MgO and SrO Coalloyed NiO for Enhancing Hole Extraction in Perovskite Solar Cells: A First-Principles Study

Abstract

NiO serves as a hole transport layer (HTL) in perovskite solar cells, yet its performance is constrained by intrinsically low hole concentration and a valence band maximum (VBM) that is too high relative to the perovskite absorber. We demonstrate a coalloying strategy with MgO and SrO to overcome the inherent trade-off between band alignment and p-type conductivity: MgO lowers the VBM to improve band alignment with perovskites, while SrO reduces cation vacancy ionization energies to enhance hole density. First-principles calculations on ternary Ni0.9465A0.0535O (A = Mg, Ca, Sr, Ba) alloys reveal that MgO most effectively lowers the VBM but deepens acceptor levels, whereas SrO minimizes defect ionization energies at the cost of raising the VBM. To overcome this limitation, we design quaternary Ni0.9465SrmMg0.0535-mO (0 ≤ m ≤ 0.0535) alloys, which offer an additional compositional degree of freedom. Most quaternary compositions exhibit VBMs below that of pure NiO. Remarkably, Ni0.9465Sr0.034Mg0.016O has a Sr vacancy ionization energy of 0.22 eV, a 54% reduction compared to the Ni vacancy ionization energy of 0.47 eV in pure NiO, and is even lower than the Sr vacancy ionization energy of 0.34 eV in the Sr-only ternary. This improvement is attributed to a synergistic strain effect that is unattainable in the binary NiO and the ternary alloy systems. This quaternary coalloyed NiO maximizes p-type doping potential while partially improving band alignment relative to pure NiO, thus representing a promising HTL candidate for high-efficiency perovskite solar cells.