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Authors: Klasen, Alexander
Title: Synthesis and Analysis of Thin Films for Perovskite Solar Cells
Online publication date: 24-Sep-2020
Year of first publication: 2020
Language: english
Abstract: Perovskite solar cells belong to a novel generation of printable solar cells that are promising for future energy production due to high efficiency and low production costs. However, fast degradation and difficult reproducibility still prevent commercialization on a large scale and calls for further research of the interplay between the different components of such a cell. Within this work, I present a guideline for how to synthesize state-of-the-art MAPbI3 based perovskite solar cells (PSC) with a record efficiency of 19.5%. I elaborate a procedure for how to study the charge transport properties through different layers of PSC with conductive scanning force microscopy (cSFM) and focus on the semiconducting metal oxides layers F:SnO2 and TiO2. UV-ozone or plasma-treatment alters the surface properties and thus the local conductance perpendicular through these layers. Moreover, different plasma-treatments alter the migration properties of oxygen vacancies differently through an F:SnO2 / TiO2 interface. Lastly, I present how cSFM can be utilized as a synthesis tool for modifying the surface of semiconducting metal oxides locally. In chapter 2, I exhibit the theory of perovskite solar cells and how the properties of the different layers influence the basic cell parameters, e.g. the short circuit current density JSC, the open-circuit voltage VOC, the fill factor FF and the efficiency. Moreover, I discuss that slow solvent evaporation of the perovskite precursor solvent hinders the reproduction of highly efficient solar cells and explain how different approaches target the issues. For the case of the compact electron transport layer TiO2, I explored the limits of increasing the solar cell performance by minimizing film thickness. Furthermore, I present an optimized combination and adaptation of several reported recipes from literature for synthesizing MAPbI3 based solar cells. In chapter 3, I expound a procedure for measuring semiconducting metal oxides as F:SnO2 and TiO2 with a peak force based conducting scanning force microscopy (cSFM). When using this procedure, the influence of tip erosion on the measured current is negligible, even after measuring over 3 million individual force-distance curves on hard materials like metal oxides. Moreover, quantitative comparable cSFM measurements can be obtained to characterize different TiO2 films that reproduce I-V characteristics of metal oxides on a local scale. Furthermore, I studied the decrease of the local conductance of TiO2 thin films over time due to contamination in ambient conditions. In chapter 4, I discuss the influence of different surface treatments on the local conductivity of F:SnO2 and TiO2 layers with cSFM. In particular, I demonstrated with X-ray photoelectron spectroscopy how surface defects like fluorine dopants and oxygen vacancies are affected by UV-ozone treatment, oxygen-plasma treatment or argon-plasma treatment. Terahertz spectroscopy measurements confirm that bulk properties of thin films remain virtually unaffected by surface treatments. Based on these insights, I present an easy procedure for increasing the local conductance of F:SnO2 substrates and TiO2 thin films by up to two orders of magnitude with UV-ozone treatment. In chapter 5, I demonstrate how the surface of TiO2 and F:SnO2 can be locally patterned with cSFM. Consecutive cSFM measurements on the same area induce local charges under the cantilever tip. Local changes in the work function also occur in humid conditions that quench surface charges, as verified with Kelvin probe force microscopy. In the second part of chapter 5, I demonstrate that oxygen plasma treatment of F:SnO2 substrates creates an undoped SnO2 interlayer that acts as an additional electric resistance at the F:SnO2 / TiO2 interface. Argon-plasma treatment removes an undoped SnO2 interlayer. In TiO2 thin films, surface-oxygen vacancies provide electrical pathways for electrons across the TiO2 surface and lead to non-local surface charging with consecutive cSFM measurement. Bulk oxygen-vacancies can migrate through the F:SnO2 / TiO2 interface. As a consequence, I demonstrate a switchable electric resistance of an F:SnO2 / TiO2 array. In conclusion, my work provides an overview of the basics of perovskite-based solar cells and a guideline for how to synthesize MAPbI3 based solar cells. I identify eight different effects that occur when measuring the local conductance of semiconducting metal oxides with cSFM and discuss how these effects could be used in future applications.
DDC: 540 Chemie
540 Chemistry and allied sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-openscience-69a3941b-2028-4f01-95ee-74cbc495de132
Version: Original work
Publication type: Dissertation
License: CC BY-ND
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Extent: XI, 135 Seiten
Appears in collections:JGU-Publikationen

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