Eppur si Muove: Proton Diffusion in Halide Perovskite Single Crystals

Abstract

Halide perovskites, HaPs, can show remarkable to outstanding properties and photovoltaic (PV) devices in which they are the absorber can show excellent performances. [1-4] Apart from the PV results, also light-emission, radiation detection, and other electronics are explored. [5-8] During their relatively short history, numerous questions have arisen about the materials' properties and functions with many central ones still under discussion, such as the proposed idea of defect tolerance which awaits experimental evidence. At the same time, already several observables have been linked to fascinating phenomena, such as anharmonicity [9] and high entropy. [10] Ion migration in HaPs belongs to the latter class and has been widely discussed. Migrating ions modify conductivity and junction architecture with consequences for the overall efficiency of junction devices, such as solar cells. A wide consensus identifies halides, specifically I − and Br − , as the migrating ions, although, still without the gold standard experiment of isotope tracing. Remarkably, with some notable exceptions, there is no mention of contributions of protons to ion migration, although their absence in HaPs containing CH 3 NH 3 + (MA +) and HC(NH 2) 2 (FA +) (as the A cation in ABX 3) would be remarkable. In the past years of our research on dynamic effects in halide perovskites (self-healing and ion Ion diffusion affects the optoelectronic properties of halide-perovskites (HaPs). Until now, the fastest diffusion has been attributed to the movement of the halides, largely neglecting the contribution of protons, on the basis of computed density estimates. Here, the process of proton diffusion inside HaPs, following deuterium-hydrogen exchange and migration in MAPbI 3 , MAPbBr 3 , and FAPbBr 3 single crystals, is proven through D/H NMR quantification, Raman spectroscopy, and elastic recoil detection analysis, challenging the original assumption of halide-dominated diffusion. The results are confirmed by impedance spectroscopy, where MAPbBr 3-and CsPbBr 3-based solar cells respond at very different frequencies. Water plays a key role in allowing the migration of protons as deuteration is not detected in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, waterdependent proton diffusion can be at the basis of the ≈7 orders of magnitude spread of diffusion (attributed to I − and Br −) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water may be related to the finding.

Comment on “Eppur si Muove: Proton Diffusion in Halide Perovskite Single Crystals”: Eppur Non si Muove: A Critical Evaluation of Proton Diffusion in Halide Perovskite Single Crystals

“…in which the authors investigate the diffusion of protons in methylammonium lead triiodide (MAPI). [ 1 ] In this study, the diffusion of D + in MAPI is reported to be 3 × 10 −10 cm 2 s −1 , which is substantially larger than a previous estimate (1.5 × 10 −15 cm 2 s −1 ). [ 2 ] Based on this, the authors claim that other reports of ion diffusion in HOIP material should be re‐examined to take into account the fast background diffusion of H + .…”

“…Methylammonium lead triiodide single crystals were grown from a solution of CH 3 NH 3 I (1 m ) and PbI 2 (1 m ) in γ‐butyrolactone. [ 1 ] The crystals were collected by gravity filtration, washed with a small volume of γ‐butyrolactone, and air‐dried protected from light. The crystals were selected based on size and placed individually in 2 mL open vials that were sealed in 25 mL vial containing 3 mL of a D 2 O solution of saturated K 2 CO 3 to maintain the relative humidity constant at 43%.…”

“…for millimeter‐size single crystals of MAPI. [ 1 ] A) Experimental data fitted assuming single crystals of different sizes [ 1 ] ( D = 3.0 × 10 −10 cm 2 s −1 , Σχ 2 = 0.47) and B) the same experimental data fitted assuming all crystals are identical in size with R = 3 µm ( D = 1.5 × 10 −15 cm 2 s −1 , Σ χ 2 = 0.51). See the Supporting Information for in‐depth statistical analysis of different models.…”

The pursuit of stability in halide perovskites: the monovalent cation and the key for surface and bulk self-healing

“…Notably, such kinetics should strongly depend on the migration speed of protons, which should also depend on the H 2 O concentration in the perovskite structure. 75,76 5 General discussion: pulling it all together…”

Revealing Charge Carrier Mobility and Defect Densities in Metal Halide Perovskites via Space-Charge-Limited Current Measurements

“… 30 − 32 However, recent studies also suggest that the influence of A site cations such as Cs + , CH 3 NH 3 + , and (NH 2 ) 2 CH + or even H + should not be neglected. 33 , 34 Overall, it is difficult to find one responsible for the ionic motion in perovskite as it largely depends on the system being studied. Nevertheless, it is clear that mobile ions are present in a large majority of metal halide perovskites and need to be accounted for.…”