IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 9, SEPTEMBER 2010 2227

Defect-Induced Breakdown in MulticrystallineSilicon 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 oflocal breakdown according to the temperature coefficient (TC)and slope of their characteristics and electroluminescence (EL)under a reverse bias. These are 1) early prebreakdown (negativeTC, 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 defect-induced breakdown (zero or weakly negative TC, moderate slope,no or weak defect luminescence), and 5) avalanche breakdown atdislocation-induced etch pits (negative TC, high slope). The lattermechanism usually dominates at a high reverse bias. The defectsleading to the etch pits are investigated in detail. In addition tothe local breakdown sites, there is evidence of an areal reversecurrent between the dominant breakdown sites showing a positiveTC. Defect-induced breakdown shows a zero or weakly negativeTC and also leads to weak avalanche multiplication. It has beenfound recently that it is caused by metal-containing precipitateslying in grain boundaries.

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

I. INTRODUCTION

LOCAL junction breakdown at low reverse biases hasbecome an important technological problem, particularlyin multicrystalline silicon solar cells. Theoretically, cells witha base doping concentration of 1 ∗ 1016 cm−3 should breakdown 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 maylead to permanent damage of modules under certain operatingconditions of these cells. If one cell of a string is shadowed,

Manuscript received January 7, 2010; revised May 26, 2010; acceptedJune 4, 2010. Date of current version August 20, 2010. This work wassupported 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 byEditor S. Ringel.

O. Breitenstein, N. Zakharov, and H. Blumtritt are with the Max PlanckInstitute of Microstructure Physics, 06120 Halle, Germany (e-mail: breiten@mpi-halle.mpg.de).

J. Bauer was with the Max Planck Institute of Microstructure Physics, 06120Halle, 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, ChristianAlbrechts University of Kiel, 24143 Kiel, Germany.

M. Kasemann, W. Kwapil, and W. Warta are with the Fraunhofer Institute ofSolar Energy Systems, 79110 Freiburg, Germany.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2010.2053866

the other cells may reverse bias this cell. The mechanismsbehind local junction breakdown, particularly their relation tomaterial defects, are not yet well understood. However, thisunderstanding becomes increasingly important since the trendin solar cell industry goes toward enabling the production ofsilicon solar cells from high-impurity [upgraded metallurgicalgrade (UMG)] feedstock. There are clear indications that highimpurity concentrations lead to breakdown at even lower volt-ages [1], [2].

This paper was extended to the contribution given at the34th IEEE Photovoltaic Specialists Conference [3] by addingnew high-resolution transmission electron microscopy (TEM)results investigating the physical nature of the line defects beingresponsible for the observed etch pits. One new author wasadded (N. Zakharov) who has contributed to these new results.

II. EXPERIMENTAL

The results presented in this paper are based on various phys-ical methods for investigating breakdown phenomena, most ofthem being imaging techniques. They were applied to a setof standard industrial acidic-etched multicrystalline cells madefrom closely neighbored wafers from the same brick, whichwere free of ohmic shunts. The material was cast by the verticalgradient freeze method from standard solar grade feedstock(not a UMG material). In addition to temperature-dependentcurrent–voltage I–V characteristic measurements, mostly darklock-in thermography (DLIT) under a reverse bias has beenused for localizing the breakdown sites. Since DLIT images canquantitatively be scaled in units of a local current density, theresults of DLIT images taken under different reverse biases andtemperatures allow the creation of images of the local temper-ature coefficient (TC) and the steepness (slope) of the reversecurrent (TC-DLIT and slope-DLIT [4]). In these techniques,the derivative of the local current density with respect to tem-perature or voltage is approximately obtained as a differencequotient, which is then normalized to the average local currentdensity. Thereby, the resulting TC or slope images show the val-ues of the relative TC or slope at the breakdown sites in units of“% current change per K” (or per V, respectively), independentof the absolute value of the breakdown current. Since for thisprocedure absolute values of reverse current and reverse voltageare used, a positive TC (slope) means an increase in absolutecurrent strength for increasing temperature (stronger reversebias). These magnitudes are appropriate to distinguish differenttypes of breakdown from each other. The local avalanchemultiplication 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 cellat two temperatures. (b) I–V characteristics of different separated regions(see text).

