Evaluation of Four Imaging 

Techniques for the Electrical 

Characterization of Solar Cells 

Preprint 

G.M. Berman, N. Call, R.K. Ahrenkiel 

and S.W. Johnston 

National Renewable Energy Laboratory 

Presented at the 2008 Materials Research Society (MRS) 

Fall Meeting 

Boston, Massachusetts 

November 30 – December 5, 2008 

Conference Paper 

NREL/CP-520-44607 

December 2008 
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Evaluation of Four Imaging Techniques for the Electrical 

Characterization of Solar Cells 

Gregory M. Berman1,3, Nathan J. Call2,3, Richard K. Ahrenkiel2,3, and Steven W. Johnston3

1

Department of Electrical Engineering, University of Colorado, Boulder, CO 80309, U.S.A. 

2

Department of Materials Science, Colorado School of Mines, Golden, CO 80401, U.S.A. 

3

National Renewable Energy Laboratory, Golden, CO 80401, U.S.A. 

ABSTRACT

 We evaluate four techniques that image minority carrier lifetime, carrier diffusion length, and 

shunting in solar cells. The techniques include photoluminescence imaging, carrier density 

imaging, electroluminescence imaging, and dark lock-in thermography shunt detection. We 

compare these techniques to current industry standards and show how they can yield similar 

results with higher resolution and in less time. 

INTRODUCTION 

 In this paper, we address four imaging techniques that measure minority carrier lifetime, 

diffusion length, and the location of shunts in a solar cell, and we compare our techniques to 

methods currently used in industry. Minority-carrier-lifetime and diffusion-length measurements 

are critical for characterizing the quality of solar materials to predict the efficiency of a device 

made from them [1-2]. Shunts can drastically lower a solar cell’s efficiency and are introduced 

in the processing of a wafer into a working solar cell [3]. 

 Minority carrier lifetime is the average time it takes for a free carrier generated in a material 

to recombine. We measure it with photoluminescence imaging (PLI) and carrier-density 

imaging (CDI). PLI probes the radiative recombination of an optically excited sample, which is 

proportional to the sample’s carrier density [4-6]. CDI measures the transmission of infrared 

(IR) light through an optically excited sample, which is also proportional to carrier density [7-8]. 

We compare these two carrier-lifetime techniques to each other and to microwave-reflection 

photoconductive decay on a Semilab tool, an industry standard that relates a material’s 

conductivity to its carrier density. 

 Carrier diffusion length is the average distance a free carrier travels in a material before it 

recombines. We measure it with electroluminescence imagining (ELI). ELI probes the radiative 

recombination of a finished solar cell that is electrically driven in forward bias [9-10]. We 

compare ELI images with light-beam-induced current (LBIC) maps [11], an industry standard 

that measures the optically induced current in a solar cell. 

 Shunts are defects created during solar cell processing that leak current. We use dark lock-in 

thermography (DLIT) to detect them. While shunts can be detected by measuring a cell’s I-V 

(current-voltage) curve, no information about a shunt’s cause or location is identified. In DLIT 

imaging, the solar cell is put under reverse bias, and the shunts are detected by a thermal 

signature, generated by current leaking across the p-n junction [12]. We demonstrate shunt 

detection and identify the defect with a scanning electron microscope. 

1 
METHODS 

Photoluminescence imaging

 PLI is the steady-state equivalent of time-resolved photoluminescence that is imaged onto a 

camera. This technique has recently been made possible by the high sensitivity and large pixel 

counts of modern Si charge-coupled-device (CCD) cameras. During PLI, carriers in a wafer or a 

finished cell are optically excited. The camera then detects a small amount of radiative 

recombination from the indirect bandgap material. Areas on the sample that have long minoritycarrier lifetimes will have a higher steady-state density of free carriers, resulting in a stronger 

radiative recombination signal. 

 We collect PLI data using a PIXIS 1024BR Si CCD camera (Princeton Instruments/Acton). 

