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

Flexible perovskite solar cells (PSCs) have attracted significant interest as promising candidates for portable and wearable devices. Copper nanowires (CuNWs) are promising candidates for transparent conductive electrodes for flexible PSCs because of their excellent conductivity, flexibility, and cost-effectiveness. However, because of the thermal/chemical instability of CuNWs, they require a protective layer for application in PSCs. Previous PSCs with CuNW-based electrodes generally exhibited poor performances compared with their indium tin oxide-based counterparts because of the neglect of the interfacial energetics between the electron transport layer (ETL) and CuNWs. Herein, an Al-doped ZnO (AZO) protective layer fabricated using atomic layer deposition is introduced. The AZO/CuNW-based composite electrode exhibits improved thermal/chemical stability and favorable band alignment between the ETL and CuNWs, based on the Al dopant concentration tuning. As a result, the Al content gradient AZO (g-AZO), composed of three successively deposited AZO layers, leads to highly efficient flexible PSCs with a power conversion efficiency (PCE) of 14.18%, whereas the PCE of PSCs with a non-g-AZO layer is 12.34%. This improvement can be attributed to the efficient electron extraction and reduced charge recombination. Furthermore, flexible PSCs based on g-AZO-based composite electrodes retain their initial PCE, even after 600 bending cycles, demonstrating excellent mechanical stability.

To resolve the inherent trade-off issue between responsivity and detectivity in FA0.9Cs0.1PbI3 perovskite photodetectors, this paper proposes a novel strategy using multifunctional self-combustion additives (urea and ammonium nitrate). During the early stages of crystallization, urea allows for the formation of a strong Lewis complex-derived low-dimensional intermediate phase; this suppresses the formation of perovskite nuclei, while ammonium ions assist the preferred grain growth along the [110] direction. During the high-temperature annealing steps, a self-combusting exothermic reaction occurs between urea as a fuel and NH4NO3 as an oxidizer, through which a locally supplied heat facilitates the removal of residual urea and byproducts. These multifunctional roles of self-combustible additives facilitate the production of high-quality, enlarged grain-structured perovskite films with improved optoelectronic properties, as confirmed by various analyses, including impedance spectroscopy and intensity-modulated photocurrent spectroscopy. The resulting FA0.9Cs0.1PbI3-based photodiode-type photodetectors exhibit outstanding performance, such as a high responsivity (0.762 A W–1) and specific detectivity (over 5.08 × 1013 Jones) at a very low external reverse bias (−0.5 V). Our findings clearly suggest that the multifunctional self-combustion additives strategy could help realize the full potential of FA1–x Cs x PbI3 as a photodiode-type photodetector.

High-performance perovskite solar cells (PSCs) are readily degradable by moisture, leading to high demand for a water-repelling efficient hole transport layer (HTL). In this study, we proposed an anionexchange approach to replace the conventional hygroscopic dopant anion bis(trifluoromethanesulfonyl)imide (TFSI − )with a hydrophobic dopant anion capable of effectively doping into a 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) matrix. By varying the size of dopant anions, we successfully controlled electrostatic interactions between spiro-OMeTAD and the dopant anion. Hexafluorophosphate (PF 6 − ) demonstrated the highest p-doping anion-exchange capability because the optimal-sized PF 6− enabled a strong electrostatic interaction between spiro-OMeTAD •+ and PF 6 − while resulting in poor affinity between Li + and PF 6 − . The resulting PF 6 − -doped spiro-OMeTAD HTL not only produced favorable energy band alignment with perovskite but also improved film conductivity. Correspondingly, the PSCs based on the PF 6 − -doped HTL exhibited a higher power conversion efficiency (PCE) of 20.78% than the reference TFSI − -based PSCs of 19.04%. Besides device performance, the superior hydrophobic nature of PF 6 − enabled the HTL to prevent water penetration into the perovskite layer, improving long-term stability against moisture. The PF 6 − -based PSCs exhibited enhanced humidity stability while maintaining 92% of the initial PCE for 1180 h at a relative humidity of 25% under ambient conditions.

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