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

Abstract

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 π–π 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 ∼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.