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| Department of Physics                                                 |
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| Fort Collins, Colorado 80523-1875                                     |
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| Telephone: (970) 491-5850                                             |
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| FAX: (970) 491-7947                                                   |
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| E-mail: <sites@lamar.colostate.edu>                                   |
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June 13, 2005

Ken Zweibel

National Center for Photovoltaics

## National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

RE: Quarterly Report II (March - May 2005)

"Characterization and Analysis of CIS and CdTe Cells"

Subcontract XXL-5-44205-03

Dear Ken,

During the past quarter, we reported a number of results at both the
Materials Research Society Spring Meeting in late March and the CdTe
National Team Meeting in early May. Most of the manuscripts or
powerpoint slides from these and other presentations are available on
our web site:

> [www.physics.colostate.edu/groups/photovoltaic](http://www.physics.colostate.edu/groups/photovoltaic)

In addition, Markus Gloeckler recently completed his PhD degree. His
thesis, which is both attached and on the web page, contains a more
extensive description of his work on CIGS grain boundaries, grading
effects, and efficiency limitations than that published or submitted for
publication.

### Cadmium Telluride Cells

**CdTe Device Physics.** Both Alan Fahrenbruch and I gave presentations
as part of the CdTe device-physics subteam's report in May. My
presentation, "CdTe Solar Cells: Basis Model and Common Deviations,"
gave my view of the standard CdTe band diagram, emphasizing that the
CdS/CdTe conduction-band offset is less optimal than that of CdS/CIGS at
1.15-eV. I also discussed the two categories of back-contact issues: (1)
hole-current impedance, which reduces photocurrent collection below
V~OC~ and forward current above V~OC~ (rollover effect), and (2) the
interaction of the back contact with the primary junction, which can
significantly reduce cell voltage. Alan's presentation, \"The Influence
of Band Profiles on Transport and Recombination\" showed the role that
the depletion layer in CdS/CdTe cells plays in determining the PV
parameters and how defect states must be considered in the context of
the depletion layer. Alan also gave a presentation with the
materials-chemistry subteam, \"Choosing Defect Properties for Device
Modeling,\" which showed the complexity of the defect properties in CdTe
and offered a possible way to simplify them for modeling purposes.

**CdTe Photoluminescence.** Caroline Corwine gave presentations both at
the MRS meeting, "Photoluminescence Studies of Cu and O Defects in
Crystalline and Thin-Film CdTe", and at the CdTe team meeting with a
similar title. In both cases, Tim Gessert and others from NREL were
co-authors. Caroline emphasized that the 1.456-eV CdTe signature that is
present in all good-quality cells is likely the result of a Cu~i~-O~Te~
donor-complex with energy about 150 meV below the conduction band.
Crystalline samples require the addition of both Cu and O to see this
line, and Li and Wei of NREL have calculated a similar energy (125 meV)
for this complex. Caroline also reported results from Manuel Romero that
showed that cathodeluminescence in the vicinity of 1.456 eV comes
preferentially from the grain-boundary region. During the next quarter,
similar measurements will be made with CdTe cells from IEC and USF.

**CdTe Stability and LBIC.** Samuel Demtsu, along with Alan Davies at
CSU and Dave Albin, Tom McMahon, and Joel Pankow at NREL, presented
"Stability of CdS/CdTe Solar Cells with Ag and Ni Back Contacts" at the
CdTe meeting. In these studies, the graphite commonly used with the back
contact was removed from NREL-fabricated cells prior to the deposition
of silver or nickel. Other cells with the graphite contact in place were
used for reference. It was seen that current and voltage were similar
with and without the graphite, but the fill-factor was reduced when the
graphite was removed, and the cells appeared to have a larger series
resistance. Initially, the Ni and Ag contacts gave similar J-V curves.
When the nickel-contact cells were subjected to elevated-temperature
stress (100C at one sun and open circuit), they showed a steady increase
in fill factor and only modest increase in non-uniformity. These
observations are consistent with the formation of a Ni-Te layer. The
silver-contact cells, on the other hand, showed less change in fill
factor, but greater increase in non-uniformity. Furthermore, they
occasionally showed a significant decrease in both voltage and
fill-factor, but recovery after additional stress. This behavior is
attributed to diffusion of silver along grain boundaries that leads to
local shorting that is at least partially relieved with additional
stress. Similar J-V and LBIC measurements as a function of stress time
with CSU cells from Sampath's group are underway, and improvements are
being made to the LBIC data collection and analysis protocols. Another
collaboration that is starting is wavelength-dependent LBIC from the
back side of bi-facial cells fabricated by Brian McCandless at IEC.

