Perovskite-based photovoltaic is one of the most promising classes of emerging solar cell technologies. This material class combines several advantageous properties, including low exciton binding energy, high charge carrier diffusion length and high optical absorption. Despite these excellent attributes, some challenges remain in perovskite research. Most notably the device stabilities and lifetimes need to be significantly improved in order to push this technology towards commercialization.
Defect physics in perovskite photovoltaics has been shown to be a main factor in understanding long-term device instabilities. However, the number of measurement techniques that can track changes in the ionic landscape during device degradation is very limited, as the perovskite layer is buried under charge extraction layers and metallic contacts. In this thesis argon gas-cluster ion beam etching is combined with x-ray and ultraviolet photoelectron spectroscopy to achieve high resolution energetic and compositional depth profiles. In contrast to most layer-to-layer techniques this method can be applied after any operation time of the photovoltaic and therefore nicely investigate potential changes in the defect landscape.
In the first part of this thesis, the impact of argon gas-cluster etching on the perovskite structure is investigated in order to identify potential damage that prevents this technique from being viable for perovskite materials. It is found that metallic lead is gradually created and a small preferential etching effect of the organic cations takes place during the depth profiling, but it is demonstrated that the major part of the crystal structure stays intact and that the energetics of the sample remains very stable. Moreover, it is demonstrated that fitting of the obtained ultraviolet photoelectron spectroscopy spectra leads to high resolution energetic and compositional depth profiles, which are suitable to identify potential loss mechanisms in full photovoltaic devices.
In the second part, we investigate the increase in device performance of a perovskite photovoltaic during the first subsequent measurements under full illumination, which is a common example of a short-term instability. Ultraviolet photoelectron spectroscopy depth profiles reveal a strong band bending effect appearing after biasing the device which consequently leads to an increase in device open-circuit voltage. Density functional theory simulations link this band bending effect to the accumulation of iodine interstitials at the interface between the perovskite and the electron transport layer.
In the final part, long-term degradation of perovskite photovoltaics is studied by investigating the impact of ionic additives on the perovskite active layer, which increases the lifetime of these devices significantly. It is found that most properties of the perovskite layer remain unaffected by the ionic additive, e.g. microstructure, energetic disorder and photoluminescence. Photoelectron spectroscopy depth profiling revealed an accumulation of iodine at the interface towards the electron transport layer, which is significantly reduced in additive-containing samples. Deep-level transient spectroscopy revealed a new mobile defect species in the ionic additive samples and at the same time a reduction of iodine diffusivity.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:79304 |
| Date | 30 May 2022 |
| Creators | Kreß, Joshua |
| Contributors | Vaynzof, Yana, Leo, Karl, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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