of illuminated lock-in thermography (MF-ILIT [4]), which isbased on imaging the local pulsed heat dissipation at a constantreverse bias for pulsed weak homogeneous illumination. At alow reverse bias, this image is essentially homogeneous anddisplays the laterally varying photocurrent. If the reverse biasis increased above the avalanche limit, in avalanche sites, thephotocurrent is locally increased. By relating the pulsed pho-tocurrent under the avalanche condition to that at a low reversebias, a quantitative image of the avalanche MF is obtained[4]. In another set of experiments, the local lifetime and thepresence of recombination-active defects in the base materialwas detected by forward-bias electroluminescence (EL) imag-ing at 1100 nm, and defect (dislocation)-induced luminescencewas imaged by forward-bias EL imaging at 1550 nm. High-resolution imaging of breakdown sites was performed by ELimaging under a reverse bias. The avalanche sites were alsomicroscopically imaged by reverse-bias lock-in electron beam-induced current in comparison with secondary electron imagingin the scanning electron microscope. By applying TEM at thedominant breakdown sites, the origin of avalanche breakdownhas been identified as avalanche breakdown caused by fieldenhancement at etch pits. The crystallographic defects leadingto the etch pits were investigated by high-resolution TEM usinga JEOL 4010 microscope.

III. RESULTS

A. I–V Characteristics

The general breakdown behavior of a solar cell is obtainedfrom its reverse-bias I–V characteristic, as shown in Fig. 1(a)for two different temperatures. For the cell type under investi-gation, typically, there is a crossover at about 13 V; at a lowerreverse bias, the current shows a positive TC, and at a higherbias, it shows a negative TC. The latter is typical for avalanchebreakdown, which, however, for a plane junction is expectedto appear only beyond −60 V. A positive TC is expected forinternal field emission breakdown, which for a monocrystallinematerial is expected only for doping concentrations of ordersof magnitude higher than 1016 cm−3. Since our cells are madefrom a multicrystalline material, it can be expected that thisearly breakdown is connected with crystal and surface defects.The transition from a positive to a negative TC points to thefact that different breakdown mechanisms exist in different

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

bias regions. Fig. 1(b) shows the room temperature I–V char-acteristics of different separated regions containing differentbreakdown sites (see the next section and Section III-F).

B. 1100-nm EL and Bias-Dependent DLIT

For distinguishing different breakdown mechanisms and re-lating them to grown-in crystal defects, bias-dependent DLITinvestigations have been made and compared with forward-biasEL images [5]. Since these EL images were obtained by usinga thermoelectrically cooled charge-coupled device camera, theydisplay the luminescence due to electron–hole recombination ata wavelength of about 1100 nm. This EL signal diminishes inthe presence of grown-in recombination-active crystal defectssuch as dislocations and grain boundaries [6]. Fig. 2 shows a setof 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 imagesare differently scaled. First weak breakdown sites appear at−5 V. They are mostly located at the edge and do not correlatewith grown-in recombination-active defects. Some of them arealso found in the cell area. In the following, we will callthese 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 breakdownsites increasingly correlate with the grown-in defects visible inthe EL image [Fig. 2(a)]. Thereby, defects with a darker ELcontrast seem to cause stronger breakdown sites. Such a site islabeled “II” in Fig. 2(c). At −15 V [Fig. 2(d)], new breakdownsites appear, showing a very steep current increase with voltage(hard breakdown) in positions where nearly no recombination-active defects are visible in EL (see, e.g., circle). Such a site islabeled “III” in [Fig. 2(d)].

