The hottest structure of 2019

At APS March last year, Pablo Jarillo-Herrero reported superconductivity in graphene bilayers. Soon after this, hundreds of publications were published on arXiv, with theorists trying to understand the phenomenon, and experimentalists trying to reproduce the effect and observe different properties on new materials.

A few weeks ago, at APS March 2019, there were more than 150 talks including the keyword “twisted bilayers” in their abstract, and google scholar finds more than 800 results for “twisted bilayer” in 2019 only.

Atomically thin materials have driven a lot of basic research in physics for the past decade, with some applications in optics and electronics. They have been proposed recently as a way to extend theMoore’s law.

Recently, new properties have been explored by assembling those atomically-thin layers each other through van der Waals bonding to form hetero-bilayers. In these structures, the electrons can no longer move freely in the planes of the atomic layers, but their behaviour (electronic states and band structure) depend on the two lattices, forming so-called Moiré-potentials, with a superlattice created at the nanometre scale. If the orientation on the two layers is changed (twisting them relative to each other), the moiré lattice changes, modifying the optical properties of the bilayers.

Interesting properties may arise with semiconducting layered materials, like TMDs (transition metal dichalcogenides). This has been reported a few weeks ago in four Nature papers. These articles investigate the impact of the moiré potential on light emission and absorption of hetero-bilayers made out of tungsten diselenide, tungsten disulphide and molybdenum disulphide. In these structures, the light emission and absorption is governed by the excitons:

Interlayer excitons

The two papers from Tran et al and Seyler et al deal with interlayer excitons, where the electrons and holes composing the exciton reside in two different layers. They use this to probe light emission effects. In the first, they show that the twist angle between the two layers can influence the polarisation and the energy of the light emission. In the second, they report individual interlayer excitons, that are trapped in the moiré potential. This allows them to detect polarised light, with a splitting of the emission bands when a magnetic field is applied.

Intralayer excitons

Another type of excitons is possible if the electron and the holes are located on the same layer within the hetero-bilayer. Particularly, this allows light absorption, with different signatures caused by the moiré potential. Jin et al report different absorption features, that they can tune with an applied voltage, revealing the role of intralayer excitons.

Hybridisation

The intralayer excitons can be hybridised into interlayer excitons, as Alexeev et al have reported. These hybridised states combine the strong absorption allowed by intralayer excitons and the stability of interlayer excitons under an electric field. These intriguing properties are allowed by the delocalisation of the conduction band over the two layers. With such excitons, the impact of the moiré potential on the optical properties (both absorption and emission) is reportedly amplified.

With all of this, it’s no wonder 2D hetero-bilayers will play a major role in future light technologies. Those four papers show a better understanding of the excitons in this hot material, with the ability to control both the absorption and the emission. My take is that the applied photovoltaics research community has a lot to earn by considering this material for further studies, as it may allow surpassing the Shockley-Queisser efficiency limit for solar cells.

Finally, I’m happy to share I’ll participate in the workshop “Moiré in Paris” on June 3-4 to discuss these incredible structures.

Why are excitons important?

In the applied physics of semiconductors, two types of carriers with similar properties but with opposite charge, take part in the optical and electrical phenomena: electrons and holes. In halide perovskite, as well as in organic semiconductors, the main carrier contributions arise from excitons. One may think about these as bound state of coupled electrons and holes moving together through the material.

The role of excitons in semiconductors

To understand excitons, one must think about the band structure of a semiconductor. In the ideal case, absorption of a phonon may raise an electron from the valence band to the conduction band, leaving a hole in the valence band. When the density of defects is large enough, there may be so-called exciton lines in the optical spectrum. These are intermediate levels located in between the conduction and valence bands, that may trap electrons or holes. Since the excitation is electrically neutral, the exciton lines do not contribute directly to electrical conduction.

Two models describe these excitons: the Wannier exciton and the Frenkel exciton.

Wannier exciton.

By doping a semiconductor with acceptor impurities, it is possible to increase the number of holes to a requested amount, thus making a p-type semiconductor. Thus, acceptor impurities are negatively charged and may attract holes, forming an impurity level just above the top of the valence band. Reciprocally, doping with donor impurities may give rise to n-type semiconductors. In this case, impurity levels are formed just below the bottom of the conduction band, by attracting the charged impurity for extra electrons. These two kinds of impurity levels create bound states for holes and at small distances below ore above the zero of their kinetic energy.

