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

Quality Characterization of 

Silicon Bricks using 

Photoluminescence Imaging 

and Photoconductive Decay

Preprint

Steve Johnston, Fei Yan, Mowafak Al-Jassim

National Renewable Energy Laboratory

Katherine Zaunbrecher

National Renewable Energy Laboratory and Colorado 

State University

Omar Sidelkheir and Kamel Ounadjela

Calisolar

Presented at the 2012 IEEE Photovoltaic Specialists Conference

Austin, Texas

June 3–8, 2012

Conference Paper

NREL/CP-5200-54115

June 2012
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1 

Quality Characterization of Silicon Bricks using Photoluminescence 

Imaging and Photoconductive Decay 

Steve Johnston,1 Fei Yan,

1 Katherine Zaunbrecher,1,2 Mowafak Al-Jassim,1

 Omar Sidelkheir,3 and 

Kamel Ounadjela3

1

National Renewable Energy Laboratory; 15013 Denver West Parkway; Golden, CO 80401, USA 

2

Colorado State University; Fort Collins, CO 80526, USA 

3

Calisolar; 985 Almanor Ave.; Sunnyvale, CA 94085, USA 

Abstract — Imaging techniques can be applied to 

multicrystalline silicon solar cells throughout the production 

process, which includes as early as when the bricks are cut 

from the cast ingot. Photoluminescence (PL) imaging of the 

band-to-band radiative recombination is used to characterize 

silicon quality and defects regions within the brick. PL images 

of the brick surfaces are compared to minority-carrier 

lifetimes measured by resonant-coupled photoconductive 

decay (RCPCD). RCPCD is a transient photoconductive 

decay technique that monitors the recombination of excess 

carriers using a frequency of about 420 MHz. Carriers are 

excited by nanosecond laser pulses of long-wavelength light in 

the range of 1150 nm. The low frequency and long penetration 

depth of light promote measurement of carriers away from the 

surface such that lifetimes of up to 100 μs are measured in 

upgraded-metallurgical-grade silicon, and up to 200 μs in 

electronic-grade silicon bricks. PL intensity shows correlation 

to lifetime in addition to the valuable spatial information from 

top to bottom of the brick and defect regions throughout the 

brick.

Index Terms — photoluminescence, imaging, infrared 

imaging, charge-carrier lifetime, photoconductivity, 

photovoltaic cells, silicon, impurities.

I. INTRODUCTION

Imaging techniques can rapidly characterize material 

quality and defect density. Photoluminescence (PL) 

imaging can be applied to silicon from the brick level to all 

wafer process steps [1–11]. At the brick level, material 

thickness allows for measurement and correlation to lifetime 

where surface recombination effects are minimized [6–7]. 

PL images of the brick can be used to calculate lifetime [7, 

10], or separate lifetime measurements can be performed. 

The lifetime can be mapped to give some spatial resolution, 

or the lifetime can be correlated at limited points to the PL 

image to generate a high-resolution lifetime image in 

minimal time [7, 11].

II. EXPERIMENT

Band-to-band PL imaging is collected using a Princeton 

Instruments Pixis 1024BR Si charge-coupled-device (CCD) 

camera cooled to about -50°C. The light source is 

composed of four 30-W, 810-nm laser diodes with 

engineered diffusers to spread out the light over the brick’s 

area. The intensity is near that of one sun, or ~100 

mW/cm2

. Figure 1 shows the PL imaging system 

configured to collect an image on the side of a 

multicrystalline Si brick.

Fig. 1. Photograph of the PL imaging system showing the camera, 

four laser diode outputs for optical excitation, and a Si brick as the 

subject of the imaging.
2 

Fig. 2. PL imaging of a brick face (left image). Lifetime decay curves are shown in the right column. 

