Investigating hematite nanostructures for photovoltaic applications

• Main Author: Hemant Kumar Mulmudi
• Other Authors: Lam Yeng Ming
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Materials::Energy materials
• Online Access: http://hdl.handle.net/10356/61537
• Institution: Nanyang Technological University

Photoelectrochemical solar cells are promising because of their low cost and fabrication process. Iron oxide (hematite phase) can be employed in forming a semiconductor/electrolyte junction to harness solar energy. Given a bandgap of 2.2 eV of Fe2O3 , a maximum of 12-13 mAcm−2 can be extracted from it. But the short hole diffusion lengths which are in the order of few nanometers pose a problem in achieving this goal. This thesis focuses on utilizing iron oxide nanostructures as a tool to resolve this problem. Solution processed method was employed in synthesizing different nanostructures of Fe2O3 on conductive substrates such as FTO. These nanostructures were directly integrated into PEC solar cells using iodine/iodide electrolyte. The limitations of such a system were systematically investigated by measuring the short circuit current with varying light intensity and impedance spectroscopy. Decoupling the absorption properties and charge transport properties in hematite nanostructures was realized by utilizing near IR absorbing organic dye (SQ02). Due to the complimentary nature of absorption of SQ02 dye, absorption and charge transport characteristics in iron oxide were studied by measuring impedance under different spectral excitations. These studies revealed that iron oxide is a good transporter of electrons when it is not excited. The bulk recombination in iron oxide is still a prevalent factor for the poor performance of these solar cells under white light illumination. Ultrathin layers of iron oxide were also deposited on tin oxide which act as host-scaffold. This approach tackles the problem of low diffusion lengths while not compromising on the absorption of iron oxide. Different thicknesses of overlayer were studied. The thickness variation was done in such a way that we could cover all the possibilities such as thickness below the hole diffusion lengths and also thickness above the hole diffusion length. Impedance spectroscopy was performed on PEC solar cells to investigate the root causes of recombination occurring in such systems.