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Halide perovskite-based solar cell (PSC) has emerged as the fastest photovoltaic material to have reached the power-conversion efficiency (PCE) of larger than 25%. Despite unprecedented growth of research interest to study the fundamental aspects as well as to optimize interfacial layers in solar cells to achieve high-performing PSCs, moisture stability still remains as roadblock to get attention PSCs deserve for commercialization and mass production. To obtain first degree of protection, there have been reports on using additives into perovskite precursors and applying protective layers directly on perovskite active materials. For instances, hydrophobic molecules such as functionalized aliphatic compounds, fullerenes, and ionic liquids have been reported to increase the moisture-resistance of PSCs when used as additives in perovskite precursor solution. Separately, hydrophilic polymers such as poly(ethylene glycol) and poly(acrylic acid) were found to maintain device stability despite their water-absorbing nature. However, the beneficial effect of amphiphilic organic molecules has been scarcely discussed. In our study, novel amphiphilic corannulene-pentakis-(triethyleneglycol-monomethyl ether)-sulfone (Cor-TEG) was utilized to explore the mutual effect of both hydrophobic corannulene and hydrophilic side chains (ethylene glycol oligomers) in increasing the moisture resistance of PSCs. It is found that the Cor-TEG additive modulated the perovskite crystal growth to form large and columnar grains with reduced defects and enhanced photocarrier extraction. Notably, the Cor-TEG modified perovskite devices showed improved stability under humid Singapore ambient air, retaining 87% of initial PCE for two weeks of exposure.

MATERIALS FOR HUMANITY (MH 21)

Amphiphilic additive for moisture-resistant perovskite solar cells

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technical paper

Lead-free halide perovskite semiconductors are necessary due to the atmospheric instability and lead toxicity associated with the 3D lead halide perovskites. However, a stable lead-free perovskite with an ideal bandgap (1.2-1.4 eV) for photovoltaics is still missing. In this work, we synthesized organic-inorganic gold halide double perovskites ((CH3NH3)2Au2X6, X = Br, I) through a solution-processed route that offers an ideal direct bandgap for photovoltaic applications. Density functional theory calculations confirm the direct nature of bandgap nature with reasonable absorption coefficients in the visible range and excellent charge transport properties. In addition, the Au-halide perovskites show high chemical stability and photoresponse. These combined properties demonstrate that Au-based halide perovskites can be a promising group of compounds for optoelectronic applications.

Direct-bandgap Mixed-valence Organic-inorganic Gold Perovskite as Visible Light Absorber

Lithium ion batteries have seen rapid development in recent years, accompanied by increasing industrial interest in electrification. Nonetheless, several issues remain, including insufficient capacity, poor safety and high raw material costs. This review utilises the periodic table to rationalise popular developments in alternative battery chemistries aimed at addressing these issues, including sodium ion, multivalent ion as well as various alloying and conversion electrodes such as lithium-silicon and metal anodes, and sulfur and oxygen cathodes. The major advantages and challenges faced by each chemistry are briefly discussed, along with the strategies being adopted to overcome these challenges and state-of-the-art developments in industry.

Alternative Chemistries to Lithium-Ion for High Energy Batteries

Energy – one of the major crises humanity is facing in today’s modern life. There is a great interest in developing micro-scaled devices that can generate electricity using ambient radiofrequency (RF) energy without the need of any bias or magnetic field. We addressed this problem and used the intrinsic quantum property of the Weyl semimetal to convert the RF energy into electricity. The whole process is wireless and there is no need for any external electrical bias (battery-free) and magnetic field.
In this work, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. The NLHE is present only when the electric field is applied along the axis of the mirror plane (a-axis of the crystal). We also observe a temperature-induced sign inversion of NLHE in TaIrTe4. Finally, based on the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless RF rectification with zero external bias and magnetic field. As shown in Fig. 1(b), an incident electric field along a-axis (Ea) generates a rectified voltage along b-axis (Vb), whereas, there is no rectification when the incident electric field is along b-axis (Eb) in consistent with the symmetry constraints of NLHE. This work opens a door to realizing room temperature applications based on the NLHE in Weyl semimetals.


