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

Non-Uniformities in Thin-Film 

Cadmium Telluride Solar Cells 

Using Electroluminescence and 

Photoluminescence

Preprint

Katherine Zaunbrecher 

Colorado State University and National Renewable 

Energy Laboratory

James Sites 

Colorado State University

Steve Johnston and Fei Yan

National Renewable Energy Laboratory

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50793

July 2011
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1 

NON-UNIFORMITIES IN THIN-FILM CADMIUM TELLURIDE SOLAR CELLS USING 

ELECTROLUMINESCENCE AND PHOTOLUMINESCENCE

Katherine Zaunbrecher1,2, Steve Johnston2, Fei Yan2, James Sites1

1

Colorado State University, Fort Collins, CO, 80526, USA 2

National Renewable Energy Laboratory, Golden, CO, 80401, USA

ABSTRACT

It is the purpose of this research to develop specific 

imaging techniques that have the potential to be fast, inline tools for quality control in thin-film CdTe solar cells. 

Electroluminescence (EL) and photoluminescence (PL) 

are two techniques that are currently under investigation 

on CdTe small area devices made at Colorado State 

University. It is our hope to significantly advance the 

understanding of EL and PL measurements as applied to 

CdTe. Qualitative analysis of defects and non-uniformities 

is underway on CdTe using EL, PL, and other imaging 

techniques.

INTRODUCTION

Not all data acquisition techniques are created equal. 

Some provide detailed, quantitative information that may 

require several hours to procure, while others may furnish

high-resolution spatial data in a matter of seconds. One 

example of the latter is imaging. Several imaging 

techniques have been developed in order to qualitatively 

assess material quality and the manufacturing process. It 

is our purpose to advance specific imaging measurements

used for quality control in the production of thin-film CdTe 

solar cells. Electroluminescence (EL) and 

photoluminescence (PL) are two such techniques that are 

currently under investigation on CdTe solar cells made at 

Colorado State University’s (CSU) Materials Engineering 

Laboratory. [1-8] These two imaging methods are 

reasonably well understood with regard to analysis of Si 

solar cells, but much less so for thin-film cells. It is our 

hope to significantly advance the understanding of EL and 

PL measurements as applied to CdTe while emphasizing 

overall cell uniformity and defect analysis.

ACQUIRING IMAGES

Imaging is a powerful diagnostic tool that can be used for

defect characterization, evaluating device performance, 

and determining overall material quality. To that effect, EL 

and PL images have been acquired on CdTe solar cells 

made at CSU. Images were taken at the National 

Renewable Energy Laboratory (NREL) using a Si chargecoupled device (CCD) camera with 1024 x 1024 pixels.

For PL imaging, an optical excitation source composed of 

four 5-Watt red light-emitting diode (LED) arrays centered 

at 630 nm is used. The LEDs emit from opposite angles to 

provide uniform excitation over the area of the sample. 

The camera then detects PL emissions as the excess 

carriers generated by the 630-nm excitation source 

recombine. Various illumination intensities were used for 

PL measurements, although an illumination of one sun is 

typical. RG1000 Schott glass filters are mounted to the 

camera lens to block reflected light from the sample and 

stage area.

The Schott glass filters are removed for EL imaging, and 

the sample is connected to a power supply where a 

forward- or reverse-bias voltage can be placed across

the cell. Again, the camera detects a signal where carriers 

are radiatively recombining, while dark regions exist where 

little or no carriers accumulate. Varying current densities 

can be used to acquire EL images. Values as low as 1–2 

mA/cm2 were used as a minimum, while current densities 

corresponding to nearly half of the short-circuit current 

densities (JSC) were used as maximum values.

Illuminated lock-in thermography (ILIT) and dark lock-in 

thermography (DLIT) were also used as evaluation 

techniques. [9-11] Lock-in thermography images were 

acquired using a FLIR/Cedip Silver SC5600 InSb infrared 

camera with built-in lock-in detection and 640 x 512 pixels. 

An 808-nm laser diode optical excitation source was used 

for ILIT. The light source (for ILIT), the applied voltage 

(DLIT), and the camera’s lock-in electronics (for both DLIT 

and ILIT) were set to acquire frames at a frequency of 6.7 

Hz. A forward bias is applied to the cell for DLIT in order to 

evaluate current flow through the device. This current 

causes a rise in temperature in the cell that is detected by 

the camera. In this way, the uniformity of the current can 

be examined. In the case of shunt detection, a reversebias voltage is used during the DLIT measurements. For a 

cell under reverse bias, shunts can create paths for larger 

currents, which can significantly heat up the region where 

a shunt is located.

