presented at the 20th european photovoltaic solar … · presented at the 20th european...

Presented at the 20th European Photovoltaic Solar Energy Conference and Exhibition, 6.- 10. June 2005, Barcelona.

RANGE OF LOSS MECHANISMS ACCESSIBLE BY ILLUMINATED LOCK IN THERMOGRAPHY (ILIT)

J. Isenberg1*, A.S.H. van der Heide2,3 and W. Warta1

1Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstr. 2, 79110 Freiburg, Germany2 ECN Solar Energy, Westerduinweg 3, 1755 LE Petten, The Netherlands

3 now at Photovoltech, Grijpenlaan 18, 3300 Tienen, Belgium*ph. +49 (0)761 4588 5321, fax +49 (0)761 4588 9250 , e-mail: [email protected]

ABSTRACT: Illuminated Lock-In Thermography (ILIT) was introduced as a further development of Dark Lock-InThermography (DLIT) in 2004 and allows to measure under conditions considerably closer to real operation conditions ofsolar cells. Recently, a number of different modes were developed, that differ in the measurement conditions chosen.This contribution investigates favorable measurement conditions for the most important parameters of a solar cell. It will beshown that adapted modes of Lock-In Thermography are well suited for the investigation of linear and non-linear shunts,bulk material quality and losses due to series resistance as e.g. contact resistance and inappropriate metalization patterns.Bulk material quality is best investigated under open circuit conditions (VOC-ILIT), whereas series resistance problemsdominate the Thermography image under short circuit conditions (JSC-ILIT). Shunts on the other hand tend to dominated themeasurement at intermediate to high voltages, thus offering the possibility to distinguish between losses due to shunting anddue to series resistance. Finally, good agreement between contact resistance images obtained by JSC-ILIT and emitterpotential maps obtained by Corescan is demonstrated.Keywords: Shunts, Qualification and Testing, Manufacturing and Processing

1. INTRODUCTION

Illuminated Lock-In Thermography (ILIT) is a patentpending [1] further development of Dark Lock-InThermography (DLIT) [2] for the characterization ofsolar cells, that was recently introduced by some of theauthors [3],[4]. It was shown that ILIT is the firstmeasurement technique that gives a quantitative andspatially resolved measure of the power losses in solarcells under operation conditions [3]. A similar idearesulting in a comparable measurement setup wasindependently realized at University of Konstanz andpublished slightly later [5].DLIT and ILIT offer a wide range of possiblemeasurement modes. The most relevant experimentalparameters, that significantly influence the cellparameters investigated and thus the resultingthermography image, are the voltage applied to the solarcell and the change of illumination intensity and voltagewith time within the Lock-In period.Originally, ILIT was mainly used under open-circuitconditions (VOC-ILIT) to investigate material quality andto measure on unfinished solar cells during the cellprocess. Additionally, it was used at the maximum powerpoint (MPP) for investigating the influence of differentloss mechanisms on cell performance. Meanwhiledifferent measurement modes have evolved which areprimarily tailored to the investigation of losses in seriesresistance [6-9].In this paper we will demonstrate that - by choosingappropriate measurement conditions - ILIT not onlyallows to investigate the most relevant loss mechanismsin solar cells, notably series resistance, parallelresistance, recombination and dark saturation currents butalso to distinguish between them.The next section will give a brief overview of the mostrelevant parameters characterizing loss mechanisms in asolar cell and advantageous measurement conditions todetermine them. After that, each measurement mode willbe discussed in more detail.

2. LOSS MECHANISMS AND THERMOGRAPHYMODES – AN OVERVIEW

As was shown before [3],[4] the most relevantmeasurement mode for a realistic evaluation of theinfluence of different loss mechanisms on overall cellperformance under operation conditions is IlluminatedLock-In Thermography (ILIT) with a voltage of VMPP

applied to the cell during the illuminated half of theLock-In period and no voltage applied during the un-illuminated half of the Lock-In period (VMPP-ILIT). Theout-of-phase image of the temperature distribution (or –90°-Image) obtained in this mode is directly proportionalto the distribution of the overall power dissipation in thesolar cell under operation conditions. An illuminationsource, that has a small spectral mismatch with AM1.5Gis advantageous for this kind of measurement.On the other hand, different measurement conditions maybe favorable, if specific parameters like dark saturationcurrent or parallel resistance are to be investigated andthe overall influence on solar cell performance is not theprime interest of the measurement. Table 1 summarizesthe most important cell parameters and measurementmodes that are particularly suited for the investigation ofeach parameter. Additional information is given on howto distinguish features observed for a particular cellparameter from other loss mechanisms. The measurementmodes discussed may be extremely helpful forinvestigating particular problems in solar cell processing.However, it should be emphasized that the thermographyimages obtained from these modes do not display arealistic distribution of overall losses in the cell underoperation.