One of the cells containing all these typical breakdown siteswas cut into small pieces so that each piece contained only

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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 thesepieces were measured with the results shown in Fig. 1(b).While the early prebreakdown site “I” shows an almost linearcharacteristic, that of the defect-induced breakdown “II” startsconducting at about −6 V and shows a moderate slope, whereasthe new breakdown site “III” starts conducting at −13 V andthen steeply increases its current. These findings have recentlybeen supported also by bias-dependent DLIT and EL investiga-tions [2], [7].

C. 1550-nm EL and TC-DLIT

In Fig. 2, it was shown that only some part of therecombination-active defects lead to stronger breakdown sites.To obtain further information about the underlying defects, thedefect-induced luminescence was investigated. Fig. 3(a) showsthe forward-bias EL image at 1550 nm taken by using anInGaAs camera with a corresponding interference filter. It doesnot image the defects causing nonradiative recombination, asthe 1100-nm image does, but rather luminescence from defect-induced bands in the band gap [6]. The most recombination-active regions [showing the strongest 1100-nm EL contrast andthe strongest DLIT signal at −12 V in Fig. 2(a) and (c)] arehighlighted in Fig. 3(a). The most intensive 1550-nm radiationis 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 willbe given in the discussion.

Since different breakdown mechanisms show characteristicTCs, the TC was imaged under different biases. The TC mea-sured at −13 V [Fig. 3(b)] shows that nearly all breakdown sites(particularly those highlighted in the 1550-nm EL image) havea negative or a close-to-zero TC at a temperature of around25 ◦C. The edge breakdown sites, on the other hand, show a

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

positive TC. Note that the normalized TC is imaged; hence, foran isolated breakdown site, the magnitude of the local currentdoes not influence the results. The local TC is determined bythe breakdown site that most strongly influences the temper-ature signal in this position. Thereby, the contribution of anadjacent weak breakdown site may be obscured. For effectivelycanceling the strong noise between the breakdown sites, whicharises due to the normalization to the average DLIT signal,all regions with an average DLIT signal below a certain valuehave been blanked to zero (red) in Fig. 3(b) and (c). ThisTC-DLIT image, which is measured at −14 V of the samecell, shows significantly more regions of a negative TC. Theymostly appear in regions where new breakdown sites appearat a higher reverse bias. In Fig. 3(b) and (c), many breakdownregions are surrounded by a bright edge. It turns out that thisis due to the influence of the region between the breakdownsites, which has been blanked out in Fig. 3(b) and (c). Indeed,in the TC-DLIT image [Fig. 3(d)], where these regions are notblanked 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-to-noise ratio also in these regions, 3 ∗ 3 pixel binning has beenapplied to the data used for Fig. 3(d). This result resolves thecontradiction between the positive TC of the overall reversecurrent [cf. Fig. 1(a)] and the negative TC of the localizedbreakdown sites [8].

D. Slope-DLIT

In addition to the TC, the slope of the local I–V character-istic can also be used to discern regions of different breakdowntypes. In Fig. 4, two slope-DLIT images obtained by using twodifferent pairs of images measured at different reverse biasesare compared. Similar to TC imaging, the magnitude of thecurrent does not influence the result of slope-DLIT for isolatedbreakdown sites. For adjacent breakdown sites, it images theslope of the site that influences the temperature signal in acertain position most strongly, and weaker breakdown sites maybe obscured. In Fig. 4(a), measurements taken at −11.5 and−13 V were used; hence, this slope refers to an average biasof −12.25 V, and for Fig. 4(a), measurements taken at −13and −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

2230 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 9, SEPTEMBER 2010

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 latticedefects according to Fig. 2(a) with a moderate slope of about70%/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 ahigh reverse bias.

