LBIC ANALYSIS OF THIN-FILM POLYCRYSTALLINE SOLAR CELLS

James R. Sites and Timothy J. Nagle

Physics Department, Colorado State University, Fort Collins, CO 80523

**ABSTRACT**

Light-beam-induced-current (LBIC) measurements are providing a direct
link between the spatial non-uniformities inherent in thin-film
polycrystalline solar cells, such as CdTe and CIGS, and the overall
performance of these cells. LBIC is uniquely equipped to produce
quantitative maps of local quantum efficiency with relative ease.
Spatial resolution of 1 m at 1-sun intensity, and return to the same
area after other measurements, is routinely achieved. A wavelength range
of 638 to 857 nm is available with diode lasers. The LBIC measurements
demonstrate that several types of effects that alter cell performance
can be traced to specific local-area features. Examples of such effects
include defects related to edges, grids, or scribes, spatial variations
in alloying, and local changes due to high-temperature stress.

# INTRODUCTION

![](media/image1.bmp) Since the late 1970's, scanned laser beams have
been used to measure photocurrent variations in multicrystalline silicon
cells \[1-3\]. In the 1980's, the two-dimensional laser scanner, which
used oscillating mirrors to produce a rastered response map, was
developed and used to map defects in thin-film solar cells \[4\].
Improvements to the two-mirror scanner reduced the laser spot size to 2
μm \[5\] and introduced modern computer techniques to capture and store
the data \[6\]. The two-mirror scanner, however, is not well suited to
quantitative studies of small areas, since it is difficult to align and
to maintain a highly focused spot over a significant area.

Instead of moving a laser beam across a solar cell, the cell can also be
moved through a fixed the laser beam, either with translation stages
\[7\] or with pieozoelectric rastering \[8\]. Use of a modern stepper
motors makes the translation-stage approach straightforward, and one can
reliably position and reposition a specific area within the beam with
uncertainty much less than 1 μm. It is highly desirable to be able focus
the laser beam near its diffraction limit and to maintain an intensity
similar to normal solar intensity. These requirements imply a laser-beam
power the order of 1 nW, which produces photocurrents the order of 1 nA.

**LBIC SYSTEM**

The LBIC configuration used to measure solar-cell response is shown in
Fig. 1 \[9-11\]. A similar LBIC facility has been reported in Ref.
\[12\]. In Fig. 1, the light source is selected from laser diodes of
different wavelengths between 638 and 850 nm. Current to the laser diode
is electronically modulated to produce an AC laser beam, and the
modulation also provides the reference signal for a lock-in amplifier.
Attenuation over a wide dynamic range is set with digital control. The
beam is expanded, the s-polarization is selected, and the beam is
sampled for both intensity and back-reflection from the cell. Beam
steering mirrors allow a parallel beam to impinge on the objective lens
(Olympus 1-UB367 SL C Plan Fluoride 40x/0.55 N.A.). This lens is
equipped with a correction collar to focus through 0 -- 2.5 mm of glass.
It has a working distance of 8.8 mm, which allows ample room for
contacts.

Figure 1. Schematic of LBIC apparatus \[from Ref. 10\].

LBIC measurements using the apparatus shown in Fig. 1 are made by
stepping the solar cell under study through the focused beam in a raster
pattern. The stepping distance is generally set to half the beam size.
Data collection takes approximately 20 min to produce a photocurrent map
of 101 by 101 points for a small area, and somewhat longer for larger
areas requiring greater travel distances.

**DATA PRESENTATION**

Our LBIC data is commonly presented in one of three formats as
illustrated in Fig. 2. The top figure shows an intensity projection of a
CIGS solar cell with

![](media/image2.png){width="3.125in" height="2.4118055555555555in"}

![](media/image3.png){width="3.0in" height="2.126388888888889in"}

![](media/image4.jpeg){width="3.0416666666666665in"
height="2.060416666666667in"}

Figure 2. Three LBIC formats (low resolution).

the grid fingers clearly visible. The middle figure for the same cell is
the most common format, the local-area quantum-efficiency (QE) map of
the area of interest. This format clearly illustrates an isolated
feature with low response. The bottom format repeats the same CIGS data
in a histogram format, which more clearly emphasizes the spread in QE
over the cell.

