**DEVICE PHYSICS OF THIN-FILM**

**POLYCRYSTALLINE CELLS AND MODULES**

### Annual Report

Phase I

September 2001 - August 2002

by

James R. Sites

Department of Physics

Colorado State University

Fort Collins, Colorado 80523

Work performed under Subcontract ADJ-1-30630-06

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

# SUMMARY

Work has been performed at Colorado State University on basic
measurements of CdTe and CI(G)S solar cells fabricated at a number of
collaborating laboratories. The goal has been to quantitatively deduce
the loss mechanisms in a number of these cells, and to make appropriate
comparisons that illuminate where progress is being made. Special
emphasis was given to the differences in CdTe cells, both as-deposited
and after elevated-temperature stress, that result from variations in
the amount of back-contact copper.

The second area of study has been further development of small-spot
measurement of micro-nonuniforities, including those that develop during
temperature stress. The facility focuses a laser beam onto a solar cell
with 1-m beam size, 1-m resolution and repeatability, and one-sun
intensity.

The third task has been the determination of defect-state densities in
CIGS cells by low-temperature capacitance measurements. Emphasis has
been given to the comparisons between CIGS absorbers fabricated by
different techniques and among different strategies for the buffer
between absorber and transparent contact.

The final task area has been numerical simulations of CdTe and CI(G)S
cells. Replication of experimental data has been successful in both
cases. The CdTe CIGS studies have illuminated the effects of thickness
variations in the CdS and CdTe layers, while more recent CIGS studies
have shown the impact of the CdS/CIGS conduction band offset on
light/dark superposition.

**TABLE OF CONTENTS**

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . 2

FIGURES AND TABLES . . . . . . . . . . . . . . . . . . . . 4

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 5

WHOLE-CELL LOSS ANALYSIS . . . . . . . . . . . . . . . . 6

CdTe Back-Contacts and Stability . . . . . . . . . . . . . . . . 6

CdTe Carrier Density. . . . . . . . . . . . . . . . . . . . . 9

Other Whole-Cell Analysis . . . . . . . . . . . . . . . . . . 10

SMALL-SPOT STUDIES . . . . . . . . . . . . . . . . . . . . 12

Calibration and Format . . . . . . . . . . . . . . . . . . . . 12

Bias Dependence . . . . . . . . . . . . . . . . . . . . . . 13

DEFECT STUDIES . . . . . . . . . . . . . . . . . . . . . . 14

CIGS Trapping States . . . . . . . . . . . . . . . . . . . . 14

NUMERICAL MODELING . . . . . . . . . . . . . . . . . . . 19

> CdTe Back Barrier . . . . . . . . . . . . . . . . . . . . . 19
>
> CdS Thickness . . . . . . . . . . . . . . . . . . . . . . 20
>
> CIGS Band Offsets . . . . . . . . . . . . . . . . . . . . . 21

