Abstract
Helical perylene diimide-based (hPDI) acceptors have been established as one of the most promising candidates for non-fullerene organic solar cells (OSCs). In this work, we report a novel hPDI-based molecule, hPDI2-CN2, as an electron acceptor for OSCs. Combining the hPDI2-CN2 with a low-bandgap polymeric donor (PTB7-Th), the blending film morphology exhibited high sensitivity to various treatments (such as thermal annealing and addition of solvent additives), as evidenced by atomic force microscope studies. The power conversion efficiency (PCE) was improved from 1.42% (as-cast device) to 2.76% after thermal annealing, and a PCE of 3.25% was achieved by further addition of 1,8-diiodooctane (DIO). Femtosecond transient absorption (TA) spectroscopy studies revealed that the improved thin-film morphology was highly beneficial for the charge carrier transport and collection. And a combination of fast exciton diffusion rate and the lowest recombination rate contributed to the best performance of the DIO-treated device. This result further suggests that the molecular conformation needs to be taken into account in the design of perylene diimide-based acceptors for OSCs.
Highlights
Solution-processed bulk heterojunction organic solar cells (OSCs) have drawn extensive attention due to their fascinating advantages, 2018 The Authors
Broad absorption in the UV–Vis region is observed for hPDI2-CN2 in dilute CF solution with a sharp onset at approximately 575 nm and several major peaks lying around 337 nm, 443 nm, 474 nm, 514 nm and 550 nm, respectively
Complementary absorption spanning from 300 to 800 nm would be achieved by incorporating PTB7-Th with hPDI2-CN2, which is desirable for the light harvesting and exciton generation of the photovoltaic devices
Summary
Solution-processed bulk heterojunction organic solar cells (OSCs) have drawn extensive attention due to their fascinating advantages, 2018 The Authors. Perylene diimide (PDI)-based small molecule is one of the most extensively investigated non-fullerene acceptors in recent years This kind of n-type organic semiconductors holds many merits for photovoltaic devices, such as ease of synthesis and reproduction, high thermal and chemical stability, excellent electron-accepting ability and high electron mobility [27,28,29,30]. The introduction of substituents at bay regions is a favourable modification strategy to enhance the performance of PDI-based devices In this strategy, some electronwithdrawing groups, such as F, CN can bring about increased electron affinity which results in lower lowest unoccupied molecular orbital (LUMO) level and superior electronic properties. This work demonstrates that helical conformation is a promising archetype for developing 3 highly efficient PDI-based acceptors and that careful morphology optimization is crucial for improving performance of hPDI-based non-fullerene OSCs
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