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Title: Optimisation of fabrication processes for stable and scalable perovskite solar cells
Author: Stringer, Michael
ISNI:       0000 0004 7651 6994
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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In recent years metal halide perovskites have become a promising photovoltaic (PV) technology, most notable for their high-power conversion efficiencies and potential for cheap, solution-processable, roll-to-roll compatible module production. In this thesis, the materials and fabrication processes that are used to make perovskite photovoltaics are investigated, developing them in such a way to make them cheaper, scalable, and transferable to high throughput manufacturing processes, whilst simultaneously aiming to achieve and maintain efficiency and stability. A family of carbazole-based conjugated polymers is identified as potential set of materials for hole selective charge transporting materials. A chemically doped polymer poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'benzothiadiazole)] (PCDTBT) hole transport layer is used with multi cation formamidinium lead iodide (FAPbI3) and methylammonium lead bromide (MAPbBr3) perovskite (FAPbI3)0.85(MAPbBr3)0.15, to achieve standard architecture devices with up to 15.9 % power conversion efficiency, with clear evidence that the chemical doping increases the conductivity and photostability of the PCDTBT. The stability of perovskite solar cells is a vital issue that must be addressed in further detail if perovskite PV is to become a commercially viable technology. Here, the importance of hydrophobic hole transport layers for perovskite solar cell stability is identified. Facile formation of a moisture free perovskite is achieved by combining the hydrophobic polymer poly(4-butylphenyldiphenylamine) (polyTPD) with a volatile methylamine bubbled acetonitrile methylammonium lead iodide (MAPbI3) perovskite solution. A multi-layer encapsulation system, comprised of a protective polyvinylpyrrolidone (PVP) interlayer and a UV-curable epoxy, is used to stabilise perovskite solar cells containing these materials, leading to MAPbI3 based inverted architecture devices with lifetimes over 1000 hours. It is also found that solvent-annealed MAPbI3 devices (which generate higher photocurrent) have reduced stability and undergo enhanced burn-in. This result demonstrates that initially enhanced device power conversion efficiency does not necessarily translate to a device having long-term stability. Triple cation CsI0.05((FAPbI3)0.83(MAPbBr3)0.17)0.95 based standard architecture perovskite solar cells are also shown to have impressive stability when encapsulated with a multilayer encapsulation system that comprises of a protective aluminium oxide (Al2O3) interlayer and a UV-curable epoxy. To pursue low-cost, scalable fabrication of perovskite solar cells, inorganic metal oxide charge transport layers have been explored. Here, the materials nickel oxide (NiO) and titanium dioxide (TiO2) have been deposited through reactive electronbeam evaporation. NiO and TiO2 are then utilised to create devices with champion power conversion efficiencies up to 15.8 % and 13.9 % respectively. Both materials are compatible with MAPbI3 and CsI0.05((FAPbI3)0.83(MAPbBr3)0.17)0.95 perovskite active layers. Critically, it is found that such metal oxides can be deposited at high speed (nm/s), and do not require a high-temperature anneal step after deposition, making reactive electron-beam evaporation compatible with roll-to-roll processing on sensitive flexible polymeric substrates. Finally, a new type of back-contact perovskite PV architecture is explored, solar micro-grooves. Here, such embossed polymeric micro-grooves are directionally coated with evaporable p- or n- type electrodes on to opposing groove walls, and then filled with the highly volatile acetonitrile solution processed MAPbI3 perovskite. These flexible, rare-metal-free, back-contact perovskite solar grooves make use of the p-type reactive electron-beam deposited NiO, and are fabricated without thermal annealing. Individual grooves act as photovoltaic devices, which achieve power conversion efficiencies of up to 7.3 %. It is demonstrated that horizontally-spaced series connected grooves act as mini-modules, which were found to build up to 15 V open circuit voltage. Crucially, these back-contact minimodules are fully functional without the use of electrode patterning techniques such as electrodeposition, laser ablation, mechanical etching, or photoresist templating.
Supervisor: Lidzey, David Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available