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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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| www.physics.colostate.edu/groups/photovoltaic                         |
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March 21, 2005

Ken Zweibel

National Center for Photovoltaics

## National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

RE: Quarterly Report I (December 2004 - February 2005)

"Characterization and Analysis of CIS and CdTe Cells"

Subcontract XXL-5-44205-03

Dear Ken,

During the past quarter, we have made progress in several areas as
described below. The manuscripts related to completed projects are
attached.

**CIGS Grain Boundaries.** Markus Gloeckler has simulated several
possible scenarios to assess conditions under which CIGS grain
boundaries (GBs) would have only minimal impact on solar-cell
performance. A common minimal-impact explanation, that there is a
positive charge sheet at the GB that repels holes, was found to lead
generally to a net performance loss. Although current may increase
slightly in this scenario, the effective junction area will also
increased, which leads to increased forward current and reduced voltage.
The explanation we consider most likely is that the CIGS valence band is
shifted downward near a GB without a significant shift in the conduction
band. 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. This scenario is also consistent with reported Kelvin-probe
measurements, but not with the charge-sheet interpretation found in
several reports. A paper (attached) is being submitted to the Journal of
Applied Physics, and Markus will give a presentation at the upcoming MRS
meeting.

**Thin CIGS and Grading.** We have begun work, simulation initially and
now experimental, on thin CIGS absorbers. The simulation work grew out
of a study of the effect of band-gap grading on CIGS cell performance
that was presented at ICTMC-14 last fall and again at the recent CIS
team meeting (Gloeckler and Sites, J. Phys. Chem. Solids, in press). The
conclusion from this work is that grading itself can have at best a
minor positive effect, and quite possibly a negative effect for standard
thickness absorbers. However, for thinner absorbers, as one would
expect, a back grading can substantially mitigate current losses related
to the thinness.

We are currently organizing our experimental thin-CIGS plans in
collaboration with Kannan Ramanathan at NREL and Bill Shafarman at IEC.
The primary two tasks for us are (1) tracking performance uniformity
with LBIC at different voltages as absorbers become progressively
thinner, and (2) loss analysis to determine whether there are basic
differences among thin-absorber cells fabricated at Upsalla, NREL, and
IEC.

**Secondary CIGS Junction Barriers.** We completed a joint project with
NREL on the impact of the conduction-band-offset (CBO) on the
performance of CdS/CIS cells. A paper by Pudov, Kanevce, Al-Thani,
Sites, and Hasoon shows that the degree of J-V distortion due to the CBO
spike that is predicted for different conditions by simulation is also
realized experimentally. The revised manuscript has now been published
in the Journal of Applied Physics (see attached), and further details
are found in Alex Pudov's PhD thesis. That paper compares the measured
J-V curves from cells fabricated with varying CdS thicknesses to the
curves predicted by simulations.

An extension of this project has been the articulation of a general rule
governing whether the CBO will cause a distortion to the J-V curve. This
rule states that with typical cell and illumination parameters,
distortion of the J-V curve occurs when the difference between the
conduction band and the quasi-Fermi level for electrons exceeds 0.48 eV
at some place in the cell when voltage is swept over the standard
operating range. This 0.48-eV energy difference is the onset of when the
secondary barrier in the junction limits the flow of current. The
critical energy will vary with cell parameters, such as effective mass,
and with illumination, but only weakly, since the effective-mass or
illumination dependence appears only in a logarithmic term. This work
will be presented at the upcoming MRS meeting by Ana Kanevce.

**Alternative Junctions With CIGS.** We have continued to explore the
reasons why alternatives to CdS may be desirable, particularly for cells
with no gallium or those with wide-gap CIGS. A previously submitted
manuscript dealing with ZnS(O,OH) and InS(O,OH) buffers (Pudov, Sites,
Contreras, Nakada, and Schock) will appear shortly in Thin Solid Films.
Similar, and possibly more robust Zn(O,OH) deposited by Raghu
Bhattacharya at NREL, has now also shown an efficiency of 18½%. A
continuing collaboration involving Falah Hasoon at NREL and Ana Kanevce
here will examine buffers made by adding modest amounts of MgO to ZnO.

