**[ENERGY CONVERSION DEVICES, INC.]{.underline}**

> **QUARTERLY TECHNICAL STATUS REPORT**

####### December 15 2004 - March 15 2005

### Subcontract Title: Development of Optically Enhanced Back Reflectors and Improved Deposition Processes for Amorphous Silicon-Based Photovoltaic Technologies

Subcontract Number: ZDJ-2-30630-22

Technical Monitor: Bolko von Roedern

Contributors: Scott J. Jones (Principal Investigator), Joe Doehler,
Masat Izu, Tongyu Liu, David Tsu, Jeff Steele and Rey Capangpangan

**Program Goals**

In this program, we plan to improve the module efficiencies through
development of a new optically enhanced Al/ZnO back reflector and
improved i-layer deposition process. In the case of the back reflector
development, a multi-layered thin film structure consisting of films
with contrasting indices of refraction placed between the Al and ZnO
layers of the back reflector will be developed. These new types of back
reflectors will be tested in amorphous silicon based single junction and
multi-junction devices. The differences in n of the different layers of
the multi-layered back reflector and electrical conduction through the
multi-layered structure will be optimized to obtain the highest
reflection values, highest currents and best cell performance. The
ultimate goal is to achieve the high currents and cell efficiencies
typically obtained with the Ag/ZnO back reflector with a new optically
enhanced back reflector that can be used in the solar module products.
For the multi-layered structure, focus will be on preparing the layers
using sputtering techniques so that this technology might be quickly
applied to ECD's present back reflector fabrication process that uses
sputtering techniques.

In the case of the i-layer, the focus will be on preparing
microcrystalline silicon based intrinsic layers for low cost, high
stable efficiency solar cells through the use of microwave plasmas. In
these studies, the effects of such deposition conditions as ion
bombardment, substrate temperature and etchant gases on the grain size
and film transparency will be studied and correlated with cell
performance.

Achievement of the goals of this program and application of these
advancements to ECD's joint venture company's production lines would
lead to an immediate improvement in module efficiencies. These advances
along with ECD's participation in the NREL a-Si teams with other
development programs will contribute to the ultimate goal of achieving
stable efficiencies of 15% using a low-cost, scalable, manufacturable
techniques and inexpensive substrates.

######### This Quarter's Results

We have been working on Al/Multi-layer(ML)/ZnO structure where ML is
ZnO/ZnOSi/Si/ZnOSi where the ZnOSi is the low index of refraction layer
and Si is the high index of refraction layer. In completing our studies,
we have explored the use of thick ZnO layers to obtain the texturing
needed for multiple light passes through the semiconductor layers while
maintaining specular Al surfaces. We have studied both Al/ZnO and
Al/ML/ZnO structures with thick top ZnO layers. Early in this reporting
period, we found that the performance of cells with these back
reflectors were quite similar. Table I shows data for a-SiGe:H cells
made nominally under the same conditions except for the use of Al/ZnO or
Al/ML/ZnO back reflectors. In this table and all of the following
tables, the Jsc values were obtained using the integrated areas under
the red portion (\>630nm) of the Quantum Efficiency plots. Only slight
differences in Voc and FF were observed. In contrast, when no texture
was used, the cells with the Al/ML/ZnO back reflectors had significantly
higher performance as is shown in Table II.

Table I.

IV data for a-SiGe cells with **textured** back reflectors. IV data
taken using 630nm filter.

+---------------+----------+--------+----------+--------+-------------+
| Back          | Voc      | Jsc    | FF       | Rs     | Pmax        |
| Reflector     |          |        |          |        |             |
|               | \(V\)    | (mA/   |          | (Ω     | (mW/cm^2^)  |
|               |          | cm^2^) |          | cm^2^) |             |
+---------------+----------+--------+----------+--------+-------------+
| Al/ZnO        | 0.593    | 10.4   | 0.576    | 12.6   | 3.55        |
+---------------+----------+--------+----------+--------+-------------+
| Al/ML/ZnO     | 0.605    | 10.4   | 0.585    | 12.5   | 3.66        |
+---------------+----------+--------+----------+--------+-------------+

Table II.