3. BULK MATERIAL QUALITY AND FIRST DIODE

Power losses due to first diode current are especiallyprominent at relatively high (around VOC) forwardvoltages for both ILIT and DLIT measurements. In ILITdirect recombination of carriers generated in the base is


Parameter Intention Method suggested Comments

J01 / τeff Detect ILIT @ VOC J01 is the direct measurand

DetectILIT or DLIT

@ ≈ VMPPLinear and non-linear shunts detected

RPDistinguish

from J0X

ILIT or DLIT@ ± ≈ VMPP

Only linear shunts will result in similar signal in forward andreverse bias

DetectILIT @ JSC or

RS-ILIT @ JSC / VMPP

Diode currents / bulk recombination effectively suppressed atlow voltages

RSDistinguishfrom J0X, RP

Compare ILIT @ JSC toILIT @ VOC / VMPP

Several methods of comparison proposed herein and byBreitenstein et al. (see e.g. [7])

DetectILIT or DLIT

@ ≈ VMPP

So called non-linear shunts, J01 may dominate at too highvoltages

J02Distinguishfrom RP, RS

See rows on RP and RS

Overall power losses ILIT @ VMPPDirectly proportional to spatial distribution of power losses;

irradiation with AM1.5G desirable

Table 1: Overview of different loss mechanisms and suggested measurement conditions. J01 and J02 are first and seconddiode currents, RP parallel resistance and RS series resistance.

Possible (direct meaning that the carriers do not cross thepn-junction). It has to be noted, that with a laterallyhomogeneous irradiation, as in our setup, this directrecombination is a “local” loss mechanism beingproportional to the irradiation intensity and thus almosthomogeneously distributed over the cell area. As definedin [3] “local” means laterally within one diffusion lengthfrom the place of electron-hole generation. Thus therecombination rate for this direct recombination isessentially constant over the cell area and cannot bedetected as inhomogeneity in ILIT. On the other hand afraction of the electron-hole pairs are separated by thepn-junction (this is the light generated current). Then, theelectrons are relatively free to move laterally in theemitter and/or front side metalization. Under VOC-conditions they may eventually be re-injected in the baseas dark saturation current J01. This is a non-localmechanism since gradients in the electrochemicalpotential in the emitter may cause electrons to travellaterally over distances considerably larger than theirdiffusion length in the base. As J01∝ Leff

-1∝τ eff-1/2 depends

strongly on material quality and minority carrier lifetime,this dark saturation current and the resulting powerdissipation in the cell is inhomogeneously distributed andmay be detected by ILIT.Additionally, it has to be considered, that - especiallyunder VOC-conditions - inhomogeneous bulk lifetimecauses the emitter potential to be different in areas ofhigh and low material quality. This results in differencesin the thermalization-losses as carriers cross the pn-junction in areas of different material quality. Forinvestigating J01 and thus bulk recombination ILITmeasurements are preferred over DLIT for two reasons:(i) the generation of electron-hole pairs is homogeneousover the area of the solar cell and (ii) measurementconditions are closer to operation conditions.An example of an ILIT image on a mc-cell under VOC

conditions is shown in figure 1. The image is almostcompletely dominated by J01, areas of high J01 and thus

low bulk lifetime appear bright, whereas low J01 and thushigh bulk lifetime results in low ILIT signal (darkcolors). More examples and detailed comparison with QEmeasurements may be found in [4].

0 20 40 60 80 1000

20

40

60

80

100ILIT[a.u.]

X [mm]

Y [m

m]

0.140.150.160.170.180.190.200.210.220.230.240.250.26

Fig.1 : ILIT -90°-Image of a multicrystalline cell, VOC-conditions. The measurement is dominated by J01.

4. LINEAR AND NON-LINEAR SHUNTS

So called “ohmic” or “linear” “Shunts” often occurunderneath the front side metalization or at the cell edges.They result in a low parallel resistance (RP) and are oftenpoint-like features. DLIT was originally developed forthe investigation of this kind of defects and is anexcellent method for detecting them. Performing ameasurement at a forward bias around VMPP, say 0.5V,and a second measurement under the same reverse biasallows to distinguish between linear shunts and other lossmechanisms, since only ohmic shunts result in a similarsignal under forward and reverse bias. The drawback isthat DLIT tends to heavily overestimate the impact ofshunts underneath the front side metalization (see e.g.[3]), which may be avoided by using ILIT instead. InILIT, typical voltages applied are again around VMPP. As


