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

> **QUARTERLY TECHNICAL STATUS REPORT**

####### September 15 2004 - December 15 2004

### 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 are currently 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 our last report,
we showed the increased solar cell performance obtained when the ML
structure was added to back reflectors using flat specular back
reflector surfaces. This month, we have further optimized the
multi-layer structure to increase the performance of the non-textured
back reflector with the increases in red light (λ\>630nm) performance
with the improvements in P~max~ for single-junction a-SiGe cells shown
in Table I. Focusing on the data beyond λ=630nm was done because the
back reflector properties affect the solar cell performance only in this
wavelength region. With i-layers having similar thicknesses, use of the
new multi-layer deposition conditions has led to red light P~max~ values
of 2.8 mW/cm^2^ as compared with 2.2 mW/cm2 values for the cells with
Al/ZnO back reflectors having no ML structure.

Table I. IV data for a-SiGe cells made with Al(specular)/ML/ZnO back
reflectors.

IV data taken using 630nm filter.

+------------+----------+----------+----------+----------+----------+
| Date       | Voc      | Jsc      | FF       | Rs       | Pmax     |
|            |          | (m       |          |          |          |
|            | \(V\)    | A/cm^2^) |          | (ohm cm) | (m       |
|            |          |          |          |          | W/cm^2^) |
+------------+----------+----------+----------+----------+----------+
| Al/ZnO     | 0.564    | 7.05     | 0.563    | 17.8     | 2.24     |
+------------+----------+----------+----------+----------+----------+
| Al/ML/ZnO  | 0.561    | 8.28     | 0.556    | 17.9     | 2.58     |
|            |          |          |          |          |          |
| 9/04       |          |          |          |          |          |
+------------+----------+----------+----------+----------+----------+
| Al/ML/ZnO  | 0.576    | 8.75     | 0.558    | 15.6     | 2.81     |
|            |          |          |          |          |          |
| 12/04      |          |          |          |          |          |
+------------+----------+----------+----------+----------+----------+

To demonstrate the correlation of increased reflection with improved
solar cell performance, we have made Al/ML/ZnO structures with ML made
with different recipes. Again to eliminate the possibility of different
degrees of texturing affecting the reflectance, the Al layers were
deposited at room temperature leading to a surface that is close to
specular. The data in Figure 1 demonstrates that for the
Al(specular)/ML/ZnO structures made under conditions A, the reflectance
values in the red portion of the light spectrum (600-1000nm) are close
to those obtained with the Al/MgF~2~/Si/MgF~2~ optical stacks that we
have previously shown are similar to Ag/ZnO back reflectors. The
Al(specular)/ML/ZnO structure (Recipe B) was made under different Si
deposition conditions which led to lower reflectance values in the
600-1000nm region.

![](media/image1.wmf)

Figure 1. Reflectance data for different type of back reflectors.

Quantum Efficiency Data for cells made with the Al(specular)/ML/ZnO back
reflectors with recipes A and B are shown in Figure 2. The reader is
reminded that these back reflectors do not have textured surfaces so the
currents are lower than typically observed for high efficiency a-SiGe
cells. In the figure, one can see that the Recipe A conditions which
leads to higher reflectivity values in the 600-1000 nm range also leads
to increased red response for the solar cells with the integrated
quantum efficiency beyond 600nm leading to an 8.75 mA/cm^2^ red current
for cells with the Recipe A back reflector while the cell with Recipe B
conditions has a red current of 7.10 mA/cm^2^.

In Table II, the solar cell properties for devices made with the Recipe
A and B back reflectors (Al/ML/ZnO) are compared with those for a solar
cell with an Al(specular)/ZnO. Other than J~sc~, the properties were
obtained by IV measurements using AM1.5 light filtered with a 630nm
cuton filter. J~sc~ values were obtained through integration of quantum
efficiency data greater than λ=630nm. The data for the cell made with
the Al(specular)/ZnO back reflector are similar to those for the device
with the Al(specular)/ML/ZnO structure made with Recipe B which had the
lower reflectivity properties. In contrast, the cell with the
Al(specular)/ML/ZnO structure made under Recipe A having the better
reflectivity values has better overall solar cell performance due to
higher solar cell currents. These results demonstrate the potential
advantages of using the Al(specular)/ML/ZnO back reflector structure
with the proper recipes and layer thicknesses to improve solar cell
performance.

![](media/image2.wmf)

Figure 2. Quantum Efficiency data for a-SiGe cells made with
Al(specular)/ZnO/ZnOSi/Si/ZnOSi/ZnO back reflectors with Recipes A and
B.

Table II.