**CdTe Transients.** Alan Fahrenbruch investigated the current transient
response to voltage and illumination steps in CdS/CdTe cells obtained
from Sampath (CSU), Gessert (NREL), and McCandless and Hegedus (IEC).
All the cells showed fully reversible dark forward-bias transients, but
their magnitude and direction depends on cell preparation. Bias and
light induced transients are small (\~ 3 to 20%) for well-behaved cells,
without rollover or cross-over, and the effect of the transients on
efficiency is small. Abnormal devices with non-optimal Cu show much
larger transients. Alan presented the results at the Spring 2005 MRS
Meeting; his manuscript for the proceedings in entitled \"Current
Transients in CdS/CdTe Solar Cells.\"

**CIGS Solar Cells**

**CIGS Grain Boundaries.** Markus Gloeckler, in collaboration with Wyatt
Metzger at NREL, completed his study of CIGS grain boundaries (GBs) and
gave a very plausible explanation of why they have only minimal impact
on solar-cell performance. It appears most likely that CIGS GBs are
benign, because the CIGS valence band is shifted downward near a GB
without a significant shift in the conduction band, and the resulting
hole barrier minimizes the GB recombination. This scenario is consistent
with direct measurements at Ohio State of significant copper depletion
near CIGS GBs, with the observation that cell performance appears to be
insensitive to the detailed nature of the CIGS GB, and with the surface
reconstruction calculations of Persson and Zunger. The commonly held
view of a positive charge sheet at the GB was shown to generally lead to
a net decrease in efficiency, since although the charge may increase
current at zero bias, it also reduces voltage and fill factor.

**Thin CIGS Absorbers.** We have planned and started a project to assess
the likely performance of CIGS cells as a function of absorber
thickness. The primary goal is to suggest strategies to mitigate
potential limitations when the CIGS thickness is below one micron. The
components of this plan are (1) the basic current loss due to thinning,
and how that loss is affected by back-surface reflection, (2) changes in
the basic efficiency vs. band gap curve when the absorber is thin, (3)
optimization of band-gap grading, which is clearly important for thin
cells, (4) the effect of back-surface recombination, (5) the effect of
the back-contact barrier, (6) grain-boundary effects, which we believe
are minimal in thicker cells, and (7) the increasing importance of
non-uniformities, which we will document with LBIC studies. Our
characterization and analysis is being done in collaboration with Kannan
Ramanathan at NREL and Bill Shafarman at IEC, who are fabricating
thin-CIGS cells.

**CIGS Conduction-Band Offset.** Building on earlier work by Alex Pudov,
Ana Kanevce has generalized the criteria for when a large
conduction-band offset distorts solar cell performance and presented the
results at the Spring 2005 MRS meeting. Her paper,
"Conduction-Band-Offset Rule Governing J-V Distortion in CdS/CI(G)S
Solar Cells," was co-authored with Gloeckler, Pudov, and myself. The
basic concept is quite simple: there needs to be sufficient electron
density at any position in the absorber, and at any bias in the cell's
operating range, to maintain a current at least equal to the desired
photocurrent. The electron density follows directly from the energy of
the quasi-fermi level relative to the conduction band. For CIGS at room
temperature and a target photocurrent of 32 mA/cm^2^, that energy
difference cannot exceed 0.48 eV. This critical energy will vary with
cell parameters, such as effective mass, and with illumination, but the
variation is weak, since these dependences appears only in a logarithmic
term.

**CIGS with ZnMgO Buffers.** Plans are now well formulated for a
collaboration involving Falah Hasoon at NREL and Ana Kanevce at CSU to
explore CIGS buffers made with mixtures of MgO and ZnO. The motivation
is that the ZnMgO system can be tuned by varying the Mg to Zn ratio to
give a large range of conduction-band offsets, which may be advantageous
for CIGS band gaps near 1.0 eV or above 1.3 eV. This procedure is
similar to tuning the ZnS/ZnO system. Ana has, for the full range of
CIGS band gaps, calculated the expected J-V curves for appropriate
values of the Mg/Zn ratio and the buffer thickness. In particular, we
predict that 10-20% Mg will be the optimal range for 1.15-eV CIGS
absorbers. So far, there is reasonable agreement with measurements from
older ZnO/CIGS cells, and we look forward to the analysis of a larger
set of ZnMgO/CIGS cells currently under fabrication.

> Sincerely,

James R. Sites

Professor

Cc: NREL Subcontracts

CSU Office of Sponsored Programs