Thermally-induced drift of A-site cations at solid-solid interface in physically paired lead halide perovskites

Abstract

The promise of hybrid organic-inorganic halide perovskite solar cells rests on their exceptional power conversion efficiency routinely exceeding 25% in laboratory scale devices. While the migration of halide ions in perovskite thin films has been extensively investigated, the understanding of cation diffusion remains elusive. In this study, a thermal migration of A‑site cations at the solid-solid interface, formed by two physically paired MAPbI₃ and FAPbI₃ perovskite thin films casted on FTO, is demonstrated through continuous annealing at comparably low temperature (100 °C). Diffusion of methylammonium (CH₃NH₃⁺, MA⁺) cations into the low‑symmetry yellow δ‑FAPbI₃ phase triggers a transition from the yellow (δ) to black (α) phase evident in the distinctive color change and verified by shifts in absorption bands and X‑ray diffraction patterns. Intermixing of the A‑site cations MA⁺ and FA⁺ (CH(NH₂)₂⁺) occurred for both systems, α‑MAPbI₃/δ‑FAPbI₃ and α‑MAPbI₃/α‑FAPbI₃. The structural and compositional changes in both cases support a thermally activated ion drift unambiguously demonstrated through changes in the absorption and X-ray photoelectron spectra. Moreover, the physical contact annealing (PCA) leads to healing of defects and pinholes in α‑MAPbI₃ thin films, which was correlated to longer recombination lifetimes in mixed MA_xFA_1-xPbI₃ thin films obtained after PCA and probed by ultrafast transient absorption spectroscopy.

Figure 1

(a) Perovskite-coated substrate in physical contact for PCA. (b) color change of the FAPbI₃ sample from yellow (as-prepared) to grey (after 20 h PCA).

Figure 2

UV–vis spectra of MAPbI₃ (a) and FAPbI₃ (b) thin films after different times of physical contact annealing (PCA) at 100 °C. PL emission of (c) MAPbI₃ before and after 80 h PCA and (d) FAPbI₃ before and after 80 h PCA. (e) Normalized plots used for the calculation of the estimated Urbach energy. (f) Calculated E_U values.

Figure 3

XRD pattern of the investigated δ-FAPbI₃ sample after 20 h PCA (red) compared with reference XRD patterns (blue and black). Phase conversion of the target sample together with residual PbI₂ impurities are observed.

Figure 4

High-resolution scans of the probed C 1 s regions of (a) pristine α‑MAPbI₃, (b) post-PCA MAPbI₃ after 60 h annealing, (c) pristine δ‑FAPbI₃, (d) post-PCA FAPbI₃ after 60 h annealing.

Figure 5

Top-view SEM images of (a) as-prepared α‑MAPbI₃ thin film, (b) post-PCA MAPbI₃ thin film, (c) α‑MAPbI₃ thin film annealed on its own (no PCA) for 80 h.

Figure 6

Transient absorption spectra of (a) pristine α‑MAPbI₃ (b) post-PCA MAPbI₃, (c) post-PCA FAPbI₃. (d) Dynamics of the thin films around the bleaching maxima. The exponential decay fittings are shown for α‑MAPbI₃ and post-PCA MAPbI₃, whereas the post-PCA FAPbI₃ data were fitted with a numerical simulation. (e) Hypothesized energy level diagram for the post-PCA FAPbI₃ dynamics.

Figure 7

(a) Color-coded 3D plot showing the intensity of each m/z over time. (b) Histogram plot showing the absolute maximum intensity of m/z values of interest. (c,d) Intensities of selected m/z signals over the experiment time. Around t = 300 s the valve was opened leading the gas flow to the detector.

Figure 8

UV–vis spectra of (a) α-MAPbI₃ and (b) α-FAPbI₃ over 80 h physical contact annealing at 100 °C. The black line in (b) shows an offset due to an instrumental error.

Figure 9

Top-view SEM images of (a) as-prepared α‑MAPbI₃, (b) post-PCA MAPbI₃ (c) post-annealing (non-PCA) MAPbI₃. Upon simple annealing there is an increase in the grain size, but complete defect healing is observed only upon PCA. The same annealing time was used for (b,c), i.e. 80 h.