High Efficiency Organic Photovoltaics with a Thick (300 nm) Bulk Heterojunction Comprising a Ternary Composition of a PFT Polymer–PC₇₁BM Fullerene–IT4F Nonfullerene Acceptor

Due to the long wavelength absorption and a better morphology of the bulk heterojunction (BHJ), replacing PC71BM with the nonfullerene electron acceptor (NFA) IT4F significantly improves the power conversion efficiency (PCE) of polymer (PFT)-based binary organic photovoltaics (OPVs) from 8.1% to 10.9% with an active layer 120 nm thick. Including PC71BM in the PFT:IT4F binary OPVs, the PFT:IT4F:PC71BM ternary OPVs further enhance the PCE up to 12.9% when the blending weight ratio of PFT:IT4F:PC71BM is 1:1:0.25, for which the BHJ thickness is 120 nm. With a fine tuning of the OPV fabrication process, a high PCE of 12.4% (vs 9.24% for the binary OPV) has been achieved by devices with a thick (300 nm) BHJ, which is rare for NFA-containing polymer OPVs. We have conducted a comprehensive study, including Förster energy transfer by absorption and emission spectroscopy, thin film photoelectron spectroscopy and (electrochemical) cyclic voltammetry for the energy level of the molecular orbitals, BHJ morphology by AFM and TEM microscopy, polymer chain crystallite and alignment orientation by 2D GIWAXS, charge carrier mobility by space-charge limited current (SCLC), dynamic photoluminescence quenching for exciton dissociation, and charge recombination by light-intensity dependence of the short-circuit current density (JSC) and open-circuit voltage (VOC). Accordingly, we have elucidated why the blending weight ratio of 1:1:0.25 of the ternary system is the best and why PFT:IT4F:PC71BM ternary OPVs achieve a respectful PCE with such a thick (300 nm) BHJ.