Finding new semi-conductor materials that emit light is essential for developing a wide range of electronic devices. But making artificial structures that emit light tailored to our specific needs is an even more attractive proposition. However, light emission in a semi-conductor only occurs when certain conditions are met.
Researchers from the University of Geneva (UNIGE), Switzerland, in collaboration with the University of Manchester, have discovered an entire class of two-dimensional materials that are the thickness of one or a few atoms. When combined together, these atomically thin crystals are capable of forming structures that emit customisable light in the desired colour.
This research, published in the journal Nature Materials (“Design of van der Waals Interfaces for Broad-Spectrum Optoelectronics”), marks an important step towards the future industrialisation of two-dimensional materials.
Semi-conductor materials capable of emitting light are used in sectors as diverse as telecommunications, light emitting devices (LEDs) and medical diagnostics. Light emission occurs when an electron jumps inside the semi-conductor from a higher energy level to a lower level. It is the difference in energy that determines the colour of the emitted light.
For light to be produced, the velocity of the electron before and after the jump must be exactly the same, a condition that depends on the specific semiconducting material considered. Only some semi-conductors can be used for light emission: for example, silicon – used to make our computers – cannot be employed for manufacturing LEDs.
“We asked ourselves whether two-dimensional materials could be used to make structures that emit light with the desired colour”, explains Alberto Morpurgo, a professor in the Department of Quantum Matter Physics, at the UNIGE Faculty of Science.
Two-dimensional materials are perfect crystals which, like graphene, are one or a few atoms thick. Thanks to recent technical advances, different two-dimensional materials can be stacked on top of each other to form artificial structures that behave like semi-conductors. The advantage of these “artificial semi-conductors” is that the energy levels can be controlled by selecting the chemical composition and thickness of the materials that make up the structure.
“Artificial semi-conductors of this kind were made for the first time only two or three years ago”, explains Nicolas Ubrig, a researcher in the team led by professor Morpurgo. “When the two-dimensional materials have exactly the same structure and their crystals are perfectly aligned, this type of artificial semi-conductor can emit light. But it’s very rare.” These conditions are so strict that they leave little freedom to control the light emitted.
Custom light
“Our objective was to manage to combine different two-dimensional materials to emit light while being free from all constraints”, continues professor Morpurgo.
The physicists thought that, if they could find a class of materials where the velocity of the electrons before and after the change in energy level was zero, it would be an ideal scenario which would always meet the conditions for light emission, regardless of the details of the crystal lattices and their relative orientation.
A large number of known two-dimensional semi-conductors have a zero-electron velocity in the relevant energy levels. Thanks to this diversity of compounds, many different materials can be combined, and each combination is a new artificial semi-conductor emitting light of a specific colour.
“Once we had the idea, it was easy to find the materials to use to implement it”, adds professor Vladimir Fal’ko from the University of Manchester.
Materials that were used in the research included various transition metal dichalcogenides (such as MoS2, MoSe2 and WS2) and InSe. Other possible materials have been identified and will be useful for widening the range of colours of the light emitted by these new artificial semi-conductors.
Tailor-made light for mass industrialisation
“The great advantage of these 2D materials, thanks to the fact that there are no more preconditions for the emission of light, is that they provide new strategies for manipulating the light as we see fit, with the energy and colour that we want to have”, continues Ubrig.
This means it is possible to devise future applications on an industrial level, since the emitted light is robust and there is no longer any need to worry about the alignment of atoms.
Source: Université de Genève
The new method of making mixed halide-perovskites results in solar cells with improved stability and performance. The new method results in better control over perovskite crystallization rates. This means the crystal structure is more ordered, in part due to researchers understanding and taking advantage of the faster crystallization of bromide relative to iodide.
The result is a material with fewer defects and less halide migration and thus less segregation of the bromide and iodide. This in turn means uniform mixing of bromide and iodide across the material, which allows the material to absorb light evenly. The end result is that solar cells made using the new method will perform better under real-world conditions.
Typical halide perovskite solution deposition uses an anti-solvent drip procedure to initiate crystallization of the halide film. The standard anti-solvent method for producing bromide-iodide mixed halide perovskite films often leads to excessive defect formation (e.g., bromide vacancies) owing to the rapid crystallization of bromide vs. iodide-perovskite phases. Simulations show that halide migration is enhanced in the presence of a large population of halide vacancies. This limits the stability of bromide-iodide mixed halide perovskites under light and heat.
In comparison to the anti-solvent approach, the gentler gas-quench method better controls crystallization, first producing a bromide-rich surface layer that then induces top-down columnar growth to form a gradient structure with less bromide in the bulk than in the surface region. The anti-solvent method does not produce such a gradient structure.
In this study, researchers from the National Renewable Energy Laboratory, the University of Toledo, and the University of Colorado Boulder demonstrated that the gas-quench method also produces fewer bromide vacancies and results in materials with a higher quality opto-electronic performance. Solar cells made using the gas-quench method retain desirable light absorption properties and provide enhanced performance in the form of a higher charge carrier mobility, higher open circuit voltage, and enhanced stability.