Three standard resolutions are used, and these are illustrated in Fig. 3
for an NREL-manufactured CIGS cell. Low resolution, shown at the top,
has a 5 mm field and a 100 μm spot. This resolution often covers a
significant portion of a typical test cell. The small square in this
view is magnified ten times for the middle

![](media/image5.png){width="3.125in" height="2.4118055555555555in"}

![](media/image6.png){width="3.125in" height="2.1847222222222222in"}

Medium

Resolution

![](media/image7.png){width="3.125in" height="2.127083333333333in"}

Figure 3. LBIC with low-, mid-, and high-resolution (100-, 10-, and
1-micron spots).

map at mid resolution (500 by 500 μm field and 10-μm spot). The area
shown in that map's small square in multiplied 10 times more for the 50
by 50 μm field with a 1 μm spot for the high resolution map at the
bottom of Fig. 3. In each case the beam was attenuated so that intensity
at the cell was comparable to one sun. This process makes it
straightforward to zoom in on a small area of particular interest and to
return to the same area even after the cell has been removed from the
apparatus for other measurements. The bottom high-resolution map, for
example, shows several features that are 2-5 microns in dimension that
are only marginally seen at lower resolution.

The quantum-efficiency maps shown in Fig. 3 all have a scale to show QE
changes of 2%. This scale is varied as needed to match the total
variation of interest. For example, 1% gradations were used in the
middle section of Fig. 2. In general, the average QE of the LBIC maps
agrees quite well with QE measured with a conventional system. All of
the data shown in Figs. 2 and 3 were taken at zero bias and a wavelength
of 638 nm, which will be taken as standard unless otherwise stated.

**TYPES OF LBIC FEATURES**

The most common LBIC features are local optical blockages that reduce
the photocurrent over small, well-defined areas. Their characteristic
signature is sharp edge definition, and they can result from inclusions
during fabrication. They are most likely what is seen in the middle
section of Fig. 2 and the bottom section of Fig. 3. In some cases, there
is an accompanying increase in reflection that gives a nearly identical
map over the same area. There are two other possibilities, however,
which have a similar signature to an optical impediment. One is a local
area that does not collect photocarriers efficiently, and the other is a
region where adjacent layers have not made good electrical contact. The
latter situation often extends over larger regions than the features
seen in Figs. 2 and 3.

The second major category of LBIC features is a leakage path, which
could be either a filamentary shunt or a weak diode. The latter tends to
be very bias dependent. In either case, the actual flaw can be very
small, but the photocurrent reduction will spread over a much larger
area. An example of the LBIC signature from a shunt in a single-crystal
cell is shown in Fig. 4. The actual shunt occurs at a single point along
a grid line in the upper left of the map. The resulting decrease in
photocurrent, however, spreads over several mm.

A third category of LBIC features is what we term external. These
features can result, for example, from blemishes in a flexible
substrate, from the pressure applied by measurement probes, or simply
from dust particles on the surface. One consequence of the large number
of possibilities for LBIC features is that it is rare to find a cell,
even the best space-qualified cell, where the LBIC map is completely
uniform.

![](media/image8.png){width="3.0in" height="2.622916666666667in"}

Figure 4. LBIC map with shunt along grid finger.

**STRESS-INDUCED FEATURES**

LBIC measurements are particularly valuable to help explain changes
sometimes seen in CdTe solar cells that have been subjected to
elevated-temperature stress. The general observation is that when the
performance of a cell changes during such stress, the changes are
essentially never uniform over the entire cell. The result is an
increase in the non-uniformity seen in LBIC measurements. An example is
given in Fig. 5, which shows a low-resolution, whole-cell map before any
stress, after 8 hours, and after 8 days at a temperature of 100ºC under
illumination and short-circuit conditions. The average QE decreased by
about 2% over 8 days, which is reflected in the current-voltage (J-V)
curve, but there were small areas where the reduction was significantly
larger.

