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.
