” avalanche breakdown appears only in regions showing no significant EL contrast, since impurity scattering would prevent avalanche multiplication. This paper can be concluded below. 1) All breakdown phenomena are field induced. Therefore, they become generally more striking for increasing net doping concentration. Moreover, they are influenced by the surface roughness, which locally increases the field strength. This is the reason why they appear in acidicetched cells at somewhat lower voltages than in alkalineetched cells [11]. 2) The edge region shows a weak leakage current with a low slope and a positive TC. It can be expected that this edge current under a reverse bias is due to hopping conduction, which, due to its characteristic temperature dependence,


3)

4)

5)

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has also been proposed to be responsible for scratchinduced reverse-bias leakage [18]. At a low reverse bias, some early prebreakdown sites in the area also appear that show a weak slope and a negative TC. Their origin is not yet known, but they are obviously not very harmful. The recombination-active defects visible in the 1100-nm forward-bias EL image are sources of relatively soft breakdown. This mechanism depends on the degree of impurity contamination [2]; hence, it should become stronger if a UMG material with a high impurity concentration is used. As any other breakdown effect, it becomes also stronger with increasing doping concentration, which is also usually higher in a UMG material. It still has to be investigated why this type of breakdown shows a negative or a close-to-zero TC. Recent investigations have shown that this breakdown may be due to metal precipitates [16]. This would explain its dependence on the degree of contamination of the defects. At a higher reverse bias, classic avalanche breakdown occurs at the tips of certain etch pits, which appear due to acidic texture etching at the sites of certain types of line defects. The reduction of the avalanche breakdown voltage from −60 V for a 1016 cm−3 material to −13 V can quantitatively be explained by the tip effect [17]. The line defects leading to the etch pits have been identified as split dislocations in [1¯ 10] direction lying in a 10-nm thin [11¯ 1]-oriented twin lamella, which are probably heavily decorated by carbon. The positive TC of the reverse current at a low reverse bias is not only due to the edge current but obviously also due to a more or less homogeneous current flowing between the local breakdown sites. Since the breakdown sites occupy only a small fraction of the whole area, the homogeneous current, although showing a very low current density, may be responsible for most of the prebreakdown current showing a positive TC. The nature of this current is not yet clear. It seems to be too large for being a diffusion saturation current. If it were a recombination saturation current, it should quantitatively correlate with the density of recombination-active defects, which has not yet been proven.

This investigation still leaves many open questions: What is the exact mechanism of the defect-induced breakdown? How can the appearance of the specific etch pits leading to avalanche breakdown be avoided? What is the nature of the early prebreakdown and the weak homogeneous leakage current? How does the breakdown behavior change if a UMG material is used? These and other open questions provide a wide field for further physical research on breakdown mechanisms. R EFERENCES [1] A. Goetzberger and W. Shockley, “Metal precipitates in silicon p-n junctions,” J. Appl. Phys., vol. 31, no. 10, pp. 1821–1824, Oct. 1960. [2] W. Kwapil, M. Kasemann, P. Gundel, M. C. Schubert, W. Warta, P. Bronsveld, and G. Coletti, “Diode breakdown related to recombination active defects in block-cast multicrystalline silicon solar cells,” J. Appl. Phys., vol. 106, no. 6, pp. 063 530-1–063 530-7, Sep. 2009.

