The perovskite field has been evolving so rapidly that it’s been hard to follow all its evolution. Here I give a list of the papers published last year that I think advanced perovskite research and marked the important research milestone. I also extended this list to other papers I’ve particularly enjoyed reading in 2018.

Halide perovskites

There are currently huge debates about whether metal halide perovskites present ferroelectric properties, as do their oxide counterparts. In a Science paper this summer, Heng-Yun Ye et al. proposed metal free halide perovskite compositions showing ferroelectricity properties. This could trigger new ferroelectric studies in this field.

Layered halide perovskites (usually called “2D-perovskites”) have been subject to a huge interest because the large organic cations used to separate different sheets of corner-sharing octahedra also provided the structure with enhanced stability against moisture and oxygen degradation. Mike Toney’s and Ted Sargent’s groups collaborated in a Nature materials paper published in September. They provide a kinetic model for the formation of those layered perovskites, providing with new insights allowing orientational and compositional control of the solar cells made out of this material.

The Rashba effect had been theorised for a long time to happen in halide perovskites, as it would explain both the high electron diffusion length and low recombination rates. In May, Kyle Frohna et al. reported in a Nature Communications paper a static Rashba effect, induced by the breaking of inversion symmetry in some phases. In their PNAS paper published in September, Daniel Niesner and co-workers provided experimental evidence of a dynamical Rashba effect, characterised by spin-splitting at elevated temperature.

In September, Roald Hoffman and Maarten Goesten published in JACS a theoretical study: “Mirrors of Bonding in Metal Halide Perovskites”. In this article they investigate the different interactions within CsPbBr₃, providing new insights both on the hydrogen bonding and the band structure of this incredible material.

Solar cell engineering

If I had to highlight a review about tandem solar cells this year, that would be undoubtedly Tomas Leijtens’ in Nature Energy, published this summer. He and his co-workers discuss the latest developments in perovskite tandem solar cells and give perspectives to move this kind of research forwald.

Condensed matter physics

Ming-Min Yang et al. reported in Science a flexo-photovoltaic effect in some non-centrosymmetric crystals like strontium titanate SrTiO₃ (a structural analogue of the photovoltaic halide perovskite CsPbBr₃). This effect is different from the typical photovoltaic effect originating from p-n junction. The presence of such an effect may allow boosting the power-conversion efficiency of solar devices. In my mind, this might provide further explanations of the already incredible solar conversion properties of halide perovskites, in the presence of the debated ferroelectric properties in these compositions.

On a completely different ground, Pablo-Jarillo-Herrero from MIT presented in two Nature papers the presence of superconductivity in twisted graphene bilayers. According to many observers, this is likely to be the breakthrough of the year, in a field that hasn’t seen such enthusiasm since the discovery of graphene in 2004.

These results took everyone in the community by surprise because although superconductivity had been previously observed in heterostructures made with graphene, they always involved another superconductive material. Here, they’ve shown that bilayers, rotated with an angle of 1.1° allowed the generation of a non-conducting state (Mott insulator) that can be turned into a superconducting state if charge carriers are added to the graphene system, under 1.7K. Interestingly, this paper caught attention because superconductivity appears in a much simpler system than what has been studied previously (cuprates), making graphene bilayers a possible Rosetta stone for the understanding of unconventional superconductivity.

In the semiconductors community, Eve Stenson from Max Plank Institute reported in PRL that a beam of positrons could boost semiconductor luminescence, more than 100 times that what an electron beam could provide. This suggests that the electron antiparticles may annihilate on collision with particles in the semiconductor, boosting the efficiency of luminescent devices.

Another paper that sparked a huge interest on Twitter has been Thapa & Pandey’s claim of the discovery of a superconducting material at -37°C. This claim has been largely criticized, notably by Brian Skinner, who discovered a weird noise pattern.

Chemistry

A large part of condensed matter chemistry is moving toward machine learning to be able to predict new molecules. In the perovskite field, this has been highlighted at MRS Fall by Marina Leite who proposed to build an open-access library, useful for all the perovskite community. With a much larger materials science background, Aron Walsh and collaborators published a Nature review detailing tools and principles for this emerging field.

Early this year, new kind of chemical reactions has been intended. Whereas typical chemical reactions are carried out with a number of molecules about the Avogadro number, a reaction on the atomic scale has been encountered by these scientits. They built 1 molecule by merging 2 atoms, trapped in laser-beams of different wavelenghts. This has been interesting because it came out just before the Nobel prize of physics, attributed to the laser tweezers, showing a great example of this technology. On a basic scale, this may be useful to understand chemistry in low pressure systems, far in the universe, and also gives perspective in the design of complicated molecules that could be engineered with such a precision. This paper is very important to design a whole new kind of chemistry.

Climate change

I started this blog by being deeply convinced of the need to tackle climate change, and that the latest research advances in solar energy could be of great help in this domain.

Without a surprise, climate science has continued to evolve this year, providing new proofs of the disaster that already started. In a Nature paper, Springmann and co-workers presented options for keeping the food system within environmental limits. This convinced me to turn vegetarian to preserve life on earth as we know it.

Others

Finally, the mainstream press has been enthusiast about a paper from two of my friends published in Nature Physics this summer. With a groundbreaking experiment, they provide new insights on a system that has been known for centuries: Leidenfrost effect. The most notable result is that they understood intrinsic movements within small droplets floating above a hot surface. I’ve been quite happy seeing such popularity for their results.

The cover photo has been taken from this Youtube video.

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