March 5, 2003

Ken Zweibel

National Center for Photovoltaics

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

RE: Quarterly Report VI (December 2002-February 2003)

Subcontract ADJ-1-30630-06

Dear Ken,

During the past quarter, we have achieved results in several areas as
described below. Additionally, Markus Gloeckler submitted his M.S.
thesis, and Tim Nagle finished his coursework M.S. program. Both of them
will receive their degrees in May, and both are continuing in the Ph.D.
program. Also in February, I wrote an LBIC section for a special issue
of the journal Progress in Photovoltaics that Ken Durose and Brian
McCandless are coordinating.

**Cell Modeling.** Our work on numerical simulation of CdTe and CIGS
cells was quite active during the past quarter. The modeling workshop I
organized adjacent to the CIS team meeting in January appears to have
been very successful. A copy of the summary report is attached. Alan
Fahrenbruch gave the lead-off talk on the value of numerical simulation,
and he also assembled a data base of the different software packages
being used for solar cell simulation. As a specific example, he used the
SCAPS software to show how cell parameters can be deduced from
comparison of capacitance calculations with measurements. Markus
Gloeckler also gave one of the talks, which described our explanation
for the failure of light/dark superposition often observed in CIGS
cells. That work will also be the basis of his poster presentation and
paper, co-authored by Caroline Jenkins and myself, for the MRS symposium
on thin-film solar cells in April.

Until recently, we have done most of our work with the AMPS and SCAPS
software. Both of these packages divide the solar cell into uniform
layers, then subdivide each layer into smaller increments for numerical
solution of the differential equations. This procedure, however, is
awkward for graded-band-gap solar cells, or other situations in which a
key parameter is continuously varying throughout a layer. We have
therefore turned to SimWindows, a package developed at the University of
Colorado, which allows parameters to be defined as a function of
position. With SimWindows Markus has done initial calculations of the
effect of band-gap grading on the performance of CIGS cells, and he has
demonstrated the progression of J-V curves with the thickness over which
the grading takes place. We are now planning what other calculations,
such as variation of carrier lifetime with band gap, would provide
insight into the design of higher band-gap cells based on alloys of CIS.

**CIGS Cells.** Alex Pudov is now spending one day a week at NREL
working with Miguel Contreras on measurement and analysis of
high-efficiency CIGS cells. He is also collaborating with Ragu
Bhattacharya and Falah Hasoon on the use of ZnS(O) instead of CdS for
the window layer. Future work will include the comparison of Zn- and
Cd-based windows as the absorber band gap is increased.

Pam Johnson's work on comparisons of selenized vs. three-stage cells and
CBD CdS vs. the Cd partial electrolyte is nearly complete. She will be
defending her Ph.D. thesis at the end of March. I am currently in
discussions with Kannan Ramanathan and Dale Tarrant to plan extensions
of this work. Also, at the CIS team meeting in January, Caroline Jenkins
and I reported on the differences we saw in J-V-T measurements of cells
completed at IEC with absorbers deposited by six different laboratories.

**CdTe Cells.** We have been heavily involved with elevated-temperature
stress measurements as part of two of the CdTe subteams. Samuel Demtsu
has completed two reports for the materials-chemistry subteam led by
First Solar. These compare the results of cells that were stressed at
First Solar for different periods of time and at different biases. With
the micro-nonuniformity subteam, we are stressing cells from Toledo and
South Florida in our lab. In this case, Samuel is tracking the full
progression of the J-V curves, and Tim Nagle is tracking changes in
performance uniformity.

Caroline Jenkins is also spending one day a week at NREL. Working with
Tim Gessert's group, she is modifying the surface of single-crystal CdTe
with small amounts of Cu and Cl so that a more reliable catalog of
photoluminescence and cathode-luminescence transitions in CdTe can be
established. Caroline is also our designated poster presenter at the
upcoming NCPV review meeting, where we will present our model of copper
addition and outdiffusion from the CdTe back contact. A copy of the
manuscript is attached.