In a compensated sample, the number of donor and acceptor impurities compensate, meaning that some of the electrons may fall from the donor to the acceptor impurity levels, annihilating the holes on this bottom level. Thus, the empty impurities are no more electrically neutral: the donors are positively charged and the acceptor negatively charged. Hence these impurities will attract or repel the remaining carriers. In this case, one may set up a hydrogen-like wave function in which electrons and holes circle about their joint centre of mass, creating a Wannier exciton. The bound carriers interact through a Coulomb interaction, screened by the dielectric constant ϵ of the material. When this dielectric constant decreases, the exciton becomes more localised, breaking up this Wannier exciton model.

The Frenkel exciton

An alternative way of thinking the exciton is in terms of excited atoms. Let’s take one of the electrons on an atom, and raise it to the excited state ϕe(𝙧). We make here a Bloch function of the form Ψ𝙠 = ∑𝙡 ei(𝙠·𝙡) ϕe(𝙧-𝙡) to allow the excitation to travel through the crystal.

This exciton can move through the crystal in a process that does not require the vacancies, interstitials or substitutional impurities to move. This model does not take into account the electron-electron interaction, and the exciton state is far more complicated in practice that the Bloch function I presented. With it, the travelling electron carries a local polarisation of the other electrons existing in the valence band. We can see it as a travelling electron carrying its own hole with it. A subsequent amount of energy would be required to separate the exciton into independent carriers, that can be collected in a solar cell.

It is important to consider that the Wannier exciton and the Frenkel excitons are opposite descriptions of the same phenomenon. Whereas the Frenkel exciton describes a single excited ionic level where the electron and hole are localised sharply on the atomic scale, the Wannier exciton represent localised electron and hole levels that extend over many lattice constant.

Excitons in halide perovskite

Given the very high density of defects in halide perovskites, is has been reported that the excitons are the main charge carrier. The dissociation energy is about 50eV, that is large enough to prevent the dissociation at RT. In these materials, the static dielectric constant ϵ is about 70kHz. Therefore we may consider the Wannier model to describe these charge carriers.

Moreover, a small effective mass of charge carriers favours large exciton radii, adequate with the Wannier model where the exciton radius is larger than the lattice constant. Because of this, it becomes clear that the exciton does not play an important role in the performance of perovskite solar cells. However, because excitons are the main carrier to travel through this material, proper understanding of their behaviour may allow progress in perovskite solar cells efficiencies.

Sources:

Love your Imperfections

At the onset of this blog, I interviewed Prof Aron Walsh about control defects within halide perovskites. The conclusion was that because charge carriers within this material behave as if there were no defects, doping the structure (i.e. adding some positively or negatively charged imperfections in the lattice) have no effect on the electrical properties. Thus, the creation of p-n junctions in halide perovskites would be made impossible, at that stage of the understanding of this class of materials.

A report in Nature Energy earlier this month has pushed this boundary further. Cui, Wei and co-workers report a homojunction of p-type and n-type MAPI perovskite, that is stable enough to produce a 21% efficient solar cell. They achieve the doping of the perovskite by deposition of the n-type layer with low iodine concentrations and the p-type under rich iodine condition. In this case, iodine vacancies are suppressed, giving room for cation vacancies, that are electron-acceptors.

However, it is now well established that halide perovskites (and particularly MAPI perovskites) are mixed ionic and electronic conductors. This would raise some questions about the long-term stability of such a structure, as cations or anions may be transported through the homojunction. Note that the conduction is even stronger under illumination, suggesting that the ionic mobility is greater, too.

Prior to this report, most of the perovskite solar cells were made in a n-i-p architecture, where the perovskite is an absorber material layered in between a hole-acceptor and an electron-acceptor material. The production of stable p-n junctions with halide perovskite surely will revolutionise the field once again, leading to a new perovskite fever.

What happened in January?

The perovskite field has been subject to an incredible momentum and is moving forward very fast. Here is a selection of interesting papers published in January.

Editorial: Perovskite photovoltaics is 10 years old

In 2009, Tsutomu Miyasaka published in JACS a paper reporting halide perovskite as a DSSC sensitizer. The editorial of Nature Energy in January reminds us of the greatest advances since then.

A decade of perovskite photovoltaics.

Persistent Ferroelectric Domains in MAPI

Garten and co-workers from NREL published in Science Advances last month this paper confirming ferroelectricity in MAPI single crystals. Ferroelectricity in halide perovskite is still a subject under debate, with many papers claiming the effect may or may not be screened in these materials. This paper claims confirmation of the existence of a ferroelectric behaviour in MAPI perovskites. The strength of this paper is the use of different techniques to draw the conclusion and may give rise to ferroelectricity control within perovskite, to tune the Fermi level or electrical response.

The existence and impact of persistent ferroelectric domains in MAPbI₃.