Room Temperature Nonlinear Hall Effect and Wireless RF Rectification in Weyl Semimetal TaIrTe4

Atomically thin Bi2O2Se has emerged as a new member in two-dimensional (2D) materials with ultra-high carrier mobility and excellent air-stability, showing great potential for electronics and optoelectronics. In addition, its ferroelectric nature renders ultra-low thermal conductivity, making it a perfect candidate for thermoelectrics. In this work, we experimentally investigate the thermoelectric performance of 2D Bi2O2Se over a wide temperature range (20 - 300 K). We observe a gate-tunable transition from polar optical phonon (POP) scattering to piezoelectric scattering, which facilitates the capacity of drastic mobility engineering in 2D Bi2O2Se. Consequently, high power factor of more than 400 uWm-1K-2 over an unprecedented temperature range (80 – 200 K) is achieved, corresponding to the persistently high mobility arising from the highly gate-tunable scattering mechanism. This finding provides a new avenue for maximizing thermoelectric performances by changing scattering mechanisms and carrier mobility over a wide temperature range.

Gate-tunable Polar Optical Phonon to Piezoelectric Scattering in Few-layer Bi2O2Se for High Performance Thermoelectrics

Metal lead halide perovskites are promising candidates for optoelectronic applications due to their high absorption coefficient, thin emission line width, solution processability, high photoluminescence efficiency, and wide color tunability. When nanostructured these perovskites offer the additional advantage of strong quantum confinement.1 In this pioneering work, triple cation perovskite Csx(MA0.17FA0.83)1−xPbBr3 (x = 0–0.15) nanocrystals were synthesised. These proved to have superior phase and environmental stability, less sensitivity to the processing conditions so high reproducibility, and high efficiency when deployed in LEDs and solar cells.2 Although these of lead halide perovskites show excellent properties they contain toxic lead. This motivated the study of stable and lead-free cesium copper iodides as optoelectronic materials.3 By synthesising these compounds as nanocrystals using a hot-injection route the phase and composition were prescribed such that Cs3Cu2I5 nanoplates gave blue emission and CsCu2I3 nanorods yield yellow emission. By mixing these components pure white emission from lead-free metal halide nanocrystals was demonstrated for the first-time. 
References:
1. J. Butkus, P. Vashishtha, K. Chen, J. K. Gallaher, S. K. Prasad, D. Z. Metin, G. Laufersky, N. Gaston, J. E. Halpert and J. M. Hodgkiss, Chemistry of Materials, 2017, 29, 3644-3652.
2. P. Vashishtha, S. A. Veldhuis, S. S. H. Dintakurti, N. L. Kelly, B. E. Griffith, A. A. M. Brown, M. S. Ansari, A. Bruno, N. Mathews, Y. Fang, T. White, S. G. Mhaisalkar and J. V. Hanna, Journal of Materials Chemistry C, 2020, DOI: 10.1039/D0TC02038A.
3. P. Vashishtha, G. V. Nutan, B. E. Griffith, Y. Fang, D. Giovanni, M. Jagadeeswararao, T. C. Sum, N. Mathews, S. G. Mhaisalkar and J. V. Hanna, Chemistry of Materials, 2019, 31, 9003-9011.



Metal Halide Nanocrystals for Optoelectronics

The search for batteries with higher energy and power densities than the current Li–ion batteries is a high priority research area in the continuing development of energy storage systems. Among the various possibilities, lithium–sulfur (Li–S) batteries have drawn a strong following because of the high theoretical specific capacity (1675 mAh g–1) and low cost (<$150 ton–1) of sulfur. The actual energy density of Li–S batteries as implemented to date is however notably lower than the theoretical value because of several technical challenges, such as the dissolution of reaction intermediate products (polysulfides), the polysulfide shuttle between the sulfur cathode and lithium anode, and the need for excess electrolyte. The use of catalysts to accelerate the polysulfide conversion to their end discharge product (Li2S) can inhibit the loss of polysulfides to the electrolyte and has the appeal of a root–cause solution. While a great deal of attention has been focused on the search for new polysulfide catalysts, relatively few have focused on strategies which can maximize the performance of catalysts developed to date. This talk introduces several strategies which can improve the catalytic activity and the sustainable use of catalysts in lithium–sulfur batteries.

Strategies for Enhancing the Polysulfide Conversion Catalysis in Rechargeable Lithium-Sulfur Batteries

Natural ceramic composites present complex microstructures that lead to tortuous crack path and confer them high toughness. Current microreinforced composites do not yet reach the level of complexity found in natural microstructures. To achieve this, we developed an automated set up for controlling 2D microplatelet orientation using magnetic fields. The setup has 4 degrees of freedom and allows fine control of platelet angles in 3D as well as position. With our setup, we can thus build microstructured porous ceramic materials (alumina) with local orientation of microplatelets, at a concentration of about 40 vol%. Other work in our group has shown that the same setup can be used to process microstructured ceramics up to 95 vol%.
Among the infinite possible microstructures, a nacre-like helicoidal arrangement was selected due to its ability to tilt and twist the crack path. Indeed, we used an analytical model based on the laminate theory to compare energy dissipation due to mixed modes of failure. Nacre-like helicoidal arrangement was predicted to exhibit the highest energy dissipation. Then, we fabricated the hybrid microstructured porous specimen fabricated and infiltrated them with a silicone matrix. Using compression tests, our hybrid dissipated 1.6 times more energy as compared to nacre-like samples of same composition. The fabrication strategy proposed here could thus be a simple route to increase the toughness of microplatelet reinforced composites.