This camera has a 1024 x 1024 array of 13 μm pixels and is back illuminated and cooled to 200 

K. The detector is deep depleted to enhance quantum efficiency beyond 1100 nm. A compact 

lens (Schneider Optics Cinegon) is mounted to the camera along with a stack of two 810 nm 

notch filters (Kaiser Optical Systems). A black glass filter (Schott RG1000) is placed between 

the two notch filters to eliminate resonance. The filter stack is designed to sufficiently attenuate 

reflected light from the 60 W, 810 nm laser diode excitation source. The fiber output from the 

laser diode is expanded to the sample area by a collimator and an engineered diffuser. 

 Measurements are made while the sample is under steady-state illumination. The image is a 

summation of all photons collected in a set exposure time. It is a spatial map of the radiative 

recombination in the sample, which is directly related to carrier concentration and carrier 

lifetime. While such images are qualitative, we show that they can be calibrated by transientlifetime measurements to yield absolute lifetime values. 

Carrier density imaging

 CDI is a direct measurement of free carriers in a process called free-carrier absorption. Freecarrier absorption occurs when excited carriers decrease the transmission of mid-to-far infrared 

light in a semiconductor. During this process, a wafer without any metallization is optically 

excited. Then an infrared camera images the change in transmission of black-body radiation 

through a sample with and without the excited carriers. 

 CDI data is acquired with a Silver 660M InSb infrared camera (ElectroPhysics/Cedip), 

which has a 640 x 512 array of 15 μm pixels and a spectral response from 3.6–5.1 μm. A builtin Stirling stage cools the detector to ~76 K. The camera has a built-in lock-in feature and a 

maximum frame rate of 100 Hz. We use the camera’s lock-in mode to collect CDI data with the 

laser diode driven by a 27 Hz square wave. The lock-in mode greatly reduces the noise of the 

measurements — a critical factor for measuring the small signals in CDI. We used a hot plate 

set at ~400 K as the black-body source. To enhance the hotplate’s emissivity, we have coated it 

with a black high-temperature paint. 

 The images we collect represent the amplitude of the Fourier transform of each pixel in time 

at the particular frequency we are collecting data. The images show the relative change in 

transmission spatially across the wafer. This transmission is proportional to the lifetime. While 

these images are qualitative by themselves, they can be calibrated by a single-point lifetime 

measurement to yield accurate lifetime images. 

2 
Electroluminescence imaging

 We collect ELI data in much the same way as PLI. We use the same Si CCD camera, though 

we take off the filter stacks since there is no laser excitation light to filter out. Others have used 

various filter arrangements to enhance ELI data and analysis [10]. In ELI, a finished solar cell is 

electrically driven in forward bias like a light emitting diode. As in PLI, the measurement is 

taken under steady-state excitation with a typical exposure time of one second. Raw ELI data is 

qualitative, but can be made quantitative by the method of data collection and analysis [9]. 

 

Dark lock-in thermography

 To detect shunts using DLIT imaging, a finished wafer is electrically driven in reverse bias. 

The reverse bias is applied to the sample by an amplifier driven by a square wave. An ideal solar 

cell under reverse bias would have very little current flow. However, imperfections in the cell 

often lead to areas where there is a lower virtual resistance through the cell’s junction, causing 

current to flow through it. This flow of current then generates heat, creating hotspots that we 

image in lock-in mode with the same thermal camera used in CDI. 

RESULTS AND DISCUSSION 

 

Comparison of PLI and CDI to transient lifetime measurements 

 Our data show that there is a strong correlation between PLI and CDI as well as with the 

actual measured lifetime on a wafer. For the absolute measurement, we use the microwavereflection–photoconductive-decay technique built into a Semilab lifetime-measurement tool. 