IV data for a-SiGe cells made with **specular** back reflectors. IV data
taken using 630nm filter.

+---------------+----------+--------+---------+-------+-------------+
| Back          | Voc      | Jsc    | FF      | Rs    | Pmax        |
| Reflector     |          | (mA/   |         |       |             |
|               | \(V\)    | cm^2^) |         | (ohm  | (mW/cm^2^)  |
|               |          |        |         | cm)   |             |
+---------------+----------+--------+---------+-------+-------------+
| Al/ZnO        | 0.564    | 7.05   | 0.563   | 17.8  | 2.24        |
+---------------+----------+--------+---------+-------+-------------+
| Al/ML/ZnO     | 0.576    | 8.75   | 0.558   | 15.6  | 2.81        |
+---------------+----------+--------+---------+-------+-------------+

To improve the performance of the textured back reflectors, we have
completed a number of experiments to alter the degree of texturing.
These include altering the chamber pressure, applied dc power, substrate
temperature and oxygen flow during ZnO depositions. All ZnO depositions
were made using a ceramic ZnO sputtering target. After this optimization
process, we compared a-SiGe:H cells using nominally the same deposition
conditions except for different back reflectors: a) Al (textured)/ZnO
made in at United Solar Ovonic's production plant in Auburn Hills, b) Al
(textured)/ZnO made in the R&D machine used in this program, c) high
performance Ag/ZnO made in United Solar's R&D machine, and d)
Al(specular)/ZnO(textured) after the optimization process. The amorphous
silicon germanium nip structures for all of these cells were made using
the same R&D equipment using nominally the same deposition conditions.
For the Al/ZnO back reflectors with textured Al surfaces and thin ZnO
layers made using United Solar Ovonics production line, P~max~ values of
3.3 mW/cm^2^ were obtained. When these conditions were repeated in the
R&D machine, lower values were obtained due to possible leaks through
the vacuum chambers due to the use of high temperatures. Through the
optimization process, we were able to improve the performance of the
Al(specular)/ZnO(textured) back reflectors with the cells having P~max~
values (filtered using 630nm filter) of 3.9 mW/cm^2^. These back
reflectors clearly out perform the back reflectors made by texturing the
Al surface and using thin ZnO layers. In particular, higher FF and Jsc
and lower Rs values have consistently been observed. The higher FF
actually makes the performance of the cells with this new back reflector
close to that for cells made in these studies with Ag/ZnO back
reflectors. These Ag/ZnO stacks were made using United Solar Ovonic's
old production line (5MW line) and may have slightly lower performance
than the back reflectors made using the United Solar Ovonic R&D system
whose performance has recently been improved (Yan et. al. MRS 2005).

Table III.

IV data for a-SiGe cells with **textured** back reflectors. IV data
taken using 630nm filter.

+--------------+----------+-----+--------+-------+--------+----------+
| Back         | Back     | Voc | Jsc    | FF    | Rs     | Pmax     |
| Reflector    | R        |     |        |       |        |          |
|              | eflector | \(  | (mA/   |       | (Ω     | (m       |
|              | De       | V\) | cm^2^) |       | cm^2^) | W/cm^2^) |
|              | position |     |        |       |        |          |
|              | Machine  |     |        |       |        |          |
+--------------+----------+-----+--------+-------+--------+----------+
| Al(T         | Pr       | 0.  | 10.30  | 0.568 | 12.6   | 3.25     |
| extured)/ZnO | oduction | 554 |        |       |        |          |
+--------------+----------+-----+--------+-------+--------+----------+
| Al(T         | R&D      | 0.  | 9.30   | 0.531 | 17.4   | 2.90     |
| extured)/ZnO |          | 587 |        |       |        |          |
+--------------+----------+-----+--------+-------+--------+----------+
| Ag/ZnO       | R&D      | 0.  | 11.94  | 0.576 | 11.2   | 3.94     |
|              |          | 575 |        |       |        |          |
+--------------+----------+-----+--------+-------+--------+----------+
| A            | R&D      | 0.  | 10.81  | 0.614 | 10.3   | 3.88     |
| l(specular)/ |          | 584 |        |       |        |          |
|              |          |     |        |       |        |          |
| Z            |          |     |        |       |        |          |
| nO(textured) |          |     |        |       |        |          |
+--------------+----------+-----+--------+-------+--------+----------+

![](media/image1.wmf){width="4.9319444444444445in"
height="3.209722222222222in"}

Figure 1.

