Periodic Reporting for period 3 - HELD (Hetero-structures for Efficient Luminescent Devices)
Reporting period: 2022-09-01 to 2024-02-29
Most optoelectronic devices, such as light-emitting diodes (LEDs) and solar cells are based on inorganic semiconducting materials. These crystalline semiconductors suffer from impurities and traps at crystal domain boundaries. Therefore, they need to be prepared as highly pure, ordered crystalline structures which requires expensive processing methods. Metal halide perovskite semiconductors are different as they have a very high defect tolerance. That is, polycrystalline films with multiple crystal domain sizes maintain high carrier mobilities and diffusion lengths. Therefore, they can be processed using simple and economic coating methods such as solution casting and sublimation. These materials are very promising for large area LEDs and solar cells enabling the reduction of electricity consumption and the generation of green electricity. The luminescence properties of metal halide perovskite films, however, are affected by the defects. This reduces the achievable efficiency in the solar cells and LEDs.
It is therefore, important to understand the relation between the perovskite structure and the luminescent properties. HELD’s main objective is the engineering of highly luminescent multiple layered heterostructures of defect tolerant perovskite semiconductors and their integration into highly efficient planar/thin film LEDs, solar cells and lasing devices.
To achieve this overall objective HELD is organized in three blocks: 1) aiming to improve the synthetic routes and processing methods to obtain high quality thin films without the use of harmful solvents. 2) using these novel preparation methods to design and develop highly emissive perovskites. 3) integrate these thin films into planar LEDs, solar cells and lasing devices.
It is therefore, important to understand the relation between the perovskite structure and the luminescent properties. HELD’s main objective is the engineering of highly luminescent multiple layered heterostructures of defect tolerant perovskite semiconductors and their integration into highly efficient planar/thin film LEDs, solar cells and lasing devices.
To achieve this overall objective HELD is organized in three blocks: 1) aiming to improve the synthetic routes and processing methods to obtain high quality thin films without the use of harmful solvents. 2) using these novel preparation methods to design and develop highly emissive perovskites. 3) integrate these thin films into planar LEDs, solar cells and lasing devices.
The work performed in HELD can be grouped around the three blocks: i) improved synthetic and processing methods, ii) application of these methods to the design and development of highly luminescent perovskites and iii) development of planar LEDs, solar cells and lasing devices using the highly emissive perovskite films.
Solvent free preparation of perovskite films is advantageous, as no harmful solvents are needed and it is easy to control the film thickness. Multiple source sublimation of perovskite precursors is the method that is used extensively in this project, however, not all perovskite precursors have a constant sublimation behaviour which is needed to reproducibly prepare the perovskites. This limits the design freedom of the perovskite composition as only a certain number of precursors lead to reproducible perovskite films. Therefore, the first achievement in HELD has been the development of a sublimation method that allows for a much wider range of perovskite precursors to be used. With this achievement, we are able to expand the type of perovskite compositions that can be deposited into thin films. Now that more perovskite precursors are accessible for use in multiple source sublimation it is important to have a method that enables a fast screening of the perovskite thin film properties. For this we have developed a combinatorial approach to perovskite film growth, such that in a single sublimation run, a wide range of perovskite compositions are obtained depending on the substrate position with respect to the sublimation sources.
A large number of compositions were evaluated, allowing to identify promising low-, intermediate- and wide- bandgap perovskites with high photoluminescence quantum yield. At the edge of the perovskite crystal grain the perovskite crystal structure ends, which unavoidably leads to different chemical entities along the crystal edge. By introducing suitable cations, it is possible to ensure that all crystal edges have the same chemical structure. This process is generally referred to as passivation. In one very successful approach we developed perovskite nanoparticles that were passivated with guanidinium bromide. Once deposited in thin films these passivated nanoparticles led to aphotoluminescence efficiency above 90 %.
Using sequential vacuum deposition, we were able to prepare heterostructures consisting of repeating perovskite-organic semiconductor stacks. In these structures amplified spontaneous emission was observed under optical excitation, which is a first important step towards the goal of lasing devices.
The vacuum deposition method, was also used extensively to study benign perovskites, that do not contain Pb. Here we have been very successful in identifying a few innovative materials with good semiconducting and luminescent properties
In this first period we have focussed on the integration of the perovskite films in photovoltaic and light-emitting devices. Essential for applications but also for device characterization is an efficient encapsulation method that is compatible with the sensitive materials constituting the device. We developed a low temperature atomic layer deposition method by which we seal the whole device with a layer of aluminum oxide that completely eliminates the interaction with water and oxygen from the air. Thanks to this sealing, the solar cells were stable for over 2000 hours when kept at 85 C. We also demonstrated that these solar cells withstand irradiation with high energy (MeV) electrons: this is among the tests applied to solar cells to evaluate their suitability for aerospace applications. This work was carried out in collaboration with Airbus Defense and Space.
Solar cells and LEDs need at least one transparent electrode to allow light to enter and escape from the device, respectively. Our very thin film (< 1 micrometer) devices need a substrate on which the active layers are deposited. Most devices employ a transparent substrate such as glass or a plastic foil that contains a transparent conductive oxide. These devices were then finished by vacuum depositing a thin metal electrode on top of the active layers. This configuration has some limitations, especially if driving electronics needs to be integrated, for example when the LEDs are used in display applications. Another configuration in which the transparent electrode is deposited on top of the active layers, is thus often preferred to maximize performance and device integration. In HELD, the semiconductor films are very thin (< 1 µm), which makes it difficult to deposit the most transparent conducting materials without damaging the underlying films and/or generating short-circuits with the bottom electrode. Using pulsed laser deposition (PLD), we have developed a method enabling us to directly deposit transparent conductive oxides on perovskite films without damaging the active layers. This will greatly enhance our capability to integrate the novel semiconductors in the targeted applications.