The major reductions seen in Fig. 5 are localized and had only a minor
effect on cell performance. In other cases, however, shunt-like changes
induced by stress are spread over a much larger area in the LBIC map,
reduce the apparent quantum efficiency a larger amount, and are
seriously detrimental to performance.

In many cases, the impact of elevated-temperature stress on both J-V and
LBIC data varies significantly with differences in cell fabrication. One
example reported earlier \[13\] was much larger non-uniformities
generated in CdTe cells made with insufficient copper in the back
contact. Another example, illustrated in histogram format in Fig. 6,
contrasts two CdTe cells that differed in another aspect of fabrication.
Both cells had very uniform QE initially. After exposure to 30 days of
elevated-temperature stress, the cell in the top panel with standard
back-contact processing showed only a modest decrease in average QE and
uniformity. However, the cell depicted in the bottom QE histogram, which
was made with part of the contact procedure omitted, showed much larger
reductions in both the magnitude and uniformity of its QE.

![](media/image9.png){width="3.25in" height="2.3958333333333335in"}

# ![](media/image10.png){width="3.2159722222222222in" height="2.4555555555555557in"}

# 

# 

![](media/image11.png){width="3.2555555555555555in"
height="2.390972222222222in"}

# 

Figure 5. LBIC for CdTe cell before stress, after 8 hours, and after 8
days.

# 

# WAVELENGTH AND BIAS VARIATIONS

The ability to vary the wavelength used for LBIC measurements is
particularly valuable if wavelength can be scanned through the absorber
band gap. One of our laser diodes was chosen so that its wavelength
could be varied from 830 to 860 nm, which spans the CdTe band gap, by
varying its temperature. An earlier report \[10\] showed that this
technique could be used with

![](media/image12.jpeg){width="2.859722222222222in" height="3.0in"}

![](media/image13.jpeg){width="2.813888888888889in" height="2.875in"}

Figure 6. Contrasting effect of stress on two CdTe cells made with and
without a key contacting step.

high-resolution LBIC to observe the band-gap variations due to spatial
variations in CdTe/CdS mixing.

Variations of LBIC maps as a function of cell bias has also proven
useful. In this case, improvements in the detection electronics were
required to adequately separate the ac LBIC signal from the dark dc
response of the cell. With care, LBIC data can now be taken at bias up
to the cell's open-circuit voltage.

An example of LBIC bias dependence is shown in Fig. 7. The top figure
shows the J-V curve from two CIGS cells made with different
concentrations of gallium \[14\]. The cell with the better-behaved J-V
curve had a uniform LBIC response at all biases. The magnitude of

![](media/image14.jpeg){width="3.0in" height="2.0368055555555555in"}

![](media/image15.png){width="3.029861111111111in"
height="2.2333333333333334in"}

![](media/image16.png){width="2.9555555555555557in"
height="2.013888888888889in"}

![](media/image17.png){width="2.901388888888889in" height="2.1875in"}

Figure 7. J-V curves of a reasonably well-behaved CIGS cell (dashed) and
a highly distorted one (solid). LBIC maps show bias dependence of the
latter.

the LBIC signal, however, did decrease when the J-V curve turned up
following the expected behavior of the apparent quantum efficiency
\[15\]. The mid-resolution LBIC response from the "distorted" J-V curve
(Fig. 7), however, showed a decrease in average magnitude at relatively
low bias. Furthermore, even at the relatively low 200-mV bias, LBIC
shows significant non-uniformity. The likely explanation is that the
large conduction-band offset responsible for the J-V kink is not uniform
in magnitude over the field of view.

**CONCLUSIONS**

LBIC has become a reliable tool for investigating a variety of
solar-cell non-uniformities. It has become much more practical in recent
years with improvements in translation stages, laser diodes, and
data-handling techniques. LBIC is particularly valuable to help explain
differences due to processing variations or changes induced by
elevated-temperature stress. It can be reliably combined with other
small-area investigation techniques to study the same area at different
labs. Finally, in our experience, poorer performing cells almost always
show larger LBIC variations indicative of poorer uniformity.

