UCLA

Organic semiconductors built on π-conjugated structures have attracted significant attention due to their distinctive optical, electronic, and mechanical properties. These characteristics position them as ideal materials for various electronic devices. Notably, their strong light absorption and efficient charge transport capabilities mark organic semiconductors as promising solutions for addressing the global energy crisis through solar energy conversion. However, precise control of “soft” nanostructures formed by the noncovalently aggregated organic semiconductors for achieving desired optoelectronic properties is challenging, compared to covalently or ionically inorganic semiconductors with rigid architectures. In Chapter 1, I will introduce basic structures and properties of organic semiconductors, especially on their molecular packing behavior. The structure-property relationship of organic semiconductors will be discussed in terms of device applications, such as the development of near-infrared donor and acceptor materials for organic photovoltaics. From molecular designs to device applications, in the following chapters, I will introduce the chemistry of several organic semiconductor aggregates constructed from twisted and nonplanar π-systems and their performances in various solar energy fields, such as photocatalytic hydrogen reaction, organic photovoltaics and perovskite solar cells. In Chapter 2, I will show the self-assembly of noncovalent π-stacked organic frameworks that shows a higher activity for photocatalytic hydrogen evolution. I will first introduce the background of noncovalent π-stacked organic frameworks, which are a subclass of porous materials that consist of crystalline networks formed by self-assembly of organic building blocks through π-π interactions. π-stacked organic frameworks based on spirofluorene as central units and 3-(dicyanomethylidene)indan-1-one as end groups demonstrate strong visible light absorption from 500 nm to 700 nm and high surface area (248 m2 g–1) with 1.8 nm hydrophilic micropores, rendering them well-suited for applications in photocatalysis. The fabricated π-stacked organic frameworks nanoparticles exhibit hydrogen evolution rate up to 152 mmol h-1g-1 at room temperature and 618 mmol h-1g-1 at 70 °C. Cryo-transmission electron microscopy further reveal the native morphology of these nanoparticles and the cocatalyst Pt loading status on them. In Chapter 3, I will discuss the singlet fission property of pentacene polymer and its application in organic photovoltaics. Singlet fission is a process that converts one singlet exciton into two triplet excitons while conserving spin. This exciton multiplication process has the potential to overcome the Shockley-Queisser limit of solar power conversion efficiency. Pentacene with high mobility has proved to be ideal organic semiconductors for singlet fission organic photovoltaics. Nevertheless, the cost-intensive vacuum deposition process and the propensity of molecular aggregation in the solid state to prematurely quench triplet excitons pose challenges for their application in photovoltaics. To address these issues, a pentacene polymer is engineered with pentacene units arranged orthogonally to the polymer backbone. This design facilitates the use of pentacene-based materials in organic photovoltaics as donor materials through solution processing. Rapid conversion of photoexcited singlets into triplet pairs, occurring on a picosecond time scale (495 ps) and further dissociate into two “free” triplet excitons in 9.8 µs are observed in the pentacene polymer via transient absorption spectroscopy. The resulting photovoltaic devices based on pentacene polymer and nonfullerene acceptors demonstrate 1.92% power conversion efficiency. In Chapter 4, I will focus on a helicene-based organic semiconductor and its application as electron transport layer in inverted perovskite solar cells. Electron transport layer materials based on fullerene tend to form large clusters and undergo dimerization when exposed to light, leading to a deterioration in electron transport capability and device degradation. The nonplanar geometry of helicenes proves effective in preventing such aggregation issues, thereby enhancing device stability. We have successfully synthesized a small-molecule n-type organic semiconductor utilizing [6]helicene. This compound was employed as the electron transport layer, n-doped by organic amines, in an inverted perovskite solar cell, achieving an impressive power conversion efficiency of over 16%.