High Phase Stability in CsPbI₃ Enabled by Pb–I Octahedra Anchors for Efficient Inorganic Perovskite Photovoltaics

Abstract

CsPbI3 inorganic perovskite has exhibited some special properties particularly crystal structure distortion and quantum confinement effect, yet the poor phase stability of CsPbI3 severely hinders its applications. Herein, the nature of the photoactive CsPbI3 phase transition from the perspective of PbI6 octahedra is revealed. A facile method is also developed to stabilize the photoactive phase and to reduce the defect density of CsPbI3. CsPbI3 is decorated with multifunctional 4‐aminobenzoic acid (ABA), and steric neostigmine bromide (NGBr) is subsequently used to further mediate the thin films' surface (NGBr‐CsPbI3(ABA)). The ABA or NG cation adsorbed onto the grain boundaries/surface of CsPbI3 anchors the PbI6 octahedra via increasing the energy barriers of octahedral rotation, which maintains the continuous array of corner‐sharing PbI6 octahedra and kinetically stabilizes the photoactive phase CsPbI3. Moreover, the added ABA and NGBr not only interact with shallow‐ or deep‐level defects in CsPbI3 to significantly reduce defect density, but also lead to improved energy‐level alignment at the interfaces between the CsPbI3 and the charge transport layers. Finally, the champion NGBr‐CsPbI3(ABA)‐based inorganic perovskite solar cell delivers 18.27% efficiency with excellent stability. Overall, this work demonstrates a promising concept to achieve highly phase‐stabilized inorganic perovskite with suppressed defect density for promoting its optoelectronic applications.

Structural Properties and Stability of Inorganic CsPbI3 Perovskites

“…Wheeler et al have shown that the incorporation of oleylammonium (OA + ) processing stabilizes the CsPbI 3 perovskite in the cubic α‐phase whereas the use of PEA + additives stabilizes the CsPbI 3 perovskite in the tetragonal β‐phase where the octahedra are slightly tilted. [ 96 ] The transformation related to the [PbI 6 ] 4− rotation was also shown in the study of Wang et al [ 92 ] Through a surface capping with 4‐aminobenzoic acid, the designated angles between PbI 6 octahedra are 111.20° and 73.93°, being closer to ideal 90° for cubic phase, respectively. However, the corresponding angles on Cs‐terminated surface are 113.83° and 65.15°, respectively.…”

“…Reproduced with permission. [ 92 ] Copyright 2020, Wiley‐VCH. b) Mechanism of the OTG‐induced phase stabilization of CsPbI 3 perovskite.…”

“…However, the corresponding angles on Cs‐terminated surface are 113.83° and 65.15°, respectively. [ 92 ] Polymers with amide groups have also been applied to stabilize the CsPbI 3 perovskite. Li et al have incorporated polyvinylpyrrolidone (PVP) into the solution processing of CsPbI 3 perovskites.…”

“…Wang et al reported a bulky organic ligand serving as organic terminal groups in which cyanoacetic acid were adopted as an anchor for bonding with the Pb–I framework of perovskite lattice. [ 92 ] It is claimed that the large 2,4‐dibutoxy phenyl unit applied in the terminal groups could efficiently hinder the octahedral tilting of the corner‐sharing [PbI 6 ] 4− through a strong sterichindrance effect. Such effect increases the formation energy of orthorhombic nonperovskite δ‐CsPbI 3 with an edge‐sharing [PbI 6 ] 4− .…”

Advances in Phase Stability of Cesium Lead Halide Perovskites

“…[ 87,88 ] Recently, Wang et al combined 4‐aminobenzoic acid incorporation with neostigmine bromide surface modification in CsPbI 3 film fabrication, which stabilized the black phase by suppressing PbI 6 octahedral rotation and reducing surface energy. [ 89 ] The modified film kept unchanged in 80% RH air for 2 h or at 80 °C in N 2 for 360 h, in stark contrast to the control CsPbI 3 film which completely turned yellow under the same aging conditions. The champion device delivered a high PCE of 18.27% and the encapsulated cells showed no obvious decay in efficiencies after 200 h storage in a 60% RH under dark conditions.…”