Sincerely,

James R. Sites

Professor

Cc: NREL Subcontracts (Attn: Carolyn Lopez)

CSU Office of Sponsored Programs (Attn: Mary Selby)

Modeling Workshop

Golden, Colorado, January 31, 2003

Associated with the National CIS R&D Team Meeting

Summary

The primary purpose of the workshop was to allow researchers who have
been doing numerical device simulation of solar cells based on CuInSe~2~
(CIS) and related alloys to compare notes on their progress. Part of
this purpose was to better define the types of problems where numerical
modeling can be particularly helpful. A second purpose was to give a
flavor of numerical device simulation to CIS researchers not currently
involved.

The three-hour workshop was organized by Jim Sites of Colorado State
University and was attended by approximately 30 researchers. The
speakers were:

Alan Fahrenbruch (ALF, Inc) "A Practical Guide to Simulation"

Angus Rockett (Univ Illinois) "Modeling from Angus' Perspective"

Markus Gloeckler (Colorado State Univ) "Numerical Modeling of CIGS Solar
Cells:

> Explanation of Superposition Failure"

Sheng Li (Univ Florida) "Modeling of CGS/CIGS Tandem Solar Cells"

Don Morel (Univ South Florida) "Defect Based Simulations in CIGS Solar
Cells"

Copies of the viewgraphs presented are being sent to the CIS team and
are available on request from <sites@lamar.colstate.edu>. Also available
is a summary description of several device-simulation software packages.

There were several general issues raised in the discussion part of the
workshop. One was the adequacy of the models in current use. A
particular point of discussion was whether a one-dimensional model is
adequate. One school of thought is that there are aspects of device
modeling which are inherently 3-D, and in analogy to historical
developments in other areas, a proper treatment of these is likely to
make current work look quite primitive. It was also pointed out that 3-D
device software does exist, and in the context of typical research
expenses, is relatively inexpensive. Others, however, made the point
that with thin-film devices, there is often very little lateral
variation on the scale of the dimensions likely to be used in a device
simulation. It was acknowledged that grain boundaries could well be an
exception, which would suggest a 2-D simulation with cylindrical
coordinates. In any case, the collective experience of the workshop
attendees with 3-D device modeling was essentially zero, and there was
agreement that, at a minimum, 3-D software should be evaluated. An
obvious line of exploration is to run the same problem on both 1-D and
3-D software.

A second general issue was the need to validate the results of numerical
device simulation. There was general agreement that fitting an
experimental current-voltage curve is insufficient. There was also
agreement that fitting an expanded data set, including temperature and
intensity variations in the current-voltage curve and/or the voltage
dependence of quantum efficiency, gives considerably increased
confidence. There is no real substitute, however, for the basic
scientific process of predicting new results and then testing them
experimentally. One of the presentations did that in a small way, and
there exist a number of specific and implicit predictions from
simulations that would seem ripe for experimental testing.

A third discussion topic was how to best use numerical simulation, and
implicitly, an understanding of its limitations. There was general
agreement that device simulation is best used to see patterns in the
response to changes of physical parameters, to evaluate new ideas for
solar cells and to assist the researcher's physical insight, rather than
to produce precise numerical fits. One specific suggestion was to
routinely look at the calculated band diagram and quasi-Fermi levels at
solar-cell operating conditions as well as at zero bias.

A fourth topic, which the workshop did not get to in depth, was
procedures for choosing modeling parameters. Many modelers have had
difficulty assigning values for defect cross-sections. There was also
recognition that the conduction-band offset between window and the CIGS
absorber is both critical and uncertain, which suggests that it should
be categorized as a fitting parameter. Some additional strategies are
seen in the viewgraphs, and more in-depth discussion of parameter
selection is a clear candidate for future workshops.

Informal feedback suggested that the modeling workshop was a valuable
experience for CIGS researchers, and several suggested that it was 5-10
years overdue. There are no specific plans for follow-on meetings, but
it seems probable that there will be future get-togethers. Meanwhile, it
is highly likely that the challenges raised at the workshop will enhance
the quality of the numerical work, the amount of experimental
verification, and the level of communication among those doing
simulations on CIGS solar cells.