High-Resolution ac Measurements of the Hall Effect in Organic Field-Effect Transistors

Abstract

We describe a high resolving power technique for Hall effect measurements, efficient in determining Hall mobility and carrier density in organic field-effect transistors and other low-mobility systems. We utilize a small low-frequency ac magnetic field (B rms < 0.25 T) and a phase sensitive (lock-in) detection of Hall voltage, with the necessary corrections for Faraday induction. This method significantly enhances signal-to-noise ratio and eliminates the necessity of using high magnetic fields in Hall effect studies. With the help of this method, we were able to obtain Hall mobility and carrier density in organic transistors with mobility as low as μ ~ 0.3 cm 2 V-1 s-1 by using a compact desktop apparatus and low magnetic fields. We have found a good agreement between Hall effect and electric field-effect measurements, indicating that contrary to the common belief, certain organic semiconductors with mobilities below 1 cm 2 V-1 s-1 can still exhibit a fully developed, bandsemiconductor like Hall effect, with the Hall mobility and carrier density matching those obtained in longitudinal transistor measurements. This suggests that even when μ < 1 cm 2 V-1 s-1 , charges in organic semiconductors can still behave as delocalized coherent carriers. This technique paves way to ubiquitous Hall effect studies in a wide range of lowmobility materials and devices, where it is typically very difficult to resolve Hall effect even in very high dc magnetic fields.

Hall Effect in Polycrystalline Organic Semiconductors: The Effect of Grain Boundaries

“…The Hall mobility, μ Hall , was evaluated by measuring a Hall voltage, V Hall , across the channel (between the Hall probes), when a magnetic field, B , is applied perpendicular to the channel carrying a longitudinal source–drain current, I SD . The Hall measurements were performed via a high‐resolution ac‐Hall technique that has been recently shown to be very effective in evaluating the intrinsic charge transport properties of low‐μ systems such as OFETs . In this method, we use a low‐frequency (<1 Hz) small ac magnetic field (r.m.s.…”

“…In ternary blends, a C 60 F 48 molecular dopant is added in small quantities, which improves the field‐effect mobility of the blended OFETs . In order to investigate the intrinsic charge carrier mobility in these devices, we used two experimental techniques applied concurrently: longitudinal charge transport measurements in four‐probe FETs and ac‐Hall effect measurements . To the best of our knowledge, this is the first observation of a Hall effect in organic semiconductor blends, revealing a band‐like transport mechanism and a relatively high intrinsic charge carrier mobility in these systems.…”

Molecular Semiconductors for Logic Operations: Dead‐End or Bright Future?

“…The signal‐to‐noise ratio being one of the most important criteria in the detection of very small Hall voltages (i.e., µV range for doped conjugated polymers), Podzorov et al improved the measurement technique involving a low frequency magnetic field modulation (AC (alternative current) measurement vs conventional DC (direct current) one) that allowed them to significantly enhance the signal‐to‐noise ratio and eliminate the necessity of using high magnetic fields. This lead to the detection of a fully developed signal in tetracene single‐crystals presenting a mobility as low as 0.3 cm 2 V −1 s −1 combined to a coherence factor of 1 despite the initial thought of hopping transport due to an absence of Hall signal in previous dc measurements . Recent progress made by Watanabe et al in the understanding of the signal‐to‐noise and flicker noise present in electrical signals of organic semiconductors clearly support the quest for understanding and smaller 1/f noise on any voltage measurements in order to probe sub 1 microvolt Hall signals .…”

“…The Lorentz force being directly proportional to the velocity of the carriers, conventional thinking leads to the conclusion that high mobility inorganic semiconductors are expected to exhibit an ideal Hall effect upon gate accumulation while lower performance organic semiconductors, mostly affected by dynamic disorder, should produce nonideal answers . Obviously, standstill deep trapped carriers for which the Lorentz force is zero won't contribute.…”

Spatially Uniform Shallow Trap Distribution in an Ultrathin Organic Transistor

“…In the development of organic device applications, electrical noise levels set important reliability metrics. Recent efforts to understand discrepancies between transistor and Hall effect carrier densities rely upon very low noise transistor operation at very low frequencies . In the realm of niche applications, optimization of noise levels in organic films will push the boundaries of the already impressive sensing capabilities of organic diode‐based magnetometers .…”