Electrical instabilities in organic semiconductors caused by trapped supercooled water

It is reported that the electrical instability known as bias stress is caused by the presence of trapped water in the organic layer. Experimental evidence as provided by the observation of an anomaly occurring systematically at around 200 K. This anomaly is observed in a variety of materials, independent of the deposition techniques and remarkably coincides with a known phase transition of supercooled water. Confined water does not crystallize at 273 K but forms a metastable liquid. This metastable water behaves electrically as a charge trap, which causes the instability. Below 200 K the water finally solidifies and the electrical traps disappear. (c) 2006 American Institute of Physics

Characterisation of different polymorphs of tris(8-hydroxyquinolinato)aluminium(III) using solid-state NMR and DFT calculations

<p>Abstract</p> <p>Background</p> <p>Organic light emitting devices (OLED) are becoming important and characterisation of them, in terms of structure, charge distribution, and intermolecular interactions, is important. Tris(8-hydroxyquinolinato)-aluminium(III), known as Alq₃, an organomettalic complex has become a reference material of great importance in OLED. It is important to elucidate the structural details of Alq₃in its various isomeric and solvated forms. Solid-state nuclear magnetic resonance (NMR) is a useful tool for this which can also complement the information obtained with X-ray diffraction studies.</p> <p>Results</p> <p>We report here ²⁷Al one-dimensional (1D) and two-dimensional (2D) multiple-quantum magic-angle spinning (MQMAS) NMR studies of the meridional (<it>α</it>-phase) and the facial (<it>δ</it>-phase) isomeric forms of Alq₃. Quadrupolar parameters are estimated from the 1D spectra under MAS and anisotropic slices of the 2D spectra and also calculated using DFT (density functional theory) quantum-chemical calculations. We have also studied solvated phase of Alq₃containing ethanol in its lattice. We show that both the XRD patterns and the quadrupolar parameters of the solvated phase are different from both the <it>α</it>-phase and the <it>δ</it>-phase, although the fluorescence emission shows no substantial difference between the <it>α</it>-phase and the solvated phase. Moreover, we have shown that after the removal of ethanol from the matrix the solvated Alq₃has similar XRD patterns and quadrupolar parameters to that of the <it>α</it>-phase.</p> <p>Conclusion</p> <p>The 2D MQMAS experiments have shown that all the different modifications of Alq₃have ²⁷Al in single unique crystallographic site. The quadrupolar parameters predicted using the DFT calculation under the isodensity polarisable continuum model resemble closely the experimentally obtained values. The solvated phase of Alq₃containing ethanol has structural difference from the <it>α</it>-phase of Alq₃(containing meridional isomer) from the solid-state NMR studies. Solid-state NMR can hence be used as an effective complementary tool to XRD for characterisation and structural elucidation.</p

High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films

Published versio

Resistive switching and noise in non-volatile organic memories

The authors found quant. criteria to characterize the states of the device: (1) pristine devices show at low bias I proportional to Vm with m = 0 pointing to trap filling and at higher bias m = 6 pointing to tunneling. The 1/f noise is characterized by 10-7 &lt;.alpha..mu. (cm2/Vs) &lt;10-5; (2) forming state is a transition between pristine and switched-state. The time dependent soft breakdown in the Al-oxide goes hand in hand with strong discrete multi level resistive switching (RTS) with a 1/f 3/2 spectrum. Once the device is switched in the high (H-) or low (L-) conductance state it never comes back to the pristine state. (3) The H- or L-state is characterized by I proportional to Vm with either m = 1 or m = 3/2. The injection model predicts the current level and the dependence of the 1/f noise on current. Reliable switched devices show mainly 1/f noise. In the L-state there is often a 1/f 3/2 contribution on top of the 1/f noise indicating multi level switching. Reliable switches between the L- and H-state are characterized by a resistance R that changes for example by a factor 30 and the relative 1/f noise, fSI/I2C1/f follows the proportionality: C1/f proportional to R with a .alpha..mu.-value of .apprx.3 10-2 cm2/Vs. The explanation from the noise for C1/f proportional to R is that the no. of carrier in the transport switches due a change of the no. of parallel conducting paths in the polymer. The onset of switching seems to be at spots of the Al / Al2O3 / polymer interface

Low-frequency noise to characterizeresistive switching of metal oxide on polymer memories

Resistive switching in aluminum-polymer diodes has been investigated by noise measurements. Quantitative criteria to characterize the diode states are: (i) Pristine state shows I ¿ Vm with m ˜ 6 at higher bias typical for tunneling. The resistance is very high, 1/f noise is very low, but the relative 1/f noise, fSI/I2 = C1/f is very high. (ii) Forming state is a time-dependent soft breakdown in the Al-oxide that results in random telegraph signal noise (RTS) with a Lorentzian spectrum or in multi-level resistive switching (MLS) with a 1/f3/2 or 1/f-like spectrum. (iii) The H- or L-state shows I ¿ Vm with m = 1 for V 1V. Deviations from ohmic behavior are explained by space charge limited current in the polymer. Reliable H- and L-states show pure 1/f noise, a resistance R that changes by at least a factor 30 and 1/f noise that follows the proportionality: C1/f ¿ R with a proportionality factor aµ(cm2/Vs) of the same level as observed in metals, polymers and other semiconductors. C1/f ¿ R is explained by switching of the number of homogeneous conducting paths in parallel. Deviations in C 1/f ¿ R are also explained. In the pristine state and even in the H-state, only a fraction of the device are is carrying current and switching seems to be at spots of the Al/Al2O3/polymer interface

Scanning Kelvin probe microscopy on organic field-effect transistors during gate bias stress

The reliability of org. field-effect transistors is studied using both transport and scanning Kelvin probe microscopy measurements. A direct correlation between the current and potential of a p-type transistor is demonstrated. During gate bias stress, a decrease in current is obsd., that is correlated with the increased curvature of the potential profile. After gate bias stress, the potential changes consistently in all operating regimes: the potential profile gets more convex, in accordance with the simultaneously obsd. shift in threshold voltage. The changes of the potential are attributed to pos. immobile charges, which contribute to the potential, but not to the current

Scanning Kelvin probe microscopy on organic field-effect transistors during gate bias stress

The reliability of org. field-effect transistors is studied using both transport and scanning Kelvin probe microscopy measurements. A direct correlation between the current and potential of a p-type transistor is demonstrated. During gate bias stress, a decrease in current is obsd., that is correlated with the increased curvature of the potential profile. After gate bias stress, the potential changes consistently in all operating regimes: the potential profile gets more convex, in accordance with the simultaneously obsd. shift in threshold voltage. The changes of the potential are attributed to pos. immobile charges, which contribute to the potential, but not to the current

Charge trapping at the dielectric of organic transistors visualized in real time and space

Scanning Kelvin probe microscopy demonstrates that water-induced charge trapping at the SiO2 dielec. - visualized in real time and space - is responsible for the commonly obsd. gate-bias-induced threshold-voltage shift in org. field-effect transistors. When a bias is applied to the electrodes, charges are injected onto the SiO2. When the contacts are grounded, the charges are released again