(C₅N₂H₁₄)GeBr₄: A 2D Organic Germanium Bromide Perovskite with Strong Orange Photoluminescence Properties

Hybrid organic–inorganic metal halide (OIMH) perovskites are regarded as potential photoluminescent (PL) materials and have attracted intensive attention. Here, we select 1-methylpiperazine as an organic component and successfully obtain a two-dimensional (2D) Ge-based OIMH perovskite, (1-mpz)GeBr₄. It features a 2D layered structure composed of distorted [GeBr₆]^4– octahedra with organic (C₅H₁₄N₂)²⁺ located between the layers. (1-mpz)GeBr₄ exhibits strong orange color under ultraviolet (UV) light and possesses good PL stability for over 2 months. The photoluminescence quantum efficiency is measured to be 7.15% at room temperature, which is the largest among all reported low-dimensional Ge-based perovskites. Experimental measurements, combined with first-principles calculations, reveal that its PL property is attributed to self-trapped excitons (STEs) from [GeBr₆]^4– groups. From the deduced structure–property relationship, Ge-based OIMH PL perovskites with good stability and high PL efficiency can be expected.