Unravelling the Self-Assembly of Diketopyrrolopyrrole-Based Photovoltaic Molecules

Abstract

The nanostructure of bulk heterojunction in an organic solar cell dominating the electron transport process plays an important role in improving the device efficiency. However, there is still a great need for further understanding the local nanostructures from the viewpoint of molecular design because of the complex alignment in the solid film. In this work, four kinds of photovoltaic materials containing a diketopyrrolopyrrole (DPP) unit combined with other different building blocks were selected and their self-assembled structures on a solid surface were studied by scanning tunneling microscopy technique in combination with theory calculations. The results reveal these DPP-based photovoltaic molecules self-assembled into different nanostructures, which strongly depend on the chemical structure, in particular the backbones and alkyl side chains. The planarities of backbones are affected both by molecule-substrate interaction and steric hindrance induced by the substituted thiophene or benzo[ b]thiophene units on DPP and porphyrin building blocks. The substituted branched alkyl side chains are out of the plane, which are influenced by the alignments of molecular backbones. In addition, the solution concentration also shows a large effect on the self-assembled nanostructures. This systematic research on the self-assembled structures of DPP-based semiconductors on a surface would provide guidance for designing materials and controlling the morphology of a donor/acceptor heterojunction system.

Low‐Bandgap Porphyrins for Highly Efficient Organic Solar Cells: Materials, Morphology, and Applications

“…tunneling microscope (STM) was used to probe the intra-and intermolecular interactions to understand the relationship between morphology adjustment and molecular design on the molecular level. [76] As shown in Figure 10c for DPPEZnP-TEH, two and even more connected trimeric arrangements formed between tetrameric and trimeric clusters. Defects surrounding these clusters (marked in green) are evident.…”

“…Defects surrounding these clusters (marked in green) are evident. [76] Some irregular cavities, arising from the lack of single molecules and changes in the molecular orientation, are evident. In Figure 10d, DPPEZnP-BzTBO assembled into rows and a 2D network forms.…”

“…In comparison to DPPEZnP-TEH, the nonplanar structure on the HOPG surface probably led to the bright elliptical spot. [76] While many questions are opened from such studies, the resolution in the details of the structures will aid in the design of new materials for higher performance.…”

“…(c,d) reproduced with permission. [76] Copyright 2018, American Chemical Society. Porphyrin is a very promising unit to construct low-bandgap materials to harness solar photons in the near-infrared region.…”

“…Copyright 2016, American Chemical Society. (c,d) reproduced with permission [76]. Copyright 2018, American Chemical Society.…”