Functional surfactants as energy valves in gradient structures of organic inorganic perovskite structures

Hybrid perovskites are considered one of the most promising semiconductor materials of our time. Their ionic composition enables low-cost and simple production at low temperatures, making them a highly demanded semiconductor for photovoltaics, but also optoelectronic applications such as LEDs, lasers or photodiodes. Their hybrid nature enables the integration of organic cations, which brings a wide range of possible materials. The classical perovskite structure permits the incorporation of small organic cations. If the given space in the structure is exceeded by the organic molecule, a layered crystalline phase with alternating arrangement of organic cations and inorganic 2D lead bromide layers is formed. This allows the use of a great variety of organic cations which become an integral part of the semiconducting material. Thus, molecules can be chosen that contribute to the functionality of the crystalline phase, for example, by using conductive conjugated π systems. Energy transfer between the components of the layered phases becomes possible and extraordinary electronic and optical properties can result. The aim of this thesis was the development of functional surfactants for the synthesis of hybrid lead halide perovskite particles with a special switchable feature. The switching was intended to introduce an energy valve into the phases, which could be switched by external stimuli (i.e. light or chemically). The obtained phases should be investigated for structural, optical and electronic properties, both before and after a switching of the ligands. For this purpose, ligands based on photoswitchable azobenzene, redox active ferrocene and conductive five-ring heterocycles were developed and their functionality was studied before and after incorporation into 2D layered hybrid perovskite phases. It was found that electronic exchange between the organic molecule and the perovskite framework is present in all the crystal phases obtained. Thus, the molecules are more than just a structural component of the phases, but contribute to the electronic properties. The oxidation of ferrocene in particular made it possible to integrate a switchable energy valve. The decisive factor is the change in the energy of the molecular orbitals, which was achieved by the oxidation. Thus, the optical and electronic properties of the semiconductor could be greatly changed. This work provides a comprehensive basis for the study of semiconducting particles with switchable ligands. Especially promising are redox-active hybrid perovskites, which emerge as a completely new class from these investigations

Design of Active Defects in Semiconductors: 3D Electron Diffraction Revealed Novel Organometallic Lead Bromide Phases Containing Ferrocene as Redox Switches

Once the optical, electronic, or photocatalytic properties of a semiconductor are set by adjusting composition, crystal phase, and morphology, one cannot change them anymore, respectively, on demand. Materials enabling postsynthetic and reversible switching of features such as absorption coefficient, bandgap, or charge carrier dynamics are highly desired. Hybrid perovskites facilitate exceptional possibilities for progress in the field of smart semiconductors because active organic molecules become an integral constituent of the crystalline structure. This paper reports the integration of ferrocene ligands into semiconducting 2D phases based on lead bromide. The complex crystal structures of the resulting, novel ferrovskite (≃ ferrocene perovskite) phases are determined by 3D electron diffraction. The ferrocene ligands exhibit strong structure-directing effects on the 2D hybrid phases, which is why the formation of exotic types of face- and edge-sharing lead bromide octahedra is observed. The bandgap of the materials ranges from 3.06 up to 3.51 eV, depending on the connectivity of the octahedra. By deploying the redox features of ferrocene, one can create defect states or even a defect band leading to control over the direction of exciton migration and energy transport in the semiconductor, enabling fluorescence via indirect to direct gap transition. © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH

Interfacial Charge Transfer Processes in 2D and 3D Semiconducting Hybrid Perovskites: Azobenzene as Photoswitchable Ligand

