Spectral conversion luminescent materials have the purpose to increase the efficiency of solar cells. By absorbing the high energy part of the solar spectrum and re-emitting the light at a lower energy where the solar cells have higher efficiency, the efficiency of the cell can be increased. Depending on the quantum efficiency of a specific solar cell, the efficiency increase can be a few % to above 10%. We develop spectral conversion materials based on metal silicon nitride materials. Silicon nitride is already used on an industrial scale as anti-reflection coating on silicon solar cells. We aim at integrating the spectral conversion function into the anti-reflection coating by exploring rare earth doped metal silicon nitride materials with the general formula MxSiyNz (M=La,Y,Ca,Sr,K metal ions) like Ca2Si5N8:Eu2+ or LaSi3N5:Yb3+. An additional advantage of silicon nitride materials is that the absorption and emission properties can be tuned to the solar spectrum and solar cell by changing the y/z-ratio.

E. van der Kolk et al., Optical Materials 33, 7, 1024-1027, 2011.

Quantum cutting materials are especially attractive spectra converters because they are able to cut a single high energy photon into two lower energy photons. In this way the classical Shockley–Queisser limit can be circumvented. Although several quantum cutting luminescent materials have been reported based on ion pairs like Yb3+/Pr3+ or Yb3+/Tb3+, they all have too weak sunlight absorption and too low quantum efficiency for application. Our aim is to fundamentally study these limitations and to develop new quantum cutters that do not have these limitations.

Promising spectral conversion materials are further developed into luminescent thin films by using a versatile RF/DC magnetron sputtering system. Films are sputtered on rotating and heated substrates using Si, Ca, Eu and Si3N4 targets on sputter guns in a reactive Ar/N2 atmosphere. Optimisation of film properties is accelerated by a combinatorial approach utilising the tilt options of the sputter guns that generate a composition gradient along the substrate. Successful films will eventually be evaluated on thin film silicon solar cells.