The intense research activities on the hybrid organic–inorganic perovskites (HOIPs) have led to the greatly improved light absorbers for solar cells with high power conversion efficiency (PCE). However, it is still challenging to find an alternative lead-free perovskite to replace the organohalide lead perovskites to achieve high PCE. This is because both previous experimental and theoretical investigations have shown that the Pb 2+ cations play a dominating role in contributing the desirable frontier electronic bands of the HOIPs for light absorbing. Recent advances in the chemical synthesis of three-dimensional (3D) metal-free perovskites, by replacing Pb 2+ with NH 4 + , have markedly enriched the family of multifunctionalized perovskites ( 30002249 Science 2018 361 151 155 ). These metal-free perovskites possess the chemical formula of A(NH 4 )X 3 , where A is divalent organic cations and X denotes halogen atoms. Without involving transition-metal cations, the metal-free A(NH 4 )X 3 perovskites can entail notably different frontier electronic band features from those of the organohalide lead perovskites. Indeed, the valence and conduction bands of A(NH 4 )X 3 perovskites are mainly attributed by the halogen atoms and the divalent A 2+ organic cations, respectively. Importantly, a linear relationship between the bandgaps of A(NH 4 )X 3 perovskites and the lowest unoccupied molecular orbital energies of the A 2+ cations is identified, suggesting that bandgaps can be tailored via molecular design, especially through a chemical modification of the A 2+ cations. Our comprehensive computational study and molecular design predict a metal-free perovskite, namely, 6-ammonio-1-methyl-5-nitropyrimidin-1-ium-(NH 4 )I 3 , with a desirable bandgap of ∼1.74 eV and good optical absorption property, both being important requirements for photovoltaic applications. Moreover, the application of strain can further fine-tune the bandgap of this metal-free perovskite. Our proposed design principle not only offers chemical insights into the structure–property relationship of the multifunctional metal-free perovskites but also can facilitate the discovery of highly efficient alternative, lead-free perovskites for potential photovoltaic or optoelectronic applications.

Two‐dimensional (2D) organic–inorganic hybrid perovskite (OIHP) ferroelectrics have attracted widespread interest in the field of optoelectronics due to the combination of excellent semiconducting and ferroelectric properties. The Curie temperature (TC), below which ferroelectricity exists, is a crucial parameter for ferroelectrics. However, the lack of research on TC tuning of 2D OIHP ferroelectrics hinders their further progress. Here, through incorporating ethylammonium (EA) as cage‐confined rotators, we obtained two 2D OIHP ferroelectrics, (IBA)2(EA)Pb2Br7 (2L; IBA=isobutylammonium), and (IBA)2(EA)2Pb3Br10 (3L). Intriguingly, TC is successfully tuned from 326 K (2L) to 370 K (3L) with increasing layer thickness. Structural and computational analyses suggest that the improvement of TC is due to the higher phase‐transition energy barrier triggered by the cage‐confined EA rotators with increased layer thickness. This work suggests that EA is an effective “cage‐confined rotator” to rationally design high‐TC 2D OIHP ferroelectrics.

Confined water molecules have attracted widespread research interest due to its versatile phase behaviors. Ammonia (NH3, isoelectronic with water) molecules are also expected to realize the delicate self-assembled hydrogen-bonded network...

Circular polarized light-dependent anomalous bulk photovoltaic effect - a steady anomalous photovoltaic current can be manipulated by changing the light helicity, is an increasingly interesting topic in contexts ranging from physics to chemistry. Herein, circular polarized light-dependent anomalous bulk photovoltaic effect is presented in achiral hybrid perovskites, (4-AMP)BiI5 (ABI, 4-AMP is 4-(aminomethyl)piperidinium), breaking conventional realization that it can only happen in chiral species. Achiral hybrid perovskite ABI crystallizes in chiroptical-active asymmetric point group m (Cs), showing an anomalous bulk photovoltaic effect with giant photovoltage of 25 V, as well as strong circular polarized light - sensitive properties. Significantly, conspicuous circular polarized light-dependent anomalous bulk photovoltaic effect is reflected in the large degree of dependence of anomalous bulk photovoltaic effect on left-and right-CPL helicity, which is associated with left and right-handed screw optical axes of ABI. Such degree of dependence is demonstrated by a large asymmetry factor of 0.24, which almost falls around the highest value of hybrid perovskites. These unprecedented results may provide a perspective to develop opto-spintronic functionalities in hybrid perovskites.

Metal nanoclusters can be anchored at defective sites of graphene sheets to strengthen their thermal stability for potential device applications. A previous transmission electron microscopy (TEM) experimental study on the morphology change of an ultrafine iron cluster embedded in a graphene nanocavity suggests that the underlying reaction mechanism is likely due to solid−solid transformation [Sci. Rep. 2012, 2, 995]. The morphology change of the Fe cluster may also assist the enlargement of the graphene nanocavity. This TEM experiment reminds us that if the anchoring Fe nanocluster within the graphene nanocavity can efficiently catalyze graphene etching at a certain operation temperature, the device application of graphene−metal nanocluster composites would be largely limited. Herein, we have performed ab initio molecular dynamics (AIMD) simulations of a triangular hexagonal close-packed (HCP) Fe 53 cluster in contact with either the edge of the graphene nanocavity or graphene nanoribbon to investigate its structural evolution and catalytic behavior at an elevated temperature (1173 K). Contrary to the previous TEM experiment, we suggest an alternative reaction mechanism, namely, the melting recrystallization for the structural transformation of Fe cluster. Moreover, we find that the molten iron cluster can etch and enlarge the graphene nanocavity. At the high temperature of 1173 K without H and O atoms, the Fe 53 cluster undergoes a phase transition from the HCP structure to a liquid-like nanodroplet while in contact with the edge of either graphene nanocavity or graphene nanoribbon. Interestingly, the Fe 53 cluster tends to saturate the graphene edges via forming Fe−C bonds but without breaking any C−C bonds within the time scale of AIMD simulations. Our reactive MD simulations show that the HCP Fe 53 cluster can complete with the reaction of carbide formation within 10 ps. Independent climbing-image nudged elastic band calculations offer additional insight into the Fe-catalyzed reaction mechanism