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[3] O. Breitenstein, J. Bauer, J.-M. Wagner, A. Lotnyk, M. Kasemann, W. Kwapil, and W. Warta, “Physical mechanisms of breakdown in multicrystalline silicon solar cells,” in Proc. 34th IEEE Photovolt. Spec. Conf., Philadelphia, PA, Jun. 7–12, 2009, pp. 181–186. [4] O. Breitenstein, J. Bauer, J.-M. Wagner, and A. Lotnyk, “Imaging physical parameters of pre-breakdown sites by lock-in thermography techniques,” Prog. Photovolt.: Res. Appl., vol. 16, no. 8, pp. 679–685, Dec. 2008. [5] J.-M. Wagner, J. Bauer, A. Lotnyk, and O. Breitenstein, “Pre-breakdown mechanisms in multicrystalline silicon solar cells,” in Proc. 23rd Eur. Photovolt. Sol. Energy Conf., Valencia, Spain, Sep. 1–5, 2008, pp. 1164–1168. [6] M. Kasemann, W. Kwapil, M. C. Schubert, H. Habenicht, B. Walter, M. The, S. Kontermann, S. Rein, O. Breitenstein, J. Bauer, A. Lotnyk, B. Michl, H. Nagel, A. Schütt, J. Carstensen, H. Föll, T. Trupke, Y. Augarten, H. Kampwerth, R. A. Bardos, S. Pingel, J. Berghold, W. Warta, and S. W. Glunz, “Spatially resolved silicon solar cell characterization using infrared imaging methods,” in Proc. 33rd IEEE Photovolt. Spec. Conf., San Diego, CA, May 11–16, 2008, pp. 1–7. [7] K. Bothe, K. Ramspeck, D. Hinken, C. Schinke, J. Schmidt, S. Herlufsen, R. Brendel, J. Bauer, J.-M. Wagner, N. Zakharov, and O. Breitenstein, “Luminescence emission from forward- and reverse-biased multicrystalline silicon solar cells,” J. Appl. Phys., vol. 106, no. 10, pp. 104 510-1– 104 510-8, Nov. 2009. [8] J.-M. Wagner, J. Bauer, and O. Breitenstein, “Classification of prebreakdown phenomena in multicrystalline silicon solar cells,” in Proc. 24th Eur. Photovolt. Sol. Energy Conf., Hamburg, Germany, Sep. 21–25, 2009, pp. 925–929. [9] S. Mahadevan, S. M. Hardas, and G. Suryan, “Electrical breakdown in semiconductors,” Phys. Stat. Sol. (A), vol. 8, no. 2, pp. 335–374, Dec. 1971. [10] R. Newman, “Visible light from a silicon p-n junction,” Phys. Rev., vol. 100, no. 2, pp. 700–703, Oct. 1955. [11] D. Lausch, K. Petter, H. von Wenckstern, and M. Grundmann, “Correlation of pre-breakdown sites and bulk defects in multicrystalline solar cells,” Phys. Stat. Sol. RRL, vol. 3, no. 2/3, pp. 70–72, Mar. 2009, DOI 10.1001/pssr.200802264. [12] T. Figielski and A. Toruñ, “On the origin of light emitted from reversebiased p-n junctions,” in Proc. 6th Int. Conf. Phys. Semicond., Exeter, U.K., Jul. 16–20, 1962, pp. 863–868. [13] J. Shewchun and L. Y. Wei, “Mechanism for reverse-biased breakdown radiation in p-n junctions,” Solid State Electron., vol. 8, no. 5, pp. 485– 493, May 1965. [14] J. Bauer, J.-M. Wagner, A. Lotnyk, H. Blumtritt, B. Lim, J. Schmidt, and O. Breitenstein, “Hot spots in multicrystalline solar cells: Avalanche breakdown due to etch pits,” Phys. Stat. Sol. RRL, vol. 3, no. 2/3, pp. 40– 42, Mar. 2009, DOI 10.1002/pssr.200802250. [15] M. Lesniak and D. B. Holt, “Defect microstructure and microplasmas in silicon avalanche photodiodes,” J. Mater. Sci., vol. 22, no. 10, pp. 3547– 3555, Oct. 1987. [16] W. Kwapil, P. Gundel, M. C. Schubert, F. D. Heinz, W. Warta, E. R. Weber, A. Goetzberger, and G. Martinez-Criado, “Observation of metal precipitates at prebreakdown sites in multicrystalline silicon solar cells,” Appl. Phys. Lett., vol. 95, no. 23, pp. 232 113-1–232 113-3, Dec. 2009. [17] S. M. Sze and G. Gibbons, “Effect of junction curvature on breakdown voltage in semiconductors,” Solid State Electron., vol. 9, no. 9, pp. 831– 845, Sep. 1966. [18] O. Breitenstein, P. Altermatt, K. Ramspeck, and A. Schenk, “The origin of ideality factors n > 2 of shunts and surfaces in the dark I– V curves of Si solar cells,” in Proc. 21st Eur. Photovolt. Sol. Energy Conf., Dresden, Germany, Sep. 4–8, 2006, pp. 625–628.

Otwin Breitenstein received the Ph.D. degree in physics from the University of Leipzig, Leipzig, Germany, in 1980. Since 1992, he has been with the Max Planck Institute of Microstructure Physics, Halle, Germany, where he investigates defects in semiconductors. Since 1999, he has been using lock-in thermography for detecting internal shunts in silicon solar cells. In 2001, he introduced this technique on a microscopic scale for isolating faults in ICs. He has contributed to the International Symposium for Testing and Failure Analysis tutorials and is an author of a book on lock-in thermography. He has published more than 100 contributions about his research in scientific journals and international conference proceedings.

