Book Description

Molecular design and synthesis play critical roles in the development of organic semiconductors for organic photovoltaics (OPVs). This dissertation describes the design, synthesis, and characterization of three classes of organic semiconductors for OPVs: p-type semiconducting polymers, n-type semiconducting polymers, and non-fullerene small molecule acceptors. The relative merits of alternative building blocks and design strategies for organic semiconductors are investigated. Complex factors governing the underlying structure-property-processing-performance relationships are discussed in detail. The fundamentals of organic semiconductors and organic solar cells, state-of-the-art materials and devices, and challenges in the design and synthesis of materials are reviewed in Chapter 1. Chapter 2 discusses the strategy of selenophene substitution as a potential method to improve photovoltaic performance of the regular thiophene-based p-type semiconducting polymers. New selenophene-containing polymers were synthesized based on a widely used polymer, PBDB-T, where the original thiophene units at various side chain and backbone positions were substituted with selenophene. This study revealed the intramolecular and intermolecular interactions related with selenophene substitution, thus provided important guidelines in designing selenophene-containing polymers. Chapter 3 presents a comparative study of the alternating naphthalene diimide-thiophene copolymer, PNDIT-hd, and naphthalene diimide-selenophene copolymer, PNDIS-hd. The effects of selenophene substitution on the intrinsic and photovoltaic blend properties of n-type semiconducting naphthalene diimide-arylene copolymers with simple donor−acceptor architecture were investigated. This study demonstrated multiple advantages of selenophene substitution including enhancing light harvesting, formation of favorable morphology, and reducing charge recombination losses in all-polymer solar cell devices. Towards enhancing the intrinsic stability of small molecule acceptors, novel tridecacyclic ladder structure was designed and realized via Friedlander condensation reactions. The tridecacyclic ladder molecule acceptors (LMAs) described in Chapter 4 combined good solubility with enhanced stabilities and high photovoltaic performance. One of the new LMAs, LTX-4Cl, demonstrated a high PCE of 11.5% with high fill factor of 0.75. This study also unraveled the significant impact of side chains and halogenations on the molecular packing characteristics of the LMAs and the resulted photovoltaic performance. Finally, the results of the above studies are summarized in Chapter 5 and an outlook is given for future development of organic semiconductors and the organic photovoltaic technology.