Charge percolation pathways in polymer blend photovoltaic diodes with sub-mesoscopic two-phase microstructures

We find that the external quantum efficiency of photovoltaic diodes based on finely mixed blends of poly-9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-l,4-phenylenediamine (PFB) and poly-9,9’- dioctylfluorene-co-benzothiadiazole (F8BT) depends strongly on the blend ratio. The peak external quantum efficiency is optimum for a PFB:F8BT ratio of 3:1. The difference of peak efficiency for this composition and a 1:1 ratio is significantly higher than the reported yield of charge-transfer excitons. From a surface topography analysis, we believe that charge percolation plays a crucial role in photocurrent efficiency in PFB:F8BT diodes. Furthermore, we present a qualitative model for different charge percolation pathways in diodes of different blend ratios.