Quinoxaline-Based Wide Band Gap Polymers for Efficient Nonfullerene Organic Solar Cells with Large Open-Circuit Voltages

다국어 초록 (Multilingual Abstract)

<P>We present here a series of wide-band-gap (<I>E</I><SUB>g</SUB>: >1.8 eV) polymer donors by incorporating thiophene-flanked phenylene as an electron-donating unit and quinoxaline as an electron-accepting co-unit to attain large open-circuit voltages (<I>V</I><SUB>oc</SUB>s) and short-circuit currents (<I>J</I><SUB>sc</SUB>s) in nonfullerene organic solar cells (OSCs). Fluorination was utilized to fine-tailor the energetics of polymer frontier molecular orbitals (FMOs) by replacing a variable number of H atoms on the phenylene moiety with F. It was found that fluorination can effectively modulate the polymer backbone planarity through intramolecular noncovalent S···F and/or H···F interactions. Polymers (P2-P4) show an improved molecular packing with a favorable face-on orientation compared to their nonfluorinated analogue (P1), which is critical to charge carrier transport and collection. When mixed with IDIC, a nonfullerene acceptor, P3 with two F atoms, achieves a remarkable <I>V</I><SUB>oc</SUB> of 1.00 V and a large <I>J</I><SUB>sc</SUB> of 15.99 mA/cm<SUP>2</SUP>, simultaneously, yielding a power-conversion efficiency (PCE) of 9.7%. Notably, the 1.00 V <I>V</I><SUB>oc</SUB> is among the largest values in the IDIC-based OSCs, leading to a small energy loss (<I>E</I><SUB>loss</SUB>: 0.62 eV) while maintaining a large PCE. The P3:IDIC blend shows an efficient exciton dissociation through hole transfer even under a small energy offset of 0.16 eV. Further fluorination leads to the polymer P4 with increased chain-twisting and mismatched FMO levels with IDIC, showing the lowest PCE of 2.93%. The results demonstrate that quinoxaline-based copolymers are promising donors for efficient OSCs and the fluorination needs to be fine-adjusted to optimize the interchain packing and physicochemical properties of polymers. Additionally, the structure-property correlations from this work provide useful insights for developing wide-band-gap polymers with low-lying highest occupied molecular orbitals to minimize <I>E</I><SUB>loss</SUB> and maximize <I>V</I><SUB>oc</SUB> in nonfullerene OSCs for efficient power conversion.</P>
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