Molecular Design and Synthesis of Organic Hole Transport Materials for Optoelectronic Applications

Abstract

Organic semiconductors offer promising potential as future materials due to their lightweight, flexibility, and finely tunable electrical properties. The delocalization of π-electrons along conjugated molecules leads to enhanced electronic interactions, ultimately reducing the bandgap and exhibiting semiconductor characteristics. These properties position organic semiconductors as a highly promising alternative to inorganic semiconductors, which face limitations due to their brittleness, finite raw material resources, and high costs. Furthermore, the unique optoelectronic properties of organic semiconductors find diverse applications in fields such as supercapacitors, lithium-ion batteries, electrochromics, thin-film transistors, LEDs, electrocatalysis, and photovoltaics. Photovoltaic devices, in particular, have gained significant importance in recent years due to their role in replacing fossil fuels and mitigating global warming. Perovskite solar cells (PSCs) have garnered special attention for their outstanding photovoltaic properties and cost-effective processing, achieving reported power conversion efficiencies (PCE) exceeding 25%. The hole transport material (HTM) plays a crucial role in the PSC structure, responsible for efficiently transporting charges within the perovskite layer and preventing charge recombination. Researchers focus on developing HTMs with desirable properties such as high charge mobility, excellent charge transport characteristics, and compatibility with the perovskite layer. However, owing to the inherently low hole mobility of Spiro-OMeTAD, which is the most frequently utilized HTM in PSCs, dopants such as lithium salt, cobalt complex, and tert-butylpyridine are inevitably employed to enhance charge transport. The incorporation of these dopants, while improving hole mobility, results in the introduction of moisture and penetration into perovskite crystals, subsequently leading to a significant acceleration in the degradation of PSCs This is the inspiration and subject of my Ph.D., my research focus was on advancing HTMs, with a particular emphasis on innovative strategies for integrating organic and inorganic materials. My systematic research approach encompassed target selection, synthesis, and characterization of advanced hole transport materials. I employed molecular simulations and computational chemistry to identify candidate molecular structures with desired charge transport characteristics, utilizing energy and electrical property calculations to predict and design their properties. After syntehsized that, the research was conducted as a process of analyzing the device characteristics in collaboration with co-workers to find out the suitability and possibility of molecular design for PSCs. Chapter 1 introduced the unique characteristics of organic semiconductors arising from their conjugated structure. It emphasized the potential of these materials as HTMs in photovoltaic devices, focusing on the operational principles and the development process of HTMs in PSCs Limitations of existing HTMs, encompassing stability and performance, were discussed. The essential elements for designing ideal HTMs, including interfacial contacts, hole mobility, charge extraction, and cost-effectiveness, were outlined, along with practical and theoretical tools for their analysis. Chapter 2 and Chapter 3 aimed to overcome challenges and limitations in HTM molecular design strategies by developing high-performance HTMs. Experimental and theoretical methods for analyzing and characterizing synthesized HTM molecules were explored. A hybrid HTM was designed and tested for compatibility with both organic and inorganic materials, showcasing efficiency and stability improvements in perovskite devices. Additionally, novel donor-acceptor (D-A) conjugated polymeric HTMs were synthesized, demonstrating exceptional stability, high charge mobility, and outstanding device performance. The research proposed the concept of locally introducing rigid segments into polymers as a unique design strategy. In addition, the significance of molecularly engineered Self-Assembled Hole Transport Layers (SAHTLs) was highlighted in advancing organic-inorganic hybrid PSCs. SAHTLs offered advantages such as reduced material consumption, tunable bandgap, and straightforward processing. By optimizing the performance of PSCs through SAHTLs, including addressing doping issues and enhancing stability, an impressive 23.6% PCE was achieved. In conclusion, these research endeavors systematically explored novel molecular structure designs to advance HTMs. The findings hold significant promise for the development of charge transport materials in photovoltaic applications and contribute to the field of organic charge transport materials.