Design and deposition of CaMoO4 as host for solar down-converters

Abstract

Silicon solar cells are approaching their maximum theoretical efficiency, but can still be improved by adapting the solar light to match the solar cell. Trough down-conversion, the energy that is above the absorption edge for silicon can be converted to more suitable energy packets. Down-conversion systems usually consist of a lanthanide pair embedded in a host material. However, efficient down-converters are difficult to synthesize, as the distance between the lanthanides must be controlled on a nanometer scale and a too high concentration of lanthanides will reduce efficiency. In the current work, CaMoO4 thin films doped with Pr3+ and Yb3+ are made using atomic layer deposition (ALD), which allows control of both position and amount of lanthanides. In order to deposit such complex oxides with ALD, the processes for the binary oxides are combined. There was no compatible process for deposition of MoOx when this work started, and the one developed was in fact area-selective. The work has also shown how design of experiments, can be used to develop ALD processes faster. Lastly, we found that the optical properties of CaMoO4:Pr3+, Yb3+ could be controlled by tuning the ALD process.