Munira Raja

Conduction processes in conjugated, highly regio-regular, high molecular mass, poly(3-hexylthiophene) thin-film transistors

Highly regio-regular poly(3-hexylthiophenes) (P3HT) thin films have been produced using the Tzrnadel method. They have head to tail counts approaching very close to 100% and high molecular mass. Thin-film transistors and Schottky diodes have been used to study the effects of counter ion and carrier density on field-effect and bulk mobility, respectively. The density of counter ions was increased using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and had a profound effect on both the field effect and bulk mobility with a ratio between the two of 102, in a similar way to DDQ in poly(β′-dodecyloxy-α,α′,-α′,α″terthienyl) (polyDOT3), but with substantially higher drift mobilities. A method is described, using Schottky barriers, of simply and accurately determining carrier drift mobility. The effect of carrier density on hole mobility is believed to be indirect, filling traps in the regions separating the highly ordered domains until trap free conduction and hence high mobility is reached.

The properties of MOS structures using conjugated polymers as the semiconductor

Data is presented from electrical measurements on MOS structures using the conjugated polymer regioregular poly-3-hexyl-thiophene (P3HT), a derivative of polythiophene. Improvement of the molecular mass and chain length is achieved using a process of fractionation. This is expected to produce an improvement in the electronic properties of the polymer. Comparison between fractionated and unfractionated P3HT on both MOS and TFT devices is presented. Metal contacts to P3HT are also studied with the formation of Schottky barriers readily observed. Current is found to fit standard theory and allows calculation of dopant levels and barrier heights all of which show good agreement with the ideal theory.

The significance of Debye length in disordered doped organic devices

The need for thin isolated active layers in organic electronic devices is becoming a critical issue for enhanced performance. Scaling-down the thickness of the layers causes a significant reduction in off-currents, however, also lowers on-currents with consequent effects on the drive capability. The Debye length is a fundamental material parameter of importance in understanding such scaling issues in devices. Here, we develop Debye length models for disordered doped organic devices operating under both accumulation and depletion regimes. The models are expressed in terms of the characteristic temperature associated with the distribution of the density of states. We show that for the same doping level, the Debye length of an organic semiconductor is smaller than that of silicon, which is most likely due to its low relative permittivity. This consequently also results in a higher ratio of the depletion width over the Debye length, suggesting the deployment of the abrupt depletion edge approximation in organics to be appropriate. Good agreement of the theory is demonstrated with the experimental data of the reverse current-voltage characteristics of P3HT Schottky diodes.

Critical Considerations in Polymer Thin-Film Transistor (TFT) Dielectrics

We present a study of aqueous and plasma anodised aluminium oxide (Al2O3) and its performance in thin film transistors (TFTs). The current through the oxide was measured with aluminium electrodes and with one of the electrode replaced by poly(3-hexylthiophene)(P3HT). The current increased by up to 2 orders of magnitude with P3HT. The current increased further when the polymer was doped with different percentages of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). It was also found to be dependent on the thickness of the polymer film. Surprisingly, the oxide current fell to its initial value when the polymer film was removed. Two mechanisms may explain the behaviour in these devices: charge injection and/or displacement. C-V plots were obtained from the MOS capacitors and were frequency dependent. They also showed substantial hysteresis, with a lateral shift along the voltage axis. This indicates the presence of a mobile species that increases with the concentration of dopant. We deduce that much of the increased gate current is associated with displacement currents induced by ion motion.

Analytical device models for disordered organic Schottky diodes and thin-film transistors for circuit simulations

Analytical device models for disordered organic Schottky diodes and thin-film transistors are presented. The models are developed taking into consideration the strong dependency of the charge mobility on carrier concentration. The drain current expressions are consequently developed in terms of the essential device parameters and applied voltages, to a power exponent of the characteristic temperature associated with the disordered nature of the semiconductor. Upon validation, better agreement of the experimental data is achieved with the disordered model rather than the conventional crystalline equation. Interestingly, under certain conditions, the disordered model reverts back to the conventional model, suggesting the latter to be a special case. Finally, to facilitate the circuit development, alternative design parameters to the mobility term are proposed.

Atomic layer deposition of Nb-doped ZnO for thin film transistors.

