The emergence of carbon dots (CDs) has opened up an exciting new field in the science and technology of carbon nanomaterials and has attracted increasing interest in recent years. Due to their diverse physicochemical properties and favourable attributes, such as quantum confinement effects and abundant surface defects, CDs and their derived hybrids have shown exciting and indispensable prospects in the energy conversion and storage fields. Considering the latest developments, in this review, we comprehensively summarize the classification and structure of CDs. Three strategies for structural engineering of CDs are presented and analyzed, in terms of the tuning of size and crystallinity, and the methodologies for surface modification and heteroatom doping, with a focus on the relationship among the synthesis methods, structure and properties of the concerned CDs. More importantly, the recent advances in energy-oriented applications of CDs, including photo- and electro-catalysis, light-emitting diodes, photovoltaic cells, lithium/sodium ion batteries and supercapacitors, will be systematically highlighted. Finally, we discuss and outline the remaining major challenges and opportunities for CDs in the future.

Design and fabrication of carbon dots for energy conversion and storage

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3. We observed hot fluorescence emission from energetic carriers with ~102-picosecond lifetimes in CH3NH3PbBr3 or CH(NH2)2PbBr3, but not in CsPbBr3. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

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Screening in crystalline liquids protects energetic carriers in hybrid perovskites

Powered by renewable energy sources such as solar, marine, geothermal and wind, generation of storable hydrogen fuel through water electrolysis provides a promising path towards energy sustainability. However, state-of-the-art electrolysis requires support from associated processes such as desalination of water sources, further purification of desalinated water, and transportation of water, which often contribute financial and energy costs. One strategy to avoid these operations is to develop electrolysers that are capable of operating with impure water feeds directly. Here we review recent developments in electrode materials/catalysts for water electrolysis using low-grade and saline water, a significantly more abundant resource worldwide compared to potable water. We address the associated challenges in design of electrolysers, and discuss future potential approaches that may yield highly active and selective materials for water electrolysis in the presence of common impurities such as metal ions, chloride and bio-organisms. Production of hydrogen fuel by electrolysis of low-grade or saline water, as opposed to pure water, could have benefits in terms of resource availability and cost. This Review examines the challenges of this approach and how they can be addressed through catalyst and electrolyser design.

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Electrolysis of low-grade and saline surface water

Graphene quantum dots from chemistry to applications

All-inorganic perovskite solar cells with high efficiency and improved stability are promising for commercialization. A multistep solution-processing method was developed to fabricate high-purity inorganic CsPbBr3 perovskite films for use in efficient solar cells. By tuning the number of deposition cycles (n) of a CsBr solution, the phase conversion from CsPb2 Br5 (n ≤3), to CsPbBr3 (n=4), and Cs4 PbBr6 (n≥5) was optimized to achieve vertical- and monolayer-aligned grains. Upon interfacial modification with graphene quantum dots, the all-inorganic perovskite solar cell (without a hole-transporting layer) achieved a power conversion efficiency (PCE) as high as 9.72 % under standard solar illumination conditions. Under challenging conditions, such as 90 % relative humidity (RH) at 25 °C or 80 °C at zero humidity, the optimized device retained 87 % PCE over 130 days or 95 % over 40 days, compared to the initial efficiency.

High-Purity Inorganic Perovskite Films for Solar Cells with 9.72 % Efficiency.

Lanthanide Ions Doped CsPbBr3 Halides for HTM-Free 10.14%-Efficiency Inorganic Perovskite Solar Cell with an Ultrahigh Open-Circuit Voltage of 1.594 V

All-inorganic perovskite solar cells (PSCs) are regarded as a promising solution to improve the environmental tolerance of emerging photovoltaics; however, the narrow light response (<550 nm) of cesium lead bromide (CsPbBr3) perovskites markedly lowers their power conversion efficiency (PCE). We present here the proof-of-concept for mixed light harvester prepared from CsPbBr3 halide and an inorganic SnS:ZnS photoactive layer (PAL) to broaden the spectral absorption from 550 nm to 700 nm. Owing to the reduced surface defects on the perovskite film, improved charge extraction, and broadened light response, the all-inorganic PSC with the structure of FTO/c-TiO2/m-TiO2/CsPb0.97Tb0.03Br3/SnS:ZnS/NiOx/carbon delivered a short-circuit current density of 8.21 mA cm−2, an open-circuit voltage of 1.57 V, a fill factor of 79.6%, and a champion PCE as high as 10.26% under one sun illumination. The preliminary results also suggest that these devices are relatively stable under persistent conditions of 80% RH or temperature of 80 °C in air. The proof-of-concept for mixed perovskite/PAL absorber provides new opportunities for a wide-spectral response and therefore markedly increased the PCE outputs.

All-inorganic CsPbBr3 perovskite solar cell with 10.26% efficiency by spectra engineering

Lattice Modulation of Alkali Metal Cations Doped Cs1−xRxPbBr3 Halides for Inorganic Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted tremendous interest because of their rapid improvement in power conversion efficiency (PCE) from the initial PCE of 3.8% for the first prototype to the certified PCE of 25.2% in 2019. However, the inherent chemical instability of organic–inorganic hybrid perovskite halides influenced by moisture, heat and ultraviolet light is still a critical issue for them to meet application-specific requirements owing to the weak-bonded organic components in the hybrid crystal structure. The use of all-inorganic perovskites CsPbI3−xBrx (x = 0, 1, 2, and 3) as light-harvesters by completely substituting organic species with inorganic Cs+ ions has been recently regarded as a promising solar conversion technology. Since the initial efficiency of 2.9% achieved in 2015, the highest PCE record for inorganic PSCs has risen to 18.4% through structure optimization, compositional engineering, interfacial engineering, solvent control and surface passivation, etc. This article is dedicated to providing an up-to-date review on the development of inorganic PSCs tailored by various inorganic perovskite materials with gradually changed optical properties and stability, as well as the film-making methods and interfacial engineering technologies. Their limited efficiencies in theory and recombination mechanisms are also predicted with a detailed balance model. Finally, we focused on the state-of-the-art strategies for enhancing the photovoltaic performance and identified new challenges and outlooks for future studies in this field.

Yuanyuan Zhao

Papers

High-Purity Inorganic Perovskite Films for Solar Cells with 9.72 % Efficiency.

Lanthanide Ions Doped CsPbBr3 Halides for HTM-Free 10.14%-Efficiency Inorganic Perovskite Solar Cell with an Ultrahigh Open-Circuit Voltage of 1.594 V

All-inorganic CsPbBr3 perovskite solar cell with 10.26% efficiency by spectra engineering

Lattice Modulation of Alkali Metal Cations Doped Cs1−xRxPbBr3 Halides for Inorganic Perovskite Solar Cells

Inorganic perovskite solar cells: an emerging member of the photovoltaic community