Narrowband exciton routing in layered perovskites

Wei et al. published in Nature Energy probed layered perovskites to understand the dynamics of energy transfer in these systems. They observe energy transfer on the scale of the picosecond. They show narrowband exciton routing with very high PLQY. Thus they produce luminescent solar concentrators with a fourfold enhancement in internal concentration.

Ultrafast narrowband exciton routing within layered perovskite nanoplatelets enables low-loss luminescent solar concentrators.

2D perovskites, really?

For a couple of years, more and more papers have been reporting the use of 2D halide perovskites. I’ve found it quite confusing. These paper usually describe slabs of corner-sharing BX₆ octahedra, separated by large organic cations. In most cases, these materials are processed as thin films or large (about 1cm×0.5cm×0.5cm) single crystals. In fact, what is said to be 2D (or quasi-2D) perovskites is always a Ruddlesden-Popper, a Dion-Jacobson or an Aurivillius phase, that are all bulk materials.

In the 2D materials community, and more generally in solid state physics, we describe two-dimensional atomic crystals as sheets thinner than the Fermi wavelength (or in some cases the De Broglie wavelength). In these materials, the electrons are allowed to move in two directions, but are confined in the c direction. Several names describe different structures, all 2D: monolayers, bilayers, van der Waals heterostructure, atomically thin layers. The most representative material of this class is graphene, that consists in a single layer of carbon atoms, that can be seen as a plane pulled out of bulk graphite.

In the perovskite community, the study of these so-called two dimensional (or quasi-two-dimensional) halide perovskites hasn’t lead, to my knowledge, to the report of any 2D electron gas. It would be very surprising to observe some of these, given the high density of defects in these materials that make large mono layers hard to achieve.

I propose to use the terminology layered perovskite, to differentiate this from the 2D materials, and hopefully allow those two different fields to move beyond current limitations and come together. No one would imagine talking about 2D carbon to describe graphite. Nonetheless, this is exactly what the perovskite community has started to do.

As well, it’s been reported “1D perovskites” and “0D” perovskites. I suggest using “perovskite nanopillers” or “perovskite dendritic structures” to describe the 1D perovskite. However, my knowledge of these 1D and 0D fields is too poor to give an accurate proposition.

Addendum (24 January 2019): Aron Walsh tweeted yesterday this commentary from Joachim Breternitz and Susan Schorr, highlighting their view about what could or couldn’t be defined as a perovskite. This reads:

In particular for the Ruddlesden–Popper phases, we would like to draw the attention of the reader to the original publications of Ruddlesden and Popper where the authors point out that these compounds contain perovskite layers. Perovskite layers are, however, not layered perovskites and no matter how small the change in semantics may appear, the latter would not be correct.

Joachim Breternitz and Susan Schorr

Indeed, what has been named layered perovskites in the oxide perovskite community are compounds of the form La2-xBaxCuO₄, Sr2VO₄, LaSrMO₄ where alternating A-site cation are considered as different layers. This is also quite confusing given that layered materials are usually considered as materials forming strong bonds in two directions and a weaker bond in the third direction, as in lead iodide (PbI₂), molybdenum disulphide (MoS₂) or tungsten diselenide (WSe₂). However, the discussion remains open!

Spin-orbit coupling in metal halide perovskites

Metal halide perovskite present outstanding electronic properties. One hypothesis that explains those properties is the presence of a very strong bulk Rashba effect, reportedly the greatest among any other materials. This blog post introduces the Rashba effect in halide perovskites, explaining the origin of both the reported static and dynamic effects. Finally, I give some details about the theory from Byschkov & Rashba and about the photogalvanic effect.

Introduction

Metal halide perovskites (structure ABX₃ where X is a halide) have been subject to an unparalleled interest in the materials science community for the past 6 years. One reason for such an interest is their incredible long carrier diffusion lengths of up to 1 micron in polycrystalline films and as high as tens of microns in single crystals, driving outstanding photovoltaic performance. It has also been observed a strong optical absorption, with incredibly low non-radiative recombination rates, leading long carrier lifetimes — up to tens of microseconds. These parameters are surprisingly high for the unavoidable presence of defects in solution-processed polycrystalline films. This differs from the image of conventional semiconductors in which defects and impurities are responsible for scattering of carriers, lowering their lifetime and diffusion length.

Therefore, many fundamental questions underlying these outstanding optoelectronic properties still remain unanswered. Particularly, the origin of carrier mobilities, of long lifetimes and very low rate of non-radiative recombination need further studies. In fact, different hypotheses have been proposed to explain both the origin of long-carrier lifetimes and low rate of non-radiative recombination:

  • The presence of a Rashba effect, from which an indirect band gap would originate;
  • A considerably low density of deep intrinsic defects, each one having high formation energies;
  • The existence of strong polaronic effects.