In the vast majority of studies on semiconductor particles one uses ligands, respectively capping agents, which bind to the external surfaces of the particles and cover it with an electrically insulating shell. Since transport of charge carrier and/ or energy across interfaces is desirable for a large number of applications, the use of pi-conjugated ligands becomes more and more interesting. Among those, compounds which show stimuli-responsive properties, particularly molecular switches are fascinating, as one hopes to be able to adjust the properties of the interfaces by demand. However, how the properties of such special ligands get influenced by the presence of a semiconductor and vice-versa is under debate. Here, ammonium-modified azobenzene compounds were selected as prototypes for molecular switches and organic-inorganic hybrid perovskites on the semiconductor side. The class of ammonium-lead-halide phases as prototypes is special, because in addition to surface functionalization of 3D crystals, organic compounds can be truly incorporated into the crystal as 2D phases yielding, for example, layered Ruddelsden-Popper phases. We present photoswitchable azobenzene ligands with varying head group lengths for the synthesis of 2D and 3D hybrid perovskite phases. Energy transfer mechanisms are influenced by the length of the molecular spacer moiety, which determines the distance between the pi-system to the semiconductor surfaces. We find huge differences in the photoswitching behaviour between the free, surface coordinated versus ligands integrated inside perovskite layers. Photoswitching of azobenzene ligands incorporated to 2D phases is nearly quenched, while the same mechanism for coordinating ligands is greatly improved, compared to the free ligands. The improvement originates from an energy transfer from the perovskite to the azobenzene, which is strongly distance dependent. This study provides evidence for the photoswitching behaviour of azobenzene as ligand for hybrid perovskites and the dependence of the head group between a chromophore and the perovskite phase.</p

Interfacial charge transfer processes in 2D and 3D semiconducting hybrid perovskites : azobenzene as photoswitchable ligand

In the vast majority of studies on semiconductor particles ligands or capping agents are used that bind to the surface of the particles covering them with an electrically insulating shell. Since the transport of charge carriers and/or energy across interfaces is desirable for a variety of applications, the use of π-conjugated ligands becomes increasingly interesting. Among them are compounds that react to external stimuli. Molecular switches in particular are fascinating because the properties of the interfaces can be potentially adjusted as required. However, there is debate about how the properties of such special ligands are influenced by the presence of a semiconductor and vice versa. Here ammonium-modified azobenzene compounds were selected as prototypes for molecular switches and organic–inorganic hybrid perovskites as semiconductor materials. The class of ammonium–lead–halide phases as prototypes is peculiar because, in addition to the surface functionalization of 3D crystals, organic compounds can actually be incorporated into the crystal as 2D phases. Thus, for example, layered Ruddlesden–Popper phases are obtained. We present photoswitchable azobenzene ligands with different head-group lengths for the synthesis of 2D and 3D hybrid perovskite phases. The energy transfer mechanisms are influenced by the length of the molecular spacer moiety, which determines the distance between the π system and the semiconductor surfaces. We find huge differences in the photoswitching behaviour between the free, surface-coordinated and integrated ligands between the perovskite layers. Photoswitching of azobenzene ligands incorporated in 2D phases is nearly quenched, while the same mechanism for surface-coordinating ligands is greatly improved, compared to the free ligands. The improvement originates from an energy transfer from perovskite to azobenzene, which is strongly distance-dependent. This study provides evidence for the photoswitching of azobenzenes as ligands of hybrid perovskites, which depends on the spacing between the chromophore and the perovskite phase.publishe

Design of Active Defects in Semiconductors: 3D Electron Diffraction Revealed Novel Organometallic Lead Bromide Phases Containing Ferrocene as Redox Switches

Once the optical, electronic or photocatalytic properties of a semiconductor are set by adjusting composition, crystal phase and morphology, one cannot change them anymore, respectively on demand. Materials enabling a post-synthetic and reversible switching of features such as absorption coefficient, band-gap or charge carrier dynamics represent are highly desired. Hybrid perovskites facilitate exceptional possibilities for progress in the field of smart semiconductors because active organic molecules become an integral constituent of the crystalline structure. We report the integration of ferrocene ligands into semiconducting 2D phases based on lead bromide. The complex crystal structures of the resulting, novel ferrovskite phases were determined by 3D electron diffraction. The ferrocene ligands exhibit strong structure directing effects on the 2D hybrid phases, which is why the formation of exotic types of face and edge sharing lead bromide octahedra is observed. The band gap of the materials ranges from 3.06 eV up to 3.51 eV, depending on the connectivity of the octahedra. Deploying the redox features of ferrocene, one can create defect states or even a defect band leading to the control over the direction of exciton migration and energy transport in the semiconductor