Polymer solar cells (PSCs) experienced a leap forward recently due to the development of non-fullerene acceptors. These novel acceptor materials possess improved photon absorption ability as well as readily tunable band structures compared to the conventional fullerenes. Perylene diimide (PDI) derivatives were among the first investigated non-fullerene acceptors for PSCs. PDI is widely adopted as building blocks for acceptor materials for its high photon absorption and electron transporting abilities, suitable and tunable energy levels, ease of synthesis and excellent photon stability. However, PDI derivatives are well known for their aggregation tendency to result in poor blend morphology, which leads to lower device efficiency than the acceptor-donor-acceptor type fused ring small molecular acceptors. To address this issue, we designed and synthesized three PDI based small molecular acceptors with a diketopyrrolopyrrole (DPP) core. The c-PDI2 and nc-PDI2 were two-PDI-armed molecules with PDI substituents attached on the carbon or nitrogen atoms of the DPP skeleton, respectively, while PDI4 was a four-PDI-armed molecule. The PDI units were predicted by molecular simulations to be positioned in altered planes forming the twisted 3D structures, which would reduce the intermolecular aggregation. Based on optical absorption, energy level, blend morphology and photovoltaic performance studies, all three molecules were found with amorphous morphology, which indicated that the aggregation tendency was efficiently suppressed. Among the three molecules, the four-armed PDI4 displayed the flatter structure with broadened electron delocalization which led to significantly increased extinction coefficient and electron transport mobility. The faster electron transport of PDI4 assured the balanced charge transport which yielded into a higher field factor (FF) over 65% in contrast to the two-armed molecules with FFs under 55% in the respective PSC devices. With additional aids from the increased photon absorption, PSCs containing PDI4 also generated substantially higher photocurrent. These improvements afforded the highest power conversion efficiency (PCE) as high as 8.45% among the three PDI derivatives, which was twice and 1.5 times of those of c-PDI2 and nc-PDI2, respectively. In comparison between the two two-armed PDI derivatives, the nitrogen-position-substituted nc-PDI2 delivered higher device performances than the carbon-position-substituted c-PDI2, also thanked to its flatter molecular arrangement and broader intra- and intermolecular electron delocalization. In our study, we successfully prevented aggregation of PDI derivatives by constructing 3D molecular structures with multiple PDI units. The numbers and substituting positions of PDIs on the DPP core were also investigated in detail, which provided valuable insights for designing of high performance PDI derivatives for PSCs.

Key words: Polymer solar cells, non-fullerene acceptor, perylene diimide, three-dimensional, structure, high efficiency