The Rashba effect would explain both the long carrier lifetimes and the intriguing low-rate of non-radiative recombination.

The Rashba effect in metal halide perovskites

Let’s start with the example of MAPI (perovskite phase where the A-site cation is methylammonium – CH₃NH₃, the metal, lead, and the halide is iodide), that is the most studied halide perovskite. In this structure, the presence of heavy atoms such as lead or iodine introduces spin-orbit coupling, that, along with inversion-symmetry breaking in the crystal, the electrons behave as if they were subject to an effective magnetic field which due to the spin-orbit coupling (SOC). This results in a momentum-dependent splitting of electron bands commonly referred to as the Rashba splitting. This effect yields a lifting of the degeneracy in k-space, inducing the shift of both the valence band (VB) maxima and conduction band (CB) minima, away from the high symmetry points of the Brillouin zone. This results in an indirect band gap, as shown in the figure below. Particularly, the splitting of the conduction band suppresses the rate of band-to-band recombination of charge carriers that has been proposed to be the origin of increased carrier lifetimes.

The spin-orbit coupling (SOC) results in a Rashba effect on the VB dispersion of MAPI perovskites. Adapted from Niesner et al (2016). (a,b) Without SOC, double spin-degenerate band with single maximum can be expected (b) is the VBM position represented in k-space. (d,e) SOC and inversion-symmetry breaking fields, the VB splits up in two spin-split bands. alpha is the Rashba parameter that quantifies the strength of the SOC.

The inversion-symmetry breaking can happen either in the bulk of non-centrosymmetric crystals, or at surfaces and interfaces. It has been originally theorised for two-dimensional electron gases (2DEG, see below). There are two main mechanisms that lead to the Rashba effect, breaking the inversion symmetry in the bulk of the MAPI crystals, one is static, the other dynamic.

The static Rashba effect

First, lead iodide sub-lattice can distort from a set of ideal corner-sharing octahedra (tetragonal structure). As recently suggested (Rakita et al, PNAS 2017), this forms a non-centrosymmetric phase that is the source of the static Rashba effect. Particularly, the room-temperature of MAPI is the tetragonal I4/mcm phase, which is not centrosymmetric, that may not be subject to spin-splitting. In other phases, when the orbitals with spin-orbit splitting are not subject to inversion symmetry, the spin-orbit coupling causes a spin-dependent shift of the electronic bands along the k-direction. The double spin-degenerate band reportedly splits into two bands, shifted in k-space by k₀. Thus, the bandgap becomes slightly indirect, as the optical transition become spin-dependent. Particularly, these effects are responsible for a high Rashba parameter, whose intensity has been reported to be ɑR = 2 E₀ / k₀ = 7± 1 eV Å in the orthorhombic perovskite and  11 ± 4 eV Å in the cubic phase . These values are among the highest one ever reported

This effect on the optical transition can be probed with a photogalvanic effect (see below), expected if coherent spin transport takes place on length scales large enough for spin-polarised currents to get through devices. That’s what Niesner and co-workers reported with their systematic study of the circular photogalvanic effect in MAPI single crystals.

The spin-splitting also causes a minimum of energy at the central high symmetry point, of depth E₀. Particularly, in the room temperature tetragonal phase, they observed (Niesner et al, 2016) a circular photogalvanic effect for excitations 110meV below the direct optical gap, indicating that the transitions between spin-polarised electronic bands happened below the direct gap.

Circular dichroism has also been observed in pump-probe spectroscopy, and spin dependence of the charge dissociation has been reported. This may allow for the creation of perovskite-based spintronic devices.

The dynamic Rashba effect

Recently, it has been proposed the existence of a dynamical Rashba effect, allowing for quantification of its magnitude. In fact, the cubic perovskite lattice allows high mobility of the organic dipolar cation that locally allow the breaking of inversion symmetry in the absence of any static distortion of the lead-iodide framework. Locally, this creates screening localisation domains that, combined with the presence of lead, provide MAPI with a giant SOC. The degrees of freedom of the molecular cation gives rise locally to a Rashba effect, fluctuating on the picosecond time scale, related to the dynamics of the methylammonium rotation. This shows that the Rashba effect exists in MAPI, regardless of whether the crystal lattice is centrosymmetric or not (i.e. is orthorhombic, tetragonal or cubic). Those dynamic structural fluctuations can occur as a result of the phonon modes or due to the interaction of the MA⁺ ions with the lead iodide framework.

Particularly, the soft nature of the lead-iodide bond allows it to be easily deformed under symmetry-breaking, due to the important fluctuation of the organic cations. At high temperature, this local structural disorder induces the dynamical Rashba effect.

