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A new type of methylammonium‐free formamidinium (FA) based perovskites is reported. The low‐dimensional perovskite films are obtained in the presence of the FACl additive, and the role of Cl is investigated through grazing‐incidence X‐ray diffraction. Solar cell devices based on (PDA)(FA)₃Pb₄I₁₃ films show extremely high thermal stability and a remarkable PCE of 13.8 %.

Abstract

Currently, most two‐dimensional (2D) metal halide perovskites are of the Ruddlesden–Popper type and contain the thermally unstable methylammonium (MA) molecules, which leads to inferior photovoltaic performance and mild stability. Here we report a new type of MA‐free formamidinium (FA) based low‐dimensional perovskites, featuring a general formula of (PDA)(FA) _n−1Pb_nI_3n+1 with propane‐1,3‐diammonium (PDA) as the organic spacer cation. The perovskite films with well‐oriented crystal grains are attained under the assistance of the FACl additive, where the role of Cl is investigated through the grazing‐incidence X‐ray diffraction technique. The photovoltaic device based on the optimized (PDA)(FA)₃Pb₄I₁₃ film demonstrates a remarkable power conversion efficiency of 13.8 %, the highest record for the FA‐based 2D perovskite solar cells. In addition, compared to (PDA)(MA)₃Pb₄I₁₃, the MA‐containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)₃Pb₄I₁₃ films and their derived devices exhibit exceedingly higher thermal stability.

Herein, the origin of figures of merit for open‐circuit voltage, short‐circuit current density, and fill factor is discussed. Three design strategies of interface engineering, bandgap engineering, and process‐control engineering are proposed. The process‐control engineering is introduced, including fabrication atmosphere, synthesis routes, architecture optimization, physical deposition condition, and chemical process with multiple degrees of freedom.

Organic/inorganic lead‐halide perovskite and its solar cells (SCs) present a new research platform for the study of special photophysical and photovoltaic (PV) characteristics across materials science, chemistry, physics, and engineering disciplines. However, the current understanding of the crystal structures, origins of figures of merit, and design strategies of SCs is inadequate. These key parameters are critical for exploring further applications of organometallic‐halide perovskite films and their SCs. Therefore, herein, the material characteristics of lead‐halide perovskite are introduced, the origins of open‐circuit voltages, short‐circuit current densities, and fill factors are explored, and three design strategies using interface engineering, bandgap engineering, and process‐control engineering for high‐quality perovskite active‐layer fabrication, outstanding efficiency, and stable SCs are summarized. Herein, process‐control engineering is introduced for the first time in perovskite SCs. Based on favorable synergistic effects, these structural features, origins of crucial parameters, and design strategies all promote the development of new schemes to explore the underlying physics, optimize functional layers and cell architectures, and improve final PV performance and device stability.

In this review, the crystallization kinetics and their effects on the performance of various types of 2D perovskite solar cells (PSCs) up to now are discussed. The crystal/natural quantum well structures and original stability for 2D perovskite are also clearly summarized. Finally, remaining challenges are discussed and possible solutions are proposed in terms of development bottlenecks for 2D PSCs.

Abstract

2D perovskites demonstrate higher moisture stability, oxygen content, thermal stability, and a significantly lower ion migration/phase transition occurrence in comparison to 3D perovskite. These advantages imply huge potential for 2D perovskite in commercial applications in the photovoltaic field. However, the horizontal arrangement of the organic layer severely hinders the transport of carriers, and thus, the power conversion efficiency of 2D perovskite solar cells (PSCs) is significantly lower than that of 3D. Controlling the crystallization orientation to achieve rapid carrier transport can effectively avoid or reduce such adverse effects. Hence, an in‐depth understanding of the formation mechanism and crystallization kinetics of 2D perovskite films is crucial to the development of high‐performing 2D PSCs. This review explores the studies conducted on crystallization kinetics, which is the key issue for 2D perovskite, and discusses their effects on the performance of various types of 2D PSCs to date. The crystal/natural quantum well structures and origin of the stability for 2D perovskite are also summarized. Finally, the remaining challenges in terms of development bottlenecks for 2D PSCs are discussed, alongside the proposal of possible solutions.

Thermal annealing of 2D/3D perovskite heterostructures leads to beneficial diffusion passivation; however, it also causes lattice expansion of the 2D perovskite. Here a novel preparation strategy, simultaneously inhibiting lattice expansion, compensating the large tensile stress of 2D perovskite, and inducing diffusion passivation, is introduced. As a result, a certified efficiency of 20.22% is obtained.

Abstract

Lattice matching and passivation are generally seen as the main beneficial effects in 2D/3D perovskite heterostructured solar cells, but the understanding of the mechanisms involved is still incomplete. In this work, it is shown that 2D/3D heterostructure are unstable under common thermal processing conditions, caused by the lattice expansion of strained 2D perovskite. Therefore an innovative fabrication technology involving a compressively strained PEA₂PbI₄ layer is proposed to compensate the internal tensile strain and stabilize the 2D/3D heterostructure. Moreover, a small amount of PEA⁺ diffusing into the grain boundaries of 3D perovskite forms 2D perovskite and passivates the defects there. Combining the effects of strain compensation and diffusion passivation, the stabilized 2D/3D perovskite solar cells deliver a reproducible and robust laboratory power conversion efficiency (PCE) of 21.31% for the p‐i‐n type devices, along with a high V _OC of 1.18 V. A certified PCE of 20.22% is confirmed by an independent national metrology institute.

