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Title: Improving the stoichiometry in lead iodide and perovskite micro-crystals, towards the fabrication of more efficient perovskite solar cells
Author: Tsevas, Konstantinos
ISNI:       0000 0004 8510 7295
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2020
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The technology of photovoltaics provides a renewable and ecofriendly way of conversion of photons to electricity. Low cost photovoltaics with high efficiency and extended life time are some of the main goals during their fabrication. Perovskite solar cells (PSC) have been promoted to potential candidates in the field of photovoltaics. In this thesis, chapter 1 provides a general background theory about the field of perovskites and their use to fabricate PSC. In chapter 2, experimental details are provided, supporting the work that was made and the relevant theory of characterization techniques used. Chapter 3, mainly focusing on a low-cost, energy efficient and water-free synthesis of lead iodide (PbI2) a material widely used for the fabrication of perovskites, by planetary ball milling (mechanochemical), controlling both stoichiometry and number of polytypic phases. Chapter 4, focusing on the stability of PSC fabricated with an optimum stoichiometry between different ions present in the photoactive layer. Synthesis of controllably doped or undoped methylammonium lead triiodide (CH3NH3PbI3) micro-crystals with various amounts of dopants of either methylammonium bromide (CH3NH3Br) or formamidinium iodide (CH(NH2)2I) or CH(NH2)2I and bromine (Br2), was achieved. Characterization techniques used, included X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM), scanning electron microscopy (SEM), UV-Vis spectroscopy, controllable chemical synthesis of micro-crystals and testing operational PSC devices. The results presented in chapter 3, indicated that the use of the sub-stoichiometric PbI2 (when iodine content is below the optimum 1 : 2 ratio of Pb : I atoms) during the fabrication of the photoactive layer in PSC devices, results in unreacted PbI2. The presence of unreacted PbI2 reduces the average power conversion efficiency (PCE) of the PSC from ~ 15.5 % to ~ 4.6 %. Whereas, the results presented in chapter 4, indicated that controllable doping of CH3NH3PbI3 micro-crystals with different incorporated ions, reveals a minimum of distortion in the resulted perovskite crystal structure. That might lead to an optimum composition in perovskites with enhanced chemical stability as function of the exposed conditions. Un-encapsulated PSC devices fabricated using controllably doped CH3NH3PbI3 micro-crystals, showed a reduction in the average PCE from ~14 (±1.5)% to ~13.7 (±0.5)%, after storage in dark and under ambient conditions for 240 hours.
Supervisor: Dumbar, Alan ; Lidzey, David ; Rodenburg, Cornelia Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available