NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Temperature-Dependent 

Photoluminescence Imaging 

and Characterization of a 

Multi-Crystalline Silicon Solar 

Cell Defect Area

Preprint

Steve Johnston, Fei Yan, Jian Li, 

Manuel J. Romero, and Mowafak Al-Jassim

National Renewable Energy Laboratory

Katherine Zaunbrecher 

National Renewable Energy Laboratory and Colorado 

State University

Omar Sidelkheir and Alain Blosse 

Calisolar

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50706

July 2011
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1 

TEMPERATURE-DEPENDENT PHOTOLUMINESCENCE IMAGING AND 

CHARACTERIZATION OF A MULTI-CRYSTALLINE SILICON SOLAR CELL 

DEFECT AREA

Steve Johnston1, Fei Yan1, Jian Li1, Katherine Zaunbrecher1,2, Manuel J. Romero1, Mowafak Al-Jassim1, Omar 

Sidelkheir3

, and Alain Blosse3 

1

National Renewable Energy Laboratory, Golden, CO, USA 2

Colorado State University, Fort Collins, CO, USA 3

Calisolar, Sunnyvale, CA, USA

ABSTRACT

Photoluminescence (PL) imaging is used to detect areas 

in multi-crystalline silicon that appear dark in band-to-band 

imaging due to high recombination. Steady-state PL 

intensity can be correlated to effective minority-carrier 

lifetime, and its temperature dependence can provide 

additional lifetime-limiting defect information. An area of 

high defect density has been laser cut from a multicrystalline silicon solar cell. Both band-to-band and defectband PL imaging have been collected as a function of 

temperature from ~85 to 350 K. Band-to-band 

luminescence is collected by an InGaAs camera using a 

1200-nm short-pass filter, while defect band luminescence 

is collected using a 1350-nm long pass filter. The defect 

band luminescence is characterized by cathodoluminescence. Small pieces from adjacent areas within the 

same wafer are measured by deep-level transient 

spectroscopy (DLTS). DLTS detects a minority-carrier 

electron trap level with an activation energy of 0.45 eV on 

the sample that contained defects as seen by imaging.

INTRODUCTION

Photoluminescence (PL) imaging of multi-crystalline 

silicon can quickly characterize wafers and cells to detect 

areas containing defects such as dense regions of grain 

boundaries and dislocation/defect clusters. [1-8] For bandto-band radiative recombination, defect areas are 

relatively dark due to non-radiative or sub-bandgap defect 

recombination. Some defects can be revealed and 

characterized directly by sub-bandgap radiative 

transitions. Silicon is known to have such sub-bandgap 

states with radiative emissions in the ~1250–1550 nm 

wavelength range. [9-12]

EXPERIMENT 

Band-to-band PL and defect band emissions are imaged 

using a FLIR SC2500 InGaAs camera with sensitivity to 

detect photons with wavelengths from 0.9 to 1.7 μm. The 

camera contains a lock-in detection option to improve the 

signal-to-noise ratio. For defect band emission imaging, a

long-pass filter with a cut-off wavelength at 1350 nm is

used to block band-to-band PL. The light source is 

composed of four 810-nm laser diodes with engineered 

diffusers to spread out the light over the sample. The 

intensity is near that of one sun, or ~100 mW/cm2

. Defect 

band PL is collected when pulsing the 810-nm laser diode 

excitation at about 7 Hz. Figure 1 shows both band-toband PL imaging (top) and defect band emission imaging 

(bottom) of part of a multi-crystalline silicon solar cell.

Figure 1 PL images of a portion of a finished multicrystalline silicon solar cell showing band-to-band PL 

(top) and defect band emission (bottom). The red 

circled area contains a high-defect-density region, 

while the blue circle represents a relatively defect-free 

area.
2 

In Fig. 1, the circled areas were cut out of the wafer using 

a Q-switched Nd:YAG pulsed laser doubled to 532 nm 

wavelength. Laser damage was smoothed by sanding the 

edges of the ~1-cm-diameter cut pieces. The blue area is 

intended to be a relatively defect-free area, while the red 

area contains a high-defect-density area. The bottom 

image shows sub-bandgap defect band luminescence 

when using a 1350-nm long-pass filter in addition to longpass RG1000 Schott glass filters used to block reflected 

810-nm laser diode light. The red circled area contains

high emissions of the sub-bandgap luminescence. A plot 

of the current vs. voltage for these cut out pieces is shown 

in Fig. 2. The piece containing the defect area (red curve) 

shows diode breakdown in reverse bias at a reduced 

voltage compared to the piece from the relatively defectfree area (blue curve).