E. Avalanche MF Imaging

An avalanche breakdown site can uniquely be identifiedby demonstrating multiplication of the photo-induced minoritycarrier flow [9]. By evaluating ILIT images measured at pulsedhomogeneous weak illumination under two different constantreverse biases, images of the local avalanche MF can be ob-tained [4]. Breakdown sites that are not influenced by the light-induced photocurrent are not displayed in this image. Onlyif the (essentially homogeneous) photocurrent is amplified incertain positions, these positions appear bright in the MF-ILITimage. As long as the higher reverse bias is below −13 V, theMF-ILIT image shows a nearly homogeneous value of 1 acrossthe whole image; hence, no measurable carrier multiplicationoccurs. Fig. 5(a) shows an MF-ILIT image of a cell used inthis 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 ofbreakdown sites newly appearing above −13 V. In Fig. 5(b), forwhich images taken at −13 and −15 V were used, displayingthe MF at −15 V, the local MF exceeds MF = 3 in some posi-tions. Here, a weak amount of carrier multiplication (M > 1)is visible in most regions, particularly in the regions containingrecombination-active defects.

F. Reverse-Bias EL Imaging

Most breakdown sites emit bright light when reverse bi-ased [6], [10]. Since reverse-bias EL imaging can detectbreakdown light with a very high spatial resolution [11], itis very useful for the exact determination of the locationof the breakdown sites. Investigations have shown that thespectrum of breakdown light strongly differs from forward-bias luminescence (radiative recombination). While forward-bias EL is spectrally centered around 1100 nm, reverse-biasEL shows a broad emission band extending into the visiblerange. Indeed, this reverse-bias luminescence can be seen in

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

the dark with the naked eye or in a light microscope. Theorigin of this luminescence is most probably bremsstrahlung;hence, it is generated by the acceleration of carriers in thebreakdown sites [12], [13]. Fig. 6(a) shows a reverse-bias ELimage 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 depen-dence of the luminescence intensity of these sites is shownin Fig. 6(b). It is visible that the early prebreakdown site “I”indeed starts to emit luminescence already at −5 V, the defect-induced breakdown “II” starts at about −11 V, and the steepbreakdown “III” starts at −13 V. We do not believe that thelight intensity is strictly proportional to the flowing current I,since it may nonlinearly depend on I and probably depends onmany other parameters, particularly on the local temperature.This inevitable temperature increase may be the reason why theluminescence 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 break-down mechanisms from each other by comparing the differentbias dependences of their EL signal. The result in Fig. 6(b)may be compared with directly measured I–V characteristicsshown in Fig. 1(b). In these directly measured characteristics,the early prebreakdown “I” shows a nearly linear characteristicinstead of the onset at −5 V, but the onsets of the defect-induced breakdown “II” and the steep breakdown “III” roughlycorrespond to those in Fig. 6(b). It cannot be excluded that thesmall pieces used for Fig. 1(b) still contain different breakdownsites, such as, e.g., edge breakdown due to the newly cutedges.

G. Electron Microscopy Results

For the investigation of the physical nature of the steepbreakdown appearing above −13 V, which leads to avalanchemultiplication, microscopic imaging of the avalanche sites isdesirable. Unfortunately, the spatial resolution of thermal imag-ing of the MF by MF-ILIT is limited by the inevitable lat-eral heat spreading in the silicon material. Therefore, localavalanche sites were imaged in the scanning electron micro-scope (SEM) in the electron-beam-induced current (EBIC)mode under a high reverse bias. Since under this condition, evenwithout any irradiation, the breakdown sites generate a strongcurrent noise signal and may overload the current amplifier,

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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 highermagnification [14].

