**Barrier Coatings and Stability of Thin Film Solar Cells**

**4th Quarterly Report - Phase II:**

**June 1, 2004 \-- August 31, 2004**

**NREL Subcontract: 44575A**

**Subcontractor: Pacific Northwest National Laboratory**

**Principal Investigator: Larry C. Olsen**

**1. OBJECTIVES/APPROACH**

The key objectives of this program are to develop low cost barrier
coatings for CdTe and CIS solar cells and to develop an improved
understanding of mechanisms affecting the stability of CdTe solar cells.
The scope of this work entails investigation of multilayer, barrier
coatings for solar cells and studies of stability issues for CdTe solar
cells. The work is structured into three main tasks: (1) Barrier layer
coatings for CdTe and CIS solar cells; (2) Modeling of mechanisms
influencing CdTe stability; (3) Experimental studies of CdTe cells
supporting Tasks 1 and 2.

**2. PROGRESS FOR THIS REPORTING PERIOD**

Efforts this past quarter concentrated on developing approaches to
applying barrier coatings to CdTe cells from Dr. Sampath\'s group at
Colorado State University (CSU) and carrying out accelerated testing of
the cells.

**2.1 Approach to Depositing Barrier Coatings to CSU CdTe Cells**

CSU has supplied devices described by Figure 1. The cell structure is
grown on tin-oxide coated glass and then defined by bead blasting such
that the diameter is 0.90 cm. The cell area is therefore approximately
0.63 cm^2^. As described in Figure 1, the CSU back contact is relatively
thick (75μm). This contact is applied with a spraying procedure with a
layer of carbon deposited first followed by a thick layer of nickel. The
contact layers are then cured. Cells are provided with a small pad of
indium attached at one of the corners of the glass substrate so that an
I-V curve can be taken by probing the Indium and the back of the cell.
Although such an I-V curve exhibits a low fill factor, accurate values
of Jsc and Voc are obtained. This approach allows CSU to identify good
cells before sending them to PNNL, and allows us to determine if the
devices have been damaged as a result of shipping.

![](media/image1.png){width="1.8574803149606298in"
height="1.8181102362204724in"}

**Figure 1.** Description of CdTe Cells fabricated by CSU.

In order to conduct accelerated tests with coated cells it was necessary
to devise a way to establish a contact to the back of a cell and then
apply a barrier coating. The approach used is described by Figure 2. The
fabrication steps are as follows:

(1.) Deposit a silver contact ring and leg to corner In pad;

\(2\) Deposit a polymer bridge;

\(3\) Deposit Ni/Ag contact leg connecting In pad in corner to back of
cell thereby forming contact to Cell back contact;

\(4\) Apply barrier coating as indicated.

The bridge is comprised of a polymer layer several microns thick. To
date barrier coatings used on the CSU CdTe cells are similar to those
deposited onto Shell Solar CIS devices. The Ni/Ag strip contacting the
back contact consists of 500 Å of Ni followed by 2000 Å of Ag. Direct
contact of Ag to the CdTe back contact was avoided due to some
researcher\'s concern over the effects of Ag diffusion in CdTe solar
cells.

![](media/image3.png){width="2.709055118110236in"
height="2.572047244094488in"}

**Figure 2.** Planned approach to applying barrier coating to CSU CdTe
cell.

**2.2 PNNL Sputtered Ni/C Contacts**

Since the CSU back contact is very rough, we were concerned that water
ingress might occur because of pin-holes in the oxide layers resulting
from the back contact layer topography. Thus, we have fabricated cells
based on the CSU cell structure using a relatively \'smooth\' back
contact deposited by RF magnetron sputtering. The sputtered contact was
established by first removing the CSU sprayed Ni-C back contact with MEK
(as directed by Kurt Barth of CSU) and then sputter depositing 500 Å to
1000 Å of carbon followed by 2000 Å of Ni (see Figure 3). Additional
deposition of Ni was done by PVD to provide adequate sheet conductance.
This procedure has resulted in cells with efficiencies greater than 10%.
The fill factors are not as good as those devices with the CSU sprayed
contact, but accelerated tests with these devices will provide valuable
information. We will consider adding silver to the sputtered Ni/C
contact in order to achieve increased fill factors, but yet maintain a
thin and smooth contact.

**Figure 3.** Illustration of process to fabricate CdTe cells with
Sputtered Back Contact (not to scale).

**2.3 Accelerated Testing of CSU CdTe Cells**

Data acquired for CdTe cells under accelerated stress testing are shown
in Figure 4. There are three types of cells on test, a bare device with
a CSU sprayed contact but no barrier coating, coated cells that have a
CSU sprayed Ni/C back contact, and coated cells that have a PNNL
sputtered back contact. The bare cell essentially degraded immediately
due to the back contact delaminating. Dr. Sampath commented that the
back contact applied for the structures tested for this work are being
improved. Thus, further studies will be forthcoming concerning testing
of bare cells.

First, consider the device with a CSU back contact that was stressed at
60ºC and 40% RH. The initial efficiency of this cell was 10.4 %, whereas
the efficiency after 1025 hours was 10.1 %. The cell performance was
essentially unaffected by the stress of 60C/40%RH. This is quite
encouraging. At this point in time, the cells with the CSU and PNNL
contacts are behaving about the same in 60ºC/90%RH environment. These
investigations will continue.

The I-V parameter showing the most significant change is the fill
factor. This would suggest a change in resistance, probably due to back
contact change. There are two areas of interest as we go forward,
namely: which properties of these cells change as result of the applied
stress; and what constitutes an effective, low cost approach to
encapsulation.

![](media/image4.wmf){width="4.927083333333333in" height="3.5in"}

**Figure 4.** Accelerated life test data for [coated cells]{.underline}
with CSU sprayed back contacts and PNNL sputtered contacts, and a [bare
cell]{.underline} with a CSU contact but no coating.