The amplitude of this dynamical effect is very similar to the one that is observed for bulk Rashba systems (static Rashba). Hence, a change in the direction of the current, that is associated with the circular photogalvanic effect at the orthorhombic-tetragonal phase transition, demonstrates that this dynamical effect has two different physical origins in the two phases.

At room temperature, the perovskite is tetragonal, thus the energy splitting between spin-polarised transition and direct optical transition increase with the temperature, as well as the amplitude of the circular photogalvanic effect. Both effect are responsible for an increase of the Rashba parameter with the temperature, that has been measured experimentally (Niesner et al, 2018), giving support for the predicted dynamical Rashba effect.

The orthorhombic lattice is centrosymmetric. Hence, the bulk Rashba effect is impossible in this low-temperature phase. Thus the temperature-independent circular photogalvanic effect that has been measured is attributed to the reduced symmetry at the surface and interfaces. 

Finally, this Rashba effect explains the reported indirect-direct band gap and thermally activated radiative recombination in tetragonal MAPI (Hutter et al, Nature 2018). Particularly, this provides an evidence for the spin-splitting mechanism at elevated temperatures, that should be seen in materials with inversion symmetry that contains heavy elements like lead, and exhibit soft phonon modes.

Further applications

The Rashba mechanism described herein would potentially be very attractive for both optoelectronics and spintronics. Notably, the exploration of Rashba physics has now been at the heart of the growing research field of spin-orbitronics that focuses on the manipulation of non-equilibrium materials properties using SO coupling.

The reported Rashba effect in metal halide perovskites could pave the way towards new applications, thanks to the unique transport properties that emerge with such an effect. The tremendous field of spin manipulation using spin-orbit coupling could notably lead to a quantum spin Hall effect, to the possibility of creating spin-orbit Torque, spin-orbit Qubits or spin transistors or even study topological insulators and Majorana fermions…

I’m looking forward to reading papers involving halide perovskites in the fantastic field of spin manipulation. Given this, I now expect perovskite to have a pivotal role in modern physics with marvellous applications.

Appendices

About the Rashba spin-orbit coupling

In non-centrosymmetric crystals, the electronic energy bands are split by spin-orbit coupling. In those systems with structural inversion symmetry breaking, SOC becomes odd in momentum p, that, in two-dimensional electron gas (2DEG), reduces to a linear dependence of the Rashba effect in momentum.

For electrons moving in an electric field (even in the absence of external magnetic field), and effective magnetic field is felt by the electrons in their frame of motion. This is called the spin-orbit field, and couples to the electron’s magnetic moment. When inversion symmetry breaking is present, this SO field becomes odd in electron momentum. Let’s take the mathematical expression to better understand this:

Let’s chose an electron with momentum p moving across a magnetic field B. This electron experiences a Lorentz force in the direction perpendicular to its motion, F= -ep×B/m and possesses a Zeeman energy μB σ·B, where σ is the vector of Pauli spin matrices, m the mass of the electron, e its charge, and μB is the Bohr magneton.

If this electron moves across an electric field E, it experiences the effective magnetic field Beff ~ E×p/mc² in its rest-frame (c is the speed of light), and this field induces a momentum-dependent Zeeman energy called spin-orbit coupling,which Hamiltonian can be written ĤSOB (E×pσ/mc². In usual crystals, the electric field can be given by the gradient of the potential: E=-V. In quantum wells — that have structural inversion symmetry breaking, along the growth direction z — the spin subbands are split in energy. The band splitting was explained by Bychkov and Rashba considering an electric field E = Ez z resulting in an effective spin-orbit coupling of the form:

ĤR = ɑR/ℏ * (z × p) · σ

Here, ɑR is the Rashba parameter. This formula has been derived for 2D plane waves, and is only phenomenological, thus do not apply as such on real systems.

About the photo-galvanic effect

The spin of electrons and holes in solid state systems has been intensively studied in quantum mechanics, and originates an outstanding number of phenomena. The dominant method to generate and investigate the spin polarisation has been optical orientation. Indeed, light propagation within a semiconducting material and scattering by inhomogeneities or mobile carriers is able to generate either a DC current (for short-circuit conditions) or a voltage (for open-circuit), that is called the photogalvanic effect.

Under illumination with a circularly polarised light, it is possible to generate a transformation of the photon angular momentum into a translational motion of free charge carriers. To better understand this phenomenon, one can imagine it as the electronic analogue of a mechanic screw or wheel, that transform a rotation into a linear motion, respectively tangential or perpendicular to the rotation momentum). This effect has a strong signature due to this circular motion, and leads to the possibility to produce helicity-dependent currents, whose behaviour upon the variation of the radiation helicity, the cristallographic orientation on the sample geometry, can be probed.

