Reversible Removal of Intermixed Shallow States by Light Soaking in Multication Mixed Halide Perovskite Films

Abstract

The highest reported efficiencies of metal halide perovskite (MHP) solar cells are all based on mixed perovskites, such as (FA,MA,Cs)Pb(I1–xBrx)3. Despite demonstrated structural changes induced by light soaking, it is unclear how the charge carrier dynamics are affected across this entire material family. Here, various (FA,MA,Cs)Pb(I1–xBrx)3 perovskite films are light-soaked in nitrogen, and changes in optoelectronic properties are investigated through time-resolved microwave conductivity (TRMC) and optical and structural techniques. To fit the TRMC decay kinetics obtained for pristine (FA,MA,Cs)Pb(I1–xBrx)3 for various excitation densities, additional shallow states have to be included, which are not required for describing TRMC traces of single-cation MHPs. These shallow states can, independently of x, be removed by light soaking, which leads to a reduction in the imbalance between the diffusional motion of electrons and holes. We interpret the shallow states as a result of initially well-intermixed halide distributions, which upon light soaking segregate into domains with distinct band gaps.

Study of transport parameters and defect states in thin film perovskites under different environments − air or vacuum − and after light-soaking

“…Since it is not the case, this result suggests that the decrease of the majority carrier mt product could be essentially due to a decrease of their mobility instead of a decrease of their lifetime. The reader may note that such a decrease of mobility has been already observed [33]. Though it concerned films with higher bromide contents than ours, a decrease of the majority carrier mobility can be considered as possible explanation of the photoconductivity decrease under light-soaking.…”

“…Evolution of the band gap with light-soaking was also observed by other groups either from small variations of the absorptance spectra [33] or from Photo Deflection Spectroscopy (PDS) [34]. They have attributed these variations to modifications of the surface of the films by creation of low gap regions and trapping states.…”

“…This decrease seems to be irreversible, at least at room temperature, in contradiction with what we observed with the ambipolar diffusion length. Some authors have suggested that light-soaking resulted mostly in a decrease of the carrier lifetime though, in some cases, a decrease of the mobility was also proposed [33]. A decrease of the carrier lifetime can be due to an increase of the recombining states or an increase of their capture cross-section or both.…”

Photoinduced Anion Segregation in Mixed Halide Perovskites

“…An open question then is the observation of other x~0.2 terminal stoichiometries in mixed cation/ mixed anion perovskites. This is notable because phase transitions are not predicted in mixed cation systems across their full compositional range [70,71]. Existence of terminal x~0.2 values is illustrated in Figure 4, which plots time-evolved photosegregated PL spectra for different thin film compositions.…”

“…Existence of terminal x~0.2 values is illustrated in Figure 4, which plots time-evolved photosegregated PL spectra for different thin film compositions. This includes MAPb(I 0.6 Br 0.4 ) 3 [40], CsPb(I 0.5 Br 0.5 ) 3 [39] [71]. Figure 4.…”