DOE PV Manufacturing Initiative
University of Texas at Austin
UT Contacts: Joe Beaman, S. Banerjee, Banerjee@ece.utexas.edu, 471-6730
Background
Current worldwide total power generation capability is ~15 TW and corresponds to annual
energy usage of ~500 Quads (1 Quad = 1015 or quadrillion BTU), with an annual increase of 1-2%.
Of this, ~80% comes from fossil fuels (oil, natural gas and coal) which will be exhausted in several
hundred years. In addition, CO2 emissions and global warming have renewed interest in “green”
sources of energy such as photovoltaics, even though current installed capacity is only a few GW.
About 1 kW / m 2 is available in a particularly sunny location (translating to ~105 TW of solar
radiation worldwide, of which about 600TW is potentially accessible), but not all of this solar
power can be converted to electricity. Much of the photon flux is at energies less than the cell band
gap, and is not absorbed. High-energy photons are strongly absorbed, and the resulting electron hole
pairs may recombine at the surface. A well-made single crystal Si cell can have about 25 percent
efficiency for solar energy conversion, providing approximately 250 W/m 2 of electrical power
under full illumination. This is a modest amount of power per unit solar cell area, considering the
cost and effort involved in fabricating a large area of Si cells. Amorphous Si thin film solar cells can
be made more cheaply, but have lower efficiencies (~10%) because of the defects in the material.
The cost/ scalability and efficiency of photovoltaic (PV) technology are obviously of
paramount importance for terrestrial applications. Currently, it costs only about 3cents per kWh for
electricity generation from fossil fuels, but about 10X that amount from amorphous Si solar cells,
and the time to recover the investment in PVs is about 4 years. In terms of scalability, at 10% cell
efficiency, approximately 3% of the land area would have to be covered with solar cells to meet the
U.S. energy needs, which would of course create other environmental problems. One approach to
obtaining more power per cell is to focus considerable light onto the cell using concentrators (CSP).
Although Si cells lose efficiency at the resulting high temperatures, GaAs and related compounds
can be used at 100ºC or higher. Clearly there is no one-size-fits-all solution to the problem. It is
important to research a spectrum of solar cell technologies spanning inexpensive, low efficiency
cells such as organic thin film cells and amorphous and/or polycrystalline silicon-germanium thin
film cells for rooftop applications, to higher efficiency but more expensive single crystal silicongermanium cells. There is need for even higher efficiency, compound semiconductor, multijunction cells and nanostructured (nanowire and nanocrystal based) devices. Although these will be
more expensive, these can be used for CSP. In such solar concentrator systems more effort and
expense can be put into the solar cell fabrication, since fewer cells are required.
The facilities at the University of Texas Microelectronics Research Center
(www.mrc.utexas.edu) include 12,000 square feet of class 100 and class 1000 cleanroom space for
crystal-growth and device processing. In addition to state-of-the-art cleanroom facilities, MRC has
15,000 square feet of characterization laboratories and office space for 15 faculty, support staff, and
150 graduate students. The cleanroom contains complete Si CMOS processing capability, including
fine-line lithography, sputter deposition, reactive-ion etching, ultrahigh purity process gases, a DI
water system, rapid thermal processing systems, wet chemistry stations, and low pressure CVD for
polysilicon, oxides, and nitrides. The cleanroom also houses the reactors for several Si and III-V
epitaxial crystal-growth techniques, including molecular beam epitaxy, metalorganic CVD, remote
plasma CVD, rapid thermal CVD, and ultrahigh vacuum CVD. The characterization laboratories
contain the apparatus for comprehensive photovoltaic optical and electrical measurements as well as
extensive computer facilities. UT faculty and their research specializations are:
 S. Banerjee: Si-Ge multi-junction, low cost, high efficiency cells
 Emanuel Tutuc, S.V. Sreenivasan, B. Korgel: Nanotextured, nanowire and nanodot PV
 Ed Yu, Seth Bank: Ultrahigh-efficiency III-V photovoltaics
 Ananth Dodabalapur- Organic and Hybrid Inorganic/Organic Thin-film solar cells
 Frank Register: Modeling and simulation
There has been much progress in the development of low cost, large area solar cell panels
using amorphous silicon (a-Si) thin film devices on glass or other substrates. These devices can be
made easily and in large area formats, and there is no need for complex focusing systems. One of
the major concerns is its low efficiency of~10% (compared to about 30% for single crystal Si and
compound semiconductor heterojunction cells). DOE has recognized that if amorphous Si solar cell
technology could achieve ~20% efficiency, under 0.7 $/ peak Watt corresponding to 0.06$ per kW2
hr, 200 W/m at AM0, and lifetime of 30 years, it should make such solar cells competitive for
large scale terrestrial applications unlike more complex technologies. However, the a-Si solar cell in
its original embodiment cannot meet the requirements and new structures such as multi-junction
tandem amorphous/microcrystalline solar cells are required. This can satisfy the demands of high
efficiency at low cost. Therefore, in collaboration with a solar cell startup, Applied Novel Devices,
Banerjee investigates alternative techniques to “peel off” thin ~25 micron single crystal films from
bulk substrates using mechanical exfoliation, and deposit amorphous/ microcrystalline films on
cheap substrates using low temperature CVD. These techniques have potential advantages over the
standard deposition techniques for solar cell applications in terms of better interfaces. This is
particularly important for the multi-bandgap amorphous solar cells proposed here because of the
presence of many interfaces in the cell. These deposition techniques also provide excellent control
over layer and doping transitions as well as the capability to incorporate a high percentage of
hydrogen in the films. Such bonded hydrogen tie up dangling bonds and improve the optical and
electrical properties of the amorphous and microcrystalline films.
Hence, it is desirable to use multi-bandgap semiconductors in solar cells which would be
capable of efficiently absorbing different parts of the solar spectrum thereby yielding higher
efficiencies than single bandgap cells. It has been shown that the theoretical maximum efficiency
with two bandgaps in series for tandem solar cells is 50 % with gaps of 1.56 eV and 0.94 eV. With
three bandgaps (1.75 eV, 1.18 eV and 0.75 eV) multijunction cells, the maximum efficiency is 56%.
One requirement for a high efficiency photovoltaic device is for the minority carrier diffusion
length to be larger than the optical absorption length. The competition between the absorption and
carrier diffusion length is the limiting factor for many solar cell devices, especially in thin film
materials. The device, shown schematically consists of an array of nanorod p-n junctions made by
nanoimprint lithography and reactive ion etching. Unlike a conventional p-n junction solar cell
where the photo-generated carriers are collected vertically, the advantage of such device is that the
photo-generated electron-hole pairs are collected laterally, which increases the electron-hole pair
collection efficiency. In this effort Tutuc and Sreenivasan fabricate and investigate the
performance of Si and GaAs nanorod array solar cells. The nanorod arrays will be fabricated using
reactive ion etching along with nanoimprint lithography. The junction will be fabricated using
dopant diffusion, and the top contact will consist of an indium-tin-oxide thin film and metal fingers.
Korgel focuses on chemical quantum dot synthesis pathways and applying them to novel, low cost
PV cells.