Impact of Donor and Acceptor Fluorination on Energy Landscape of Organic Bulk Heterojunction Solar Cells

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Abstract
End‐group fluorination of the well‐known non‐fullerene acceptor (NFA) ITIC extends the absorption spectrum to the near‐infrared (NIR), increasing the solar cells’ photocurrent compared to the non‐fluorinated version. Here, ITIC and its fluorinated variants (asymmetric ITIC‐2F * and symmetric ITIC‐4F) were synthesized, and the effect of fluorination on physicochemical, optical, electronic, and photovoltaic properties was systematically investigated. Fluorination increases the acceptors’ electron affinity (EA), consequently reducing the device photovoltage. The NFA's molecular quadrupole moment (QM) increases with the degree of fluorination, which improves exciton dissociation and reduces charge recombination at the donor‐acceptor (D‐A) interface. Simultaneously, the NFA ionization energy (IE) increases, facilitating hole transfer to the donor. Both effects improve the internal quantum efficiency (IQE) of solar cells upon NFA fluorination. Donor fluorination increases the donor's IE, potentially hampering hole transfer. Unfortunately, cyclic voltammetry (CV) recordings of single materials fail to accurately describe the energetic conditions at D‐A heterojunctions. Here, the unique capability of energy‐resolved electrochemical impedance spectroscopy (ER‐EIS) to detect the density of states (DOS) shows that the donor's IE is lower in blends than in the pristine material, thereby enabling hole transfer. In combination with the increased NFAs EA, enhanced charge generation quantum yields are obtained; however, ultimately, a trade‐off exists between photocurrent and photovoltage.

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