Control over the bulk heterojunction (BHJ) morphology is a key issue in the development of stable and highly efficient organic photovoltaics (OPV). While most approaches to control BHJ morphology involve the use of annealing or solvent additives, these approaches are very limited. In contrast, molecular engineering approaches have the potential to offer unprecedented control over BHJ morphology by controlling the covalent connectivity and self-assembly of the BHJ components. Herein, we describe a novel “flexible linker” strategy to control the BHJ morphology in OPV. Efficient diketopyrrolopyrrole based donor small molecules (DPP(TBFu)<sub>2</sub>) were chosen as a model system to explore the effect of the aliphatic linker connectivity on the self-assembly, crystallinity, and charge transport in neat films and BHJ devices. Different linking positions have been tested with a pair of dimers and a polymer which all retain the original DPP conjugated core. The flexibly-linked polymer was found to be an amorphous and electronically inactive nucleation promoter while dimers exhibited varying degrees of crystallinity and hole mobility similar to DPP(TBFu)<sub>2</sub>. Differential scanning calorimetry and X-ray diffraction suggest that these new flexibly linked dimers promote alternative polymorphic crystal structures, opening new ways to engineer the crystal structure. In addition, we show that the flexible linker strategy can lead to significant improvement of the BHJ stability under thermal stress, by controlling the crystallization of the donor phase.

Aliphatically linked conjugated small molecules for crystal structure engineering and stability enhancement in organic solar cell

253rd National Meeting (2017)

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