Grazing-incidence wide-angle x-ray diffraction experiments confirm that proper choice of the fullerene casting co-solvent yields well-ordered interdispersed bulk heterojunction (BHJ) morphologies without the need for subsequent thermal annealing or the use of trace solvent additives (e.g., diiodooctane). Both semicrystalline (e.g., P3HT (poly(3-hexylthiophene-2,5-diyl)) and entirely amorphous (e.g., PSDTTT (polydithiophene-2,6-diyl)-alt-(2,5-bis(4,4'-bis(2-octyl)dithienosilole-2,6-diyl)thiazolothiazole)]) conjugated polymers can be processed into highly efficient photovoltaic devices using the solvent-blend SqP design rules.

In this way solar cell power conversion efficiencies are obtained for the PTB7 (polybenzodithiophene-2,6-diyl)(3-fluoro-2-thienothiophenediyl)]) and PC61BM (phenyl-C61-butyric acid methyl ester) materials combination that match those of blend-cast films. By tuning the Flory-Huggins χ parameter of the 2-CP/co-solvent blend, it is possible to optimally swell the polymer of interest for fullerene interdiffusion without dissolution of the polymer underlayer. Starting with a base solvent that has high fullerene solubility, 2-chlorophenol (2-CP), ellipsometry-based swelling experiments are used to investigate different co-solvents for the fullerene-casting solution.

Design rules are presented for significantly expanding sequential processing (SqP) into previously inaccessible polymer:fullerene systems by tailoring binary solvent blends for fullerene deposition.

Sequential processing