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Three kinds of charged star‐shaped conjugated macroelectrolytes, named as PhNBr, TPANBr, and TrNBr, are synthesized as electron‐collecting interlayers for inverted polymer solar cells (i‐PSCs). Based on these well‐defined structured interlayer materials, the light soaking (LS) effect observed in i‐PSCs was studied systematically and accurately. The general character of the LS effect is further verified by studying additional i‐PSC devices functionalized with other common interlayers. The key‐role of UV photons was confirmed by electrochemical impedance spectroscopy and electron‐only devices. In addition, the ultraviolet photoelectron spectroscopy measurements indicate that the work function of the indium tin oxide (ITO)/interlayer cathode is significantly reduced after UV treatment. In these i‐PSC devices the LS effect originates from the adsorbed oxygen on the ITO substrates when oxygen plasma is used; however, even a small amount of oxygen from the ambient is also enough for triggering the LS effect, albeit with a weaker intensity. Our results suggest that the effect of adsorbed oxygen on ITO needs to be considered with attention while preparing i‐PSCs. This is an important finding that can aid the large‐scale manufacturing of organic solar cells via printing technologies, which do not always ensure the full protection of the device electrode substrates from oxygen. [ABSTRACT FROM AUTHOR]