TY - GEN A1 - Kot, Małgorzata A1 - Gawlińska‐Nęcek, Katarzyna A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - Prospects of improving efficiency and stability of hybrid perovskite solar cells by alumina ultrathin films T2 - Small N2 - Over the last few years, the influence of low temperature (≤80 °C) and, in particular, of room temperature, atomic layer deposited alumina (ALD‐Al2O3) on the properties of the underlying hybrid perovskites of different compositions and on the efficiency and stability of the corresponding perovskite solar cells (PSCs) is extensively investigated. The main conclusion is that most probably thanks to the presence of intrinsic defect states in the ALD‐Al2O3 and in the perovskite layers, charge transfer and neutralization are possible and the entire lifetime of the PSCs is thus improved. Moreover, the migration of mobile ions between the layers is blocked by the ALD‐Al2O3 layer and thus the occurrence of hysteresis in the current density–voltage characteristics of the PSCs is suppressed. Considering the uniform and nondestructive surface coverage, low thermal budget, small amount of material required, and short duration of the established ALD‐Al2O3 deposition on top of hybrid perovskites, this additional, but fully solar cell technology‐compatible, process step is most likely the most effective, cheapest, and fastest way to improve the efficiency and long‐term stability of PSCs and thus increase their marketability. KW - Perovskite solar cells (PSC) KW - Atomic layer deposition (ALD) KW - Photoelecton spectroscopy (PES) KW - Aluminum oxide Y1 - 2025 U6 - https://doi.org/10.1002/smll.202408435 SN - 1613-6810 VL - 21 IS - 12 PB - Wiley ER - TY - GEN A1 - Kot, Małgorzata A1 - Gawlińska‐Nęcek, Katarzyna A1 - Pożarowska, Emilia A1 - Henkel, Karsten A1 - Schmeißer, Dieter T1 - Photosensitivity and carrier densities of perovskite solar absorbers T2 - Advanced science N2 - Dark and light current–voltage characteristics of perovskite solar absorbers are analyzed in terms of their carrier densities. The analysis reveals p‐type large polarons as a dominant carrier type in the investigated perovskite solar cells. The mechanism causing photosensitivity is attributed to the dissociation (and pairing) of bipolarons to large polarons (and vice versa) that are controlled by the internal potential Γ. As an example, the polaron concept is tested for a formamidinium lead triiodide perovskite solar cell. The individual steps of the data analysis are demonstrated and determine the ionicity factor of this perovskite film, quantify the density of the large polarons, and predict the gain and loss of photo‐induced carriers. It is deduced that a reversible light‐on/off operation can only occur when the bias voltage never exceeds a critical value of the internal potential. The results gained in this study suggest that the novel analysis can be successively applied on different hybrid perovskite materials, too. KW - Bipolarons KW - Ionicity factor KW - Large polarons KW - Perovskite solar cells Y1 - 2025 UR - https://advanced.onlinelibrary.wiley.com/doi/epdf/10.1002/advs.202412711 U6 - https://doi.org/10.1002/advs.202412711 SN - 2198-3844 VL - 12 IS - 16 SP - 1 EP - 8 PB - Wiley CY - Hobken, New Jersey ER - TY - GEN A1 - Morales, Carlos A1 - Gertig, Max A1 - Kot, Małgorzata A1 - Alvarado, Carlos A1 - Schubert, Markus Andreas A1 - Zoellner, Marvin Hartwig A1 - Wenger, Christian A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - In situ X‐ray photoelectron spectroscopy study of atomic layer deposited cerium oxide on SiO₂ : substrate influence on the reaction mechanism during the early stages of growth T2 - Advanced materials interfaces N2 - Thermal atomic layer deposition (ALD) of cerium oxide using commercial Ce(thd)4 precursor and O3 on SiO2 substrates is studied employing in‐situ X‐ray photoelectron spectroscopy (XPS). The system presents a complex growth behavior determined by the change in the reaction mechanism when the precursor interacts with the substrate or the cerium oxide surface. During the first growth stage, non‐ALD side reactions promoted by the substrate affect the growth per cycle, the amount of carbon residue on the surface, and the oxidation degree of cerium oxide. On the contrary, the second growth stage is characterized by a constant growth per cycle in good agreement with the literature, low carbon residues, and almost fully oxidized cerium oxide films. This distinction between two growth regimes is not unique to the CeOx/SiO2 system but can be generalized to other metal oxide substrates. Furthermore, the film growth deviates from the ideal layer‐by‐layer mode, forming micrometric inhomogeneous and defective flakes that eventually