I have a general question about tandem solar cells. So tandem solar cells are solar cells with multiple p-n junctions in them. However, why can't we have a single junction solar cell with multiple wavelength absorbers in them? That is to say, why do tandem solar cells need to be split into two separate subcells? To be clear, I'm talking about this from the viewpoint of organic solar cells. So I'm imagining a bulk heterojunction solar cell, where the absorbing section is a blend of two different wavelength absorbers. I have a suspicion that it has to do with the relative LUMO energy levels between the high and low wavelength absorbers and the charge-transfer process that transfers an electron from the donor's LUMO to the acceptor's LUMO. But I can't quite reason it out to my satisfaction. Any ideas?