Current Voltage Characteristics and Excess Noise at the Trap Filling Transition in Polyacenes

Experiments in organic semiconductors (polyacenes) evidence a strong super quadratic increase of the current–voltage (I–V) characteristic at voltages in the transition region between linear (Ohmic) and quadratic (trap-free space-charge-limited current) behaviors. Similarly, excess noise measurements at a given frequency and increasing voltages evidence a sharp peak of the relative spectral density of the current noise in concomitance with the strong superquadratic I–V characteristics. Here, we discuss the physical interpretation of these experiments in terms of an essential contribution from ¯eld-assisted trapping-detrapping processes of injected carriers. To this purpose, the fraction of ¯lled traps determined by the I–V characteristics is used to evaluate the excess noise in the trap-¯lled transition (TFT) regime. We have found an excellent agreement between the predictions of our model and existing experimental results in tetracene and pentacene thin ¯lms of di®erent length in the range 0:65 35 m.