Revealing conflicting effects of solvent additives on morphology and performance of non-fullerene organic photovoltaics under different illuminance conditions

Abstract

The use of solvent additives is effective for controlling the morphology and efficiency of non-fullerene bulk-heterojunction (BHJ) organic photovoltaics (OPVs) under both indoor and outdoor conditions. In this study, we examined the effects of solvent additives on the morphology and photovoltaic operation of non-fullerene OPVs under different illuminance conditions. Solvent additives (e.g., 1,8-diiodooctane) improved molecular packing and charge transport in non-fullerene BHJ layers, increasing the power conversion efficiency (PCE) under 1-sun illumination. However, incorporating the solvent additives promoted the formation of a crystallization-induced roughened surface and an isolated-non-fullerene domain in the intermixing region. These unexpected morphological features increased the degree of trap-assisted charge recombination and leakage current, significantly reducing the PCE of non-fullerene OPVs at low light levels corresponding to indoor lighting. Understanding the effect of solvent additives inspires us to further control the morphological features of the non-fullerene OPVs by fine-tuning the solvent-additive concentration beyond the globally accepted level. This was effective for optimizing the indoor PCE of non-fullerene OPVs (21.71%), which exceeded that of the conventionally accepted reference (18.02%). This study provides guidelines for understanding the critical roles of solvent additives and optimizing the morphology of non-fullerene BHJs for high-performance indoor OPV applications.