Impact of Perovskite Surface Termination Dependent Molecular Interaction on Interfacial Energetics and Phase Stability.

The interface between the perovskite and the charge transport layer is the cornerstone of high-performance perovskite optoelectronics. Perovskite surface termination, as the major component of the interface, plays a decisive role in interfacial properties and stability. However, direct experimental evidence on the impact of the perovskite surface termination remains unclear, since it is highly dependent on the fabrication procedures and interfacial molecular interactions. Herein, we deliberately prepare perovskite films with two different terminations (PbX2 and AX) by tuning the precursor ratio and investigate the evolution of stability and interfacial properties at the perovskite/C60 interface upon interaction with a widely used passivator (i.e., piperazine). Our results reveal that the perovskite films with the two bare terminations possess oppositely oriented vertical surface dipoles. In tandem with higher defect density, the PbX2 termination possesses a less favorable interfacial energy level alignment and lower structural stability than the AX termination. This intrinsic difference is modified significantly by the interaction with piperazine. Electron transfer from the perovskite to piperazine flips the surface dipole orientation at both terminations. Piperazine also passivates the defects on the PbX2 termination while inducing lattice distortion on the AX. Under the synergy of these two mechanisms, the piperazine interaction leads to enhanced electron extraction and structural stability for the PbX2-terminated films but accelerated degradation in the AX ones. These effects can be well-correlated with the evolution of efficiency and stability of assembled inverted solar cells. This work discloses the multifaceted role of perovskite surface termination in interfacial properties and structural stability, providing inspiring insights into the realization of high-performance perovskite-based optoelectronics.