Periodic Reporting for period 4 - HELD (Hetero-structures for Efficient Luminescent Devices)
Reporting period: 2024-03-01 to 2024-08-31
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.
Two novel solvent-free deposition methods have been developed that lead to high quality perovskite thin films and are compatible with large scale industrial deposition speeds. Luminescent perovskite films have been obtained using the vacuum based deposition methods with a wide range of compositions including lead free varieties. These films have been used to prepare efficient solar cells (with power conversion efficiencies reaching 21.8 %) and LEDs (reaching external quantum efficiencies of 23 %). We furthermore achieved optically pumped lasing in multilayer perovskite films.
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.
Two novel solvent-free deposition methods have been developed that lead to high quality perovskite thin films and are compatible with large scale industrial deposition speeds. Luminescent perovskite films have been obtained using the vacuum based deposition methods with a wide range of compositions including lead free varieties. These films have been used to prepare efficient solar cells (with power conversion efficiencies reaching 21.8 %) and LEDs (reaching external quantum efficiencies of 23 %). We furthermore achieved optically pumped lasing in multilayer perovskite films.
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.
Vacuum based preparation of perovskite films is advantageous, as no harmful solvents are needed and it is easy to control the film thickness. However, not all perovskite precursors have a constant sublimation behaviour which is needed to reproducibly prepare the perovskites. 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 allowing to expand the type of perovskite compositions that can be vacuum deposited. The second achievement is 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. By introducing suitable cations, it is possible to ensure that most crystal edges have the same chemical structure. 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 a photoluminescence efficiency above 90 % and integrated into LEDs lead to 23 % external quantum efficiency (a world record at the time of publication).
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 practical deposition speed of co-sublimed hybrid organic-inorganic perovskites was determined to be around 40 nm/min. As this is most likely not sufficient for large scale production we then investigated another method called close space sublimation (CSS). Using a home build system we achieved very promising solar cells exhibiting power conversion efficiencies of 18 %. This method is used in the proof of concept project APERITIF to further optimize and increase its market opportunities. Besides scientific publications we have disseminated this result to industrial parties who have expressed interest.
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
The perovskite films were incorporated in photovoltaic and light-emitting devices. An efficient encapsulation method that is compatible with the sensitive materials constituting the device is important. 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: indicative that they may be suitable for aerospace applications. This has led to a bilateral research project with Airbus Defense and Space with the aim to prepare cells and evaluate them in space using mini-satellites.
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. 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 thin active layers make it difficult to deposit 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 has led to the development of efficient bifacial and semitransparent perovskite solar that can be used in power plants and building integrated photovoltaics, respectively.
Vacuum based preparation of perovskite films is advantageous, as no harmful solvents are needed and it is easy to control the film thickness. However, not all perovskite precursors have a constant sublimation behaviour which is needed to reproducibly prepare the perovskites. 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 allowing to expand the type of perovskite compositions that can be vacuum deposited. The second achievement is 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. By introducing suitable cations, it is possible to ensure that most crystal edges have the same chemical structure. 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 a photoluminescence efficiency above 90 % and integrated into LEDs lead to 23 % external quantum efficiency (a world record at the time of publication).
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 practical deposition speed of co-sublimed hybrid organic-inorganic perovskites was determined to be around 40 nm/min. As this is most likely not sufficient for large scale production we then investigated another method called close space sublimation (CSS). Using a home build system we achieved very promising solar cells exhibiting power conversion efficiencies of 18 %. This method is used in the proof of concept project APERITIF to further optimize and increase its market opportunities. Besides scientific publications we have disseminated this result to industrial parties who have expressed interest.
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
The perovskite films were incorporated in photovoltaic and light-emitting devices. An efficient encapsulation method that is compatible with the sensitive materials constituting the device is important. 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: indicative that they may be suitable for aerospace applications. This has led to a bilateral research project with Airbus Defense and Space with the aim to prepare cells and evaluate them in space using mini-satellites.
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. 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 thin active layers make it difficult to deposit 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 has led to the development of efficient bifacial and semitransparent perovskite solar that can be used in power plants and building integrated photovoltaics, respectively.
Most of the results obtained in HELD are beyond the current state of the art. All perovskite films are prepared without solvent which is important in view of production, as the coordinating solvent used to dissolve the precursors are either banned for production in the European Union, or costly (and energy intensive) to remove from the films.
I will mention the in my opinion most important achievement that are beyond the state of the art.
• The close-space sublimation approach leading to high quality perovskite films and solar cells.
• The luminescent multilayer heterostructure of perovskite/organic semiconductor stacks.
• Passivation of perovskite films using molecular additives leading to photoluminescence efficiency above 80%.
• The damage-free deposition of transparent conductors on top of perovskite and organic semiconductor films.
• Perovskite based LEDs with external quantum efficiency of 23 %.
I will mention the in my opinion most important achievement that are beyond the state of the art.
• The close-space sublimation approach leading to high quality perovskite films and solar cells.
• The luminescent multilayer heterostructure of perovskite/organic semiconductor stacks.
• Passivation of perovskite films using molecular additives leading to photoluminescence efficiency above 80%.
• The damage-free deposition of transparent conductors on top of perovskite and organic semiconductor films.
• Perovskite based LEDs with external quantum efficiency of 23 %.