title: Nanostructured n-Type Electrodes Based on Anodic Aluminum Oxide for Solar Cell Application
title: Nanostrukturierte n-Typ-Elektroden auf Basis von anodischem Aluminiumoxid für Solarzellenanwendungen
Creators: name: Döhler, Dirk
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Climate change is becoming more and more visible due to extreme weather events in all parts of the world. To counteract these it is necessary to cover the energy demand by climate-neutral, renewable energy production. In this context, the conversion of solar energy is of great interest, since, unlike wind or water energy, it can be used more cost-effectively even by private individuals on a smaller scale to produce energy. However, the production of silicon-based photovoltaic systems is still very energy intensive. An alternative is perovskite solar cells, which use mixed organic-inorganic lead compounds as absorber material. These have already deliver comparable efficiency to silicon-based solar cells, but are not yet stable over the long term. To improve this, it is necessary to better understand the effects occurring in perovskite solar cells, such as ion migration. For this purpose, a model system is developed in the course of this work, which simulates the transition from a perovskite solar cell with planar geometry of the n-semiconductor electrode to a mesoporous geometry. First, the nanostructuring of the electrode by electrochemical anodization of aluminum on a conductive glass substrate is investigated. Here, the anodization is complemented by a patterning step using nanosphere lithography. This leads to a more homogeneous anodization and a significantly more ordered pore structure. The solar cells based on the transparent electrodes with ordered pore structure show a higher efficiency and current density than the cells based on electrons with disordered pore structure. A linear relationship is found between the proportion of pores with a good electrical contact and the current density. Furthermore, the length of the pore structure of the transparent electrodes is changed to simulate the transition from a planar to a mesoporous perovskite solar cell. For this purpose, an anodization with oxalic acid as electrolyte is chosen to achieve the largest possible surface area. The average pore diameter is 83 nm before and 51 nm after the deposition of 16 nm TiO2, which is used as an n-type semiconductor. The pore length is varied between 100 and 500 nm. Planar and mesoporous perovskite solar cells are used as control samples. It is demonstrated by UV-Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction and scanning electron microscopy that the three electrode systems, planar, mesoporous and pore geometry are basically comparable in their structure and optical properties. Electrical measurements show an increase in short-circuit current density with increased specific surface area of the n-type electrode, indicating better charge extraction. The rearrangement of ions at the electrode interface is decreased for cells with larger interface area. Through the described model system, it is shown that planar cells are not qualitatively but quantitatively different from their mesoporous counterpart. It is also shown that, compared to a mesoporous perovskite solar cell, a significantly smaller TiO2 interface is sufficient to achieve the same efficiency of charge extraction.