References:

The cover photo shows ARPES on beamline 5-4 at SSRL. Courtesy of SLAC National Accelerator Laboratory. ARPES is a technique allowing to observe the band dispersion.

The papers that made 2018

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.

New year, new milestones

Happy new year to all my readers! I’m sure 2019 will bring new exciting advancement in the field. At a minimum, there’ll be new stability developments and increased efficiency of halide perovskite solar cells. There also will be more and more fundamental studies. I’ll try to keep pushing with more new posts this year.

I’ve been carrying research in two different labs in 2018 so it’s been a bit hard to manage writing blog posts, engage with the community and curate a twitter feed. Now that I’m carrying out my own research at the University of Manchester for the next 3 years, things will go smoothly.

This year I’d like to talk here about Rashba-Dresselhaus effects and spin-physics within perovskites, the use of machine learning to predict perovskite properties and materials, and the all expanding field of so-called “2D perovskites” (even though I prefer the terminology layered perovskites to describe those systems). More to come pretty soon.

Finally, here is a video on Scientific Video Protocols about the history of perovskite photovoltaics, featuring Prof Henry Snaith:

A Few Notes on Photo-induced Phase Segregation

Tandem solar cells are a very promising technology that would allow to overpass the Shockley-Queisser efficiency limit for photovoltaic devices. They can be manufactured by stacking two solar devices, one of low band-gap (to absorb low-energy photons), on top of which is placed a high band-gap absorber (that would absorb the high-energy photons, therefore reducing the heat losses inside the solar cell). Mixed halide perovskite are particularly interesting for the assembly of those tandem solar cells, because compositional arrangements can allow acute band-gap tuning.

In perovskite of the form ABX₃, where X is a mixture of halide ions, generally iodide and bromide, researchers have observed a phase segregation under illumination. In fact, the halide ions form clusters inside the perovskite, leading to a hysteresis cycle in the J-V characteristic curves and other instabilities. This issue is currently one of the most important to address before perovskite can hit the market.

Here is a collection of a few papers on the subject, that I have found particularly interesting:

  • Eric T. Hoke et al. reported first the photo-induced phase segregation effect. (October 2014)
  • Christopher Eames et al. proposed a mechanism explaining the ion migration in perovskites. (February 2015)
  • Dan Slotcavage, from the same group, continued research on effect. (September 2016)
  • Sergiu Draguta et al. propose a model suggesting that the phase segregation is driven by the band-gap reduction of the iodide-rich phase. (September 2016)
  • Connor G. Bischak et al. proposed that the effect originates in a polaron. (October 2016)
  • Alex J. Barker et al. reported that the role of defects is of utter importance. They hypothesise that the phase segregation is driven by the generation of charge carrier gradients through the thickness of the film. (March 2017)
  • Felix Lang et al. proposed that radiation introduce phase-segregation. (May 2017)
  • Ute B. Cappel et al. measured enrichment of bromide ions at the surface of thin films upon illumination. (July 2017)
  • Gergely F. Samu et al. reported that light-soaking influenced the segregation. (July 2017)
  • Carolin M. Sutter-Fella et al.: review about phase segregation (February 2018)
  • John M. Howard et al. probed a water-induced phase segregation (April 2018)
  • Connor G. Bischak et al. proposed that the photo-induced phase segregation is controlled by tunable polaron distortion, suggesting that phase segregation may be an intrinsic effect of mixed-halide perovskites. (May 2018)

The cover photo is taken from Bischak et al. in ACS Energy Letters. It shows the evolution of perovskite films emission wavelengths (blue = homogeneous film, yellow = iodine clusters).

Band-gap Tuning for Tandem Perovskites

Tuning the active materials for the engineering of tandem solar cells is an arising field of research among the perovskite community. In this post, I present the main targets of tandem solar cells engineering: band-gap tuning. Then, I showcase two solutions that have been reported by students I am currently working with.

Tandem Solar Cells or the Necessity of Wide Band-gaps

Metal halide perovskites, of the form ABX₃ (where A can be methyl ammonium, formamidinium, caesium or a mixture of both, B lead or tin and X a halide, typically iodide, bromide, or mix of both), are gaining much attention for thin film photovoltaics, thanks to their long carrier diffusion lengths, their defect tolerance enabling versatile solution (or vapour) processing and a very strong optical absorption. The efficiencies of so-called perovskite solar cells are comparable to the one achieved with the incumbent silicon technologies.

