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Shunt resistance (RSH) is obtained via reproducing the real photovoltaic performance of organic solar cells based on the full model of the Shockley equation. With an increasing illumination intensity, the RSH of devices decreases markedly, which is attributed to an increase in the free volume of their photoactive layer. The thermal vibration of molecules (polymeric segments) is markedly enhanced under high-intensity illumination (photoexcitation), loosening intermolecular packing and thereby increasing the free volume of the photoactive layer. In addition, the larger surface roughness of the photoactive layer markedly decreases the RSH of the device. Under 1 sun AM 1.5 G illumination, the values of RSH in state-of-the-art organic solar cells are estimated to be about 500–3000 Ω cm2, while under 500 lux illumination, RSH is calculated to be ∼26 000 Ω cm2. This study points out that it is crucial to significantly increase RSH in organic solar cells under illumination, which is very helpful for efficiency improvement under low-intensity illumination and the large-area fabrication of organic photovoltaics. [ABSTRACT FROM AUTHOR]