One of the key issues with improving the efficiency of conjugated polymer-based photovoltaics is controlling the nm-scale morphology of the two active components. Typically, efficiency is optimized by trial-and- error: component weight ratios, solvent additives, post-processing thermal annealing, etc. are all used to attempt to obtain the desired bulk heterojunction (BHJ) interpenetrating network. In this talk, we discuss some of our recent work aimed at using alternate, more reproducible methods for controlling BHJ morphology. We present results showing new amphiphilic semiconducting polymers and fullerenes that spontaneously self-assembly into co-axial cylindrical micelles in solution, and thus provide a built-in connected network when cast into gels that form thin films. Photoexcitation of the polymer in these systems produces separated charges on sub-ps time scales, and the separated charges are stable for weeks to months in air-free environments. The stability of the charge-separated pair can be controlled by changing the gel morphology. We also show that sequential processing (SqP) of the polymer and fullerene components can provide an exquisite degree of control of the nm-scale morphology of a BHJ. By first casting the polymer layer and then intercalating the fullerene in a second casting step from a solvent that swells but does not dissolve the polymer, it is easy to create reproducible BHJ solar cells without the need for solvent additives or post-deposition processes such as thermal annealing. We show that there are simple design rules for finding the best solvents for SqP by measuring the Flory-Huggins interaction parameter, eliminating the need for trial-and-error to produce optimized polymer photovoltaics.

Avoiding the kinetics of the bulk heterojunction: Sequential processing and self-assembly for conjugated, polymer-based photovoltaics

251st National Meeting (2016)

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