15 January 2004

Dr. Harin S. Ullal

MS 3212

National Center for Photovoltaics

National Renewable Energy Laboratory

1617 Cole Blvd

Golden, CO 80401

Subject: RDJ-2-30630-20 (Research Leading to High Throughput
Manufacturing of Thin-Film CdTe PV Modules); Q5 Quarterly Report.
Deliverable D.2.1

Principal Investigator: Dr. Rick Powell

Dear Dr. Ullal,

During the fifth quarter of the subject subcontract the following work
was performed:

# Vapor Transport Deposition (VTD)

**Reactor Development**

During this reporting period, work continued on a vapor transport
deposition distributor in which the vaporization and distribution
functions are separated (VTD-2 type). We demonstrated that chemical
contamination can be eliminated by operating within a restricted
temperature window and by using barrier coatings. Vapor sealing problems
have been encountered in a number of the VTD-2 type distributor designs
however. These problems are due to the difficulty in maintaining
mechanical tolerances in mating ceramic components at high temperature.
After assessing a number of candidate designs, we decided to focus on a
design that has the least pressure difference across the mating
surfaces. Initial test have been conducted for CdS. Film uniformity was
good but material utilization was poor. Moreover distributor behavior
with respect to flow and temperature was anomalous.

In the large area coating equipment, films with varying microstructure
are produced. In addition the VTD technique has formed films with a
range of microstructures over time and between deposition chambers. A
set of experiments designed to revisit the parameters that control
microstructure was begun with specific interest into the use of
nucleation layers.

Work also continued on precise powder metering equipment needed to form
thin CdS via VTD deposition. We have demonstrated that an alternative
feeder can reduce film thickness variation for small time scales but
have not yet shown this equipment to be superior in all respects. Tests
are underway to document the detail behavior of the feed equipment.
Initial tests have shown that the new feed equipment is capable of feed
rates with a variance of \<35mg/min over the course of several hours.
While promising, we need control that is at least 2 to 3 times better.

## Reactor Modeling

During this quarter theoretical reactor modeling work (at CSM) focused
on the addition of a particle sublimation module to the numeric
simulation. A convective sublimation model was developed to predict the
behavior of a single CdTe particle in a specified flow field for various
particle sizes, velocities, and temperatures. The effect of convection
on the sublimation process is described using the extended film model,
which describes the particle mass loss and the heat required by the
sublimation process. The particle diameter and temperature can then be
updated. Particle temperature is determined using a lumped capacitance
analysis. This sublimation model has been added to a previously
developed model for tracking a single particle through a specified flow
field. Preliminary results demonstrate the influence of process
parameters such as temperature and pressure in the heater on particle
sublimation dynamics.

## Non-uniformity losses

Work is underway (at UT) to estimate the non-uniformity losses in
individual cells and large-area modules. Quantitative estimations of
non-uniformity losses at a cell level have been made. These calculations
require estimates of the fraction of bad micro-diodes. This fraction has
been estimated based on measured distributions of cell V~oc~ on modules
that have been scribed into \~1cm^2^ cells. From the range (0.3 -- 0.5
V) and the standard deviation (65-130 mV) of the distributions, a
fraction of bad diodes between 5 and 10% has been calculated.

The non-uniformity loss in a cell has then been estimated by numerically
modeling a bimodal distribution of diodes connected in parallel. The
poor diode's V~oc~ is set to the lowest value found in the experimental
distribution and the good diode's V~oc~ is set to the highest value
found in the experimental distribution. Using the fraction of bad diodes
estimated above, the efficiency loss relative to a hypothetical cell
with a uniform distribution of diodes with the average V~oc~ was
calculated. With some degree of uncertainty related to unknown
micro-diode equivalent circuit parameters, we can assert that the
non-uniformity loss for a cell is always in the range of 10-20%.

Additional work is underway using micro I/V mapping by AFM and V~oc~
distributions on areas continuously covered with very thin back metal
layers to further describe the bad micro-diode fraction, characteristics
and region of influence.

At the module level, only a qualitative understanding of module losses
has been developed so far. We have estimated the non-uniformity
dimensions both in perpendicular direction (\~ 0.5 cm) and along linear
cell (\~ 2cm), the latter related to the characteristic scribe
resistance. While semi quantitatively we can assert that this leads to
the relative non-uniformity loss in the range of 20-30% on the module
level, more robust simulations are called upon to estimate the module
non-uniformity loss. We have been developing software to simulate the
equivalent circuit shown in Figure 1. Progress has been impeded because
of generic problems related to iterations of exponentially strong
functions in diode I/V characteristics. Currently we have obtained more
robust SPICE software to remedy this problem with results expected in
the few months.

![](media/image1.wmf){width="6.355555555555555in"
height="1.7631944444444445in"}

Figure 1. Equivalent circuit of large-area module with interconnects

## Film characterization

Work continued on expanding the use of IV measurements at various
temperatures (IVT). Ideally these measurements would aid in the
determination of loss mechanisms and related process steps.
Traditionally "equivalent" circuit parameters are extracted from IVT
data since these parameters can be related to standard semiconductor
junction models and can more clearly be separated as to the causal
process step.

