1. The Science of Solar Cells May 15, 2008

2. Announcements

3. Review of the Lab Report Handouts Excel demo

4. Sunlight to Electricity So we are somehow converting light, which has a certain energy, to a flow of electrons (current) So the question is: How does this conversion process take place? This was the difficulty people had with trying to make a solar cell…until around 1954 in Bell Labs And it turns out our old friend, the PN junction was the missing link that when discovered, made the first Si solar cell possible back in 1954 Some Application solar radiation DC Electric Current out of PV device e - e - e -

5. Silicon Material Solid Si forms as a regular crystal lattice of material, forming covalent bonds between the Si atoms Si

6. Silicon Material Solid Si forms as a regular crystal lattice of material, forming covalent bonds between the Si atoms If light with enough energy hits Silicon, these bonds can be broken momentarily, freeing one of the electrons that made up the bond and leaving behind a hole So, in this case, the electron does not get very far before it is attracted back to the hole and recombines with the hole to form the bond again Remember the electron has a negative charge, the hole has a positive charge So current does not flow Si solar radiation

7. Detour: Forming an Electric Field An electric field forms in the vicinity of any electric charge A simple way to form an electric field is like this: Now let’s take a look at two situations and see what effect the electric field has: 1. A negatively charged object in the electric field 2. A positively charged object in the electric field This simple concept turns out to be key to the operation of a Si solar cell, the key that alluded researchers for many years… In many cases the simplest ideas are the Nobel Prize winning solutions! - +

8. Silicon Material Now consider the same situation we were looking at before, but now let there be an electric field acting inside the Si material Now, when the same light hits the material and again momentarily breaks the bond— what will happen? Solar radiation again breaks the bond Now though, electron is free to flow or conduct through the material So an electric field would be useful to separate the electron from the hole after the light breaks the bond! It would allow current to flow! How can we generate an electric field inside the Si? (hint: it will involve the PN junction) solar radiation Si + -

9. Silicon Quick chemistry crash-course! (déjà-vu, I know) This time we’ll be looking specifically at Silicon, the common material used for solar cells today. But the basic concept extends to other types of solar devices and materials Silicon (Si): Group 4  4 electrons in outer shell Si

10. Silicon Material Solid Si forms as a regular crystal lattice of material, with the Si atoms sharing 8 electrons between them Si Let’s see how we can form a PN junction in Si material

11. The Silicon PN Junction: N Doping What if a Si atom is replaced with a Phosphorus (P) atom? Atomic number of 15: Meaning it has 15 total protons (positive charges) and 15 total electrons (negative) giving a net zero charge Remember, P is in Group 5 So P has 5 electrons in its outer shell P 5 outer (valence electrons)

12. The Silicon PN Junction: N Doping What if a Si atom is replaced with a Phosphorus (P) atom? Remember, P is in Group 5 So P has 5 electrons in its outer shell Si

13. The Silicon PN Junction: N Doping What if a Si atom is replaced with a Phosphorus (P) atom? Remember, P is in Group 5 So P has 5 electrons in its outer shell Electrons are the mobile charge carriers Si ‘doped’ with P is known as N-type since the carriers of charge are electrons--which have a Negative charge N-type Si Si PP P P

14. The Silicon PN Junction: P Doping What if a Si atom is replaced with a Boron (B) atom? Atomic number of 5: Meaning it has 5 total protons (positive charges) and 5 total electrons (negative charges) giving the atom a net zero charge And remember, B is in Group 3 So Mg has 3 electrons in its outer shell B 3 outer (valence electrons)

15. The Silicon PN Junction: P Doping What if a Si atom is replaced with a Boron (B) atom? Remember, B is in Group 3 So B has 3 electrons in its outer shell Si

16. The Silicon PN Junction: P Doping What if a Si atom is replaced with a Boron (B) atom? Remember, B is in Group 3 So B has 3 electrons in its outer shell Do you think this material will allow current to flow? Yes, now there is a free electron that is free to move and carry charge SiN ‘doped’ with Mg is known as P-type since the carriers of charge are ‘holes’--which have an effectively Positive charge P-type Si Si BB B B

