ИСТИНА

Blends of conjugated polymers with small-molecule acceptors are widely used as active layers oforganic solar cells. The blend morphology is of high importance for the deviceperformanceand is strongly dependent on the interplay between the donor-donor, acceptor-acceptor and donor-acceptor interactions.Some conjugated polymersform charge transfer complexes(CTCs)with small-molecule acceptors modifying donor-acceptor interaction1.CTCscan be inherited from solution into thefilmresulting in dramatic changes in the film photophysics2.However, a systematical study of the role of the donor-acceptor charge-transfer interaction in the blend morphology was not performed yet. In this work, we address the effectof the donor-acceptorinteraction on the film morphology and photophysics for blends of model conjugated polymer MEH-PPV with electron acceptors of the fluorene series.The donor-acceptor interaction was tuned by varyingthe acceptor electron affinity(EA), since CTC is stronger for higher EA.We observedastepwise morphology change when the acceptor EAexceeds a threshold value.Specifically, the X-ray diffraction data(Fig. 1a,b)indicatean abrupt crossover from the segregated polymer and acceptor phases fortheblends with low EA(< 3.7 eV)acceptors to the semicrystalline intermixed structurefor the blendswith high EA(> 3.7 eV)acceptors. According to Raman spectroscopy data(Fig. 1b,black dots), this crossover is accompanied by a stepwise increase in ground-state charge transferfor high-EAacceptors.Similar stepwise behavior wasobserved in polymer photoluminescencequenching(Fig. 1b, blue triangles) showing the close relation between the morphology and photophysics.To explain our data, we proposea morphological model based on the Flory-Hugginstheory.