PHASE II PLANS . . . . . . . . . . . . . . . . . . . . . . 25

COMMUNICATIONS . . . . . . . . . . . . . . . . . . . . . 26

Publications . . . . . . . . . . . . . . . . . . . . . . . 26

Presentations . . . . . . . . . . . . . . . . . . . . . . . 26

Degrees . . . . . . . . . . . . . . . . . . . . . . . . . 27

Additional Reports . . . . . . . . . . . . . . . . . . . . . 27

**FIGURES**

Figure 1. CdTe J-V, QE, and C-V when back-contact copper is varied . . .
. . 6

Figure 2. CdTe elevated-stress variations with copper amount . . . . . .
. . 7

Figure 3. Variation in micro-nonuniformity with stress and copper . . .
. . . 8

Figure 4. Capacitance analysis of a typical CdTe cell . . . . . . . . .
. . 9

Figure 5. Low-resolution QE maps and distributions . . . . . . . . . . .
12

Figure 6. High-resolution bias dependence of CdTe defect . . . . . . . .
. 13

Figure 7. J-V comparison of Cd PE and CdS CIGS cells . . . . . . . . . .
14

Figure 8. QE comparison of same cells . . . . . . . . . . . . . . . 15

Figure 9. Capacitance comparison of same cells . . . . . . . . . . . .
16

Figure 10. C-f at varying temperatures for Cd PE and CdS cells . . . . .
. . 17

Figure 11. Resulting activation-energy plot . . . . . . . . . . . . . .
18

Figure 12. Band picture suggested by C-f-T measurements . . . . . . . .
. 18

Figure 13. Representative CdTe J-V curves . . . . . . . . . . . . . . 19

Figure 14. Effect of CdS thickness on J~L~ . . . . . . . . . . . . . . .
21

Figure 15. CIGS J-V fits illustrating non-superposition . . . . . . . .
. . 22

Figure 16. Effect of E~C~ on CIGS diode barrier . . . . . . . . . . . .
23

**TABLES**

Table 1. CdTe hole density from many cells . . . . . . . . . . . . . .
10

**INTRODUCTION**

The objectives of the Colorado State University program have been to (1)
quantitatively separate individual performance-loss mechanisms in CdTe
and CI(G)S solar cells using existing experimental and analytical
techniques, (2) expand the tool set for such measurement and analysis
and (3) suggest fabrication approaches or modifications to minimize the
losses. Much of the work performed during the past year is described in
this report, or in the publications and referenced in the final section,
but in many cases more detailed information can be found on our website:
[www.colostate.edu/orgs/pvlab](http://www.colostate.edu/orgs/pvlab).

The experimental and analytical work in this report has largely been
done by a dedicated group of graduate students. Pamela Johnson has
worked primarily on comparisons of defects in CIGS cells made with
different buffer strategies. Alex Pudov has done comparitive analysis of
the highest efficiency CIGS cells and has coordinated a study of CdTe
cells made with varying amounts of copper in the back contact. Markus
Gloeckler has refined the micro-nonuniformity system and has
successfully explained the effect of conduction band offset on
light/dark non-superposition in CIGS cells. Caroline Jenkins has
completed her M.S. degree and has analyzed a large number of CdTe and
CIGS cells. Samuel Demtsu has coordinated the installation of a solar
simulator and performing some of the CdTe measurements. Tim Nagle has
done much of the recent micro-nonuniformity work and has significantly
upgraded the software. In addition, Affiliate Prof. Alan Fahrenbruch has
continued the development of AMPS modeling of CdTe cells.

Prof. Sites\' group has actively participated in the NREL-sponsored
National CdTe and CIS R&D Teams. It has had productive collaborations
with Prof. Sampath\'s group at Colorado State, as well as with
researchers at Aoyama Gakuin University, Colorado School of Mines, First
Solar Inc., Global Solar Energy, Florida Solar Energy Center, Institute
of Energy Conversion, Moldova State University, National Renewable
Energy Laboratory, Shell Solar Industries, University of Illinois,
University of Oregon, University of South Florida, University of Toledo,
and Washington State University.

**WHOLE-CELL LOSS ANALYSIS**

## CdTe back-contacts and stability. Alex Pudov, Samuel Demtsu, and Markus Gloeckler tracked several properties of CdTe cells made by Prof. Sampath's group with varying amounts of back-contact copper. They examined how the amount of copper affected both the as-deposited cells and their vulnerability to temperature stress. Fig 1. (from Publication 5) shows the variation in the J-V, QE, and C^-2^-V curves with copper amount. 

![](media/image1.wmf){width="3.1381889763779527in"
height="2.158267716535433in"}![](media/image2.wmf){width="2.9925196850393703in"
height="2.127952755905512in"}![](media/image3.wmf){width="0.8118110236220473in"
height="0.5692913385826772in"}![](media/image4.wmf){width="2.9653543307086614in"
height="2.127952755905512in"}

## 

## 

## Figure 1. J-V, QE, and C-V measurements for CdTe cells with five back-contact copper concentrations. "1 min Cu" corresponds to the equivalent of 10 Å.

