Soft interfaces within hybrid perovskite solar cells: Real-time dynamic tracking of interfacial electrical property evolution by EIS

Despite the significant increase in power conversion efficiency (PCE) achieved recently, the fundamental physics behind some of the photovoltaic anomalies in perovskite solar cells (PSCs) is rarely investigated. The interfaces within PSCs, especially the two key interfaces related to the perovskite (electron transport layer/perovskite and perovskite/hole transport layer), play a vital role in their overall performance and stability since they are especially susceptible to external stimuli. Herein, high efficiency planar PSCs were fabricated, and the property evolution of the above-mentioned two key interfaces was real-time dynamically traced via electrochemical impedance spectroscopy (EIS). The results indicate that the two interfaces under a small bias are variable, and even last for hours due to ion migration within the PSCs. Specifically, the charge transfer resistance of the two interfaces and the bulk charge transport resistance of the perovskite layer continually increased, while their capacitances (interfacial and dielectric) displayed non-linear changes. Subsequently, ion accumulation occurred at the two interfaces due to electromigration, and a gradual variation in the dielectric property of the perovskite layer was observed at a definite stage and reached a steady state finally. Interestingly, the impedance characteristics could recover within hours after the bias was removed, indicating that ion migration is reversible within PSCs. Therefore, the "soft interface" mechanism was proposed to well elucidate the unique attributes of PSCs. These findings provide novel and deep insight into the unique interfacial properties within PSCs, which advances our understanding of the underlying physical mechanism in photovoltaics.