Synthetic Approaches for the Development of Organic Molecules and their Application

Renewable energy has emerged as the foremost alternative for energy supply, driven by the increasing energy demand and addressing concerns over climate change. Solar energy utilization has experienced gradual growth over the past 11 years, making it as one of the fastest-growing renewable energy technologies. Perovskite solar cells have shown remarkable progress in converting sunlight energy into electricity, with an efficiency increasing from 3.8% in 2009 to a recent peak of 26.1%. However, several challenges hinder their commercialization, including the optimization of perovskite materials, deposition methods, device architecture, and the development of cost-effective charge-transporting materials. Hole-transporting materials (HTMs) are critical components of perovskite solar cells, playing a pivotal role in achieving high efficiencies, enhancing device stability, and overall performance. Moreover, they are responsible for reducing production costs, making HTMs essential for advancing perovskite solar cells towards commercial viability. The intricate relationship between these materials and the surrounding layers, particularly the perovskite layer, has been examined.

The overall objective of this thesis is to design and synthesise novel HTMs to explore the intricate relationship between their molecular structure and their effects on the surrounding layers. Through this, the research aims to gain a fundamental understanding of the requirements for effective HTMs and contribute to enhancing the overall performance of perovskite solar cells.

First, eleven new organic carbazole-based HTMs have been synthesised through rational design and molecular engineering, and subsequently characterised. The new HTMs are obtained from relatively inexpensive starting materials employing a simple preparation method, avoiding the need for expensive and complex purification techniques. Their chemical structures were verified by ¹H and ¹³C NMR spectroscopy and mass spectrometry. We evaluated their optical, thermal and electrochemical properties using various techniques such as TGA, UV-vis, CV and PL.

The HTMs were used in regular or inverted perovskite solar cells (PSCs). The fabrication methodology of the devices consisted in the deposition of the different layers comprising the device via spin-coating deposition and evaporation in high vacuum, complemented by thermal treatments. The characterisation of the solar cells was carried out by measuring current-voltage (JV) curves using a solar simulator and advanced optoelectronic techniques. In addition, the interfaces between the substrate/HTM and HTM/perovskite were studied via charge transfer processes, field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy and surface wettability. The new carbazole derivatives showed interesting differences in their behaviour in perovskite-based devices.

In summary, the thesis explores numerous molecules, demonstrating how subtle modifications of their molecular structure –such as replacing electron-donating groups or adjusting the length of the π-conjugated system by incorporating phenyl groups– can significantly influence material properties and improve photovoltaic performance. To obtain high-performance results in perovskite solar cells, it is necessary to tune several desirable characteristics of the hole-transporting materials, such as steric effects, well-aligned energy levels, hydrophobicity, and stability. The thesis outlines rational design strategies using simple synthetic schemes with straightforward procedures. These findings contribute to the advancement of various materials, serving as a basis for the development of even more efficient alternatives and accelerating the commercialisation of innovative solar cell technologies.

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