Layered Germanium Hybrid Perovskite Bromides: Insights from Experiments and First-Principles Calculations

Metal halide perovskites are maturing as materials for efficient, yet low cost solar cells and light-emitting diodes, with improving operational stability and reliability. To date however, most perovskite-based devices contain Pb, which poses environmental concerns due to its toxicity; lead-free alternatives are of importance to facilitate the development of perovskite-based devices. Here, the germanium-based Ruddledsen-Popper series (CH3(CH2)3NH3)2(CH3NH3)n-1GenBr3n+1 is investigated, derived from the parent 3D (n = ?) CH3NH3GeBr3 perovskite. Divalent germanium is a promising, nontoxic alternative to Pb2+ and the layered, 2D structure appears promising to bolster light emission, long-term durability, and moisture tolerance. The work, which combines experiments and first principle calculations, highlights that in germanium bromide perovskites the optical bandgap is weakly affected by 2D confinement and the highly stereochemically active 4s2 lone pair preludes to possible ferroelectricity, a topic still debated in Pb-containing compounds.