Publication date: 18th June 2020
Perovskite compounds exhibit exquisite electronic features for photovoltaic applications. Incorporation of those materials into solid state solar cells allowed the recording of very high power conversion efficiencies (PCEs) above 22%, which are comparable to the current commercial available materials. However in order for perovskite compounds to reach eventually the market, some severe limitations have to be addressed. Those are not other than their inherent environmental instability and their composition of toxic elements (e.g. Pb). In this work we address both those important issues by the discovery of a new family of 3D perovskites, namely “hollow” perovskites, with chemical formula (A)1-x(en)x(M)1-0.7x(I)3-0.4x, (A = methylammonium (MA), formamidinium (FA); M = Sn, Pb, en = ethylenediammonium)[1-2]. Incorporation of en cations in the 3D perovskite structure leads to massive M and I vacancies in the 3D [MX3] framework, thus the term hollow. By adjusting the percentage of en in the structure we were able to fine tune the optical properties of the corresponding materials, maintaining at the same time the desired 3D structure dimensionality. These hollow perovskites exhibit significantly blue-shifted direct band gaps over a very wide energy range, from 1.25-1.51 eV for Sn-based perovskites and from 1.53-2.1 eV for the Pb-based analogues. DFT calculations revealed that as metal halide fragments are eliminated from the perovskite structure, the bands themselves become less disperse (i.e. narrower) due to the reduced lengths of fragments with M-I overlap. A most important outcome from this synthetic strategy is the superior enhancement of the air stability of the corresponding materials. The Sn based (MA)0.6(en)0.4(Sn)0.72(I)2.84 perovskite is stable in air for at least 9 days, while the (FA)0.61(en)0.39(Pb)0.727(I)2.844 perovskite is stable in air for more than 300 days. Both lifetimes are the highest reported for a 3D ASnX3 and a 3D FAPbI3 phase respectively. This family of perovskite compounds poses as a new platform of promising light absorbers that can be utilized in single junction or tandem solar cells.
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