Influence of Nanoscale Phase Separation on the Charge Generation Dynamics and Photovoltaic Performance of Conjugated Polymer Blends:  Balancing Charge Generation and Separation

Abstract

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 ∼20 nm, significantly larger than the exciton diffusion length of ∼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 computersimulated 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.

Bicontinuous minimal surface nanostructures for polymer blend solar cells

“…As there are interfaces close to each other throughout the device, charge generation is uniformly spread so there are only small charge gradients, minimising the diffusion current. The range of FF values is similar to experimental findings 44 and for the limiting case of a bilayer structure (not shown) FF is 0.54. Fig.…”

“…10,43,44 From spectral data, this blend has been calculated to absorb 5.8% of the AM 1.5 solar emission spectrum, with an assumed Gaussian optical field profile. 9 We set w ex R 0 6 = 0.02, such that simulations of excitons in a single component material with energetic disorder s = 0.062 eV gives a diffusion length L ex of 6 nm 45 in a lifetime t ex of 500 ps.…”

“…Other parameter values are k diss = (100 fs) À1 , 49 k r = 1 Â 10 6 s À1 , e = 4, V bi = 1.3 V. D inj = 0.8 eV for electrons and 0.1 eV for holes based on the HOMO/LUMO levels of the polymers and the work functions of the electrodes. [50][51][52] The uncertainty in the hole injection barrier height was overcome by fitting dark current modelling to published experimental results, 44 ensuring the charge density and local field near the electrodes are correct. Fig.…”

“…Another problem is domain purity as minority components in the blend domains can have a measurable impact on the results as they act as dissociation, and hence recombination, centres. Recent experimental studies have found that the domain size of an optimised morphology can exceed the exciton diffusion length 44 even though domain purity 490% is common. 55,56 We repeated the calculations of IQE in the blend morphologies, this time creating additional islands by swopping voxels in the domains of each polymer, and then plotted the results against the original feature sizes to mimic experiment where the islands are invisible under AFM.…”

Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

“…The success of the P3HT:F8TBT system in particular may be attributed to the high charge-carrier mobility of the P3HT phase, which promotes efficient charge separation, [7] overcoming the problem of geminate recombination that has plagued many all-polymer blends. [8] Indeed, the higher fill factor and larger optimum film thickness can both be attributed to the reduction in required electric field strength to separate electron-hole pairs localized at blend interfaces. [9] Similar to P3HT/ fullerene blends, [10] annealing is required to optimize device performance.…”