Energy dissipation in composites with hybrid nacre-like helicoidal microstructures

The solutionization of Inconel 718 (IN718) is often terminated by rapid cooling at a rate of air cooling or faster. However, in this study, a slow cooling method utilizing furnace cooling was employed for selective laser melted (SLM) micron-TiC reinforced IN718 to investigate the interactions between the TiC particles and the matrix. Four holding temperatures of 975, 1075, 1175, and 1275 °C were chosen while the holding time was kept constant at 1 hour. It was found that the addition of micron-TiC facilitated the formation of nano-sized Ti-rich precipitates in as-printed samples. The difference in microhardness between reinforced and non-reinforced IN718 increased with increasing holding temperature, from 14.9% higher at 975 °C to 50.3% higher at 1275 °C. This was mainly attributed to the prevention of solute segregation to the grain boundary and the reduction of the susceptibility to hot cracking after adding TiC particles. The results indicate that the solute materials (Nb, Mo, and Si) were absorbed by the Ti-rich particles through Ostwald ripening mechanism instead of diffusing to the grain boundaries. The degree of absorption increased with increasing holding temperature. Moreover, grain coarsening at high solutionizing temperature was reduced substantially with the presence of Ti-rich particles due to Zener pinning. Thus, this provides a better understanding for choosing a suitable heat treatment profile for IN718 composite.

Mitigation of solute segregation after solutionization in selective laser melted Inconel 718 / micron-TiC composite

Electrification could provide many benefits towards the environment like lessening the dependence on non-renewable resources and reducing emissions when applied in the space of transportation. However, their battery packs take up a significant amount of space, decreasing the cargo capacity of EVs and payload of electric planes. Therefore, electrification of transportation options is driven largely by the advancement of battery technology and light-weighting composites with the goal of achieving a higher energy while reducing weight and saving space. 
To that end, the concept of providing structural power in the form of multifunctional structural batteries has been deemed as a potentially viable strategy to achieve both high energy density, light-weighting and space savings simultaneously. However, the scientific problems facing the development of multifunctional materials for structural batteries are especially hard to solve due to the seemingly incongruent material requirements for energy storage and structural stability. Structural battery constituents include the structural electrodes and the structural electrolytes. In particular, achieving good mechanical and conductivity properties on structural electrolyte is especially challenging due to the complicated electrochemical processes involved in batteries and the stringent mechanical requirements of structural components. In this work, a sulfonated polyether ether ketone, an engineering thermoplastic, grafted with polyethylene glycol (SPEEK-g-PEG) was studied for its multifunctional capabilities. A conductivity of 10-6 S cm-1 was achieved with SPEEK-g-PEG (SPEEK grafted with PEG 750) free-standing film paving the way for the use of engineering thermoplastics as multifunctional structural battery electrolytes.

Towards the design of multifunctional composites for structural battery applications

Polymer composites reinforced with a thin film made of carbon nanotubes (CNTs), also called as buckypapers, have been first reported in 2004 [1]. The CNTs in the films form an entangled, interconnected and porous network hold together by the van der Waals forces. The films retain most of the properties associated with CNTs, including high specific surface area, electrical and thermal conductivities, while they allow to bypass the agglomeration and processing issues faced by traditional processing methods based on melt compounding [2]. The reported applications of buckypapers include batteries, fuel cells, filters, strain sensing and lightning striking protection thus incorporating them into composites could allow for new multifunctional materials [3]. The main drawback of the buckypapers is their preparation method, which is currently based on filtration or Chemical Vapour Deposition, therefore making the preparation of the films slow, expensive and size restricted [4]. In this talk a new approach towards manufacturing of buckypaper composites will be presented. The method relies on making the thin films using a dispersion of carbon nanotubes directly on the epoxy layer, thus saving the time required for vacuum filtration. With the aid of data from Dynamic Mechanical Analysis tests, this presentation will explain the advantages and limitations of this manufacturing method. A future outlook of the buckypaper composites will be given, focusing on the multifunctionality, scalability and additive manufacturing of these materials.

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