Engineering metal oxides for UV-stable perovskite solar cells

This thesis is a study of the role of the metal oxide electron transporting material in perovskite solar cells. The central research question is how the metal oxide influences the stability of perovskite solar cells and how the metal oxide can be modified to improve the stability. The thesis begins by providing four background chapters. Chapter 1 explains the need for solar energy and what perovskite solar cells can contribute to a clean energy future. Chapter 2 covers the working principles of solar cells, whereas Chapter 3 provides more detail about the history and architecture of the perovskite solar cell. Chapter 4 introduces the concept of nanostructuring metal oxides, using a block-copolymer template. The key experimental techniques and methods used throughout this work are described in Chapter 5. These introductory sections serve as the foundation for the four major experimental chapters of the thesis. Chapter 6 reports on the use of neodymium doped TiO2 in perovskite solar cells. Chapter 7 and Chapter 8 aim to increase the efficiency of SnO2 based perovskite solar cells, by making use of a novel electrode material and doping SnO2 with gallium. Chapter 9 covers the spontaneous coalescence of perovskite crystals, highlighting the extraordinary properties of perovskites.Diese Arbeit untersucht die Anwendung von Metalloxiden als Elektronentransportmaterial in Perowskit-Solarzellen. Die zentrale Forschungsfrage ist, wie Metalloxide die Stabilität von Perowskit-Solarzellen beeinflussen und wie Metalloxide, beispielsweise durch gezielte Dotierung, modifiziert werden können, um die Stabilität von Solarzellen zu verbessern. Die Arbeit beginnt mit vier einleitenden Hintergrundkapiteln. Kapitel 1 erklärt die zunehmende Notwendigkeit von Solarenergie und den Beitrag von Perowskit- Solarzellen zu einer sauberen Energiezukunft. Kapitel 2 stellt die Arbeitsprinzipien von Solarzellen vor, während Kapitel 3 mehr Details über die Geschichte und den Aufbau von Perowskit-Solarzellen enthält. Kapitel 4 leitet insbesondere das Konzept der Nanostrukturierung von Metalloxiden unter Verwendung einer Blockcopolymerschablone ein. Die für diese Arbeit benutzten experimentellen Methoden werden ausführlich in Kapitel 5 beschrieben. Diese einleitenden Kapitel dienen als Grundlage für die vier experimentellen Hauptkapitel dieser Arbeit. Kapitel 6 beschreibt die Verwendung von Neodymdotiertem TiO2 in Perowskit-Solarzellen und den Einfluss der Dotierung auf den Wirkungsgrad. Kapitel 7 und Kapitel 8 zielen darauf ab den Wirkungsgrad SnO2-basierter Perowskit-Solarzellen zu steigern, indem einerseits ein neuartiges Elektrodenmaterial verwendet wird und anderseits SnO2 mit Gallium dotiert wird. Kapitel 9 beschreibt die spontane Koaleszenz von kollidalen Perowskit-Kristallen, eine der aussergewöhnlichen Eigenschaften von Perowskitmaterialien

Block Copolymer-based Photonic Pigments: Towards Structural Non-iridescent Brilliant Coloration

Creating color through the self-assembly of specific building blocks to fabricate photonic morphologies is a promising and intriguing approach to reproducing the flamboyant visual effects and dynamic properties observed in the natural world. However, the complexity and lack of robustness in the manufacture of these nanostructured materials hinder their technical exploitation on a large scale. To overcome such limitations, here we present a novel methodology to create bioinspired photonic pigments as dispersed and micrometer-scale particles based on highly ordered concentric lamellar microspheres made of block copolymers. First, we introduce the fabrication protocol and the advantages of this approach compared to the traditional colloidal self-assembly. Then, we discuss some possible future research directions focused on developing hybrid organic-inorganic photonic pigments with enhanced dielectric contrast, reduced scattering, and specific functionalities. Finally, we speculate on possible applications for these structures that go beyond their use as simple photonic pigments

3D tomographic analysis of the order-disorder interplay in the Pachyrhynchus congestus mirabilis weevil

The bright colors of Pachyrhynchus weevils originate from complex dielectric nanostructures within their elytral scales. In contrast to previous work exhibiting highly ordered single-network diamond-type photonic crystals, we here show by combining optical microscopy and spectroscopy measurements with 3D FIB tomography that the blue scales of P. congestus mirabilis differ from that of an ordered diamond structure. Through the use of FIB tomography on elytral scales filled with Pt by electron beam-assisted deposition, we reveal that the red scales of this weevil possess a periodic diamond structure, while the network morphology of the blue scales exhibit diamond morphology only on the single scattering unit level with disorder on longer length scales. Full wave simulations performed on the reconstructed volumes indicate that this local order is sufficient to open a partial photonic bandgap even at low dielectric constant contrast between chitin and air in the absence of long-range or translational order. The observation of disordered and ordered photonic crystals within a single organism opens up interesting questions on the cellular origin of coloration and studies on bio-inspired replication of angle-independent colors.Comment: 13 pages, 10 figure

Insect adhesion on rough surfaces: analysis of adhesive contact of smooth and hairy pads on transparent microstructured substrates.

