In 2D halide perovskites, the choice of the A-site perovskitizer cations is not entirely limited by the so-called Goldschmidt tolerance factor. The stabilizing effect of the spacer cations makes it possible to tolerate larger A-site cations as perovskitizers. By solving the single-crystal structures of (BA)₂(A)Pb₂I₇ where BA = butylammonium, and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or guanidinium (GA), we have proven that the relatively large DMA and GA can serve as the A-site cation of 2D perovskites despite the fact that they are too large to formally satisfy the Goldschmidt rule. As the size of the A-site cations increases, the Pb−I bonds are elongated, which reduces the orbital overlap of Pb s- and I p-orbitals, thus decreasing the electronic bandwidths and increasing the bandgaps, as confirmed by DFT calculations. This increases the local distortions of the PbI₆ coordination environment and leads to a decrease of PL intensity and shortening of PL lifetime. The expanded cages also result in soft and anharmonic lattices, as supported by Raman spectra. Exciton-exciton annihilation processes occur in these systems with the rates being slower in the structures with larger A cations. The bromide perovskite forms a similar n = 2 structure with DMA. More interestingly, n = 3 phases can form with ethylammonium (EA) and methylhydrazinium (MHz) as A-site cations and BA as the spacer. Their structures are more distorted than the n = 2 phase, thus exhibit similar bandgaps as the (BA)₂(DMA)Pb₂Br₇ compound. Understanding the structure-property relationships of these compounds will help illustrate how the large A-site cations may influence optical properties of both 2D and 3D perovskites and facilitate their future optoelectronic applications.

Jacky Even

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