Event

IDEAZ (NANOTECHNOLOGY DEPARTMENT)

Submitted by

Udit kumar (COG14/03257) Sourabh hirau (COG14/08151)

IIT Roorkee IIT Roorkee

Udit.dhananjay.shah865@gmail.com skhirau@gmail.com

Contact no – 8979523753 8126130183

Metamaterial Antireflective coating on Solar cell –A review article

Introduction

Energy is the key issue these days, fossil fuels is our major sources of energy. Following the

current rate of consumption of fossil fuel, it will last in next 30-50 years. Energy is the

backbone of industrialized economy. Nuclear power is costly and risky. So many countries

have shown their interests in renewable sources of energy.

Now a day solar energy has emerged as a potential source of energy. There are different ways

to harness and use solar energy. Solar cell is one of the best ways to harness; it directly

converts solar energy to electrical energy. On an average single p-n junction solar cell

efficiency is not more than 20%. As it cannot use the whole solar spectrum; making a high

efficiency solar cell is always a challenging task. A lot of work has been done to choose an

appropriate semiconductor for appropriate energy gaps to match complete solar spectrum.

There is multi-junction solar cell which gives us better utilization of the solar spectrum.

Metamaterial has a huge advantage in this field. These are the artificially engineered

materials with property not found in nature. These materials are extremely useful in

controlling the path of the light. Many authors were intended to propose Metamaterials as

antireflective coating with traditional p-n junction solar cell.

Antireflective Techniques

The current anti-reflective technologies are classified into three broad categories

1. The film anti-reflective coatings

2. Micron-scale Texturing and light trapping

3. Sub-wavelength surface texturing

The most common technique used today is the use of thin film-reflective coatings (ARCs).

These coatings are used to minimise reflectance almost completely at a particular wavelength

through destructive interference of reflected light. Double or even multi-layer ARCs are also

used to minimise reflection over multiple frequencies.

Micron scale texturing and light trapping is also a potential mean of antireflection through

utilization of “double bounce” effect. In this light reflected from one part of textured surface

is directed onto another part of the surface and light incident more than once on solar cell.

Now a day Metamaterial is used to serve this purpose (but with different mechanism).

Metamaterial –an introduction

A Metamaterial is a macroscopic/nano composite of periodic or non-periodic structure,

whose function is due to both the artificial cellular architecture and chemical composition.

These materials exhibit some properties, which are not usually shown by the naturally

occurring materials.

By using artificial structure we attain negative permeability and also permittivity, earlier it

only works in microwave region of light but with nano level structures now it works in

visible frequencies too.

Generally they are left-handed material.

Figure-1

Negative reflective index materials

Figure-2

It happens due to the simultaneous existence of

1. Negative permeability

2. Negative permittivity

Figure-3

A typical Metamaterial

The resonating wires give the material The resonating loops give the material

negative permeability negative permittivity

The resonating wire and resonating loops has the most critical role in Metamaterials. Wires is

for electrical response and resonating loops are for magnetic response, together they give us

negative reflective index as shown in figure. Generally it is the structure which play key role

in deciding the property of Metamaterial.

Metamaterials application in solar cell

• Photonic metamaterial is used in multi band solar cell.

• In designing high efficiency solar cell (composite metamaterial)

• light trapping applications

• Anti-reflective coatings

Metamaterial as antireflective coating

Many pioneering works has been done in this area few of them are described below

Metamaterials of Sawtooth Structure According to Nicholas X. Fang, the Brit (1961) and

Alex (1949) d’Arbeloff, Associate Professor of the Department of Mechanical Engineering,

MIT, the thinnest materials used to fully capture light are limited to a very narrow range of

wavelengths and the angles of incidence. They proposed a design composed of a pattern of

wedge-shaped ridges whose widths are precisely tuned to slow and capture light of a wide

range of wavelengths and the angles of incidence. These Metamaterials could be made

extremely thin, saving weight and cost. Also, Kin Hung Fung, an MIT postdoc has proposed

a design of multilayer sawtooth structure to absorb a wide range of frequencies with an

efficiency of more than 95 percent.

Figure-4

Fig: Sawtooth Tapered ridges, made from alternating layers of metal and insulating material

deposited on a surface, can produce a metamaterial that is tuned to a range of specific

frequencies of light. Light of different wavelengths is absorbed by the material at different

levels, where the light's wavelength matches the width of the ridges. Designed in MIT’s

Department of Mechanical Engineering

Solar cell

A simple si solar cell works in breakdown region when photon respective to the band gap

falls on it leads to charge separation and one electron goes into the conduction band from

valence band and that way we get current

Fig: a simple p-n junction solar cell

(Not getting into detail of solar cell working) The kind of solar cell used commonly without

any anti reflective coating are quiet less efficient.

Figure-5: showing solar panel installed in a area

Efficiency calculation some commonly used solar cell without antireflective coating are

following-

Disclaimer – The data provided in the tables are based on good faith on the authors.

The above data show clearly the efficiency of commonly used solar cell is not more than 20%

in almost all type of cell.

Solar cell with antireflective coating

Metamaterial anti reflective coating is applied to enhance the performance of solar cell

Fig: showing a schematic diagram of multi junction solar cell with anti-reflective coating

Efficiency of solar cell after coating are described below

The % reflectance and transmittance characteristics with frequency of light ha show above,

which clearly will result into significant increase in efficiency

Above chart shows light absorption due to different type of metamaterial coating

Conclusion

Metamaterial artificially engineered material is very useful in manipulating the path of light

and take advantage of it. We observe a significant rise in absorption coefficient after applying

antireflective coating which leads to better efficiency of solar cell.

References

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