High efficiency all-polymer solar cells realized by the synergistic effect between the polymer side-chain structure and solvent additive

By adopting a series of donor-acceptor (D-A) polymers containing Benzo[1,2-b: 4,5-b'] dithiophene (BDT) and thieno[3,4-c] pyrrole-4,6-dione (TPD) with different numbers of alkyl aromatic side-chains, we demonstrate a high optimized PCE of 4.35% for all-polymer solar cells by incorporating an n-type polymer N2200. Through systematic characterization of tapping mode atomic force microscopy (AFM), 2-dimensional grazing-incidence X-ray diffraction (2d-GIXD), photoluminescence spectra and peak force-kelvin probe force microscopy (PF-KPFM), we have shown that the introduction of alkyl aromatic side chains to the donor polymer backbone is beneficial for the intermolecular pi-pi stacking and hence improves the polymer crystallinity as well as hole mobility. More importantly, we discovered that conjugated side-chains and additives can work synergistically to restore the intermolecular stacking of donor-acceptor polymers in the as-cast amorphous blend film and meanwhile develop fine phase segregation for efficient exciton dissociation and transport. As a result, the donor polymer PTP8 with fully alkyl aromatic side chains demonstrated an improved short-circuit current density (J(sc)), a high open-circuit voltage (V-oc) of similar to 1.00 V and a power conversion efficiency (PCE) of 4.35% after the addition of 0.5% DIO, which is among the highest reported efficiencies for all polymer solar cells.