Ok, if you remember Schoolhouse Rock you’ll have a certain groovy tune in your head for the rest of the day. I think it’s best to take a moment and lay down some basic descriptions and definitions of the parts of a p-n junction for those of us who want to know about currently available PV technologies. I mention these topics in passing in the other entries, and here’s where we can establish a baseline.
There is a lot of solid, scientifically validated information out on the web dealing with classic semiconductor physics. Just doing a search on “p-n junction” yields pages of websites with information, educational tools, java applications, etc. I recommend the simple yet clear descriptions at the Georgia State science site: HyperPhysics; especially the section on Semiconductor Concepts. This is a silicon-dedicated site, but the physics apply to other materials as well.
And if you really want to get a deep abstract interpretation of these things, read more on the double layer of plasma physics. Note: I use the basis of plasma physics rather than the basis of electrochemistry in this respect, because the definition of the double layer model in electrochemistry assumes a fixed surface charge or surface charge layer. This model works well enough for an oxide particle in aqueous solution, but is limiting in trying to explain mirrored effects of charge attenuation for solid/solid semiconductor interactions.
Bulk semiconductor materials:
p-type: a semiconductor that contains impurities (adding atoms, missing atoms, replacing atoms) such that the major charge carriers are positively charged electron holes. Given that the majority carriers are so prevalent relative to the electrons, if you “add” more holes from an outside source, they will be able to diffuse freely through the p-type material with a very low probability of recombining with an electron (i.e. “hole transparent”).
i-type: an “intrinsic” semiconductor. This material contains no impurities and the populations of electrons and electron holes are the same. Electron-hole pairs (termed excitons) recombine easily because they are oppositely charged and opposites attract.
n-type: a semiconductor that contains impurities (adding atoms, missing atoms, replacing atoms) such that the major charge carriers are negatively charged electrons. Given that the majority carriers are so prevalent relative to the holes, if you “add” more electrons from an outside source, they will be able to diffuse freely through the n-type material with a very low probability of recombining with a hole (i.e. “electron transparent”).
Junction (read “interface”):
Placing two materials in atomically-close contact such that their physical properties at the interface are very different from that of the bulk material. Hence, the term junction is a materials science vehicle for talking about an electrochemical interface between solids.
Electrochemical Potential
Two things to keep in mind: each material is a reservoir for a certain dominant species of charge carriers, and the respective charge carriers are oppositely charged on either side of the junction. So you have two potentials that make the holes and electrons “want” to drift and diffuse into each other. First is an electrostatic potential ( “opposites attract”, so there is electrostatic drift of charges). Second is a high concentration gradient at the interface that drives one to pour into the other like two waterfalls (the diffusion from high chemical potentials). In a p-n junction the dominant component is the electrostatic potential. This is not true for a vast majority of electrochemical interfaces (electrochemical ultracapacitors, plant and algal photosystems, neuronal charge transfer) where chemical potential gradients prevail.
Space Charge
An excess or deficiency of electrons/holes/ions that build up in one region of a material (such as at the interface or junction). The layer of this excess/deficiency found at the interface of a junction is called the space charge layer (or double layer). In a p-n junction, this is best described as a deficiency. When a p-type semiconductor is connected to an n-type semiconductor via a junction, an electrochemical driving force or potential causes the charges to move toward one another and recombine, making a neutral zone called the depletion region.
Depletion Region
Another way to talk about the space charge layer at the junction (if we can talk about holes with respect to electrons, we can talk about depletion regions with respect to space charge layers). Opposites attract, right? Right–and then they recombine and there are no charges. As an analogy, imagine if you were to roll hot coals and ice cubes into one another–in the end you would have neither hot coals nor ice cubes (in fact, you get low energy mud). The same thing occurs for electron-hole pairs. They recombine and form a layer of no charge, which inhibits an more drift of electrons and holes across the junction. In reality this conjugate pouring tapers off (attenuates) and each material is left with a deficiency of electrons or holes (a space charge layer) that would not have existed without the junction.
Electric Field
Where you have a junction with a p-type and an n-type material (in the dark), a space charge layer (the insulating depletion region) rapidly develops. In response to this insulating layer, an electrostatic potential is maintained across the junction (0.6-0.7 V in silicon). This works just like a dam to hold back the waterfall event. When light of the appropriate energy level hits the p-n junction and is absorbed, electron-hole pairs are generated. What happens to these pairs? Well, if they have nowhere to go (no potential to separate them) they just recombine. But because the electric field is present, the charges feel a pull to pour over the “dam” and flow down the circuit.
The electric field from this potential is a cornerstone of First Generation Photovoltaics. In that particular technology, if you don’t have a “field” you cannot harvest the photovoltaic effect, because there is no driving force to separate photogenerated charges. But remember that the full scope of driving forces are described as the electrochemical potential, not just the electrostatic potential. Newer devices like Advanced Photovoltaics (and very efficient organisms called plants, algae, and photosynthetic bacteria) take advantage of chemical potentials as well.
So, do we have to define a solar cell as a p-n junction? NO, but they are really common in today’s technology.