beam pulsing at about 1 kHz as well as ac coupling to the cur-rent amplifier and lock-in processing of the EBIC signal havebeen used [14]. These investigations were made on the materialof the region framed in Fig. 5(b), which showed significantavalanche multiplication. Fig. 7 shows a lock-in EBIC imageat 0 V at low magnification [Fig. 7(a)] together with lock-inEBIC images taken in the region framed in Fig. 7(a) at −15 Vat different magnifications [Fig. 7(b) and (c)], in comparisonwith a secondary electron (SE) image [Fig. 7(d)] of the re-gion of Fig. 7(c). In the 0-V EBIC image [Fig. 7(a)], onlythe surface structure and a grain boundary are visible. Under−15 V [Fig. 7(b)], in one grain, bright lines appear, which inhigher magnification [Fig. 7(c)] can be resolved as rows of sin-gle bright spots. These spots represent sites of locally increasedEBIC. Traditionally, these sites are also called “microplasma”and are known to be due to avalanche multiplication of thebeam-induced current [15]. It will be shown below that this isindeed the case here. The SE image [Fig. 7(d)] shows that, inthe 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 resolutionSEM images in Fig. 8(a) and (c). In Fig. 8(c), the region framedin Fig. 8(a) is shown with the object inclined, so that the SEMis looking vertically into the etch pits. It is visible that thispair of pits is lying within a planar defect, which is probablya stacking fault or a twin lamella. Between the two arrows inFig. 8(c), a TEM cross-sectional specimen was cut out of thematerial by focused ion beam (FIB) preparation. A bright-fieldTEM image of this specimen is shown in Fig. 8(d). This imageshows a line defect, which proves that these are indeed etchpits. A higher magnification dark-field TEM image of anothertip is shown in Fig. 8(b). In addition, this image demonstratesthat these pits are really line-defect-induced etch pits. Here, alsoother dislocations can be seen, which did not lead to etch pits.Obviously, under the acidic etching conditions used for thesecells, only certain types of line defects are leading to etch pits.The high-magnification TEM image Fig. 8(b) shows that the tipradius is about 20 nm.

Fig. 8. (a) SEM image of a region containing etch pits. (b) High-magnificationdark-field TEM image of a similar etch pit [14]. (c) Enlarged view of the regionmarked 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 beingresponsible for the avalanche breakdown are aligned in rowsshowing a clear etch line, the line defects leading to the etchpits are obviously lying within a planar defect. This is alsovisible in Fig. 8(c). The acidic etching solution was selectednot to lead to etch pits at dislocations. Indeed, Fig. 8(b) showsthat usual dislocations (e.g., at the bottom left) are not leadingto these etch pits. Obviously, the line defects leading to the etchpits are no ordinary dislocations. In the following, the nature ofthese line defects and of the planar defect containing them areinvestigated in detail by high-resolution TEM.

A planar TEM specimen was prepared by FIB below a regionshowing a linear row of etch pits, similar to those shown inFig. 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 imageof 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, islying in the 〈111̄〉 plane and is weakly visible edge-on. Theline defects leading to the etch pits are labeled “D.” Theseline defects are running in the 〈11̄0〉 direction; hence, they areinclined to the image plane, leading to the observed “zigzag”contrast in Fig. 9(a). Fig. 9(b) shows the planar defect betweenthe line defects in high resolution with the specimen tilted intothe 〈101〉 direction. It clearly shows that the planar defect is a180◦ twin lamella being about 10 nm thick.

Fig. 9(c) shows a high-resolution image of a line defect lyingwithin this lamella. Here, the specimen orientation was again〈112〉, and the twin boundaries are not visible under this imag-ing condition. The line defect consists of a diagonally lyinglamella (between the arrows “S”), which probably representsa split dislocation with a stacking fault in between. In thestrain 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 planardefect. (a) Overview image shown in the 〈112〉 direction. (b) Planar defectshown in the 〈101〉 direction. (c) Line defect shown in the 〈112〉 direction.

In this region, the lattice parameter is reduced compared withthat of the matrix by 1.5%. We assume that this reduction iscaused by implementation of carbon impurities. According toVegard’s law, the carbon atom content of this region should beabout 2.5%, which would be far above the carbon solubility.Thus, according to these observations, the line defects leadingto the etch pits are dislocations in the [11̄0] direction split intotwo partials with a stacking fault in between, lying in a [111̄]-oriented twin lamella, which are probably heavily decoratedby carbon. The Burgers vector of the line defect could notyet be determined. The incorporation of metal in this defectseems to be less probable since metal precipitates should showanother atomic structure than the silicon matrix and a clearinterface to it.

