Restricting Anion Migrations by Atomic Layer-Deposited Alumina on Perovskite Nanocrystals while Preserving Structural and Optical Properties

Abstract

Lead halide perovskite (LHP) nanocrystals (NCs) offer an easy tunability of band gap via anion exchanges, enabling their usage in tandem optoelectronic devices with graded band gaps. However, instantaneous anion migrations at the interfaces of different LHP layers impede the formation of welldefined interfaces. We deposited an ultrathin alumina (Al 2 O 3−x ) layer at the interface of CsPbBr 3 −CsPbI 3 NC films by using atomic layer deposition (ALD) and demonstrated that low-temperature ALD alumina has negligible impact on the structural or optical properties of CsPbBr 3 NCs except agglomeration. ALD alumina can restrict anion migration for months but cannot be prevented permanently. The rate of anion migration significantly decreases with an increase in the Al 2 O 3−x layer thickness on CsPbBr 3 NC films, which follows first-order kinetics. Density functional theory (DFT) calculations showed that the iodide ion can migrate through oxygen vacancies in the Al 2 O 3−x layer with an activation energy of 1.54 eV. This strategy provides new insight into fabricating halide perovskite-based tandem optoelectronics devices.

Insights into the Cation Migration Kinetics across 2D/3D Perovskite Interfaces and Strategy for Its Prevention

“…We expected that the ALD alumina layer could prevent cation migration. The alumina (Al 2 O 3– x ) layer of thickness 6.4 ± 0.1 nm was deposited on MAPbBr 3 films by thermal ALD at 70 °C . The alumina layer did not affect the morphology and emission of MAPbBr 3 film (refer to Figure S2), the same as CsPbBr 3 nanocrystal films .…”

Prevention of ion migration in lead halide perovskites upon plugging the anion vacancies with PbSe islands

“…This is attributed to the removal of surface bromide ions in the form of oleylammonium bromide, which creates more defects in NCs upon washing with antisolvent. 39 The relatively minor reduction in PL intensity observed in SP/MS-CPB suggests that the surface bromide remains largely intact indicating PbSe effectively plugs the halide ion vacancies of CsPbBr 3 and offers protection from the external environment under ambient conditions. 43 In PbSe decorated halide perovskite NCs, as the halide vacancies are not created under harsh experimental conditions, we believe that these vacancies are unlikely to form during device operation.…”

“…37 Balakrishna's group inhibited the halide ion exchange in tandem devices via passivating the perovskite's defects using polyvinylpyrrolidone 38 while Santra's group prevented the ion migration at the interfaces of CsPbBr 3 –CsPbI 3 using an ultrathin alumina shell. 39 In this context, utilizing metal chalcogenides for plugging halide vacancies can serve as a potential strategy to prevent the migration of halide ions due to the highly covalent nature of the Pb–chalcogen bond that renders it stable. Recently, a few studies discussed about the passivation of surface defects of halide perovskites via NHSs formation with PbE (E = S, Se).…”

“…46 Nonetheless, it is a well-known fact that in pristine halide perovskites, halide vacancies are further created under light exposure and other environmental stresses, facilitated by the detachment of surface halide in the form of oleylammonium halide. 39 Since halide vacancies facilitate anion migration, effective sealing of these vacancies and preventing their further formation during device operation would significantly impede halide ion migration. To ensure effective passivation of halide vacancies in SP/MS-CPB, we subjected the NCs to multiple washings using the antisolvent ethyl acetate.…”

“…For long term PL studies, a thin film of poly methylmethacrylate (PMMA) was spin coated on top of the heterostructure to protect the CPI from moisture, as it is prone to degradation under ambient conditions. 36,39 In a similar manner, we fabricated the MS-CPB/MS-CPI NHSs, in which smaller PbSe islands are present on the perovskite surface. The PXRD patterns and their cross-sectional field-emission scanning electron microscopy (FESEM) are given in Fig.…”