Here, a new methodology for analyzing the charge-density dependence of carrier mobility in organic semiconductors, applicable to the low-charge-density regime (10(14)-10(17) cm(-3)) corresponding to the operation conditions of many organic optoelectronic devices, is reported. For the P3HT/PCBM blend photovoltaic devices studied herein, the hole mobility mu is found to depend on charge density n according to a power law mu(n) alpha n(delta), where delta = 0.35. This dependence is shown to be consistent with an energetic disorder model based upon an exponential tail of localized intra-band states.