PhosphorusPhosphorus (P) is widely used as n-type dopant for siliconSilicon (Si) to form the emitter layer in wafer-based siliconSilicon solar cellsSolar cells . The main purpose of this work is to investigate the influence of P dopingDoping on the structural and mechanical properties of siliconSilicon . CASTEPCASTEP program, which uses the density functional theory (DFT), with a plane-wave basis, is used to study the structural, electronic, and mechanical properties of undoped and P-doped Si (Si1−xPx for 0.0001 ≤ x ≤ 0.05). The density of states (DOS), band structure, elastic constants, bulk modulus $$ \left( B \right) $$ , Young’s modulus (E), Shear modulus $$ \left( G \right) $$ , and Poisson’s ratio (v) were all calculated. It is found that brittleness of Si increased by P dopingDoping .