High Efficiency and High Rate Deposited Amorphous Silicon-Based Solar
Cells

PHASE III First Quarter

Technical Progress Report

September 1, 2003 to November 30, 2003

**NREL Subcontract No. NDJ-2-30630-08**

Subcontractor: The University of Toledo

Principal Investigator: Xunming Deng

> Department of Physics and Astronomy
>
> University of Toledo, Toledo, OH 43606
>
> \(419\) 530-4782
>
> dengx@physics.utoledo.edu

Contract technical monitor: Dr. Bolko von Roedern

**Expansion of UT's Fabrication Capability for Triple-junction Amorphous
Silicon Based Solar Cells**

Contributors: H. Povolny and X. Deng

Triple junction solar cells have been fabricated at UT using a
multi-chamber rf PECVD deposition system made by GSI in 1990. The
original system consisted of two PECVD deposition chambers and a load
chamber. Figure 1 shows a photo of the original system. Several years
ago, the load chamber was converted to become a dual purpose chamber: 1)
designated deposition chamber for the n-type Si:H based materials and 2)
the load lock chamber.

The three PECVD deposition chambers produce high quality amorphous and
microcrystalline silicon and silicon germanium alloy with the each of
the doped layers, n and p, deposited in a dedicated deposition chamber.
The chamber dedicated to intrinsic silicon and silicon germanium alloys
has been equipped with a VHF power supply and impedance matching network
in order to explore the deposition process and film quality at higher
frequency. This allows us to deposit high-quality materials at higher
deposition rates than can be achieved with standard RF PECVD.

![](media/image1.jpeg){width="6.0in" height="3.140277777777778in"}

Figure 1. A photo of UT's PECVD system before expansion.

A novel Hot-Wire CVD chamber was also added several years ago to explore
high deposition rate microcrystalline silicon as well as high deposition
rates of amorphous silicon. Figure 2 shows a photo of the HWCVD system.
The chamber uses a coiled filament design which leads to extremely high
gas disassociation rates. There are three separate gas inlets, one
directed through the coiled filament, an annular gas inlet just above
the filament, and an annular gas inlet just below the substrate. This
allows us to study the effects of gas injection position on film growth
and film properties. Finally, the chamber includes RF cathode so that
PECVD can be done as well.

![](media/image2.jpeg){width="4.266666666666667in" height="3.2in"}

Figure 2. A photo of UT's HWCVD chamber integrated to the multi-chamber
PECVD system.

During this program period, we have carried out a major expansion to
this deposition system to include 4 sputter deposition chambers and an
additional load lock chamber. With this expansion, all of the 12 layers
in a triple-junction solar cells, Al/ZnO/nipnipnip/ITO, can be made
without vacuum break at the interfaces. Figure 3 shows a AutoCAD drawing
of the system before the expansion and Figure 4 shows the AutoCAD
drawing of four additional chambers (1 load lock and 3 sputter chambers
for Al, ZnO and ITO, as previously designed).

The four sputter chambers were added for the purpose of making back
reflector and top contact layers, as well as other metal and oxide
layers, without vacuum break. Each sputter chamber has a sample rotation
stage so that we can evenly coat a four inch square substrate using only
a three inch diameter sputter target. A second gas inlet allows for
reactive sputtering and an easily accessible bottom flange allows for
the possibility of incorporating other processes into our system with
minimal difficulty or downtime.

![](media/image3.png){width="4.000694444444444in"
height="2.8305555555555557in"}

Figure 3 AutoCAD drawing of the PECVD/HWCVD system before recent
expansion.

![](media/image4.png){width="5.375694444444444in" height="5.26875in"}

Figure 4. AutoCAD drawing of the multi-chamber deposition system with
additional chambers (1 load lock and 3 sputtering chambers for Al, ZnO
and ITO layers, as previously planned).

The load-lock chamber is equipped with a substrate elevator capable of
holding up to ten individually selectable substrates at one time. This
is vital for any system of this size and versatility so that multiple
chambers can be used at the same time without interfering with each
other and without requiring a vacuum break. Partially completed devices
can be stored in the load chamber until the next chamber and operator
are ready for them without interfering with the workings of any of the
other chambers. The elevator is equipped with a manipulator arm so that
substrates can be moved between holders with different masks in situ.
With appropriate masks, this allows us to perform combinatorial design
of experiment in addition to simple patterning of individual layers.

Over all, the expanded combination HWCVD/PECVD/VHFPECVD/Sputter
deposition system with integral load-lock incorporating multiple
substrate capability with interchangeable masks provides remarkable
versatility to carry out a variety of key studies in various aspects of
photovoltaic devices.

The design and the optimization of this four additional sputter chambers
are supported by NREL while the acquisition of the system components and
the installation were supported by a research grant from Air Force
Research Laboratory. Figure 5 is a photo of the entire system with nine
chambers\--3 PECVD, 1 HWCVD, 4 Sputtering, and 1 Load lock chambers.
Initial films have already been made using these new sputter chambers.
The optimization is underway and will be reported extensively in the
next Quarterly Report.

![](media/image5.jpeg){width="5.997916666666667in"
height="4.024305555555555in"}