Last Combinatorial Screening of perovskite composition via sublimation
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.
Schematic of the combinatorial approach to rapidly evaluate perovskite compositions in thin films
First perovskite-organic semiconductor heterostructures
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.
SEM cross-section image and sketch of a perovskite-organic semiconductor heterostructure
Soft deposition of transparent conductors using pulsed laser deposition
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 an industrial pulsed laser deposition (PLD) tool from SOLMATES BV, 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
Pulsed Laser Deposition. A. schematic of its functioning, a high energy laser pulse hits the target of the material to be deposited, which then forms a plasma and reaches the substrate above it. B Image of the tool, inset shows the plasma formed after the laser pulse hits the indium tin oxide target.
Bifacial-perovskite based solar cells where the top transparent electrode (ITO) was deposited by PLD. A: Schematic of the device architecture and the chemical structures of the used organic semiconductors. B: Image of our test substrate with 16 individual pixels. C: current density versus voltage characteristics of the cells under simulated sunlight (AM 1.5) and compared with a reference device employing a metallic Ag top electrode.