Synergistic Role of Water and Oxygen Leads to Degradation in Formamidinium-Based Halide Perovskites

Abstract

Mixed-cation metal halide perovskites have shown remarkable progress in photovoltaic applications with high power conversion efficiencies. However, to achieve large-scale deployment of this technology, efficiencies must be complemented by long-term durability. The latter is limited by external factors, such as exposure to humidity and air, which lead to the rapid degradation of the perovskite materials and devices. In this work, we study the mechanisms causing Cs and formamidinium (FA)-based halide perovskite phase transformations and stabilization during moisture and air exposure. We use in situ X-ray scattering, X-ray photoelectron spectroscopy, and first-principles calculations to study these chemical interactions and their effects on structure. We unravel a surface reaction pathway involving the dissolution of FAI by water and iodide oxidation by oxygen, driving the Cs/FA ratio into thermodynamically unstable regions, leading to undesirable phase transformations. This work demonstrates the interplay of bulk phase transformations with surface chemical reactions, providing a detailed understanding of the degradation mechanism and strategies for designing durable and efficient perovskite materials.

Figure 1

Humidity-induced structural phase transformations measured by in situ GIWAXS. The structural phases analyzed are (a) tetragonal β–perovskite of space group P4/mbm (left), 2H FAPbI₃ hexagonal nonperovskite phase of space group P6₃/mmc (center), and orthorhombic δ−CsPbI₃ nonperovskite phase of space group Pnma (right). Phase transformations over time of CsFA β–perovskite exposed to (b) H₂O/air, (d) H₂O/N₂, and (f) dry air, where (c, e, and g) are the corresponding integrated areas of the main scattering peaks of each phase.

Figure 2

XPS spectra of the peaks: (a) N 1s, (b) I 3d, and (c) O 1s of CsFA perovskite, FAI, and PbI₂ films without (w/o) and after H₂O/air exposure. For the CsFA perovskite films, panel (d) shows the atomic ratio of iodine(I), cesium (Cs), nitrogen (N, FA), or oxygen (O) normalized to lead (Pb), for pristine films and after H₂O/air exposure. Full peak deconvolution and details for the CsFA films can be found in Figures S10 and S11 and Tables S2 and S3, for FAI films in Figure S12, Tables S4 and S5, and for PbI₂ films in Figure S13 and Tables S6 and S7. CsFA perovskite films w/o H₂O exposure (green) were fabricated in a nitrogen glovebox and exposed to a room atmosphere while mounting the XPS measurement. CsFA films after H₂O/air exposure (blue) were fabricated in a nitrogen glovebox, exposed to humidity, and then measured.

Figure 3

DFT calculations of iodide oxidation and superoxide formation on the perovskite surface. Panel (a) visualizes the formation of IO_n^– species upon oxidation of surface iodide ions [Rx. 1]; from left to right: hypoiodite, IO^–; iodite, IO₂^–; iodate, IO₃^–; and periodate, IO₄^–. All oxidized species are explicitly highlighted, and reaction energies are given (see the computational methods for details). (b) Reaction mechanism [Rx. 2 and 3] of (left) lead(II) iodate, Pb(IO₃)₂, formation by consumption of oxygen molecules and (right) removal of Pb(IO₃)₂ resulting in a surface vacancy V_PbI2. The following color code is used for the atomic representations: purple, I; cyan, Pb; blue, N; green, Cs; gray, C; white, H; and red, O.

Figure 4

Proposed surface and bulk mechanism for CsFA perovskites exposed to H₂O/air. (I) H₂O is adsorbed on the CsFA surface, promoting the loss of FAI (g). Surface vacancies are created, leading to preferential oxygen binding sites, favoring the oxidation of iodide and energetically favorable formation of Pb(IO₃)₂ (II), which will create a PbI₂ vacancy (III). Surface vacancies and the loss of FAI will lead to faster phase segregation and phase transformations from mixed-cation perovskite into single-cation nonperovskite phases (IV).

Figure 5

The PEAI capping layer is used to stabilize the perovskite in H₂O/air. Panel (a) shows H₂O adsorption energy on a CsFA surface and CsFA-PEAI surface calculated by DFT. From GIWAXS measurements in Figure S27, (b) is the integrated area of the main scattering peak of each phase for films exposed to H₂O/air. (c) Shows the 2D GIWAXS patterns from the surface and bulk measurements after 600 min of exposure to H₂O/air for the (left) PEAI-treated CsFA films and (right) untreated CsFA. A PEAI-treated CsFA perovskite layer in a solar cell with (d) n-i-p architecture. Panel (e) shows the current density–voltage curve and stabilized PCE of a high-efficiency device, and (f) shows the statistics of the PCE in box plots for the CsFA-untreated and PEAI-treated, w/o and with H₂O exposure, under 1 sun illumination. The inset pictures show the device after H₂O/air exposure before depositing the Spiro-OMeTAD and Au layers.