- Title
- Lewis base passivation in perovskite solar cells
- Creator
- Cook, Andre
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Metal-halide perovskites are semiconducting materials with the unique combination of low temperature fabrication and tunable bandgaps. The tunable bandgap makes them an excellent candidate for use in tandem solar cells, where multiple solar cells are used in combination to provide a high power density. In particular, the use of a perovskite solar cell (PSC) on a silicon solar cell provides the potential for a consumer priced tandem solar cell. Affordable high power density cells will be critical for area limited and power intensive applications such as electric vehicles. PSCs can be fabricated using low temperature methods such as solution processing or vapour deposition, which can result in a perovskite film with a high surface defect density. These defects typically exist as edge ion vacancies on the surfaces of the perovskite grains, creating a charge imbalance that can trap charge carriers, which reduces power output. Surface defects can also lead to heterogeneity in the perovskite film, which can accelerate degradation. To address these effects, the exposed surface of the perovskite can be treated with polar molecules or ions to balance the charge, commonly referred to passivation treatments. The effect of passivation treatments on the performance and heterogeneity within perovskite films will form the basis of this thesis. In Chapter 3 the effect of pyridine and PbI2 on perovskite films is investigated. This work produced a publication showing that pyridine treatment causes the formation of passivating PbI2 grains on the perovskite surface. This was shown by comparing confocal fluorescence microscope (CFM) images above and below the absorption cut off for PbI2. Where low PL emission regions at 405 nm excitation became high PL emission regions at 559 nm excitation. This was due to surface PbI2 grains blocking the excitation of the perovskite at 405 nm, then at 559 nm the PbI2 was transparent allowing the underlying PbI2 passivated perovskite to be excited. This simple technique provided clear evidence for the formation and passivation due to PbI2 on the perovskite film. The effect of trioctylphosphine oxide (TOPO) passivation on the heterogeneity of MAPbI3 and mixed (Cs0.05(FA0.85MA0.15)0.95Pb(Br0.15I0.85)3) perovskite films was investigated via CFM imaging in Chapter 4. By varying excitation wavelength and intensity it was shown that the heterogeneity visible in the MAPbI3 films was mostly due to a carrier confinement effect. This was a consequence of confocal imaging, rather than defects. In the mixed perovskite film the heterogeneity was due to PbI2 grains. TOPO treatment caused morphological changes for both types of perovskite, resulting in smaller grains for the MAPbI3 film and a lack of PbI2 grains in the mixed perovskite. This is explained by the TOPO inhibiting surface ion migration, which prevents grain coalescence for MAPbI3 and prevents PbI2 formation in the mixed perovskite. In Chapter 5 a series of para functionalized pyridines was used to examine the effect of Lewis base strength on passivation capability. The strongest Lewis base increased the PL emission and lifetime the most, indicating effective passivation. However, it reduced device power output, due the formation of a charge barrier shown via contact potential difference (CPD) imaging. The weaker Lewis bases did not have a significant effect on device PCE, making the effect of Lewis base strength on passivation and device performance unclear, leading to a broader scope for the final chapter. For Chapter 6 an in depth study into passivation using a Lewis acid, a Lewis base and a surfactant (TOPO) on MAPbI3 and mixed perovskite films was performed. This showed the Lewis acid to be a better passivator for the MAPbI3 film and TOPO to be better for the mixed perovskite. The Lewis base was shown to act as an electron extraction layer, via the change in CPD with light intensity, making it unsuitable for passivating the perovskite-hole collector interface. For the MAPbI3 film, TOPO inhibiting grain coalescence reduced the photovoltage response. This combination of results shows that surface treatments have effects beyond defect passivation that are different for the two perovskite blends. Together these results show that surface treatments used to passivate defects will affect other properties of the perovskite film. Restructuring of the perovskite film can be caused by treatments like pyridine, which caused PbI2 grain formation. Restructuring can also be prevented if the treatment stops surface ion migration, with this effect being beneficial for the mixed perovskite by stopping PbI2 grain formation and detrimental for MAPbI3 by stopping coalescence. Notably, both Lewis acid and base treatments caused increases in PL intensity and lifetime on both types of perovskite, meaning both anionic and cationic vacancies are present on the perovskite surface. This information will guide further research and development of passivation treatments, allowing for control of surface ion migration and passivation of multiple types of defects.
- Subject
- perovskite solar cells; photovoltaics; passivation; confocal flourescence microscope; atomic force microscopy; scanning kelvin force probe
- Identifier
- http://hdl.handle.net/1959.13/1494541
- Identifier
- uon:53825
- Rights
- Copyright 2023 Andre Cook
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 7 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 337 KB | Adobe Acrobat PDF | View Details Download |