Isolating and quantifying the impact of domain purity on the performance of bulk heterojunction solar cells†
In solution-processed organic bulk heterojunction (BHJ) solar cells, the purity of the phase-separated domains is known to play an important role in determining device function. While the effects of domain purity have been investigated by tuning of the BHJ morphology, such tuning typically results in several parameters (for example domain size and crystallinity) being varied at once. Here we show that by varying the time between spin-coating and the application of an anti-solvent treatment, the domain purity of the polymer-rich phase in PBDTTT-EFT:PC71BM blends can be tuned while keeping other morphological parameters constant. This unique approach enables the effect of domain purity on device function to be isolated and quantified. Over the purity range explored, solar cell power conversion efficiency is observed to monotonically increase from 7.2% to 9.6% with increasing domain purity, with the cell fill factor most affected by changes in domain purity. Employing transient photovoltage measurements we find that purer phases result in a reduction in the rate constant of bimolecular recombination. A more thorough treatment is also presented on the relationship between the total scattering intensity (derived from resonant soft X-ray scattering measurements) and domain purity. In particular it is shown that domain purity does not scale linearly with total scattering intensity requiring an initial estimate of absolute domain composition.