Imagine you have commissioned 100 different manufacturers to make you 100 different arbitrary Lego knockoffs; each manufacturer picks a color, thickness, and a peg pitch (spacing between the pegs). Each manufacturer only makes exactly one kind. Some manufacturers' legos will fit together, and some won't.
Some manufacturers make theirs very cheaply and in mass quantities, and some will take years to deliver a very small number at very high prices.
As a solar cell designer, you want to make a stack of these legos to form a rainbow. It has to go from blue on the top to red on the bottom, and it has to stack together without too much force.
Think of a brand-X terrestrial Home Depot crystalline solar cell as just yellow lego bricks. They aren't the whole rainbow (and green would actually be a closer match to sunlight if you could only pick one color) but they're cheap.
Gallium Arsenide legos are green, but they're really hard to make. Germanium legos are red, and it turns out that the green Gallium Arsenides fit on them really well. Yellow Silicon ones, on the other hand, don't fit well with either.
So that brings us to two. Indium Phosphide legos are blue and so are Gallium Phosphide legos. But neither of those fit well on the green Gallium Arsenide; one's lego pins are too dense, the other too sparse. It took a long time for a manufacturer to come up with the right blend, thickness, and color, but they were able to come up with a lego that is blue, made from a mix called Indium Gallium Phosphide, and stacks nicely on top of the green Gallium Arsenide. So that's 3.
The fourth layer might be Indium Gallium Arsenide Nitride (let's just call it orange), shoved between the existing layers; somehow making a mix of a good quality lego, but one that makes the right color, right thickness, and right pin pitch.
Now to translate to real physics: The pin pitch is the lattice parameter of each of these crystals, or the distance between individual atoms. If you attempt to epitaxially grow (grow on top of in the same fashion) a different compound than what already exists there, it tends to work ok if the lattice parameters are close. If they're radically different, you can get growth but it's highly disordered and ends up making a lousy layer and a lousy solar cell.
The way a solar cell works is it absorbs only at one single frequency.
Photons below that frequency are lost and not used at all. If the photon is above the frequency then the energy up to that cutoff is used and the rest is wasted (either emitted as a new photon or as heat).
Meaning if you set the target frequency at infrared then you loose all the additional energy UV has over infrared.
By making multiple layers (junctions) you waste less energy, the more the better (in theory anyway).
Some manufacturers make theirs very cheaply and in mass quantities, and some will take years to deliver a very small number at very high prices.
As a solar cell designer, you want to make a stack of these legos to form a rainbow. It has to go from blue on the top to red on the bottom, and it has to stack together without too much force.
Think of a brand-X terrestrial Home Depot crystalline solar cell as just yellow lego bricks. They aren't the whole rainbow (and green would actually be a closer match to sunlight if you could only pick one color) but they're cheap.
Gallium Arsenide legos are green, but they're really hard to make. Germanium legos are red, and it turns out that the green Gallium Arsenides fit on them really well. Yellow Silicon ones, on the other hand, don't fit well with either.
So that brings us to two. Indium Phosphide legos are blue and so are Gallium Phosphide legos. But neither of those fit well on the green Gallium Arsenide; one's lego pins are too dense, the other too sparse. It took a long time for a manufacturer to come up with the right blend, thickness, and color, but they were able to come up with a lego that is blue, made from a mix called Indium Gallium Phosphide, and stacks nicely on top of the green Gallium Arsenide. So that's 3.
The fourth layer might be Indium Gallium Arsenide Nitride (let's just call it orange), shoved between the existing layers; somehow making a mix of a good quality lego, but one that makes the right color, right thickness, and right pin pitch.
Now to translate to real physics: The pin pitch is the lattice parameter of each of these crystals, or the distance between individual atoms. If you attempt to epitaxially grow (grow on top of in the same fashion) a different compound than what already exists there, it tends to work ok if the lattice parameters are close. If they're radically different, you can get growth but it's highly disordered and ends up making a lousy layer and a lousy solar cell.