Enhanced Optoelectronic Properties of Silicon Clathrate Films via Sodium Extraction by Vacuum Annealing

Silicon clathrates, which exhibit a direct or near‐direct bandgap, represent a promising alternative to conventional diamond cubic silicon for optoelectronic applications. They combine the elemental abundance and nontoxicity of Si with a more favorable bandgap and higher optical absorption at room temperature. Their open cage‐like frameworks allow for reversible guest atom incorporation with minimal lattice distortion, making them attractive not only for optoelectronics but also for energy storage applications. However, conventional synthesis methods typically require the use of sodium as a structure‐directing agent to stabilize the metastable clathrate lattice during thermal decomposition. The resulting sodium incorporation degrades the optoelectronic properties and complicates device integration. Herein, a nontoxic, scalable, vacuum‐annealing approach is utilized that effectively removes sodium from silicon clathrates films without the use of hazardous chemicals, such as iodine, HF, or SF 6 . Optoelectronic characterization reveals a significant improvement in semiconducting behavior, including enhanced surface photovoltage response, marking a critical step toward developing sodium‐free silicon clathrate thin films as efficient light absorbers. This vacuum annealing process is compatible with large‐area substrates and offers a key step toward a safer, and more sustainable approach to integrating exotic Si phases into the next‐generation optoelectronic devices.