coalesce for deposit thicknesses above 10 nm. The ALD‐cerium oxide films present less order and a higher density of defects than films grown by physical vapor deposition techniques, likely affecting their reactivity in oxidizing and reducing conditions. KW - Atomic Layer Deposition (ALD) KW - Cerium oxide KW - In-situ X-ray Photoelectron spectroscopy (in-situ XPS) KW - Growth model KW - Substrate influence Y1 - 2025 U6 - https://doi.org/10.1002/admi.202400537 SN - 2196-7350 VL - 12 IS - 5 SP - 1 EP - 13 PB - Wiley CY - Weinheim ER - TY - GEN A1 - Morales, Carlos A1 - Tschammer, Rudi A1 - Pożarowska, Emilia A1 - Kosto, Julia A1 - Villar‐Garcia, Ignacio J. A1 - Pérez‐Dieste, Virginia A1 - Favaro, Marco A1 - Starr, David E. A1 - Kapuścik, Paulina A1 - Mazur, Michał A1 - Wojcieszak, Damian A1 - Domaradzki, Jarosław A1 - Alvarado, Carlos A1 - Wenger, Christian A1 - Henkel, Karsten A1 - Flege, Jan Ingo T1 - Hydrogen sensing via heterolytic H₂ activation at room temperature by atomic layer deposited ceria T2 - ChemSusChem : chemistry, sustainability, energy, materials N2 - Ultrathin atomic layer deposited ceria films (<20 nm) are capable of H2 heterolytic activation at room temperature, undergoing a significant reduction regardless of the absolute pressure, as measured under in‐situ conditions by near ambient pressure X‐ray photoelectron spectroscopy. ALD‐ceria can gradually reduce as a function of H2 concentration under H2/O2 environments, especially for diluted mixtures below 10 %. At room temperature, this reduction is limited to the surface region, where the hydroxylation of the ceria surface induces a charge transfer towards the ceria matrix, reducing Ce4+ cations to Ce3+. Thus, ALD‐ceria replicates the expected sensing mechanism of metal oxides at low temperatures without using any noble metal decorating the oxide surface to enhance H2 dissociation. The intrinsic defects of the ALD deposit seem to play a crucial role since the post‐annealing process capable of healing these defects leads to decreased film reactivity. The sensing behavior was successfully demonstrated in sensor test structures by resistance changes towards low concentrations of H2 at low operating temperatures without using noble metals. These promising results call for combining ALD‐ceria with more conductive metal oxides, taking advantage of the charge transfer at the interface and thus modifying the depletion layer formed at the heterojunction. KW - Atomic Layer Deposition KW - Ceria KW - Hydrogen Sensing KW - X-Ray photoelectron spectroscopy KW - Raman spectroscopy KW - Resitive sensor Y1 - 2025 U6 - https://doi.org/10.1002/cssc.202402342 SN - 1864-5631 VL - 18 IS - 13 SP - 1 EP - 13 PB - Wiley-VCH CY - Weinheim ER - TY - GEN A1 - Morales, Carlos A1 - Tschammer, Rudi A1 - Guttmann, Dominic A1 - Chavarin, Carlos Alvarado A1 - Ruffert, Christine A1 - Henkel, Karsten A1 - Wenger, Christian A1 - Flege, Jan Ingo T1 - Bottom-up strategy to develop ultrathin active layers by atomic layer deposition for room temperature hydrogen sensors compatible with CMOS technology T2 - MikroSystemTechnik Kongress 2025 : Mikroelektronik, Mikrosystemtechnik und ihre Anwendungen - Nachhaltigkeit und Technologiesouveränität : proceedings : 27.-29. Oktober 2025, Duisburg Y1 - 2025 SN - 978-3-8007-6614-7 SN - 978-3-8007-6615-4 SP - 71ff. PB - VDE VERLAG GmbH CY - Berlin ER - TY - GEN A1 - Kao, Ming-Chao A1 - Schewe, Lukas Paul A1 - Akhtar, Arub A1 - Vlad, Alina A1 - Keller, Thomas F. A1 - Henkel, Karsten A1 - Anooz, Saud Bin A1 - Popp, Andreas A1 - Galazka, Zbigniew A1 - Flege, Jan Ingo A1 - Stierle, Andreas A1 - Vonk, Vedran T1 - Surface termination of β-Ga₂O₃(100) as-cleaved single crystals T2 - Applied physics letters N2 - The surface of β-Ga₂O₃ single crystals cleaved along their (100) plane is investigated using surface x-ray diffraction and atomic force microscopy. The results show the surface to consist of a single, so-called B-termination, which means that the crystal cleaves at planes formed by edge-sharing oxygen octahedra, thereby breaking the longest and weakest Ga–O bonds. Refinement of the atomic positions results in small displacements from the bulk structure, at most approximately 0.01 Å. Atomic force microscopy suggests that relatively large terraces form together with steps of half the a-axis length of approximately 0.6 nm, which means that terraces have the same atomic termination, related by the crystal symmetry. These results are important as a fundamental property of β-Ga₂O₃ when processed or used in various semiconductor applications. KW - X-ray KW - Gallium oxide Y1 - 2026 U6 - https://doi.org/10.1063/5.0309937 SN - 0003-6951 VL - 128 IS - 7 SP - 1 EP - 5 PB - AIP Publishing CY - Melville, NY ER -