Solar Cells; An invention that if used properly, could power houses and cars (life long) for a lot less money then it takes to power a house by electricity or to power a car by gasoline. In this presentation I will explain the technology behind solar cells.

By Sriram Gaddamanugu

School Year of 2004 to 2005

•Photovoltaic cells are cells that convert light into usable energy

•“Photo” means light and “Voltaic” means electricity

•Before photovoltaic cells were used only on space probes and satellites, but now they are used in many things including, houses, cars, calculators, lanterns and many more things.

•Photovoltaic Cells are made of semiconductors

•Semiconductors are made of silicon

•The energy of the light knocks electrons loose which allows them to flow freely

•They have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction

•This flow of electrons is called current

•By placing metal contacts on the top and bottom of the PV cell, the current can be used to power objects

• Silicon has special chemical properties, especially when in its crystalline form

• An atom of silicon has 14 electrons, and are arranged in 3 different valance electron shells

• Pure silicon is a poor conductor because none of its electrons• are free to move about since all the shells are filled up• Silicon in a solar cell is slightly modified so it will work in a solar

cell

• Silicon that is modified intentionally is called impure• This means other atoms have been mixed with silicon atoms• Without impurities, silicon wouldn’t work in a solar cell• Silicon in Solar Cells contains about one atom of phosphorus,

for every million silicon atoms (phosphorus has 5 atoms, not 4)• It would still bond with silicon but, the phosphorus still has one

extra atom that doesn’t bond with a silicon atom• But there is still a positive proton in the phosphorus nucleus

holding it down

•When energy is added to pure silicon a few electrons break free of their bonds and leave their bonds

•A hole is left behind each time this happens

•These electrons that have been knocked loose wander randomly looking for a hole to fill

•These wandering electrons are called Free Carriers

•Free carriers carry electrical current

•There are so few of free carriers in silicon, however, that they aren’t very useful

•That is why impure silicon mixed with phosphorus is used

•This is because it takes a lot less energy to knock loose one of the “extra” phosphorus atoms because one of the electrons isn’t tied up in a bond

•As a result, most of the electrons break free, and there are many more free carriers in the impure silicon than you would in pure silicon

•The process of adding impurities on purpose is called doping

•Doped silicon is called N-Type Silicon--The N stands for negative

•N-Type silicon is a much better conductor than pure silicon

•Actually only part of the cell is N Type, the other part is doped with boron, which has only 3 electrons in its outer shell

•This part is called P Type silicon---The P stands for positive

•Instead of having free carriers, P Type has free holes

•These holes are only slots that aren’t filled with electrons, so they carry a positive charge

•Remember, every PV cell has at least one electrical field

•Without one, the PV cell wouldn’t work

•This field forms when the N and P Type silicon come into contact

•When they do, the free carriers in the N side try to fill up the free slots on the P side.

•Before contact, the silicon was electrically neutral

•But when the slots and free carriers mix at the junction between the N and P Type silicon, the neutrality is disrupted.

•Right at the junction, the two mix and form a barrier, which makes it harder and harder for electrons on the N side to cross the P side

•Eventually it is impossible for N electrons to cross over to the P side, and an electrical field is formed

•This field pushes electrons from the P side to the N side, but doesn’t allow it the other way around

•When light, in the form of photons, hits our cell, its energy frees electrons from bonds with the slots

•If this happens close enough, the field will send the electron to the n side, and the hole to the P side

•If provided with an external path, the electrons will flow through the path, to unite with slots that the field sent

•The electron flow makes current, and the energy field makes voltage.

•With both of these, Power is created

•There are a couple of final steps before you can use the cell.

•First thing you have to do is to add anti-reflective coating

•This is because silicon is shiny, and light reflects off of shiny materials.

•And if light gets reflected, it won’t get absorbed by the PV cell

•This reduces reflection losses to less than 5%

•The final step is placing a glass cover plate on the cell.

•What this does is it protects the cell to a certain degree from damage.

•Such damages might be from scratches and exposure to liquids

•PV modules are made by connecting several PV cells

•They are used to achieve high levels of power

•They are usually put in a sturdy frame, complete with a glass cover and positive and negative terminals on the back

Hello, My name is Sriram, I am a dedicated Duke Fan, My favorite Football teams are the Atlanta Falcons and the Minnesota Vikings. My favorite professional basketball teams are the Cleveland Cavaliers and The Phoenix Suns. My favorite Bands are Black Sabbath, ACDC, Kiss, My Chemical Romance, Simple Plan, Good Charlotte, System of Down, Linkin Park, and Green Day. My favorite Pro Football Player is Michael Vick, My favorite College Basketball Player is Rashaad McCants (even if he is from UNC, he is a fun player to watch), and My favorite Pro Basketball Player is LeBron James. And My favorite songs are Back In Black by ACDC, Lose Yourself by Eminem, Headstrong by Trapt, American Idiot by Green Day, and Iron Man by Black Sabbath. I have one sister (who I tend to hate) and my favorite Sports are Football, Basketball, and Tennis. I aspire to be a professional Tennis player when I grow up, although I seriously doubt I could be. I also would be a college graduate of Duke University.