June 9, 2003

Ken Zweibel

National Center for Photovoltaics

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

RE: Quarterly Report VII (March - May 2003)

Subcontract ADJ-1-30630-06

Dear Ken,

During the past quarter, we have achieved results in several areas as
described below. In addition, Pamela Johnson successfully defended her
PhD thesis, "The Effect of Trapping Defects on CIGS Solar-Cell
Performance." She will spend June and July preparing part of that work
for journal publication. Two students, Alan Davies and Jun Pan, who have
just completed their first year of coursework, have joined the research
group.

**Cell Modeling.** We have had two formal presentations of our work on
numerical simulation. The MRS paper in April, "Explanation of Light/Dark
Superposition Failure in CIGS Solar Cells," identified the critical
parameters for superposition failure in this case and used this
framework for a careful analysis of experimental J-V-T data. This paper,
authored by Markus Gloeckler, Caroline Jenkins, and myself, is attached.
The second presentation, "Numerical Modeling of CIGS and CdTe Solar
Cells: Setting the Baseline," was made at the World Conference on
Photovoltaic Energy Conversion in Japan. This second paper, authored by
Markus Gloeckler, Alan Fahrenbruch, and myself, was designed to provide
a starting point that allows straightforward paths to the simulation of
various features seen in real cells. A copy of this paper is also
attached.

We have identified several specific modeling issues for focus in the
next few months. Most involve secondary barriers in CIGS and CdTe cells,
and our goal is to produce a general understanding of the effects of
such barriers. For both materials, there has been considerable recent
work in Europe, so it is very timely to try to integrate the various
conclusions that have been developed. In the case of CIGS, the important
secondary barrier is that due to the conduction band offset, and two
specific issues are (1) how gallium and sulfur variations affect the
offset and cell performance, and (2) whether the typical indium-rich
surface layer has a significant impact on the band structure and cell
performance. For CdTe, there can be secondary-barrier effects form the
back contact that Alan Fahrenbruch has been pursuing, but there are also
front junction effects that have a major impact on the apparent quantum
efficiency, and there is a question of how much the CdS/CdTe mixing
affects the band diagram and performance.

**CIGS Cells.** Alex Pudov assisted in the analysis of the
record-setting non-CdS cells made jointly by NREL and Aoyama Gakuin
University. A copy of the paper presented at WCPEC-3,
"ZnO/ZnS(O,OH)/Cu(In,Ga)Se~2~/Mo Solar Cell with 18.6% Efficiency," is
attached. Work on the performance and stability of such cells is
continuing. Mr. Pudov has also analyzed a set of CIGS cells made by
Falah Hasoon and Hamda Al-Thani at NREL with gallium concentration
ranging from 0 to 100% (6/4/03 report to Hasoon), and he has begun J-V-T
measurements on those cells. Additionally, he has made extensive
measurements on Zn(S,O)/CIGS cells made at NREL by Raghu Bhattacharya.
As deposited, these cells often show the "red-kink" effect under white
light, which makes sense, because the carrier density of the wider-gap
ZnS is much less affected than CdS by short-wavelength photons.

Pam Johnson, in collaboration with David Cohen and Jennifer Heath in
Oregon and Kannan Ramanathan at NREL, is preparing for publication the
comparisons of defects in selenized vs. three-stage cells and in CBD-CdS
vs. Cd-partial-electrolyte cells.

**CdTe Cells.** Samuel Demtsu and Tim Nagle have completed whole-cell
and small-area measurements of CdTe cells from First Solar, Toledo, and
South Florida at selected time intervals during elevated-temperature
stress cycles. The results will be presented at the CdTe team meeting in
July. Additionally, I have been working with Peter Meyers and others to
define "third-level metrics", CdTe-cell loss parameters that facilitate
comparison studies. The goal is that these metrics have both have a
clear physical interpretation and can be relatively straightforwardly
measured. Again we are working toward a presentation in July.

Caroline Jenkins, working with Tim Gessert and others at NREL, has
prepared single crystal CdTe samples with several combinations of
surface etching and the addition of copper or chlorine. Variations in
photoluminescence from these samples appear to allow us to separate the
effect of each, at least for crystalline CdTe. Caroline will give a
poster presentation at the regional AVS meeting in June and has
submitted an abstract for the international CdTe meeting in September.

Sincerely,

James R. Sites

Professor

Cc: NREL Subcontracts (Attn: Carolyn Lopez)

CSU Office of Sponsored Programs (Attn: Mary Selby)