Download SOLAR CELLS Global reserves are running out sharply because of

SOLAR CELLS
Global reserves are running out sharply because of using fossils fuels for conventional energy
production. Moreover, the idea of switching to another alternative energy sources comes
into consideration because of greenhouse gasses emissions, the increase of the production
costs of non-recyclable fossil fuels and unreliable geopolitical constraints. Using sun as a
source of energy generation could be a solution of energy demand. Electricity
(photovoltaics) [1] and hydrogen (water splitting) [2, 3] generation from solar energy is the
main focus. We will consider the main advantages of nanastructured based solar cells where
solar cell is a device that converts sunlight energy to electricity based on the photovoltaic
effect. Distributed power, satellite systems and portable devices can be a good example of
photovoltaic cells application.(7) .
Photons in the sunlight hit the solar panel and are absorbed by semiconductor materials,
such as silicon. When the photon is absorbed its energy is then transmitted to an electron in
the crystal lattice. This energy “excites “the electron into the conduction band in which it is
free to move around within the semiconductor. This entails the formation of a hole and
allows electrons of neighboring atoms to occupy it and, therefore, leave another hole
behind. Consequently, photons absorbed create mobile electron-hole pairs. The target in
solar cells is obtaining electricity and in order to fulfill this target, carriers must be driven in
opposite directions. There are two main ways of charge separation in solar cells: drift of
carrier, which is driven by an electrostatic field established across the device, and diffusion
of carriers from zones of high carrier concentration to zones of low carrier concentration.
A common solar cell is constructed through a p-n junction made of silicon. When a piece of
p-doped silicon is in contact with a piece of n-doped silicon, the diffusion of carriers occurs;
the electrons of the n-doped silicon fill the holes of the p-doped silicon inducing an electric
field that creates a diode. This diode promotes a charge flow, known as drift current that
opposes and balances out the diffusion of carriers. The application of a positive biased
voltage entails the injection of DC current by diffusion of electrons from the n-side and holes
from the p-side. The region where this phenomenon is produced is called depletion region
because it no longer contains any mobile charge carrier carriers.
As we have seen in this paper nanostructure solar cells offer many advantages compared to
traditionally used solar cells such as the ability to exceed a single junction solar cell efficiency
(which can be achieved by implementing new concepts), the possibility to overcome
practical limitations in existing devices (as tailoring properties of the materials or solving the
problems of lattice miss match) or the potential for low cost devices. However, there still are
many challenges for nanostructured solar cells on the way, as the incompletely understood
mechanisms behind them, their lack of designing rules or the transportation problem.
Overcoming these challenges is the goal in order to take advantage of the full potential
offered by nanostructured solar cells and, therefore, obtaining a high efficient solar source.
Prepared by
O.F.M.ABDUL GALIB
Source
http://www.eecis.udel.edu/~honsberg/Refs/Nano-Hawaii.pdf
[1] K. Kalyanasundaram and M.Grätzel, P. Indian Acad. of Sci.-Chem.Sci., 109, 447 (1997).
[2] A. Fujishima and K. Honda, Nature,49, 37 (1972).
[3]T. Bak, J. Nowotny, M. Rekas, and C.C. Sorrell, Int. J. Hydrogen Energ., 27,991 (2002).
[4]M. Fischer, Int. J. Hydrogen Energ., 11,495 (1986).
[5]M. Grätzel, Nature, 414, 338 (2001).
[6]M. Grätzel, Coord. Chem. Rev., 111,167 (1991).
[7]R. W. Miles, Vacuum, 80, 1090(2006).
[8] C.B. Honsberg,”Approaches for Ultra-High Efficiency
Solar Cells,” Proceedings DOE Solar ProgramReview, November, 2004
[9] S. Kolodinski, J.H. Werner, T. Wittchen, and H.J. Queisser, “Quantum efficiencies
exceeding unity due to impact ionization in silicon solar cells,” Applied Physics Letters, vol.
63, no. 17, p. 2405 7, (1993)
[10] R.D. Schaller, and V.I. Klimov, “High efficiency carriermultiplication in PbSe nanocrystals:
implications for solarenergy conversion,” Physical Review Letters, vol. 92, no.
[11] T. Trupke, M.A. Green, and P. Wurfel, “Improving solar cell efficiencies by up-conversion
of sub-band-gap light,”Journal of Applied Physics, v. 92, no. 7, p. 4117-22, (2002).
[12]Luque, and A. Martí, “A Metallic Intermediate Band High Efficiency Solar Cell
[13]Progress in Photovoltaics,vol. p. 73-86, (2001).
[14]R.T. Ross, “Efficiency of hot-carrier solar energy converters,” Journal of Applied Physics,
vol. 53, no. 5,p. 3813-18, (1982).