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CHAPTER 1
Introduction
Today Environment pollution, Green House Gas Emission, Energy Crisis are some of the most discussed issues in the world than never before. It is estimated that 17 billion mega Watts of electricity is required in the world annually; 730 billion metric tonnes of Carbon-dioxide is released to the atmosphere in the world last year and numbers are growing. Some of the resultant effects are global warming, climatic changes which are not only affecting humans but also other living beings as well as entire bio-diversity of the earth. It is a pitiful fact that the major contribution to above mentioned numbers are by developed and developing countries, the nations which are contributing more for the advancement of science and technology in the world.
The science and technology is growing exponentially if we observe the advancement in past decades and so is the pollution. Therefore it is true that if we do not apply science and technology for the above mentioned issues more and more, man has to face the dark period in the near future. Considering the last sentence, one field that is evolving aggressively from past few years and promising to provide energy conserving and generation solutions in the field of electronics is “Green Photonics”.
Green Photonics help protect the environment. The impact of the green photonics is not only in Clean Energy Generation and Solid State Lighting but also other interesting applications in Energy Loss reduction, Environmental Sensing, Light Pollution effects etc. Thus it is one of the promising fields in electronics for energy conservation.
The growing importance of this area in photonics has led to organization such as OIDA (Opto-Electronics Industry Development Association) and OSA (Optical Society of America), organizing special conferences in the area and bringing out specific reports on Green Photonics share of the opto-electronic market. As per OIDA, while the CAGR (Compounded Annual Growth Rate) for global optoelectronics is forecast to be 3.4%, the Green photonics share is forecast to be very encouraging 19.6%. This translates into revenue of $493 billion for optoelectronics components by 2020, of which $261 billion or 53% is the green photonics market share.
CHAPTER 2
Clean Energy Generation through Solar Technology – Photovoltaic
The most environmental friendly source of energy is undoubtedly the Sun. In one hour, the sun deposits 13.6 TW of radiation on earth – more energy than is consumed in a whole year (13 TW). Solar Technology has been around for a long time but its relevance is more today than it was ever before.
In 1950s, scientists at bell laboratories serendipitously discovered that silicon doped with certain impurities is very sensitive to light. This discovery led to photovoltaic as a source of energy generation for space. With the consensus that continued reliance on fossil fuels result in global warming, solar technology has become very relevant. The critical question remains the economic viability. Research which is continuing at a frantic pace in exotic areas such as Nano-photonics, photonic crystals and Plasmonics may provide new answers.
Figure (1):- Amorphous Silicon PV panel
Silicon Photovoltaic (PV) cells include those based on single crystal, multi crystalline and amorphous thin films. Non silicon based Thin films are also being actively studied. Solar cells have evolved from single crystal based cells to thin film planar solutions. This leads to cost reduction. A major route to improving solar cell efficiencies is by improving light trapping in solar absorber layers. The basic aim is to increase the number of photons absorbed by the solar cell and to use the maximum efficiency of each photon. Photonic technology is very important for this. The conventional silicon photovoltaic cells have the efficiency of 7 to 13 per cent.
Traditional light trapping schemes involve a textured metallic back reflector that also introduces losses at optical wavelengths. Light trapping in silicon solar cells mainly falls into two categories, the first being the reduction of front surface reflection and the second involve increasing the optical path length of light within the cell.
Concept of Photonics
Various photonic concepts can be applied to help in increase the optical efficiency of the PV systems. The photonic principles that are used for increasing efficiency are
1. Spectral selectivity: sunlight consists of various wavelengths. The efficiency of absorption of the wavelength can vary depending on the material of the solar cell. This varying sensitivity to different parts of the spectrum to different solar cells or it is used to trap light after a spectral shift. This spectral shift is used to increase the light guiding efficiency.
2. Angular selectivity: angular confinement of the light rays can also be used to increase efficiency. The angle at which the light is incident on the surface of the solar cells affects the PV system in many ways. The correct angular incident of light will ensure that we get efficient light trapping.
3. Diffractive grating: Gratings are used to achieve a defined change of the direction of the internal radiation. This results in path length enhancement of the internal light and consequently in increased absorption and quantum efficiency.
CHAPTER 3
Photonic Crystals
Photonic crystals use the above mentioned principles and helps increasing the efficiency. Photonic crystals are periodic optical nanostructures that are designed to affect the motion of photons. Photonic crystals are composed of periodic dielectric or metallo-dielectric nanostructures that affect the propagation of electromagnetic waves (EM) by defining allowed and forbidden electronic energy bands. Essentially, photonic crystals contain regularly repeating internal regions of high and low dielectric constant. Photons (behaving as waves) propagate through this structure or not depending on their wavelength. Wavelengths of light that are allowed to travel are known as modes, and groups of allowed modes form bands. Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena such as inhibition of spontaneous emission, high-reflecting Omni-directional mirrors and low-loss-wave guiding. Nano structured diffractive gratings formed by photonic crystals have been designed to increase the optical path length of light within the solar cell.
In the construction of 2-D photonic crystal, alternating layers of material with different refractive indices are stacked to form a structure that is periodic along two dimensions. The parameter that determines the band structure is the refractive index contrast and thickness of the constituting layers. Nano structured diffractive grating formed by photonic crystals has been designed to increase the optical length of light within the solar cell.
Working
Different materials have different sensitivity for different wavelength range of light. In the same manner PV cells are also more sensitive to particular wavelength of light i.e. the longer wavelength. The photonic crystals as a band pass filters reflect wavelengths of range 400-1100 nm and direct longer wavelengths at the interface between two adjacent cells and also direct the light into sensitive areas of the PV cells. By this the absorption rate of the photons are greatly increased and also the efficiency. This is of the range 8 to 30 % practically depending on the type of the photonic crystal used.
Figure (3):- Thermal image of the silicon PV cells with 2-D photonic crystal layer
The above figure is thermal image of the silicon PV cells with 2-D photonic crystal layer. One can observe the variations in the colour of the PV cells according to the change in the angle of deposition of the 2-D PC layer.
Types of Photonic Crystals
By the criteria of structure, the photonic crystals can be divided into three types namely
1. 1-D photonic crystals
2. 2-D photonic crystals
3. 3-D photonic crystals
2-D Photonic Crystals
Figure (4):- 2-D Photonic Crystals
Thomas Krauss made the first demonstration of a two-dimensional photonic crystal at optical wavelengths. This opened up the way for photonic crystals to be fabricated in semiconductor materials by borrowing the methods used in the semiconductor industry. Today, such techniques use photonic crystal slabs, which are two dimensional photonic crystals "etched" into slabs of semiconductor; total internal reflection confines light to the slab, and allows photonic crystal effects, such as engineering the photonic dispersion to be used in the slab. Research is underway around the world to use photonic crystal slabs in integrated computer chips, to improve the optical processing of communications both on-chip and between chips.