We have collected PL images on multicrystalline silicon 

bricks that are 156 mm x 156 mm and roughly 27 cm tall. No 

surface-passivation treatment is applied for either the imaging 

or lifetime measurements. An example of a brick PL image is 

shown in Fig. 2. This brick is from the corner of a cast ingot, 

and the image is of the face where the left edge is an outside 

edge of the ingot. The image shows how PL intensity 

decreases at the bottom, side, and top regions due to impurities 

diffusing into the silicon from the crucible and impurities 

segregating to the top during cooling. The network of dark 

lines within the bulk of the brick correspond to dislocation 

networks and grain boundaries that propagate during the 

casting process. These defect clusters contain extended 

defects that can accumulate impurities and form precipitates. 

All of these dark defect regions have low PL emission due to 

the high recombination of carriers through other non-radiative 

defect states. The image shows two bright streaks from the 

bottom corners that extend toward the center due to strong 

reflections of the excitation light. These artifacts of the 

imaging could likely be reduced or eliminated with improved 

excitation lighting configuration.

The PL image of the brick surface is compared to minoritycarrier lifetimes measured by resonant-coupled photoconductive decay (RCPCD) [12]. RCPCD is a transient photoconductive decay technique that monitors the recombination 

of excess carriers using a frequency of about 420 MHz. A 

photograph of the RCPCD excitation coil near the silicon 

brick is shown in Fig. 3. Carriers are excited by nanosecond 

laser pulses of long-wavelength light near 1150 nm. The light 

travels through the hole in the box and through the coil, which 

is positioned roughly 1 mm from the brick. The low 

frequency and long penetration depth of light promote
3 

Fig. 3. Photograph of the RCPCD coil antenna near a silicon brick.

measurement of carriers away from the surface such that 

lifetimes are dominated by bulk recombination after initial 

surface recombination takes place. Absorption of 1150-nm 

wavelength light in silicon and penetration of 420-MHz 

radiation into typical 1016 cm-3 doping give a measurement 

depth of 3 to 5 mm from the surface of the brick. The 

measurement carrier frequency has adequate bandwidth to 

resolve decay rates down to about 50 ns.

On the brick image shown in Fig. 2, colored circles indicate

the RCPCD measurement area (12-mm diameter spot) and 

steps of 12 mm from top to bottom. The photoconductive 

decay curves are plotted on a logarithmic scale such that a 

linear fit of slope is the exponential decay time constant, or 

carrier lifetime. This transient decay gives a direct 

measurement of lifetime that does not require assumptions 

about photo-generation or measurement of light intensity, as 

do techniques based on measured photoconductivity [13]. The 

plots of Fig. 2 show various decay curves from top to bottom 

of the brick. The fits are made at lower injection levels after 

initial surface recombination no longer dominates the decay 

curve. At the top edge, the decay times are very short, leading 

to lifetimes less than 1 μs. The top plot shows how lifetimes 

increase from the top edge toward the center of the brick. The 

middle plot then shows how lifetimes, after reaching their 

peak in the lower middle portion of the brick, then become 

shorter again toward the bottom edge of the brick. Lifetimes 

of up to ~200 μs are measured in this electronic-grade silicon 

brick, whereas lifetimes of up to ~100 μs are more typical for 

upgraded metallurgical-grade, or solar-grade, silicon. In the 

bottom plot of Fig. 2, the PL intensity down the center of the 

brick is plotted on the left axis, while the measured carrier 

lifetime values are plotted on the right axis. The scales of 

each axis are adjusted to show correlation and account for 

background counts in the image. Using the axes scales, a 

linear factor can be calculated and used to convert the PL 

image to a minority-carrier lifetime image, as shown in Fig. 4. 

This results in a high-resolution lifetime image [6–7] that can 

be acquired quickly based on 1-minute or less PL exposure 

times and a few seconds for RCPCD measurement points to 

correlate the image to lifetime. Large variations of doping 

may need to be accounted for separately, because the PL, in 

addition to excess carrier density, is also proportional to 

doping [9, 11]. Overall, PL intensity shows correlation to 

lifetime and provides valuable spatial information from top to 

bottom of the brick and for defect regions seen throughout the 

surfaces of the brick. 

