device stability impedes its commercialization, mainly stemming from the chemical decomposition of regular 3D perovskites in damp environment. [3,4] Encapsulation techniques can slow down the degradation process, but the essential approach to tackle this issue is to find stable perovskite materials capable of achieving long-term stability. [5] In contrast to traditional 3D counterparts, quasi-2D layered perovskites have shown enhanced stability owing to the bulkier and hydrophobic organic molecule in the structure, and have been applied both in photovoltaics and light-emitting diodes. [6][7][8][9] Quasi-2D perovskites take the generic structural formula of L 2 S n−1 M n X 3n+1 , where n is an integer, M is a divalent metal, X is a halide anion, and L and S are organic cations with large and small sizes, respectively. [10] Layered structures are usually formed by inserting the large-sized organic cation spacers into the inorganic sheets of corner-sharing [MX 6 ] octahedra. These quasi-2D compounds can be regarded as natural formed quantum-well (QW) structures, in which the semiconducting inorganic sheets act as the wells and the organic dielectric layers correspond to the Quasi-2D layered organometal halide perovskites have recently emerged as promising candidates for solar cells, because of their intrinsic stability compared to 3D analogs. However, relatively low power conversion efficiency (PCE) limits the application of 2D layered perovskites in photovoltaics, due to large energy band gap, high exciton binding energy, and poor interlayer charge transport. Here, efficient and water-stable quasi-2D perovskite solar cells with a peak PCE of 18.20% by using 3-bromobenzylammonium iodide are demonstrated. The unencapsulated devices sustain over 82% of their initial efficiency after 2400 h under relative humidity of ≈40%, and show almost unchanged photovoltaic parameters after immersion into water for 60 s. The robust performance of perovskite solar cells results from the quasi-2D perovskite films with hydrophobic nature and a high degree of electronic order and high crystallinity, which consists of both ordered large-bandgap perovskites with the vertical growth in the bottom region and oriented smallbandgap components in the top region. Moreover, due to the suppressed nonradiative recombination, the unencapsulated photovoltaic devices can work well as light-emitting diodes (LEDs), exhibiting an external quantum efficiency of 3.85% and a long operational lifetime of ≈96 h at a high current density of 200 mA cm −2 in air.

repeating in three dimensions and typically comprising organic cations (A) such as methylammonium (MA) or formamidinium (FA), metallic cations (M) (usually Pb 2+ or Sn 2+ ), and halide ions (X) (I − , Br − , Cl − ) according to the formula AMX 3 . [8][9][10] These materials possess many attractive properties for photovoltaic applications including a high light-absorption coefficient, high charge-carrier mobility, compatibility with low-cost solution processing, and potential ease of high-volume fabrication on flexible substrates using conventional roll-to-roll (R2R) printing and coating technologies. [8][9][10][11][12][13] However, these so-called "3D-perovskites" suffer from a low environmental stability, caused by weak lightinduced interactions between the organic cations and surrounding halide anions, and a susceptibility to hydrolytic reactions of the organic cations on exposure to moisture. [14] This has been one of the main limiting factors in the commercialization of perovskitebased solar cells and other optoelectronic devices. [15,16] More recently, Ruddlesden-Popper layered perovskites (R 2 A n−1 M n X 3n+1 ; n = 1 → ∞), particularly the subset with n ≤ 4 comprising 2D stacks of 3D perovskites, have garnered more attention due to their interesting optoelectronic properties [17][18][19] and considerably higher stability compared to 3D perovskites. [20][21][22][23] These 2D-perovskite materials are typically prepared by utilizing larger organic cations (R) such as phenylethylammonium or butylammonium (BA) in the formulation. The longer alkyl chains or aromatic moieties of these larger cations not only tune the material optoelectronic properties, 2D organic-inorganic hybrid Ruddlesden-Popper perovskites have emerged recently as candidates for the light-absorbing layer in solar cell technology due largely to their impressive operational stability compared with their 3D-perovskite counterparts. The methods reported to date for the preparation of efficient 2D perovksite layers for solar cells involve a nonscalable spincoating step. In this work, a facile, spin-coating-free, directly scalable dropcast method is reported for depositing precursor solutions that self-assemble into highly oriented, uniform 2D-perovskite films in air, yielding perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9% (certified PCE of 14.33% ± 0.34 at 0.078 cm 2 ). This is the highest PCE to date for a solar cell with 2D-perovskite layers fabricated by nonspin-coating method. The PCEs of the cells display no evidence of degradation after storage in a nitrogen glovebox for more than 5 months. 2D-perovskite layer deposition using a slot-die process is also investigated for the first time. Perovskite solar cells fabricated using batch slot-die coating on a glass substrate or R2R slot-die coating on a flexible substrate produced PCEs of 12.5% and 8.0%, respectively.