IV. DISCUSSION

From all these investigations, the following physical pictureof breakdown in acidic-etched multicrystalline silicon solarcells emerges: At low voltages (here below −10 V), some verysoft prebreakdown sites appear, which are not correlated to anyrecombination-active crystal defects, since in neighboring cells,they appear in different positions. They show a negative TC anda low slope (not shown here). According to the curve of “I”shown in Fig. 1(b), their characteristic is essentially linear sothat they are not very dangerous. At biases beyond −14 V, theyare negligible. Their physical origin is not yet identified. At anintermediate reverse bias (here up to −13 V), also relativelysoft breakdown with moderate slope occurs mainly at sites ofrecombination-active crystal defects, which are most probablydislocations or grain boundaries. These sites are the same inneighboring cells; hence, they are due to grown-in crystal de-

fects. Two groups may be distinguished: Defects with a weaker1100-nm forward-bias EL contrast, which show a strong1550-nm defect-induced EL signal, seem to be only weaklycontaminated by impurities and also show weaker breakdownsites. More strongly contaminated defects show only weak1550-nm luminescence, most probably due to stronger non-radiative recombination in these sites, and lead to strongerbreakdown sites. It had been shown that these breakdown siteseven appear at a flat surface, but then at a somewhat higherreverse bias [11]. The latter is a general finding for alkaline-etched cells, which we attribute to the higher surface roughnessof acidic-etched surfaces. Originally, we proposed that thephysical mechanism being responsible for these defect-inducedbreakdown sites is a kind of breakdown implying defect statesin the gap. A well-known mechanism of this type is trap-assisted tunneling. However, since the gap energy reduces withincreasing temperature, trap-assisted tunneling should show apositive TC of the current, just as internal field emission (Zenerbreakdown) does. However, we have observed nearly zero oreven negative TCs in these sites. Moreover, at least at higherbiases, the regions of recombination-active crystal defects alsoshow weak avalanche multiplication. Therefore, it could alsobe possible that a kind of “trap-assisted avalanche” breakdownmechanism 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]. Theseprecipitates are preferentially forming in grain boundaries. Ithas to be concluded that metal precipitates are at least oneimportant reason for defect-induced breakdown.

At higher reverse biases, here above −13 V, very hard (steep)breakdown occurs in new sites outside of recombination-activedefects. Mostly in these regions and only in this bias range,strong avalanche multiplication occurs. Microscopic investiga-tions have proven that this hard breakdown is due to avalancheeffects at the tips of etch pits, many of them being arrangedin lines. At these tips, the p-n junction should be bowl shapedwith a radius of 300 nm, which according to the literature[17] quantitatively explains the reduction of the breakdownvoltage from −60 to −13 V. The acidic etchant industrially usedis selected not to produce etch pits at dislocations. However,obviously, the special type of decorated dislocations shown inFig. 9 leads to etch pits also for this etchant. It is understandablethat “classic” avalanche breakdown appears only in regionsshowing no significant EL contrast, since impurity scatteringwould prevent avalanche multiplication.

This paper can be concluded below.

1) All breakdown phenomena are field induced. Therefore,they become generally more striking for increasing netdoping concentration. Moreover, they are influenced bythe surface roughness, which locally increases the fieldstrength. This is the reason why they appear in acidic-etched cells at somewhat lower voltages than in alkaline-etched cells [11].

2) The edge region shows a weak leakage current with a lowslope and a positive TC. It can be expected that this edgecurrent under a reverse bias is due to hopping conduction,which, due to its characteristic temperature dependence,

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has also been proposed to be responsible for scratch-induced reverse-bias leakage [18].

3) At a low reverse bias, some early prebreakdown sites inthe area also appear that show a weak slope and a negativeTC. Their origin is not yet known, but they are obviouslynot very harmful.

4) The recombination-active defects visible in the 1100-nmforward-bias EL image are sources of relatively softbreakdown. This mechanism depends on the degree ofimpurity contamination [2]; hence, it should becomestronger if a UMG material with a high impurity con-centration is used. As any other breakdown effect, itbecomes also stronger with increasing doping concentra-tion, which is also usually higher in a UMG material. Itstill has to be investigated why this type of breakdownshows a negative or a close-to-zero TC. Recent investiga-tions have shown that this breakdown may be due to metalprecipitates [16]. This would explain its dependence onthe degree of contamination of the defects.

