Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Antireflection coating with plasmonic metal nanoparticles
for photovoltaic applications
Zbigniew Starowicz, Marek Lipiński
Institute of Metallurgy and Materials Science of Polish Academy of Science, Cracow
Photovoltaics is dynamically growing field of science and industry.
Every year significant growth of installed power of photovoltaic
systems is observed. Two major factors stand up against
world wide popularity of PV: low efficiency and high production
cost, which is mainly concerned with materials costs. Many technological
and physical tricks have been implanted to the solar
cells to improve their capabilities of conversion solar radiation
into electricity. To deal with high materials costs photovoltaic
thin film technology have been invented. One of the methods of
improving cells efficiency is reduction of reflected and non-absorbed
photons. Conventionally antireflection coating and surface
texturisation is used for that matter. Texturisation of semiconductor
surface means creating a few micron-high geometric
figures, shapes on the surface to enable multiple reflection of
light beams. For thin film technology implementation of surface
texture is impossible due to size of figures exceeding total
cell thickness. Here plasmonics as a new solution have been
proposed.
Plasmonics is a new branch of science which provides tools
for confinement of light in nanoscale objects. Generally speaking
special type of interaction of light with metallic objects is the origin
of many phenomena. In the solar cells they can provide improvement
of carriers generation due to near field around the object or
efficient scattering of light and extending the optical path length
[1]. This paper refers only to second feature of plasmonic structures.
These structures can be used in any kind of solar cells
[2]. In the last few years many authors have reported their achievements.
For example Matheu in his work announced 2,8 and
8,8% increase of efficiency after addition of 100 and 150 nm silver
nanospheres [3].
Physical explanation of this phenomenon is closely related to
internal structure of metals. Immovable atomic cores sit in the
node of crystalline net and electron cloud is filling the space around.
Electric field applied to small metallic object cause the electron
move according to field direction, leaving positively charged
cores behind, then the object act as a dipole. When light wave
falls on the particle smaller than wavelength, oscillating electric
field cause the electron cloud is oscillating too (Fig. 1). The
quantum of oscillation of different sign charges in metal is a quasi-particle
known as “plasmon”. For wave of frequency close to
metal plasmon frequency (for spherical shape equals square
www.livenano.org/wp-content/uploads/2011/03/plasmon.jpeg
Fig. 1. Electromagnetic wave passing by two metal nanospheres
root of three times value of plasmon frequency) this oscillations
take the form of resonance. For photovoltaic applications great
role play aluminum and silver because of their high density of
valance electrons [6], also gold and copper as their resonance
is placed in visible range.
Oscillating electrons scatter the incident beam in the direction
of semiconductor substrate preferable, as there is much greater
number of optical modes than in air. Placing particles on both
sides of the cell this can provide light trapping effect as the is an
angular scattering.
Scattering abilities can be calculated from the following formula:
(1)
as well as absorption from:
(2)
Where alpha is polarization:
(3)
dependent on particle volume (size) and particle and surrounding
medium dielectric functions. Scattering abilities change with frequency
of incident light. In the Figure 2 it was shown how scattering
cross-section normalized to the particles size varies with
different wavelengths and size for silver spheres.
This plot should be understood as for example: 20nm diameter
sphere scatters light from 18 times larger area than its geometrical
size at the resonance wavelength of 475 nm.
Crucial feature of the particle is scattering abilities better than
the absorption ones.
Q sca
= C sca
/(C sca
+ C abs
) (4)
On that basis we can sey that small particles absorb stronger than
bigger. For 100nm sphere scattering is large in the broad part
of spectrum and the value of Q sca
exceeds 90%. Larger particles
have also resonances of the higher mode than dipole – quadrupole,
hexapole – recognized as picks moved towards shorter wavelengths.
Second important parameter is effectiveness of scattering
the light to substrate that equals:
f = f substrate
/f total
(5)
It is more likely that light would be send to medium of higher value
of refractive index, this aspect was well covered by Catchepole
and Polman. Position of the particle above semiconductor
substrate is compromise between scattering cross-section which
grow as apart increases and parameter f which drops down [4].
Final optical properties of the plasmonic structure depends on
many variables like: particle size, shape, position above substrate
and distribution as well as the refractive indexes of surrounding
media [5].
Plasmonic structures composed of array of nanoparticles fixed
to the substrate are obtained by thorough, time demanding
and expensive methods. We chose to our experiments nanoparticles
from colloidal solutions. In that way size of particles can
be precisely fixed. Particle will be placed in the semiconductor
substrate by the spin-coating method or deep coating method.
Elektronika 6/2012 105