Physical structure of Al-pSi metal-insulator semiconductor solar cells

Abstract

Investigations have been made of Al‐pSi metal‐insulator semiconductor (MIS) cells fabricated by depositing Al films onto silicon substrates with thermally grown SiO₂ surface films on the order of 18 Å thick. These studies have involved depth concentration profiling and ellipsometric measurements to understand the physical structure, electrical and optical characterization of aluminum films, and efforts to optimize an Al‐Si cell with a SiO antireflection (AR) coating. Depth concentration profiles show that the correct physical model is not Al/SiO₂/Si, but Al/Al₂O₃(Si)/Si with the Al₂O₃ interfacial layer being 15 Å thick. The Al film reduces the SiO₂ to yield Al₂O₃+Si for an interfacial film. Investigations to optimize the Al‐Si cell structure involved using optical constants measured for Al films to determine AR layer thicknesses, which maximize cell photocurrent for each Al film thickness. Analytical studies were also conducted concerning sheet resistance effects to determine the required grid design to maximize the fill factor. These analytical and experimental studies are used to project an ultimate, practical AM1 efficiency of 11–12% for Al‐pSi MIS cells based on a continuous metal front layer and a silicon resistivity of 2–5 Ω cm.