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Incorporating a low dimensional (LD) perovskite capping layer on top of a perovskite absorber, improves the stability of perovskite solar cells (PSCs). However, in the case of mixed-halide perovskites, which can undergo halide segregation into single-halide perovskites, a systematic study of the capping layer's effect on mixed-halide perovskite absorber is still lacking. This study bridges this gap by investigating how the 1D perovskite capping layers on top of MAPb(IxBr1−x)3 (x = 0, 0.25, 0.5, 0.75, 1) absorbers affect the films' stability. We utilize a new method, dissimilarity matrix, to investigate the image-based stability performance of capping-absorber pair compositions across time. This method overcomes the challenge of analyzing various film colors due to bandgap difference in mixed-halide perovskites. We also discover that the intrinsic absorber stability plays an important role in the overall stability outcome, despite the capping layer's support. Within the 55 unique capping-absorber pairs, we observe a notable 1D perovskite material, 1-methoxynaphthalene-2-ethylammonium chloride (2MeO–NEA–Cl or 9-Cl), that improves the stability of MAPbI3 and MAPb(I0.5Br0.5)3 by at least 8 and 1.5 times, respectively, compared to bare films under elevated humidity and temperature. Surface photovoltage results also show that the accumulation of electrostatic charges on the film surface depends on the capping layer type, which could contribute to the acceleration/deceleration of degradation.

Singlet fission can split a high energy singlet exciton and generate two lower energy triplet excitons. This process has shown near 200 percent triplet exciton yield. Sensitizing solar cells with singlet fission material, it can potentially increase the power conversion efficiency limit from 29 percent to 35 percent. Singlet fission in the tetracene is known to be efficient, and the energy of the triplet excitons are energetically matched to the silicon bandgap. In this work, we designed an optical measurement with an external magnetic field to determine the efficiencies of triplet exciton transfer from tetracene to silicon. Using this method, we have found that a passivation layer of 8 angstroms of hafnium oxynitride on silicon allows efficient triplet exciton transfer around 133 percent.

The role of the alkali metal cations in halide perovskite solar cells is not well understood. Using synchrotron-based nano-x-ray fluorescence and complementary measurements, we found that the halide distribution becomes homogenized upon addition of cesium iodide, either alone or with rubidium iodide, for substoichiometric, stoichiometric, and overstoichiometric preparations, where the lead halide is varied with respect to organic halide precursors. Halide homogenization coincides with long-lived charge carrier decays, spatially homogeneous carrier dynamics (as visualized by ultrafast microscopy), and improved photovoltaic device performance. We found that rubidium and potassium phase-segregate in highly concentrated clusters. Alkali metals are beneficial at low concentrations, where they homogenize the halide distribution, but at higher concentrations, they form recombination-active second-phase clusters.

The role of the alkali metal cations in halide perovskite solar cells is not well understood. Using synchrotron-based nano-x-ray fluorescence and complementary measurements, we found that the halide distribution becomes homogenized upon addition of cesium iodide, either alone or with rubidium iodide, for substoichiometric, stoichiometric, and overstoichiometric preparations, where the lead halide is varied with respect to organic halide precursors. Halide homogenization coincides with long-lived charge carrier decays, spatially homogeneous carrier dynamics (as visualized by ultrafast microscopy), and improved photovoltaic device performance. We found that rubidium and potassium phase-segregate in highly concentrated clusters. Alkali metals are beneficial at low concentrations, where they homogenize the halide distribution, but at higher concentrations, they form recombination-active second-phase clusters.

Lead halide-based perovskite thin films have attracted great attention due to the rapid increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplet-triplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross-sections throughout the visible spectrum, and the strong spin-orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the passivating ligands required for colloidal sensitizers, which can hinder exciton transport through large scale arrays and reduce the triplet transfer efficiency to the annihilator. Our bilayer device exhibits an upconversion efficiency in excess of 3% under 785 nm illumination at an incident power of ∼88 W/cm[superscript 2]. Keywords: lead halide perovskites; upconversion; free carriers; triplet excitons; charge-separated state; triplet-charge annihilation; triplet-triplet annihilation, National Science Foundation (U.S.) (Contract EEC-1041895), National Science Foundation (NSF) (Grant EEC-1041895), National Science Foundation Graduate Research Fellowship (Grant 1122374), US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (Award DE-FG02-07ER46454)

Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly concentrated aggregates. Synchrotron-based X-ray-beam-induced current and electron-beam-induced current of solar cells show that Rb clusters do not contribute to the current and are recombination active. Our findings bring light to the beneficial effects of alkali metal halides in perovskites, and point at areas of weakness in the elemental composition of these complex perovskites, paving the way to improved performance in this rapidly growing family of materials for solar cell applications.

Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly concentrated aggregates. Synchrotron-based X-ray-beam-induced current and electron-beam-induced current of solar cells show that Rb clusters do not contribute to the current and are recombination active. Our findings bring light to the beneficial effects of alkali metal halides in perovskites, and point at areas of weakness in the elemental composition of these complex perovskites, paving the way to improved performance in this rapidly growing family of materials for solar cell applications.

Impurity contamination in thin-film solar cells remains an uncertain risk due to the little-known impact of impurities on recombination. Building upon previous work, in which we intentionally contaminated lead halide perovskite (LHP) solar cells with iron, we further examine the distribution and charge state of iron-induced defects in LHP films using synchrotron-based X-ray techniques. X-ray absorption measurements suggest that iron-rich regions, which form among iron feedstock concentrations that exceed 100 ppm, most closely resemble the chemistry of Fe2O3. Iron distributed within the bulk may form a mix of Fe2+and Fe3+, the latter of which is not expected to be recombination active, potentially allowing LHPs to incorporate more iron than traditional semiconductors. X-ray beam induced current measurements show little correlation between the presence of iron-rich regions and charge collection, which further suggests low recombination activity at these sites. These results further elucidate the recombination behavior caused by iron incorporation in LHP films, revealing insight into how inhomogeneous incorporation of impurities may mitigate photovoltaic performance degradation., National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR-1419807), United States. Department of Energy. Office of Science User Facility (Contract No. DE-AC02-06CH11357), National Science Foundation (U.S.) (NSF EECS Award No. 1541959), Martin Family Society of Fellows for Sustainability, National Science Foundation (U.S.). Graduate Research Fellowship (Grant No. 1122374), National Science Foundation (U.S.). (CA No. EEC-1041895)

Supported nanostructures on novel support materials attract great attention in the development of tunable conversion and storage systems for renewable energy vectors. Their great potential is related to a high material efficiency and properties that can be tuned by size, shape and interaction with support and solvents. However, advanced preparation methods are required to overcome limitations regarding the ordering of the active layer as well as its stability. In this thesis, several approaches are explored to generate stable functional nanostructures at the solid-liquid interface: i) Bottom-up self-assembly of hydrogen-bonded, porous mono- and bilayer networks for cluster and nanoparticle trapping; ii) Implementation of all-carbon, transparent films that support ordered, photoabsorbing organic monolayer networks with photovoltaic response; iii) Immobilization of Pt clusters on engineered, transparent Au films for indirect plasmonic activation; iv) Immobilization of wet-chemically synthesized Pt nanoparticles on selfassembled thiol monolayers on Au for electrocatalysis with tunable decoupling from the substrate. All these nanostructures were studied in a twofold way - on the atomic scale by scanning tunneling microscopy techniques, as well as on the macroscopic scale by investigating the photovoltaic, photocatalytic or electrocatalytic activity. In this way, a sustainable platform for fundamental studies of supported nanostructures at the solid-liquid interface could be established that links mechanistic insight to performance., Nanostrukturierte Materialien, geträgert auf neuartigen Substraten, sind interessant für die Entwicklung von Speicher- und Energieumwandlungssyteme für erneuerbare Energietechnologie. Ihr großes Potential ist der hohen Effizienz der Materialien, sowie deren Eigenschaften geschuldet, welche durch Größe, Struktur und Wechselwirkung mit Trägermaterialien und Lösungsmitteln optimiert werden können. Allerdings erfordern diese Materialien neuartige Herstellungsmethoden, um die Limitierung, welche durch die Ordnung von aktiven Schichten und deren Stabilität entsteht, zu überwinden. In dieser Arbeit werden verschiedene Herangehensweisen zur Herstellung von stabilen, funktionalen Nanostrukturen an der fest-flüssig Grenzfläche getestet: i) Selbstorganisation von porösen, ein- und zweilagigen Netzwerken, die durch Wasserstoffbrückenbindungen stabilisiert werden, zur Verankerung von Clustern und Nanopartikeln; ii) Implementierung transparenter Filme auf Kohlenstoffbasis auf denen geordnete, einlagige organische Netzwerke mit photovoltaischen Eigenschaften aufgebracht werden können; iii) Immobilisierung von Pt Clustern auf transparenten Goldfilmen für indirekte plasmonische Aktivierung; iv) Immobiliserung von nass-chemisch hergestellten Pt Nanopartikeln auf selbstorganisierenden Thiol-Schichten auf Gold für Elektrokatalyse mit systematischer Entkopplung vom Trägermaterial. Alle nanostrukturierten Systeme wurden auf der atomaren Skala mit Hilfe von Rastertunnelmikroskopie sowie auf makroskopischer Skala charakterisiert, wobei die photovoltaische, photokatalytische oder elektrokatalytische Aktivität untersucht wurde. So wurde ein Modellsystem für Grundlagenstudien an der fest-flüssig Grenzfläche von geträgerten Nanomaterialien geschaffen, mit dessen Hilfe mechanistische Informationen mit katalytischer Aktivität korreliert warden können.