The higher copper cells shown in Fig. 1 have efficiencies just over 11%,
but those with less copper have lower efficiencies. Quantum efficiency
is nearly identical for all the cells, and capacitance curves imply an
increasing-thickness transition region between the CdTe and the metallic
contact as more copper is used. The temperature dependence of the J-V
curves shows more "rollover" at reduced temperatures as the copper
content is increased.

Another significant cell difference with varying copper content is shown
below in Fig. 2. The effect of elevated-temperature stress is quite
modest when sufficient copper is used, but quite severe with smaller
amounts.

![](media/image5.wmf){width="3.2838582677165356in"
height="5.374409448818898in"}

**Figure 2. Increasing J-V losses with temperature stress (100ºC at
short-circuit).**

There were also major spatial nonuniformities in the photocurrent from
the cells with low copper content. Fig. 3 shows 500x500-μm maps of the
photocurrent, expressed as quantum efficiency, before and after one-week
stress for three of the cells. The performance deterioration shown in
Fig. 2, in addition to being much greater in the low-copper cells,
clearly evolves more quickly in some regions of the cells than others.

![](media/image6.png){width="2.8909448818897636in"
height="2.048031496062992in"}![](media/image7.png){width="2.998031496062992in"
height="2.106299212598425in"}![](media/image8.png){width="2.998031496062992in"
height="2.0291338582677168in"}

**Figure 3. QE maps of three copper-level cells before and after
stress.**

**CdTe Carrier Density.** A survey was made of a large number of CdTe
cells analyzed at Colorado State by capacitance measurements since 1989
(Presentation 4). In many cases the C^-2^ vs V data has two distinct
regions as shown in the left part of Fig. 4. The relatively flat region
in reverse bias corresponds to the CdTe being nearly fully depleted, and
the capacitance essentially measures its thickness. Near zero and into
forward bias, the depletion edge moves into the bulk CdTe and the
inverse slope is proportional to its carrier density. The same data is
shown to the right in the hole density vs distance from the junction
format. The density is in the low 10^14^ range in the bulk CdTe, but
increases rapidly as the depletion edge approaches the back contact. The
CdTe thickness deduced, 3.3 μm in this case, is generally close to that
measured directly.

![](media/image12.wmf){width="2.997916666666667in"
height="2.7006944444444443in"}![](media/image13.wmf){width="2.951388888888889in"
height="2.839583333333333in"}

**Figure 4. Capacitance analysis of a typical CdTe cell.**

In many cases the C^-2^-V data corresponding to the left part of Fig. 4
shows only one of the two regions. If the CdTe is thicker, one sees only
the steep region, and if very thin, only the flat region. Clearly it is
important to correctly identify which region is seen, and this can be
done by noting the extrapolated voltage intercept of the measured
region. One can also estimate the CdTe density when only the flat region
is present in the data by forcing an intercept at a physically plausible
built-in potential of perhaps 1 volt.

Table 1 lists the CdTe hole densities and thicknesses deduced from cells
from many different labs over several years. The estimated accuracy of
the densities is about a factor of two. It is notable that all the
densities are between 10^14^ and 10^15^ cm^-3^. As far as I know, no one
has strong evidence for higher densities, and hence I would conclude
that there is a fundamental limitation to the polycrystalline CdTe hole
density that may preclude significantly higher cell voltages.

**Table 1. CdTe Hole Density from C-V. \"Typical\" Cells.**

**CdTe Electronic**

**Manufacturer Year Hole Density CdTe Thickness V~OC~**

Ametek\* 1989 4 10^14^ cm^-3^ 1.5 m 720 mV

Photon Energy 1991 8 10^14^ 2 790

Univ South Florida 1991 4 10^14^ 3 850

Solar Cells, Inc 1996 1 10^14^ 3.3 825

Golden Photon 1996 1 10^14^ 3 810

Univ South Florida 1996 1 10^14^ 5 840

Inst Energy Conv 1996 4 10^14^ 3 800

School of Mines 1996 4 10^14^ 2 790

NREL 1996 2 10^14^ 4 820

Colorado State\* 2001 4 10^14^ 1.8 790

First Solar 2001 4 10^14^ 3 830

Univ South Florida 2001 4 10^14^ 4 850

Univ Toledo\* 2001 2 10^14^ 2 810

\*Hole density determination required extrapolation for the thinner CdTe
cells.

