Current-voltage characteristics in organic semiconductor crystals: space charge vs. contact-limited carrier transport (original) (raw)
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Trap distribution and space-charge limited currents in organic crystals
Journal of Physics and Chemistry of Solids, 1969
A simplified theory of space-charge Iimited currents in a crystal with the 'diffuse trap level' has been discussed. The equations derived have been applied to experimental current-voltage and current-temnerature characteristics in anthracene crystals. The hole trap band having the bottom at -0.76 eV approximately has been found.
Exploiting diffusion currents at Ohmic contacts for trap characterization in organic semiconductors
Organic Electronics, 2014
Studying space-charge limited currents enables fundamental insight into the properties of charge carrier transport. However, in unipolar devices with Ohmic contacts, diffusion of charge carriers from the contacts into the intrinsic layer can dominate the current-voltage (J-V) characteristics, especially when the devices are thin as in organic electronic devices ($100 nm). Thus, the common approximation of drift-only trap-limited currents (J $ V lþ1 ) caused by an exponential distribution of traps is not applicable for determination of the trap distribution. Here, we show by numerical drift-diffusion simulations of unipolar devices with p-doped injection layers (p-i-p devices), how diffusion currents affect the J-V power law depending on the intrinsic layer thickness for typical transport parameters of organic semiconductors. As the thickness dependence of the power law is characteristic of the trap distribution, the distribution can be determined from a simple variation of the device thickness.
Journal of Applied Physics, 2018
Single-carrier devices are an excellent model system to study charge injection and charge transport properties of (doped) transport layers and to draw conclusions about organic electronics devices in which they are used. By combining steady-state and impedance measurements at varying temperatures of hole-only devices with different intrinsic layer thicknesses, we are able to determine all relevant material parameters, such as the charge mobility and the injection barrier. Furthermore, the correlation and sensitivity analyses reveal that the proposed approach to study these devices is especially well suited to extract the effective doping density, a parameter which cannot be easily determined otherwise. The effective doping density is crucial in organic light-emitting diodes (OLEDs) for realizing efficient injection, charge balance, and lateral conductivity in display or lighting applications. With the fitted drift-diffusion device model, we are further able to explain the extraordin...
Charge Transport in Organic Semiconductors
Chemical Reviews, 2007
Modern optoelectronic devices, such as light-emitting diodes, fieldeffect transistors and organic solar cells require well controlled motion of charges for their efficient operation. The understanding of the processes that determine charge transport is therefore of paramount importance for designing materials with improved structure-property relationships. Before discussing different regimes of charge transport in organic semiconductors, we present a brief introduction into the conceptual framework in which we interpret the relevant photophysical processes. That is, we compare a molecular picture of electronic excitations against the Su-Schrieffer-Heeger semiconductor band model. After a brief description of experimental techniques needed to measure charge mobilities, we then elaborate on the parameters controlling charge transport in technologically relevant materials. Thus, we consider the influences of electronic coupling between molecular units, disorder, polaronic effects and space charge. A particular focus is given to the recent progress made in understanding charge transport on short time scales and short length scales. The mechanism for charge injection is briefly addressed towards the end of this chapter. Keywords Charge carrier mobility Á Charge transport Á Organic semiconductors Á Molecular model Á Gaussian disorder model Á SSH model Á Organic optoelectronics Contents
2011
The orientational dependence of charge carrier mobilities in organic semiconductor crystals and the correlation with the crystal structure are investigated by means of quantum chemical first principles calculations combined with a model using hopping rates from Marcus theory. A master equation approach is presented which is numerically more efficient than the Monte Carlo method frequently applied in this context. Furthermore, it is shown that the widely used approach to calculate the mobility via the diffusion constant along with rate equations is not appropriate in many important cases. The calculations are compared with experimental data, showing good qualitative agreement for pentacene and rubrene. In addition, charge transport properties of core-fluorinated perylene bisimides are investigated.
Electrical characterization of organic semiconductors by transient current methods
Synthetic Metals, 2001
This paper presents a method for characterizing the transport properties of organic semiconductors by means of large-signal electrical transients. For validating the method, poly(phenylene-vinylene) (PPV) was chosen as a test material, thanks to its widely investigated properties. Large voltage steps are applied to Au/PPV/Au test structures and, from the resulting current transient, the carrier mobility is measured. Transient phenomena are shown to give insight on the physical properties of the material and on the technological characteristics of the device, while requiring a simpler test system in comparison to other methods, like impedance spectroscopy and time-of-¯ight measurements.