March 5, 2004

Ken Zweibel

National Center for Photovoltaics

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

RE: Quarterly Report X (December 2003 - February 2004)

"Device Physics of Thin-Film Polycrystalline Solar Cells"

Subcontract ADJ-1-30630-06

Dear Ken,

During the past quarter, we have achieved results in several areas as
described below. As usual, we have been pleased to have discussions and
direct collaborations with several colleagues throughout the thin-film
polycrystalline community.

**Cell Measurement and Analysis.** In the CdTe area, we have been
working primarily with cells fabricated at First Solar, Inc. One project
done jointly with Chris Ferekides of USF and Steve Hegedus of IEC was to
compare cells taken from production modules that were made with
variations in the fabrication process. The cells ranged in efficiency
between 7 and 10½%, and the results were quite consistent among the
three labs. The measurements performed were dark and light
current-voltage including hysteresis and temperature effects, quantum
efficiency including voltage effects, capacitance, and photocurrent
uniformity. Primary efficiency limitations as expected were the blue
loss in the CdS and the series resistance. Most of the modules showed
similar parameters at the cell level, but one exception, apparently made
with thin CdS, showed higher blue collection and total current, lower
voltage with only weak temperature dependence, less photocurrent
uniformity, and nearly full depletion. Details of these results were
reported at the recent CdTe team meeting.

A second presentation at the CdTe team meeting, by Samuel Demtsu and
myself, was a systematic, step-by-step approach to break down the
current, voltage, and fill-factor into a set of twelve individual loss
mechanisms. The record-efficiency CdTe cell (16.5%) and a typical
First-Solar production cell (9.6%) were used as examples. In each case
the losses were quantified, and the larger ones highlighted. Such losses
can be better quantified for cells in the 15% range, but the process
works reasonably well for cells the order of 10%. Examination of
existing cells was also used to define a target cell of 19% efficiency
as a mid-term goal.

With CIGS cells, a major project by Alex Pudov has been to better
understand the current-voltage phenomena known as the "red kink". The
cells used were provided by Miguel Contreras and Falah Hasoon at NREL,
as well as by colleagues in Germany and Japan. The principle of the red
kink is that the conduction-band offset between CdS and CIGS can produce
a current-limiting barrier that can severely distort the J-V curve
\[Eisgruber et al, Solar Energy Mat. Solar Cells 53, 367 (1998)\].
Generally, the photoconductivity of CdS will mitigate the problem under
white light, so that it is primarily seen when the blue photons are
suppressed. Hence, the term "red kink". One practical consequence is
that a CdS/CIS bottom tandem cell will not see blue photons and may
therefore be compromised in performance. Alex has shown that the kink is
most pronounced with CIS and progressively diminishes with higher Ga
concentration. He has also shown that the kink is present at the higher
Ga concentrations when larger band-gap ZnS/O or InS/O windows are used,
and that the red/blue transition in each case corresponds to the window
band gap. Furthermore, when the red kink is pronounced, there is also a
smaller, but non-negligible distortion under white light, which leads to
a modest reduction in efficiency. The response time constant for a
change from red to white light is the order of seconds, but the reverse
from white to red is the order of hours. An abstract of this work has
been submitted to the E-MRS Conference in May.

In a project to improve cell analysis, Markus Gloeckler has written a
software package called CurVA. This package allows one to overlay
current-voltage data expressed in four different formats with curves
from the parameterized solar-cell diode equation. It is basically an
extension of a process developed by Jim Phillips at IEC, but it allows
considerable user interaction. In particular, sliders on the screen
allow real-time adjustment of each parameter and of the range of data
being fit. We are using this software extensively, and it is also being
used by Jim Phillips and by NREL.

In other cell measurement and analysis work, we are hosting Thomas
Schulmeyer, a PhD student from the Technical University of Darmstadt,
who has performed a number of XPS and UPS measurements to experimentally
determine the band alignment in CIGS cells. At Colorado State, he is
characterizing the device properties of the cells used in the
photoemission studies.

**Small-Spot Measurements.** Tim Nagle has continued with
light-beam-induced-current (LBIC) measurements. One major project as
part of the CdTe nonuniformity team effort was to measure sets of cells
with and without the First-Solar interfacial layer (IFL) used to improve
the back contact. He found that the as-deposited cells with the IFL had
good J-V characteristics, little cell-to-cell variation, and good
photocurrent uniformity. Elevated-temperature stress (90ºC for up to 30
days) reduced the fill-factor and the voltage of the IFL cells, but had
no significant effect on the uniformity. The cells without the IFL had
as-deposited voltages ranging from 200 to 800 mV and poorer uniformity.
Under stress, the higher voltage non-IFL cells degraded in performance
similarly to the IFL cells, but the 200 mV ones changed less. In neither
case did the original poor uniformity change significantly. This work
was also presented at the recent CdTe team meeting.