5) At a higher reverse bias, classic avalanche breakdownoccurs at the tips of certain etch pits, which appear dueto acidic texture etching at the sites of certain types ofline defects. The reduction of the avalanche breakdownvoltage from −60 V for a 1016 cm−3 material to −13 Vcan quantitatively be explained by the tip effect [17].

6) The line defects leading to the etch pits have been iden-tified as split dislocations in [11̄0] direction lying in a10-nm thin [111̄]-oriented twin lamella, which are proba-bly heavily decorated by carbon.

7) The positive TC of the reverse current at a low reversebias is not only due to the edge current but obviouslyalso due to a more or less homogeneous current flowingbetween the local breakdown sites. Since the breakdownsites occupy only a small fraction of the whole area, thehomogeneous current, although showing a very low cur-rent density, may be responsible for most of the prebreak-down current showing a positive TC. The nature of thiscurrent is not yet clear. It seems to be too large for beinga diffusion saturation current. If it were a recombinationsaturation current, it should quantitatively correlate withthe density of recombination-active defects, which hasnot yet been proven.

This investigation still leaves many open questions: What isthe exact mechanism of the defect-induced breakdown? Howcan the appearance of the specific etch pits leading to avalanchebreakdown be avoided? What is the nature of the early pre-breakdown and the weak homogeneous leakage current? Howdoes the breakdown behavior change if a UMG material isused? These and other open questions provide a wide field forfurther physical research on breakdown mechanisms.

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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 recombinationactive defects in block-cast multicrystalline silicon solar cells,” J. Appl.Phys., vol. 106, no. 6, pp. 063 530-1–063 530-7, Sep. 2009.

[3] O. Breitenstein, J. Bauer, J.-M. Wagner, A. Lotnyk, M. Kasemann,W. Kwapil, and W. Warta, “Physical mechanisms of breakdown in mul-ticrystalline 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 physicalparameters 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-breakdownmechanisms in multicrystalline silicon solar cells,” in Proc. 23rdEur. 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 charac-terization 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 multicrystal-line 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 pre-breakdown 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 breakdownin 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.

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[12] T. Figielski and A. Toruñ, “On the origin of light emitted from reverse-biased 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 breakdownradiation 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: Avalanchebreakdown 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 insilicon 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 ofmetal precipitates at prebreakdown sites in multicrystalline silicon so-lar cells,” Appl. Phys. Lett., vol. 95, no. 23, pp. 232 113-1–232 113-3,Dec. 2009.

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[18] O. Breitenstein, P. Altermatt, K. Ramspeck, and A. Schenk, “The originof ideality factors n > 2 of shunts and surfaces in the dark I– V curves ofSi 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 inphysics from the University of Leipzig, Leipzig,Germany, in 1980.

Since 1992, he has been with the Max PlanckInstitute of Microstructure Physics, Halle, Germany,where he investigates defects in semiconductors.Since 1999, he has been using lock-in thermographyfor detecting internal shunts in silicon solar cells. In2001, he introduced this technique on a microscopicscale for isolating faults in ICs. He has contributed tothe International Symposium for Testing and Failure

Analysis tutorials and is an author of a book on lock-in thermography. He haspublished more than 100 contributions about his research in scientific journalsand international conference proceedings.

2234 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 9, SEPTEMBER 2010

Jan Bauer received the Diploma in physics from theUniversity of Halle, Halle, Germany, in 2006, for aninvestigation on shunting precipitates in a Si solarcell material, and the Ph.D. degree in solar cell char-acterization, particularly under a reverse bias, fromthe Max Planck Institute of Microstructure Physics,Halle, in 2009, after investigating the electrical prop-erties of nanowires.

He is currently with CaliSolar Inc., Berlin,Germany.

Jan-Martin Wagner received the Ph.D. degreefrom the University of Jena, Jena, Germany, in2004, where he did first-principles calculations ofgroup-III nitrides.

