Heiße Ladungsträger in Zinn-Perowskit Solarzellen um Effizienzlimits zu durchbrechen
Zusammenfassung der Projektergebnisse
The synthesis of high quality single crystals of different tin-based perovskites was intended to be the starting point for an in-depth characterisation of their properties, the identification of the best-suited material for hot carrier applications, and subsequently for the optimisation of thin films to demonstrate a successful extraction of hot charge carriers from a tin-based perovskite absorber material. This approach was severely impeded by the difficulty to fabricate single crystals of high quality and appropriate size. Despite intensive collaboration with experts in the field, the obtained samples exhibited only modest material quality and were too small for use in crucial research techniques that would have allowed for an in-depth understanding. The actually performed work thus focused mainly on thin films: their characterisation, optimisation, and application. The importance of the material quality for the observation could be highlighted and several approaches for boosting the performance of tin-based HaP materials were reported. These included efforts to mix tin with strontium or lead to improve the material characteristics or to vary processing protocols for obtaining high quality samples of neat tinbased perovskites. Running through all the key studies is how strongly determined the photophysical properties, in particular the lifetime of hot carriers, are by the underlying material quality. Hot carriers are virtually absent in films cast through protocols commonly employed for lead-based perovskites and special solution additives or processing tricks need to be employed here. We obtained a description of the crystal structure of FASnI3, which was subsequently used for calculations of the electronic band structure and for theoretical studies on carrier dynamics and for evaluating the performance limits of classical solar cells based on tin-based perovskites. Given their narrower band gap, they are often assumed to offer a higher theoretical power conversion efficiency than their lead-based counterparts, but we could show that this is only true in the limit of thick absorber layer. Tin-based solar cells exhibit a significantly weaker absorption and therefore require thicknesses beyond 300 nm to overcome the potential of lead-based devices. This weaker absorption at the onset of its spectrum is furthermore at the core of another surprising observation: tin-based materials are very bright and exhibit a short luminescence lifetime. According to our findings, this is tightly interlined with the much broader onset of the absorption, not found for lead. As an important side-project, the photophysics of two-dimensional perovskites were studied with a particular focus on defect states and carrier phonon interactions, as these were key concepts relevant also for the theory of hot charge carriers. In a surprising event, halide-related defects could be identified to give rise to broad emission bands of 2D perovskites, contradicting the commonly invoked concept of self-trapped excitons. This unexpected finding was one of the fortuitous highlights of the project and was subsequently picked up by several news outlets, including AAAS EurekAlert!, phys.org, or engineersonline. Given the experience with tin-based materials, we addressed an almost complete void in the literature on tin-based two-dimensional perovskites and showed how the defect chemistry varies strongly between lead-based and tinbased 2D compounds, giving rise to the complete absence of broad emission bands in the latter. Two-dimensional perovskites were shown to exhibit high energy luminescence from hot excitons, whose previous models were solely based on observations made for lead-based compounds and could now be extended to their tin-based counterparts. Beyond photophysical studies, these insights into two-dimensional perovskites were used to fabricate light-emitting diodes with luminescence in the blue spectral region, which has remained a challenging field for fabrication of efficient devices.
Projektbezogene Publikationen (Auswahl)
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Cooling, scattering and recombination - the role of the material quality for the physics of tin halide perovskites, Adv. Funct. Mater. 29, 1902963 (2019)
S. Kahmann, S. Shao, M. A. Loi
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Extrinsic nature of the broad photoluminescence in lead iodide-based Ruddlesden Popper perovskites, Nat. Commun. 11, 2344 (2020)
S. Kahmann, E. Tekelenburg, H. Duim, M. E. Kamminga, M. A. Loi
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Mechanism of crystal formation in Ruddlesden-Popper Sn-based perovskites, Adv. Funct. Mater. 30, 2001294 (2020)
J. Dong, S. Shao, S. Kahmann, D. Hermida-Merino, G. H. ten Brink, M. A. Loi, G. Portale
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Negative Thermal Quenching in FASnI3 Perovskite Single Crystals and Thin Films, ACS Energy Lett. 5, 2512 (2020)
S. Kahmann, O. Nazarenko, S. Shao, O. Hordiichuk, M. Kepenekian, J. Even, G. R. Blake, M. V. Kovalenko, M. A. Loi
