(Invited) Processing of Perovskite Materials for Optoelectronic Devices
2023-12-22

Among materials used for thin film technologies, perovskite materials have emerged as one of the rising stars for optoelectronic applications. They show great promise for several applications like e.g. photovoltaics, lasers, light emitting diodes or X-ray detectors. Their success is founded on the feature of some perovskite materials to be largely tolerant to the formation of defects, which enables functional layer deposition from liquid precursor inks. In order to exploit the extraordinary optoelectronic properties, it is necessary to adapt the material processing to the requirements of the application. For a large spectrum of devices, homogeneous, high quality, large grain poly-crystalline perovskite layers are required. Accordingly, our work strives to cover the whole spectrum of the layer formation ranging from the perovskite precursor ink chemistry, the deposition process to post-processing and re-crystallization. Regarding the precursor ink, we spotlight the complex formation of the lead halide species by nuclear magnetic resonance and electrical conductivity measurements. We found a strong and systematic impact of the choice of (Lewis base) solvent on the formation of lead complexes in the precursor solution. Surprisingly, a strong difference in the complex formation, not necessarily translates to notable differences in the resulting layers. Indeed, the difference observed in solvation and coordination appears to become less for increased concentrations of the precursor ink. As such we assume, that throughout the drying process, the impact of lead-solvent complexes observed in the ink might diminish. To add another parameter, we also included Lewis-base additives into our investigations, that evidently modify the shape and size of the crystallites in the resulting layers. Despite their far higher donor number, we found the additive to only marginally impact on the lead core in the liquid precursor. Rather, by in-situ GIWAXS characterization, we could show that the intake of thermal energy during the annealing process proves crutial for changes in the crystal formation. After a gas-quenching process, where the perovskite phase is already present, the formation process with and without additive did not show notable differences. Subsequent thermal annealing of the deposited layers on the other hand allows the coordination between lead and the additive and affords a re-organization of the perovskite phase. During the deposition process, anyhow, the process technology (like gas quenching) is still important to form a homogeneous and pinhole-free initial film, which then can reorganize to form a high-quality perovskite layer. In the second part of the presentation, we will show, that the strategy of thermally assisted reorganization is not limited to the initial annealing step. By using a thermal hot-pressing process, we are also able to trigger the perovskite re-organization and re-crystallization even if the perovskite deposition and formation process is already finished. We performed post-processing by providing thermal energy, while simultaneously limiting the available space where potential perovskite decomposition products could leak, by pressing it with a flat stamp. As a result, the perovskite recrystallizes and forms extremely smooth layers that comprise large grains and express remarkable optical quality. We show this strategy to be applicable to a large variety of different hybrid as well as all-inorganic perovskite materials. Employing patterned stamps, we also utilize this strategy to directly imprint photonic nanostructures (e.g. Bragg gratings) into the perovskite layers. We will show how additive driven reorganization and thermal imprint enable a variety of optoelectronic devices ranging from light harvesting (single-junction and tandem perovskite based solar cells) to stimulated light emission (perovskite lasers).