**Other whole cell analysis.** Additional CdTe whole-cell projects have
included J-V-T characterization (7/02 Report by Demtsu) of a large
number of First Solar cells as part of the Defect Chemistry working
group of the CdTe national team. We have also been involved in two
international CdTe collaborations. One with Moldova State University has
been characterization of cells fabricated there by CSS. Results have
been fair, with the primary cell difficulty being the back-contact
barrier. The second project has been with EcoSolar in India (7/02 Report
by Jenkins) where we also see a back-contact problem, but otherwise
decent cells.

In other studies involving CIGS cells, we have published a paper
(Publication 2) with Tokio Nakada of Aoyama Gakuin University (AGU). The
paper was a comparison among the ZnS/CIGS cells made at AGU, the highest
efficiency NREL cells, and the best single-crystal silicon cells. We
have also done characterization for Unisun (12/01 Report by Jenkins) and
with the ZIS-window layer cells fabricated at EPV (5/02 Report by
Jenkins).

# SMALL-SPOT STUDIES

Our small-spot photocurrent measurement apparatus has proven to be
valuable in a number of applications. One of them was illustrated in the
previous section describing the differences in CdTe cells with amount of
copper used in the back contact. Additional results produced by Tim
Nagle and Markus Gloeckler are described in this section.

**Calibration and Format.** One point not made in the previous section
is that the QE measured by the small-spot apparatus agrees quite well
with the whole-cell QE from conventional measurements. Similarly,
measurements made on the same cell at different times, as illustrated in
Fig. 5, generally maintain their calibration within about 1%. Also

No Stress 8 hrs Stress 8 days Stress

![](media/image14.png){width="2.25in" height="1.9854166666666666in"}

![](media/image16.png){width="3.334722222222222in"
height="2.4555555555555557in"}

**Figure 5. Evolution of a QE map with elevated-temperature stress.**

shown in Fig. 5 is the QE distribution expressed as the number of
reduced areas (defects) of different sizes and expressed as a histogram.

![](media/image19.png){width="3.2153543307086614in"
height="2.3901574803149606in"}![](media/image20.png){width="3.246456692913386in"
height="2.37992125984252in"}![](media/image21.png){width="3.2153543307086614in"
height="2.161811023622047in"}![](media/image22.png){width="3.2255905511811025in"
height="2.161811023622047in"}**Bias Dependence.** As with conventional
QE measurements, the small-spot maps are generally done at zero-bias.
Fig. 6, however, shows two lower-response areas at a series of biases.
The signature of one such area varies greatly with bias, and
progressively more of the surrounding area is reduced in photocurrent.
This signature is clearly that of a shunt, though our attempts at a
detailed fit have not yet been successful. The smaller lower-response
area to the right does not change significantly with bias and is thus
due to a high resistance area, or possibly an optical obstacle.

**Figure 6. Bias dependence of two reduced-response areas. Field is
100x100 μm.**

# DEFECT STUDIES

**CIGS trapping states.** The primary defect-studies project has been
carried out by Pam Johnson in collaboration with NREL, Siemens (now
Shell) Solar Industries, and the University of Oregon (see Publications
1, 4, and 7). Direct comparisons were made between cells completed on
the same absorber using traditional CBD CdS and a partial electrolyte
solution (Cd PE) without the sulfur. The latter leads to a modified CIGS
surface, but no CdS layer. Both NREL and Siemens (Shell) absorbers were
used. Fig. 7 contrasts the J-V curves, in both linear and logarithmic
format, of the two types of cells on NREL CIGS absorbers. The PE cell
clearly gains in current, but loses in voltage.

