The Influence of Polymer Sequence on the Formation of Bulk-Heterojunctions in Organic Solar Cells

The rapid development of polymer solar cell technologies has lead to the implementation of multi-chromophore polymer structures, such as donor-acceptor, block, or statistical polymers that provide better energy-gap tuning ability or spectrum coverage, but also make controlling morphology more complicated. We compare the morphology and solar cell device performance of two fullerene acceptor with diffident crystallinity, PCBM and ICBA, and two selenophene-thiophene copolymers that have the same degree of polymerization and composition, and differ only in their sequence (statistical vs. block copolymers). P3HS-b-P3HT spontaneously undergoes phase separation and P3HS-s-P3HT does not. P3HS-b-P3HT performs best when the intrinsic self-assembled nanostructure is the most perturbed. P3HS-s-P3HT does not undergo intrinsic phase separation, and vapor annealing can be used to optimize the polymer:fullerene morphology, where better nanostructure is well correlated with the best device. On the other hand, different fullerene structures not only have distinct morphology evolution, but also affect the self-assembly of polymers. Inspired by previous results, we also propose a "blocky" structure that incorporates selenophene into a donor-acceptor copolymer in order to improve molecular ordering.