ANALYSIS

The current focus of this research, after having acquired

images, is evaluating cell uniformity and identifying nonuniformities that may appear. These characteristics 

include a variation in EL and PL intensities over the entire 

area of the cell as well as local defects that appear as both 

dark and bright regions and vary in size and shape. While 

these non-uniformities are present using both techniques, 

there are few features that overlap in PL and EL.

Progress has been made to categorize the general types 

of features seen in CdTe using EL and PL imaging. On 

one hand, one can look at the overall uniformity and note 

the linearity of the EL and PL intensity based on current 

density and illumination intensity, respectively. On the 
2 

other hand, local defects and their origins can be 

identified. When examining these features, one can often 

recognize that their origins are due either to the inherent 

properties of the material or as a result of the fabrication 

process. However, these two categories are not 

necessarily mutually exclusive.

While examining the overall uniformity of each cell, there 

is a linear response of the EL intensity with the amount of 

current through the cell; the PL intensity is also linear with 

the illumination level, as seen in Figures 1 and 2. This 

intensity, used to characterize the brightness of the signal, 

is determined by the average pixel counts. A baseline 

parameter has also been developed to characterize the 

overall uniformity of a cell—the full-width at half-maximum 

of the intensity distribution over the peak intensity. When

plotted against current density, it shows a positive slope 

for each device. The curves from different cells, however, 

do not overlap one another as in the case of EL intensity 

versus current density.

Figure 1. EL intensity versus illumination level for 

three different devices

Figure 2. PL intensity versus illumination level of cells 

with varying processing steps

ILIT was another imaging technique used to look at cell 

uniformity. However, contrary to PL and forward-bias EL, it 

was not a useful technique to characterize current 

uniformity. The ILIT images revealed little spatial variation, 

except when resulting from non-uniformity in the light 

source shining on the cells.

When comparing the overall effectiveness of the images in 

determining cell uniformity, it is interesting to note that EL 

images tend to display more detail than PL counterparts. 

Both have been seen to contain gradients in intensity as 

well as local defects and other features; however, there is 

a sense of higher resolution of certain defects when using 

EL that appears to be due to properties of cell materials.

Turning from uniformity analysis to feature 

characterization, some common local defects that are 

seen in both PL and forward-bias EL images include 

scratches on the glass, patterns on the transparent 

conducting oxide (TCO) from detergent residues and 

diverse artifacts from the cleaning process, and local 

gradients due to material quality. There are also various 

dark points on the images where there is a lack of carrier 

accumulation in those regions. This may occur as a result 

of a high number of defect states being present or 

conductive paths due to weak diodes or shunts. Some 

features also act as simple optical obstructions and thus 

block the signal to the camera.

EL and PL, however, may give different qualitative 

information for certain features. For instance, there are 

bright spots in some of the forward-bias EL images that do 

not appear in PL. They appear in areas of higher current 

density where response to the bias current is greater than 

that of the overall cell. These spots are located mostly 

around the edges of the devices and typically saturate the 

camera. Some may be attributed to shunts at the cell 

edge, while the few appearing in the inner part of the cells 

may be areas of better local performance or decreased 

performance due to shunting. In order to see what fraction 

of these spots were caused by shunting, both reverse-bias 

EL and DLIT images were compared to the forward-bias 

EL images containing the spots; it was found that less 

than half of these areas correspond to shunts. When 

comparing the two methods, however, more shunts were 

found using DLIT than reverse-bias EL. And while some 

shunts appeared much more severe than others in the 

same cell using one technique, this was not necessarily 

true of the other.

Although the effect of defects on device performance has 

not yet been quantitatively evaluated for CdTe solar cells 

using EL and PL imaging, preliminary analysis shows 

correlation between intensity and select performance 

parameters. For the case of several devices processed 

with various treatments, there was a correlation between 

the EL intensity and fill factor for all of the cells that had 

not yet undergone the full Cu treatment or the final 

annealing and then another correlation for the cells that 

were made with these final treatments. A similar 

correlation was also found between EL intensity and cell 

efficiency. These data are shown in Figures 3 and 4.

Once initial EL and PL measurements were taken on 

CdTe solar cells made at CSU, a set of devices was made 
3 

in order to aid in identifying defect origins and to begin 

cataloging defect types. Intentional scratching and 

deposition of impurities were the first few types of defects 

studied. These scratches and impurities were placed on 

the TCO, in the CdS, and in the CdTe before the 

placement of the back contact on several different 

substrates. The scratches were made with various 

materials. Intentional impurities included dust particles, 

fingerprints, and detergent residue.

Figure 3. EL intensity versus fill factor for devices with 

varying processing steps

Figure 4. EL intensity versus efficiency for devices 

with varying processing steps

In the case of scratches made using glass, metal, and 

ceramic, there are regions that appear dark in the EL 

images where material has been completely removed. 

There are also brighter areas around the edges of the 

scratches in the EL images where material may have been 

partially removed and current is focused in those regions. 

