Correlating the Efficiency and Nanomorphology of Polymer Blend Solar Cells Utilizing Resonant Soft X-ray Scattering

Enhanced scattering contrast afforded by resonant soft X-ray scattering (R-SoXS) is used to probe the nanomorphology of all-polymer solar cells based on blends of the donor polymer poly(3-hexylthiophene) (P3HT) with either the acceptor polymer poly((9,9-dioctytfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazolel-2',2 ''-diyl) (F8TBT) or poly(N,N'-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-12-bithiophene)) (P(NDI20D-T2)). Both P3HT:F8TBT and P3HT:P(NDI20D-T2) blends processed from chloroform with subsequent annealing exhibit complicated morphologies with a hierarchy of phase separation. A bimodal distribution of domain sizes is observed for P3HT:P(NDI20D-T2) blends with small domains of size 10 nm that evolve with annealing and larger domains of size 100 nm that are insensitive to annealing. P3HT:F8TBT blends In contrast show a broader distribution of domain size but with the majority of this blend structured on the 10 nm length sole. For both P3HT:P(NDI20D-T2) and P3HT:F8TBT blends, an evolution in device performance is observed that is correlated with a coarsening and purification of domains on the 5-10 nm length scale. Grazing-Incidence wide-angle X-ray scattering (GI-WAXS) is also employed to probe material crystallinity, revealing P(NDI20D-T2) crystallites 25-40 nm in thickness that are embedded In the larger domains observed by R-SoXS. A higher degree of P3HT crystallinity Is also observed in blends with P(NDI20D-T2) compared to F8TBT with the propensity of the polymers to crystallize in P3HT:P(NDI20D-T2) blends hindering the structuring of morphology on the sub-10 nm length scale. This work also underscores the complementarity of R-SoXS and GI-WAXS, with R-SoXS measuring the size of compositionally distinguishable domains and GI-WAXS providing information regarding crystallinity and crystallite thickness.