Reducing Energy Disorder in Perovskite Solar Cells by Chelation.

In inverted perovskite solar cells (PSCs), the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is a widely used electron transport material. However, a high degree of energy disorder and inadequate passivation of PCBM limit the efficiency of devices, and severe self-aggregation and unstable morphology limit the lifespan of devices. Here, we design a series of fullerene dyads FP-Cn (n = 4, 8, 12) to replace PCBM as an electron transport layer, where [60]fullerene is linked with a terpyridine chelating group via a flexible alkyl chain of different lengths as a spacer. Among three fullerene dyads, FP-C8 shows the most enhanced molecule ordering and adhesion with the perovskite surface due to the balanced decoupling between the chelation effect from terpyridine and the self-assembly of fullerene, leading to lower energy disorder and higher morphological stability relative to PCBM. The FP-C8/C60-based devices using Cs0.05FA0.90MA0.05PbI2.85Br0.15 as a light absorber show a power conversion efficiency of 21.69%, higher than that of PCBM/C60 (20.09%), benefiting from improved electron extraction and transport as well as reduced charge recombination loss. When employing FAPbI3 as a light absorber, the FP-C8/C60-based devices exhibit an efficiency of 23.08%, which is the champion value of inverted PSCs with solution-processed fullerene derivatives. Moreover, the FP-C8/C60-based devices show better moisture and thermal stability than PCBM/C60-based devices and maintain 96% of their original efficiency after 1200 h of operation, while their counterpart PCBM/C60 maintains 60% after 670 h.

A penta-silicene nanoribbon-based 3D silicon allotrope with high carrier mobility and thermoelectric performance.

Motivated by the successful synthesis of penta-silicene nanoribbons using various experimental techniques, we design a new 3D silicon allotrope, labeled cco-Si48, by assembling such nanoribbons, confirm its dynamical, thermal and mechanical stabilities, and further study its electron/phonon transport and linear optical properties based on the state-of-the-art theoretical calculations. We find that cco-Si48 is a direct bandgap semiconductor with a gap of 1.46 eV, exhibiting a high hole mobility in the magnitude of 103 cm2 V-1 s-1 and a low lattice thermal conductivity of 6.33 W m-1 K-1 at 300 K. Unlike the commonly reported n-type silicon-based materials with high thermoelectric performance, the p-type cco-Si48 outperforms its n-type counterpart in the thermoelectric figure of merit (ZT) value with a considerable value of 0.57 at 800 K. We further demonstrate that the electron-phonon interactions play a critical role in determining the optimal carrier concentrations for the peak ZT values. This work expands penta-silicene nanoribbons to their 3D assembled structure with new features and applications.

A New 3D Carbon Allotrope Composed of Penta-graphene Nanotubes with Low Lattice Thermal Conductivity.

Motivated by the unique geometries and novel properties of penta-graphene (PG) and its derivatives, we propose a new stable 3D carbon allotrope, penta-C72, which is composed of PG nanotubes by connecting adjacent tubes in a tip-to-tip manner. Using first-principles calculations, we confirm its dynamical, thermal, and mechanical stabilities and find that penta-C72 is semiconducting with an indirect bandgap of 2.12 eV. Its lattice thermal conductivities at 300 K are found to be anisotropic with values of 97.32 and 179.35 W/mK along the x and z directions, respectively, which are much lower than that of diamond (2664.93 W/mK) and carbon nanotube-based bct-C4 (1411.02 W/mK). A detailed analysis of both harmonic and anharmonic properties suggests that the soften acoustic phonon modes, the low Young's modulus, and strong anharmonicity are the key factors for the low lattice thermal conductivity. The study expands the family of carbon materials by assembling PG nanotubes.

Homogeneous Phase Distribution in Q-2D Perovskites via Co-Assembly of Spacer Cations for Efficient Light-Emitting Diodes.

Quasi-2D (Q-2D) perovskites are promising candidates to apply in light-emitting diodes (LEDs). However, delicate control on crystallization kinetics is needed to suppress severe phase segregation. Here, the crystallization kinetics of Q-2D perovskites are investigated via in situ absorbance spectroscopy and for the first time find the multiphase distribution is governed by the arrangement, rather than diffusion, of spacer cations at the nucleation stage, which associate with its assembling ability determined by molecular configuration. A "co-assembly" strategy is conceived by combining co-cations with different configuration characteristics, where bulky cations disturb the assembling between slender cations and lead-bromide sheet, contributing to homogeneous emitting phase with effective passivation. Correspondingly, in the phenylethylammonium (PEA+ )-based Q-2D perovskites ( = 3), homogeneous phase distribution is achieved by incorporating co-cation triphenylmethaneammonium (TPMA+ ), the branching terminals of which suppress cations assembling into low-n phases and afford adequate cations as passivating ligands. Therefore, the champion external quantum efficiency of the LED device reaches 23.9%, which is among the highest performance of green Q-2D perovskite LEDs. This work reveals that the arrangement of spacer cations determines the crystallization kinetics in Q-2D perovskites, providing further guidance on the molecular design and phase modulation of Q-2D perovskites.

