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Title: Vapour-based growth of inorganic compounds for next-generation, stable photovoltaics
Author: Lee, Lana Catherine
ISNI:       0000 0004 8501 2835
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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This thesis investigates the growth of stable inorganic compounds for photovoltaic applications using vapour-based fabrication techniques. In the literature section, chemical vapour deposition is identified as a versatile family of growth techniques through which a wide range of materials can be grown, and morphologies accessed. Of these, furnace based chemical vapour deposition holds the advantages of ease of set up and simplicity of equipment. Meanwhile, owing to the removal of lengthy vacuum and gas purging steps relative to temporal atomic layer deposition and the possibility of continuous substrate feeds, atmospheric pressure spatial atomic layer deposition reactors are identified as promising tools for high-throughput fabrication of metal oxide films, offering up to two orders of magnitude improvements to deposition rates over vacuum-based growth. For these reasons, both techniques are utilised for work in this thesis. Following this, a review of recent studies into low-toxicity bismuth containing photovoltaic absorber materials is presented. Most of these are predicted to form defect states close to the band edges such that carriers trapped in these states can be readily thermally de-trapped rather than undergoing Shockley-Read-Hall recombination across the bandgap. BiOI is identified as a promising photovoltaic material amongst compounds such as methylammonium bismuth iodide and bismuth-based double perovskite structured compounds due to its relatively favourable bandgap (1.9 eV), small predicted electron and hole effective masses and large dielectric constant. Part A of the results and discussion section focuses on the growth, materials properties and application of BiOI in solar cells. Despite its predicted tolerance to defects, BiOI solar cells have traditionally performed poorly due to sub-optimal film morphology. In this thesis, compact BiOI thin films consisting of large crystallites are grown by chemical vapour deposition at atmospheric pressure in a tube furnace. Incorporation into photovoltaic devices almost doubles the efficiency of literature reports, achieving a record efficiency of 1.8 %, as well as the highest external quantum efficiency for any bismuth-based photovoltaic material of 80 % (at λ = 460 nm). Additionally, device performance is stable after 3 days storage in ambient conditions under laboratory illumination. Growth at higher temperature is then explored to assess whether the structural properties of BiOI can be improved. A decrease in Urbach energy from 70 to 40 meV occurs when the deposition temperature is raised from 360 ºC to 500 ºC, proposed to enable higher device open-circuit voltage. However, micron-sized particles form on the film surface due to unwanted gas phase reactions at temperatures ≥ 400 ºC, limiting their potential application. Finally, films are annealed under vacuum to deliberately introduce iodine deficiency. Phase purity is maintained despite up to 40 % iodine loss at the film surface, whilst, over the same measurement area in photoelectron spectroscopy measurements, the Fermi level of BiOI remains constant after 48 hours of vacuum annealing at 100 ºC, indicating that iodine loss does not induce a doping effect and suggesting BiOI shows tolerance to iodine deficiency. Part B of the results and discussion section focuses on high throughput fabrication of NiOx thin films for application as a hole transport layer in perovskite solar cells. This aims to overcome the throughput limitations of solution and vacuum based batch processing routes towards NiOx which have been used in p-i-n perovskite devices. Deposition conditions for NiOx film growth using the Vertical Cambridge University Close Proximity reactor are established. Using growth curves, growth is determined to occur via a decomposition mechanism, typical of chemical vapour deposition reactions. Additionally, growth rates over 30 times faster than vacuum-based temporal atomic layer deposition are achieved. Incorporation into p-i-n organolead halide perovskite devices gives a champion efficiency of 16.6 %, which is comparable to the best devices using solution- and vacuum- based NiOx processing techniques. An annealing strategy to reduce the defect concentration in NiOx films is identified, whilst preliminary experiment exploring alternative solar cell device architecture which result in improved device currents are discussed.
Supervisor: MacManus-Driscoll, Judith ; Hoye, Robert Sponsor: Centre for Doctoral Training in New and Sustainable Photovoltaics (CDT-PV)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Photovoltaics ; solar cell ; bismuth oxyiodide ; BiOI ; chemical vapour deposition ; CVD ; spatial atomic layer deposition ; AP-SALD ; ALD ; defect tolerance ; non-toxic ; bismuth ; nickel oxide ; NiO ; inorganic solar cell