A fill factor (FF) in polymer solar cells is dependent on bimolecular recombination of charges especially in a thick active layer.  The thick active layer is favorable for harvesting many more photons in the solar light, but is unfavorable for collecting charge carriers to each electrode.  Rather, bimolecular recombination is dominant and hence lowers FF.  Recently, several polymer solar cells have been reported to exhibit high FFs even with a thick active layer.  In this study, we have discussed FF in polymer solar cells based on PNTz4T^[1], a naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole (NTz)-based polymer, or its fluorinated derivatives and a C70 fullerene derivative (PC₇₁BM).  In these systems, the position of fluorine atoms impacts the thickness dependence of FF.  The solar cell based on PNF4T, in which the fluorine atoms were introduced in the NTz unit, exhibits high FF even with a thick active layer.  The solar cells based on PNTz4TF2 and PNF4TF2, in which the fluorine atoms were introduced in the thiophene units, exhibit low FF with a thick active layer.  In order to discuss the difference in the FFs, we studied bimolecular recombination losses by measuring transient photovoltage/transient photocurrent (TPV/TPC) of these solar cells with a thin or thick active layer.

 Bimolecular recombination reduction factor is given by the ratio of bimolecular recombination rate constant (k_rec) to diffusion-limited Langevin rate constant (k_L) ζ = k_rec / k_L.  Here, k_rec was evaluated from charge lifetime (τ_n) and charge density (n) measured by TPV/TPC methods.  In addition, k_L was evaluated from charge mobility measured by the space-charge limited current (SCLC) method.  For the thin devices, ζ was evaluated to be on the order of 10⁻², which indicates that bimolecular recombination is substantially suppressed in all the thin devices.  Among them, ζ of PNTz4T and PNF4T-based devices is smaller than that of PNTz4TF2 and PNF4TF2-based devices, which is consistent with the thickness dependence of FF for each device.  For the thick devices, ζ was evaluated to be on the order of 10⁻² for PNTz4T, PNF4T, and PNTz4TF2, and to be on the order of 10⁻¹ for PNF4TF2.  This finding indicates that bimolecular recombination is still suppressed in the PNTz4T, PNF4T, and PNTz4TF2-based thick devices but rather is close to the Langevin recombination in the PNF4TF2-based thick device.  This is mainly because charge lifetime is shorter in the thick cell than in the thin solar cell based on PNF4TF2.  Such a thickness-dependent charge lifetime suggests that the active layer morphology would be dependent on the layer thickness.  We therefore conclude that it is required for achieving high FF even with a thick active layer not only to suppress bimolecular recombination but also to maintain desirable film morphology for charge transport.