Abstract
In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π–π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing efficient charge transport and high power conversion efficiency. Here, we demonstrate that side-chain engineering of non-fullerene acceptors could drive the fused-ring backbone assembly from a π–π stacking mode to an intermixed packing mode, and to a non-stacking mode to refine its solid-state properties. Different from the above-mentioned understanding, we find that close atom contacts in a non-stacking mode can form efficient charge transport pathway through close side atom interactions. The intermixed solid-state packing motif in active layers could enable organic solar cells with superior efficiency and reduced non-radiative recombination loss compared with devices based on molecules with the classic end-group π–π stacking mode. Our observations open a new avenue in material design that endows better photovoltaic performance.
Highlights
In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π–π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing efficient charge transport and high power conversion efficiency
While the recent emergence of advanced Non-fullerene small molecule acceptors (NFAs) has significantly boosted the power conversion efficiency (PCE) of BHJ organic solar cells (OSCs) by extending the absorption range, improving the charge generation efficiency and minimizing the recombination-related losses, current understanding of charge carrier dynamics and photo-physics is still strongly limited by the empirical model, in which a long-range structural ordering induced by end-group π–π stacking of NFAs is critical for efficient charge transfer and extraction as well as high photovoltaic efficiency
This statement is true but eliminates the charge-transfer/transport channels induced by close atom interactions, as observed in highquality organic crystals based on semiconductors free from π–π overlap (Herringbone packing) and systematically investigated in this work
Summary
In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π–π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing efficient charge transport and high power conversion efficiency. Multiple molecular packing motifs are regarded as essential in guiding different transport channels in small molecule organic semiconductors, as seen from previous research of polyaromatic hydrocarbons or thioacences that herring bond structure, onedimensional slip stack, and two-dimensional brick layer packing induce different transport properties and anisotropy[25] Such detailed molecular stacking manipulation can hardly be achieved in NFAs, partially due to the large size of NFA molecules that retard molecular crystallization, resulting in technical difficulties to obtain single crystals. Molecular and crystal engineering allow us to combine the two solid-state packing motifs together in a BHJ blend, leading to a PCE of 13.7%, surpassing the single-mode interaction dictated function We believe these observations are vital in new organic semiconductor material design that better performance can be obtained through detailed crystalline structure manipulation
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