![](media/image23.wmf){width="3.129166666666667in"
height="3.048611111111111in"}

![](media/image24.wmf){width="3.832638888888889in"
height="2.5701388888888888in"}

**Figure 7. J-V comparison, linear and log formats, between CdS and
Cd-PE cells.**

The efficiencies in Fig. 7 are approximately 14% for the Cd PE cell and
15% for the CdS cell. The Cd PE cell has the larger current, but the
smaller voltage. The current difference is primarily due to the
absorption of high-energy photons by the CdS layer and can best be seen
in Fig. 8, which compares the quantum efficiencies. The QE comparison
also shows that the Cd PE cell is also slightly superior in current at
all wavelengths, has a stronger interference pattern (shown more clearly
in reflection curves), and has a red fall-off indicating a slightly
larger effective band gap.

![](media/image25.wmf){width="4.333333333333333in"
height="3.6493055555555554in"}

**Figure 8. Quantum efficiency comparison of cells from Fig. 7.**

The voltage contrast, best illustrated by the logarithmic plot in Fig.
7, shows that not only is V~OC~ reduced for the Cd PE cell, but the
difference becomes larger at maximum power, because the Cd PE cell has a
somewhat larger diode quality factor A (2.0 vs 1.7). About 20 mV of the
80-mV difference at maximum power is due to the band-gap difference, but
the remaining 60 mV represents a weaker junction, most likely the result
of increased carrier recombination.

The junction difference implied by the J-V curves above is also
reflected in capacitance measurements. Fig. 9 shows the C-V contrast
between the two cells and the resulting contrast in carrier densities.
After a frequency range was identified where dispersion was minimal, 100
kHz was selected for C-V. The inset shows C^-2^ vs V, and the primary
plot depicts CIGS hole density vs distance from the junction, as
extracted from the C^-2^-V data for the two cells. The Cd PE treatment
led to a lower carrier density than the standard CdS cell completion.

![](media/image26.wmf){width="4.129861111111111in" height="3.03125in"}

**Figure 9. Capacitance-voltage contrast (inset) and resulting carrier
densities.**

At reduced temperatures, capacitance-frequency data can be used to
examine trapping levels more directly. The absorbers for these
measurements were deposited at Siemens (Shell), but the cells were
completed with CdS layers and with Cd PE treatment using the same
equipment as with the NREL absorbers. The light J-V and the QE curves
were qualitatively similar to Fig. 7 and 8, but the efficiencies of the
cells were about 3% lower in each case.

The low-temperature measurements were performed at the University of
Oregon with the assistance of David Cohen and Jennifer Heath.
Capacitance vs frequency for the two types of cell is shown in Fig. 10
over a broad range of temperature. The basic feature of the curves is a
transition between the low-frequency capacitance, which includes
carriers moving in and out of trapping states, and the high-frequency
capacitance, which is only that due to the depletion width since it
exceeds the rate at which traps can be populated and depopulated. As
temperature is lowered, the transition occurs at lower frequencies,
since the population/depopulation rate is slower, and the corresponding
response times increase from the s range to the ms range. The primary
contrast for the two types of cell was that at the same temperature, the
Cd PE cells exhibited a systematically higher transition frequency for
trapping carriers.

![](media/image27.wmf){width="3.782638888888889in"
height="3.1951388888888888in"}![](media/image28.wmf){width="3.8048611111111112in"
height="3.4902777777777776in"}

**Figure 10. Capacitance vs frequency for Cd PE and CdS cells.**

If the transition from low- to high-frequency capacitance is due to a
single trapping level, it can be used to deduce an activation energy
E~a~ for that level. Fig. 11 shows the contrast, which suggests that the
primary Cd PE trapping level differs from that in CdS cells.

![](media/image29.wmf){width="3.536111111111111in"
height="3.4270833333333335in"}

**Figure 11. Temperature-scaled transition frequencies and resulting
E~a~.**

The two levels from Fig. 11 are shown on the band diagram in Fig. 12.
The horizontal scale represents the region of spatial accessibility, and
there is the possibility that the dominant level may change closer to
the actual junction.

