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

Abstract

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.

Ag/In lead‐free double perovskites

“…A massive effort was headed toward the simultaneous substitution of Pb, A-site cation and/or anion; however, combinations including a single B-site replacement capable to preserve the abovementioned virtues of lead perovskites remained elusive so far (promising exceptions are Sn-based perovskites [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] for solar cells and Ge-based layered perovskites [45][46][47][48][49][50] for LEDs).…”

Germanium‐Based Halide Perovskites: Materials, Properties, and Applications

“…Through DFT calculations of the electronic structure, the authors attributed the weak dependance of band gap from quantum confinement to the peculiar nature of the germanium orbitals, leading to band structure of layered RP germanium bromide perovskites similar to the parent 3D perovskite, differently to the case of analogous lead-based compounds. [41] The PEA 2 GeI 4 (PEA=C 6 H 5 (CH 2 ) 2 NH 3 + ) 2D perovskite has been reported by Cheng an co-workers as one of the first examples of layered iodide Ge-based perovskite. [42] This compounds crystallizes in a triclinic crystal system (space group P1) with organic PEA + cations intercalated as spacers between the [GeI 6 ] 4À octahedral planes.…”

“…). [41] Analogously to the Pbcounterparts, the crystal structure of these materials consist of the stacking of n inorganic perovskite layers, intercalated with bulky butylammonium cations acting as spacers. The authors focused on the n = 1 and 2 of this series with the aim of highlighting the quantum confinement effects and comparing their properties with MAGeBr 3 .…”

“…Figure 8 reports the optical properties of the synthetized materials showing the reflectance and PL spectra for n = 1, 2, and 1 (3D) compounds. [41] The band gaps extracted from the Tauc plots of absorption demonstrate a progressive blue-shift moving from MAGeBr 3 (2.85 eV) to BA 2 GeBr 4 (2.95 eV), and to BA 2 MAGe 2 Br 7 (2.98 eV), as expected from increased quantum confinement of optical excitations within the layers by reducing n. [41] Photoluminescence decays in time-resolved measurements, demonstrated similar initial decay time for all the samples as possibly due to nonradiative recombination through defects. Through DFT calculations of the electronic structure, the authors attributed the weak dependance of band gap from quantum confinement to the peculiar nature of the germanium orbitals, leading to band structure of layered RP germanium bromide perovskites similar to the parent 3D perovskite, differently to the case of analogous lead-based compounds.…”

“…d) PL decay time of the BA 2 MA n-1 Ge n Br 3n + 1 compounds (for n = 1, 2, 1) at room temperature. Reproduced from reference[41] with permission from Wiley-VCH.…”