We present a novel insight on the light-soaking effect, in which the open-circuit voltage (V_oc) of inverted polymer solar cells improves with increase in light irradiation time. We previously have found that there are two kinds of the light soaking effects such as a short-circuit current density dependent type and a V_oc dependent type. The expression of these effects needed to irradiate the light containing UV component. However, for 4-(triphenylphosphonio)butane-1-sulfonate (TPPBS)-modified indium tin oxide (ITO)/poly(3-hexylthiophene-2,5-diyl) (P3HT): [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au inverted cell, we first observed an increase in the V_oc by irradiating white light not containing UV component. After pre-treatment by applying forward bias under UV-cut light, the V_oc was improved even when the performance was evaluated by irradiating UV-cut light, that is, the V_oc values of 0.36, 0.42, 0.46 and 0.54 V were obtained by holding at forward voltages of 0, 0.3, 0.6, and 1.0 V, respectively. We discussed about this pre-treatment effect from the viewpoint of desorption of oxygen molecules adsorbing on ITO.