![](media/image30.wmf){width="3.7506944444444446in"
height="2.8006944444444444in"}

**Figure 12. Band-diagram representation of trapping levels deduced from
C-f-T.**

**NUMERICAL MODELING**

**CdTe back barrier.** Alan Fahrenbruch has used the Penn State
developed AMPS software, as well as SCAPS from Europe, to continue
modeling the different J-V scenarios resulting from the height of the
CdTe back-contact barrier and its interrelation with the thickness and
carrier density of the CdTe absorber layer. Fig. 13 contrasts the
different J-V signatures expected under different combinations of those
three parameters. All other parameters were held constant, and the
calculated curves are quite consistent with what is seen experimentally.

**Figure 13. Effect of barrier height, CdTe thickness, and hole density
on J-V.**

The squares (open for light, filled for dark) represent the preferred
situation where the back barrier is relatively small and the carrier
(hole) density in the CdTe is near the top of its experimentally
observed range. There is good light/dark superposition in this case and
the largest efficiency. With a reduced carrier density, but retaining
the small barrier height, the curves (open and filled circles) retain
the clean diode shape and good superposition, but are shifted to lower
voltages as expected since the Fermi level is now futher from the
valence band.

If the back-barrier height is increased enough to be significant, as
seen in some cells as fabricated and others after elevated-temperature
stress, there are two effects can be seen in both the modeling and in
experimental results. If the CdTe carrier density is high enough that
the CdTe is not fully depleted, the "rollover" effect is present and is
depicted in Fig. 13 by open and filled diamonds. Current is severely
limited above V~OC~, and the curve is somewhat washed out in the power
quadrant leading to a reduced fill factor. In circuit terms, the back
contact is a secondary diode with polarization opposite that of the
primary photodiode.

Finally in Fig. 13, the open and closed triangles represent the
situation where there is both (1) a large contact barrier and (2) a
carrier density sufficiently small, combined with a small CdTe thickness
(2 m in this case), that the depletion of the primary diode overlaps
that of the back-contact diode. The consequence of the overlap is that
the flat-band voltage, and hence V~OC~, is reduced. Ironically, the
"rollover" effect is much less pronounced, because the depletion overlap
effectively reduces the height of the back-contact barrier.

**CdS Thickness.** A long-standing mystery about both CdTe and CIGS
solar cells is the lack of apparent collection from the photons that are
absorbed in the CdS layer. Alan Fahrenbruch (Publication 6) has now
calculated how much such collection would be expected as a function of
CdS thickness. He varied key parameters (CdS carrier density,
recombination-center density, surface recombination velocity, and the
CdS/contact band offset) over experimentally plausible values. In
general the model predicted a modest amount of CdS collection.
Furthermore, it was observed that the QE signature with such collection
is difficult to distinguish from that of a slightly thinner CdS layer
with no collection. Hence, the assumption that CdS is "dead" with
respect to collection is called into question.

The open circles in Fig. 14 show photocurrent vs CdS thickness
calculated with physically reasonable parameters for a CdTe cell. A
similar figure can be generated for CIS or CIGS cells. The open
triangles are the photocurrent under the assumption that the CdS layer
is indeed dead. The difference between these two sets of points is the
photocurrent from the CdS, depicted with open squares. This photocurrent
first increases with thickness as more absorption takes place, but then
decreases as the probability of recombination becomes greater. Changes
in the parameters listed above lead to differences in the possible
current from CdS collection. In particular, a larger band offset between
CdS and the front contact significantly decreases the CdS collection and
pushes the circle points down towards the triangles on Fig. 14.

**Figure 14. Effect of CdS thickness on J~L~.**

**CIGS Band Offsets.** Another long-standing mystery is why some CIS and
CIGS cells show very good superposition between light and dark J-V
curves, while others have a pronounced voltage shift. When superposition
fails, it does so in the direction of the dark curve predicting a larger
V~OC~ than that realized by the light curve. It is not uncommon in fact
for the V~OC~ predicted by the dark curve to be larger than physically
plausible.