Fig. 4. The PL image is converted to a lifetime image through 

correlation to RCPCD-measured minority-carrier lifetimes. 
4 

Another example of characterizing Si brick lifetimes by 

imaging and mapping is shown in Fig. 5. This brick is from 

the edge of an ingot where one side has increased impurities 

due to diffusion from the crucible during casting. The bottom 

quarter had been cropped off due to poor quality, but the 

pieces were placed back together for the imaging and mapping 

measurements. The top row shows PL images of each face of 

the brick, and each face was also scanned to measure RCPCD 

along the center line as was shown in Fig. 2. The color scale 

indicates values of lifetime as measured by RCPCD and 

correlated to the PL. Lifetime values of up to 70–80 μs were 

measured by RCPCD. This brick is 265 mm tall, and so, with 

the camera’s 1024 x 1024 CCD array, the resolution is 

roughly 260 μm per pixel. Artifacts of strong reflections are 

shown at the bottom corners, as were seen in Fig. 2.

The middle row of Fig. 5 shows microwave-reflection 

photoconductive decay (μ-PCD) lifetime maps measured on a 

Semilab system. The Semilab system uses a carrier frequency 

of 10 GHz and an excitation wavelength of 950 nm, so the 

depth of measurement is only about 0.5 mm from the surface 

into the brick. The small excitation spot size of about 1-mm 

diameter provides for fairly good spatial resolution. Using 

0.5-mm steps, the lifetime mapping requires about 45 minutes

Fig. 5. Lifetime maps of the four side-faces of a Si brick. The top row shows PL imaging that is converted to lifetime values through 

correlation to RCPCD lifetime scans down the center of each face. The middle row shows lifetime maps acquired using a Semilab microwave 

reflection photoconductive decay system. The bottom row shows lifetime maps from a Sinton Instruments brick lifetime measurement system.
5 

for each side. However, because the penetration depth of the 

measurement is lower, the recombination of carriers is greatly 

affected by the surface, and the measured, or effective, carrier 

lifetimes are shorter [11]. The highest lifetimes on this brick 

reported by the Semilab tool are about 6 μs. 

The bottom row of Fig. 5 shows lifetime maps acquired 

from a Sinton Instruments brick lifetime measurement system

using the quasi-steady-state photoconductance (QSSPC) 

measurement technique [13]. This system uses a sensor with a 

4 mm x 30 mm coil to define the measurement area, and the 

entire brick was scanned with five sweeps. The Sinton system 

uses filtered light so that it is predominantly in the infrared, 

and it has a depth sensitivity of about 2.5 mm. Although this 

depth sensitivity of QSSPC exceeds that of the Semilab μPCD, unpassivated surfaces are reported to still have a strong 

effect on the measurement results [13]. The maximum 

measured lifetimes of about 25 μs are lower than those of 70–

80 μs measured by the transient-based RCPCD technique. 

This is a similar result to a brick lifetime comparison of 

QSSPC and transient modes reported previously [13].

III. CONCLUSION

Photoluminescence images on silicon bricks can be 

correlated to lifetime measured by photoconductive decay and 

could be used for high-resolution characterization of material 

before wafers are cut. The RCPCD technique has shown the 

longest lifetimes of any of the lifetime measurement 

techniques we have applied to the bricks. RCPCD benefits 

from the low-frequency and long-excitation wavelengths used. 

In addition, RCPCD is a transient technique that directly 

monitors the decay rate of photoconductivity and does not rely 

on models or calculations for lifetime. The measured lifetimes 

over brick surfaces have shown strong correlations to the PL 

image intensities; therefore, this correlation could then be used 

to transform the PL image into a high-resolution lifetime map. 

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.

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