Self-Diffusion Effect Assisted TiO₂/Li₃PO₄ Electron Selective Passivating Contact in Silicon Solar Cells Approaching 23% Efficiency

Carrier selective contacts with passivation effects are considered to have a significant influence on the performance of crystalline silicon (c-Si) solar cells. It is essential for electron selective contact materials to meet the requirements of ultra-low contact resistance and excellent passivation effects. This work introduces a stack layer of Lithium Phosphate (Li₃PO₄) /Titanium Dioxide (TiO₂) as a new electron selective passivating contact. It is found that the stack achieves an impressive contact resistivity (ρ_c) of 0.128 mΩ cm² on n-type c-Si substrates with resistivity ranging from 1 to 3 Ω cm (14.6 mΩ cm² for the n-Si/a-Si:H/Li₃PO₄/TiO₂/Al contact). Furthermore, it effectively reduces the surface recombination parameter (J₀) to less than 4 fA by incorporating a 6 nm a-Si:H(i) layer. The characterization of the n-Si/Li₃PO₄/TiO₂ interface reveals that phosphorus diffusion into silicon plays a crucial role in achieving the ultra-low contact resistivity, while the presence of PO₄ ^3- groups helps in fixing hydrogen atoms to maintain the desired chemical passivation effect. Finally, a silicon heterojunction solar cell (SHJ) with a rear full-area configuration of a-Si:H/Li₃PO₄/TiO₂/Al is successfully demonstrated achieving an impressive power conversion efficiency of 22.89%. The result proves the efficacy of employing hydrogen-rich low-work function metal oxide stacks as electron selective passivating contacts.