Lock-in Thermography

Use of Solar Photovoltaic (SPV) for Street Lighting Systems (SLS) is an ideal application of SPV for illumination of streets and crossroads located in areas that are not connected to the power grid. This paper presents the performance and degradation analysis of two pairs of the Champion PV modules which were used for SLS installed at Solar Energy Centre, India during the year 1990. These PV modules have experienced minimal degradation in their power output even after operating for 21 years. The power loss observed in all modules is less than 10%, which is less than the specified level in IEC 61215 standard. These modules showed unique behavior and the most surprising aspect is that only one of the modules in each pair is affected by browning inspite of installed at same location and under similar environmental conditions. A detailed investigation has been performed by using the I-V measurement and characterization techniques like Lock-in thermography and Electroluminescence imaging.

The attention of the present study was focused on the aid provided by lock-in thermography for non-destructive evaluation of aerospace materials and structures. The experimental analysis was performed by testing several specimens, which were made of different materials employed in the fabrication of aircraft (composites, hybrid composites, sandwiches, metals) and which included the most commonly encountered kinds of damage (delamination, impact damage, fatigue failure).

In most industrial type solar cell processes the edge isolation is an important step. After most emitter diffusion techniques, especially the POCl 3 diffusion, the front contact is connected with the back contact through the emitter around the edge of the solar cell. As a common technique in industry this shunt is removed by plasma etching of the wafer stack. Other techniques which were investigated are: the grinding of the wafer edge with sandpaper, the cutting of isolation trenches with a wafer dicing saw and the laser separation by inserting trenches, the latter two techniques applied on both sides of the wafer. All these technologies are compared concerning their impact on the solar cell performance of industrial type screen printed solar cells. Using IV measurement (illuminated and dark), laser beam induce current (LBIC) and lock-in thermography it is shown that isolation by laser still suffers from too low fill factors and sawing, grinding, or plasma etching are preferable. A good correlation between decreasing shunt resistance and increasing amount of detected shunts using in-lock thermography or LBIC is found.

This paper treats an experimental study that focusses on optical infrared thermography for non-destructive testing of composites through lock-in and flash excitation. Different fiber reinforced plastics with various artificial defects have been investigated. Three different postprocessing techniques are applied, namely fast Fourier transform (FFT), principal component analysis (PCA) and thermographic signal reconstruction (TSR). A comparison between the different excitation and post processing methods is performed, and their strengths and weaknesses in detecting artificial defects in composites are evaluated and discussed.

Imaging techniques are applied to multi-crystalline silicon bricks, wafers at various process steps, and finished solar cells. Photoluminescence (PL) imaging is used to characterize defects and material quality on bricks and wafers. Defect regions within the wafers are influenced by brick position within an ingot and height within the brick. The defect areas in as-cut wafers are compared to imaging results from reverse-bias electroluminescence and dark lock-in thermography and cell parameters of near-neighbor finished cells. Defect areas are also characterized by defect band emissions. The defect areas measured by these techniques on as-cut wafers are shown to correlate to finished cell performance.

Attention of this paper is devoted to testing and evaluation of carbon-fibre-reinforced polymer specimens with lock-in thermography. Several specimens were prepared by a vacuum infusion technique using a low-viscosity epoxy system; delaminated areas were simulated by the insertion of thin kapton diskettes at a given depth through the thickness. Artificial defects had different shapes (circular or elliptical) and dimensions. Inserts were located at different depths and, in some cases, by overlapping more than one kapton-shaped layer; i.e. two or more concentric discs, each of different diameter. A preliminary test campaign was performed by differential scanning calorimetry and rheometry to find optimal processing parameters to achieve fully cured state of matrix and to avoid the formation of defects in accordance with aerospace standards for composites manufacturing. Specimens were non-destructively inspected with optical lock-in thermography; results were presented in terms of phase images. All inserts were discovered and also well outlined under difficult conditions such as in the case of thin overlapped foils.

The correlation between the spatially resolved carrier lifetime of multicrystalline silicon and the spatially resolved monochromatic solar cell efficiency is investigated by means of microwave-detected photoconductance decay (MW-PCD) measurements and illuminated lock-in thermography (ILIT). Local monochromatic solar cell efficiencies are determined from ILIT measurements under short-circuit conditions and at the maximum power point of the cell. The resulting efficiency images are compared with efficiency images obtained from MW-PCD lifetime images of unprocessed neighbouring wafers using PC1D simulations. We observe a qualitative correlation between the measured and the simulated efficiency images. Areas with reduced efficiency are found in the same locations using both methods. However, the dynamic range in the monochromatic efficiency is larger for the images obtained from ILIT measurements. Possible explanations for this difference are a change in carrier lifetime during cell processing and varying lifetimes on microscopic scales, leading to averaging faults in the lifetime images.