**CdTe Cell Operation.** We have been correlating the general form of a
CdTe-cell band diagram with various undesirable features, such as
reduced voltage, "rollover", and apparent reduction in current
collection near maximum power, that are often observed in experimental
J-V curves. Jun Pan, Markus Gloeckler, and Alan Fahrenbruch have
performed a number of simulations that show how the overall band picture
of a CdTe cell, and the details of its J-V curve respond to variations
in back-contact barrier, CdTe thickness, and CdS carrier density. To
date, we have confirmed or expanded several of the explanations from the
University of Toledo, but contrary to that work, we have not found a
need to modify the small cliff in the CBO deduced from photoemission
studies. Victor Karpov and I plan to present the areas of agreement, and
those of disagreement, at the CdTe team meeting in May.

**CdTe LBIC Studies.** Samuel Demtsu and Alan Davies have make
systematic J-V and light-beam-induced-current (LBIC) measurements,
before and after elevated-temperature stress, on CdTe cells fabricated
at NREL with different back contacts. They find that removal of the
graphite typically used, followed by deposition of Ni or Ag, degrades
both the uniformity and the overall cell response. Furthermore, both the
J-V and the uniformity of the cells with the metal contacts following
graphite removal are more degraded by stress than those with the
graphite in place. Similar measurements are underway with cells made by
Sampath's group at Colorado State. In the course of these studies, the
importance of forward-bias LBIC to identify "weak-diode" areas and their
effect on performance has become very clear. The forward-bias protocol
we have adopted, which normalizes the effects of several J-V features,
is to consistently measure forward-bias LBIC at the voltage where J is
half of J~L~.

**CdTe PL Studies.** Caroline Corwine, with Tim Gessert and colleagues
at NREL, continues to explore the origin of the 1.456-eV CdTe
photoluminescence signature that is present in all good-quality cells.
Working with crystalline samples, Caroline finds that that the
characteristic line requires the addition of both copper and oxygen. She
also finds that the energy of this line does not show a dependence on
excitation energy, which strongly indicates that it results from a
defect-to-band transition. Furthermore, its measured energy is
consistent with a transition between the Cu~i~-O~Te~ donor-complex
energy calculated by Li and Wei at NREL and the CdTe valence band. A
revised manuscript (attached) has been submitted to Applied Physics
Letters, and Caroline will also be making a presentation at the MRS
meeting at the end of March.

**PVSC Presentations.** I presented an invited talk at the 31^st^
Photovoltaics Specialists Conference in January, "LBIC Analysis of
Thin-Film Polycrystalline Solar Cells." The talk, co-authored by Tim
Nagle, was a review that focused on several CdTe and CIGS LBIC
applications that we have developed at Colorado State. Also at PVSC,
Samuel Demtsu presented a talk entitled, "Quantification of Losses in
CdS/CdTe Solar Cells." Copies of both those manuscripts are attached. In
addition, members of our group were co-authors on four PVSC-31 papers
presented by other groups:

T.J. McMahon, T.J. Berniard, D.S. Albin, and S.H. Demtsu, "Excess Dark
Currents and Transients in Thin-Film CdTe Solar Cells: Implications for
Cell Stability and Encapsulation of Scribe Lines and Cell Ends in
Modules."

S.D. Feldman, L. Mansfield, T.J. Ohno, V. Kaydanov, J.D. Beach, and T.J.
Nagle, "Non-uniformity Mitigation in CdTe Solar Cells: The Effects of
High-Reistance Transparent-Conducting-Oxide Buffer Layers."

D.S. Albin, T.J. McMahon, T.J. Berniard, J. Pankow, S.H. Demtsu, and R.
Noufi, "Experiments Involving Correlations Between CdTe Solar-Cell
Fabrication History and Intrinsic Device Stability."

T.A. Gessert, S.Smith, T. Moriarity, M. Young, S. Asher, S. Johnson, A.
Duda, C. DeHart, and A.L. Fahrenbruch, "Evolution of CdS/CdTe Device
Performance During Cu Diffusion."

**Student Progress.** In January, Alex Pudov successfully defended his
PhD thesis, "Impact of Secondary Barriers on CuIn~1-x~Ga~x~Se~2~
Solar-Cell Operation", which can be found in full on our web site. His
degree will be awarded in May. In February, Tim Nagle and Ana Kanevce
passed their Preliminary Examinations, which formally admits each of
them to PhD candidacy.

> Sincerely,

James R. Sites

Professor

Cc: NREL Subcontracts

CSU Office of Sponsored Programs