Multimodal Characterization of Non-Fullerene Organic Solar Cells Based to Assess the Effectiveness of Solvent Plasticizers

Organic solar cells (OSCs) are attractive due to their low-cost, processing simplicity, lightweight, flexibility and earth abundant materials. Synthesis and fabrication of non-fullerene acceptors (NFAs) allowed the power conversion efficiency (PCE) of OSCs to rise, approaching 20%. Solvent plasticizers have been successfully used as solvent additive or via vapor solvent annealing for example to maximize PCE in many fullerene based OSCs. Solvent plasticizers are used in most of the high performing NFA OSCs including PM6:Y6 systems. Whether usage of plasticizers is an effective fabrication method of NFAs requires further investigation, especially to examine potential effects on long-term stability and industrial viability of this class of materials. Here, we exploited synchrotron X-ray techniques and electron microscopes to examine the effects of plasticizers on active-layer morphologies in multiple NFA devices. Our findings suggest that as the amount of plasticizer solvents increases; domain size, purity and molecular packing improve in a favorable way for charge generation and extraction. However, we find a universal issue among all the investigated NFA systems when exceeding the optimal mount of plasticizers. Those NFAs seem to have a propensity to massively aggregate and crystalize even with a slight increase in the amount solvent, which hinders charge separation and overall device performance. This suggests that fabrication of NFAs devices with solvent plasticizers has to be done carefully to avoid undesired nanostructures.