We present physical and electrical characterization of niobium-doped zinc oxide (NbZnO) for thin film transistor (TFT) applications. The NbZnO films were deposited using atomic layer deposition. X-ray diffraction measurements indicate that the crystallinity of the NbZnO films reduces with an increase in the Nb content and lower deposition temperature. It was confirmed using X-ray photoelectron spectroscopy that Nb5+ is present within the NbZnO matrix. Furthermore, photoluminescence indicates that the band gap of the ZnO increases with a higher Nb content, which is explained by the Burstein-Moss effect. For TFT applications, a growth temperature of 175 °C for 3.8% NbZnO provided the best TFT characteristics with a saturation mobility of 7.9 cm2/Vs, the current On/Off ratio of 1 × 108, and the subthreshold swing of 0.34 V/decade. The transport is seen to follow a multiple-trap and release mechanism at lower gate voltages and percolation thereafter.

Impact of universal mobility law on polycrystalline organic thin-film transistors

We have developed novel analytical models for polycrystalline organic thin-film transistor (OTFT) by employing new concepts on the charge carrier injection to polysilicon thin-films. The models, also incorporate the effect of contact resistance associated with the poor ohmic nature of the contacts. The drain current equations of the OTFT, both in the quasi-diffusion and quasi-drift regimes, predict temperature dependencies on essential material and device parameters. Interestingly, under the drift regime, the polycrystalline OTFT model reveals similar power dependencies on the applied voltages, to those of purely disordered model developed by utilizing the universal mobility law (UML). Such similarities are not thought to be coincidental since the effect of gate voltage on surface potential is influenced by the Fermi level pinning in the grain boundary. Nonetheless, the best fits on the data of 6,13-bis(tri-isopropylsilylethynyl) OTFTs are attained with the proposed polycrystalline rather than the disorde...

Modeling of polymer Schottky diodes for real device applications

Organic Schottky diodes are widely used in a number of real applications such as OLEDs according to J. H. Burroughhes et al. (1990), G. Gustafsson et al. (1992) and M. Granstrom et al. (1995) and also as an integral part of RF tags. Schottky diodes also provide a simpler way of obtaining the doping density and bulk mobility of the charge carriers in a material. It is thus important to understand and consequently be able to model their behavior. With the aid of experimental data, the behavior of polymer Schottky diodes is analyzed here in detail.

Improved Thin Film Transistor (TFT) performance using fractionated Poly-3-hexylthiophene (P3HT)

By using the Trznadel method, it has been possible to increase molecular weight, the regio-regularity, and probably to decrease the residual catalyst of poly-3-hexylthiophene thin films. The drift mobility of holes, normal to the surface of cast films, in air, has been found using Schottky diodes, and field-effect mobility has been measured with Thin-Film Transistors. Three types of film have been studied using the two methods of assessing mobility. The as-synthesised films are compared with those that have been fractionated. The third set of films involves doping with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The doped films show a field effect mobility of 0.2 cm 2 V -1 s -1 : all others being lower. Field effect mobility is approximately two orders of magnitude greater than that in the bulk normal to the plane of the film. Doping levels in the films are found to be similar, probably because of the process conditions.

Design and simulation of a high-gain organic operational amplifier for use in quantification of cholesterol in low-cost point-of-care devices

This paper presents circuit design and simulations of a high gain organic Op-Amp, for use in quantification of real cholesterol, in the range of 1-9 mM. A 7-stage inverter chain is added onto the design so as to enhance the amplifier gain. The circuit adapts p-channel transistors only (PMOS) design architecture with saturated loads, simulated on a conventional platform, using appropriate OTFT model and associated parameters. The effect of variation in threshold voltage on circuit operation is also examined. For a supply voltage of ±15 V, the DC output voltage is found to be within an acceptable range of -1 V to -12.5 V, with a highest open loop gain of 83 dB. The closed loop gain is also in agreement with theoretical values, in the range of 1.5 dB to 39 dB, with corresponding bandwidths of 770 Hz to 275 Hz respectively. The latter gain of 39 dB and/or gain-bandwidth product of 10.63 kHz is currently the highest reported in the literature, for this lower supply voltage. The amplifier offers adequate quantification factor, with linear sensitivity of -0.7 V/mM. This paper is the first to adapt organic circuit designs in quantification of cholesterol, with promising outputs, for implementation in low-cost sensor systems.