Quantum Efficiency Plots for A-SiGe:H cells with different back
reflectors.

Figure 1 shows quantum efficiency data for a-SiGe:H cells made with the
production Al(textured)/ZnO back reflectors and with the
Al(specular)/ZnO(textured). From this figure, the increased response in
the red portion of the spectrum with the use of the
Al(specular)/ZnO(textured) back reflector can be seen.

In addition to the bottom cells, we have also compared the
Al(specular)/ZnO (thick) with the production Al/ZnO and the 5MW Ag/ZnO
using triple-junction structures. The triple-junction structures were
optimized for Ag/ZnO back reflectors. One can see from the data in the
table that the QE middle+bottom for the cells with Al(specular)/ZnO
(thick) back reflectors are roughly 1.5-1.6 mA/cm2 larger than the
values for the cells with the 30MW production back reflector and only
0.4-0.5 mA/cm2 lower than the values for the cells with Ag/ZnO back
reflectors.

Table IV.

Data for triple-junction cells with different back reflectors

+-------+-----------+--------+-----+----+-----+------+------+------+
| Run   | Back      | Back   | P   | QE | QE  | QE   | QE   | QE   |
| \#    | Reflector | Ref    | max |    |     |      |      |      |
|       |           | lector |     | T  | Mid | Bo   | Mid+ | T    |
|       |           | Depo   | (   | op | dle | ttom |      | otal |
|       |           | sition | mW/ |    |     |      | Bo   |      |
|       |           | M      |     | Ce | C   | Cell | ttom |      |
|       |           | achine | cm^ | ll | ell |      |      |      |
|       |           |        | 2^) |    |     |      | C    |      |
|       |           |        |     |    |     |      | ells |      |
+-------+-----------+--------+-----+----+-----+------+------+------+
| 3D    | Al(t      | Prod   | 10  | 7. | 7   | 7.51 | 1    | 2    |
| -2395 | extured)/ | uction | .29 | 09 | .86 |      | 5.37 | 2.46 |
|       |           |        |     |    |     |      |      |      |
|       | ZnO       |        |     |    |     |      |      |      |
+-------+-----------+--------+-----+----+-----+------+------+------+
| 3D    | Al(s      | R&D    | 10  | 7. | 8   | 8.75 | 1    | 2    |
| -2395 | pecular)/ |        | .85 | 24 | .26 |      | 7.01 | 4.25 |
|       |           |        |     |    |     |      |      |      |
|       | ZnO(      |        |     |    |     |      |      |      |
|       | textured) |        |     |    |     |      |      |      |
+-------+-----------+--------+-----+----+-----+------+------+------+
| 3D    | Ag/ZnO    | R&D    | 11  | 7. | 8   | 9.16 | 1    | 2    |
| -2395 |           |        | .23 | 31 | .27 |      | 7.44 | 4.74 |
+-------+-----------+--------+-----+----+-----+------+------+------+
| 3D    | Al(t      | Prod   | 10  | 7. | 7   | 7.46 | 1    | 2    |
| -2403 | extured)/ | uction | .09 | 13 | .37 |      | 4.83 | 1.96 |
|       |           |        |     |    |     |      |      |      |
|       | ZnO       |        |     |    |     |      |      |      |
+-------+-----------+--------+-----+----+-----+------+------+------+
| 3D    | Al(s      | R&D    | 10  | 7. | 7   | 8.32 | 1    | 2    |
| -2403 | pecular)/ |        | .93 | 30 | .99 |      | 6.31 | 3.61 |
|       |           |        |     |    |     |      |      |      |
|       | ZnO(      |        |     |    |     |      |      |      |
|       | textured) |        |     |    |     |      |      |      |
+-------+-----------+--------+-----+----+-----+------+------+------+

The improved efficiencies with the Al(specular)/ZnO(textured) back
reflector is likely related to a different feature shape and size on the
top surface of the back reflector. Figure 2 displays Scanning Electron
Micrographs of the Al(specular)/ZnO(textured) back reflector and the
Al(textured)/ZnO back reflector from the production machine. Larger and
sharper features having more of a pyramidal look are seen for the
Al(specular)/ZnO(textured) back reflector while rounded features and a
more porous structure is seen for the production back reflector. The
smaller features for the production back reflector can also been seen in
the Atomic Force Micrographs shown in Figure 3.

