Biomedical Engineering Reference
In-Depth Information
designed to be most effective at wavelengths
∼
600 nm and are less effective for the near-infra-
red and other visible wavelengths, which are
present in a large portion of the incident solar
energy. These films also exhibit poor thermal
stability due to the mismatch of thermal expan-
sion coefficient between SiN
x
and Si. The PECVD
process is also costly.
Subwavelength structures such as pores and
periodic and stochastic features can be used to
realize graded refractive indices at the surface of
semiconductor materials that are used in opto-
electronic devices
[23-30]
. These structures can
potentially provide broadband antireflection,
but the increased surface area of these features
can increase the number of defect sites where
electron-hole recombination can occur, which
can be detrimental to the efficiency of photovol-
taic devices
[31]
.
Glass and plastics such as polycarbonate and
polymethylmethacrylate (PMMA) are impor-
tant optical substrates. Transparent substrates
suffer less severe reflective loss than silicon and
other semiconductor materials; a loss of
∼
4% at
the air/glass interface can degrade the perfor-
mance of devices with multiple components and
optical interfaces. Substrates that have a refrac-
tive index of approximately 1.5 (e.g., glass)
should ideally be coated with a material with a
refractive index of 1.22, based on the Fresnel
equation. Unfortunately, materials with this low
refractive index are rare. As a result, magnesium
fluoride with a refractive index of 1.38 is widely
used as a single-layer ARC. But magnesium-
fluoride coatings are not suitable for polymers
due to the high tensile growth stress and the
poor mechanical properties of fluoride thin films
at low polymer-processing temperatures
[32,
33]
. Fluoropolymers with low refractive indices
can also be used as ARCs
[34-37]
.
To reduce reflection, expensive multilayer
ARCs are typically used. The solution and tem-
perature requirements needed for depositing
these coatings make them incompatible with
many substrate materials such as plastics
[38]
.
To maximize the suppression of reflection, the
refractive index of a film interposed between
two materials should be the geometric mean of
their refractive indices. The thickness of the film
should be a quarter wavelength (i.e., a quarter
of the wavelength of light at the specific location
where reflection reduction is desired) to take
advantage of interference. Light that is reflected
at the coating-substrate interface will be half of
a wavelength out of phase from the incident
light that is reflected from the coating-air inter-
face, resulting in destructive interference and
reduced reflectance
[4]
.
One important application of the quarter-
wavelength ARC is to improve the conversion
efficiency of solar cells. Solar cells collect photons
from sunlight and convert them into electric
power
[5]
. Materials that are used to construct
solar cells tend to have high refractive indices
(e.g., crystalline silicon has a refractive index of
∼
3.5), which result in high reflectance
[6]
.
Reflected light reduces the number of photons
that can be used to form the electron-hole pairs
that drive the current in solar cells. As a result,
one of the goals in fabricating solar cells is to
reduce the amount of light that is reflected. To
accomplish this goal, texturing and antireflection
coatings are used. Texturing (usually at the geo-
metrical-optics scale) reduces overall reflection
by directing diffuse reflections back into the sub-
strate. This is usually done by chemical etching
using potassium hydroxide (KOH) or inorganic
acids
[7, 8]
. Though not quite as common as wet
etching, reactive ion etching (RIE) and lasers can
also be used to texture silicon
[9-20]
.
Bioinspired texturing at the multiwavelength
scale has been demonstrated to improve the
light-harvesting capabilities of crystalline silicon
solar cells
[21, 22]
. Typical antireflection coatings
used on silicon are silicon nitride (SiN
x
) and tita-
nium dioxide (TiO
2
) films.
Quarter-wavelength SiN
x
films deposited by
plasma-enhanced chemical vapor deposition
(PECVD) are the industrial standard for ARCs
on crystalline-silicon solar cells
[5, 6]
. They are
