Basic Research Needs for Solar Energy Utilization - Office of ...

Amorphous Silicon. From its discovery in the early 1970s, hydrogenated amorphous silicon (a-
Si:H) has stimulated a large worldwide effort to develop various semiconductor device
applications. The main advantage of a-Si:H for PV is the low materials and manufacturing costs.
The films can be deposited on ordinary window glass, as well as on flexible substrates (stainless
steel or polyimide), to make PV products such as flexible roofing shingles, semitransparent
modules for windows or skylights, and portable power modules. The basic cell structure is a p-in
structure, with an undoped (intrinsic) a-Si:H layer between two very thin doped layers. This
structure produces an electric field throughout the cell to separate photogenerated electrons and
holes and produce electricity. The main disadvantage of a-Si:H is the modest efficiency that has
been achieved to date. Stable, small-area cell efficiencies reach 13%, but the best module
efficiency is 10.2% (for 1,000-cm 2 area). Most a-Si:H power modules sold today have stable
efficiencies of between 6% and 8%. The higher-efficiency designs use dual- or triple-junction
cell structures to capture more of the available sunlight (the band gap of a-Si:H ranges from 1.6
to 1.8 eV, depending primarily on hydrogen content). Bottom cells can be either lower-band-gap
a-Si:H, a-SiGe:H, or microcrystalline silicon.
The high absorption coefficient of a-Si:H stems from its lack of crystalline order. However, this
same disorder limits cell efficiencies and results in a self-limiting degradation known as the
Staebler-Wronski effect, or “light-induced instability.” No fundamental solution to this problem
has yet been found, so solar cells are made with thin undoped layers to minimize this degradation
in efficiency (about 15–25% decrease from the initial efficiency). Commercial products are
stable after about one month of exposure to sunlight and are always rated at their ultimate
(“stable”) efficiencies.
Films of a-Si:H are made by chemical vapor deposition from SiH4 onto a 200–300°C substrate
using a radio-frequency plasma (13.56 MHz) or dc plasma. Deposition rates are typically
0.1-0.3 nm/s, meaning that a 0.6-μm-thick solar cell is deposited in less than one hour. Both
batch deposition (typically for glass substrates) and roll-to-roll deposition (typically for flexible
substrates) are used in production. The largest thin-film manufacturing facility today produces
30 MW/yr of a-Si:H.
Cadmium Telluride. One of the most promising approaches for the fabrication of low-cost, highefficiency
solar cells is cadmium telluride (CdTe). This material has nearly the ideal band gap for
a single-junction device, and efficient solar cells have been made by a variety of potentially
scalable and low-cost processes; these include close-spaced sublimation (CSS), high-rate
physical vapor deposition, spraying or screen printing, solution growth, and electrodeposition.
The record cell (Figure 66) is a 16.5%-efficient device, whereas the best commercial-size
module is 11% efficient (typical commercial products are in the 7–9% range).
Although concerns are often raised about the environmental, health, and safety aspects associated
with cadmium and tellurium, advocates of this technology believe these are mainly issues of
“perception.” Extensive studies have been performed concerning the manufacturing, deployment,
and even decommissioning of CdTe modules. The results indicate that all safety issues can be
handled at a very modest cost, including the recycling of cadmium and tellurium from old
modules.
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