Non-fullerene acceptors (NFAs) have become one of the most important research topics in the field of organic solar cells (OSCs), due to their highly tunable optoelectronic and morphological properties. Recently, the rapid development of Y-series NFAs has enabled a paradigm shift in the OSC field with the device efficiencies exceeding 17%. Among various chemical modifications on Y-series NFAs, side-chain engineering and core engineering are two versatile strategies to manipulate intermolecular packing, blend film morphology, and processability of the materials. Therefore, in this thesis, I will introduce a variety of NFAs with similar Y6 molecule and gain more insights into the structure-property relationship of NFAs and the performance of the OSCs. This thesis is constituted of five cha...[ Read more ]

Non-fullerene acceptors (NFAs) have become one of the most important research topics in the field of organic solar cells (OSCs), due to their highly tunable optoelectronic and morphological properties. Recently, the rapid development of Y-series NFAs has enabled a paradigm shift in the OSC field with the device efficiencies exceeding 17%. Among various chemical modifications on Y-series NFAs, side-chain engineering and core engineering are two versatile strategies to manipulate intermolecular packing, blend film morphology, and processability of the materials. Therefore, in this thesis, I will introduce a variety of NFAs with similar Y6 molecule and gain more insights into the structure-property relationship of NFAs and the performance of the OSCs. This thesis is constituted of five chapters:

Chapter I introduces the general background of OSCs, the related device structures, working mechanisms, performance parameters, active layer materials and the motivation of this thesis.

Chapter II concentrates on engineering the outer side chains of Y6 on the flanking thienothiophene units with 4-hexylphenyl (PhC6) and 6-phenylhexyl (C6Ph) chains. Use of PhC6 can enhance the steric effect between the attached phenyl and the end group, which in combination with the additional conjugation effect provided by the linking phenyl leads to upshifted energy levels and increased V_OC as a result. Again, substitution with the bulkier PhC6 unprecedentedly improves film-morphology with reduced paracrystalline disorder and long period and increased root-mean-square composition variations as well, leading to increased electron and hole mobilities and suppressed monomolecular recombination with J_SC and fill-factor (FF) simultaneously enhanced. The PM6:BTP-PhC6-based devices yielded a higher efficiency value of 16.7% than the PM6:BTP-C6Ph-based one (15.5%).

Chapter III investigates the influence of the orientation of side chains on the properties of NFAs and the performance of the organic solar cells (OSCs). Three isomeric NFAs named o-BTP-PhC6, m-BTP-PhC6, and p-BTP-PhC6 are designed by changing the substitution positions and thus orientations of the side chains attached to the central core. The studies show that the optimal side-chain orientation can be achieved by the meta-positioned hexylphenyl group (of the m-BTP-PhC6 molecule), which introduces significant beneficial effects on optical absorption, intermolecular packing, and phase separation of the NFAs. By pairing a donor polymer PTQ10 with m-BTP-PhC6, device efficiencies of 17.7% can be achieved.

Chapter IV deciphers the role of chalcogen-containing heterocycles in non-fullerene acceptors for OSCs. In this study, a family of NFAs named BPF-4F, BPT-4F, and BPS-4F incorporating various chalcogen-containing heterocycles was designed and synthesized. These NFAs exhibited dramatic differences in their photovoltaic performances with the best device efficiency of 16.8% achieved by the thiophene-based cells, which was much higher than the furan-based ones (12.6%). In addition, the selenophene-based NFA showed a red-shifted absorption relative to the furan- and thiophene-based ones and obtained a reasonably high efficiency of 16.3% owing to an improved J_SC.

Chapter V is the conclusions of this thesis and the outlook for the high-performance OSCs based on NFAs.