On the understanding of energy loss and device fill factor trade-offs in non-fullerene organic solar cells with varied energy levels

Fill factor (FF) is an important parameter governing the power conversion efficiency (PCE) in non-fullerene organic solar cells (NF–OSCs), which however is less studied than the other two parameters (short-circuit current J_sc and open-circuit voltage V_oc). To understand how energy offsets, exciton and charge carrier dynamics impact the FF, four groups of bulk heterojunctions (BHJs) NF–OSCs are investigated with FFs varying from 0.61 to 0.78 under progressive changes of HOMO-HOMO offsets (Δ_HOMOs, from 0.09 to 0.24 eV). By pump-probe optical spectroscopy, we find that the FF exhibits a positive dependence on Δ_HOMO and charge-separated state lifetime (τ_CS) in the blends, a result of inhibited back charge transfers and recombination at the donor-acceptor interface under higher Δ_HOMOs. Moreover, we observe a fast charge extraction with decreased sensitivity to internal electric-fields in high-FF devices. Despite these merits, the gains of FF are at the expense of increasing the voltage loss to non-radiative recombination in our studied systems. The combined results suggest that remaining appropriate energetic offsets is essential for controlling the carrier dynamics with longer-lived CS-states, restraining charge back transfer and reducing charge recombination toward high FFs and photovoltaic efficiencies.