Structures and properties of alkali metal hybrid halide perovskites containing dabconium or piperazinium

Abstract

Hybrid halide perovskites have received much attention over the past two decades due to their realisation in optoelectronic applications. In this study, hybrid halide alkali metal perovskites that contained either piperazinium (piperazine-1,4-diium) or dabconium (1,4-diazabicyclo[2.2.2]octane-1,4-diium) were studied crystallographically and subsequently, their optical band gaps and solid-state fluorescence properties were measured. Specifically, the alkali metal halides NaCl, NaBr, NaI, KCl, KBr, KI, CsCl, CsBr and CsI were employed in combination with the aforementioned organic dications. Moreover, diffuse reflectance spectroscopy was used to measure the optical band gaps of the materials. Sixteen perovskite structures were determined, nine of which are novel. Eight novel dabconium-containing perovskite structures and one piperazinium-containing structure were obtained. The dabconium-series exhibited one of two structural dimensionalities, either a 3D perovskite structure (six in total) or a 1D ABX3-type perovskite structure (five in total). Similarly, the piperazinium-series also exhibited either a 3D structure (four in total) or a 1D ⟨100⟩-type perovskite structure (one structure). In addition, the piperazinium-series was found to generally crystallise with water molecules included in the crystal structure, whereas the dabconium-series did not. The dabconium-containing structures crystallised in a wide range of phases, including monoclinic, orthorhombic, trigonal, and hexagonal phases, while the piperazinium-containing structures were obtained in one of two phases (3D structures in the orthorhombic phase and the 1D structure in the monoclinic phase). Structural trends were identified in both families. The band gaps of the materials from both series were determined to exceed 3.00 eV and hence the materials are unsuited for application as sensitisers in perovskite solar cells. The materials could be classified as either semi-conductors (band gap below 5.0 eV) or insulators (band gap exceeding 5.0 eV). Subsequently, their solid-state fluorescence spectra were measured, and it was determined that none of the perovskite materials obtained in this study exhibited fluorescence at room temperature. However, because of their wide band gap, they may find application in white-light emission devices, such as perovskite light-emitting diodes. Furthermore, though the materials were not suitable as sensitisers in perovskite solar cells, they show promise for application as electron transmitting materials and hence may still be considered in the domain of perovskite solar cells. Notably, structural and property tuneability was illustrated for a specific example of the dabconium-series. It was shown that the material's structure and band gap could be engineered by careful consideration of the precursor constituents, based on the structural trends identified. This tuneability of materials is much desired in the field of materials science. Finally, several avenues for future work, including synthetic extensions, additional property measurements and other potential optoelectronic applications, were identified form the results of this study and show that the perovskite family tree is still revealing new blossoms each day.