In lock-in ͑modulated͒ thermography the lateral thermal diffusivity can be obtained from the slope of the linear relation between the phase of the surface temperature and the distance to the heating spot. However, this slope is greatly affected by heat losses, leading to an overestimation of the thermal diffusivity, especially for thin samples of poor thermal conducting materials. In this paper, we present a complete theoretical model to calculate the surface temperature of filaments heated by a focused and modulated laser beam. All heat losses have been included: conduction to the gas, convection, and radiation. Monofilaments and coated wires have been studied. Conduction to the gas has been identified as the most disturbing effect preventing from the direct use of the slope method to measure the thermal diffusivity. As a result, by keeping the sample in vacuum a slope method combining amplitude and phase can be used to obtain the accurate diffusivity value. Measurements performed in a wide variety of filaments confirm the validity of the conclusion. On the other hand, in the case of coated wires, the slope method gives an effective thermal diffusivity, which verifies the in-parallel thermal resistor model. As an application, the slope method has been used to retrieve the thermal conductivity of thin tubes by filling them with a liquid of known thermal properties.

Recent progress in the metallisation of poly-silicon thin-film solar cells on glass, created by solid phase crystallisation (SPC) of evaporated amorphous silicon (EVA), revealed that shunting through sub-micron holes (density 100-200 mm S2 ) in the films causes severe shunting problems when the air-side metal contact is deposited onto these diodes, by creating effective shunting paths between the two highly doped layers of EVA cells. We present evidence of these pinholes by optical transmission and focussed ion beam (FIB) microscopic images and confirm the point-like pinhole shunts using lock-in thermographic images. The latter revealed that the Al rear electrode induces strong ohmic shunts below the grid lines and a high density of weak nonlinear shunts away from the grid lines. Two distinctly different approaches are shown to reduce the shunting problem to a negligible level: (i) to contact only a small fraction of the rear Si surface via a point contacting scheme, whereby the metal layer needs to be thin (<1 mm) and the fractional area coverage small (<5%), and (ii) to deposit line contacts in a bifacial interdigitated scheme, whereby a thick layer of metal is deposited followed by a wet-chemical etching step that effectively reduces shunting by preferentially etching away the shunting paths. Test devices with an area of 1 cm 2 achieve pseudo fill factors ( pFF) of above 75% and diode ideality factors of below 1Á3, demonstrating that the proposed methods are well suited for the metallisation of the rear surface of EVA solar cells.

This article presents the lock-in thermography measurement technique applied to encapsulated organic heterojunction solar cells, built from poly(3-hexylthiophene) (P3HT) and 1-(3-methaoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM). The realizable temperature resolution grants the possibility to visualize even very weak thermal losses in solar cells. The thermal behavior of the cells is demonstrated for different applied voltages. Especially, parallel and serial resistances can be spatially resolved. In order to explain the nonuniform behavior of the serial resistance a simplified replacement circuit of the organic cell is created, according to the Kirchhoff laws. The matrix of series and parallel resistivities allows the simulation of the currents flowing vertically through the semiconducting layer. Bias dependent simulations reveal that the considerable difference of the sheet resistances between the indium-tinoxide (ITO) and aluminum contacts is responsible for the experimentally observed inhomogeneous temperature profiles of the organic solar cells.

Abstract: Pulsed Phase Thermography,(PPT) has been successfully applied for defect detection purposes on a variety of materials. A great deal of work has been done to evaluate the potential of PPT for quantitative applications, using for instance statistical methods, Neural Networks or wavelets (Ibarra-Castanedo et al., 2004). However, calibration requirements and lengthy computation subroutines, preclude their use on most NDT

The detection limit of infrared thermographic investigations can be improved down to 10 K by using a highly sensitive high-speed infrared camera in an on-line averaging lock-in thermography system. Together with a microscope objective, this allows lock-in thermography to be used as a simple and sensitive technique to localize the sites of leakage currents and other heat sources in electronic components. The practical realization of a novel lock-in thermography system is described and both test measurements and practical applications are introduced. The detection limit for surface-near local heat sources in silicon is a few microwatts with a spatial resolution down to 5 m. Leakage sites in several microelectronic structures are imaged and assigned to the layout of the integrated circuit by comparing direct images with lock-in ones. The direct comparison of an averaged and background-subtracted stationary thermogram with a lock-in one, both measured under similar conditions at the same sample, clearly demonstrates the gain in information obtained by using lock-in thermography.