![](media/image2.jpeg){width="2.8805555555555555in"
height="2.8805555555555555in"}
![](media/image3.jpeg){width="2.921527777777778in"
height="2.921527777777778in"}

Al(specular)/ZnO(textured) Production Al(textured)/ZnO

Figure 2.

Al(specular)/ZnO(textured)
![](media/image4.jpeg){width="4.810416666666667in"
height="3.607638888888889in"}

Al(textured)/ZnO

![](media/image5.jpeg){width="4.810416666666667in"
height="3.607638888888889in"}

Figure 3.

Atomic Force Micrographs for different back reflectors.

![](media/image6.wmf){width="6.43125in" height="3.3354166666666667in"}

Figure 4.

The dependence of QE values for bottom and triple-junction cells made
with Al(specular)/ZnO(textured) back reflectors on the number of
substrate passes across the ZnO target (and thus the ZnO thickness).

Figure 4 shows the dependence of the quantum efficiency for a-SiGe:H
bottom cells and the QE middle+bottom values for triple-junction cells
as a function of the number of passes over the ZnO sputtering target,
which is proportional to the ZnO thickness. Both the bottom cell QE and
the QE middle+bottom for the triple-junction cells behaved similarly
with the number of passes. As the number of passes increased from 10 to
24, the QE values increased as did the cell efficiencies. Between 24 and
35 passes, the QE values are relatively constant with maybe a slight
decrease when 40 passes were used. In the plot, we also included data
for cells made using the back reflectors from the production machine
again demonstrating the improvement with the Al(specular)/ZnO(textured)
back reflector. Attempts to improve the QE and efficiencies for the back
reflectors with thinner ZnO layers (10-18 passes) through high
temperature anneals were not successful, and the QE values remained
relatively unchanged, as can be seen from the data in Figure 4.

Requiring a certain ZnO thickness, we have explored the option of
increasing the ZnO deposition rate. Table V shows a-SiGe:H bottom cell
data for cells with Al(specular)/ZnO(textured) back reflectors made at
different ZnO target currents. As the applied power is lowered from the
standard conditions no change in the red Pmax is observed. Raising the
applied power above the standard conditions leads to lower currents and
lower Pmax.

Table V.

IV data for a-SiGe cells with **textured** back reflectors. IV data
taken using 630nm filter.

+--------------+--------+---------+------+------------+--------------+
| Applied      | Voc    | Jsc     | FF   | Rs         | Pmax         |
| Power        |        |         |      |            |              |
|              | \(V\)  | (mA     |      | (Ω cm^2^)  | (mW/cm^2^)   |
| (relative)   |        | /cm^2^) |      |            |              |
+--------------+--------+---------+------+------------+--------------+
| 0.5          | 0.590  | 11.2    | 0    | 11.5       | 3.92         |
|              |        |         | .592 |            |              |
+--------------+--------+---------+------+------------+--------------+
| 1            | 0.599  | 10.6    | 0    | 10.7       | 3.91         |
|              |        |         | .613 |            |              |
+--------------+--------+---------+------+------------+--------------+
| 2            | 0.581  | 9.60    | 0    | 12.4       | 3.39         |
|              |        |         | .609 |            |              |
+--------------+--------+---------+------+------------+--------------+

In the next quarter, we plan to to further develop this new back
reflector fabrication process by:

1)  making further attempts to increase the ZnO deposition rate without
    a loss in cell efficiency,

2)  attempting to reproduce the results observed in the R&D machines in
    a large area roll-to-roll line,

3)  testing the use of ZnAl targets rather than ZnO ceramic targets to
    make the high performance Al(specular)/ZnO(textured) back
    reflectors.