  Perovskite solar cells (PSCs) have shown remarkably improved power-conversion
efficiency of around 25%. However, their working principle remains arguable and
the stability issue has not been solved yet. In this report, we revealed that
the working mechanism of PSCs is explained by a dominant pn junction occurring
at the different interface depending on electron transport layer, and charges
are accumulated at the corresponding dominant junction initiating device
degradation. Locations of a dominant pn junction, the electric field, and
carrier-density distribution with respect to electron-transport layers in the
PCS devices were investigated by using the electron-beam-induced current
measurement and Kelvin probe force microscopy. The amount of accumulated
charges in the devices was analyzed using the charge-extraction method and the
degradation process of devices was confirmed by SEM measurements. From these
observations, we identified that the dominant pn junction appears at the
interface where the degree of band bending is higher compared to the other
interface, and charges are accumulated at the corresponding junction where the
device degradation is initiated, which suggests that there exists a strong
correlation between PSC working principle and device degradation. We highlight
that an ideal pin PSC that can minimize the degree of band bending should be
designed for ensuring long-term stability, via using proper selective contacts
