Experimental Characterization of Organohalide Lead Perovskites for Photovoltaic Applications

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Titel: Experimental Characterization of Organohalide Lead Perovskites for Photovoltaic Applications
Author: Musazay, Abdurrahman
Medarbetare: Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten, Institutionen för fysik
Nivå: pro gradu-avhandlingar
Abstrakt: Perovskites are a class of materials that possess many interesting properties with a wide range of technological applications in the field of optoelectronics and photovoltaics. In recent years, perovskites have gained considerable attention as an inexpensive and easy-to-synthesize light absorbing material for so-called organic-inorganic solar cells. In this study we wish to examine the structural and electronic properties of CH3NH3PbI3 organohalide lead perovskites. Charge transport behaviour between the light harvesting perovskite and the underlying electron transport mesostructure are some of the factors that affect the Power Conversion Efficiencies (PCE) of these devices. Therefore, advanced characterization methods were used to investigate the structural and electronic changes that may occur at the interface. Scanning electron microscopy (SEM) was used to survey the structure and morphology of the samples. It was found that the titania grain sizes were 20-25 nm in size and the perovskite grain sizes from 200 nm to 500 nm. The samples were prepared using a solution processing method, which is widely considered as one of the most cost effective ways for crystal growth. However, our studies show that this method does not provide a full perovskite coverage of the surface (14.4% of surface uncovered) which reduces the light harvesting yield. X-ray diffraction (XRD) was employed to study the crystal structure of the sample. It was concluded that the titania was in the anatase phase and the perovskite in a tetragonal crystal system (space group: I4/mcm), with a cell size of a=8.89 A and c=12.68 A. Moreover, our XRD results reveal the existence of a PbI2 crystal phase, indicating an incomplete conversion of the precursors to the perovskite phase. In order to probe the changes that occur at the interface and to elucidate the electron transport mechanisms, X-ray photoelectron spectroscopy (XPS) was conducted and the core-level spectra was investigated. A shift of 0.44 eV in the binding energy of the Ti 2p line was observed between the titania samples and the titania/perovskite. We hypothesize the origin of this shift to be due to a local screening effect, or the formation of a barrier between the perovskite and the titania that is hindering charge transport and is preventing the compensation for the surface charges lost during photoionization. Based on the findings presented in this thesis we suggest, as a possible research direction for the future, UV Photoelectron Spectroscopy (UPS) for constructing the band alignment schemes with the PbI2 layer included and a thorough investigation of the substrate effects and the synthesis routes on the charge transport dynamics of these systems.
Permanenta länken (URI): URN:NBN:fi-fe2017112252113
Datum: 2015
Discipline: Physics

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