Delafossite Oxide Materials for Efficient and Stable Inverted Perovskite Solar Cells

Abstract

This thesis thoroughly examines the development and use of copper and silver delafossite oxide materials as hole transport layers (HTLs) in inverted perovskite solar cells (IPSCs). The study seeks to tackle significant issues related to the efficiency, stability, and scalability of perovskite solar cells by investigating the viability of these inorganic oxides as substitutes for traditional organic HTLs. The research centres on synthesising, characterising, and integrating delafossite oxides into IPSCs to assess their viability for photovoltaic applications. Delafossite oxides, comprising CuMO₂ and AgMO₂ (M = Fe, Co, Mn, Ga, Al, Cu, Cr, Ni, In, Sc), were synthesised by hydrothermal methods at low temperatures (<210 °C) and pressures (<20 atm). The synthesis process was optimised using NaOH as a mineraliser to improve phase purity, regulate crystal size, and guarantee structural stability. A thorough characterisation was conducted utilising X-ray diffraction (XRD) and scanning electron microscopy (SEM) to examine structural and morphological attributes, thermogravimetric analysis (TGA) to determine thermal stability and Ultraviolet-Visible (UV-Vis) spectroscopy to assess optical properties, including bandgap energy and light absorption. The findings indicated that Cu-based and Ag-based delafossites exhibit the necessary structural integrity, optical properties, and thermal stability for hole transport layers in inorganic perovskite solar cells. Thin film deposition processes, including spin coating, spray pyrolysis, and doctor blading, were utilised to integrate these materials into devices by fabricating homogenous films on Fluorine-doped Tin Oxide (FTO) substrates. The synthesised films were examined for their structural, optical, electrical, and electrochemical characteristics. Traditional HTL, like NiO and CuO, served as reference points for comparison. Photoluminescence (PL) and carrier lifetime assessments revealed effective charge transfer in certain delafossites, notably AgCoO₂ and CuCoO₂. These materials exhibited favourable characteristics for hole extraction and charge conveyance. Nonetheless, device-level integration exposed problems like surface roughness, interface compatibility, and the deterioration of MAPbI₃ perovskite layers. Although certain delafossite-based devices had promise, their performance measures, particularly power conversion efficiency (PCE), fell short of expectations due to these obstacles. The thesis is organised into seven chapters, each focussing on a distinct facet of the topic. Chapter 1 presents the work's introduction, motivation, and objectives, emphasising the potential of delafossite oxides as alternatives to conventional organic hole transport layers (HTLs). Chapter 2 extensively examines perovskite solar cells, detailing their topologies, operational principles, and the function of hole transport layers, emphasising delafossite materials. Chapter 3 delineates the experimental methodologies employed for synthesising and characterising delafossite oxides alongside the fabrication of thin films and solar cell devices. Chapter 4 examines the synthesis, structural characterisation, and performance assessment of Cu-based delafossite oxides. In contrast, Chapter 5 parallels similar investigations for Ag-based delafossite oxides, highlighting its enhanced conductivity and stability relative to the copper-based variants. Chapter 6 details the manufacturing and characterisation of solar cell devices utilising CuMO₂ and AgMO₂ thin films, assessing their photovoltaic performance and pinpointing significant obstacles in device optimisation. Chapter 7 concludes by summarising the principal findings, examining their ramifications, and delineating future research avenues to improve the economic feasibility of delafossite-based HTLs.