$f^{-\gamma}$ Current Fluctuations in Organic Semiconductors: Evidence for Percolation

The $f^{-\gamma}$ sloped current noise power spectra, observed in organic semiconductors, have been interpreted within a {\em variable range hopping} mechanism of the fluctuations. The relative current noise power spectral density ${\cal S}(f)=S_I(f)/I^2$ exhibits a maximum at the {\em trap-filling transition} between the {\em ohmic} and the {\em space-charge-limited-current} regime [Phys. Rev. Lett., {\bf 95}, 236601, 2005]. Here, we discuss the electronic conditions determining the crossover from ohmic to space-charge-limited transport. These arguments shed further light on the need to adopt a {\em percolative} fluctuation picture to account for the competition between insulating and conductive phases coexisting at the {\em transition}, where small changes in the external bias lead to dramatic effects in the fluctuations.