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B6.03 - Surface Passivation of Inorganic Substrates by Multifunctional Polymeric Interfaces 
December 2, 2014   10:45am - 11:00am

Several applications of polymer-based materials require the fabrication of a hybrid interface between polymer thin films and inorganic substrates like silicon wafers. The properties of this interface are determined by how well the polymer covalently bonds with the inorganic substrate, thereby satisfying dangling bonds and passivating electrically active interface traps present on the surface. We demonstrate here that a variant of the chemical vapor deposition (CVD) polymerization technique, namely initiated chemical vapor deposition (iCVD) can covalently graft a polymer film onto a silicon substrate by initiating a chemical reaction between the surface hydride bonds on silicon and the functional (vinyl or ethynyl) groups of the vapor-delivered monomer. While the CVD process used for depositing inorganic passivating films like silicon nitride requires high temperature (400 - 450 oC), the iCVD method offers a solvent-free, low temperature (20-25 oC) alternative, which retains delicate organic functionalities in the monomers, enabling the fabrication of hybrid and multifunctional polymer-based devices. Following the iCVD passivation process, the minority carrier lifetimes of a functionalized silicon surface showed over three-fold improvement (upto 94 microseconds at 1x1015 cm−3 carrier injection levels) compared to bare silicon, suggesting passivation of surface traps. Surface passivation results obtained by iCVD are reproducible and stable during storage in air for over 100 hrs. The roles played by chemical bonding and field effect charges in the observed surface passivation were examined through attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) and x-ray photoelectron spectroscopy (XPS) of the functionalized silicon surface. Such a process of chemical passivation when applied to silicon solar cell surfaces can reduce surface recombination of minority carriers, increase solar cell efficiency and lower overall cost per watt. We further show that iCVD polymerization can be used to graft and pattern functional polymer films of hundreds of nanometers thickness on top of this passivated silicon surface. Grafted poly(ethylene glycol diacrylate) (PEGDA) films are dielectric and serve as anti-reflection coating (ARC) layers in solar cells. Grafted poly(para diethynyl benzene) (PPDEB) films show semi-conducting behavior and their aromatic rings can be used to graft poly(ethylenedioxythiophene) (PEDOT) films using Friedel Crafts catalysts like FeCl3. These grafted PEDOT films can serve as polymer based current collecting electrodes with excellent conductivity approaching 1400 S/cm. Finally, scale-up of the iCVD process has previously been demonstrated in a roll-to-roll system. The novel iCVD passivation can thus provide a simple, commercially viable platform to engineer high quality polymer surfaces and interfaces in the semiconductor, solar and biomedical industries.

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