Insect climbing footpads are able to adhere to rough surfaces, but the details of this capability are still unclear. To overcome experimental limitations of randomly rough, opaque surfaces, we fabricated transparent test substrates containing square arrays of 1.4 µm diameter pillars, with variable height (0.5 and 1.4 µm) and spacing (from 3 to 22 µm). Smooth pads of cockroaches (Nauphoeta cinerea) made partial contact (limited to the tops of the structures) for the two densest arrays of tall pillars, but full contact (touching the substrate in between pillars) for larger spacings. The transition from partial to full contact was accompanied by a sharp increase in shear forces. Tests on hairy pads of dock beetles (Gastrophysa viridula) showed that setae adhered between pillars for larger spacings, but pads were equally unable to make full contact on the densest arrays. The beetles' shear forces similarly decreased for denser arrays, but also for short pillars and with a more gradual transition. These observations can be explained by simple contact models derived for soft uniform materials (smooth pads) or thin flat plates (hairy-pad spatulae). Our results show that microstructured substrates are powerful tools to reveal adaptations of natural adhesives for rough surfaces

Intracellular free sodium and potassium, post-carbachol hyperpolarization, and extracellular potassium-undershoot in rat sympathetic neurones

Double-barrelled ion-sensitive microelectrodes were used to record the free intracellular Na+- and K+-concentrations ([Na+]i, [K+]i) and to determine their relation to changes in membrane potential and extracellular K+ ([K+]e) in rat sympathetic ganglia. The application of 50 μmol/l carbachol resulted in an elevation of [K+]e followed by a post-carbachol [K+]e-undershoot. The membrane depolarization of the sympathetic neurones was associated with an increase in [Na+]i and a decrease in [K+]i. A membrane hyperpolarization and a recovery of [K+]i and [Na+]i to their baseline levels were observed during the [K+]e-undershoot. The time course of the [K+]e-undershoot correlated exactly with the duration of the rise in [Na+]i and decrease of [K+]i. No K+-reuptake occurred in the presence of ouabain. These data confirm, by direct measurements of intracellular ion concentration changes, the contribution of the Na+, K+-pump to the post-carbachol membrane hyperpolarization and [K+]e-undershoot

3D Nanostructured Conjugated Polymers for Optical Applications

This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/adfm.201502392The self assembly of block-copolymers into the gyroid morphology was replicated into 3D nanostructured conjugated polymers. Voided styrenic gyroidal networks were used as scaffolds for the electrodeposition of two poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives and poly(pyrrole) (PPy). The careful choice of solvents and electrolytes allowed the excellent replication of the initial self-assembled morphology into self-supporting gyroidal conjugated polymer networks. The nanostructured films were employed to fabricate electrochromic devices, exhibiting excellent colour contrast upon switching, with fast switching speeds. The versatility and reliability of this method was demonstrated by the creation of switchable Fresnel zone plates, with which the focussing of light can be switched on and off.We acknowledge the EPSRC EP/G060649/1 for funding. This study was supported by the Nokia Research Centre Cambridge

Analysing photonic structures in plants.

The outer layers of a range of plant tissues, including flower petals, leaves and fruits, exhibit an intriguing variation of microscopic structures. Some of these structures include ordered periodic multilayers and diffraction gratings that give rise to interesting optical appearances. The colour arising from such structures is generally brighter than pigment-based colour. Here, we describe the main types of photonic structures found in plants and discuss the experimental approaches that can be used to analyse them. These experimental approaches allow identification of the physical mechanisms producing structural colours with a high degree of confidence

Bio-inspired materials to control and minimise insect attachment

More than three quarters of all animal species on Earth are insects, successfully inhabiting most ecosystems on the planet. Due to their opulence, insects provide the backbone of many biological processes, but also inflict adverse impacts on agricultural and stored products, buildings and human health. To countermeasure insect pests, the interactions of these animals with their surroundings have to be fully understood. This review focuses on the various forms of insect attachment, natural surfaces that have evolved to counter insect adhesion, and particularly features recently developed synthetic bio-inspired solutions. These bio-inspired solutions often enhance the variety of applicable mechanisms observed in nature and open paths for improved technological solutions that are needed in a changing global society

Porous translucent electrodes enhance current generation from photosynthetic biofilms.

Some photosynthetically active bacteria transfer electrons across their membranes, generating electrical photocurrents in biofilms. Devices harvesting solar energy by this mechanism are currently limited by the charge transfer to the electrode. Here, we report the enhancement of bioelectrochemical photocurrent harvesting using electrodes with porosities on the nanometre and micrometre length scale. For the cyanobacteria Nostoc punctiforme and Synechocystis sp. PCC6803 on structured indium-tin-oxide electrodes, an increase in current generation by two orders of magnitude is observed compared to a non-porous electrode. In addition, the photo response is substantially faster compared to non-porous anodes. Electrodes with large enough mesopores for the cells to inhabit show only a small advantage over purely nanoporous electrode morphologies, suggesting the prevalence of a redox shuttle mechanism in the electron transfer from the bacteria to the electrode over a direct conduction mechanism. Our results highlight the importance of electrode nanoporosity in the design of electrochemical bio-interfaces