Versatile 1-D Nanostructures for Green Energy Conversion …
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molecular beam epitaxy process (PAMBE) are used. The method requires an ultrahigh vacuum of the order of 10
−8 torr and low deposition rate, which is approximately
10 nm/min that minimizes the lattice mismatch (epitaxy) between the growing crystal
and the substrate. Ultra-pure solid sources, such as gallium (Ga) or indium (In), are
heated in a Knudsen effusion cell where they sublimate and the beam of evaporated
atoms reach the substrate without any gas-phase reaction. On the substrate, kept at
controlled temperatures, the beams may react with each other and condense to form 1D nanostructures of high crystallinity. Such pure and strain-free materials are required
for applications such as LASERS and light-emitting diodes (LEDs) (Chen et al.
2011; Krishnapriya 2017; Lu et al. 2013). Also, the variety of pure semiconductor
1-D nanostructures can be grown by various low-cost chemical techniques (Kamble
et al. 2014, 2015a, b; Patil 2017).
The top-down and bottom-up approaches of growth are followed, both in physical and chemical synthesis techniques. Physical technique includes: (1) evaporation
technique, (2) sputtering technique, (3) lithography processes, (4) hot and cold processes, (5) spray pyrolysis, (6) inert gas-phase condensation technique, (7) pulsed
laser ablation. While chemical techniques include: (1) lyotropic liquid crystal technique, (2) electrochemical deposition, (3) electroless deposition, (4) hydrothermal
and solvothermal techniques, (5) sol–gel technique, (6) chemical vapor deposition, (7) chemical bath deposition (CBD), (8) successive ionic layer adsorption and
reaction (SILAR), and so on.
Both the physical and chemical techniques have their own advantages and limitations. But chemical methods are preferred and have an important role in developing
large-scale 1-D nanostructures which can be attributed to the following properties
of chemical techniques: (1) good chemical homogeneity as it offers mixing at the
molecular level and (2) its versatility in designing and synthesizing new materials
which can be further refined into the final end products (Tiwari et al. 2012).
3 1-D Nanostructures for Photovoltaic Solar Cells (PSC)
The basic phenomenon in PSC includes the excitation of electrons in semiconductor
to higher energy level due to incident photons, and these photo-excited electrons
and their positive counterparts (holes) are able to generate an electrical current in
an external circuit. However, an asymmetry in the electronic system is required to
separate these positive and negative charges. Bringing two different (e.g. p- and ntype) semiconducting materials into contact with each other can cause this. Also the
device concepts like Schottky diodes or sandwich of a low-doped semiconductor in
between two different metal electrodes, and photo-electrochemical device are some
of the other possibilities. Depending on the device concept and material used for
separation of photo-generated charge carriers there are different types/generations of
solar cells like silicon solar cells which comes in first generation, thin film solar cells
from second generation, from third generation dye-sensitized solar cells (DSSC),
quantum dot sensitized solar cell (QDSSC), polymer heterojunction solar cell, and
335
molecular beam epitaxy process (PAMBE) are used. The method requires an ultrahigh vacuum of the order of 10
−8 torr and low deposition rate, which is approximately
10 nm/min that minimizes the lattice mismatch (epitaxy) between the growing crystal
and the substrate. Ultra-pure solid sources, such as gallium (Ga) or indium (In), are
heated in a Knudsen effusion cell where they sublimate and the beam of evaporated
atoms reach the substrate without any gas-phase reaction. On the substrate, kept at
controlled temperatures, the beams may react with each other and condense to form 1D nanostructures of high crystallinity. Such pure and strain-free materials are required
for applications such as LASERS and light-emitting diodes (LEDs) (Chen et al.
2011; Krishnapriya 2017; Lu et al. 2013). Also, the variety of pure semiconductor
1-D nanostructures can be grown by various low-cost chemical techniques (Kamble
et al. 2014, 2015a, b; Patil 2017).
The top-down and bottom-up approaches of growth are followed, both in physical and chemical synthesis techniques. Physical technique includes: (1) evaporation
technique, (2) sputtering technique, (3) lithography processes, (4) hot and cold processes, (5) spray pyrolysis, (6) inert gas-phase condensation technique, (7) pulsed
laser ablation. While chemical techniques include: (1) lyotropic liquid crystal technique, (2) electrochemical deposition, (3) electroless deposition, (4) hydrothermal
and solvothermal techniques, (5) sol–gel technique, (6) chemical vapor deposition, (7) chemical bath deposition (CBD), (8) successive ionic layer adsorption and
reaction (SILAR), and so on.
Both the physical and chemical techniques have their own advantages and limitations. But chemical methods are preferred and have an important role in developing
large-scale 1-D nanostructures which can be attributed to the following properties
of chemical techniques: (1) good chemical homogeneity as it offers mixing at the
molecular level and (2) its versatility in designing and synthesizing new materials
which can be further refined into the final end products (Tiwari et al. 2012).
3 1-D Nanostructures for Photovoltaic Solar Cells (PSC)
The basic phenomenon in PSC includes the excitation of electrons in semiconductor
to higher energy level due to incident photons, and these photo-excited electrons
and their positive counterparts (holes) are able to generate an electrical current in
an external circuit. However, an asymmetry in the electronic system is required to
separate these positive and negative charges. Bringing two different (e.g. p- and ntype) semiconducting materials into contact with each other can cause this. Also the
device concepts like Schottky diodes or sandwich of a low-doped semiconductor in
between two different metal electrodes, and photo-electrochemical device are some
of the other possibilities. Depending on the device concept and material used for
separation of photo-generated charge carriers there are different types/generations of
solar cells like silicon solar cells which comes in first generation, thin film solar cells
from second generation, from third generation dye-sensitized solar cells (DSSC),
quantum dot sensitized solar cell (QDSSC), polymer heterojunction solar cell, and