Figure 2 Current vs. voltage curves for the laser-cut 

regions. The red curve represents the piece 

containing the high-defect-density region, while the 

blue curve represents the relatively defect-free piece.

PL imaging of the high-defect-density red circle area of 

Fig.1 is shown in Fig. 3. Images at selected temperatures 

are shown for both band-to-band imaging (left column) 

and defect band imaging (right column). The sample was 

cooled in a cryostat using liquid nitrogen. A laser diode 

with an 810-nm wavelength and approximately one-sun

intensity was used to illuminate the sample. The FLIR

InGaAs camera was used to collect the images as 

temperature was varied. A relatively defect-free area is 

circled in blue on the lowest temperature band-to-band PL 

image. The average PL intensity within this area is plotted 

as a function of temperature and represented by the blue 

circle points in Fig. 4. The red outlined rectangular area of 

Fig. 3 represents a high-defect-density region, and this 

area is averaged and represented by the red square 

markers in the plots of Fig. 4. For the defect band images 

of Fig. 3, a green triangle roughly outlines the high-defect 

area. The average defect band intensity is also plotted in 

Fig. 4 using green triangle markers, which are referenced 

to the right axis.

Figure 3 PL imaging at selected temperatures 

performed on the red cut-out piece from Fig 1. The left 

column shows band-to-band PL, while the right 

column shows sub-bandgap defect band emissions. 

The red outlined rectangular area contains high defect 

density, and the blue circular area represents a 

relatively defect-free area. The green triangle is used 

to define average defect band emissions.
3 

Figure 4 Average PL intensities (band-to-band on left 

axis and defect band on right axis) of the areas 

defined in Fig. 3 are plotted as a function of 

temperature (top). The band-to-band intensities are 

divided by the temperature-dependent B-coefficient 

for silicon in the bottom graph. The bottom plot also 

shows the defect band intensities divided by a

temperature-dependent B-coefficient factor. 

The band-to-band PL intensities for both the defect and 

defect-free areas are shown to increase with temperature 

in the top graph of Fig. 4. The defect band intensity, 

however, is shown to decrease with increasing 

temperature. The B-coefficient for radiative recombination 

in silicon has a known temperature dependence. [13,14] 

The average band-to-band PL intensity can account for 

this variation by dividing the PL values by the Bcoefficient. The results are plotted on the bottom graph of 

Fig. 4. There is now a near-linear increase in band-toband PL with temperature. This trend is due to the 

temperature dependence of the minority-carrier lifetime, as 

shown in Fig. 5. Minority-carrier lifetime has been 

measured on this cut-out area of the finished cell by the 

open-circuit voltage decay technique. [15,16] When 

compared over the same temperature range, the factor of 

increase in the average PL intensity is comparable to the 

factor of increase in lifetime, such that dividing PL intensity 

by lifetime yields a roughly flat, temperature-independent 

set of values.

Figure 5 Temperature-dependent minority-carrier 

lifetime of the blue-circled area of Fig. 1 as measured 

by open-circuit voltage decay.

As shown in Fig. 4, the defect band points are divided by 

an equivalent B-coefficient factor based on temperaturedependent bandgap [17,18], whose values similarly 

decrease with temperature like those of the band-to-band 

PL B-coefficient. The defect band PL intensity data shows 

a change in slope at approximately 210 K (top graph in 

Fig. 4). The temperature-dependent, bandgap-based 

corrected data then shows a peak at this value (bottom 

graph of Fig. 4), which suggests a defect signature.

