Injection-limited and space charge-limited currents in organic semiconductor devices with nanopatterned metal electrodes

Abstract

Charge injection at metal-organic interfaces often limits the electric current in organic light-emitting diodes without additional injection layers. Integrated nanopatterned electrodes may provide a way to overcome this current injection limit by local field enhancements leading to locally space charge–limited currents. We compare electrical characteristics of planar and nanopatterned hole-only devices based on the charge transport material NPB with different thicknesses in order to investigate the nanopattern’s effect on the current limitation mechanism. Integration of a periodic nanograting into the metal electrode yields a current increase of about 1.5 to 4 times, depending on thickness and operating voltage. To verify the experimental results, we implement a finite element simulation model that solves the coupled Poisson and driftdiffusion equations in a weak form. It includes space charges, drift and diffusion currents, non-linear mobility, and charge injection at the boundaries. We find in experiment and simulation that the planar devices exhibit injection-limited currents, whereas the currents in the nanopatterned devices are dominated by space charge effects, overcoming the planar injection limit. The simulations show space charge accumulations at the corners of the nanopattern, confirming the idea of locally space charge–limited currents.

Morphological Electrical and Hardness Characterization of Carbon Nanotube-Reinforced Thermoplastic Polyurethane (TPU) Nanocomposite Plates

“…As shown in Figure 5, the use of logarithm scales in both the I-and Vaxes allowed us to identify at least two linear tendencies in both curves: in the low voltage region, both systems show linear behavior with a slope close to 1 (I ~ V), a characteristic value for an ohmic conduction mechanism; in the high voltage region, ohmic behavior is still present in the TPU plate with 7 wt% in MWCNT, while the system with 5 wt% presents a slope of 1.6, indicating the presence of a non-ohmic conduction. Comparing these results with those found in the IS experiments, the ohmic behavior of the TPU/MWCNT plate with 7 wt% is associated with the percolation conduction mechanism that seems to dominate the electrical behavior of the nanocomposite; however, for a TPU/MWCNT plate with 5 wt%, the non-ohmic behavior observed at the high-voltage region can be associated with competition of ohmic conduction by the electron percolation process and the trap-limits space charge conduction in those regions where a tunneling injection process occurs, taking into consideration that tunneling conduction can be transformed into space charge conduction when the excitation voltage is increased [54,55]. As shown in Figure 4, R 0 exhibits the highest conductivity (σ 0 ~0.01 S/cm), and it is almost constant for 1, 3 and 5 wt% MWCNT loading (for 7 wt%, the error value is greater than the adjusted R 0 ; thus, we have omitted it), showing that its value does not depend on the MWCNT content, such that it is caused by the resistance of the Au electrodes and wire [35].…”

Hole transport through an isotype amorphous-crystalline Ge2Sb2Te5 heterojunction under forward bias

“…But, if the rate at which holes are supplied by the injecting contact exceeds the maximum rate at which they can be transported through the heterojunction, there will be an accumulation of holes at the contact. This space-charge counteracts the electric field applied across the device so that the hole current density will be limited (i.e., space-charge-limited operation) [27].…”

“…Supposing d a-GST ? x a-GST so that the slope across the a-GST QNR is modest in magnitude, it is reasonable to make the approximation in equation (27). Therefore, based on the following definitions…”

A study of bottom-contact organic thin-film transistors based on source/drain tunneling structure

“…These methods can not only improve the energy level, but also can reduce the interface defects, so as to reduce source/drain contact resistance. It is noteworthy of mentioning that, the total OTFT resistance of OTFT is made up of the sum of the channel resistance and the contact resistance, the latter being determined by the carrier injection at organic semiconductor/metal electrode interface [21]. In this article, an electrode decorating method by electrode treatment and insulating tunneling layer introduction at source/drain contact is proposed to suppress large contact resistance due to low crystalline quality at organic semiconductor/metal contact interface of bottom-contact OTFT.…”