in DLIT a comparison to a measurement with the samevoltage applied in reverse direction is used to distinguishlinear shunts from e.g. dark saturation currents. Figure 2(left) shows an ILIT Image of Cz-cell 1 at 502 mV thatfeatures dark saturation currents J01, linear (ohmic) shuntsand shunts with non-linear characteristics. By subtractingtwo times the image taken at 500mV reverse voltage fromthis image a new image (figure 2 - right) is obtained.Ohmic shunts disappear and comparison with figure 2(left) allows to distinguish between ohmic and non-linearshunts. The main non-linear shunts were identified bythis analysis and are marked with arrows in the left imageof figure 2. The factor of two is most likely due to seriesresistance effects: In DLIT switching from +VMPP to -VMPP results in both the direction of the current andvoltage to be changed. In ILIT on the contrary thedirection of the current is the same at both voltages. Thusthe voltage difference between contacts and shuntposition causes the absolute value of the voltage at theshunt to be larger than VMPP in forward direction, butsmaller in reverse direction. For an ohmic shunt thisdifference in absolute voltage at the shunt position maywell explain the differences observed in thethermography images. Of course it can not be completelyexcluded that there is a slight diode like behavior of aparticular shunt besides its dominant ohmic nature.Finally, it should be noted that the difference image couldbe realized in one ILIT measurement if the reversevoltage is chosen slightly higher than the forwardvoltage.

Fig. 2: ILIT –90°-Image of a Cz solar cell at 502 mV(left). Same image but with two times the image taken at–500 mV subtracted (right).

5. SERIES RESISTANCE

Series resistance (RS) mainly arises from contactresistance, emitter sheet resistance and disruptions,constrictions, etc. of the grid. RS may be observed byILIT as the heating caused by current flowing through aresistor. Favorable measurement conditions are at orclose to short circuit (JSC-ILIT). Under these conditionsloss mechanisms like shunts or dark saturation currentsare effectively suppressed as the emitter potential is closeto 0. Thus most of the power is dissipated bythermalization of the minority carriers in the base andwhile crossing the pn-junction. These processes result ina quite high, homogeneous background signal.As the currents flowing at JSC are high, Joule heating inseries resistance of the metalization is rather prominentand often the most significant inhomogeneous features ofthe measurement. Examples may be found in [4].Locally increased contact resistance may lead to locallyincreased emitter potential and redistribution of thecurrent flows in the emitter and the front side

metalization. This may lead to two contrary effects in thethermography images for not too large areas of highcontact resistance: Firstly, the increased emitter potentialin the badly contacted area results in a decrease of thepotential difference over the pn-junction and thus indecreased thermalization heating and consequentlydecreased thermography signal in the poorly contactedarea. Secondly, the redistribution of current patterns maylead to currents flowing within the emitter towardsregions with lower contact resistance and only there intothe metalization. This causes locally increased jouleheating, in particular in a “corona like ring” around badlycontacted regions and at places where significant poweris dissipated at the semiconductor-metal interface due tothe redistribution of current patterns.

0 20 40 60 80 1000

20

40

60

80

100 ILIT -90°-Im[a.u.]

X [mm]

Y [m

m]

0.150.160.170.180.190.200.210.220.230.240.25

0 20 40 60 80 1000

20

40

60

80

100 Emitter Potential[mV]

X [mm]

Y [m

m]

06.0012.018.024.030.036.042.048.054.060.066.072.078.084.090.0

Fig. 3: JSC-ILIT -90°-Image of a multicrystalline solarcell with inhomogeneous contact resistance (upper).Emitter potential map of the same solar cell measured byCorescan (lower). From [8].

Figure 3 displays a measurement showing all of theseeffects: The upper image shows an ILIT image takenunder short circuit conditions on a multicrystalline solarcell with inhomogeneous contact resistance, the lowerimage displays the emitter potential map on this cell atshort circuit as obtained with Corescan [10]. Excellentcorrelation between the JSC-ILIT image and the Corescanmap is found. As predicted, the areas of high emitterpotential in Corescan result in a low thermography signalsurrounded by a corona of higher thermography signal.Investigating the 0°-Image (not shown) reveals that the“corona” is not a homogeneous ring of increased jouleheating, but that the highest power dissipation is found atthe intersections of the metalization with the edges of the


poorly contacted region. This is most likely due to jouleheating resulting from current crowding at thesemiconductor-metal interface at these places which iscaused by the extra current coming from the poorlycontacted emitter region and entering the grid here.A different method for the detection of series resistancelosses was proposed by Breitenstein et al. [6],[7].Constant illumination is used and the voltage ismodulated between 0V and around VMPP. A comparisonbetween the two methods can be found in [8].

