Accessing vertical orientation of two-dimensional (2D) perovskite films is key to achieving high-performance solar cells with these materials. Herein, we report on solvent-vapor annealing (SVA) as a general postdeposition strategy to induce strong vertical orientation across broad classes of 2D perovskite films. We do not observe any local compositional drifts that would result in impure phases during SVA. Instead, our experiments point to solvent vapor plasticizing 2D perovskite films and facilitating their surface-induced reorientation and concomitant grain growth, which enhance out-of-plane charge transport. Solar cells with SVA 2D perovskites exhibit superior efficiency and stability compared to their untreated analogs. With a certified efficiency of (18.00 ± 0.30) %, our SVA (BDA)­(Cs0.1FA0.9)4Pb5I16 solar cell boasts the highest efficiency among all solar cells with 2D perovskites (n ≤ 5) reported so far.

at deep sub-wavelengths, [2] leading to a variety of applications including nanolasers, [3] sensors, [4] bioimaging, [5] surfaceenhanced Raman spectroscopy, [6,7] color displays, [8] and others. [9][10][11][12] To improve the performance of today's photonic systems, researchers have extensively investigated the fundamental relation between the wave vector and energy of an SPP wave-named dispersion relation. This quantity describes the propagation of light within a material (i.e., medium), extremely relevant for the abovementioned optical devices. Therefore, understanding the dispersion relation can allow the design of optical materials with superior response, ranging from 2D van der Waals to oxides and metals. Concerning 2D materials, it can uncover the origin of tunable polaritons in hyperbolic metamaterials based on graphene and hexagonal boron nitride (h-BN), which is due to the hybridization of SPP and surface phonon polaritons. [13] As another example, by utilizing the band-edge mode of the dispersion relation in metallic nanocavities, [3] lasing with a 200 times enhancement of the spontaneous emission rate of the dye has been reached. [14] As a class of emerging photonic materials, noble metal alloys with permittivity and localized surface plasmon resonances not achievable by pure metals [9,15,16] have been proposed as alternative candidates for plasmonics [12,[17][18][19][20] because of their tunable dielectric functions, which make it possible to engineer the alloy composition to attain optical properties that will meet desired resonances. In turn, this tunability could be used to Surface plasmon polaritons (SPPs) enable the deep subwavelength confinement of an electromagnetic field, which can be used in optical devices ranging from sensors to nanoscale lasers. However, the limited number of metals that satisfy the required boundary conditions for SPP propagation in a metal/dielectric interface severely limits its occurrence in the visible range of the electromagnetic spectrum. We introduce the strategy of engineering the band structure of metallic materials by alloying. We experimentally and theoretically establish the control of the dispersion relation in Ag-Au alloys by varying the film chemical composition. Through X-ray photoelectron spectroscopy (XPS) measurements and partial density-of-states calculations we deconvolute the d band contribution of the density-of-states from the valence band spectrum, showing that the shift in energy of the d band follows the surface plasmon resonance change of the alloy. Our density functional theory calculations of the alloys band structure predict the same variation of the threshold of the interband transition, which is in very good agreement with our optical and XPS experiments. By elucidating the correlation between the optical behavior and band structure of alloys, we anticipate the fine control of the optical properties of metallic materials beyond pure metals. Band Structure EngineeringThe ORCID identification number(s) for the author(s) of this article can b...

Marina S. Leite and co‐workers (article number https://doi.org/10.1002/adom.201800218) demonstrate the alloying of Ag and Au for band structure engineering. The tunability of the optical dispersion relation is measured by reflectivity and valence band spectra. The band structure calculations corroborate the experiments, while elucidating the energy shift of interband transitions. The image represents the optical measurements in the Ag–Au system.

Thermal evaporation is a promising technique for the fabrication of uniform perovskite films over large areas that forgo the use of hazardous solvents. However, evaporation equipment, particularly at the laboratory scale, is often shared between different materials systems and it is, thus, important to understand the potential impact that halide perovskite evaporation can have on other films and devices processed in the same chamber. Here, we observe that evaporation of perovskite precursors such as PbI2 and CsI results in significant iodine contamination that is not efficiently removed by conventional decontamination procedures such as solvent cleaning, chamber bakeout, and foil replacement. X-ray photoelectron spectra show that this iodine contamination can incorporate itself into organic and metal films grown in the same chamber, which degrades the performance of thermally evaporated organic photovoltaic cells by ∼90%. To remove the contamination and restore the performance of other optoelectronic devices grown in the same chamber, a total resurfacing/replacement of all interior evaporator surfaces was required.

Perovskite CsPbI3 is a promising photovoltaic absorber material, thanks to its ideal bandgap for Si-tandem solar cell applications and its excellent thermochemical stability compared with hybrid organic–inorganic perovskites. However, CsPbI3 has its own stability challenges as its photoactive β- and γ-polymorphs are thermodynamically unstable at room temperature compared with the yellow non-perovskite δ-phase. Stabilizing CsPbI3 has, thus, been the subject of considerable research in recent years. While some approaches, such as alloying with halides and reducing crystalline domain size, have proven effective in improving phase stability, these benefits have, thus far, come at the expense of photovoltaic efficiency compared with the state-of-the-art CsPbI3 solar cells. In this perspective, we discuss the progress and limitations of inorganic perovskite stabilization techniques and look forward at how to achieve inorganic perovskite solar cells with both commercially viable efficiencies and lifetimes.