f<Superscript>-γ</Superscript> current fluctuations in organic semiconductors: evidence for percolation

The f<Superscript>-γ</Superscript> sloped current noise power spectra, observed in organic semiconductors, have been interpreted within a variable range hopping mechanism of the fluctuations. The relative current noise power spectral density <InlineEquation ID="Equ1"> <EquationSource Format="TEX">${\cal S}(f)=S_I(f)/I^2$</EquationSource> </InlineEquation> exhibits a maximum at the trap-filling transition between the ohmic and the space-charge-limited-current regime [Phys. Rev. Lett. <Emphasis Type="Bold">95, 236601 (2005)]. Here, we discuss the electronic conditions determining the crossover from ohmic to space-charge-limited transport. These arguments shed light on the need to adopt a percolative fluctuation picture to account for the competition between insulating and conductive phases coexisting at the transition, where small changes in the external bias lead to dramatic effects in the fluctuations. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2006