Electrical conductivity measurements were performed with thin (50 μ) single crystals of p‐terphenyl, p‐quaterphenyl, and anthracene supplied with aqueous electrodes, one of which was an iodine‐iodide solution (acceptor electrode), and the other an iodide solution. The results strongly indicate that the acceptor electrode can form ohmic contact for hole injection into these crystals and that space‐charge‐limited currents can be drawn through them. The crystals were found to contain hole‐trapping states the location‐in‐energy of which can be approximated by a decreasing exponential distribution above the valence band. The measurements showed that the hole mobility in p‐terphenyl is about 3×10−2 cm2/v sec, is independent of the field at least up to about 4×104 v/cm, and that the hole‐trap concentration is at least 1013 cm−3. The acceptor electrode used does not form ohmic contact to crystals of naphthalene and diphenyl; an explanation for this is proposed. Some theoretical aspects of ohmic contact formation ...

Space‐Charge‐Limited Currents in Organic Crystals

Current–voltage, impedance, and transient conductance measurements have been carried out on indium-tin-oxide/poly(phenylene vinylene)/Al light emitting diodes. In these devices injection and transport is expected to be dominated by positive carriers. Fowler–Nordheim tunneling theory cannot account for the temperature dependence, the thickness dependence, or the current magnitude of the current–voltage characteristics. Space-charge limited current theory with an exponential distribution of traps is however in extremely good agreement with all of the recorded current–voltage results in the higher applied bias regime (approximately 0.7⩽V/d⩽1.6×106 V cm−1). This gives a trap density Ht of 5(±2)×1017 cm−3 and the product of μNHOMO of between 1014 and 5×1012 cm−1 V−1 s−1. Assuming NHOMO is 1020 cm−3 gives an effective positive carrier mobility between 10−6 and 5×10−8 cm2 V−1 s−1. The characteristic energy Et of the exponential trap distribution is 0.15 eV at higher temperatures (190⩽T⩽290 K), but this decreases...

Space-charge limited conduction with traps in poly(phenylene vinylene) light emitting diodes

Current injection in insulators provides a valuable technique for obtaining such information about defect states in insulators as their density, their energetic location in the forbidden gap, and their cross sections for capture of free electrons and holes. It also enables determination of free carrier drift mobilities. The physical principles underlying one-carrier space-charge-limited current injection, both steady-state and transient, and steady-state, two-carrier, largely neutralized current injection are discussed. Experimental verifications of the different types of current injection are also discussed briefly.

https://iopscience.iop.org/article/10.1088/0034-4885/27/1/307/pdf

Volume-controlled current injection in insulators

Anomalous room temperature magnetoresistance in organic semiconductors

Numerical methods have been used to solve the bipolar current problem for a single emitting layer between electrodes with explicit injection characteristics. We consider ohmic and tunneling contacts at the anode and/or cathode for various ratios of hole to electron mobility. Diffusion is included and found to have minimal effect on the recombination efficiency. The recombination profile is dictated mainly by the ratio of mobilities, less so by contacts. Maximum efficiency is obtained for two ohmic contacts. When the injection is imbalanced, higher efficiencies are achieved when the majority carrier has the lower mobility. At sufficiently high voltages, the current tends towards balance, allowing for maximum efficiency.

The roles of injection and mobility in organic light emitting diodes

The space‐charge‐limited electrical behavior of a trap‐free insulator containing both mobile electrons and holes is investigated under the approximation of neglecting diffusion current When both contacts are ohmic, the dependence of two‐carrier space‐charge‐limited current on voltage and interelectrode spacing remains the same as in the one‐carrier case; ie, the current varies as the square of the voltage and as the inverse cube of the spacing For given voltage and spacing, however, the two‐carrier current is usually orders of magnitude larger than the one‐carrier current This is in striking contrast to the case of space‐charge‐limited flow of electrons and positive ions in a vacuum The two‐carrier current in an insulator can be reduced to the size of the one‐carrier current either by making one contact highly blocking or by reducing the mobility of one of the carriers to zero While the first approach yields in detail the well‐known one‐carrier case, the second approach differs from the one‐carrier

W. Ruppel

Papers

Two‐Carrier Space‐Charge‐Limited Current in a Trap‐Free Insulator