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Thrust 2: Organic Electronics

Research Highlights

Power Conditioning for Organic Solar Cells
A new method for extracting power from organic photovoltaic cell arrays has been developed.  The technique, called  Time-Domain Array Reconfiguration (TDAR)  improves overall system efficiency by 30%-50% over fixed-array techniques.  These efficiency improvements are enabled by a novel approach to harnesses efficiency at the system level, analyzing the trade-off between maximizing the efficiency of each PV cell, operating the power converter circuits at a high efficiency, while maintaining a low system complexity.
Highly Conductive Transparent Electrodes using Metallic and Semiconducting Carbon Nanotubes
The use of carbon nanotubes which have been separated into metallic or semiconducting electronic type have been utilized to create transparent conductive electrodes.  Through effective doping procedures using thionyl chloride and nitric acid, we are able to shift the Fermi Level of the CNTs over the range of 1 eV, and obtain sheet resistances below 80 W/sq.  Conductive tip AFM studies show highly uniform electroactivity over the surface of the CNT electrodes unlike ITO.
Theoretical Modeling of Exciton-Dissociation and Charge-Recombination Processes in Organic Solar Cells
The exciton-dissociation and charge-recombination processes in organic solar cells based on pentacene/C60 heterojunctions have been investigated by means of quantum-mechanical calculations. The electronic couplings and the rates of exciton dissociation and charge recombination have been evaluated for several geometrical configurations of the pentacene/C60 complex, which are relevant to bilayer and bulk heterojunctions.
Operation Mechanism for Polymer Light-emitting Electrochemical Cells (LECs)
Performed the first, concerted scanning Kelvin probe and optical microscopy experiments on μm-gap planar LECs operated under conditions relevant to vertical LECs.  We varied the work function of the electrode metal used for our devices and noted changes to the potential profile and the corresponding electroluminescence profile
Low voltage, high gain, flexible organic complementary inverters
Flexible organic complementary inverters have been fabricated on plastic substrates. The performance of the organic NMOS transistor (> 2 cm2/Vs) exceeds that of a-Si. The corresponding PMOS transistor has a field-effect mobility (0.33 cm2/Vs). The small difference in mobility could be compensated by adjusting the channel width. By using a high-k gate dielectric layer fabricated by Atomic Layer Deposition (ALD), devices operating voltages below 5 V could de demonstrated. The inverters exhibit balanced voltage transfer curves with gain values as high as 180 and show good stability under bending.
Controlled externally-initiated synthesis of P3HT
Controlled externally-initiated synthesis of P3HT has been exchanged using a ligand exchange protocol. The process developed in our group provides P3HT with a narrow polydispersity (<1.1), 100% regioregularity, and 100% initiation efficiency. This is the highest degree of control achieved to date for the synthesis of P3HT.
Ultrasound-Induced Enhancement of the Field Mobility of Poly(3-hexylthiophene)
In a study of the field effect mobility of conjugated polymers we have routinely sonicated CHCl3 solutions of poly(3-hexylthiophene) for ca. 5 min prior to fabrication of thin film devices. While a solution of P3HT in CHCl3 is bright orange, the color changes to dark brown upon ultrasonication. Surprisingly, films prepared by spin coating the ultrasonicated polymer solution on bottom-contact FET devices in air resulted in an increase of the field effect mobility by approximately two orders of magnitude.
Thermochemical Nanolithography Now Allows Patterning of Multiple Chemicals on a Chip and of Organic Electronic Materials The ability to write functional nanostructures with high speed, in ambient conditions with high fidelity is particularly desirable in the design and fabrication of future nanoelectronic, nanophotonic, and biosensing devices.
Control of Effective Work Function for Metal and Oxide Contact Materials Demonstrated control of the effective work function of a variety of OLED, OPV and OFET contact materials, using both ω-functionalized alkanethiols (for metals such as Au, Ag) and both alkane- and aryl-phosphonic acids (PAs) for metals oxides such as ITO. Work function (Φ) can be tuned over a range of ca. 1.5 eV. Control of Φ is achieved either by choice of terminal functional group, or dilution of one modifier in another in the monolayer film.
Formation of Hole-Selective Polymer Layers on ITO, with Control of Work Function Through Electrochemical Doping Demonstrated a novel method for the formation of "hole-selective" contacts in organic solar cells, based on electrodeposited poly(thiophenes) whose morphologies are controlled by the type of electrodeposition, and which can be controllably doped, to tune the population of "charge transfer states" in the as-deposited polymer, and control its effective work function. Optimal doping of these layers provides for enhanced efficiencies in subsequently deposited small molecule OPVs (e.g. pentacene/C60; TiOPc/C60 heterojunctions)
Ambipolar Polymer Field-Effect Transistors and Integrated Complementary Circuits
High-mobility ambipolar field-effect transistors and high-gain inverters have been demonstrated by simple solution casting of a new polymer semiconductor. The transistors showed high mobilities of both electrons and holes (0.04 cm2/Vs and 0.003 cm2/Vs) and high on/off current ratios (103-105). The circuit performance represented by output voltage gain of a complementary inverter (~30) is the best among reported values from such single-component polymer semiconductors. This result suggests that high-performance organic electronic devices can be achieved without patterning of p- and n-type semiconductors.