Context
The family of halide perovskites encompasses all crystalline materials derived from the ideal perovskite structure, and results from the self-assembly of inorganic fragments [MX6]n- where M is a divalent metal and X a halogen ion (I, Br, Cl) and A cations, either organic in the case of hybrid perovskites, or inorganic such as cesium. Depending on the nature of the A cation and the stoichiometry of the compound, the inorganic framework involves 3D, 2D, 1D or 0D connectivity inducing low-dimensional electronic structures, passing from a 3D structure to that of quantum wells, quantum wires or quantum boxes respectively. In particular, layered 2D hybrid perovskites, of formula A2MX4, with A an organic cation, have been studied for some twenty years for their exceptional optical properties, but also for the great flexibility offered in terms of elaboration, self-assembly and chemical synthesis.
The story of halide perovskites for photovoltaics accelerated sharply in mid-2012, after some initial Japanese and Korean results, under the simultaneous impetus of 2 teams: the EPFL team (M. Grätzel) and the Oxford team (H. Snaith). Record photovoltaic yields are rapidly reaching 10% (2012), 15% (2013), 20% (2014), 22% (early 2016), 25.7% (2022). Such a slope of development is completely unprecedented in the history of photovoltaics, which is why, as early as 2013, the journal Science described hybrid perovskites as a “breakthrough” material. In 2016, this craze was confirmed with the potential for halide perovskites to be included in silicon cells, to make tandem cells: in 2022, a record efficiency of 31.3% was already achieved in a silicon/perovskite tandem cell. Perovskite/perovskite tandem cells are also being produced, with a record efficiency of 26.4% in 2022.
The rise of halide perovskites is certainly driven by the field of photovoltaics, but halide perovskites are interesting materials in many other respects. As early as 2014, we saw the first demonstrations of PeLEDs (hybrid perovskite-based LEDs), a first demonstration of white luminescence, a first indication of amplified spontaneous emission (ASE: Amplified Spontaneous Emission), and since 2015, articles on laser applications have been multiplying. CsPbX3 nanoparticles have enabled halide perovskites to enter the field of quantum optics. Another important application that emerged in 2014 is the use of halide perovskites as X-ray photodetectors. The addition of magnetic-type functionalities to the molecule points to a forthcoming entry into the field of spintronics: significant Rashba effect, 2D perovskites based on Cu, Mn, Fe, Cr, multiferroism, ferroelectricity, etc. Finally, as of 2016, new applications for halide perovskites are appearing in the literature: water splitting, thermoelectric power generation, nonlinear optics, piezoelectric energy recovery, field-effect transistors, RAM memories, physics-biology interface, etc.
The reason for this dazzling success lies in the fact that halide perovskites are a new class of semiconductors combining a set of properties rarely found all in the same material: a high absorption coefficient, bandgap energy easily adaptable from near-IR to visible, excitonic properties adaptable to the different applications targeted, long charge carrier diffusion lengths, high defect tolerance. What’s more, all these properties are associated with solution deposition methods, under mild temperature and pressure conditions, which are inexpensive and compatible with large surfaces.
However, the industrial development of optoelectronic components based on halide perovskites presents a number of hurdles that need to be overcome: the presence of lead and chemical stability (sensitivity to humidity and light). In particular, although enormous progress has been made in a short space of time in terms of stability, this is still insufficient for mass production.
The Groupement De Recherche (GDR) HPero proposes an approach that mixes fundamental and applied aspects in equal measure, so as to create a synergy likely to develop new concepts as well as propose new potentialities in terms of applications. To make decisive progress on the various issues, GDR HPero is organized around 6 scientific axes.
Material engineering
The development of halide perovskites in photovoltaics, photonics and other emerging fields calls for the design of new derivative materials offering added value, notably in terms of increased stability, reduced lead content, enhanced performance, use of solvents or more environmentally-friendly deposition methods.
Key areas of research include the use of additives and the adjustment of the chemical composition of 3D perovskites, the surface treatment of perovskite layers including the formation of 2D nanolayers, and the synthesis of lead-depleted or lead-free perovskites, the preparation of 2D perovskites incorporating functional cations, the synthesis of composite materials made from two types of halide perovskites, the development of perovskite nanocrystals, and the search for more environmentally-friendly solvents.
To achieve these objectives and accelerate the discovery of stable compounds with specific characteristics, it is desirable to take advantage of the great chemical flexibility of halide perovskites, by developing new machine learning approaches coupled with robotic platforms for synthesis and characterization.
Structural studies and defects
The intrinsic physical behavior of hybrid perovskites is highly dependent on the underlying crystal structure (dimensionality, symmetry, topology, disorder) and its dynamics. Recent advances show a move towards a genuine structural engineering approach. Controlling defects in these semiconductor materials is essential for obtaining high-performance optoelectronic devices that are stable over time.
The main difficulty in understanding and measuring the role of defects stems from the intrinsically “soft” nature of hybrid perovskites, which gives rise to dynamic states of these defects as a function of numerous parameters. Such studies are highly multidisciplinary, involving chemical (material synthesis, thin films, crystals, etc.), physical (transport, emission, recombination, etc.), technological (devices, interfaces, etc.) and theoretical aspects. A detailed understanding of these defects requires studies at the atomic scale, hence the importance of the presence within the GDR of large instruments such as the SOLEIL synchrotron or irradiation or implantation instruments (controlled creation of defects).