The use of other imaging techniques such as reverse-bias 

DLIT helped to identify shunts in these regions. Most of 

the shunting appeared to be a result of the metal 

scratches and dust placed on the TCO before CdS 

deposition. The dust also appeared as bright regions in 

PL. The graphite scratch was, for the most part, an optical 

obstruction that blocked any PL and EL signal generated 

in that region. Most of the regions where the material had 

been scratched by ceramic appeared dark in both PL and 

EL. Glass scratches and the fingerprints placed on the 

TCO had bright and dark features in both EL and PL 

images. And in the regions where detergent residue was 

left, EL revealed bright and dark regions, while PL 

produced the dark patterns that were often seen in images 

that had been previously taken. Examples of metal, 

graphite, and glass scratches and their effects on PL, EL, 

and reverse-bias DLIT are shown in Figure 5. 

In an attempt to see how PL measurements would fare as 

an in-line instrument for early material quality evaluation, 

five unfinished substrates were fabricated at CSU and 

then imaged at NREL. These substrates included a film 

that had only the TCO, CdS and CdTe, one with a CdCl2

treatment after CdTe deposition, and films with varying Cu 

and annealing treatments. After the initial PL imaging, the 

films were then made into full devices. They were then 

imaged again using PL, EL, and DLIT. The images of the 

unfinished films and finished devices were then compared. 

While features could be detected as early as the CdTe 

deposition, PL signal greatly increased after the CdCl2 

treatment, and the contrast was such that most features 

were more distinct and more detailed characteristics were 

easily seen. And features that appeared in the initial PL 

imaging of the films could be seen in the images of the 

finished device.

With all of the data collected over the past year in hand, 

the next stages of detailed analysis can begin. Particularly 

illuminating is that even during the initial stages of imaging 

CdTe devices, beginning with identifying features and 

drawing qualitative conclusions, these techniques do 

indeed provide reasonably informative spatial information. 

As a next step, particular focus will be placed on defect 

and uniformity analysis. Quantitative impacts of defects 

that more commonly occur during the fabrication 

processes will be given particular attention. Areas of 

decreased performance will be examined in-depth as well 

in an attempt to distinguish between weak diode effects, 

variation in series resistance, and increased local 

recombination due to shunting.

SUMMARY

Qualitative analysis of defects and non-uniformities in 

CdTe is underway using EL and PL imaging techniques. In 

order to evaluate the capability of both techniques to 

characterize the overall quality of the cells, we examine

the EL and PL intensities and compare them to cell 

performance. PL and EL intensities are plotted as a 

function of illumination intensity and current density, 

respectively, with both showing linear responses. EL 

intensity is also plotted against fill factor and efficiency, 

with correlations present for devices made with similar 

processing. Studies of defects and their imaging 

signatures were also performed, and an example of a cell 

made with intentional scratches. Identifying the origins and 

effects of defects and non-uniformities to EL and PL may 

lead to better devices in the future.
4 

Figure 5. Metal, graphite, and glass scratches imaged 

(from top to bottom): (a) EL, 6.6 mA/cm2

, 30 s 

exposure; (b) PL, one sun, 30 s exposure; (c) room 

light with 2RG1000 Schott glass filters; and (d) DLIT

ACKNOWLEDGEMENTS

The authors would like to thank Pavel Kobyakov and 

Jason Kephart of Colorado State University’s Material 

Engineering Laboratory and Keegan Barricklow of Abound 

Solar for making the CdTe solar cells. This work was 

supported by the U.S. Department of Energy under 

Contract No. DE-AC36-08-GO28308 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.

[4] T. Trupke, J. Nyhus, and J. Haunschild, 

“Luminescence imaging for inline characterization in 

silicon photovoltaics”, Phys. Status Solidi RRL 5, 2011, pp. 

131-137.

[5] 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] 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.

[8] 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.

[9] O. Breitenstein, M. Langenkamp, O. Lang, and A. 

Schirrmacher, “Shunts Due to Laser Scribing of Solar 

Cells Evaluated by Highly Sensitive Lock-in 

Thermography”, Sol. Ener. Mat. & Sol. Cells 65, 2001, pp. 

55-62.

[10] O. Breitenstein, J. Bauer, J.-M. Wagner, and A. 

Lotnyk, “Imaging Physical Parameters of Pre-Breakdown 

Sites by Lock-in Thermography Techniques”, Prog. 

Photovolt: Res. Appl. 16, 2008, pp. 679-685.

[11] O. Breitenstein, W. Warta, and M. Langenkamp, 

“Lock-in Thermography: Basics and Use for Evaluating 

Electronic Devices and Materials”, 2nd Ed., SpringerVerlag, Berlin, 2010. 

EL

PL

photo

DLIT