A biphenylene nanoribbon-based 3D metallic and ductile carbon allotrope.

Assembling two-dimensional (2D) sheets for three-dimensional (3D) functional materials is of current interest. Motivated by the recent experimental synthesis of 2D biphenylene [Science372 (2021) 852], we propose a new porous 3D metallic carbon structure, named T48-carbon, by using biphenylene nanoribbons as the building block. Based on state-of-the-art theoretical calculations, we find that T48-carbon is not only dynamically, thermally, and mechanically stable, but also energetically more favorable as compared with some other theoretically predicted carbon allotropes. Especially, T48-carbon exhibits mechanical anisotropy, ductility and intrinsic metallicity. A detailed analysis of electronic properties reveals that the metallicity mainly comes from the pz-orbital of sp2-hybridized carbon atoms. This work shows the promise of design and synthesis of 3D biphenylene-based metallic carbon materials with novel properties.

Design of Three-Dimensional Metallic Biphenylene Networks for Na-Ion Battery Anodes with a Record High Capacity.

Na-ion batteries (NIBs) capture intensive research interest in large-scale energy storage applications because of sodium's abundant resources and low cost. However, the low capacity, poor conductivity, and short cycle life of the commonly used anodes are the main challenges in developing advanced NIBs. Here, stimulated by the recent successful synthesis of biphenylene [Science 2021, 372, 852], we show that these problems can be curbed by assembling armchair biphenylene nanoribbons of different widths into three-dimensional architectures, which lead to homogeneously distributed nanopores with robust structural and mechanical stability. Through density functional theory and molecular dynamics calculations combined with the tight-binding model, we find that the assembled 3D biphenylene structures are metallic and thermally stable up to 2500 K, where the metallicity is further identified to originate from the pz-orbitals (π-bonds) of the sp2 carbon atoms. Especially, the optimal assembled structures HexC28 (HexC46) deliver a gravimetric capacity of 956 (1165) mA h g-1 and a volumetric capacity of 1109 (874) mA h mL-1, which are much higher than those of graphite and hard carbon anodes. Moreover, they also show a suitable average potential, negligible volume change, and low diffusion energy barrier. These findings demonstrate that assembling biphenylene nanoribbons is a promising strategy for designing next-generation NIB anodes.

Two Birds with One Stone: Interfacial Engineering of Multifunctional Janus Separator for Lithium-Sulfur Batteries.

Li-dendrite growth and unsatisfactory sulfur cathode performance are two core problems that restrict the practical applications of lithium-sulfur batteries (LSBs). Here, an all-in-one design concept for a Janus separator, enabled by the interfacial engineering strategy, is proposed to improve the performance of LSBs. At the interface of the anode/separator, the thin functionalized composite layer contains high-elastic-modulus and high-thermal-conductivity boron nitride nanosheets and oxygen-group-grafted cellulose nanofibers (BNNs@CNFs), by which the formation of "hot spots" can be effectively avoid, the Li-ion flux homogenized, and dendrite growth suppressed. Meanwhile, at the interface between the separator and the cathode, the homogenously exposed single-atom Ru on the surface of reduced graphene oxide (rGO@Ru SAs) can "trap" polysulfides and reduce the activation energy to boost their conversion kinetics. Consequently, the LSBs show a high capacity of 460 mAh g-1 at 5C and ultrastable cycling performance with an ultralow capacity decay rate of 0.046% per cycle over 800 cycles. To further demonstrate the practical prospect of the Janus separator, a lithium-sulfur pouch cell using the Janus separator delivers a cell-level energy density of 310.2 Wh kg-1 . This study provides a promising strategy to simultaneously tackle the challenges facing the Li anode and the sulfur cathode in LSBs.

Light-programmable manipulation of DC field in Laplacian Meta-devices.

Impressive progresses have been achieved in the field of metamaterial to mimic the illusion or camouflage effects in nature. These include invisible cloaks and many other cloak-based illusion meta-devices. However, to date many experiments only present single, static or discretized functionalities. The dynamical control of multiple kinds of illusion signals can only be achieved by embedding complex active sources directly connected to external stimuli, leading to limited on/off switching effect in a contact fashion. Here, we experimentally demonstrate a distinct scheme to incorporate multi-functions into one passive Laplacian DC meta-device, assisted by light illumination. It is shown that light-programmable cloaking, full illusion, and partial illusion can be achieved on the same device without physical contact of the heating pads or electric bias, at the cost of only four kinds of natural bulk materials with homogeneous parameters throughout. A DC network is fabricated to demonstrate the proof of concept, with measurement results in good agreement with the numerical simulations. The proposed scheme may open a new avenue to the non-contact multiphysical control of multi-illusion functions for Laplacian fields.