This technique measures the conductivity of a material, which is proportional to the carrier 

density, by detecting the change in reflection of microwaves off the sample. The lifetime is then 

found from the carrier-concentration decay rate after a laser pulse illuminates the sample. It then 

takes ~6 minutes to map a 5-inch standard wafer at a resolution of 1 mm. Resolution is limited 

to the excitation light cross-section incident on the sample. A PLI image of the same wafer is 

collected in ~1 second at a resolution of ~125 μm. Likewise, a CDI image is made in ~10 

seconds with a resolution of ~250 μm. The resolution is ultimately limited by the diffraction 

limit of light, which is ~1 μm for PLI and ~5 μm for CDI. The disadvantage of these imaging 

techniques is that they do not yield quantitative results. The lack of quantitative results is shown 

in Fig. 1, where only the Semilab map has a scale for the lifetime. 

 Figure 1. Lifetime of CZ Si wafer with induced impurities acquired from (a) a Semilab lifetime 

scanner, (b) photoluminescence imaging, and (c) carrier-density imaging. 

3 
4 

 To obtain a quantitative analysis of the imaging techniques, 

we compare identical points on a wafer with each technique 

and plot them against each other. We find that there is a linear 

relationship between the pixel counts, the signal strength at a 

single pixel, and the measured lifetime values on the Semilab 

tool. By measuring two different points on a wafer with the 

Semilab tool, we can create a calibration constant to transform 

both PLI and CDI data into quantitative lifetime values, as 

shown in Fig. 2. 

Figure 2. Comparison of PLI and CDI data with Semilab lifetime measurements at discrete 

points on a CZ Si wafer to calibrate image values to absolute lifetime.

Comparison of ELI to LBIC diffusion-length measurements

 The standard method of mapping carrier-diffusion length in a finished cell is through the 

LBIC technique. Mapping a 5-inch square wafer takes ~6 hours at a resolution of 250 μm, as 

seen in Fig. 3(a) and (b). For the same wafer, ELI images can be collected in ~1 second at a 

resolution of ~125 μm, as seen in Fig. 4(a) and (b). Although the correlation between the LBIC 

measurement and ELI may not appear linear, Fig. 3(c) and 4(c) illustrate the similarity between 

them. Such correlations have been quantified in detail previously [9]. 

 

6000

4000

2000

0

250 500 750

L (μm)

Semilab LBIC

(c)

(a) (b)

Figure 3. LBIC carrier-diffusion-length map for (a) a low-efficiency wafer and (b) a highefficiency wafer. (c) Comparitive histogram of the diffusion lengths of (a) and (b). 

 

25

20

15

10

5

0

x103

2000 4000 6000

EL Pixel Counts

EL Imaging

(c)

(a) (b)

Figure 4. Electroluminescence imaging of the relative carrier-diffusion length for (a) a lowefficiency wafer and (b) a high-efficiency wafer. (c) Comparative histograms of the image pixel 

counts of (a) and (b). 

7000

6000

5000

4000

3000

2000

1000

0

PL image counts

0 100 200 300

Semilab lifetime ( μs)

30

25

20

15

10

5

0

CDI Image Counts

 PLI

 CDI 
Shunt Detection and Analysis using DLIT

 Using DLIT, we can find the location of all the major shunts on a wafer and gain insight into 

their cause as shown in Fig. 5. We can determine which shunts are leaking the most current from 

the amplitude image [Fig. 5(b)]. In the phase image [Fig. 5(c)], we can see how fast heat is 

spreading from different points in the sample, which allows us to resolve smaller shunts. 

 

Figure 5. Thermographic shunt detection in a multicrystaline Si wafer at 4 V reverse bias. (a) 

Thermal image. (b) Lock-in amplitude image. (c) Lock-in phase image. Units are arbitrary. 

 To get a better understanding of what may be causing shunts, we use a microscopic objective 

on our thermal camera to view specific shunts at a higher optical resolution. Often a shunt can 

be seen on a gridline where the metallization has punctured through the emitter layer. In some 

cases, a bright shunt in the shape of a line corresponds to a crack in the wafer that was filled in 

during metallization, causing a catastrophic shunt that ruins the device. For the case shown in 

Fig. 6, an aluminum particle has punctured through the emitter layer, creating a path for current 

to flow. In the zoomed-in DLIT image, the shunt seems asymmetric. A scanning electron 

microscope image [Fig. 6(b)] shows that this asymmetric shunt corresponds to an embedded 

aluminum particle. 

