Physical Fundamentals And Characteristics Of Solar Cells

Materials

Certain materials for solar cells, called semiconductors have their valence electrons bound to atoms with energies very similar to those of the photons that make up sunlight. When it impinges on the semiconductor its photons break the bonds and the valence electrons are free to move about the semiconductor. Something similar happens also with the broken link, called “holes” that jump from one atom to another can also move rather freely.

These free electrons (negative) and these holes (positive) created at points where there is light, tend to diffuse to dark regions and therefore less densely them. However, both particles move in the same direction do not lead to electricity, and sooner or later recombine restoring the broken link. However, if somewhere near the region where these pairs of electrons and holes recombine have been created by restoring the broken link. However, if somewhere near the region where these pairs of electrons and holes recombine have been created by restoring the broken link. However, if somewhere near the region where these pairs of electrons and holes have been created, it creates an electric field inside the semiconductor, this field separates electrons from holes, making each circle in the direction opposite and therefore resulting in a net electric current in the meaning of the electric field.

Ways to create a strong electric field

There are several methods for establishing an electric field inside a solid. All are linked to the concept of contact potential that appears when two materials with different electron affinity. It is natural that may exist, therefore, countless pairs of different materials can provide a contact potential. Moreover, since different electron affinity is what determines the appearance of the contact potential may be considered an electric field even with a single material such that two contiguous regions of a sample has been treated or contaminated due to have different affinity. It will be said in the latter case which has a “homo-union” and “hetero-junction”, in the case of different materials. When a hetero-junction is made up of a metal and a semiconductor is called Schottky barrier.

In conventional solar cells, the electric field separator is achieved in the transition zone, or union of two regions of a silicon crystal that had been chemically treated unequally: one was phosphorus dopant (n region) and another with boron (region p). This appeared an electric field directed toward the area n p which tends to send the items to the area n and holes toward the area p. All this can be seen in Figure 4.

Structure of a solar cell

Figure 4 can be seen the concrete constitution of a conventional silicon solar cell. A bar crystalline silicon doped with boron, cut into disks with a thickness of approximately 0.3 mm. One side is heavily doped with phosphorus by high temperature diffusion from a gaseous atmosphere rich in phosphorus, so that this element enters the silicon with the highest concentration of boron that it contained to a depth of approximately 0.3 microns . Above this layer is deposited and a metal grill on the back of the cell with a continuous layer. Both layers serve to facilitate making electrical contacts in both regions.

Operating Mode

When light shines on the upper surface of the cell some links are broken, generating electron-hole pairs. If this generation is produced at a lesser distance from the junction of what is called diffusion length, on average, before or after these carriers will be separated by the strong electric field that exists in the union, moving the electron towards the ny area p hole into the area, leading consequently to a current from the area na area p. If an electron and a hole was found across the junction before they recombine, losing heat as the light energy they had absorbed.

One of the factors constraining the effective conversion of light into electrical energy is that which arises from the mismatch between the energy of the photons in the solar spectrum and the energy needed to break the bond of an electron in a given material. Photons with energy well below that required to break a bond will not be absorbed and lost. The very energy of their energy spent in breaking the bond of an electron (i.e. to create an electron-hole pair) and the rest give kinetic energy to that hole and electron. The kinetic energy is lost as heat quickly because of collisions of these carriers with atoms of the material. The recoverable energy electron-hole pair is generated at most equal to the potential energy due to the field at the junction.

Since the solar spectrum is quite wide and most of the photons have energies between the 3.1 eV and 0.7 eV is not possible to achieve very high yields with a single material. Figure 6 shows the maximum yields obtained with different semiconductor materials in response to this effect and assuming no pre combinación.

Materials with band gap of 1.5 eV would be the best, given the solar spectrum. But since the performance depends not only on that, but also of manufacturing technologies and material of the cell, a silicon cell is more efficient real one of CdTe.

Conversion efficiency

The conversion efficiency of a solar cell is defined as:

The maximum theoretical yield of 95% is achievable given that the solar spectrum comes from a gas of photons at 6000 º C to work at a temperature of 300 º C. This value is unattainable in practice if you use a single cell from a semiconductor material. Solar radiation is not monochromatic, but has a rather broad spectral distribution. In the area of land extending approximately from the ultraviolet (3500 Å) to the near infrared (2 m). The sum of the powers for each of these frequencies is what we call the incident solar power, Pin. But not all frequencies are usable by a particular photovoltaic material because it is transparent over a wavelength.

In highly absorbent materials, the current generated is large, but the open circuit voltage is low, for being the band gap of semiconductor. Only by dividing the solar spectrum in various frequency blocks and putting a cell spectrally adapted in each block may exceed the limit of 25% which can be seen as insurmountable barrier for systems with one constant band gap cell.

Effect of series resistance

An ideal solar cell generates electricity that can be provided in full to load. A real cell, however, possess a certain resistance series that will lose some power. The series resistance changes the shape of the curve VI so that Pmax is reduced compared to the same cell with Rs = 0. Figure 7 shows a curve for various values of Rs.

The negative effect of the series resistance becomes important in cells that receive concentrated light because the power dissipated in a resistance is 2R.

Light Reflectance

One element that affects the absorption of light by the cells, apart from the intrinsic optical properties of the material, is the reflection on the cell surface due to the refractive index discontinuity at the interface.

The refractive index of semiconductors is quite high, from 3.5 to 4.5, which produces a very important reflection of light on the surface. This reduces the efficiency of the cell, since it only absorbs 60% -70% of incident light. This problem is generally resolved by depositing thin transparent anti-reflective, with optical thickness of about one quarter of the wavelength of light, allowing takeovers easily reach 90%.

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