  The ability to reduce energy loss at semiconductor surfaces through
passivation or surface field engineering has become an essential step in the
manufacturing of efficient photovoltaic (PV) and optoelectronic devices.
Similarly, surface modification of emerging halide perovskites with quasi-2D
heterostructures is now ubiquitous to achieve PV power conversion efficiencies
(PCEs) > 22% and has enabled single-junction PV devices to reach 25.7%, yet a
fundamental understanding to how these treatments function is still generally
lacking. This has established a bottleneck for maximizing beneficial
improvements as no concrete selection and design rules currently exist. Here we
uncover a new type of tunable passivation strategy and mechanism found in
perovskite PV devices that were the first to reach the > 25% PCE milestone,
which is enabled by surface treating a bulk perovskite layer with hexylammonium
bromide (HABr). We uncover the simultaneous formation of an iodide-rich 2D
layer along with a Br halide gradient achieved through partial halide exchange
that extends from defective surfaces and grain boundaries into the bulk layer.
We demonstrate and directly visualize the tunability of both the 2D layer
thickness, halide gradient, and band structure using a unique combination of
depth-sensitive nanoscale characterization techniques. We show that the
optimization of this interface can extend the charge carrier lifetime to values
> 30 {\mu}s, which is the longest value reported for a direct bandgap
semiconductor (GaAs, InP, CdTe) over the past 50 years. Importantly, this work
reveals an entirely new strategy and knob for optimizing and tuning
recombination and charge transport at semiconductor interfaces and will likely
establish new frontiers in achieving the next set of perovskite device
performance records.
