Multivalent MoOX via doping enables highly hole selective and stable crystalline silicon heterojunction solar cells

Molybdenum trioxide (MoO 3-X , X < 3) stands out as a highly effective hole-selective layer for crystalline silicon (c-Si) solar cells, owing to its high work function ( W F ), high optical transmittance, and easy preparation. However, it is very prone to generating oxygen vacancies ( V O ) under external stimulus (air, heat) or contacting the neighbor layers in the devices, which dramatically decrease the W F , leading to the significant reduction of hole selectivity and instability of the solar cells. In this work, multivalent MoO X (Mo 4+ , Mo 5+ , Mo 6+ ) is proposed as hole selective layer by fine controlling the oxidation duration of the precursor solution. The Mo 4+ ions in the film are eliminated and V O are reduced by the introduction of Al 3+ or Nb 5+ doping, contributing to the reduction of contact resistivity of MoO X /p-Si heterojunction from 120.7 to 29.6 or 26.4 mΩ cm 2 . Simultaneously, the recombination current density is decreased from 232 to 110 or 106 fA/cm 2 with Al 3+ or Nb 5+ doping. The MoO X :Nb 5+ /p-Si heterojunction solar cell has achieved a champion conversion efficiency of 20.22%, moreover, it shows much more stable than the MoO 3-X /p-Si heterojunction device. This work provides a feasible strategy of designing hole selective layers for highly efficient and stable c-Si solar cells.