The paper describes the quantitative determination of the size and the location of a subsurface defect from the phase image of lock-in infrared thermography. Phase (or temperature) difference between a defect area and a sound area indicates the qualitative ...

This paper reviews the latest results in application and development of infrared imaging methods for fast and spatially resolved silicon solar cell characterization. Infrared imaging methods comprise electroluminescence (EL) imaging, photoluminescence (PL) imaging, and lock-in thermography (LIT). We report on new insights into the nature of local series resistances and important observations on local junction breakdown in industrial multicrystalline silicon solar cells. Significant improvements have been achieved in the applicability of infrared imaging methods for in-line application in silicon solar cell production. It was demonstrated that quantitative values for local reverse currents in hot-spots can be easily obtained in 10 milliseconds. Quantitative series resistance images were obtained in 800 milliseconds with a good potential to reduce the measurement time to below 500 milliseconds.

Infrared lock-in thermography allows to image shunts very sensitively in all kinds of solar cells and also to measure dark currents flowing in certain regions of the cell quantitatively. After a summary of the physical basis of lock-in thermography and its practical realization, four types of quantitative measurements are described: local I-V characteristics measured thermally up to a constant factor (LIVT); the quantitative measurement of the current through a local shunt; the evaluation of the influence of shunts on the efficiency of a cell as a function of the illumination intensity; and the mapping of the ideality factor n and the saturation current density J 0 over the whole cell. The investigation of a typical multicrystalline solar cell shows that the shunts are predominantly responsible for deterioration of the low-light-level performance of the cell, and that variations of the injection current density related to crystal defects are predominantly determined by variation of J 0 rather than of n.

Lock-in thermography is applied to image Joule heating and Peltier-type heat transport separately. Images obtained for a multicrystalline silicon solar cell are quantitatively evaluated using an integration method. The results are interpreted in terms of diffusion and electron/hole drag contributions. The approach presented is especially interesting where the thermal contact resistance to the sample is a problem and where versatility with respect to sample geometry is needed. A further advantage of the method is that it does not need any separate power or temperature calibration.

Imaging techniques are applied to multi-crystalline silicon bricks, wafers at various process steps, and finished solar cells. Photoluminescence (PL) imaging is used to characterize defects and material quality on bricks and wafers. Defect regions within the wafers are influenced by brick position within an ingot and height within the brick. The defect areas in as-cut wafers are compared to imaging results from reverse-bias electroluminescence and dark lock-in thermography and cell parameters of near-neighbor finished cells. Defect areas are also characterized by defect band emissions. The defect areas measured by these techniques on as-cut wafers are shown to correlate to finished cell performance.

The local breakdown of commercial silicon solar cells occurring at reverse voltages of only 3-4 V has been investigated by means of current-voltage measurements, dark lock-in thermography, and reverse-biased electroluminescence (ReBEL) with a spatial resolution on the micrometer-scale. It is shown that the origin of the local breakdown (so-called type I) can be traced back to a contamination of the wafer surface with Al particles prior to the phosphorous diffusion step. A model is presented explaining that the spectral maximum of ReBEL is within the visible range.

In modulated photothermal experiments the lateral thermal diffusivity can be obtained from the slope of the linear relation between the phase of the surface temperature and the distance to the heating spot. However, this slope is greatly affected by heat losses so that the measured thermal diffusivity is overestimated, especially for thin samples of poor thermal conducting materials. In this paper we definitely identify the physical mechanism responsible for the overestimation of the diffusivity as heat conduction to the surrounding gas. Accurate measurements of the thermal diffusivity using the "slope method" have been obtained by keeping the sample in vacuum.

IGBTs with embedded current monitors, i.e. realized by separating a small part of the main device emitter and using it as the current sense terminal, are currently used to integrate intelligent power modules (IPMs). In a previous paper [Breglio G, Irace A, Napoli E, Spirito P, Hamada K, Nishijima T, et al. Study of a failure mechanism during UIS switching of planar PT-IGBT with current sense cell. Microelectron Reliab 2007;47(9-11):1756-60] we have demonstrated how, during UIS switching in particular circuit configurations, the interplay between the sense-emitter cell and the rest of the device can lead to latch-up of the lateral p-n-p bipolar transistor and current focalization in the sense-emitter cell which finally causes device failure. In this paper, we show how the location of this very localized failure spot can be very accurately determined with the aid of a very sensitive lock-in thermography setup. The main advantage of this approach is the direct applicability to the failed device without the need of time consuming sample preparation as in other failure analysis (FA) techniques.

