I'm not working in solar cells but do research in optics and have a general understanding of this (also from discussions with people who understand this better)

I'm not working in solar cells but do research in optics and have a general understanding of this (also from discussions with people who understand this better). The main limiter for solar cells from silicon is the overlap of the bandgap with the spectrum of the sunlight and also the loss in efficiency when cells get hot. So what people do is they use multiple materials (often in different layers) that cover different parts of the spectrum to absorp most of the sunlight. Researchers tend to have a good idea which these are (although there is research in creating new organic ones), but the challenge is to combine this in fabrication with the silicon and in a cheap easy to fabricate way and with materials that don't degrade over time. So short answer researchers have a general idea what needs to be done, but as usual the devil is in the details. Again not an expert in the exact area, so anyone who is please correct if something isn't right.