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In this work, a range of nonfullerene organic solar cells comprising two perylene diimide (PDI)-based small molecule acceptors in combination with four representative polymer donors have been investigated and compared. In addition to significant differences in the power conversion efficiency, the energy losses of photovoltaic devices vary widely for these two PDI-based acceptors when paired with different donors. The sensitive Fourier-transform photocurrent spectroscopy (FTPS) and electroluminescence (EL) measurements have been performed to quantify their respective energetic offsets (ΔE_offset) and energy losses, with the aim of understanding the distinct energy losses in the studied organic blends. By comparing these results, we find that with decreasing ΔE_offset both nonradiative recombination loss and radiative recombination loss due to the charge-transfer state absorption are suppressed; as a result …