To further characterize the band-to-band PL and defect 

band emissions, we have performed cathodoluminescence (CL) mapping to collect high-resolution 

spatial and spectral data. CL was measured on a roughly 

200 μm x 200 μm defect-containing area from the red 

circled piece of Fig. 1. The sample is cooled to 

approximately 78 K. Figure 6 shows the CL mapping 

results for band-to-band transitions near 1130 nm (top 

image) and for defect band transitions near 1400 nm 

(lower image). The top image shows a dark line 

representing a grain boundary or dislocation where 

recombination through impurity and/or defect states is 

high. In the lower image, this same dark feature is evident, 

and sub-bandgap emissions are revealed very near to this 

line. Spectral data is gathered at three points in the vicinity 

of this defect area. The first point, p1 (shown in blue), 

represents an area where defect band emissions are very 

weak. The corresponding spectrum shown in the bottom 

graph of Fig. 6 shows a peak centered at ~1100 nm for the 

band-to-band radiative transition but no sub-bandgap 

signal. The other points, p2 and p3 (shown in red and 

orange in Fig. 6), correspond to areas where defect band 

emission is relatively high. The spectrums corresponding 

to these spots show peaks of defect band emissions in the 

1300–1600 nm range. 
4 

Figure 6 Cathodoluminescence mapping is performed 

on a roughly 200 μm x 200 μm defect-containing area 

from the red circled piece of Fig. 1. The top image 

shows intensities at ~1130 nm, while the lower image 

represents defect band intensities with wavelengths in 

the 1300–1600 nm range. The bottom graph shows 

spectrums from the points labeled in the defect band 

image.

A small, roughly 1 mm x 2 mm sample was cleaved from a 

nearby defect region for deep-level transient spectroscopy 

(DLTS). A sample of this size or smaller is required for 

DLTS due to limitations on the value of capacitance for 

this measurement. Figure 7 shows band-to-band 

electroluminescence imaging [19,20] (top) and defect 

band emissions (bottom) when a forward bias is applied to 

this sample. This piece shows similar defect structures to 

the neighboring piece used for temperature-dependent 

imaging.

Figure 7 Electroluminescence images acquired with 

forward bias show band-to-band emissions in the top 

image and defect band emission in the bottom image. 

The top image is partially obscured due to the large 

probe pin contacting the grid line.

DLTS data was collected using a reverse bias of 5 V and 

filling pulses to 0.2 V forward bias. A positive peak, 

corresponding to a minority-carrier electron trap, was 

evident. Analysis of the DLTS data is used to generate the 

Arrhenius plot shown in Fig. 8., where the DLTS-detected

peak corresponds to a defect level with an activation 

energy of 0.45 eV. A similarly prepared and sized sample 

from a defect-free area did not show a discernible peak 

when also measured by DLTS.
5 

Figure 8 DLTS data from the sample shown in Fig. 7 is 

analyzed to construct an Arrhenius plot. The slope

corresponds to an activation energy of 0.45 eV, while 

the intercept relates to a capture cross section of 

1.2x10-12 cm2

. 

SUMMARY

Temperature-dependent PL imaging was collected on 

samples from a multi-crystalline silicon solar cell. A defectcontaining region was compared to a relatively defect-free 

region using both band-to-band PL and defect band 

emissions. The temperature dependence of the band-toband PL is due to the temperature variations of the Bcoefficient for radiative recombination and minority carrier 

lifetime. The temperature dependence of the defect band 

emissions shows a potential defect signature. CL mapping 

showed the defect band spectrum had a peak in the 

1300–1600 nm range. DLTS was used to measure a 

minority-carrier electron trap with an activation energy of 

0.45 eV on a defect-containing sample.

ACKNOWLEDGEMENT

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08GO28308 with the 

National Renewable Energy Laboratory and with support 

from the American Recovery and Reinvestment Act.

REFERENCES

[1] T. Trupke, R. A. Bardos, M. C. Schubert, and W. 

Warta, “Photoluminescence Imaging of Silicon Wafers”, 

Appl. Phys. Lett. 89, 2006, 044107.

[2] T. Trupke, R. A. Bardos, M. D. Abbott, F. W. Chen, J. 

E. Cotter, and A. Lorenz, “Fast Photoluminescence 

Imaging of Silicon Wafers”, 32nd IEEE PVSC and WCPEC4, 2006, pp. 928-931.

[3] W. McMillan, T. Trupke, J. W. Weber, M. Wagner, U. 

Mareck, Y. C. Chou, and J. Wong, “In-line Monitoring of 

Electrical Wafer Quality Using Photoluminescence 

Imaging”, 25th EPVSEC/5th WCPEC, Valencia, Spain, 

2010, pp. 1346-1351.

[4] J. Haunschild, M. Glatthaar, M. Demant, J. Nievendick, 

M. Motzko, S. Rein, and E. R. Weber, “Quality control of 

as-cut multicrystalline silicon wafers using 

photoluminescence imaging for solar cell production”, Sol. 

