Rehermann, C.; Merdasa, A.; Suchan, K.; Schröder, V.; Mathies, F.; Unger, E.L.: Origin of Ionic Inhomogeneity in MAPb(IxBr1–x)3 Perovskite Thin Films Revealed by In-Situ Spectroscopy during Spin Coating and Annealing. ACS Applied Materials & Interfaces 12 (2020), p. 30343-30352
10.1021/acsami.0c05894
Abstract:
Irradiation-induced phase segregation in mixed methylammonium halide perovskite samples such as methylammonium lead bromide-/iodide, MAPb(IxBr1-x)3, is being studied intensively, since it limits the efficiency of wide band gap perovskite solar cells. It has been postulated that this phenomenon depends on the intrinsic ionic (in)homogeneity in samples induced already during film formation. A deeper understanding of the MAPb(IxBr1-x)3 formation processes and the influence of the halide ratio, solvents, and perovskite precursor composition as well as the influence of processing parameters during deposition, e.g. by spin-coating and annealing parameters, is still lacking. Here, we use a fiber-optic based optical in-situ setup to study the formation processes of the MAPb(IxBr1-x)3 series on a sub-second timescale during spin-coating and thermal annealing. In-situ UV-vis measurements during spin-coating reveal the influence of different halide ratios, x, in the precursor solution on the phase crystallizing preferentially. Pure bromide samples directly form a perovskite phase, samples with high iodide content form a solvate intermediate phase and samples with a mixed stoichiometry between 0.1 ≤ x ≤ 0.6 form both. This leads to heterogeneous formation process via two competing reaction pathways, that leads to a heterogeneous mixture of phases, during spin-coating and rationalizes the compositional heterogeneity of mixed bromide/iodide samples even after annealing.