To help choose among various data analysis techniques, we convened a
small conference of CdTe team members including UT, CSU, USF, ALF, and
IEC. Generally the group concluded that the IVT signature of current
First Solar devices is problematic for conventional analysis. Moreover
typical IVT curves show additional features in far forward bias that are
not described by simple equivalent circuits. Thus we have decided to
focus on the use of critical points in the extended IV curve rather than
analysis based on an equivalent circuit.

# Stability Issues 

## Accelerated Life Testing

During this reporting period we continued to build the correlation
between the current 56 day accelerated life test (ALT) procedure (56
days, open circuit, \~65ºC, continuous \~0.8 sun illumination) and a
more rapid test. The rapid test consists of subjecting module segments
to continuous illumination at higher temperature (\~120ºC) and
frequently recording IV measurements *in-situ.* The module segments are
strips that are cut so that most of the typical device non-uniformity is
sampled. The normal multi-vapor metal halide lamps used in light soak
stations provide the illumination. Data analysis consists of extracting
a saturated efficiency value from a fitted decay curve.

In order to verify the approach modules from an experiment with 9
different conditions were cut in half with one half tested with the
standard light-soak procedure and strips from the other half tested
using the rapid ALT technique. Figure 2 shows comparison between current
light soak relative saturation efficiency value for half-modules as
determined after 28 days of light soaking and relative saturation
efficiency values as determined from ALT stressed strips

Since the correlation appears promising and there are enormous benefits
of a rapid test, implementation of the new procedure for process
engineering and monitoring of standard product is underway.

![](media/image2.jpeg){width="5.122916666666667in" height="4.0in"}

Figure 2. Comparison of rapid ALT technique (ordinate) with the standard
ALT method (abscissa).

## Stress Induced Changes/Back Contact Issues

To date we have not found a clear marker and corresponding mechanism(s)
for the stress induced changes in the device. However, we have recently
identified an important back contact process variable. Modest and simple
changes to this variable result in dramatic improvements in device
performance and performance fluctuation. Figure 3 shows the effect for a
significant population of modules. The effect is quite reproducible and
early indications are that modules produced with Process B are more
stable. Consequently the effort of understanding stress-induced changes
has shifted to understanding the physical mechanism of this process
step. We believe that this work will help advance the understanding of
stress-induced changes.

![](media/image3.wmf){width="6.495138888888889in"
height="3.7819444444444446in"}

Figure 3. Effect of newly identified back contact process variable.

Work continues to understand the ramifications of the CdCl~2~ treatment
on the back contact interface. A joint experiment has been designed with
the "Chlorine" sub-team. The objective of this experiment is a
comparison between wet and vapor CdCl~2~ activation treatments. Each of
the participating groups received material pulled from the FS-production
line after CdTe deposition. The samples were taken from coater runs with
900Å and 1100Å CdS. Each group will perform CdCl~2~ wet and vapor
activation treatments following procedures and techniques established at
their facilities. Surface analysis after CdCl~2~ treatment will include
GI-XRD and XPS measurements. The participants will complete devices with
their specific back-contact technologies. Basic device characterization
along IV- and QE-measurements will be performed. CdCl~2~ activated
material will be returned to FS for device completion considering the FS
back-contact application technology. Device characterization will
include monitoring of the long-term performance under OC light soaking
at 65°C.

## Chemical Analysis:

A used inductively coupled plasma atomic emission spectrometer (ICP-AES)
has been acquired and refurbished. The ICP will be used for analysis of
trace impurities that influence initial and long-term device
performance. The ICP will also be used as well in defining
specifications and developing procedures for quality control of
in-coming raw materials such as CdS, CdTe, CdCl~2~ and NPR.

Special emphasis is focused on the Cu up-take of the device, as Cu is
added as a dopant during back-contact preparation and is introduced in
uncontrolled amounts into the semiconductor layers during CdCl~2~
activation treatment. In addition, non-negligible amounts of Cu are
usually found in raw materials CdS, CdTe, CdCl~2~.

# Environmental, Health, and Safety

First Solar's recycling activities have continued with the following
materials being recycled; glass, filter-cake containing semiconductor
and back contact metals, dust fines generated from the air filtering
system, and copper lead cords. All of these recycling activities
originate with Ohio companies.

In November of 2003, we visited a recycling company in Pennsylvania to
evaluate their capability to recycle spent modules, manufacturing scrap,
and by-products generated from our current recycling processes.
Essentially this vendor would merge our hazardous waste streams into
existing larger-volume industrial recycling streams. They appear to be a
very promising additional avenue for our recycling needs. Economic
negotiations are currently underway to determine future relations with
this company.

Recycling discussions and analysis of sample waste streams are underway
with a company in Germany to determine if an overseas recycling
opportunity can be established. This communication has just begun.

Activities for generating a testing method for HEPA filtration continue
with several different aerosol particles being evaluated for quantity
and size to determine if they can be used as a filter testing challenge.

First Solar is currently establishing an electronic version for annual
Hazard Communication and Cadmium Compliance safety training. The program
is being designed to present current job specific First Solar issues in
a visual and interactive format to assist in communicating the
safety-training message. Training for employees will take place in
January of 2004 and this program will then be incorporated into the new
employee safety orientation training session.

Best Regards,

RC Powell

Principal Investigator

First Solar, LLC

cc: Ms. Vicki Riddell

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