Understanding structural aspects requires a multi-scale approach in space and time, looking at single crystals (bulk behavior), thin films and devices. Systematization of structure/property studies of bulk materials (as a function of temperature) is needed to provide a solid understanding of the role played by certain structural parameters (distortion, disorder, symmetry). Structural dynamics (exciton and polaron formation, polar reorientation) require appropriate pump-probe approaches. Structural mapping operando (under illumination) on thin films or devices will enable us to understand the effects of degradation, ion migration, phase separation and mesoscopic structuring (ferroelastic domains), using synchrotron techniques (nano-diffraction, nano-EXAFS).
Physical properties
Many physical properties have yet to be explored, as progress in the development of a wide variety of materials constantly opens new doors. Scientific issues such as excitonic, polaronic and polaritonic effects, the study of phonons, charge carrier dynamics and transport, and spin properties are likely to undergo numerous developments in the future. Understanding them is fundamental to optimizing new materials and imagining new fields of application. The physics of interfaces is also set to gain in importance, as perovskites are now integrated into heterostructures and thin-film architectures of increasing complexity.
The use of theoretical approaches to understanding the physical properties of halide perovskites accompanies the numerous experimental approaches to describing structural, optical and transport properties, thanks to approaches covering several scales, from the material (ab initio approaches), its interfaces to nano-objects (semi-empirical approaches) and complete devices (drift diffusion).
The special feature of this research area is its close interaction with all the other areas, in the form of feedback between fundamental properties/materials/opto-electronic components.
Interfaces
Interface design and modification remains a crucial issue for the development of semiconductor-based devices: it is an essential tool for exploiting the potential of halide perovskite-based optoelectronics, particularly for photovoltaic devices and light-emitting diodes. Indeed, the remarkable improvements in the performance and stability of perovskite solar cells recently observed can be attributed mainly to a careful choice of the nature and implementation of interfaces in the stack.
The studies to be carried out focus on the relationship between microstructure and the physical, chemical and optoelectronic properties of perovskite-based thin films, including their surface properties (chemistry, reactivity, etc.), minimization of non-radiative recombination losses at interfaces, and energy alignment processes at interfaces between perovskite and transport layers. To carry out these studies, characterization methods adapted to studying interfaces will be deployed, such as impedance spectroscopy, spectral photoluminescence possibly time-resolved or imaging, Raman and infrared spectroscopies, microsctructural characterizations such as XRD 1D, 2D and EXAFS (SOLEIL, ESRF), photoemission spectroscopy such as X-ray photoelectron spectroscopy (XPS), hard X-ray photoemission spectroscopy (HAXPES), ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES).
Photovoltaics
The efficiency of hybrid perovskite halide solar cells has certainly been one of the fastest-growing in the history of photovoltaic research. Nevertheless, there is a significant performance gap between small-area cells and modules. A great deal of research and engineering effort is therefore needed to master the crystallization of perovskite materials on large surfaces, and thus improve module performance while increasing size. In addition to efficiency, the stability of halide perovskite-based devices currently represents a major challenge in this field. While efficiency can be defect-tolerant, defects in PSCs participate in a variety of chemical reactions with molecules in the environment leading to degradation. Fundamental research and engineering approaches to understanding and circumventing material/device degradation are therefore the key topics underway.
The main priorities are :
- Ageing studies: Understanding the main degradation mechanisms by coupling local measurements of optoelectronic and chemical/structural properties of the perovskite; implementing solutions to increase device lifetime: e.g. intrinsic stability, encapsulation.
- Integration of perovskites into tandem devices, including the design of photogenerated carrier extraction layers, the recombinant junction and, if required, passivation solutions for the various interfaces.
- Work on deposition techniques potentially applicable on an industrial scale.
Emerging developments
Widely used for photovoltaic energy conversion, halide perovskites are now being considered for a wide range of applications. Some of these developments are already well underway. Halide perovskites are emerging as a serious alternative to III-V semiconductors in the field of photonics: halide perovskite-based light-emitting diodes are approaching the 30% external quantum efficiency threshold, and micro-lasers based on various cavity structures with perovskites as the gain material have been demonstrated. Perovskite nanocrystals could play a crucial role in the development of room-temperature single-photon sources for applications in quantum cryptography and communication. Finally, there are now a significant number of demonstrations of perovskite photodetectors paving the way for new applications such as visible light communications.hybrid or inorganic perovskites are also promising for the detection of ionizing radiation (applications in large-area medical radiography or gamma spectrometry).
Other developments are still in the very early stages: photocatalysis in the broadest sense (fuel cells, CO2 reduction, water-splitting), systems for electrochemical energy storage (batteries, super-capacitors, etc.) and energy recovery (piezoelectric, thermoelectric properties, etc.), components for spintronics, etc.), applications in terahertz electronics (nano-antennas, optical switches, etc.) and sensors, etc. In many application situations, the current hurdles once again concern the demonstration of photochemically stable and non-toxic materials, and the development of efficient forming processes compatible with large surfaces.
