No abstract

Accounting for the perovskite ionic transport and reactions reveals the importance of the electron blocking (hole transporting) layer in determining device stability.

The mixed ionic–electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts—such as hysteresis or device degradation—leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼10 21 cm –3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

Ion migration into blocking layers toward the metallic electrodes is studied within a semiconductor device model framework. We find that ion leakage into the blocking layers and their accumulation at the electrode interface are significantly affected by the electronic injection barrier at the contact. Specifically, we find that if the device structure promotes, under light, hole (electron) accumulation within the perovskite layer, these excess holes (electrons) would release an almost equivalent number of cations (anions) into the transport layers toward the contacts. Our analysis suggests that it would be beneficial to include intentional doping of the blocking layers and that it should follow the “just enough” strategy.

Ion diffusion into the BLs changes the device from symmetric-intrinsic to asymmetric-doped with all internal parameters changed.