Correlated Mobility Fluctuations and Contact Effects in p-Type Organic Thin-Film Transistors

Low-frequency noise (LFN) has been used in order to gain insight into the physical properties of the materials involved in organic thin-film transistors (OTFTs) fabrication, often with contradictory results. Besides the physical origin of noise, contact effects on noise have been a source of concern and discussion. In this paper, we report on accurate LFN measurements in p-type staggered top-gate OTFTs over four decades of channel current, from the subthreshold to the strong accumulation region. The measured spectra follow a clear 1/ $f$ behavior attributed to the trapping/detrapping of channel charge carriers into interface and oxide defects, while the influence of noise sources at contacts is found to be negligible. However, contacts affect the measured noise by a nonnegligible differential resistance. Noise data are interpreted in the context of a multitrap correlated mobility fluctuations (CMFs) model, showing that noise is dominated by acceptor-like traps. Despite the low mobility ( $\mu _{{\mathrm{eff}}}\sim $ 2 cm2/V/s), the large scattering parameter ( $\alpha \sim 10 ^{7}$ Vs/C) produces an increase of the noise at the higher currents due to CMFs. The product $\alpha \mu _{{\mathrm{eff}}} \approx 2\cdot 10^{7}$ cm2/C, which measures the strength of CMFs, is similar to what was reported for a-Si:H and much higher with respect to crystalline silicon MOSFETs revealing a strong correlation between CMFs and the state of disorder of the active layer.