Thermally induced molecular graft copolymerization of the zwitterionic monomer, N,N′-dimethyl-(methylmethacryloyl ethyl)ammonium propanesulfonate (DMAPS), with the ozone-preactivated poly-(vinylene fluoride) (PVDF) was carried out in a mixed solvent of N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO). The chemical composition of the resulting PVDF with grafted DMAPS polymer (PDMAPS) side chains, or the PDMAPS-g-PVDF copolymers, was analyzed by elemental analysis. An increase in the [DMAPS]/[-CH2CF2-] molar feed ratio used for graft polymerization gave rise to an increase in the graft concentration of the DMAPS polymer in the copolymer. Microfiltration (MF) membranes were prepared from the DMSO solutions of the copolymers by phase inversion in aqueous media of different ionic strength and temperature. The surface composition and morphology of the MF membranes were investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. The mean pore size of the membrane decreased with the increase in graft concentration of the PDMAPS-g-PVDF copolymer. However, it increased with an increase in the ionic strength of the casting bath. Due to the anti-polyelectrolyte effect, the permeability of aqueous solutions through the PDMAPS-g-PVDF MF membranes exhibited a dependence on electrolyte concentration. The flow rate was observed to decrease as the electrolyte concentration of the permeate was increased.

3D/2D hybrid perovskite systems have been intensively investigated to improve the stability of perovskite solar cells (PSCs), whereas undesired crystallization of 2D perovskite during the film formation process could undermine the structural stability of 2D perovskite materials, which causes serious hysteresis of PSCs after aging. This issue is, however, rarely studied. The stability study for 3D/2D hybrid systems to date is all under the one-direction scan, and the lack of detailed information on the hysteresis after aging compromises the credibility of the stability results. In this work, by correlating the hysteresis of the hybrid PSCs with the 2D crystal structure, we find that the prompt 2D perovskite formation process easily induces numerous crystal imperfections and structural defects. These defects are susceptible to humidity attack and decompose the 2D perovskite to insulating long-chain cations and 3D perovskite, which hinder charge transfer or generate charge accumulation. Therefore, a large hysteresis is exhibited after aging the 3D/2D hybrid PSCs in an ambient environment, even though the reverse-scan power conversion efficiency (PCE) is found to be well-preserved. To address this issue, alkali cations, K+ and Rb+, are introduced into the 2D perovskite to exquisitely modulate the crystal formation, which gives rise to a higher crystallinity of 2D perovskite and a better film morphology with fewer defects. We achieved PCE beyond 21% due to the preferable charge transfer process and reduced nonradiative recombination losses. The structural features also bring about impressive moisture stability, which results in the corresponding PSCs retaining 93% of its initial PCE and negligible hysteresis after aging in an ambient atmosphere for 1200 h.

Introducing layered quasi-2D perovskite phases into a conventional 3D perovskite lightabsorbing matrix is a promising strategy for overcoming the limited environmental stability of 3D perovskite solar cells. Here, we present a simple drop-casting method for preparing hybrid perovskite films comprising both quasi-2D and quasi-3D phases, formed using phenylethylammonium or iso-butylammonium as spacer cations. The film morphology, phase purity, and crystal orientation of the hybrid quasi-2D/3D perovskite films are improved significantly by applying a simple N 2 blow-drying step, together with inclusion of methylammonium chloride as an additive. An enhanced power conversion efficiency of 16.0% is achieved using an isobutylammonium-based quasi-2D/3D perovskite layer which, to our knowledge, is the highest recorded to date for a quasi-2D/3D perovskite solar cells containing a non-spin-cast perovskite layer prepared under ambient laboratory conditions.