One of the best performing semiconducting polymers used in bulk heterojunction devices isPCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Herewe report the formation of bilayer ordering in PCDTBT, and postulate that this structural motifis a direct consequence of the polymer’s molecular design. This bilayer motif is composed ofa pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is instark contrast to the monolayer ordering found in other conjugated polymers. The crystallinebilayer phase forms at elevated temperatures and persists after cooling to room temperature.The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties,may guide the synthesis of new materials with improved optoelectronic properties.
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