We report the indoor performance of flexible organic photovoltaic devices utilizing quasi-amorphous ZnO/Ag/ZnO as the transparent conducting electrode. A ZnO/Ag/ZnO electrode with specific thickness values of 40/9/50 nm provides excellent transparent conducting electrode properties with transmittances up to 92% in the visible region, a sheet resistance of 4.8 Ω/sq, and a root-mean squared surface roughness value of 2.1 nm. In addition, the micro-cavity effect and quasi-amorphous structural properties of the ZnO/Ag/ZnO electrode allow further enhanced light absorption and mechanical stability, respectively. Poly (3-hexylthiophene):indene-C60 bisadduct photoactive layer-based inverted organic photovoltaics with the ZnO/Ag/ZnO (40/9/50 nm) electrode yield an averaged power-conversion efficiency of 12.3% under a light-emitting diode lamp with a luminance of 500 lux, which is 20% greater than the power-conversion efficiency value of the reference organic photovoltaics with an indium tin oxide electrode. Furthermore, the same organic photovoltaics on flexible polyethylene terephthalate substrates exhibit excellent mechanical stability (i.e., 92% of the initial power-conversion efficiency value is maintained even after 400 bending cycles with a bending radius of 9.55 mm), with averaged power-conversion efficiency values of 10.2% under the 500-lux light-emitting diode.