Elucidation of the Formation Mechanism of Highly Oriented Multiphase Ruddlesden–Popper Perovskite Solar Cells

Abstract

The crystallographic orientation and phase distribution of two-dimensional Ruddlesden−Popper perovskites (2D-RPPs) should be carefully controlled to obtain high-performance 2D-RPP-based optoelectronic devices. However, these characteristics are still unclear. Herein, we systematically examine the formation mechanism of highly oriented multiphase 2D-RPPs. We argue that the 3D-like perovskites containing small organic cations nucleate first with out-of-plane (111) preferential orientation, followed by the further growth of twodimensional perovskites incorporating bulky organic cations owing to the difference in the solubility between small and bulky cations. This spatial segregation of organic cations across the film depth induces the formation of multiple perovskite phases, which produces n-value-graded 2D-RPP films with continually upshifted band energy alignment. Highly oriented multiphase 2D-RPP films with isobutylammonium (isoBA 2 (Cs 0.02 MA 0.64 FA 0.34 ) 4 Pb 5 I 16 ) were successfully employed as a photoabsorbers for perovskite solar cells (PSCs), exhibiting remarkable efficiency of over 16% and significantly enhanced environmental stability compared with their three-dimensional counterparts.O rganic−inorganic hybrid perovskite materials have shown useful optoelectronic properties for application in various devices, including photodetectors, light-emitting diodes, and solar cells. 1−8 Despite their tremendous potential, the intrinsic instability of organic− inorganic hybrid perovskites against moisture, heat, and light limits their commercialization. 9,10 Recently, two-dimensional Ruddlesden−Popper perovskites (2D-RPPs) have been recognized as a new class of materials that enable high performance and long-term stability. Furthermore, 2D-RPPs have a more widely tunable optoelectronic properties, which originate from their structural versatility and quantum confinement effect, and thus, offer a broader application range than their three-dimensional (3D) counterparts. 11−14 The crystal structure of 2D-RPPs is derived from typical 3D perovskite materials with an ABX 3 composition, where A is an univalent organic cation and B is a divalent metal cation, which are octahedrally coordinated with halide ions X. With the introduction of bulky alkylammonium spacer cations, the chemical composition of 2D-RPPs is expressed as A′ 2 A n−1 B n X 3n+1 (n = 1, 2, 3, ..., ∞), where n is the number of inorganic octahedra layers, which are sandwiched between spacer cations A′ to form unit building blocks. 15 The building

Charge‐Carrier Transport in Quasi‐2D Ruddlesden–Popper Perovskite Solar Cells

“…[127] Recently, Jang et al investigated the mechanism of oriented multiphase 2DRP perovskite films with iso-BA 2 (Cs 0.02 MA 0.64 FA 0.34 ) 4 Pb 5 I 16 by delaying the crystallization process. [128] Based on the LaMer principle, in a cold antisolvent bath, the solubility difference between small and bulky organic cations leads to continuous crystallization, resulting in high-n perovskite phases evolving at the bottom of the 2DRP perovskite films, while low-n phases evolved at the top surface with the participation of large spacer organic cations, which produced a continuous upward shifting band structure, thus enhancing charge separation and transport. Consequently, the cold antisolvent method provides an effective way to achieve an orderly phase distribution in 2D perovskite films containing multiple spacer cations and other mixed A-site cation components.…”

Multiple‐Ring Aromatic Spacer Cation Tailored Interlayer Interaction for Efficient and Air‐Stable Ruddlesden–Popper Perovskite Solar Cells

“…[29] These low-n-value phases that existed in the bottom of the layered 2DRP perovskite significantly hindered the growth of the 2DRP main phases, suggesting the cornersharing inorganic sheets grow along both parallel and vertical directions. [30] This kind of multiphase structure with randomly oriented crystals could lead to poor charge carrier transport.…”

Investigation of the S-Shaped Current–Voltage Curve in High Open-Circuit Voltage Ruddlesden–Popper Perovskite Solar Cells

“…The steady-state photoluminescence (PL) spectroscopy measurements were collected with an excitation wavelength of 450 nm from both the front-(perovskite film side) and back-side (substrate side) illumination to reveal the spatial phase composition of the as-fabricated 2D-RPP films based on different HTLs (NiO x and PEDOT:PSS). With an excitation wavelength of 450 nm, the penetration depth into layered perovskite films can be estimated to be ∼150 nm based on the absorption spectra, which is smaller than the film thickness of ∼350 nm, indicating that the fluorescence signals collected are mainly from the same side of films as the photoexcitation (Jang et al, 2020).…”