It is well known that cross-linking of polyethylene molecules into three-dimensional networks improves ma teriaI properties; in particular, it enhances impact strength, thermal performance and chemical resistance. A product of good characteristics can be obtained if the manufacturing process is adequate and ali involved machines are working properly. In this context, infrared thermography as a remote imaging technique may be a valuable tool to monitor the polyethylene cross-Iinking processes. Experimental tests were carried out on specimens without treatment and on specimens treated with either chemical processes or electron beam irradiation. Our results prove that lock-in thermography can detect local non-uniformity of material characteristics due to either the extrusion or cross-linking processes, and material differences Iinked to the different compound.

Solar cells showing too large pre-breakdown currents, which might flow for unintentional reverse biasing by local shading, cannot be used since they would lead to thermal damage of the module. To be able to reduce the number of off-specification cells, the origin of the reverse current needs to be investigated. Pre-breakdown mechanisms in semiconductors exhibit characteristic temperature dependences. We present temperature-dependent lock-in thermography (LIT) measurements as well as illuminated LIT investigations under reverse bias that facilitate the identification of the type of breakdown observed at certain sites. There are both local regions with positive and with negative temperature coefficient of the breakdown current. It turns out that grown-in recombinative crystal defects lead to regions of local soft pre-breakdown sites. However, outside of these regions there are often some additional sites where a hard breakdown occurs which may even dominate at higher reverse bias. Lock-in EBIC images show that in the latter regions the cell surface, as a result of the acidic texturization etch, contains cone-shaped holes which act as avalanche breakdown sites.

Inspection of aerospace components has always been a challenge. Infrared thermography has demonstrated to be a useful tool for this matter. In this paper, we offer a comparative study involving three active techniques: pulsed thermography, lock-in thermography and vibrothermography. Some of these techniques have proven to be more effective than others for a specific type of system. We compare the experimental results from these three techniques as applied to two typical aerospace parts: honeycomb structures and Glare. The later is perhaps the most challenging of all as will be pointed out. Some insights are provided regarding the most suitable technique for a number of typical situations.

In this article, the theoretical and experimental aspects of three active thermography approaches: pulsed thermography (PT), lock-in thermography (LT), and vibrothermography (VT), are discussed in relation to the nondestructive evaluation (NDE) of honeycomb sandwich structures. For this purpose, two standard specimens with simulated defects (delaminations, core unbonds, excessive adhesive, and crushed core) were tested, and results were processed, examined, and compared. As will be pointed out, the similarities and differences between these active approaches allow conclusions to be made about the most suitable approach for a particular application. In addition, results from NDE inspection by X-rays and c-scan ultrasounds are provided and discussed for reference.

This paper was published in Procedures of SPIE, Thermosense XXIX vol. 6541and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations ...

Several non-destructive testing methods, lock-in thermography, ultrasonic inspection, microtomography and microradiography, were used to assess the manufacturing quality of joints between carbon fiber reinforced carbon composites and Cu/ CuCrZr. The results revealed that ultrasonic inspection is critical since carbon composites and copper have a significant difference in the acoustic impedance; moreover this technique is sensitive to irregular shaped joined surfaces; microtomography and microradiography offer qualitative information on the joint, since carbon is significantly less X-rays sensitive than copper. Lock-in thermography gives information on thermal continuity at interface. Non destructive test results have been validated by destructive tests (morphological analysis and mechanical testing).

The non-ideal behavior of the dark current-voltage (I-V) characteristics of typical silicon solar cells is characterized by (1) an unexpectedly large recombination current, often characterized by an ideality factor larger than 2, (2) an ohmic characteristic at low reverse bias, and (3) pre-breakdown at a reverse bias far below the expected breakdown voltage. Experimental evidence, especially from lock-in thermography results, shows that all these features are due to currents flowing locally in the edge region, or at certain extended crystal defects like grain boundaries. Detailed investigations on local breakdown sites in industrial solar cells are introduced. Though a realistic theory of these processes is still missing, a unified explanation of non-ideal dark I-V characteristics is presented and several theoretical approaches to explain different aspects of this non-ideal behavior are discussed.