As a Postdoctoral Fellow at the same Institute ofSolid-State Theory, he did calculations of ultrathinSi/SiO2 multiquantum-well structures as candidatesfor third-generation solar cells. In 2007, he joinedthe Max Planck Institute of Microstructure Physics,Halle, Germany, where he worked on the character-ization of solar cells. Since July 2010, he has been

with Christian Albrechts University of Kiel, Kiel, Germany.

Nikolai Zakharov received the Ph.D. degree inphysics from Moscow Physical Engineering Insti-tute, Moscow, Russia, in 1975. His Ph.D. work wasfocused on “formation of structural defects in semi-conductors during decomposition of solid solution.”

From 1969 to 1993, he was a Senior Researcherwith the Institute of Crystallography, Russian Acad-emy of Sciences, Moscow. From 1991 to 1993, hewas a Researcher with Lawrence Berkeley Labo-ratory, Berkeley, CA. Since 1993, he has been aResearcher with the Max Planck Institute of Mi-

crostructure Physics, Halle, Germany.

Horst Blumtritt received the Diploma in physicsfrom Martin-Luther-University, Halle, Germany,in 1974.

From 1974 to 1991, he was with the Institute ofSolid State Physics and Electron Microscopy, Halle,doing applied research on semiconductors and de-vices. In particular, he studied the defect activity byinvestigating EBIC contrast mechanisms. From 1992to 2006, he was a member of the Laboratory forElectron Microscopical Services for quality control,failure analysis, and R&D in micro- and optoelec-

tronics in Halle. Since 2007, he has been with the Max Planck Institute ofMicrostructure Physics, Halle, doing FIB applications.

Andriy Lotnyk received the Ph.D. degree from theUniversity of Halle, Halle, Germany, in 2007. Hedid his Ph.D. work at the Max Planck Institute ofMacrostructure Physics, Halle, from 2004 to 2007.His thesis work was focused on investigations ofsolid-state reactions in electroceramic systems.

Starting from the middle of 2007, he spent almost1.5 years as a Postdoctoral Fellow with the MaxPlanck Institute of Microstructure Physics in thegroup of Dr. Breitenstein, where he performed TEMinvestigations of defects in solar cells. Since 2009,

he has been a Permanent Staff Member of the research group “Synthesis andReal Structure of Solids” at the Faculty of Engineering, Christian AlbrechtsUniversity of Kiel, Kiel, Germany, and a Coordinator of the TEM center at theNanolab Kiel.

Dr. Lotnyk was the recipient of the Otto Hahn Medal from the Max PlanckSociety in 2008 for his work.

Martin Kasemann is currently working toward thePh.D. degree in the field of advanced characteriza-tion of solar cells with the Fraunhofer Institute ofSolar Energy Systems and the University of Freiburg,Freiburg, Germany.

Since his first publication in the late 2006, hehas authored or coauthored 35 journal and confer-ence papers and has given three invited and plenarytalks at international photovoltaic conferences. He iscurrently in charge of launching a distance-learningMaster of Science Program in Photovoltaics at the

University of Freiburg.

Wolfram Kwapil received the Diploma in physicsfrom the University of Karlsruhe, Karlsruhe,Germany, in 2006. He is currently working towardthe Ph.D. degree with the Fraunhofer Institute forSolar Energy Systems, Freiburg, Germany.

His research topics include the prebreakdown be-havior of multicrystalline silicon solar cells, the im-pact of metal precipitates present in the space chargeregion on solar cell parameters, and the precipitateevolution 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 En-ergy Systems (ISE), Freiburg, Germany, in 1985 andis currently the Head of the group Characteriza-tion and Simulation/CalLab and the Deputy Headof the department Silicon Solar Cells-Developmentand Characterization, Fraunhofer ISE. His researchinterests comprise development of characterizationtechniques and application for crystalline silicon ma-

terials and solar cells, silicon material properties and impact on solar cellperformance, simulation of solar cells and cell processing, as well as solar cellcalibration with the highest precision.

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