A recently discovered new family of 3-D halide perovskites with the general formula (A)1–x(en)x(Pb)1–0.7x(X)3–0.4x (A = MA, FA; X = Br, I; MA = methylammonium, FA = formamidinium, en = ethylenediammonium) are referred as “hollow” perovskites owing to extensive Pb and X vacancies created on incorporation of en cations in the 3-D network. They exhibit exceptional environmental stability and enhanced photovoltaic performance compared to conventional 3-D perovskites. The “hollow” strategy allows for fine tuning of optical and electronic, and transport properties. Although there is information about the kinetic stability of these materials, there is no information on their thermodynamic stability. We recently performed extensive thermochemical studies, using room temperature solution calorimetry, to identify the thermodynamic properties of three different families of hollow perovskites en/FAPbI3, en/MAPbI3 and, en/FAPbBr3. Bromide analogs are more stable compared to iodide analogs in the FA based hollow perovskites. The least stable (FA)1–x(en)x(Pb)1–0.7x(I)3–0.4x gains stability on incorporation of the en cation, whereas (FA)1–x(en)x(Pb)1–0.7x(Br)3–0.4x becomes less stable. This behavior is attributed to the difference in the 3D cage size in the bromide and iodide analogs. Configurational entropy which arises from randomly dispersed cation and anion vacancies may play a significant role in stabilizing these hollow perovskite structures despite small difference in their formation enthalpies.

Entropy stabilization in 3D “hollow” halide perovskites

ACS Spring 2022

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