We study ion migration in 2D lead halide perovskites of varying dimensionality using scanning Kelvin probe microscopy (SKPM). We perform potentiometry on micrometer-scale lateral junctions in the absence of injected charge, and we compare how ion motion varies between prototypical two-dimensional n-butylammonium lead iodide perovskites (BA 2 PbI 4 , n = 1), and methylammonium-incorporated quasi-2D perovskites (BA 2 MA 3 Pb 4 I 13 , ∼⟨n⟩ = 4) under the effects of illumination and temperature. We attribute the observed slow dynamics to relaxation of the bias-induced ionic charge distributions at different temperatures, and we extract the activation energies associated with the ionic motion in each case. Finally, we propose an explanation for these phenomena by hypothesizing that ion motion in purely-2D BA 2 PbI 4 perovskite films is dominated by paired halide and halide vacancy, whereas for quasi-2D BA 2 MA 3 Pb 4 I 13 perovskites, the ion motion is a combination of both halide and methylammonium (vacancy) migration. These data show that dimensionality in these systems plays a critical role in ion dynamics.

We demonstrate reduced surface recombination velocity (SRV) and enhanced power-conversion efficiency (PCE) in mixed-cation mixed-halide perovskite solar cells by using (3-aminopropyl)­trimethoxysilane (APTMS) as a surface passivator. We show the APTMS serves to passivate defects at the perovskite surface, while also decoupling the perovskite from detrimental interactions at the C60 interface. We measure a SRV of ∼125 ± 14 cm/s, and a concomitant increase of ∼100 meV in quasi-Fermi level splitting in passivated devices compared to the controls. We use time-resolved photoluminescence and excitation-correlation photoluminescence spectroscopy to show that APTMS passivation effectively suppresses nonradiative recombination. We show that APTMS improves both the fill factor and open-circuit voltage (V OC), increasing V OC from 1.03 V for control devices to 1.09 V for APTMS-passivated devices, and leads to a PCE increase from 15.90% to 18.03%. We attribute the enhanced performance to reduced defect density resulting in suppressed nonradiative recombination and lower SRV at the perovskite/transport layer interface.

Interfacial passivation with bulky organic cations such as phenetylammonium iodide has enabled high performance for metal halide perovskite optoelectronic devices. However, the homogeneity of these interfaces and their formation dynamics are poorly understood. We study how Ruddlesden−Popper 2D phases form at a 3D perovskite interface when the 2D precursors are introduced via solution or via vapor. When using vapor deposition, we observe uniform coverage of the capping layer and the formation of a predominantly n = 2 Ruddlesden−Popper phase. In contrast, when using solution deposition, we observe the presence of a mixture of n = 2 and n = 1 in the film and the formation of aggregates of the organic cations. As a result of the better phase purity and uniformity, vapor deposition enables higher median solar cell performance with narrower distribution compared to solution-treated films. This study provides fundamental information that the perovskite community can use to better design capping layers to achieve higher charge extraction efficiencies.

We use scanning probe microscopy to study ion migration in formamidinium (FA)-containing halide perovskite semiconductor Cs0.22FA0.78Pb­(I0.85Br0.15)3 in the presence and absence of chemical surface passivation. We measure the evolving contact potential difference (CPD) using scanning Kelvin probe microscopy (SKPM) following voltage poling. We find that ion migration leads to a ∼100 mV shift in the CPD of control films after poling with 3 V for only a few seconds. Moreover, we find that ion migration is heterogeneous, with domain interfaces leading to a larger CPD shift than domain interiors. Application of (3-aminopropyl)­trimethoxysilane (APTMS) as a surface passivator further leads to 5-fold reduction in the CPD shift from ∼100 to ∼20 mV. We use hyperspectral microscopy to confirm that APTMS-treated perovskite films undergo less photoinduced halide migration than control films. We interpret these results as due to a reduction in the halide vacancy concentration after APTMS passivation.

No abstract