Carrie L. Donley

Effects of field dependent mobility and contact barriers on liquid crystalline phthalocyanine organic transistors

Copper phthalocyanine (CuPc) transistors were fabricated using the Langmuir-Blodgett technique to produce bottom contact organic field effect transistors (OFETs) on silicon substrates. The resultant devices were measured and the performance was analyzed using a two-dimensional numerical simulation of the device structure. A hole barrier at the Au∕phthalocyanine source and drain contacts was seen from the experimental data. The numerical simulations were used to extract a barrier height of 0.415eV at the Au∕phthalocyanine contacts. Also, a Frenkel-Poole mobility model was used to account for the drain current in the transistors and a high field mobility of 0.018cm2∕Vsec was extracted from the experimental data. The resultant device parameters were compared to simple analytical results and the benefits of enhanced two-dimensional modeling of OFETs are shown.

Broadband Spectroelectrochemical Attenuated Total Reflectance Instrument for Molecular Adlayer Studies

The study of thin film molecular architectures is an increasingly important endeavor with respect to the development and characterization of materials ranging from liquid crystalline displays to receptor-based biosensors. Here we describe an apparatus capable of simultaneously acquiring broadband spectroscopic and electrochemical information on molecularly thin films deposited on a transparent electrode surface through a multiple internal reflection geometry. To demonstrate the capabilities of this system, the spectroelectrochemical behavior of a single, neutral copper phthalocyanine bilayer was evaluated.

Anisotropic optical constants of electroluminescent conjugated polymer thin films determined by variable-angle spectroscopic ellipsometry

This article reports on in-plane and out-of-plane refractive index (n) and extinction coefficient (k) values measured using variable-angle spectroscopic ellipsometry for poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) thin films of different molecular weights (Mn=9−255 kg∕mol), both in the pristine and annealed states. The in-plane n and k values are generally larger than the out-of-plane values for all pristine films leading to a measurable optical anisotropy which becomes much stronger as F8BT molecular weight increases. This indicates that polymer chains lie preferentially in the plane of the substrate and this configuration is more energetically favorable for longer polymer chains. Upon annealing, a larger reduction in kout-of-plane than in kin-plane is measured leading to a further increase in optical anisotropy. A redistribution of oscillator strengths and a broadening toward lower energies in absorption spectra are also observed indicating significant restructuring of F8BT chains upon anne...

Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities

The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir–Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10 -6 cm 2 ·V -1 ·s -1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10 -4 cm 2 ·V -1 ·s -1 . Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad.

Probing thin-film morphology of conjugated polymers by Raman spectroscopy

We use Raman spectroscopy to investigate the thin-film morphology of conjugated polymers [poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT)] in terms of the polymer chain conformation at interfaces with quartz, a crosslinked benzocyclobutene derivative, polyvinylphenol, and poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate). The polymer chains near the substrate interface adopt a more planar conformation (lower torsion angle between fluorene and benzothiadiazole units) than chains in the bulk of the film for all substrates studied. On annealing, polymer chains both near the interface and in the bulk of the film adopt more planar conformations than in their pristine states but to a different degree. The influence of F8BT molecular weight on polymer chain conformation near the substrate interface is also examined. These results are confirmed by additional absorption and photoluminescence measurements.

Organic photovoltaic cells containing discotic liquid crystalline phthalocyanines

We report on the photovoltaic properties of solar cells containing a new discotic liquid crystalline material (DL-CuPc) based on copper phthalocyanine. In addition to being soluble, these materials can self-organize into highly ordered structures which can lead to good transport properties that can potentially be superior to those of amorphous materials. Increase in short-circuit current density and fill factor was obtained by thermal annealing of spin-coated DL-CuPc layer in bi-layer solar cells based on junction between DL-CuPc and C60. These improvements are explained by change in structure and morphology upon thermal annealing.

Spatial control of the recombination zone in ambipolar light-emitting polymer transistors

Ambipolar organic field-effect transistors (FET) are interesting as building blocks for low power complementary circuits in organic electronics. Another intriguing feature of ambipolar FETs is the recombination of holes and electrons within the channel, which leads to the formation of excitons that can relax radiatively and thus emit light. We have recently demonstrated that ambipolar charge transport is a generic feature in a wide range of polymer semiconductors when appropriate injection electrodes and trapfree dielectrics are used. Among these materials are those that are generally used in light-emitting diodes and thus show high photoluminescence efficiencies. Here we demonstrate ambipolar light-emitting field-effect transistors based on the conjugated polymer OC1C10-PPV (poly(2-methoxy-5-(3,7-dimethyloctoxy)-p-phenylenevinylene)) as the semiconducting and emissive layer. OC1C10- PPV shows efficient electron and hole transport with field-effect mobilities of 3⋅10-3 cm2/Vs and 6⋅10-4 cm2/Vs, respectively. Electrons and holes are injected from calcium and gold source and drain electrodes, respectively, and recombine radiatively within the transistor channel leading to visible light emission. We can actively control the position of the recombination zone through the applied gate and source-drain bias in both constant and variable current mode and thus move the emission zone from the source through the channel to the drain electrode and vice versa. The intensity of light emitted from the channel is proportional to the drain current with efficiencies comparable to those of LEDs based on OC1C10-PPV.

A Novel Non-destructive Interfacing Technique for Molecular Scale Switching Junctions

Characterization of molecular scale electronic devices generally involves deposition of a metal top electrode onto an organic film. During the evaporation process, high-energy granules of metals may lead to unwanted reactions between the organic molecules and deposited top electrodes. This can cause, as commonly reported, lasting damage which leaves most junctions either shorted or an open circuit. To overcome this important issue of physical damage to the molecules, we developed a novel technique of interfacing molecules by laying a prefabricated metallic electrode on top of a monolayer of molecules which were deposited onto another set of electrodes. A monolayer of cadmium stearate ((C17H35COO)2Cd) was deposited, using the Langmuir-Blodgett technique, onto platinum (Pt) electrodes which further rested on 200nm of oxide used for isolation. A separate set of aluminum with native oxide (Al/AlOx) electrodes were fabricated on a different oxide free, highly p-doped substrate and was gently placed on the cadmium stearate monolayer. Electrical connection to the top electrode was accomplished by probing the backside of the highly doped wafer whereas the bottom electrodes were individually addressed. Pressure was applied to ensure firm contact between the molecules and the top electrodes. Preliminary results showed an observed switching voltage of 3V-4V. Most of the devices with Al as the top electrodes exhibited a gradual progression from switching on the positive side, closing or no longer switching, and then switching towards the negative side.