Space-Charge-Limited Current Fluctuations in Organic Semiconductors

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Low-frequency current fluctuations are investigated over a bias range covering ohmic, trap-filling and space-charge-limited current regimes in polycrystalline polyacenes. The relative current noise power spectral density S(f) is constant in the ohmic region, steeply increases at the trap-filling transition region and decreases in the space-charge-limited-current region. The noise peak at the trap-filling transition is accounted for within a continuum percolation model. As the quasi-Fermi level crosses the trap level, intricate insulating paths nucleate within the ohmic matrix, determining the onset of non-equilibrium conditions at the interface between the insulating and conducting phase. The noise peak is written in terms of the free and trapped charge carrier densities.