Last but not least, in collaboration with the group of Prof. Lee from Seoul National University and using films composed of highly luminescent perovskite nanoparticle, we have developed very high efficiency LEDs, exhibiting over 22 % external efficiency (a world record at the time of publication).
Solvent free preparation of perovskite films is advantageous, as no harmful solvents are needed and it is easy to control the film thickness. Multiple source sublimation of perovskite precursors is the method that is used extensively in this project, however, not all perovskite precursors have a constant sublimation behaviour which is needed to reproducibly prepare the perovskites. This limits the design freedom of the perovskite composition as only a certain number of precursors lead to reproducible perovskite films. Therefore, the first achievement in HELD has been the development of a sublimation method that allows for a much wider range of perovskite precursors to be used. With this achievement, we are able to expand the type of perovskite compositions that can be deposited into thin films. Now that more perovskite precursors are accessible for use in multiple source sublimation it is important to have a method that enables a fast screening of the perovskite thin film properties. For this we have developed a combinatorial approach to perovskite film growth, such that in a single sublimation run, a wide range of perovskite compositions are obtained depending on the substrate position with respect to the sublimation sources.
A large number of compositions were evaluated, allowing to identify promising low-, intermediate- and wide- bandgap perovskites with high photoluminescence quantum yield. At the edge of the perovskite crystal grain the perovskite crystal structure ends, which unavoidably leads to different chemical entities along the crystal edge. By introducing suitable cations, it is possible to ensure that all crystal edges have the same chemical structure. This process is generally referred to as passivation. In one very successful approach we developed perovskite nanoparticles that were passivated with guanidinium bromide. Once deposited in thin films these passivated nanoparticles led to aphotoluminescence efficiency above 90 %.
Using sequential vacuum deposition, we were able to prepare heterostructures consisting of repeating perovskite-organic semiconductor stacks. In these structures amplified spontaneous emission was observed under optical excitation, which is a first important step towards the goal of lasing devices.
The vacuum deposition method, was also used extensively to study benign perovskites, that do not contain Pb. Here we have been very successful in identifying a few innovative materials with good semiconducting and luminescent properties
In this first period we have focussed on the integration of the perovskite films in photovoltaic and light-emitting devices. Essential for applications but also for device characterization is an efficient encapsulation method that is compatible with the sensitive materials constituting the device. We developed a low temperature atomic layer deposition method by which we seal the whole device with a layer of aluminum oxide that completely eliminates the interaction with water and oxygen from the air. Thanks to this sealing, the solar cells were stable for over 2000 hours when kept at 85 C. We also demonstrated that these solar cells withstand irradiation with high energy (MeV) electrons: this is among the tests applied to solar cells to evaluate their suitability for aerospace applications. This work was carried out in collaboration with Airbus Defense and Space.
Solar cells and LEDs need at least one transparent electrode to allow light to enter and escape from the device, respectively. Our very thin film (< 1 micrometer) devices need a substrate on which the active layers are deposited. Most devices employ a transparent substrate such as glass or a plastic foil that contains a transparent conductive oxide. These devices were then finished by vacuum depositing a thin metal electrode on top of the active layers. This configuration has some limitations, especially if driving electronics needs to be integrated, for example when the LEDs are used in display applications. Another configuration in which the transparent electrode is deposited on top of the active layers, is thus often preferred to maximize performance and device integration. In HELD, the semiconductor films are very thin (< 1 µm), which makes it difficult to deposit the most transparent conducting materials without damaging the underlying films and/or generating short-circuits with the bottom electrode. Using pulsed laser deposition (PLD), we have developed a method enabling us to directly deposit transparent conductive oxides on perovskite films without damaging the active layers. This will greatly enhance our capability to integrate the novel semiconductors in the targeted applications.
Last but not least, in collaboration with the group of Prof. Lee from Seoul National University and using films composed of highly luminescent perovskite nanoparticle, we have developed very high efficiency LEDs, exhibiting over 22 % external efficiency (a world record at the time of publication).
Most of the results obtained in HELD so far are beyond the current state of the art. Just to mention a few: the novel perovskite processing method, the combinatorial approach to perovskite materials screening, the luminescent multilayer heterostructure of perovskite/organic semiconductor stacks and the damage-free deposition of transparent conductors on top of perovskite and organic semiconductor films.
In the remaining second half of the project we expect to extend upon these results, in particular making use of the novel processes and evaluation schemes to develop even more luminescent perovskites covering a wide range of bandgaps. These will then be integrated in thin film solar cells, LEDs and cavity structure for lasing devices. We will also engage with industrial partners to extend the prospect of perovskite solar cells for space application and in general to advocate for solvent-free industrial processing methods that can be used to prepare a wide range of thin film perovskite compositions.
In the remaining second half of the project we expect to extend upon these results, in particular making use of the novel processes and evaluation schemes to develop even more luminescent perovskites covering a wide range of bandgaps. These will then be integrated in thin film solar cells, LEDs and cavity structure for lasing devices. We will also engage with industrial partners to extend the prospect of perovskite solar cells for space application and in general to advocate for solvent-free industrial processing methods that can be used to prepare a wide range of thin film perovskite compositions.