# ACKNOWLEDGMENTS

The authors gratefully acknowledge Dr. Jason Hiltner, who built much of
the current LBIC facility. They also thank Markus Gloeckler and Alan
Davies for their contributions to the LBIC facility. Funding for the
work described here was provided by the U.S. National Renewable Energy
Laboratory.

# REFERENCES

\[1\] J.D. Zook, R,B. Maciolik, and J.D. Heaps, "Effects of Grain
Boundaries in Polycrystalline Solar Cells," *Appl Phys. Lett.* **37**,
1980, pp 223-226.

\[2\] C.V. Hari Rao, H.E. Bates, and K.V. Ravi, "Electrical Effects of
SiC Inclusions in EFG Silicon Ribbon Solar Cells, *J. Appl. Phys.*
**47**, 1976, pp 2614-2620.

\[3\] C. Belouet, J. Hervo, R. Matres, N.T. Phuoc, and M. Pertus, "
Growth and Characterization of Polysilicon Layers Achieved by the
Ribbon-Against-Drop Process," *Proc. 13^th^ IEEE Photovoltaics
Specialists Conf.*, 1978, pp 131-136.

\[4\] J.E. Phillips, R.W. Birkmire, and P.G Lasswell, "Stability of
Thin-Film Cu~2~S-Bassed Solar Cells," *Proc. 16^th^ IEEE Photovoltaics
Specialists Conf.*, 1982, pp 719-722.

\[5\] S. Damaskinos, "An Improved Laser Scanning Technique for
Evaluation of Solar Cells: Application to CuInSe~2~/(Cd,Zn)S Devices,"
*Solar Cells* **23,** 1989, pp 151-156.

\[6\] R.J. Matson, K.A. Emery, I.L. Eisgruber, and L.L. Kazmerski, "The
Large-Scale Laser Scanner: Mill-Characterization o f Photovoltaic Cells
and Modules," *Proc. 12^th^ European PVSEC*, 1994, pp 1222-1225.

\[7\] W.D. Sawyer, "An Improved Method of Light-Beam-Induced Current
Characterization of Grain Boundaries," *J. Appl. Phys.* **59**, 1986, pp
2361-2368.

\[8\] S.A. Galloway, A.W. Brinkman, K. Durose, P.R. Wilshaw, and A.J.
Holland, "A Study of the Effects of Post-Deposition Treatment on
CdS/CdTe Thin-Film Solar Cells Using High-Resolution OBIC," *Appl. Phys.
Lett.* **68**,1996, pp 3725-3727.

\[9\] J.F. Hiltner and J.R. Sites, High-Resolution Laser Stepping
Measurements on Polycrystalline Solar Cells," *Proc. 16^th^ European
PVSEC*, 2000, pp 630-633.

\[10\] J.F. Hiltner and J.R. Sites, Micron-Resolution Photocurrent of
CdTe Solar Cells Using Multiple Wavelengths," *Proc. Mat. Res. Soc.*
**668**, 2001, pp H9.8.1-6.

\[11\] K. Durose. et. al., "Physical Characterization of Thin-Film Solar
Cells," *Prog. Photovolt: Res. Appl.* **12**, 2004, pp 177-217.

\[12\] J. Martin, C. Fernandez-Lorenzo, A. Poce-Fatou, and R. Alcantara,
"A Versatile Computer-Controlled High-Resolution LBIC System," *Prog.
Photovolt: Res. Appl.* **12**, 2004, pp 283-295.

\[13\] C.R. Corwine, A.O. Pudov, M. Gloeckler, S.H. Demtsu, and J.R.
Sites, "Copper Inclusion and Migration from the Back Contact in CdTe
Solar Cells, *Solar Energy Mat. Solar Cells* **82**, 2004, pp 481-489.

\[14\] A.O Pudov, F.S Hasoon, A. Kanevce, H.A. Al-Thani, and J.R. Sites,
"Secondary Barriers in CdS/

CuIn~1-x~Ga~x~Se~2~ Solar Cells," *J. Appl. Phys.*, in press.

\[15\] J.R. Sites, H. Tavakolian, and R. A. Sasala, "Analysis of
Apparent Quantum Efficiency," *Solar Cells* **29**, 1990, pp 39-48.