Also not an expert, but from my reading it's all engineering. The physics is sufficiently well understood and variations can be simulated. It's about preventin

Also not an expert, but from my reading it's all engineering. The physics is sufficiently well understood and variations can be simulated. It's about preventing reflections at various surface boundaries (or more generally, controlling reflections everywhere) and structuring things so that different wavelengths go to the right layers, and preventing recombination, and a zillion other details. Material quality and process control to get the right surface properties are very important. The cells that make it out of the lab are those where the cells are robust and processes can be adapted and scaled to mamufacturing quantities, competitively with current products.