Band Alignment and Interfacial Stability of Co₃O₄ vs NiO as a Hole Transport Layer with FA_0.4MA_0.6PbI₃ Perovskite

The unstable cubic phase of halide perovskites (ABX₃) and the poor interfacial quality between their absorbing layer and the hole transport layer (HTL) cause the long-term instability of halide perovskite solar cells (PSCs). To stabilize the intrinsic cubic perovskite structure, mixing CH₃NH⁺ (MA⁺) and CH(NH₂)⁺ (FA⁺) large organic ions at the A site is frequently used. Although NiO offers better stability than organic HTLs, such as poly(triaryl-amine) (PTAA), the stability of NiO-based PSCs still remains an issue, primarily due to the formation of interfacial Ni vacancies at the NiO/perovskite interface. In this theoretical study, by analyzing Co₃O₄/FA_0.4MA_0.6PbI₃ and NiO/perovskite interfaces, we show that Co₃O₄ offers greater benefits as an HTL material than NiO for three main reasons. First, Co₃O₄/FA_0.4MA_0.6PbI₃ shows a type II band alignment with a small valence band offset (0.13 eV), whereas NiO/FA_0.4MA_0.6PbI₃ interfaces give type I band alignments. Second, Co₃O₄/FA_0.4MA_0.6PbI₃ interfaces show higher adhesion energy (1.48 J/m²) than NiO/FA_0.4MA_0.6PbI₃ interfaces, indicating enhanced interfacial stability. Third, the formation of interfacial Co vacancies in NiO/FA_0.4MA_0.6PbI₃ presents greater difficulty due to their higher formation energy of 1.75 eV compared to the Ni vacancies in NiO/FA_0.4MA_0.6PbI₃, suggesting better stability under environmental conditions. FA_0.4MA_0.6PbI₃ also shows higher adhesion energies with Co₃O₄ or NiO than those for MAPbI₃. Therefore, we suggest that the combination of Co₃O₄ as the HTL and FA_0.4MA_0.6PbI₃ as the light-absorbing layer holds great potential for achieving PSCs with long-term stability.