6. SHUNTS AND SERIES RESISTANCE

Shunts, non-linear or linear, often occur underneath thefront-side metalization due to over-firing of (screen-printed) front-side contacts. From a technological pointof view it is therefore desirable to distinguish betweenshunts and losses in series resistance, notably in contactresistance. This is best achieved by comparing an ILIT-image taken under JSC-conditions to an ILIT-image takenat higher voltages, e.g. under VOC-conditions (VMPP oraround would also be acceptable). Series resistances arebest observed under JSC-conditions, whereas all diodetype losses tend to dominate the ILIT-image at highervoltages. Figure 4 shows an example, where thecomparison was performed by dividing the imageobtained under JSC-conditions through the image takenunder VOC-conditions. The cell investigated received adeliberately inhomogeneous firing of the contacts with atoo low firing temperature on the left hand side of thecell. Thus a region with interwoven areas of high and lowcontact resistance resulted on the left hand side of thecell, whereas good contact formation was made in the restof the cell.In the analysis presented in figure 4, bright spots indicaterelatively high local signal under short circuit conditionsand thus most likely series resistance problems, whereasdark spots indicate locally higher signal under open-circuit conditions, which is most likely due to shunting orlocally low bulk material quality. Thus the dark spots inthe central and right part of this cell indicate shuntingproblems, whereas the bright spots most likely displaylocal series resistance problems. The interwoven area ofhigh and low contact resistance results in a dotted imageas the JSC-ILIT signal is very inhomogeneous due to thestructure of this area.The analysis is unfortunately not as straight forward formulticrystalline solar cells, as their thermography signalunder open circuit conditions tends to have stronginhomogeneities due to the bulk material quality. Thusthe effect of low contact resistance in the JSC-ILIT imagemay easily be covered by the stronger lateral differencesin the VOC-ILIT image. It may be desirable to interpretthe individual JSC-ILIT image for contact resistance insuch cases and use images which are obtained similar toFigure 4 for the investigation of point like shunts only.Another alternative might be to resort to lower voltages,but even at VMPP bulk material quality may causesignificant inhomogeneities in the thermography image.If the difference between the two images is taken ascomparison, then images as in figure 4 may be obtainedin one measurement using a constant illumination sourceand applying a modulated voltage of VOC and 0V in thetwo halves of the Lock-In period respectively. Thistechnique is very close to the one proposed by Rappich et

al. [11] and was first suggested by Breitenstein et al. [6]for modulation between 0V and VMPP.

20 40 60 80 100 1200

20

40

60

80

100

120

X [mm]

Y [m

m]

Fig. 4: Cz cell with deliberately inhomogeneous contactresistance: JSC-ILIT image divided by VOC-ILIT Image.Dark spots indicate shunting, whereas bright spotsindicate for example series resistance problems.

7. CONCLUSION

Illuminated Lock-In Thermography (ILIT) and itsdifferent measurement modes were discussed. It wasshown that a wide variety of parameters and lossmechanisms in solar cells is accessible anddistinguishable by Lock-In Thermography if themeasurement conditions are chosen carefully.Appropriate experimental conditions for the detectionand identification of dark saturation currents, ohmicshunts and losses in series resistance were proposed.Additionally possibilities to distinguish these lossmechanisms from each other were demonstrated.

REFERENCES[1] J. Isenberg, S. Riepe, W. Warta, Deutsches Patentamt

2002, DE 102 40 060[2] O. Breitenstein, M. Langenkamp, Lock-in

Thermography - Basics and Use for FunctionalDiagnostics of Electronic Components", Springer2003, ISBN 3-540-43439-9

[3] J. Isenberg, W. Warta, J. Appl. Phys. 95(9), 2004 p.5200

[4] J. Isenberg, W. Warta, Prog. in Photovolt. 12(5),2004 p.339

[5] M. Kaes, S. Seren, T. Pernau, G. Hahn, Prog. inPhotovolt. 12(5), 2004 p.355

[6] O. Breitenstein, J.P. Rakotoniaina, J. Appl. Phys. 97,2005 p.074905

[7] O. Breitenstein, J.P. Rakotoniaina, A.S.H. van derHeide, J. Carstensen, accepted by Prog. in Photovolt.,2005

[8] J. Isenberg, A.S.H. van der Heide, W. Warta,accepted by Prog. In Photovolt. 13, 2005

[9] J. Isenberg, A.S.H. van der Heide, W. WartaProc. 31st IEEE-PVSC, Orlando (2005), in print

[10] A.S.H. van der Heide, A. Schönecker, J.H. Bultman,W.C. Sinke, Prog. in Photovolt. 13, 2005, p.3

[11] J. Rappich, M. Mueller, F. Schneider, H. Tributsch,Solar Eng. Mat. 53, 1998 p.205

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