Efficient non-fullerene organic photovoltaics printed by electrospray via solvent engineering.

Developing scalable processing methods with low material waste is still one of the remaining challenges for organic photovoltaics (OPVs) to become a practical renewable energy source. Here we report the first study on printing active layers of OPVs containing non-fullerene acceptors (NFA) by electrospray (ES). The properties of solvent significantly influence the interfacial morphology of ES-printed organic thin-films, and solvent engineering is essential to facilitate the formation of efficient active-layer films. We introduce low-vapor-pressure non-halogen solvent O-xylene (OXY) into the high-vapor-pressure solvent of chloroform (CF) to form a binary solvent system with appropriate evaporation time, electric conductivity and solubility. The characteristic times of the ES process using binary solvents are quantified to provide insights into the dynamic formation of thin films. Longer droplet evaporation time with decent solubility collectively decrease the roughness and domain size of the polymer: NFA blend films, thus increase the photo current and fill factor of the ES-printed OPV devices. The ES-printed active layers show enhanced crystallinity and phase separation of NFA molecules than the spin-coated films. The champion cell with ES-printed PTB7-Th:FOIC active layer exhibits PCE of 9.45%, which is on par with the spin-coated cells and is among the highest of spray deposited organic solar cells to date. This work demonstrates that electrospray is an effective method to prepare OPVs on NFAs.