Solar Energy is not a Solution to America’s Energy Crisis
With the growing cost of fuel for cars and the rolling blackouts of last summer, the need for an alternative, cost-effective, environment friendly energy source is escalating. Many possible solutions have been presented, such as nuclear power, wind power, and hydrogen fuel cells; prevalent among these is solar power.
Solar cells directly convert photons from the sun into electricity (Wikipedia). Solar cells that convert both solar and non-solar light are called photovoltaic cells (Wikipedia 2006). They are made of semi conducting materials, most often silicon (Aldous 2006). The simplest photovoltaic cells power watches and calculators while larger, more complex systems can add to the power grid and provide power for homes.
How Solar Power Works
Most solar cells are made of crystalline silicon. Pure silicon has 14 electrons that form 3 different tiers around the individual atom. In the last tier there are only 4 electrons, but as each tier desires 5 electrons there is one missing from this tier. To fill the last spot the electrons will join together to share their electrons. This linking is what forms the crystalline structure, which is vital for photovoltaic cells. (Aldous 2006)
Solar Energy 3
Pure silicon is not a good conductor for electricity because it has no free electrons, so we add impurities to the silicon. The process of adding impurities is called doping. Phosphorus is an impurity that can be added into the silicon used for solar cells. This type of impure silicon is called N-type and is much better as a conductor than pure silicon. This is because phosphorus has five electrons in its final tier so every atom of phosphorus has one electron not shared with another atom. This electron is easier to knock lose when photonic energy is added to the silicon. (Aldous 2006)
Only one part of the photovoltaic cell is doped by phosphorus, the other part is usually doped by boron, to become P-type silicon. Boron only has three electrons in the outer tier, so instead of having extra electrons it has holes. These holes are just the absence of electrons so they have a positive charge and can move about similarly to electrons. (Aldous 2006)
An electric field forms when N-type and P-type silicon are put in contact. The free electrons on the N side see the holes in the P side and try to fill them in. All the free electrons of the N-type silicon do not how ever fill up all the free holes in the P-type silicon. At the junction between the two sides the electrons and holes mix and form an electric field as a barrier. This causes the electrons to be able to flow from the P side to the N side but not in the other. (Aldous 2006)
When light hits the solar cell, the energy frees the electron/hole pairs. Each photon will free one electron and create one hole. When this happens close to the electron field or if a hole and an electron wander into the field’s range of
Solar Energy 4