During the past decade, tremendous progress has been made in the field of polymer solar cells (PSCs) as they show unique properties, which include mechanical flexibility, lightweight, and the capability of processing using low-cost fabrication methods such as roll-to-roll printing. However, the performance of PSCs still lags behind compared to inorganic solar cells such as silicon solar cells. Moreover, the structure-property-performance relationships of PSCs have not been fully understood, which are critically important to achieve high-efficiency PSCs. In this thesis, the material design of both polymer donors and non-fullerene acceptors, as well as their high-efficiency devices are discussed by studying four high-efficiency PSC systems. Firstly, a facile method to fine-tune po...[ Read more ]

During the past decade, tremendous progress has been made in the field of polymer solar cells (PSCs) as they show unique properties, which include mechanical flexibility, lightweight, and the capability of processing using low-cost fabrication methods such as roll-to-roll printing. However, the performance of PSCs still lags behind compared to inorganic solar cells such as silicon solar cells. Moreover, the structure-property-performance relationships of PSCs have not been fully understood, which are critically important to achieve high-efficiency PSCs. In this thesis, the material design of both polymer donors and non-fullerene acceptors, as well as their high-efficiency devices are discussed by studying four high-efficiency PSC systems. Firstly, a facile method to fine-tune polymer properties and blend morphology was demonstrated in Chapter 2. By employing donor polymers with a mixture of two even-numbered, randomly distributed alkyl chains, the polymer properties and blend morphology can be systematically tuned between two regular polymers, and a high power conversion efficiency (11.1%) can be achieved. This approach promotes the scalability of donor polymers and thus facilitates the commercialization of PSCs. Secondly, a nonfullerene semitransparent tandem organic solar cell was developed in Chapter 3, which exhibits a broad absorption from 300 to 1000 nm and achieves a record efficiency of 10.5% with an average transmittance of 20%. Importantly, this high performance was enabled by a novel narrow-bandgap nonfullerene acceptor named IEICS-4F exhibiting a strong crystallinity and high electron mobility. Thirdly, efficient all-polymer solar cells (all-PSCs) based on a polymer acceptor named PFBDT-IDTIC was demonstrated in Chapter 4. By combining PFBDT-IDTIC with a fluorinated donor polymer (PM6), a high power conversion efficiency of 10.3% can be achieved, which is the highest value reported for single-junction all-PSCs at that time. This high performance can be attributed to its good absorption property and high electron mobility of PFBDT-IDTIC. Further improvements of all-PSCs were achieved by a new polymer acceptor, PT-IDTTIC in Chapter 5. A record efficiency of 12.06% is achieved with a small voltage loss of 0.52V, which is the smallest voltage loss for all-PSCs to date. The structural novelty of PT-IDTTIC lies in the further extended fused ring core (indacenodithienothiophene), which enables extended absorption into the near-IR region and also increases the electron mobility of the polymer.