Two-dimensional (2D) semiconducting materials have been studied extensively for their interesting excitonic and optoelectronic properties arising from strong many-body interactions and quantum confinement at 2D limit. Most of these materials have been inorganic, such as transition metal dichalcogenides, phosphorene, etc. Organic semiconductor materials, on the other hand been investigated for their excellent electrical conductivity and low dielectric coefficients for similar applications in the thin film or bulk material phase. The lack of crystallinity in the thin film and bulk phases has led to ambiguity over the excitonic and electronic/optical band gap characteristics. The recent emergence of 2D organic materials has opened a new domain of high crystallinity and controlled morphology, allowing for the study of low-lying excitonic states and optoelectronic properties. They have been demonstrated to have different excitonic properties compared with the Wannier–Mott excitons in inorganic 2D materials. Here we present our recent experimental observations and analysis of 2D organic semiconductor materials. We discuss the role of high-crystalline and morphology-controlled growth of single-crystalline materials and their optoelectronic properties. The report explains the Frenkel (FR) and charge-transfer (CT) excitons and subsequent light emission and absorption properties in organic materials. The true nature of low-lying excitonic states, which arises from the interaction between CT and FR excitons, is experimentally studied and discussed to reveal the electronic band structure. We then discuss the pure FR behaviour we observed in J–type aggregated organic materials leading to coherent superradiant excitonic emissions. The supertransport of excitons within the organic materials, facilitated by their pure FR nature, and the delocalization of excitons over a large number of molecules are also demonstrated. Finally, we discuss the applications and our vision for these organic 2D materials in fast organic light-emitting diodes, high-speed excitonic circuits, quantum computing devices, and other optoelectronic devices.

Nickel–yttria stabilized zirconia (Ni–YSZ) cermet is the most commonly used anode in solid oxide fuel cells (SOFCs). The current article provides an insight into parameters which affect cell performance and stability by reviewing and discussing the related publications in this field. Understanding the parameters which affect the microstructure of Ni–YSZ such as grain size (Leng et al 2003 J. Power Sources 117 26–34) and ratio of Ni to YSZ, volume fraction of porosity, pore size and its distribution, tortuosity factor, characteristic pathway diameter and density of triple phase boundaries is the key to designing a fuel cell which shows high electrochemical performance. Lack of stability has been the main barrier to commercialization of SOFC technology. Parameters influencing the degradation of Ni–YSZ supported SOFCs such as Ni migration inside the anode during prolonged operation are discussed. The longest Ni-supported SOFC tests reported so far are examined and the crucial role of chromium poisoning due to interconnects, stack design and operating conditions in degradation of SOFCs is highlighted. The importance of calcination and milling of YSZ to development of porous structures suitable for Ni infiltration is explained and several methods to improve the electrochemical performance and stability of Ni–YSZ anode supported SOFCs are suggested.

To reach the target of carbon neutral, a transition from fossil energy to renewable energy is unavoidable. Photovoltaic technology is considered one of the most prominent sources of renewable energy. Recently, metal halide perovskite materials have attracted tremendous interest in the areas of optoelectronic devices due to their ease of processing and outstanding performance. To date, perovskite solar cells (PSCs) have shown high power conversion efficiency up to 25.7% and 31.3% for the perovskite-silicon tandem solar cells, which promises to revolutionize the PV landscape. However, the stability of PSCs under operating conditions has yet to match state-of-the-art silicon-based solar cell technology, in which the stability of the absorbing layer and relevant interfaces is the primary challenge. These issues become more serious in the larger area solar modules due to the additional interfaces and more defects within the perovskite. Bilayer perovskite film composed of a thin low dimensional perovskite layer and a three-dimensional perovskite layer shows great potential in fabricating solar cells with high efficiency and stability simultaneously. In this review, recent advancements, including composition design and processing methods for constructing bilayer perovskite films are discussed. We then analyze the challenges and resolutions in deposition bilayer perovskite films with scalable techniques. After summarizing the beneficial effect of the bilayer structure, we propose our thinking of feasible strategies to fabricate high efficiency perovskite solar modules with a long lifetime. Finally, we outline the directions for future work that will push the perovskite PV technology toward commercialization.

