Band Alignment and Interfacial Stability of Co3O4 vs NiO as a Hole Transport Layer with FA0.4MA0.6PbI3 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/FA0.4MA0.6PbI3 and NiO/perovskite interfaces, we show that Co3O4 offers greater benefits as an HTL material than NiO for three main reasons. First, Co3O4/FA0.4MA0.6PbI3 shows a type II band alignment with a small valence band offset (0.13 eV), whereas NiO/FA0.4MA0.6PbI3 interfaces give type I band alignments. Second, Co3O4/FA0.4MA0.6PbI3 interfaces show higher adhesion energy (1.48 J/m2) than NiO/FA0.4MA0.6PbI3 interfaces, indicating enhanced interfacial stability. Third, the formation of interfacial Co vacancies in NiO/FA0.4MA0.6PbI3 presents greater difficulty due to their higher formation energy of 1.75 eV compared to the Ni vacancies in NiO/FA0.4MA0.6PbI3, suggesting better stability under environmental conditions. FA0.4MA0.6PbI3 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 FA0.4MA0.6PbI3 as the light-absorbing layer holds great potential for achieving PSCs with long-term stability.