Other LBIC projects during the past quarter included a joint study with
the School of Mines to look at cells where back-contact copper was
applied only in local areas. The same cell regions were investigated
with electroluminescence (EL) at CSM and with LBIC by Tim Nagle here.
The clear result is that EL is significantly enhanced under the copper
contacts, but the photocurrent from the same spatial region is decreased
about 1% at mid-wavelengths and about 2½% nearer the band gap. These
results were reported at the recent CdTe meeting by Scott Feldman of
CSM, but at least in my opinion, the explanation is not yet complete. In
a second CSM presentation at the team meeting, Tim measured the
photocurrent uniformity of CSM cells with and without a TCO bilayer and
found, as expected, that the bilayer improved the uniformity.

**Defect Studies.** Caroline Corwine's photoluminescence (PL) work with
Tim Gessert, Pat Dippo, and others, at NREL has focused on the impact of
copper and chlorine impurities in single-crystal CdTe. Emphasis to date
has centered on copper, and we have extensive PL data on two peaks
correlated with the copper inclusion. Both show an intensity depencence
that implies that they result from donor-to-acceptor transitions.
Caroline reported these results at the recent CdTe meeting. However,
since the impurities are added by diffusion, control experiments are
very critical, and there has been some ambiguity in the results to date.
We are therefore repeating some of the PL measurements and are looking
carefully at information deduced from other types of measurements on the
same CdTe samples. For the next round of PL measurements, Caroline is
being trained to make the measurements with the NREL equipment.

**Numerical Simulation.** The primary simulation project continues to be
the impact of conduction-band offset on the J-V curves of CdS/CIGS
cells. This work by Markus Gloeckler has explored the well-documented
V~OC~ weakness at band gaps above 1.3 eV. The proposed model shows that
even small amounts of forward-current recombination near the diode
interface can limit the cell's voltage. An abstract has been submitted
to this year's E-MRS conference. At the same conference, I have been
invited to speak on the more general subject of the device physics
involved in the match of window and absorber materials for chalcogenide
cells.

A second simulation project related to CdS/CIGS, also being done by
Markus Gloeckler, is the effect of absorber grading. The model used
assumes that Ga-concentration, and hence band gap can increase towards
the front of the absorber and/or towards the back. The critical
parameter appears to be the position of the minimum band gap. If it is
close to the window (about 0.1 μm), there should be a net increase in
efficiency. On the other hand, if the minimum is near 0.5 μm, roughly
the depletion width, there will be a modest current-blocking barrier in
forward bias, and the efficiency should be somewhat decreased.

One CdTe simulation project carried out by Alan Fahrenbruch has been a
comparison of different strategies to lower the back-contact barrier.
The configurations he investigated included a heavily doped p^+^-CdTe
layer, a p^+^-Cu~2-x~Te layer, a p^+^-ZnTe layer, and an interfacial
dipole. Alan discussed the impact of each of these at the recent CdTe
team meeting. In a second CdTe-simulation area, Markus Gloeckler's paper
on apparent-quantum-efficiency effects has been accepted for publication
in the Journal of Applied Physics, and he will give a poster
presentation at the European PV conference is May.

A major new effort in solar-cell simulation is to move to more
sophisticated software that allows two- and three-dimensional modeling.
The software package, called DESSIS, was purchased by NREL just over a
year ago. It is useful in a number of areas, but in particular, Markus
Gloecker in collaboration with Wyatt Metzger has just started using it
for device-physics studies. Two questions we hope to answer are (1)
whether one can determine the impact of a grain boundary on device
performance, and (2) under what circumstances does a 1-D simulation give
essentially the same information as a 2-D approach.

**Degree Progress.** Markus Gloeckler and Caroline Jenkins both passed
their PhD prelim exams, and Samuel Demtsu is scheduled for his in early
March. These exams require a presentation of research results to date
and a plan for the proposed thesis. In addition, Alan Davies and Jun Pan
passed their MS final exams, consisting of critical reviews of two
assigned journal articles. Their degrees will be awarded in May.

Sincerely,

James R. Sites

Professor

Cc: NREL Subcontracts (Attn: Carolyn Lopez)

CSU Office of Sponsored Programs (Attn: Mary Selby)