Fig. 15 shows a set of light and dark J-V data from a good-quality CIGS
cell made at the Institute of Energy Conversion. Three different
temperatures are shown, and apart from the lack of light/dark
superposition, the curves are well behaved. The temperature dependence
of the light curves, as expected, is about --1.9 mV/K. The temperature
dependence of the dark curves, however, is --3.3 mV/K, which means that
not only is the implied V~OC~ implausible, but it becomes more
implausible as temperature is reduced.

![](media/image31.wmf)

# Figure 15. CIGS data (points) at three temperatures (298 K, left, 268 K, and 238 K) and numerical fits (lines) that illustrate failure of light/dark superposition.

The physical model that Markus Gloeckler, also using the AMPS software,
has used to successfully fit the data points in Fig. 15 with the solid
curves is the combination of a CdS/CIGS conduction band offset,
photogeneration of carriers in the CdS, and significant trap densities.
Other parameters, such as the TCO/CdS band offset and the specific
carrier and trap densities, affect the results, but are not central to
the superposition failure.

Fig. 16 shows the calculated conduction band under different conditions
for the cell that corresponds to the J-V curves above. The top curves at
room temperature illustrate four conditions: zero bias light and dark,
and +0.6 V bias light and dark; the bottom curves are at 238 K. The
CdS/CIGS offset is taken to be +0.4 eV, consistent with +0.3 predicted
by theory. At zero bias, the CdS part of the band forms a sharp
triangular peak, or barrier. Photogeneration by the light, which follows
from the parameters used, transfers more of the depletion to the CIGS
and pushes the CdS barrier to lower energy. For small positive voltages,
the barrier has little or no effect on carrier transport, but near
V~OC~, the barrier does start to impede current flow. Because of the
photogeneration, however, it does so at a lower voltage in the dark that
it does in the light. Hence the dark J-V curve experiences

![](media/image32.wmf)

![](media/image33.wmf)

Figure 16. Calculated conduction band of (left to right) TCO/CdS/CIGS
cell. Top is at 298 K; bottom at 238 K. Two biases, light and dark shown
in both cases.

current limitation while the light one does not. Much the same thing
happens at lower temperature, except that the light/dark difference of
the CdS barrier's peak energy is greater, and hence the voltage shift of
the dark J-V curve is greater. Since the Fig. 15 curves are quite
sensitive to the peak energy, some sample-to-sample variation is both
expected and observed.

The assumption of photogeneration in the CdS has been tested
experimentally with a filter that blocks light with energy above the CdS
band gap and hence precludes CdS photogeneration. The result, which is
predicted by the model, was a reduction in both the negative current
below V~OC~ and the positive current above V~OC~. The extreme case of
J-V distortion under red light has been labeled "the red kink" in the
literature and can be reproduced numerically by a CdS barrier with
larger peak energy.

#  

# PHASE II PLANS

Much of the work planned for Phase II will follow naturally from the
work summarized above. It will involve continued collaboration with our
team partners, and it will continue to focus on the basic information
needed to assist commercialization of thin-film photovoltaics.

Whole-cell analysis will give major emphasis to stability issues in CdTe
cells. One issue that will receive considerable attention is whether
reduced performance can be attributed solely to reduction in
back-contact copper. In collaboration with NREL and elsewhere, we plan
to utilize a combination of electrical, luminescence, and compositional
studies to better track what is actually changing during
elevated-temperature stress.

Photocurrent micro-nonuniformity studies will emphasize the additional
information available from variations in light intensity and electrical
bias, and they will continue to assist in the documentation of
stress-induced changes. A new initiative will be to perform nonunifomity
measurements on the same small area of a cell by different techniques at
different laboratories.

Defect studies in CIGS will expand the types of buffer layers that Pam
Johnson has examined. Currently, Alex Pudov is compiling a comparison of
the reported results from a large number of buffer strategies.
Additionally, the low-temperature apparatus under construction in our
laboratory should be ready shortly. A new initiative with CdTe is for
Caroline Jenkins to work part time at NREL in the analysis of
transitions seen in luminescence and to help plan new experiments to
better utilize such measurements.

