Photo-physical Properties of Perovskites on Curved Surfaces

Organic-inorganic metal halide perovskites (MHPs) have recently attracted increasing attention as highly-efficient light-harvesting materials for photovoltaic applications. The technical ease of solution processing of these materials is one of their major advantages on the route towards fabrication of low-cost solar cells and optoelectronic devices. However, the precise control of crystallization and morphology of MHPs deposited from solutions, considered crucial for enhancing the final photovoltaic performance, still remains challenging.

Firstly, the PhD thesis focuses on novel and thus far unexplored issues of crystallization mechanisms and photo-physical characterization of polycrystalline MAPbI3 and MAPbBr3 coated via one-step solution processing onto the surface of elongated, cylinder-shaped, quartz substrates with unprecedentedly high curvatures (diameters of 80 to 1800 ÎŒm). The obtained results are then compared with the ones gathered for deposits of polycrystalline MAPbI3 and MAPbBr3 on planar substrates.

Secondly, the deposits of polycrystalline MAPbI3 and MAPbBr3 on cylindrical substrates are used to track the changes in photoluminescence (PL) and photocurrent (PC) of these materials in the presence of various gaseous media. In particular, the advantages offered by small cross-sectional dimensions and the cylindrical geometry of thus obtained deposits of MAPbI3 and MAPbBr3 made it possible to easily combine them with simple gas-flow observation chambers having both optical and electrical accesses. This approach enabled to perform simultaneous measurements of PL and PC for polycrystalline deposits of MAPbI3 and MAPbBr3 exposed to the precisely controlled flow of different gaseous media, including technologically important oxygen (O2) and nitrogen (N2). Thus, an insight could be gained into the role and importance of surface defects for the general optoelectronic properties, such as photo-brightening and photo-bleaching, as well as long-term stability of these materials.

Thirdly, a study of selective photo-bleaching of MAPbI3 under illumination with specific wavelengths is performed. For the first time this type of experiments were performed under precisely controlled atmosphere of either O2 or N2, additional insight could be gained on the influence of these gaseous media on the mechanism of selective, wavelength-dependent photo-bleaching in MAPbI3.

Lastly, a study of the photo-induced charge transfer in a model system of the MAPbI3/TiO2 interface is carried out. Specifically, a contactless technique based on a combination of low-temperature electron spin resonance (ESR) with in situ illumination is used to directly track the photo-carriers at the MAPbI3/TiO2 interface. In particular, this approach shows that the ESR signal intensity of paramagnetic defects in TiO2 (Ti3+-related centers) markedly changes upon illumination of the MAPbI3/TiO2 interface. It is then inferred that the presented novel methodology can be used to monitor the flow of light-excited electrons from MAPbI3 to TiO2.

Altogether, in addition to exploring morphological and photo-physical aspects of thin polycrystalline films of two archetypal MHPs, that is MAPbI3 and MAPbBr3, coated onto strongly curved substrates, this PhD dissertation also provides a novel approach for designing MHPs-based devices. In particular, it paves the way for designing sensitive differential gas sensors, which are expected to function under various environmental conditions.

A multi-technique approach to study the microstructural properties of tin-based transparent conductive oxides

Federica Landucci

Transparent conductive oxides (TCOs) are semiconductor-like materials that exhibit high electrical conductivity and high optical transparency combined. They are adopted in various applications ranging from gas sensors, to electrochromic windows, to photovoltaic cells. Indium-based TCOs represent the industry standard. Nevertheless, indium is among the less abundant elements in the earth crust and forecasts based on its current consumption envisage an urgent need to replace it. Tin-based TCOs are a promising alternative, since their opto- electronic characteristics mimic the ones of indium-based materials. This thesis aims to investigate the link between optoelectronic and microstructural properties of tin dioxide and zinc tin oxide (ZTO) with a composition Zn0.05Sn0.30O0.65 and their stability when submitted to thermal treatments. Indeed, lots of practi- cal applications require the TCO to operate in high temperature conditions. To conduct this study, a combination of analytical techniques, such as transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X- ray diffraction (XRD), electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC) was employed. Amorphous SnO2 and ZTO were deposited by RF sputtering and annealed up to 1050°C in different atmospheres. The influence of annealing temperature and atmosphere were decoupled and led us to an in-depth comprehension of the mechanisms governing the optoelectronic properties of both materials. When annealed in air, between room temperature and 300°C, ZTO exhibits increased mobility and carrier concentration with respect to the as-deposited state. This increase, investigated with DSC, was ascribed to a structural relaxation that allows point defects to release electrons in conduction band. Between 300°C and 500°C atmospheric oxygen passivates oxygen vacancies, drastically decreasing the carrier concentration and therefore causing a large drop of the conductivity. EPR experiments allowed to ascribe the drop in conductivity to the decrease of carrier concentration, which occurs slightly before the phase change. At 570°C (and 930°C for the case of vacuum annealing) the phase change occurs and the ZTO crystallizes in the rutile form of SnO2. The material becomes completely insulating. When the temperature is increased to 1050°C, evaporation of zinc is observed. In order to improve the electrical conductivity of ZTO at high temperature, a doping strategy was implemented starting from DFT calculations conducted by a partner group, who screened among the entire periodic table, which elements are the best candidates to act as n-dopants for ZTO. Bromine and iodine were retained, since they were found to be the most energetically favorable to become substitutional defects for a tin site. An exploratory doping route is therefore presented and the treated samples analyzed with TEM, EDX and UV-VIS-IR spectroscopy. Finally, the structural properties of an indium-based TCO (zirconium-doped indium oxide) were investigated and used as a benchmark to propose a crystallization model for the tin-based, as well as the indium-based materials. The influence of pa- rameters such as the material thickness, annealing atmosphere and temperature and deposition pressure are discussed for both materials.

