Probing the Degradation Chemistry and Enhanced Stability of 2D Organolead Halide Perovskites

Abstract

Recent work on quasi-2D Ruddlesden–Popper phase organolead halide perovskites has shown that they possess many interesting optical and physical properties. Most notably, they are significantly more stable when exposed to moisture when compared to the typical 3D perovskite methylammonium lead iodide (MAPI); direct evidence for the chemical source of this stability remains elusive, however. Here, we present a detailed study of the superior moisture stability of a quasi-2D Ruddlesden–Popper perovskite, n-butylammonium methylammonium lead iodide (nBA-MAPI), compared to that of MAPI, and examine a simple, yet efficient, methodology to improve the stability of MAPI devices through the application of a thin layer of nBA-MAPI to the surface. By employing a variety of analytical techniques (photoluminescence, time-of-flight secondary ion mass spectrometry, cyclic voltammetry, X-ray diffraction) we determine that the improved stability of Ruddlesden–Popper perovskites is a consequence of a unique degradation pathway which produces a passivating surface layer, composed of increasingly stable phases of the 2D perovskite, via disproportionation. Our work establishes that this protective material isolates the bulk of the perovskite from a newly identified hydration layer which is found to accumulate at the C60/perovskite interface of full devices, slowing further hydrolysis reactions that would damage the device. As MAPI devices degrade quickly without any protection, a surface treatment of nBA-MAPI is an efficient way to delay device deterioration by creating an artificial 2D surface layer that similarly inhibits interaction with the hydration layer.

Secondary Ion Mass Spectrometry (SIMS) for Chemical Characterization of Metal Halide Perovskites

“…The authors focused on the C60/perovskite interface in the depth profiles based on the assumption that the surface of the perovskite film undergoes significant change during degradation. [116] A hydration layer at the surface of the perovskite layer was observed in all three devices by ToF-SIMS depth profiles, which is due to the interaction of water and perovskite that induces surface degradation of perovskite and forms the hydration layer, implicating the role of hydration in the degradation of perovskite. [116] Further isotopic studies with D 2 O vapor were performed to investigate the difference in the interaction with water of MAPbI 3 and nBA-MAPI.…”

“…[116] A hydration layer at the surface of the perovskite layer was observed in all three devices by ToF-SIMS depth profiles, which is due to the interaction of water and perovskite that induces surface degradation of perovskite and forms the hydration layer, implicating the role of hydration in the degradation of perovskite. [116] Further isotopic studies with D 2 O vapor were performed to investigate the difference in the interaction with water of MAPbI 3 and nBA-MAPI. [116] ToF-SIMS shows the difference in the penetration of water after exposure the MAPbI 3 and the nBA-MPI devices to D 2 O vapor for 1 h. [116] Figure 14a shows an increase of OH − and OD − in the hydration region of MAPbI 3 device, while no OD − hydration layer was observed in nBA-MAPI device.…”

“…[116] This result indicates a low reactivity with water of the nBA-MAPI device than the MAPbI 3 device, suggesting the formation of a protective layer at the nBA-MAPI surface that reduces the interaction of bulk perovskite with the hydration layer and improves the stability of quasi-2D nBA-MAPI perovskite. [116] The degradation of perovskite due to the oxygen diffusion was also revealed by ToF-SIMS. [117] Figure 14b,c shows the slices oxygen distribution through a MAPbI 3 film and a MAPbI x Cl 3-x film at a depth of ≈200 nm after aging the films in dry air under dark for 30 min, which indicate that the oxygen diffuses into the perovskite films.…”

“…[115] Wygant et al systematically investigated the degradation of 3D (MAPbI 3 ), 2D (nBA-MAPbI), and 2D/3D perovskite using ToF-SIMS. [116] The authors found that the MAPbI 3 degradation undergoes a hydration pathway and nBA-MAPbI and 2D/3D perovskite undergo a disproportionation. The authors focused on the C60/perovskite interface in the depth profiles based on the assumption that the surface of the perovskite film undergoes significant change during degradation.…”

“…[116] Further isotopic studies with D 2 O vapor were performed to investigate the difference in the interaction with water of MAPbI 3 and nBA-MAPI. [116] ToF-SIMS shows the difference in the penetration of water after exposure the MAPbI 3 and the nBA-MPI devices to D 2 O vapor for 1 h. [116] Figure 14a shows an increase of OH − and OD − in the hydration region of MAPbI 3 device, while no OD − hydration layer was observed in nBA-MAPI device. [116] This result indicates a low reactivity with water of the nBA-MAPI device than the MAPbI 3 device, suggesting the formation of a protective layer at the nBA-MAPI surface that reduces the interaction of bulk perovskite with the hydration layer and improves the stability of quasi-2D nBA-MAPI perovskite.…”