Authors: Willem Vos & Devashish Sharma
Silicon is the basic material that is used in our smart
phones, in optical sensors, or in solar cells on our roofs. It is a
major outstanding challenge that silicon absorbs incident light only
weakly, especially in the red part of the visible spectrum. Recently,
using extensive computations, scientists from the University of Twente
in the Netherlands have discovered that a 3D nanostructured back
reflector greatly increases the absorption. The back reflector is also
made of silicon which is convenient to integrate with ultrathin silicon
films. Consequently, next generation devices can be made ultrathin,
which allows new devices to be much more flexible and compact. The
results appear in the leading journal Optics Express that is published
by the Optica society (formerly the Optical Society, OSA).
Figure
1: Schematic illustration of a thin photovoltaic cell where the
absorption of light is increased thanks to a 3D nanostructured back
reflector - a 3D photonic band gap crystal - that thoroughly recycles
the unabsorbed light. Hence, even an ultrathin photovoltaic cell could
harvest much more light (than a cell without such a back reflector) and
thus generate more energy to drive an external appliance, like the
schematic lamp.
When
incident light is absorbed by a plate semiconducting material like
silicon, negatively charged electrons are excited from the lower-energy
valence band to the higher-energy conduction band and similar for
positively charged holes (that represent the lack of electrons). By
attaching electrodes to the plate, the electrons and holes are harvested
and sent into an electric circuit to drive a useful appliance. This
process notably occurs inside a solar cell, see Figure 1, where the
harvested current serves to power an LED for ambient lighting. While
thick silicon plates are widely used, thin silicon films are enjoying a
rising popularity on account of their obvious sustainability, since they
require much less material, less resources, and lower cost.
Unfortunately, however, thin and ultrathin silicon films hardly absorb
light, especially at long wavelengths in the visible spectrum where the
sun radiates a lot. In other words, thin silicon films are not “black”.
Therefore the team set out to study how a back reflector could recycle
unabsorbed light, and become highly absorbing, or “black”.
Figure 2: Absorption enhancement, equal to the ratio of absorption with a back reflector and absorption without
back reflector, for an 80 nm ultrathin silicon layer. The red curve
pertains to the ultrathin film with a 3D photonic band gap back
reflector. The blue dashed curve pertains to an ultrathin film with a,
hypothetical, perfect metal back reflector. The green line indicates the
reference level (= 1) of the ultrathin film without any back reflector.
As
a back reflector, the Twente team studied a diamond-like photonic
crystal composed of two sets of perpendicular pores, shown in Figure 1.
Such photonic crystals are known to have a record-wide 3D photonic band
gap. As a result, the team indeed finds that this crystal is a truly
omnidirectional, broadband, and polarization-robust back reflector. Lead
author Devashish effuses: "Our extensive computations reveal that the
photonic back reflector yields a striking 9.15 times enhanced absorption
even for a 80 nanometer ultrathin film (see Figure 2). Our devices are
up to 80% lighter than bulk silicon, due to the porosity of the photonic
structure, jokingly referred to as “holeyness”".
Figure
3: Absolute absorption spectra (in %) for an 80 nm ultrathin silicon
layer. The red curve pertains to the ultrathin film with a 3D photonic
crystal back reflector and the blue dashed curve pertains to the
ultrathin film with a perfect metal back reflector. The green line
represents the absorption of an ultrathin film without any back
reflector, with the black solid line as the reference level (or 0 %).
Figure
4: Schematic of a thin charge-coupled device (CCD) whose absorption
could also be enhanced by a 3D photonic band gap back reflector. Hence,
such a CCD becomes more efficient at collecting light.
Group
leader Vos explains: "Such a strong absorption in a thin silicon film
(see Figure 3) can also be interpreted in a quantum physical picture,
namely that the photonic crystal acts as a colored electromagnetic
vacuum below the absorbing film. The absorption of incident light is so
strongly boosted that ultrathin silicon would effectively turn black".
The Twente team also projects that their holey 3D inverse
woodpile structures offer application potential for compact on-chip
sensors, photodiodes, and charge-coupled devices (CCD) for cameras (see
Figure 4).
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