Influence of nanoscale phase separation on the charge generation dynamics and photovoltaic performance of conjugated polymer blends: Balancing charge generation and separation

Through controlled annealing of intimately mixed blends of the polyfluorene copolymers poly(9,9'-dioctylfluorene-co-bis(N,N'-(4,butylphenyl))bis(N,N'-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) we observe the change in charge generation dynamics and photovoltaic performance as the length of nanoscale phase separation is varied from 5 nm or less to greater than 40 nm. We find that device efficiency is optimized for a phase separation of similar to 20 nm, significantly larger than the exciton diffusion length of similar to 5-10 nm. Femtosecond time-resolved transient absorption measurements confirm that the charge generation time is longer and charge generation efficiency is lower in films with a more evolved morphology. Photoluminescence quantum efficiency is also observed to monotonically increase with annealing temperature consistent with a decrease in exciton dissociation resulting from a coarsening of phases. Using a Monte Carlo model of exciton diffusion and dissociation in computer-simulated structures, we infer that the domains have purity of > 95% and find good agreement between the observed photoluminescence quenching and measured domain sizes. Charge transport studies of single-carrier devices show that charge transport through the blend does not significantly improve as device performance. improves, and photocurrent is observed to scale linearly with light intensity independent of blend morphology and device geometry. We conclude that the recombination of geminate charge pairs is limiting device performance, with the optimum phase separation of 20 nm balancing the efficiency of charge generation and charge separation.