Michael Cölle

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

We report on organic n-channel field-effect transistors and circuits based on C60 films grown by hot wall epitaxy. Electron mobility is found to be dependent strongly on the substrate temperature during film growth and on the type of the gate dielectric employed. Top-contact transistors employing LiF∕Al electrodes and a polymer dielectric exhibit maximum electron mobility of 6cm2∕Vs. When the same films are employed in bottom-contact transistors, using SiO2 as gate dielectric, mobility is reduced to 0.2cm2∕Vs. By integrating several transistors we are able to fabricate high performance unipolar (n-channel) ring oscillators with stage delay of 2.3μs.

Reproducible resistive switching in nonvolatile organic memories

Resistive switching in nonvolatile, two terminal organic memories can be due to the presence of a native oxide layer at an aluminum electrode. Reproducible solid state memories can be realized by deliberately adding a thin sputtered Al2O3 layer to nominal electron-only, hole-only, and bipolar organic diodes. Before memory operation, the devices have to be formed at an electric field of 109V∕m, corresponding to soft breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. The polymer acts as a current limiting series resistance.

Low-Voltage Ring Oscillators Based on Polyelectrolyte-Gated Polymer Thin-Film Transistors

There has been a remarkable progress in the development of organic electronic materials since the discovery of conducting polymers more than three decades ago. Many of these materials can be processed from solution, in the form as inks. This allows for using traditional high-volume printing techniques for manufacturing of organic electronic devices on various flexible surfaces at low cost. Many of the envisioned applications will use printed batteries, organic solar cells or electromagnetic coupling for powering. This requires that the included devices are power efficient and can operate at low voltages. This thesis is focused on organic thin-film transistors that employ electrolytes as gate insulators. The high capacitance of the electrolyte layers allows the transistors to operate at very low voltages, at only 1 V. Polyanion-gated p-channel transistors and polycation-gated n-channel transistors are demonstrated. The mobile ions in the respective polyelectrolyte are attracted towards the gate electrode during transistor operation, while the polymer ions create a stable interface with the charged semiconductor channel. This suppresses electrochemical doping of the semiconductor bulk, which enables the transistors to fully operate in the field-effect mode. As a result, the transistors display relatively fast switching (≤ 100 µs). Interestingly, the switching speed of the transistors saturates as the channel length is reduced. This deviation from the downscaling rule is explained by that the ionic relaxation in the electrolyte limits the channel formation rather than the electronic transport in the semiconductor. Moreover, both unipolar and complementary integrated circuits based on polyelectrolyte-gated transistors are demonstrated. The complementary circuits operate at supply voltages down to 0.2 V, have a static power consumption of less than 2.5 nW per gate and display signal propagation delays down to 0.26 ms per stage. Hence, polyelectrolyte-gated circuits hold great promise for printed electronics applications driven by low-voltage and low-capacity power sources.

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

The reliability of organic 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 observed, 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 observed shift in threshold voltage. The changes of the potential are attributed to positive immobile charges, which contribute to the potential, but not to the current.

Separate charge transport pathways determined by the time of flight method in bimodal polytriarylamine

Time of flight photocurrent transient studies on thin films of bimodal polytriarylamine (PTAA) show two distinct and separate arrival times for hole transport in the same sample at a single field. The corresponding mobilities differ by two orders of magnitude, typically μfast∼10−3 cm2 V−1 s−1 and μslow∼10−5 cm2 V−1 s−1 at room temperature, and are measured parametric in electric field and temperature. The mobility data are analyzed using the correlated disorder model by Novikov, yielding a fitting parameter set. The two conduction paths are believed to come about as a result of phase segregation between the shorter and longer polymer chains with the shorter chains giving rise to the faster conduction pathways (as confirmed by results obtained for monomodal, shorter, and longer chain PTAA, by sample thickness scaling of the photocurrents and by reversal of the illuminated electrode). Separate arrival times are also obtained in a blend of the two short and long chain monomodal polymers. The phase separation...

LOW-FREQUENCY NOISE TO CHARACTERIZE RESISTIVE 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 αμ(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.