A luminescent materials that can cut one high energy photon into two lower energy photons can break the well-known Shockley-Queisser limit of solar cells in a cost-effective way and enhance cell efficiency up to a factor of 1,4 which is a huge step in the field of PV. Finding such a photon multiplying material is a holy grail in luminescence materials research. The challenge is to absorb all solar light by a first ion, with an energy higher than two times the band-gap of the solar cells and successively transfer the energy in two steps to a neighbouring second ion that emits a photon with an energy close to the band-gap of the cell. This energy transfer approach has been successful by combining two rare earth ions, but optimised phosphors have never reached external efficiencies above 100%. We are exploring new avenues to find the ultimate down-converting material.