 

Figure 6. Images of a shunt caused by an embedded aluminum particle using (a) a thermal 

camera in lock-in mode with ~50X magnification and (b) a scanning electron microscope. 

CONCLUSIONS 

 We have demonstrated imaging methods for minority carrier lifetime (PLI, CDI) and 

diffusion length (ELI) that give much faster and higher resolution data than traditional singlepoint mapping techniques. For carrier lifetime imaging, we have shown that a single-point 

calibration can easily transform a qualitative image into actual lifetime values. These imaging 

techniques enable the possibility of higher-level quality control and defect analysis of solar cell 

materials in in-line production processes. 

5 
6 

 We have used DLIT to find the location of shunts on a wafer and trace them back to specific 

points in the production process. In industry, this information could be used to address problems 

in production and increase the yield of working wafers with higher efficiencies. 

ACKNOWLEDGMENTS 

 We thank CaliSolar Inc. for providing solar cells, Dr. Jian Li and Mr. Jerry Tynan for their 

assistance in the experimental set-up, Mr. Bobby To for taking the SEM images, and Dr. 

Mowafak Al-Jassim and Dr. Dean Levi for coordinating this work. This work was supported by 

the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the National 

Renewable Energy Laboratory (NREL) and under NREL’s Solar America Initiative PV 

Incubator program. 

REFERENCES 

 1. D. K. Schroder, Semiconductor Material and Device Characterization, (John Wiley & Sons, 

Inc., New York, 1990). 

 2. J. W. Orton and P. Blood, The Electrical Characterization of Semiconductors: 

Measurement of Minority Carrier Properties (Academic Press, Inc., San Diego, CA 1990). 

 3. O. Breitenstein and M. Langenkamp, Lock-in Thermography – Basics and Uses for 

Functional Diagnostics of Electrical Components (Springer, New York, 2003). 

 4. Y. Koshka, S. Ostapenko, I. Tarasov, S. McHugo, and J. P. Kalejs, Appl. Phys. Lett. 74, 

1555 (1999). 

 5. M. Tajima, Z. Li, S. Sumie, H. Hashizume, and A. Ogura, Jpn. J. Appl. Phys. 43, 432 

(2004). 

 6. T. Trupke, R. A. Bardos, M. C. Schubert, and W. Warta, Appl. Phys. Lett. 89, 044107 

(2006). 

 7. M. Bail, J. Kentsch, R. Brendel, and M. Schulz, Proceedings of the 28th IEEE-PVSC, 

Anchorage, AK, 99 (2000). 

 8. J. Isenberg, S. Riepe, S. W. Glunz, and W. Warta, J. Appl. Phys. 93, 4268 (2003).

 9. T. Fuyuki, H. Kondo, T. Yamazaki, Y. Takahashi, and Y. Uraoka, Appl. Phys. Lett. 86, 

262108 (2005). 

10. P. Wurfel, T. Trupke, M. Rudiger, T. Puzzer, E. Schaffer, W. Warta, and S. W. Glunz, 22nd

European Photovoltaic Solar Energy Conference and Exhibition, Milan, Italy (2007). 

11. O. Palais, J. Gervais, E. Yakimov, and S. Martinuzzi, Eur. Phys. J. AP 10, 157 (2000). 

12. O. Breitenstein in 17th Workshop on Crystalline Silicon Solar Cells and Modules: Materials 

and Processes, edited by B. L. Sopori (2007). 
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14. ABSTRACT (Maximum 200 Words)

We evaluate four techniques that image minority-carrier lifetime, carrier diffusion length, and shunting in solar cells. 

The techniques include photoluminescence imaging, carrier-density imaging, electroluminescence imaging, and dark 

lock-in thermography shunt detection. We compare these techniques to current industry standards and show how 

they can yield similar results with higher resolution and in less time. 

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PV; imaging techniques; minority-carrier lifetime; carrier diffusion length; shunting; solar cells; photoluminescence 

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