17. The PN Junction Revisited N-type material has negatively charged free electrons able to carry charge P-type material has positively charged free ‘holes’ able to carry charge When a p-material is brought into contact with a n-material, the resulting device is called a PN junction Let’s look in further detail at what happens when this PN junction forms… P-type + + + + + + + N-type - - - - - - -

18. The PN Junction Revisited (In Further Depth) What will happen when the positively charged holes meet up with the negatively charged electrons at the PN junction? (when the PN junction is created) Hint: What happens when you spray cologne on one of a room Diffusion: Movement of particles from a region of high concentration to one of low concentration Electrons diffuse to P side take the spot of the holes Holes diffuse to the N side to cancel out electrons P-type + + + + + + + N-type - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + +

19. The PN Junction Revisited (In Further Depth) What will happen when the positively charged holes meet up with the negatively charged electrons at the PN junction? (when the PN junction is created) Hint: What happens when you spray cologne on one of a room Diffusion: Movement of particles from a region of high concentration to one of low concentration Electrons diffuse to P side take the spot of the holes Holes diffuse to the N side to cancel out electrons A region is left surrounding the PN junction that is depleted of free electrons and holes— called the ‘Depletion Region’ P-type + + + + + + + N-type - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + +

20. The PN Junction Revisited (In Further Depth) But what’s left behind when the electrons leave the n side and the holes leave the p side? An electric field between the positively charged P atoms and the negatively charged B atoms forms automatically when the PN junction is made! This electric field prevents electrons from recombining with holes when light from the sun breaks a bond P-type + + + + + + + N-type - - - - - - - - - - - - - - - P+P+ - - - + + + + + + + + + + + + + B P Before electron diffusion- B atom: zero charge Before electron diffusion- P atom: zero charge B - After electron diffusion- B atom with extra electron: net negative charge P + After electron diffusion: P atom missing an electron: net positive charge P+P+ P+P+ P+P+ P+P+ B- B-B- - +

21. The PN Junction Revisited (In Further Depth) P-type + + + + + + + N-type - - - - - - - - - - - - - - - P+P+ - - - + + + + + + + + + + + + + B P Before electron diffusion- B atom: zero charge Before electron diffusion- P atom: zero charge B - After electron diffusion- B atom with extra electron: net negative charge P + After electron diffusion: P atom missing an electron: net positive charge P+P+ P+P+ P+P+ P+P+ B- B-B- - + solar radiation breaks bonds Creates electrons and holes: Electric field sweeps electrons to the right and holes to the left

22. Silicon Material Now consider the same situation we were looking at before, but now let there be an electric field acting inside the Si material Now, when the same light hits the material and again momentarily breaks the bond— what will happen? Solar radiation again breaks the bond Now though, electron is free to flow or conduct through the material So an electric field would be useful to separate the electron from the hole after the light breaks the bond! It would allow current to flow! How can we generate an electric field inside the Si? (hint: it will involve the PN junction) solar radiation Si + -

23. Power of the Sun Video Time allowing (10 min)

24. Lessons From the Lab Does what you saw in the lab make sense with what you’ve learned today? Voltage stays constant—dependent on the solar material Current changes with light intensity--more electrons from more light

25. Summary Separation of charges is key to the operation of a solar device In Silicon solar cells, the electrons and holes are separated using a PN junction

26. Another Way to Think About it: Energy Band Diagram e-e- e-e- e-e- e-e- e-e- e-e- e-e- Valence Band: Energy level of outer (valance) electrons when they are being used to form a bond Conduction Band: The next available energy level of electrons above the valance band where they are broken free of the bond and can conduct through the material solar radiation with the right energy Energy level diagram for Silicon Bandgap Energy: (E GAP ) The Approximate energy needed to break a Si bond Increasing Energy This is why different materials absorb different parts of the sun’s energy!