Two market solutions can be proposed for perovskites: either challenging the incumbent silicon or use perovskite as a layer that can be stacked on top of the silicon cell, boosting its efficiency. For this later solution, a wide band-gap needs to be achieved with perovskites, in order to maximise the energy collection. In fact, the silicon solar cells have a very low band-gap. It means that every incident photon having an energy higher than the silicon band-gap will be collected, but the energy difference (the energy higher than the band-gap), will be dissipated as heat loss. Thus the need of a layer that would extract a portion of higher voltage photo-generated carriers will be valuable in order to overpass the fundamental efficiency limit for single junctions solar cells, commonly known as the Shockley-Queisser limit.

Because the perovskite has a tunable band-gap, that can be controlled with halide substitution (for example), they tend to be considered as the best solution for more efficient solar cells. Their ability to be solution-processed or vapour-deposited would allow benefiting high quality scalable inexpensive integration on existing devices. State of the art perovskite-silicon tandem solar cells achieved record efficiencies of 23.6% for monolithically integrated cells, and 26.4% for mechanically stacked configuration. Attempts to make perovskite-perovskite tandem solar cells have also been tried with record efficiencies of 18.5%, but these rely largely on the development of more efficient and more stable low-end-gap perovskite absorbers (1.1 to 1.3eV). The PCE figures are expected to grow significantly with a deeper understanding of the materials.

All these aspects make the development of perovskites for tandem devices an active field of research.

Band-gap Tuning Induced via Compositional Engineering

In order to achieve higher band-gap solar devices, compositional tuning of the solar cells has been tried by the Materials Science & Engineering group led by Mike McGehee. His students, Bush et al. describe their result in their paper: “Compositional Engineering for Efficient Wide Band-Gap Perovskites with Improved Stability to Photo-induced Phase Segregation”.

They explore various compositions of hybrid perovskites of the form FA1-xCsxPb(I1-yBry)3. They investigate the performance of solar cells as well as the photo-stability by varying the composition between formamidinium (FA) and caesium (Cs) in the perovskite A-site and between iodide and bromide on the X-site. With that, they show that increasing the concentration in caesium into the solid-solution thin film results in higher VOC and greater photo-stability, as it raises the bandgap. This can be compared with the previously known raise of bromide, that yields higher band-gap, with a fewer photo-stability and VOC.

That article might be particularly important in the development of high-efficiency perovskite tandem solar cells, as the authors identify stable compositions with high band-gap (1.68 to 1.75eV), that demonstrate high device efficiencies.

Band-gap Tuning Induced with Change of the Lattice Constants

Variation of those A-site cation changes the band-gap. Prasanna et al. proposed an interpretation in their JACS paper: “Band-gap Tuning via Lattice Contraction and Octahedral Tilting in Perovskite Materials for Photovoltaics”. By exploring both lead and tin on the B-site, they allow different octahedral sizes, as the length of the metal-halide bond changes. In this paper, they show that the use of a smaller A-site cation can distort the perovskite lattice in two different ways: either by tilting the BX₆ octahedra (pictured on the right) or by contracting the lattice isotropically (depicted on the left). The first effect yields to a change of the metal-halide orbital overlap, that varies the band-gap. The second results in a relative contraction that increases the orbital overlap, thus cutting down the band-gap.

scheme: two strategies
Two strategies implemented by Prasanna et al: either octahedral tilting enlarging the band-gap, or lattice contraction yielding to a smaller one.

Those two strategies can be achieved with partial substitution of the large FA cation with the smaller Cs. With lead halide perovskites, this results in octahedral tilting but with tin-based materials to lattice contraction, due to the smaller size of the tin.

Hence the authors provide a framework to tune the band-gap as well as the valence and conduction band positions by controlling the A-site cation composition.

In order to achieve high efficiency perovskite-based tandem solar cells, it is necessary to adjust the band-gap of the top cells. As shown in this post, this can be improved through control of the cation composition, resulting in higher band-gaps. These results will, for sure, be needed for future high-efficiency solar panels.

The cover picture is from Michigan Engineering on Flickr. Scheme from Prasanna et al.

Proposal: Graphene Sheets with Halide Perovskite Layers

Some months ago, I had conversations with some researchers to propose them a graduate project proposal that could be done within the scope of a PhD. I have been admitted at the University of Manchester to carry out this project under the supervision of Prof Sir Andre K. Geim and Prof Irina V. Grigorieva. I have decided to publish this proposal here, as it may give ideas to other students.

Controlling photons and charge carriers interactions in graphene sheets with halide perovskite layers

Julien Barrier
julien.barrier@espci.org

ABSTRACT. In the pursuit of highly efficient photovoltaic absorbers, halide perovskites are real prospects, but they lack stability and recombinations limit their efficiency. To fix current shortcomings, I propose to widen the range of investigations. In particular, I am seeking to advance the optoelectronic properties of graphene sheets with metal halide perovskite layers. Lowering recombination rates is a challenge for perovskites that may be addressed by transferring charge carriers from the perovskite to the graphene layers. Various synthetic strategies would be methodically elucidated in the aim of enhancing the efficiency of both perovskite solar cells, transistors and light emitters. This proposal may have remarkable outcomes for energy efficient technologies.

