Date of Award

8-2019

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Dr. Clement Burns

Second Advisor

Dr. Lisa Paulius

Third Advisor

Dr. Asghar Kayani

Fourth Advisor

Dr. Ramakrishna Guda

Keywords

Perovskites, CH3NH3PbI3, impedance spectroscopy, solar cells, doping, RBS

Abstract

Owing to their high-power conversion efficiency (PCE), easy processability, and low fabrication cost, organic lead halide perovskites (OLHP) are emerging as a most promising photovoltaic technology. However, toxicity of lead (Pb) is a major concern for further development. Therefore, it is essential to explore nontoxic metals to replace lead in these materials. In the current research work, nontoxic Mn2+, Na+ and Ba2+ are doped at 1, 5 and 10% mole concentrations to partially substitute Pb2+ in methyl ammonium lead iodide (CH3NH3PbI3 or MAPbI3) perovskite systems, and the effects of doping on structural, optical, electronic and dielectric properties are investigated.

In the first part of the work, thin films of Mn2+ and Na+ doped MAPbI3 are fabricated from perovskite precursor solutions containing metal acetates, using a single step spin coating technique. They are characterized using Rutherford back scattering spectroscopy (RBS), x-ray diffraction (XRD) and ultraviolet-visible spectroscopy (UV-Vis spectroscopy). RBS shows a reduction of surface roughness upon addition of dopants to the perovskite films, which is favorable for solar cell performances. XRD data confirms that the room temperature tetragonal structure of the pristine material is preserved, but there is a shrinkage of the lattice as smaller sized Mn2+ and Na+ dopants are introduced. In addition, formation of smaller grains with Mn2+ doped samples are observed whereas with the Na+ dopants such effects are not observed at the studied dopant levels. UV-Vis absorption measurements confirm that these metals can be used to replace Pb in pristine material at 1, 5 and 10% dopant concentrations without significantly altering the remarkable absorption property of the pristine material. Furthermore, a procedure is optimized to fabricate solar cells with FTO/c-TiO2/mp-TiO2/Perovskite/P3HT/Au configuration and a PCE of 16.7 % with short circuit current density (Jsc) of 28.3 mA/cm2, open circuit voltage (Voc) of 0.99 V and fill factor of (FF) of 0.58 for pristine material is obtained.

In the second part of the research, Ba2+ doped MAPbI3 polycrystals (MAPb1-xBaxI3 with x= 1%,5% and 10%) are successfully synthesized and their structural, calorimetric, ionic conductivity, and dielectric properties are investigated using XRD spectroscopy, differential scanning calorimetry (DSC) and impedance spectroscopy (IS). No new structures are formed upon doping with Ba2+ ions, however it results a lattice expansion as shown by the XRD study. Moreover, as deduced from DSC study on the doped crystals, Ba2+ shows no effect on the tetragonal to cubic phase transition temperature of the pristine material. The IS measurements are used to find the dark conductivity and the dielectric constants of the doped crystals at different temperatures. The conductivity arises mainly from vacancy mediated iodide ion (I-) migration and the increase in ionic conductivity with dopants is ascribed to lattice distortion caused by the isovalent ion doping. The possibility of an increase in the number of I- vacancies due to lowering of defect formation energies in the distorted lattice is suggested to explain the observed conductivity. In addition, the bulk dielectric constant increases with increasing Ba2+ dopant amounts. This phenomenon is correlated to the ordering of MA dipoles and the distortions of the Ba sites in the lattice. Finally, the temperature dependence of the dielectric constants is observed for all the samples and it is attributed to thermal effects on orientation polarization of MA molecules.

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Dissertation-Open Access

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