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Jan Bauer received the Diploma in physics from the University of Halle, Halle, Germany, in 2006, for an investigation on shunting precipitates in a Si solar cell material, and the Ph.D. degree in solar cell characterization, particularly under a reverse bias, from the Max Planck Institute of Microstructure Physics, Halle, in 2009, after investigating the electrical properties of nanowires. He is currently with CaliSolar Inc., Berlin, Germany.

Jan-Martin Wagner received the Ph.D. degree from the University of Jena, Jena, Germany, in 2004, where he did first-principles calculations of group-III nitrides. As a Postdoctoral Fellow at the same Institute of Solid-State Theory, he did calculations of ultrathin Si/SiO2 multiquantum-well structures as candidates for third-generation solar cells. In 2007, he joined the Max Planck Institute of Microstructure Physics, Halle, Germany, where he worked on the characterization of solar cells. Since July 2010, he has been with Christian Albrechts University of Kiel, Kiel, Germany.

Andriy Lotnyk received the Ph.D. degree from the University of Halle, Halle, Germany, in 2007. He did his Ph.D. work at the Max Planck Institute of Macrostructure Physics, Halle, from 2004 to 2007. His thesis work was focused on investigations of solid-state reactions in electroceramic systems. Starting from the middle of 2007, he spent almost 1.5 years as a Postdoctoral Fellow with the Max Planck Institute of Microstructure Physics in the group of Dr. Breitenstein, where he performed TEM investigations of defects in solar cells. Since 2009, he has been a Permanent Staff Member of the research group “Synthesis and Real Structure of Solids” at the Faculty of Engineering, Christian Albrechts University of Kiel, Kiel, Germany, and a Coordinator of the TEM center at the Nanolab Kiel. Dr. Lotnyk was the recipient of the Otto Hahn Medal from the Max Planck Society in 2008 for his work.

Martin Kasemann is currently working toward the Ph.D. degree in the field of advanced characterization of solar cells with the Fraunhofer Institute of Solar Energy Systems and the University of Freiburg, Freiburg, Germany. Since his first publication in the late 2006, he has authored or coauthored 35 journal and conference papers and has given three invited and plenary talks at international photovoltaic conferences. He is currently in charge of launching a distance-learning Master of Science Program in Photovoltaics at the University of Freiburg.

Nikolai Zakharov received the Ph.D. degree in physics from Moscow Physical Engineering Institute, Moscow, Russia, in 1975. His Ph.D. work was focused on “formation of structural defects in semiconductors during decomposition of solid solution.” From 1969 to 1993, he was a Senior Researcher with the Institute of Crystallography, Russian Academy of Sciences, Moscow. From 1991 to 1993, he was a Researcher with Lawrence Berkeley Laboratory, Berkeley, CA. Since 1993, he has been a Researcher with the Max Planck Institute of Microstructure Physics, Halle, Germany.

Horst Blumtritt received the Diploma in physics from Martin-Luther-University, Halle, Germany, in 1974. From 1974 to 1991, he was with the Institute of Solid State Physics and Electron Microscopy, Halle, doing applied research on semiconductors and devices. In particular, he studied the defect activity by investigating EBIC contrast mechanisms. From 1992 to 2006, he was a member of the Laboratory for Electron Microscopical Services for quality control, failure analysis, and R&D in micro- and optoelectronics in Halle. Since 2007, he has been with the Max Planck Institute of Microstructure Physics, Halle, doing FIB applications.

Wolfram Kwapil received the Diploma in physics from the University of Karlsruhe, Karlsruhe, Germany, in 2006. He is currently working toward the Ph.D. degree with the Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany. His research topics include the prebreakdown behavior of multicrystalline silicon solar cells, the impact of metal precipitates present in the space charge region on solar cell parameters, and the precipitate evolution during high-temperature steps.

Wilhelm Warta received the Diploma and Ph.D. degrees in physics from the University of Stuttgart, Stuttgart, Germany, in 1978 and 1985, respectively. He joined the Fraunhofer Institute for Solar Energy Systems (ISE), Freiburg, Germany, in 1985 and is currently the Head of the group Characterization and Simulation/CalLab and the Deputy Head of the department Silicon Solar Cells-Development and Characterization, Fraunhofer ISE. His research interests comprise development of characterization techniques and application for crystalline silicon materials and solar cells, silicon material properties and impact on solar cell performance, simulation of solar cells and cell processing, as well as solar cell calibration with the highest precision.