Influence of Sodium\nConcentration on the Optoelectronic\nProperties of Silicon Clathrate Films

Due to their original optoelectronic properties and cage-like\nstructure,\ntype-II silicon clathrate (Na<sub><i>x</i></sub>Si<sub>136</sub>) films are investigated as a new silicon-based material for energy\napplications. In view of the devices and applications, understanding\nthe energy levels of these clathrates is crucial and needs to be investigated\nas they vary strongly with Na concentration. In this study, we determined\nthe optical bandgap energy, work function, ionization energy, and\nsurface photovoltage for a wide range of compositions ranging from\nNa<sub>0.1</sub>Si<sub>136</sub> to Na<sub>23</sub>Si<sub>136</sub>. Our findings reveal that the semiconducting properties and potential\nas solar cell absorbers are dramatically dependent on the Na concentration,\nwith nearly Na-free cages showing the most promise for device applications.