Our numerical modeling of CdTe cells by Alan Fahrenbruch and CIGS cells
by Markus Gloeckler has matured considerably during the past year. In
both cases, we will refine the match to experimental results,
particularly those made over a range of temperatures and light
intensities. We are also planning a mini-workshop on numerical modeling
in conjunction with the next CIS team meeting.

**COMMUNCATIONS**

**Publications**

1\. P.K. Johnson, A.O. Pudov, J.R. Sites, K. Ramanathan, F.S. Hasoon,
and D.E. Tarrant, "Comparison of Trap States Bewteen CIGSS/CdS/ZnO and
Cd PE CIGSS/ZnO Cells," European PVSEC **17**, 1035 (2001).

2\. A.O. Pudov, J.R. Sites, and T. Nakada, "Performance and Loss
Analyses of High-Efficiency Chemical Bath Deposition
(CBD)-ZnS/Cu(In~1-x~Ga~x~)Se~2~ Thin-Film Solar Cells," Jpn. J. Appl.
Phys. **41**, L672 (2002).

3\. J.R. Sites, "Quantification of Losses in Thin-Film Polycrystalline
Solar Cells," Solar Energy Mat. and Solar Cells **74**, xxx (2002).

4\. P.K. Johnson, A.O Pudov, J.R. Sites, K. Ramanathan, F.S. Hasoon, and
D. Tarrant, "Interface Properties of CIGS(S)/Buffer Layers Formed by the
Cd-Partial Electrolyte Process," IEEE PVSC **29**, xxx (2002).

5\. A.O. Pudov, M. Gloeckler, S.H. Demtsu, J.R. Sites, K.L. Barth, R.A.
Enzenroth, and W.S. Sampath, " Effect of Back-Contact Copper
Concentration on CdTe Cell Operation," IEEE PVSC **29**, xxx (2002).

6\. A.L. Fahrenbruch, "Comparison of Experimental Data and AMPS Modeling
of the Effects of CdS Layer Thickness on the CdS/CdTe Solar Cell," IEEE
PVSC **29**, xxx (2002).

7\. K. Ramanathan, F.S. Hasoon, S. Smith, H. Al-thani, J. Allleman, H.S.
Ullal, J. Keane, P.K. Johnson, and J.R. Sites, "Properties of Cd and Zn
Partial Electrolyte Treated CIGS Solar Cells," IEEE PVSC **29**, xxx
(2002).

8\. B.E. McCandless and J.R. Sites, "Cadmium Telluride Solar Cells,"
Chapter 15 in *Handbook of Photovoltaics,* A. Luque and S. Hegedus, Ed.
(Wiley, 2002).

## Presentations

1\. J.R. Sites, "Solar Electricity," Colorado State Physics Colloquium,
September 2001.

2\. P.K. Johnson, "Trap State Comparison," Munich, October 2001.

3\. J.R. Sites, "Solar Electricity," Broomfield Rotary Club, February
2002.

4\. J.R. Sites, "Capacitance Basics," CdTe Team Meeting, Cocoa FL, March
2002.

5\. A.O. Pudov, "Variations in Back-Contact Copper," CdTe Team Meeting,
Cocoa FL, March 2002.

6\. A.L. Fahrenbruch, "Modeling CdTe Cells," CdTe Team Meeting, Cocoa
FL, March 2002.

7\. M. Goleckler, "Micro-nonuniformities with LBIC," CdTe Team Meeting,
Cocoa FL, March 2002.

8\. P.K. Johnson, "CdS vs Cd-PE,\" CIS Team Meeting, New Orleans, May
2002.

## Graduate Degree

Caroline Jenkins (M.S., May 2002) Coursework/Research Project

## Additional Reports

Caroline Jenkins Unisun Characteriztion December 2001

Markus Gloeckler FSEC Micro-nonuniformities March 2002

Caroline Jenkins EPV ZIS/CIS Characterization May 2002

Samuel Demtsu First Solar CdTe Charaterization July 2002

Caroline Jenkins EcoSolar CdTe Characterization July 2002