  Design and modification of the interfaces, always a critical issue for
semiconductor devices, has become the primary tool to harness the full
potential of halide perovskite (HaP)-based ones. In particular the outstanding
improvements in HaP solar cell performance and stability can be primarily
ascribed to a careful choice of the interfacial layout in the layer stack. In
this review we describe the unique challenges and opportunities of these
approaches (section A). For this purpose, we first elucidate the basic physical
and chemical properties of the exposed HaP thin film and crystal surface
(section B). We then lay out the energetic alignment processes to adjacent
transport and buffer layers (section C) and finally elaborate on the impact of
the interface formation on how well/poor a device functions. Based on those
sections we then present a road map for the next steps in interfacial design
principles for HaP semiconductors (section D).
