While photovoltaic (PV) devices based on three-dimensional (3D) perovskites, (Cs, MA, FA)Pb(I, Br)3 (MA=methylammonium, FA=formamidinium), have attracted substantial recent interest, because of the unprecedented rise in power conversion efficiency to values above 20%, the perovskite family offers profound structural and electronic flexibility beyond these 3D systems . A key theme for the current talk will center around how the organic cation either directly or indirectly impacts the photophysical and thermal properties of the given semiconductor. In one recent example, acene-based organic cations template single-layer <100>-oriented perovskites, in which the details of the organic cation and the hydrogen bonding interaction between the lead(II) halide (PbX42- X=Cl, Br, I) perovskite layers and the organic cations fine tune the position of the band edge and luminescence . More complex organic cations (e.g., oligothiophenes) lead to hybrids in which the conduction and valence bands are comprised of states that principally derive from inorganic and organic components, respectively, opening the possibility of creating electronic properties akin to quantum well systems. In another example, specific choice of organic cation in the layered lead(II) halide perovskites lowers the melting temperature below the decomposition point, ultimately creating hybrid perovskites that can be melt-processed into thin-film form  and providing a promising avenue to solvent-free film deposition of functional perovskite semiconductors. As seen by these and other examples, the independent and diverse tunability of the organic and inorganic structural components is expected to provide (and is already providing) many exciting opportunities beyond the 3D hybrid perovskite systems.
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