Supported nanostructures on novel support materials attract great attention in the development of tunable conversion and storage systems for renewable energy vectors. Their great potential is related to a high material efficiency and properties that can be tuned by size, shape and interaction with support and solvents. However, advanced preparation methods are required to overcome limitations regarding the ordering of the active layer as well as its stability. In this thesis, several approaches are explored to generate stable functional nanostructures at the solid-liquid interface: i) Bottom-up self-assembly of hydrogen-bonded, porous mono- and bilayer networks for cluster and nanoparticle trapping; ii) Implementation of all-carbon, transparent films that support ordered, photoabsorbing organic monolayer networks with photovoltaic response; iii) Immobilization of Pt clusters on engineered, transparent Au films for indirect plasmonic activation; iv) Immobilization of wet-chemically synthesized Pt nanoparticles on selfassembled thiol monolayers on Au for electrocatalysis with tunable decoupling from the substrate. All these nanostructures were studied in a twofold way - on the atomic scale by scanning tunneling microscopy techniques, as well as on the macroscopic scale by investigating the photovoltaic, photocatalytic or electrocatalytic activity. In this way, a sustainable platform for fundamental studies of supported nanostructures at the solid-liquid interface could be established that links mechanistic insight to performance., Nanostrukturierte Materialien, geträgert auf neuartigen Substraten, sind interessant für die Entwicklung von Speicher- und Energieumwandlungssyteme für erneuerbare Energietechnologie. Ihr großes Potential ist der hohen Effizienz der Materialien, sowie deren Eigenschaften geschuldet, welche durch Größe, Struktur und Wechselwirkung mit Trägermaterialien und Lösungsmitteln optimiert werden können. Allerdings erfordern diese Materialien neuartige Herstellungsmethoden, um die Limitierung, welche durch die Ordnung von aktiven Schichten und deren Stabilität entsteht, zu überwinden. In dieser Arbeit werden verschiedene Herangehensweisen zur Herstellung von stabilen, funktionalen Nanostrukturen an der fest-flüssig Grenzfläche getestet: i) Selbstorganisation von porösen, ein- und zweilagigen Netzwerken, die durch Wasserstoffbrückenbindungen stabilisiert werden, zur Verankerung von Clustern und Nanopartikeln; ii) Implementierung transparenter Filme auf Kohlenstoffbasis auf denen geordnete, einlagige organische Netzwerke mit photovoltaischen Eigenschaften aufgebracht werden können; iii) Immobilisierung von Pt Clustern auf transparenten Goldfilmen für indirekte plasmonische Aktivierung; iv) Immobiliserung von nass-chemisch hergestellten Pt Nanopartikeln auf selbstorganisierenden Thiol-Schichten auf Gold für Elektrokatalyse mit systematischer Entkopplung vom Trägermaterial. Alle nanostrukturierten Systeme wurden auf der atomaren Skala mit Hilfe von Rastertunnelmikroskopie sowie auf makroskopischer Skala charakterisiert, wobei die photovoltaische, photokatalytische oder elektrokatalytische Aktivität untersucht wurde. So wurde ein Modellsystem für Grundlagenstudien an der fest-flüssig Grenzfläche von geträgerten Nanomaterialien geschaffen, mit dessen Hilfe mechanistische Informationen mit katalytischer Aktivität korreliert warden können.