Solution-Processed Materials for Flexible, Large-Area Electronics

The performance of polymer based electronic devices has shown great improvements over the past years. Polymer light emitting diodes (LEDs) show high luminescence efficiency and thin film transistors (TFTs) show field effect mobilities of 0.1 cm2/Vs. Polymeric materials are easily processed from solution providing the potential of a vacuum free fabrication process. Methods used to deposit and integrate solution-processed materials to fabricate TFT backplanes by ink-jet printing are presented. The materials studied allow the development of an all-additive process in which materials are deposited only where their functionality is required. A systematic study on device structure is presented, including the use of polymeric electrodes and the influence of the microstructure of thin films on device performance. The use of blends of the polymer with an encapsulating material is shown to be advantageous in providing additional flexibility in the process. By controlling the surface energy of the substrates we cause an array of isolated and encapsulated thin-film-transistors (TFTs) to spontaneously assemble from a semiconductor/dielectric polymer blend solution. Silver nanoparticles are used as gate and data metals, the semiconductor used is a polythiophene derivative (PQT-12), and the gate dielectric is a polymer. The maximum processing temperature used is 150°C, making the process compatible with flexible substrates. The I_ON/_OFF ratio is 10⁵-10⁶, and TFT mobilities of 0.05 cm2/Vs were obtained. The electrical stability of the all-printed transistors was compared to conventional fabrication methods and it is shown to be acceptable for array operation. Polymer blends were also used in the fabrication of photovoltaic devices. Conventional fluorescence, fluorescence scanning near-field optical microscopy and atomic force microscopy have been combined to relate film morphology with photovoltaic and photoluminescence efficiencies as a function of surface treatment, concentration, and processing conditions. The optimized self-organization process, in conjunction with phase compositions, has resulted in the demonstration of the highest photovoltaic efficiencies reported for the polyfluorene materials.

Biography
Ana Claudia Arias is currently a Member of Research Staff at PARC Inc (Palo Alto Research Center), Palo Alto, CA. At PARC she uses inkjet printing techniques to fabricate organic active matrix display backplanes for paper-like displays. She came to PARC from Plastic Logic in Cambridge, UK where she led the semiconductor group, working in collaboration with chemical companies to develop polymeric semiconductors for thin film transistors. She did her PhD on semiconducting polymer blends for photovoltaic devices at the University of Cambridge, UK. Prior to that, she received her master and bachelor degrees in Physics from the Federal University of Paraná in Curitiba, Brazil. Her research work in Brazil focused on the use of semiconducting polymers for light emitting diodes. She the author of numerous papers and patents in the field of organic based electronic and optoelectronic devices and in printed electronics.