Chemical Analysis of the Interface between Hybrid Organic–Inorganic Perovskite and Atomic Layer Deposited Al₂O₃

Abstract

Ultrathin metal oxides prepared by atomic layer deposition (ALD) have gained utmost attention as moisture and thermal stress barrier layers in perovskite solar cells (PSCs). We have recently shown that 10 cycles of ALD Al2O3 deposited directly on top of the CH3NH3PbI3–xClx perovskite material, are effective in delivering a superior PSC performance with 18% efficiency (compared to 15% of the Al2O3-free cell) with a long-term humidity-stability of more than 60 days. Motivated by these results, the present contribution focuses on the chemical modification which the CH3NH3PbI3–xClx perovskite undergoes upon growth of ALD Al2O3. Specifically, we combine in situ Infrared (IR) spectroscopy studies during film growth, together with X-ray photoelectron spectroscopy (XPS) analysis of the ALD Al2O3/perovskite interface. The IR-active signature of the NH3+ stretching mode of the perovskite undergoes minimal changes upon exposure to ALD cycles, suggesting no diffusion of ALD precursor and co-reactant (Al(CH3)3 and H2O) into the bulk of the perovskite. However, by analyzing the difference between the IR spectra associated with the Al2O3 coated perovskite and the pristine perovskite, respectively, changes occurring at the surface of perovskite are monitored. The abstraction of either NH3 or CH3NH2 from the perovskite surface is observed as deduced by the development of negative N–H bands associated with its stretching and bending modes. The IR investigations are corroborated by XPS study, confirming the abstraction of CH3NH2 from the perovskite surface, whereas no oxidation of its inorganic framework is observed within the ALD window process investigated in this work. In parallel, the growth of ALD Al2O3 on perovskite is witnessed by the appearance of characteristic IR-active Al–O–Al phonon and (OH)–Al=O stretching modes. Based on the IR and XPS investigations, a plausible growth mechanism of ALD Al2O3 on top of perovskite is presented.

Enhanced Nucleation of Atomic Layer Deposited Contacts Improves Operational Stability of Perovskite Solar Cells in Air

“…Several studies have reported that excessive exposure to metal-organic ALD precursors can damage the perovskite material by etching away organic cations from the perovskite surface. [55][56][57][58] We used the same analysis to investigate the nucleation of ALD SnO 2 on C 60 that has been coated with a thin layer of PEIE. The thickness of the PEIE layer is approximately 2 nm as measured by spectroscopic ellipsometry and can be controlled by varying the concentration of the PEIE solution ( Figure S3).…”

Barrier Designs in Perovskite Solar Cells for Long‐Term Stability

“…Insulting ALD-Al 2 O 3 has been demonstrated to be efficient barriers. [179][180][181][182][183] For example, Koushik et al deposited an ultra-thin (≈2 nm) Al 2 O 3 layer on top of the MAPbI 3−x Cl x perovskite using the ALD method to enhance the resistance of PSC to moisture (Figure 7f). It should be noted that this moisture barrier is required to be thin enough to allow the tunneling of carriers.…”

Inorganic Materials by Atomic Layer Deposition for Perovskite Solar Cells

“…Based on previous reports, each pulse step (the metal precursor and the co-reactant) influences the perovskite active layer in somewhat contradicting ways. While a study reported to not observe any degradation of the perovskite after exposure of repeated pulses of TMA and H 2 O at 80 °C, several studies reported the loss of nitrogen, implying etching of MA + from the perovskite active layer [ 55 ]. A study found that TMA partial pressures of 0.1 Torr can etch MAPbI 3 at 75 °C, and observed continual mass loss of perovskite at high TMA exposures of 3 Torr at 25 °C [ 56 ], suggesting that variations in ALD process parameters result in very different perovskite surfaces, which may explain the discrepancies in literature.…”

“…Several reports demonstrated successful encapsulation of PSC devices by ALD single materials or nanolaminates of multiple stacks of alternating materials by ALD and/or organic materials. For example, encapsulated semitransparent PSC devices with a bilayer of 50-nm Al 2 O 3 -coated polyethylene terephthalate (PET) resulted in stable devices based on storage in ambient air for over 45 days [ 55 ].…”