Space-Charge-Limited Current Fluctuations in Organic Semiconductors

Low-frequency current fluctuations are investigated over a bias range covering {\em ohmic}, {\em trap-filling} and {\em space-charge-limited current} regimes in polycrystalline polyacenes. The relative current noise power spectral density ${\cal S}(f)$ is constant in the {\em ohmic} region, steeply increases at the {\em trap-filling transition} region and decreases in the {\em space-charge-limited-current} region. The {\em noise peak} at the {\em trap-filling transition} is accounted for within a {\em 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 {\em noise peak} is written in terms of the free and trapped charge carrier densities.