High-grade efficiency III-nitrides semiconductor solar cell
Introduction
In a solar cell, the carriers in excess are created by solar light rays. A given photon introduces a jump of an electron from an occupied state in the valence band to a free state in the conduction band, the so-called fundamental absorption of a given material. Hence, the more a material absorbs, the more the generation of conducting electron is augmented. In solar cells, one tries to use the most absorbing semiconductors in order to improve the generation photocurrent.
A solar cell converts solar ray energy into electrical energy. It is presented as dark color plaque because it absorbs the luminous energy.
The performances of a solar cell can be emphasized mainly by its ability to absorb different energies solar spectrum photons, by its capability to transform this absorbed energy in a potential energy as free carriers, by its aptitude to scatter these charges in the materials without a consequential recombination, by its capacity to collect the freed carriers supplying an electrical power debited in an external load and by the faculty of these carriers to cross a potential barrier generating a force constituting the voltage.
The semiconductor layer is placed under an internal electrical field due to the PN junction. Solar cells are organized as nets and mounted on panels. During their installation, their orientation and their inclination are definite.
A solar cell is a photodiode functioning without an external bias. The generated photocurrent is debited on a load.
The semiconductors used in our paper are of direct transitions, meaning that the electron of valence relies on the top of the valence band, which corresponds to the bottom of the conduction band. There is only one obstacle corresponding to the bandgap value of the considered semiconductor to cross. This energy is given by the incident photon hitting the electron and giving it the necessary energy. There, the photon must possess a wavelength at least equal to the ratio between Boltzmann's constant times light's celerity over the considered bandgap. The considered semiconductor must “work” at the wavelength corresponding to the maximum solar radiance in order to obtain the best efficiency. The seasonal performances vary with the inclination degree and the daily performances depend essentially upon the orientation to the south. This latter is calculated according to the installation latitude and altitude.
Section snippets
Materials used
The main characteristic is that in the Wurtzite (hexagonal) configuration, III-nitrides semiconductors have direct bandgaps, and this to multiplies their optical properties, such as radiations and direct transitions, compared to the other III–V semiconductors. The III–N semiconductors have large direct bandgaps energies going from 0.8 eV for the InN passing by 3.5 eV for the GaN to 6.3 eV for the AlN. They appear to have wide direct bandgaps throughout the alloy composition range, strong atomic
Data
The solar radiation intensity above the atmosphere reaches 1353 kW m−2, with a spectrum centered near 495 nm wavelength. Therefore, at the ground level, this spectral density is reduced to only near 1000 kW m−2. Ozone absorbs mean UV in the 0.2–0.4 μm range. Oxygen possesses two narrow weak attenuation bands at 0.69 and 0.76 μm. Water steam possesses seven absorption bands, three of which are strong in the mean infrared at 0.9, 1.15 and 1.3 μm. Carbon dioxide absorbs at three narrow bands in the
Resolution methods
Voltage and current distributions in PN junctions’ semiconductor devices use the following physical equations and boundary conditions [10].
The Poissons’ equation is expressed bywhere ϕn and ϕp are the electrons and holes Fermi levels, respectively. ψ is the electrostatic voltage, where n and p are electrons and holes free carriers densities, respectively.
ND+ and NA− are the doping concentrations, ε is the electrical permittivity, ni
Results
In our study, the series resistance of the cell and current losses have been neglected, since it was considered as being an ideal cell.
The main goal of this study is to show that the used semiconductor, in occurrence In0.22Ga0.78N works in the maximum of the solar irradiance, and that its absorption coefficient, being very important in the order of 5×104 cm−1, added to the low refractive index, conducting to a well-reduced reflection coefficient, the majority of incident radiation is effectively
Conclusion
Finally, one can say that new III-nitrides semiconductor solar cells seem to be very adequate, because their very large range of frequency permits their optimization, playing on the x-stoichiometric coefficient in their ternary or quaternary alloys, in order to center their bandgap on the maximum of solar irradiation. Their application would be very pertinent if one can idealize their behavior by controlling the doping level of the emitter and especially the base.
We think, also, that the
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