Control of active layer morphology and performance of organic solar cells by engineering aggregation and crystalline properties of photoactive materials

Abstract

Organic solar cells (OSCs) are attracting significant interests of the researchers owing to their numerous advantages. Based on flexibility, lightweight, eco-friendliness, solution processed large-scale production, it is considered as a promising platform for power sources of next generation wearable/portable electronic devices. Development of conjugated organic materials for better performance and long-term stability of the device is the most important task to improve the commercial viability of the OSCs. Recently, two classes of OSCs i.e. all-polymer solar cells (all-PSCs) and non-fullerene small molecule acceptor (NFSMA)-PSCs are the rising stars replacing PCBM acceptors that suffered from weak light absorption and mechanical robustness and the power conversion efficiency (PCE) has increased to 20%. More comprehensive understanding of the two effective systems in terms of the relationship between the chemical structure of the active materials and the device performance is required. Since the active layer of OSCs is constructed by formation of bulk heterojunction (BHJ) of donor and acceptor materials, the blend morphology plays a critical role in determining the device performance. Formation of optimal blend morphology is associated with very complicated aspects including adequate domain size, purity, donor/acceptor interface, and molecular orientation, and etc. During the polymer blend mixing and film formation process, thermodynamics and kinetics interplay simultaneously and thus it is the most important to carefully design photoactive materials to control both of them. In this dissertation, control of two essential properties, i.e., aggregation and crystallinity, of polymer donor and NFSMA by chemical modification and their impacts on blend morphologies and device performance are demonstrated. Side-chain engineering strategy is adopted to the alternating copolymer based on thienothiophene benzo[1,2-b:4,5-b']dithiophene (BDT) and thienothiophene π-bridged N-alkylthieno[3,4-c]pyrrole-4,6-dione (ttTPD) repeating units, and the synthesized polymer donors are utilized for all-PSCs and NFSMA-PSCs to investigate different responses depending on the system. It is observed that the polymer aggregation property play a more decisive role in all-PSCs because the long chains of polymer materials seriously limit the entropic contribution of mixing process. On the other hand, optimizing crystallinity and hole mobility of polymer donors are critical in improving NFMSA-PSC performance. The photovoltaic performance given by novel polymer donors are the highest level among TPD-based polymer donors and is competitive even in light of the state-of-the-art research trends. Lastly, we developed a series of Y acceptors by incorporation of 2D outer side chains, in order to control the molecular rigidity and enhance processability in non-halogenated solvents.