The objective of this study is to determine the photovoltaic properties of inverted organic photovoltaic devices (OPVs) that employ indium-tin oxide (ITO) modified by ethoxylated polyethylenimine (PEIE) as an electron collecting electrode under indoor lighting conditions. Owing to the reduced current generation at low light intensity, possessing a sufficiently large shunt resistance (R-P) associated with a low leakage current is of utmost importance for performance maximization. The photovoltaic properties under this lighting condition were optimized by controlling the thickness of the PEIE layers, and the electrical, optical, and surface properties of the PEIE-modified ITO were determined. Inverted OPVs based on poly(3-hexylthiophene)-indene-C-60 bisadduct (P3HT-ICBA) with an 8.5-nm-thick PEIE layer yielded a maximum power-conversion efficiency (PCE) of 13.9% under an LED light with a luminance of 5001x. Meanwhile, the same device showed poor performance with a PCE of 3.1% under 1 sun illumination owing to the insulating nature of PEIE.