Penn Electric Racing Team

How Solar Cells Work

Solar cells are capable of turning solar radiation into electricity. Silicon solar cells generally operate at an efficiency of 18% or less. This is a measure of the amount of the sunlight energy that is turned into usable electrical energy. Silicon solar cells are made of either single or polycrystalline structures. Each is manufactured in different ways. Single crystals are sliced from larger crystals grown from a single ingot. Polycrystalline cells are cast in a mold and generally have a less perfect crystal structure leading to less efficiency.

Each atom of Silicon has a nucleus closely surrounded by two full shells of electrons. The third and outermost shell has only four of eight positions filled by electrons. When in its crystal form Silicon atoms are tightly packed allowing each nucleus to share its outer, valence electrons with those of the four other closest Silicon atoms. The crystal appears very uniform with each atom sharing all its valence electrons.

To create a solar cell the thin slices of Silicon crystal must be modified. P-type and N-type Silicon must be created by allowing a small concentration of another element to diffuse into the Silicon crystal structure. P-type uses Boron, 3 valence electrons, and N-type uses Phosphorus, 5 valence electrons. The two types are created separately, thus there are no regions that have both Boron and Phosphorus within the Silicon. Wherever a Boron atom exists there is a vacancy in the crystal lattice because it can only bond to neighboring Silicon atoms in 3 places. Conversely, the Phosphorus doped Silicon has extra free electrons floating around each Phosphorus nucleus. Solar cells are arranged with alternating sections of P-type and N-type Silicon.

In the regions that the two types of Silicon are touching there is a rearrangement of the free electrons and the vacancies of the Phosphorus and the Boron. In this area, called the P-N junction the free electrons of the N-side move to occupy the vacancies on the P-side. No nuclei are moving only the electrons and vacancies surrounding them. On both sides of the P-N junction the atoms have full valence shells but on the N-side there are Phosphorus nuclei with an extra proton each and on the P-side there are Boron nuclei with one less proton each. The number of electrons is equal on both sides since all vacancies have been filled. This leaves the N-side with a net positive charge and the P-side with a net negative charge. Silicon is a semiconductor not a pure metal so this charge doesn’t spread out. It stays localized at the P-N junction and an electric field has been created.

If the solar cell is put in the sun photons will strike the surface of the Silicon and pass their energy on to electrons. A typical photon can eject one electron from its nucleus creating a free electron and a vacancy. These free electrons will feel the effect of the electric field. They are pushed towards the junction on the N-side and away from the junction on the P-side. Likewise, the vacancy, which has a net positive charge, will be pulled towards the junction on P-side and pushed away from it on the N-side. This upsets the balance of electric charge. If an external current path connects the two sides of the cell the electrons can be swept back to the P-side to reunite with their vacancies. As sun continues to hit the Silicon cell a steady flow of electrons or current is created. The power generated by the cell is the product of this current and the voltage created by the cell’s electric field.

In order to have enough power to charge a battery or run most electric devices one needs more than the typical .5 V supplied by each cell. To get around this, the cells can be connected in series so that the voltages add. A string of 12 cells in series can generate approximately 6 V. In applications where even more power is needed strings of cells can be linked together. These sub-arrays often need some sort of control system to regulate and stabilize the power and voltage levels.

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