FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

Abstract

Mixed‐halide wide‐bandgap perovskites are key components for the development of high‐efficiency tandem structured devices. However, mixed‐halide perovskites usually suffer from phase‐impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA+)‐based mixed‐halide perovskites, MAPb(I0.6Br0.4)3, the halide composition of the spin‐coated perovskite films is preferentially dominated by the bromide ions (Br−). Additional thermal energy is required to initiate the insertion of iodide ions (I−) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA+) in the precursor solution, it can effectively facilitate the I− coordination in the perovskite framework during the spin‐coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high‐crystallinity perovskite film with high Br− content. As a result, high‐quality MA0.9FA0.1Pb(I0.6Br0.4)3 perovskite film with a bandgap (Eg) of 1.81 eV is achieved, along with an encouraging power‐conversion‐efficiency of 17.1% and open‐circuit voltage (Voc) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.

Preventing phase segregation in mixed-halide perovskites: a perspective

“…9,42,67,70,72,73 An alternative, highly promising method for limiting halide segregation is an improvement in the crystallinity of the perovskite, i.e. creating films with a larger average grain size, 9,42,53,72,75 which may affect properties at the grain boundaries such as trap density, strain, or ion migration channels. 54,68,97 In addition, passivation of electronic trap states through the use of various additives has also been shown to be successful at mitigating halide segregation, 73,104,105,124,125 with potassium iodide being particularly effective.…”

“…67 While engineering the composition of the A-site cation in the mixed-halide perovskite crystal presents perhaps the most promising method in which to completely prevent, or at least slow the progression of, halide segregation in working perovskite solar cells and LEDs, the reason for the changes in photo-stability is still a matter of debate. One explanation proposed is that the composition of the A-site cation is strongly linked to the crystallinity and quality of the resulting perovskite film, 9,42,70,75 and therefore the choice of A-site cation has a large influence on a number of factors that could possibly be integral to halide seg- have suggested that polaronic interactions (interactions between excited charge carriers and the ionic perovskite lattice) are both the driving factor behind halide segregation, and are affected by the composition of the A-site cation in the perovskite crystal. 54,69 Regardless of the underlying cause, given the observed strong influence of the A-site cation on the photostability of the perovskite, engineering the perovskite stoichiometry is clearly an important tool for eliminating halide segregation from metal halide perovskite materials.…”

“…38 The films created with a halide excess showed improved stability under ∼20 mW cm −2 (∼0.2 sun) illumination compared to the stoichiometric films, according to measurements of PL and photothermal deflection spectra. However, the incorporation of excess halide was also found to turn the material more bromide-rich because of the preferential binding of lead to bromide (as compared to iodide), 75,141 leading to blueshifts in the PL emission peak and absorption edge. 38 Ruth et al found that the speed at which halide segregation occurred in MAPb(Br 0.4 I 0.6 ) (3−y) films increased as the halide content was reduced from excess (y < 0)…”

“…9,42,53,72,75 Analysis of comparable perovskite films made via slightly different processing methods has revealed a clear impact of crystalline quality on the propensity of the material to commence halide segregation under illumination. In one such study,Hu et al showed that deposition of MAPb(Br 0.27 I 0.73 ) 3 perovskite films on a non-wetting, poly[bis(4phenyl)(2,4,6trimethylphenyl)amine] (PTAA) surface yielded larger grain size and improved photostability than those of films deposited on a poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) surface 53.…”

Halide Segregation in Mixed-Halide Perovskites: Influence of A-Site Cations

“… 3 , 20 , 23 , 30 While the remixing dynamic is thought to occur for entropic reasons, 21 , 31 the cause of the initial segregation is still much debated, with several potential mechanisms having been proposed. 32 Empirically, many factors have been found to affect the halide segregation mechanism, such as crystallinity, 10 , 24 , 33 − 35 trap state density, 20 , 23 , 36 − 43 and the average stoichiometry of the perovskite. 6 , 10 , 11 , 44 , 45 Interestingly, the composition of the A-cation site appears to have a particularly strong effect on the extent to which halide segregation occurs.…”