Solar Cells - part 1 of I have no idea how many
So before I go into some really interesting stuff, and by interesting stuff I mean stuff that has to do with liquid crystals, I thought I’d take a moment to cover a little bit of ground on photovoltaics.
Now while I am going to be talking about photovoltaics, I’m going to make the caveats:
1) I’m a chemist, not a physicist. More importantly, I’m not a semiconductor physicist. That’s stuff my father does and I swore up and down as a kid that I would never do anything my father ever did. Oh Fate, you are a cruel mistress. If my explanations are overly simplistic, just bear with me. The reality of the situation is that it is much, much more complex than what I am presenting, but this is just to give you a basic picture of how things work (from a first principles standpoint) so we can get to the stuff with liquid crystals. If you want more than what I’m providing, you can always grab what you need, courtesy of the website: http://britneyspears.ac/physics/basics/basics.htm .
2) I’m only going to be talking about photovoltaics that are referred to as “bulk heterojunction” photovoltaics. There are a lot of different types of solar technologies (dye sensitized, inorganic, etc) but I’m only going to be talking about one very specific flavor of solar technology, the stuff commonly referred to (by people in the biz) as “bulk heterojunction” photovoltaics.
Alright. So with that junk out of the way, here we go:
The basic idea behind a photovoltaic device is to take incoming energy from the sun and turn around and get electrons flowing after some black box stuff happening in between. The bottom line is to take sunlight, shine it on some magic stuff, and then take note that electrons are moving and generating current. Sounds pretty simple, right? Plants have been harvesting sunlight and making biomass out of the bargain for quite a while, so it shouldn’t be too much of a stretch to take light and make electricity, right?
Well, almost.
The way a solar cell works is to use incoming light and have the photons from the sunlight generate an excited state in the photovoltaic material. The excited state manifests itself in a bound hole-electron pair that is commonly referred to as an “exciton.” The exciton then migrates through the photovoltaic material, and at some point is separated into the constituent cationic charge and electron. The two constituents trundle off to their respective places at the electrodes and, lo and behold, you generate current in that way. Sounds pretty straightforward so far, but unfortunately, there are a few twists in this that make things a bit more complicated.
The basics process, as I described a paragraph ago, for how a bulk heterojunction photovoltaic device is:
1) Absorb that photon! Light shoots through some atmosphere (in the lab or outdoors) and a photon blasts into the device. The photon generates an exciton as a result. That’s pretty much all I have to say about that.
2) Exciton migration! Okay, so you made one of those fancy bound hole-electron pairs that we refer to as an exciton. What happens? Well, that sucker has to go somewhere before it decides to simply recombine and emit a photon, or get quenched by some other non-emissive mechanism and become useless. The exciton will generally move a distance on the order of 5-10 nanometers in a random fashion in that bulk heterojunction. Hopefully it finds a heterojuntion interface in that time since that’s required to do anything productive.
3) Charge separation! So your exciton found an interface. Hooray! Once it finds an interface, the exciton can be charge separated - that is to say, the hole (the positive charge) and the electron can go their merry way, independent of each other. Sequestering the hole and electron by having each migrate to the appropriate electrode is how we generate current, which is electricity that we can use.
There it is in a nutshell from a first-principles perspective - how solar cells work. This is most applicable to bulk heterojunction photovoltaics, but the fundamental basis for how any solar cell works has a great deal of overlap with what I laid out in (pseudo)detail.
Next time, I’ll go into some background into some simple thin film devices being researched for photovoltaics, particularly organic photovoltaics.
Now while I am going to be talking about photovoltaics, I’m going to make the caveats:
1) I’m a chemist, not a physicist. More importantly, I’m not a semiconductor physicist. That’s stuff my father does and I swore up and down as a kid that I would never do anything my father ever did. Oh Fate, you are a cruel mistress. If my explanations are overly simplistic, just bear with me. The reality of the situation is that it is much, much more complex than what I am presenting, but this is just to give you a basic picture of how things work (from a first principles standpoint) so we can get to the stuff with liquid crystals. If you want more than what I’m providing, you can always grab what you need, courtesy of the website: http://britneyspears.ac/physics/basics/basics.htm .
2) I’m only going to be talking about photovoltaics that are referred to as “bulk heterojunction” photovoltaics. There are a lot of different types of solar technologies (dye sensitized, inorganic, etc) but I’m only going to be talking about one very specific flavor of solar technology, the stuff commonly referred to (by people in the biz) as “bulk heterojunction” photovoltaics.
Alright. So with that junk out of the way, here we go:
The basic idea behind a photovoltaic device is to take incoming energy from the sun and turn around and get electrons flowing after some black box stuff happening in between. The bottom line is to take sunlight, shine it on some magic stuff, and then take note that electrons are moving and generating current. Sounds pretty simple, right? Plants have been harvesting sunlight and making biomass out of the bargain for quite a while, so it shouldn’t be too much of a stretch to take light and make electricity, right?
Well, almost.
The way a solar cell works is to use incoming light and have the photons from the sunlight generate an excited state in the photovoltaic material. The excited state manifests itself in a bound hole-electron pair that is commonly referred to as an “exciton.” The exciton then migrates through the photovoltaic material, and at some point is separated into the constituent cationic charge and electron. The two constituents trundle off to their respective places at the electrodes and, lo and behold, you generate current in that way. Sounds pretty straightforward so far, but unfortunately, there are a few twists in this that make things a bit more complicated.
The basics process, as I described a paragraph ago, for how a bulk heterojunction photovoltaic device is:
1) Absorb that photon! Light shoots through some atmosphere (in the lab or outdoors) and a photon blasts into the device. The photon generates an exciton as a result. That’s pretty much all I have to say about that.
2) Exciton migration! Okay, so you made one of those fancy bound hole-electron pairs that we refer to as an exciton. What happens? Well, that sucker has to go somewhere before it decides to simply recombine and emit a photon, or get quenched by some other non-emissive mechanism and become useless. The exciton will generally move a distance on the order of 5-10 nanometers in a random fashion in that bulk heterojunction. Hopefully it finds a heterojuntion interface in that time since that’s required to do anything productive.
3) Charge separation! So your exciton found an interface. Hooray! Once it finds an interface, the exciton can be charge separated - that is to say, the hole (the positive charge) and the electron can go their merry way, independent of each other. Sequestering the hole and electron by having each migrate to the appropriate electrode is how we generate current, which is electricity that we can use.
There it is in a nutshell from a first-principles perspective - how solar cells work. This is most applicable to bulk heterojunction photovoltaics, but the fundamental basis for how any solar cell works has a great deal of overlap with what I laid out in (pseudo)detail.
Next time, I’ll go into some background into some simple thin film devices being researched for photovoltaics, particularly organic photovoltaics.