Local pre-breakdown sites in solar cells can be studied by lock-in thermography (LIT). Three new LIT techniques are proposed and demonstrated here, which are TC-DLIT for studying the local temperature coefficient of pre-breakdown sites, Slope-DLIT for measuring the normalized local slope of the I-V characteristics, and MF-ILIT for imaging the local carrier multiplication factor. First results on multicrystalline silicon cells show that the pre-breakdown mechanism cannot completely be described by the conventional impact ionization and internal field emission models.

We study the emission of light from industrial multicrystalline silicon solar cells under forward and reverse biases. Camera-based luminescence imaging techniques and dark lock-in thermography are used to gain information about the spatial distribution and the energy dissipation at pre-breakdown sites frequently found in multicrystalline silicon solar cells. The pre-breakdown occurs at specific sites and is associated with an increase in temperature and the emission of visible light under reverse bias. Moreover, additional light emission is found in some regions in the subband-gap range between 1400 and 1700 nm under forward bias. Investigations of multicrystalline silicon solar cells with different interstitial oxygen concentrations and with an electron microscopic analysis suggest that the local light emission in these areas is directly related to clusters of oxygen.

Solar cells showing too large pre-breakdown currents, which might flow for unintentional reverse biasing by local shading, cannot be used since they would lead to thermal damage of the module. To be able to reduce the number of off-specification cells, the origin of the reverse current needs to be investigated. Pre-breakdown mechanisms in semiconductors exhibit characteristic temperature dependences. We present temperature-dependent lock-in thermography (LIT) measurements as well as illuminated LIT investigations under reverse bias that facilitate the identification of the type of breakdown observed at certain sites. There are both local regions with positive and with negative temperature coefficient of the breakdown current. It turns out that grown-in recombinative crystal defects lead to regions of local soft pre-breakdown sites. However, outside of these regions there are often some additional sites where a hard breakdown occurs which may even dominate at higher reverse bias. Lock-in EBIC images show that in the latter regions the cell surface, as a result of the acidic texturization etch, contains cone-shaped holes which act as avalanche breakdown sites.

Inspection of aerospace components has always been a challenge. Infrared thermography has demonstrated to be a useful tool for this matter. In this paper, we offer a comparative study involving three active techniques: pulsed thermography, lock-in thermography and vibrothermography. Some of these techniques have proven to be more effective than others for a specific type of system. We compare the experimental results from these three techniques as applied to two typical aerospace parts: honeycomb structures and Glare. The later is perhaps the most challenging of all as will be pointed out. Some insights are provided regarding the most suitable technique for a number of typical situations.

Don't we all Enjoy the Wonderful technology advancements of our day and age, the amazing smart phones and computer tablets, home security, just to name a few of the unbelievable advancements we enjoy in our day and age, but if each and every one of us enjoy all those things that make our life easier, then wouldn't we want to help push the age of technology further by any means possible, instead of hindering the worlds progress by raising the cost of higher education tuition at University's every year that goes by, making it harder, an less appealing for people to want to put forth the effort necessary to get their education, an education the world needs and is counting on each individual to get, and as we raise the cost of getting a higher education we don't realize or think about how at some point every person that is alive on this earth at some point will be dead, that includes the people responsible for inventing all these amazing technology's, so someone at some point has to replace those people, or our technology advancements will cease to advance any further, and if we continue to discourage people from getting the education they need by raising the cost of tuition each year that goes by, then as each year goes by fewer and fewer people will continue to put forth the effort necessary to get the education that we are all counting on each other to get, and more people will begin settling for a job that seems to get them by, so instead of being greedy and letting our decisions be made by figuring how we can pinch a few more penny's out of the pockets of each person we meet, then let's make our decisions with the goal in mind to make the world better, but not just for ourselves and our day and age but a long lasting effect that will continue to make the world a better place long after we have all lived a long wonderful life for many years to come

Lock-in thermography is applied to image Joule heating and Peltier-type heat transport separately. Images obtained for a multicrystalline silicon solar cell are quantitatively evaluated using an integration method. The results are interpreted in terms of diffusion and electron/hole drag contributions. The approach presented is especially interesting where the thermal contact resistance to the sample is a problem and where versatility with respect to sample geometry is needed. A further advantage of the method is that it does not need any separate power or temperature calibration.

