Measurement of Charge-Density Dependence of Carrier Mobility in an Organic Semiconductor Blend

Here, a new methodology for analyzing the charge‐density dependence of carrier mobility in organic semiconductors, applicable to the low‐charge‐density regime (1014–1017 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 µ is found to depend on charge density n according to a power law µ(n) ∝ nδ, where δ = 0.35. This dependence is shown to be consistent with an energetic disorder model based upon an exponential tail of localized intra‐band states. The charge‐density dependence of the carrier mobility of P3HT/PCBM solar cells is examined using charge‐extraction measurements combined with short‐circuit photocurrent data. The current increases sub‐linearly with charge density consistent with a charge density dependent carrier mobility μ(n). This charge density dependence is the same as determined for the bimolecular recombination coefficient k(n). This agrees with a multiple trapping model in which the carrier mobility and bimolecular recombination coefficient are both charge density dependent μ(n) ∝ k(n).