Perovskite nanocrystals for optoelectronics and photovoltaics

Thesis

Lead halide perovskite nanocrystals, APbX3, are an astounding candidate for optoelectronic and photovoltaic applications. Their tunable band gap, through both composition and quantum size effects, allows for a wide colour gamut spanning the visible to infra-red region. The colour purity achievable through narrow full width at half maximum (FWHM) in their emission spectra, and bright emission with reports of near unity photoluminescence quantum yield (PLQY), place lead halide perovskite nanocrystals at the forefront of research into display technologies. Despite a vast research effort into these materials over the past five years, questions remain regarding their surface chemistry, optimal synthesis for compositional breadth, and their stability.

In this thesis, a mixed cation mixed anion lead halide perovskite nanocrystal synthesis is investigated. Although fine tuning of the band gap is achieved, the miscibility of the inorganic cation, cesium (Cs), and organic cation, formamidinium (FA), is unclear due to concurrent changes in size and shape of the nanocrystals across the compositional range. To resolve this, a new synthesis to perovskite nanocrystals is investigated utilising a phosphine oxide, with the aim to simplify the ligand shell. This is motivated to decouple size and shape control from composition in a mixed cation perovskite nanocrystal synthesis. All inorganic CsPbX₃ nanocrystals are synthesised through this phosphine oxide assisted synthesis, and achieve up to 70 % PLQY whilst maintaining monodispersity at high reaction yields close to the theoretical limit. A simple oleate-only ligand shell is achieved, and confirmed through nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectroscopy (XPS). The synthesis is translated across to form FAPbBr₃ nanocrystals. Although similar PLQYs are achieved, the size and shape control is lost. This lack of control is attributed to the interaction of the labile ammonium protons on FA with the ligands used in the synthesis, which is determined via NMR.

Authors: Ashton, OJ

• DOI: 10.5287/ora-gobo2oxjo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: optoelectronics; halide; lead; nanocrystal; synthesis; colloidal; perovskite
• Subjects: Synthesis; Chemistry; Optoelectronic devices; Physics; Materials; Semiconductor nanocrystals