The Role of Energy Offsets on Charge Photogeneration Dynamics in Y‐Series Molecules‐Based Polymer Solar Cells

Recent research has revealed that low‐energy offset polymer solar cells (PSCs) are capable of a power conversion efficiency of over 19%. However, it is unclear how energy offsets and the charge photogeneration process are correlated. Herein, the effect of energy offsets on charge photogeneration dynamics for Y‐series molecules (Y5, Y6, Y10, and BTP‐4F‐12)‐based PSCs with the variations of the lowest unoccupied molecular orbital energy offsets (ΔE_LUMO) of 0.11–0.42 eV and the highest occupied molecular orbital energy offsets (ΔE_HOMO) of 0.08–0.23 eV utilizing steady‐state and time‐resolved spectroscopies is studied. The steady‐state measurement shows that the probability of photoluminescence quenching via energy transfer for the donor exciton reduces with the increasing ΔE_LUMO. It is found that even in PM6:Y6 with the highest ΔE_LUMO, ≈18% of PM6 exciton dissociated via the path of “energy transfer first and then hole transfer,” manifesting the energy transfer also plays a vital role in the process of exciton dissociation. Furthermore, it is found that the PM6 exciton can efficiently dissociate under the ΔE_LUMO of 0.11 eV. After photoexcitation of the Y‐series molecule acceptors, the exciton dissociation efficiency enhances with the increase of ΔE_HOMO. Besides, the higher energy offsets, the lower charge recombination rate in the ultrafast timescale has been found from the transient absorption measurement. These findings reveal that energy offsets are important for charge photogeneration and recombination in an ultrafast timescale for Y‐series molecule‐based PSCs, which may shed light on the design of high‐performance PSCs.