EPFL2019

Nanoscale Phase Segregation in Supramolecular π-Templating for Hybrid Perovskite Photovoltaics from NMR Crystallography

Paramvir Ahlawat, Loï Charles Carbone, Brian Irving Carlsen, Manuel Cordova, David Lyndon Emsley, Ulf Anders Hagfeldt, Michael Allan Hope, Farzaneh Jahanbakhshi, Dominik Józef Kubicki, Yuhang Liu, Jovana Milic, Aditya Mishra, Marko Mladenovic, Toru Nakamura, Ursula Röthlisberger, Rashmi Runjhun, Thomas Schneeberger, Shaik Mohammed Zakeeruddin

The use of layered perovskites is an important strategy to improve the stability of hybrid perovskite materials and their optoelectronic devices. However, tailoring their properties requires accurate structure determination at the atomic scale, which is a challenge for conventional diffraction-based techniques. We demonstrate the use of nuclear magnetic resonance (NMR) crystallography in determining the structure of layered hybrid perovskites for a mixed-spacer model composed of 2-phenylethylammonium (PEA+) and 2-(perfluorophenyl)ethylammonium (FEA+) moieties, revealing nanoscale phase segregation. Moreover, we illustrate the application of this structure in perovskite solar cells with power conversion efficiencies that exceed 21%, accompanied by enhanced operational stability.

2021

Material Development for Perovskite/Silicon Tandem Photovoltaics

Peter Joseph Fiala

The developed world is built on the fact that energy is readily available and functionally infinite. The electricity from the wall, the gas at the station, and the heat in our homes are reliable and low-cost. But this comfort is so far only possible through the combustion of fossilized carbon sources. This is not a sustainable system. Not only are these sources finite, but the long-term effects of this much combustion are catastrophic for our planet and species. Transitioning from current energy production to sustainable sources will require the development and deployment of a diverse group of technologies. Chief among these is the generation of electricity from sunlight, or photovoltaics (PV).Most modern PV is based on crystalline silicon. Silicon PV alone could be scaled to provide the necessary energy production, but this could be realized more quickly and efficiently if more advanced technologies are used. The defining metric is the cost per generated electricity. In the case of silicon, the current cost breakdown of materials, manufacturing, installation, and system costs leaves little room for further price reduction; and light-to-electricity efficiencies are near their practical limit. Of the proposed alternatives, the pairing of silicon with a thin film of lead-halide perovskite in a multijunction or "tandem" architecture is one of the most promising. This splits the solar spectrum in two, and raises the theoretical maximum efficiency from 33.3% to 45.1% based on each material better using its segment than any one material could use the full spectrum. The central goal of this thesis was therefore to advance the capability of this type of solar cell while remaining within the constraints of industrially-plausible production methods.Towards this goal, we first isolated perovskite layers to study what limits their performance. This led to improvements in the bulk material quality, the interfaces with adjacent materials, and the layer morphology. We reduced bulk disorder via balancing the annealing temperature, time, and environment. The interfaces were improved by selecting passivating materials which were compatible with our deposition methods. We further observed a complex interplay between the materials used and the environment of the annealing, which illuminated critical dependencies of device performance and gave us a more complete understanding of our material. Finally the morphology was improved via high-temperature annealing. This was enabled by a surface-binding additive which reduced the detrimental effects of such an anneal while preserving the benefits, resulting in a perovskite with less internal resistance. From there, we moved outside of the perovskite layer and developed a light scattering ZnO layer which increased the optical performance and absorption of our cells. Returning to the main goal of the thesis, the lessons learned from work on isolated perovskites were applied in tandems on textured silicon. We additionally increased the perovskite layer thickness and made the bottom-cell more robust to processing damages. Together, the result was a stabilized efficiency of 27.3%. This is better than any silicon cell to date, but work remains to achieve 30% and beyond, extend material stability, scale up the deposition area, and transfer these capabilities to industry. Together, the work done as a part of this thesis represents a step forward in the journey towards carbon-neutral energy production.