Ener. Mat. & Sol. Cells 94, 2010, pp. 2007-2012.

[5] T. Trupke, J. Nyhus, and J. Haunschild, “Luminescence 

imaging for inline characterization in silicon photovoltaics”, 

Phys. Status Solidi RRL 5, 2011, pp. 131-137.

[6] T. Trupke, J. Nyhus, R. A. Sinton, and J. W. Weber, 

“Photoluminescence imaging on silicon bricks”, 

Proceedings of the 24th European Photovoltaic Solar 

Energy Conference, Hamburg, Germany, 2009, pp. 1029-

1033.

[7] B. Mitchell, T. Trupke, J. W. Weber, and J. Nyhus, 

“Bulk minority carrier lifetimes and doping of silicon bricks 

from photoluminescence intensity ratios”, J. Appl. Phys. 

109, 2011, 083111.

[8] J. Haunschild, M. Glatthaar, S. Riepe, and S. Rein, 

“Quality control using luminescence imaging in production 

of mc-silicon solar cells from UMG feedstock”, 35th IEEE 

PVSC, 2010, pp. 812-816.

[9] I. Tarasov, S. Ostapenko, C. Haessler, and E. U. 

Reisner, "Spatially resolved defect diagnostics in 

multicrystalline silicon for solar cells," Mat. Sci. & Engr. B 

71 (1-3), 51-55 (2000).

[10] S. Ostapenko, I. Tarasov, J. P. Kalejs, C. Haessler, 

and E.-U. Reisner, “Defect monitoring using scanning 

photoluminescence spectroscopy in multicrystalline silicon 

wafers”, Semicond. Sci. Technol. 15, 2000, pp. 840-848.

[11] I. Tarasov, S. Ostapenko, W. Seifert, M. Kittler, J. 

P. Kaleis, “Defect diagnostics in multicrystalline silicon 

using scanning techniques”, Physica B 308-310, 2001, pp. 

1133-1136.

[12] M. Kittler, W. Seifert, T. Arguirov, I. Tarasov, and S. 

Ostapenko, “Room-temperature luminescence and 

electron-beam-induced current (EBIC) recombination 

behaviour of crystal defects in multicrystalline silicon”, Sol. 

Ener. Mat. & Sol. Cells 72, 2002, pp. 465-472.

[13] H. Schlangenotto, H. Maeder, and W. Gerlach, 

“Temperature Dependence of the Radiative 

Recombination Coefficient in Silicon”, Phys. Stat. Sol. (a) 

21, 1974, pp. 357-367.

[14] T. Trupke, M. A. Green, P. Wurfel, P. P. Altermatt, A. 

Wang, J. Zhao, and R. Corkish, “Temperature 

dependence of the radiative recombination coefficient of 

intrinsic crystalline silicon”, J. Appl. Phys. 94, 2003, pp. 

4930-4937.
6 

[15] B. R. Gossick, “Post-Injection Barrier Electromotive 

Force of p-n Junctions”, Phys. Rev. 91, 1953, pp. 1012-

1013.

[16] S. R. Lederhandler and L. J. Giacoletto, 

“Measurement of Minority Carrier Lifetime and Surface 

Effects in Junction Devices”, Proceedings of the IRE 43, 

1955, pp. 477-483.

[17] W. Bludau, A. Onton, and W. Heinke, “Temperature 

dependence of the band gap of silicon”, J. Appl. Phys. 45, 

1974, pp. 1846-1848.

[18] D. Z. Garbuzov, “Reradiation effects, lifetimes and 

probabilities of band-to-band transitions in direct A3B5

compounds of GaAs type”, Journal of Luminescence 27, 

1982, pp. 109-112.

[19] T. Fuyuki, H. Kondo, T. Yamazaki, Y. Takahashi, and 

Y. Uraoka, “Photographic Surveying of Minority Carrier 

Diffusion Length in Polycrystalline Silicon Solar Cells by 

Electroluminescence”, Appl. Phys. Lett. 86, 2005, 262108.

[20] T. Fuyuki, H. Kondo, Y. Kaji, A. Ogane, and Y 

Takahashi, “Analytic Findings in the Electroluminescence 

Characterization of Crystalline Silicon Solar Cells”, J. Appl. 

Phys. 101, 2007, 023711.