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Title: Halide perovskites for photovoltaics and light-emitting diodes
Author: Zhao, Baodan
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Halide perovskite solar cells, with rapid efficiency improvements from ~10% to ~23% in 6 years, have attracted significant attention due to their remarkable performance, low processing cost and their potential to become a strong alternative candidate to silicon solar cells. Significant development has also been achieved in halide perovskite-based LEDs with EQE improved from below 1% to ~20% in less than 4 years. This remarkable progress can mainly be attributed to the optimisation of halide perovskite properties. This dissertation focuses on the correlation between optical and electrical properties of halide perovskites and their remarkable performance. Bandgap tunabilities of halide perovskites in blue to green regions through mixing Br-and Cl-and in near infra-red region by substituting Pb2+ with Sn2+ are demonstrated. The absorption and PL spectra are consistent with each other supporting the bandgap tunability. Corresponding EL spectra, which are consistent with their PL spectra, are also demonstrated for blue to green regions. Terahertz measurements coupled with PLQE and transient PL results reveal that the high carrier mobilities are the main reason behind the high efficiency of tin-rich samples. A novel perovskite-polymer-bulk heterostructure is introduced and studied comprehensively. Correlations between their optoelectronic properties and remarkable performance on timescales ranging from femtosecond to microsecond are presented. Transient optical spectroscopy reveals the energy transfer from 2D regions to 3D regions happens in 1 ps. The 20% EQE of the LEDs based in this structure is consistent with conventional thin-film optical models giving internal quantum efficiency of ~100%. This in agreement with near-unity PLQE value of the pristine emissive layer material and the dominant bimolecular recombination process observed in nanosecond-scale transient PL measurements. Two typical interfacial engineering methods to improve the quality of halide perovskite and device performance are then presented. Optimised NiOx is adopted to improve the anode interface. From transient photovoltaic measurements, we find the charge collection ability of NiOx is superior to that of PEDOT:PSS. This is also the main reason behind their better photovoltaic device performance. A unique anti-solvent treatment with additive modifies both the bulk and surfaces of halide perovskites and improves the device performance significantly. Transient PL and PLQE measurements demonstrate that non-radiative recombination pathways are significantly reduced.
Supervisor: Friend, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Halide Perovskites ; Photovoltaics ; Light-emitting Diodes