Correlating anisotropy in polarized resonant soft X-ray scattering of block copolymer active layers with organic photovoltaic device performance

The use of Resonant Soft X-ray Scattering (RSoXS) allows the study of domain spacing and molecular orientation of conjugated polymers in the active layer of organic photovoltaics through tuning of the X-ray energy and polarization. Using the conjugated block copolymer system of poly(3-hexylthiophene) with acceptor blocks poly(fluorene-alt-dithienylbenzothiadiazole), (PFTBT), poly(carbazole-alt-dithienylbenzothiadiazole) (PCDTBT), and poly(phenylene-alt-dithienylbenzothiadiazole) (PPDTBT), along with alkyl side chain-added analogs PFT6BT, PCT6BT, and PPT6BT, we can examine the effects that morphological changes have on electronic device performance. The addition of hexyl side chains to the acceptor blocks decrease the degree of molecular order, as measured by the scattering anisotropy from RSoXS. This anisotropy has been shown to define the strength of alignment of chains with respect to the block copolymer interface within nematic domains, and can be used to directly compare long-range order between polymers. Based on our early findings, we believe that suppression of the long-range order is directly related to lower fill factors and lower device efficiencies. Studies into the charge carrier mobilities and crystallinity support these findings across the polymer systems.