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Title: Improving the sustainability of organic and perovskite photovoltaic cells
Author: Kwak, Chankyu
ISNI:       0000 0004 5989 3077
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2016
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Many researchers have studied conjugated polymer-based photovoltaic cells and perovskite-based photovoltaic cells. They have shown lower efficiencies than inorganic photovoltaic cells so far. However, they are attractive because of their potential low cost and easy process. In order to fabricate organic photovoltaic cells, organic solvents are typically used, which results in significant waste solvent being produced. These are moderately expensive and many are toxic. Perovskite photovoltaics commonly incorporate lead, which is toxic and may hinder their adoption. This thesis aims reduce the need for organic solvents during organic photovoltaic cell manufacture by employing water-soluble conjugated polymers as an alternative. It also seeks to improve the efficiency of the devices such the less solvents are required per Watt produced. Reducing the usage of organic solvents would reduce fabrication and solvent treatment costs. Bismuth perovskites are also studied for use in perovskite photovoltaic cells to replace the toxic lead with a less toxic material. The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer used in both types of solar cells has been characterised in order to understand the influence of moisture and oxygen in air on the layer. Eight different thermally annealed PEDOT:PSS films were fabricated namely: as cast, 50, 75, 100, 125, 150, 175, and 200 °C. UV-vis absorption and conductvity were measured. Absorption intensity increased very slightly as thickness increased. In order to develop fabrication skills and understand the principles of these devices, P3HT bulk-heterojunction photovoltaic cells were prepared. The devices were fabricated with different blend ratios both in air and in an oxygen free glovebox. P3HT:PCBM blend ratios of 1:0.6 and 1:0.8 showed the best efficiencies. In this thesis, the synthesis of a new low band gap polyelectrolyte based on fluorene and dithiano-benzothiadiazole is described. Poly[(9,9-bis(4-sulfonatobutyl sodium) fluorene-alt-phenylen)-ran-(4,7-di-2-thienyl-2,1,3-benzothiadiazole-alt-phenylene)] is an anionic charged conjugated polyelectrolyte and was synthesised via Suzuki-cross coupling. Sulfonate groups were introduced to help the low band gap polyelectrolyte to dissolve in water. The aim was a new bulk-heterojunction material to be applied in organic photovoltaic cells. It has a strong absorption peak at 372 nm, a weaker one at 530 nm and a photoluminescence emission peak at 647 nm. Although the conjugated polyelectrolyte did not show any photovoltaic effects as an active layer, it resulted in an improvement of efficiency when used as an additive in the PEDOT:PSS hole transporting layer in the devices. There is an efficiency gain as a result of improved carrier generation and charge transport across the interface into the hole transporting layer which is optimised at a CPE concentration close to 5 mg/ml. Improving the efficiency will improve the sustainability of the devices by reducing the materials required and waste produced per Watt of power produced. Although lead-based perovskites have shown high performance in photovoltaic cells, they have led to concerns regarding their toxicity. Hybrid perovskites with reduced lead content are currently being investigated as a strategy to overcome this issue and to this end we evaluate the use of bismuth as a possible candidate for lead substitution. A series of hybrid perovskite films with the general composition MA(PbyBi1-y)I3-xClx were characterised by their basic optical and structural properties using UV-vis spectroscopy, scanning electron microscopy and grazing incidence wide angle X-ray scattering. The bismuth perovskite precursors form a perovskite crystal structure upon annealing, with a corresponding optical bandgap, for MABiI3, of around 2 eV. Whilst the structural and optical characterisation is promising, preliminary photovoltaic cell tests show power conversion efficiencies below 0.01% with a maximum VOC of 0.78 V. It was suggested that such low overall efficiencies reflect a competition between precursor conversion and material de-wetting from the substrate that occurs during perovskite formation, the overall outcome of which is severely limited photocurrent. In the context of current processing methods, these factors may limit the general applicability of hybrid bismuth perovskites in photovoltaic applications. A blend ratio of 3:1 MAI:BiCl3 used to make a perovskite based photovoltaic cell and annealed at 90 °C showed the best results in this research but it was very low efficiency.
Supervisor: Dunbar, Alan ; Ebbens, Stephen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available