Solar cells showing too large pre-breakdown currents, which might flow for unintentional reverse biasing by local shading, cannot be used since they would lead to thermal damage of the module. To be able to reduce the number of off-specification cells, the origin of the reverse current needs to be investigated. Pre-breakdown mechanisms in semiconductors exhibit characteristic temperature dependences. We present temperature-dependent lock-in thermography (LIT) measurements as well as illuminated LIT investigations under reverse bias that facilitate the identification of the type of breakdown observed at certain sites. There are both local regions with positive and with negative temperature coefficient of the breakdown current. It turns out that grown-in recombinative crystal defects lead to regions of local soft pre-breakdown sites. However, outside of these regions there are often some additional sites where a hard breakdown occurs which may even dominate at higher reverse bias. Lock-in EBIC images show that in the latter regions the cell surface, as a result of the acidic texturization etch, contains cone-shaped holes which act as avalanche breakdown sites.

This paper reviews the latest results in application and development of infrared imaging methods for fast and spatially resolved silicon solar cell characterization. Infrared imaging methods comprise electroluminescence (EL) imaging, photoluminescence (PL) imaging, and lock-in thermography (LIT). We report on new insights into the nature of local series resistances and important observations on local junction breakdown in industrial multicrystalline silicon solar cells. Significant improvements have been achieved in the applicability of infrared imaging methods for in-line application in silicon solar cell production. It was demonstrated that quantitative values for local reverse currents in hot-spots can be easily obtained in 10 milliseconds. Quantitative series resistance images were obtained in 800 milliseconds with a good potential to reduce the measurement time to below 500 milliseconds.

The conventional quantitative evaluation of dark lock-in thermography (DLIT), electroluminescence (EL), and photoluminescence (PL) images of solar cells is based on the model of independent diodes, where each image pixel is assumed to be connected with the terminals by an independent series resistance. In reality, however, the solar cell represents a 2-dimensional resistance-diode network. In this work solar cells containing well-defined spatial distributions of the saturation current density J 01 and also containing J 02 -type and ohmic shunts are modeled for various externally applied biases and illumination conditions realistically as a 2-dimensional resistance-diode network. The resulting local diode voltage distributions are converted into DLIT, EL and PL images, which are further processed by conventional evaluation methods, which rely on the simple model of independent diodes. These are the so-called "Local-IV" method for the DLIT analysis, which may be supported by EL results to obtain series resistance images, and "C-DCR" for the PL analysis. This leads to calculated images of the local effective series resistance R s and of J 01 . Regarding the resulting R s images, PL shows the expected series resistance distribution and is not affected by the shunt regions. The DLIT-EL R s images instead yield expected values only in the homogeneous regions, which are not affected by the assumed shunts. DLIT-EL determines higher values of R s in local shunt regions and lower values around these regions and in spatially extended shunt regions. Regarding the J 01 images both methods again give the expected results if J 01 is distributed homogeneously. However, in the shunted regions, PL suffers from balancing currents within the emitter and DLIT from optical blurring. By comparing local and extended regions of increased J 01 we find that DLIT approximates the expected J 01 value better than PL, which clearly underestimates even extended local maxima of J 01 . For a local current analysis of silicon solar cells we recommend the use of DLIT for the determination of J 01 images and PL for the determination of R s images.

Local pre-breakdown sites in solar cells can be studied by lock-in thermography (LIT). Three new LIT techniques are proposed and demonstrated here, which are TC-DLIT for studying the local temperature coefficient of pre-breakdown sites, Slope-DLIT for measuring the normalized local slope of the I-V characteristics, and MF-ILIT for imaging the local carrier multiplication factor. First results on multicrystalline silicon cells show that the pre-breakdown mechanism cannot completely be described by the conventional impact ionization and internal field emission models. Figure 4. MF-ILIT image of the solar cell measured at room temperature using V low ¼ 13 V and V ¼ 15 V. The scaling in (a) is from 1 to 4 and in (b) from 1 to 1Á5

IGBTs with embedded current monitors, i.e. realized by separating a small part of the main device emitter and using it as the current sense terminal, are currently used to integrate intelligent power modules (IPMs). In a previous paper [Breglio G, Irace A, Napoli E, Spirito P, Hamada K, Nishijima T, et al. Study of a failure mechanism during UIS switching of planar PT-IGBT with current sense cell. Microelectron Reliab 2007;47(9-11):1756-60] we have demonstrated how, during UIS switching in particular circuit configurations, the interplay between the sense-emitter cell and the rest of the device can lead to latch-up of the lateral p-n-p bipolar transistor and current focalization in the sense-emitter cell which finally causes device failure. In this paper, we show how the location of this very localized failure spot can be very accurately determined with the aid of a very sensitive lock-in thermography setup. The main advantage of this approach is the direct applicability to the failed device without the need of time consuming sample preparation as in other failure analysis (FA) techniques.