  Halide perovskite light absorbers have great advantages for photovoltaics
such as efficient solar energy absorption, but charge accumulation and
recombination at the interface with an electron transport layer (ETL) remains a
major challenge in realizing their full potential. Here we report the
experimental realization of a zipper-like interdigitated interface between a
Pb-based halide perovskite light absorber and an oxide ETL by the PbO capping
of the ETL surface, which produces an atomically thin two-dimensional metallic
layer that can significantly enhance the perovskite/ETL charge extraction
process. As the atomistic origin of the emergent two-dimensional interfacial
metallicity, first-principles calculations performed on the representative
MAPbI$_3$/TiO$_2$ interface identify the interfacial strain induced by the
simultaneous formation of stretched I-substitutional Pb bonds (and thus Pb-I-Pb
bonds bridging MAPbI$_3$ and TiO$_2$) and contracted substitutional Pb-O bonds.
Direct and indirect experimental evidences for the presence of interfacial
metallic states are provided, and a non-conventional defect-passivating nature
of the strained interdigitated perovskite/ETL interface is emphasized. It is
experimentally demonstrated that the PbO capping method is generally applicable
to other ETL materials including ZnO and SrTiO$_3$, and that the zipper-like
interdigitated metallic interface leads to about two-fold increase in charge
extraction rate. Finally, in terms of the photovoltaic efficiency, we observe a
volcano-type behavior with the highest performance achieved at the
monolayer-level PbO capping. The method established here might prove to be a
general interface engineering approach to realize high-performance perovskite
solar cells.
