Evaluation of photovoltaic properties of nanocrystalline-FeSi₂/Si heterojunctions

In this paper, an application of nanocrystalline iron disilicide (NC-FeSi₂) combined with nanocrystalline-Si (NC-Si) in a heterostructured solar cell is introduced and numerically evaluated in detail. The proposed cell structure is studied based on an experimental investigation of photovoltaic properties of NC-FeSi₂/crystalline-Si heterojunctions, composed of unintentionally-doped NC-FeSi₂ thin film grown on Si substrate. Photoresponse measurement of NC-FeSi₂/crystalline-Si heterojunction confirmed ability of NC-FeSi₂ to absorb NIR light and to generate photocarriers. However, collection of these carriers was not so efficient and a radical improvement in design of the device is required. Therefore, a modified device structure, comprising of NC-FeSi₂ layer sandwiched between two heavily-doped p- and n-type NC-Si, is suggested and numerically evaluated. Simulation results showed that the proposed structure would exhibit a relatively high conversion efficiency of 25%, due to an improvement in collection efficiency of photogenerated carriers in the NC-FeSi₂ and NC-Si layers. To attain such efficiency, defect densities in NC-FeSi₂ and NC-Si layers should be kept less than 10¹⁴ and 10¹⁶ cm⁻³ eV⁻¹, respectively. Remarkable optical and electrical properties of NC-FeSi₂, employed in the proposed structure, facilitate improving device quantum efficiency spectrum providing significant spectrum extension into the near-infrared region beyond Si bandgap.