Solid polymer electrolytes (SPEs) possess several merits including no leakage, ease in process, and suppressing lithium dendrites growth. These features are beneficial for improving the cycle life and safety performance of rechargeable lithium metal batteries (LMBs), as compared to conventional non-aqueous liquid electrolytes. Particularly, the superior elasticity of polymeric material enables the employment of SPEs in building ultra-thin and flexible batteries, which could further expand the application scenarios of high-energy rechargeable LMBs. In this perspective, recent progresses on ion transport mechanism of SPEs and structural designs of electrolyte components (e.g. conductive lithium salts, polymer matrices) are scrutinized. In addition, key achievements in the field of single lithium-ion conductive SPEs are also outlined, aiming to provide the status quo in those SPEs with high selectivity in cationic transport. Finally, possible strategies for improving the performance of SPEs and their rechargeable LMBs are also discussed.

Photosynthetic organisms harness solar radiation to produce energy-rich compounds from water and atmospheric CO₂ via exquisite supramolecular assemblies, which offers a design principle for highly efficient artificial photocatalytic systems. As an emerging research field, significant effort has been devoted to self-assembled supramolecular materials for photocatalytic H₂ production and CO₂ reduction. In this review, we introduce the basic concepts of supramolecular photocatalytic materials. After that, we will discuss recent advances in the preparation of supramolecular photocatalytic materials from zero-dimension to three-dimension which include molecular assemblies, micelles, hybrid nanoparticles, nanofibers, nanosheets, microcrystals, lipid bilayers, supramolecular organic frameworks, supramolecular metal-organic frameworks, gels, and host-guest metal-organic frameworks, etc. Furthermore, we show the recent progress in the photocatalytic properties of supramolecular photocatalytic materials, i.e. photocatalytic proton reduction, water splitting, CO₂ to HCOOH, CO₂ to CO, CO₂ to CH₄ conversions, etc. Finally, we provide our perspective for the future research, with a focus on the development of new structures and highly efficient photocatalysis.

Organic–inorganic halide perovskites have been intensively investigated as potential photovoltaic materials due to their exceptional optoelectronic properties and their successful applications in perovskite solar cells (PSCs). However, a large number of defect states still exist in the PSCs so far and are detrimental to their power conversion efficiencies (PCEs) and stability. Here, an effective strategy of incorporating single-crystalline graphene quantum dots (GQDs) into the perovskite films is proposed to passivate the defect states. Intriguingly, the GQD-modified perovskite films exhibit purer phase structure, higher quality of morphology, and higher electrical conductivity when compared with the control perovskite films. All of the advantages caused by the incorporation of the GQDs lead to fast carrier separation and transport, long carrier lifetime, and low nonradiative recombination in the PSCs based on the GQD-modified perovskite films. As a result, this kind of PSC displays an increase in all photovoltaic parameters, and its PCE shows an enhancement of more than 20% when compared with the control PSC. Moreover, this novel PSC is demonstrated to have long-term stability and resistibility against heat and moisture. Our findings provide an insight into how to passivate the defect states and enhance the electrical conductivities in the perovskites and pave the way for their further exploration to achieve higher photovoltaic performances.

Transition metal nitrides (TMNs), including titanium nitride (TiN), exhibit remarkable application prospects as anodes for durable high-rate lithium-ion batteries (LIBs). Regrettably, the absence of simple synthesis methods restricts their further development. Herein, a facile and low-cost molten salt synthesis strategy was proposed to prepare carbon-anchored TiN nanoparticles as an advanced anode material for LIBs with high rate capabilities. This nanosized TiN obtained is ∼5 nm in size and well-distributed onto carbon plates, which could release a reversible capacity of ∼381.5 mAh g⁻¹ at 0.1 A g⁻¹ after 250 cycles and ∼141.5 mAh g⁻¹ at 1.0 A g⁻¹ after 1000 cycles. Furthermore, it was confirmed that the conversion reaction between TiN and Li-ions happened during the electrochemical reaction process, resulting in the formation of Li₃N and Ti. This unique microstructure attributed from TiN nanoparticles anchored by carbon could support the structural volume during cycling. This work highlights the method superiority of TiN prepared via a molten salt synthesis strategy as an anode for LIBs with impressive rate performances.