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

+-------------------+-----+---------+---------+-------+-------------+
| Back Reflector    | Voc | Jsc     | FF      | Rs    | Pmax        |
|                   |     |         |         |       |             |
|                   | \(  | (mA     |         | (Ω    | (mW/cm^2^)  |
|                   | V\) | /cm^2^) |         | c     |             |
|                   |     |         |         | m^2^) |             |
+-------------------+-----+---------+---------+-------+-------------+
| Al/ML/ZnO (Recipe | 0.  | 8.75    | 0.558   | 15.6  | 2.81        |
| A)                | 576 |         |         |       |             |
+-------------------+-----+---------+---------+-------+-------------+
| Al/ML/ZnO (Recipe | 0.  | 7.10    | 0.562   | 18.0  | 2.28        |
| B)                | 572 |         |         |       |             |
+-------------------+-----+---------+---------+-------+-------------+

To achieve high efficiency performance, it is well known that the back
reflector structure must have a textured scattering interface/surface to
achieve enhanced cell currents through increased light passes through
the intrinsic layers. Addition of the ZnOSi/Si/ZnOSi structure alone
will not lead to high performance cells and thus the high reflective
properties with this new optically enhanced multi-layer back reflector
must be achieved along with a textured back reflector surface to achieve
high solar cell efficiencies.

In contrast to the solar cell with specular surface back reflectors,
smaller improvements in bottom cell performance have been achieved thus
far in the program with the Al/ML/ZnO back reflector structure. Table
III compares IV data for a-SiGe cells with Al/ZnO and Al/ML/ZnO textured
back reflectors. These back reflectors have been textured by increasing
the top ZnO thickness. Again the IV measurements were made using AM1.5
light filtered using a 630nm cuton filter. Theses cells were chosen
because they have the best overall performance for the type of back
reflector used. The cell with the Al/ZnO back reflector has lower V~oc~
and FF likely due to slightly higher Ge contents in the intrinsic layer.
Thus there may be an improvement in the J~sc~ with the Al/ML/ZnO back
reflector however it is masked by lower Ge contents in the intrinsic
layer of the solar cell. However, the overall P~max~ value is higher for
the Al/ML/ZnO back reflector showing there is some improvement with this
new type of back reflector structure.

Considering the data for the specular back reflectors (See Table I),
higher improvements might be expected for the cells with the Al/ML/ZnO
back reflectors that are textured. However, we have started making these
textured back reflectors and further optimization should lead to more
significant increases with the use of the ML structure.

Table III.

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

+--------------+---------+----------+--------+-----------+-----------+
| Back         | Voc     | Jsc      | FF     | Rs        | Pmax      |
| Reflector    |         |          |        |           |           |
|              | \(V\)   | (m       |        | (Ω cm^2^) | (         |
|              |         | A/cm^2^) |        |           | mW/cm^2^) |
+--------------+---------+----------+--------+-----------+-----------+
| Al/ZnO       | 0.593   | 10.5     | 0.556  | 14.3      | 3.43      |
+--------------+---------+----------+--------+-----------+-----------+
| Al/ML/ZnO    | 0.605   | 10.4     | 0.585  | 12.5      | 3.66      |
|              |         |          |        |           |           |
| Recipe A     |         |          |        |           |           |
+--------------+---------+----------+--------+-----------+-----------+

There are a few potential reasons why the improvement in device
performance is not as large when the back reflector is textured. To
create a textured back reflector, both the use of a thick top ZnO as
well as use of high substrate temperatures during Al growth to texture
the Al surface prior to ZnO growth have been used. In these studies, it
was thought that minimizing the surface roughness at the interfaces of
the ZnOSi/Si/ZnOSi optical stack might be important to achieve high
reflectivity. Thus, thick ZnO have initially been used to achieve the
textured scattering surface for the back reflector. Texturing the Al
surface with high temperature growth will likely lead to a different
type of textured surface which might not affect the reflectivity of the
multi-layer stack and lead to higher performance module.

With the addition of the textured surface (either with the thick ZnO or
the textured Al), layer thicknesses in the multi-layer stack might need
to be re-optimized to achieve high solar cell efficiencies. Little of
this work has yet to be done in these studies.

Also a different scattering scheme which does not involve the formation
of rough interfaces might need to be developed to achieve high
efficiency solar cells with the Al/ML/ZnO back reflector. Adding a
porous TCO layer or altering the present layers to have low density
regions or voids that would act as scattering sources might need to be
done. The low density regions/voids would have to have sizes on the
order of 600-1000nm. Developing materials with these size scatterers
that would be conductive enough to achieve low solar cell series
resistances and maintain these properties during all of the solar cell
fabrication steps might be a difficult task. Thus focus in future
studies will be on the testing of textured Al surfaces and
re-optimization of the Al/ML/ZnO stacks.