In article number https://doi.org/10.1002/aenm.2020015672001567, Yu‐Ching Huang, Wei‐Fang Su, and co‐workers demonstrate a facile process for the mass‐production of perovskite solar cells. The process integrates slot‐die coating and near‐infrared irradiation heating techniques, combined with a new perovskite precursor formula to rapidly produce large‐area perovskite solar cells and modules in air.

Polymer additives in perovskite solar cells are found to act as barriers at the perovskite grain boundaries and hinder the ion migration, improving the device stability under both light irradiation and electrical stressing. The polymer incorporated perovskite solar cells have significantly increased electrical‐field tolerance with an increase in breakdown voltage from −0.4 to −2 V.

Abstract

Passivation of organometal halide perovskites with polar molecules has been recently demonstrated to improve the photovoltaic device efficiency and stability. However, the mechanism is still elusive. Here, it is found that both polymers with large and small dipole moment of 3.7 D and 0.6 D have negligible defect passivation effect on the MAPbI₃ perovskite films as evidenced by photothermal deflection spectroscopy. The photovoltaic devices with and without the polymer additives also have comparable power conversion efficiencies around 19%. However, devices with the additives have noticeable improvement in stability under continuous light irradiation. It is found that although the initial mobile ion concentrations are comparable in both devices with and without the additives, the additives can strongly suppress the ion migration during the device operation. This contributes to the significantly enhanced electrical‐field stress tolerance of the perovskite solar cells (PVSCs). The PVSCs with polymer additives can operate up to −2 V reverse voltage bias which is much larger than the breakdown voltage of −0.5 V that has been commonly observed. This study provides insight into the role of additives in perovskites and the corresponding device degradation mechanism.

The novel SiO₂/ZrO₂ coating of the CsPbBr₃ emitting core leads to highly emissive and stable hybrid perovskite nanoparticles of high interest for color downconverting purposes in light‐emitting diodes. Indeed, these devices feature a high luminous efficiency (≈75 Lm W⁻¹) and outstanding stabilities of about 200 and 700 h operating at 100 and 10 mA under ambient conditions, respectively.

Abstract

Significant advances are realized in perovskite‐converted hybrid light‐emitting diodes (pc‐HLEDs). However, long‐living devices at high efficiencies still represent a major milestone with average stabilities of <200 h at ≈50 lm W⁻¹ under low applied currents (<15 mA). Herein, a dual metal oxide‐coated CsPbBr₃@SiO₂/ZrO₂ composite is prepared in a one‐pot synthesis through the kinetic control of the sol–gel reaction, followed by a gentle drying process in air. These hybrid nanoparticles show photoluminescence quantum yields of ≈65% that are stable under temperature, ambient, and irradiation stress scenarios. This is translated to pc‐HLEDs with a near‐unity conversion efficiency at any applied current, high efficiencies around 75 lm W⁻¹, and one of the most remarkable stabilities of ≈200 and 700 h at 100 and 10 mA, respectively. In addition, the device degradation mechanism is thoughtfully rationalized comparing devices operating under ambient/inert conditions. As such, this work provides three milestones: i) a new room temperature one‐pot protocol to realize the first SiO₂/ZrO₂ metal oxide coating that effectively protects the emitting perovskite nanoparticle core, ii) one of the most stable and efficient pc‐HLEDs operating under ambient condition at any applied current, and iii) new insights for the degradation of pc‐HLEDs.

Aiming at stable and efficient perovskite light‐emitting diodes (PeLEDs), this work proposes an all‐inorganic strategy involving an insulator–perovskite–insulator device structure and cascade ZnS‐ZnSe electron transport layers, which improve charge‐injection efficiency and suppress the electric‐field‐induced ion migration channels. The findings provide an addressable approach access to future commercialization of PeLEDs.

Abstract

Stability issue is one of the major concerns that limit emergent perovskite light‐emitting diodes (PeLEDs) techniques. Generally, ion migration is considered as the most important origin of PeLEDs degradation. In this work, an all‐inorganic device architecture, LiF/perovskite/LiF/ZnS/ZnSe, is proposed to address this imperative problem. The inorganic (Cs_{1− x} Rb _x )_{1− y} K _y PbBr₃ perovskite is optimized with achieving a photoluminescence quantum yield of 67%. Depth profile analysis of X‐ray photoelectron spectroscopy indicates that the LiF/perovskite/LiF structure and the ZnS/ZnSe cascade electron transport layers significantly suppress the electric‐field‐induced ion migrations of the perovskite layers, and impede the diffusion of metallic atoms from cathode into perovskites. The as‐prepared PeLEDs display excellent shelf stability (maintaining 90% of the initial external quantum efficiency [EQE] after 264 h) and operational stability (half‐lifetime of about 255 h at an initial luminance of 120 cd m⁻²). The devices also exhibit a maximum brightness of 15 6155 cd m⁻² and an EQE of 11.05%.