Garnet-type solid-state electrolytes (SSEs) are particularly attractive in the construction of all-solid-state lithium (Li) batteries due to their high ionic conductivity, wide electrochemical window and remarkable (electro)chemical stability. However, the intractable issues of poor cathode/garnet interface and general low cathode loading hinder their practical application. Herein, we demonstrate the construction of a reinforced cathode/garnet interface by spark plasma sintering, via co-sintering Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) electrolyte powder and LiCoO₂/LLZTO composite cathode powder directly into a dense dual-layer with 5 wt% Li₃BO₃ as sintering additive. The bulk composite cathode with LiCoO₂/LLZTO cross-linked structure is firmly welded to the LLZTO layer, which optimizes both Li-ion and electron transport. Therefore, the one-step integrated sintering process implements an ultra-low cathode/garnet interfacial resistance of 3.9 Ω cm² (100 °C) and a high cathode loading up to 2.02 mAh cm⁻². Moreover, the Li₃BO₃ reinforced LiCoO₂/LLZTO interface also effectively mitigates the strain/stress of LiCoO₂, which facilitates the achieving of superior cycling stability. The bulk-type Li|LLZTO|LiCoO₂-LLZTO full cell with areal capacity of 0.73 mAh cm⁻² delivers capacity retention of 81.7% after 50 cycles at 100 μA cm⁻². Furthermore, we reveal that non-uniform Li plating/stripping leads to the formation of gaps and finally results in the separation of Li and LLZTO electrolyte during long-term cycling, which becomes the dominant capacity decay mechanism in high-capacity full cells. This work provides insight into the degradation of Li/SSE interface and a strategy to radically improve the electrochemical performance of garnet-based all-solid-state Li batteries.

Rhodium-containing compounds offer a fertile playground to explore novel materials with superconductivity (SC) and other fantastic electronic correlation effects. A new ternary rhodium-antimonide La₂Rh_3+δSb₄ (δ≈1/8) has been synthesized by a Bi-flux method. It crystallizes in the orthorhombic Pr₂Ir₃Sb₄-like structure, with the space group Pnma (No. 62). The crystalline structure appears as stacking the two-dimensional RhSb₄- and RhSb₅-polyhedra networks along b axis, and the La atoms embed in the cavities of these networks. Band structure calculations confirm it as a multi-band metal with a van-Hove singularity like feature at the Fermi level, whose density of states are mainly of Rh-4d and Sb-5p characters. The calculations also imply that the redundant Rh acts as charge dopant. SC is observed in this material with onset transition at T^c_on≈0.8 K. Ultra-low temperature magnetic susceptibility and specific heat measurements suggest that it is an s-wave type-II superconductor. Our work may also imply that the broad Ln₂Tm_3+δSb₄ (Ln = rare earth, Tm = Rh, Ir) family may host new material bases where new superconductors, quantum magnetism and other electronic correlation effects could be found.

Quasi-two-dimensional (quasi-2D) perovskites are promising materials for potential application in light-emitting diodes (LEDs) due to their high exciton binding energy and efficient emission. However, their luminescent performance is limited by the low-n phases that act as quenching luminescence centers. Here, a novel strategy for eliminating low-n phases is proposed based on the doping of strontium bromide (SrBr₂) in perovskites, in which SrBr₂ is able to manipulate the growth of quasi-2D perovskites during their formation. It was reasonably inferred that SrBr₂ readily dissociated strontium ions (Sr²⁺) in dimethyl sulfoxide solvent, and Sr²⁺ was preferentially adsorbed around [PbBr6]⁴⁻ through strong electrostatic interaction between them, leading to a controllable growth of quasi-2D perovskites by appropriately increasing the formation energy of perovskites. It has been experimentally proved that the growth can almost completely eliminate low-n phases of quasi-2D perovskite films, which exhibited remarkably enhanced photoluminescence. A high electroluminescent efficiency matrix green quasi-2D perovskite-LED (PeLED) with a pixel density of 120 pixels per inch fabricated by inkjet printing technique was achieved, exhibiting a peak external quantum efficiency of 13.9%, which is the most efficient matrix green quasi-2D PeLED so far to our knowledge.

Chalcogenide phase-change materials (PCMs), in particular, the flagship Ge2Sb2Te5 (GST), are leading candidates for advanced memory applications. Yet, GST in conventional devices suffer from high power consumption, because the RESET operation requires melting of the crystalline GST phase. Recently, we have developed a conductive-bridge scheme for low-power phase-change application utilizing a self-decomposed Ge-Sb-O (GSO) alloy. In this work, we present thorough structural and electrical characterizations of GSO thin films by tailoring the concentration of oxygen in the phase-separating GSO system. We elucidate a two-step process in the as-deposited amorphous film upon the introduction of oxygen: with increasing oxygen doping level, germanium oxides form first, followed by antimony oxides. To enable the conductive-bridge switching mode for femtojoule-level RESET energy, the oxygen content should be sufficiently low to keep the antimony-rich domains easily crystallized under external electrical stimulus. Our work serves as a useful example to exploit alloy decomposition that develops heterogeneous PCMs, minimizing the active switching volume for low-power electronics.