Unraveling the Morphology of High Efficiency Polymer Solar Cells Based on the Donor Polymer PBDTTT-EFT

The microstructure of the polymer PBDTTT-EFT and blends with the fullerene derivative PC71BM that achieve solar conversion efficiencies of over 9% is comprehensively investigated. A combination of synchrotron techniques are employed including surface-sensitive near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and bulk-sensitive grazing-incidence wide angle X-ray scattering (GIWAXS). A preferential face-on orientation of PBDTTT-EFT is observed in the bulk of both pristine and blend thin films, with pi-pi stacking largely normal to the substrate, which is thought to be beneficial for charge transport. At the surface of the blend, a slight edge-on structure of the polymer is observed with side-chains aligned normal to the substrate. The effect of the solvent additive 1,8-diiodooctane (DIO) on solar cell efficiency and film microstructure is also investigated, where the addition of 3 vol% DIO results in an efficiency increase from approximate to 6.4% to approximate to 9.5%. GIWAXS studies indicate that the addition of DIO improves the crystallization of the polymer. Furthermore, atomic force microscopy and transmission electron microscopy are employed to image surface and bulk morphology revealing that DIO suppresses the formation of large PC71BM aggregates.