Background

In the past five years, metal halide perovskites have drawn huge excitement from the photovoltaics research community because of their high power conversion efficiency. The 2D-structured perovskite demonstrate promising stability properties [Wang 2017], large carrier mobility [Brenner 2016], strong light absorption, superb photo- and electro-luminescence [Stranks 2017, Dou 2015] and strong quantum confinement effects. These properties have enabled LEDs [Yuan 2016, Friend 2017], lasers and photodetectors [Li 2017] powered with perovskite materials.

In the realm of 2D materials, the use of stacked layers, forming so-called van der Waals heterostructures, has empowered in-band structure engineering [Geim 2013] to create tunnel junctions with unparalleled performances. To date, the best materials for building such heterostructures have been graphene, frequently achieving superior performances for surface science, endowing it with favourable electronic, optical, thermal and mechanical properties [Novoselov 2016].

This proposal aims to create heterostructures based on layered 2D perovskite with graphene sheets. This challenge has been addressed with thin film perovskites and has shown excellent FET performance but poor stability in ambient conditions [Cheng 2015]. Here, two dimensional perovskite may be a promising alternative for enhanced stability and attractive electronic and optical properties [Yang 2016]. Moreover, 2D perovskite can be bound to graphene sheets by various techniques as I present in this research proposal.

Project Details

Scope of the project

I propose to investigate various bonding strategies between 2D perovskite layers and graphene sheets to create stacks and assess their properties. Various synthetic methods as well various bond types may be explored: van der Waals, ionic, covalent and hydrogen bonds [Liu 2012].

The question to be addressed is: to what extend does the nature of the bonding between graphene sheets and 2D perovskite layers influence the optoelectronic properties of the stack?

Approaches

Graphene and perovskite space groups are different therefore stacking the two materials in a heterostructure will induce lattice strain. However, interface modelling studies [Guo 2017] showed that this kind of heterostructure can be stable.
Van der Waals heterostructures will be manufactured via physical deposition under vacuum. Ionic heterostructures will be developed by doping the graphene structure to make it more or less electroattractive. Hydrogen and covalent bonding will involve structure modification, enabled by the 2D perovskite where the crystallographic structure is not influenced by the cation [Weidman 2016]. Thus both the excitonic energy and the bandgap will stay unchanged. However, it may require the modification of graphene via grafting organic cations molecules on its surface.

Van der Waals heterostructures of graphene and perovskite have been theorised [Guo 2017]. It has been predicted that the electronic structure of both 2D halide perovskite and graphene will be preserved after stacking. To my knowledge, there has never been any attempt to carry out the practical experiment.

With grafting methods, I anticipate that recombination rates may be reduced by transferring charge carriers from the perovskite to the graphene layers. This may lead to higher efficiency solar cells. Finally, I anticipate that the different binding approaches will tune the position of the Fermi level by modifying the interface between the two layers. The orbital overlapping may be altered, as well as the electronic configuration at the interface.

Methodology

Characterisation of the manufactured materials would be carried out with X-Ray Diffraction techniques, as well as observations with Scanning and Transmission Electron Microscopy. The optoelectronic properties would be explored via photoluminescence spectroscopy; charge transport with microwave conductivity measurements; surface states with transient absorption spectroscopy; and the band structure with Compton and Raman scattering.

Timeline

The first year of the PhD would be dedicated to exploring the property range through methodical testing, with the aim of identifying the most efficient process for keeping the heterojunction stable. During the two following years, I would focus on the optoelectronic properties of the manufactured materials. The various bonding investigations would be carried out simultaneously in order to be able to compare results.

Objectives and Final Outcomes

I believe that combining two outstanding materials may lead to unparalleled results. Covalent bonding will affect the graphene conjugation system, therefore compromising some of its properties. I suppose that non-covalent interactions may preserve all of its electronic properties. The afforded assembly with two-dimensional perovskite structures may certainly give rise to a new class of layered semiconductors, as the assembly with the bulk perovskite counterparts already gave astonishing performances [Cheng 2015].

I am convinced that the perovskite is a class of materials that has the potential to impact the future of modern civilisation. My investigations may lead to meaningful industrial opportunities with great impact on energy saving and energy collection.

References

 

You may have realised that there were two shortcomings in this proposal. How would you manage to fix it?

The cover picture is from Charles van Meter on Flickr.