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The overall objective of this project was to develop polymer electrolyte materials using initiated chemical vapor deposition (iCVD) for dye sensitized solar cells (DSSC). Critical to this application is the need to effectively fill the mesopores of the dye sensitized TiO₂ nanoparticle network, which acts as the photoanode, to create proper contact with the polymer electrolyte. Thus, we have relied on iCVD as a means to synthesize and grow the polymer within the mesoporous cavities by direct vapor-to-solid free radical polymerization without the use of any liquid medium. This precludes wettability and infiltration issues with using ready-made polymer solutions arising from surface tension and steric hindrance. Specifically, iCVD relies on introducing the monomer and initiator into a reaction chamber as a continuous stream of vapors. The initiator vapor contacts an array of heated filament wires (200-400 °C) to generate free radical species, which then interact with adsorbed monomer on surfaces to initiate polymerization and polymer growth. To promote polymerization, the substrate surface is maintained at near room temperature (0-30 °C) to facilitate adsorption. iCVD decouples the activation and deposition temperatures allowing controlled reaction that follows conventional free radical polymerization, thus allowing broad utilization of synthetic polymer chemistry to design materials. In addition, with the liquid-free environment at moderately low pressures (10-1000 Pa), the monomer and initiator species have long mean free paths for reaching down high aspect ratio features, thus allowing very uniform and conformal coating around the substrate geometry.

We have successfully synthesized poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(glycidyl methacrylate) (PGMA) using a state-of-the-art iCVD reactor system. Spectroscopically, these iCVD polymers are identical to liquid-synthesized counterparts. Importantly, we have achieved our goal of growing these polymers within mesoporous TiO₂ nanoparticle networks, with complete pore filling within cavities 20-25 nm in diameter of up to 12 µm in layer thickness. This has been done by fundamentally understanding transport and reaction behavior during iCVD. We have found that transport needs to account for both gas diffusion and surface diffusion, with the latter being a significant component in mesoporous materials, while reaction needs to account for the chain propagation reaction rate constant. We were able to estimate the diffusion and reaction time constants as a function of relative pressure z (=P_M/P_M,sat), the ratio of partial pressure of monomer in the gas phase to its saturated vapor pressure at the deposition temperature, which is essentially a measure of surface monomer concentration in the Henry’s law limit. We found that there is an intermediate z regime in which diffusion is faster that reaction, i.e. reaction limited. By performing iCVD under this region, we were able to completely fill the pores of mesoporous TiO₂ with both PHEMA and PGMA chemistries. This is a significant improvement compared to wet infiltration techniques, which suffer from pore blocking and plugging even with layer thicknesses of less than 2 µm.

With the new ability to infiltrate polymers into mesoporous nanostructures, we have successfully integrated iCVD polymers into DSSCs. The motivation was to replace the standard liquid electrolyte since liquids suffer from leakage and cause corrosion. Importantly, we have found that DSSCs integrated with iCVD PHEMA show higher power conversion efficiencies compared to standard liquid cells at all TiO₂ electrode thicknesses studied. This increase is due to the significantly higher open circuit voltages for the polymer cells. Through electrochemical impedance spectroscopy measurements, we attribute this to a decrease in charge recombination at the electrode-electrolyte interface in the polymer cells. We believe the tight contact of the polymer with the TiO₂ surface reduces surface trap states, reducing the likelihood of charge recombining with the redox couple in the electrolyte, which is a major loss mechanism in liquid cells. Thus, we have achieved our goal of enhancing solar cell performance with the integration of iCVD polymers.

This project has enabled the setup of the state-of-the-art iCVD reactor system, provided training of three Ph.D. graduate students, and offered independent research opportunities to two undergraduates as well as five high school students, including six from underrepresented groups (women, disabled). This work has enabled one publication in ECS Transactions, one paper in press in Thin Solid Films, and one manuscript under review in Nano Letters. A book chapter titled “Vapor Deposition Polymerization” wa also published in the Encyclopedia of Chemical Processing. This work has been presented at major conferences, including meetings of the American Institute of Chemical Engineers, the Electrochemical Society, the American Chemical Society, and the Materials Research Society. In addition, this work has been presented at international conferences, including the International Hot-Wire Chemical Vapor Deposition and the EuroCVD conferences. Importantly, this starter grant has successfully led to a continuation grant in the form of the NSF CAREER Award, which aims to build on the groundwork laid here. Ultimately, we aim to understand fundamental science and develop viable technologies for delivering sustainable energy solutions to society.

Bibliographic Citations

[1] Bose, R. K.; Nejati, S.; Lau, K. K. S. Initiated chemical vapor deposition (iCVD) of hydrogel polymers. ECS Transactions 2009, 25, 1229-1235.

[2] Nejati, S.; Lau, K. K. S. Integration of polymer electrolytes in dye sensitized solar cells by initiated chemical vapor deposition. Thin Solid Films (in press).

[3] Nejati, S.; Lau, K. K. S. Pore filling of nanostructured electrodes in dye sensitized solar cells. Nano Letters (submitted).

[4] Anthamatten, M.; Lau, K.K.S. Vapor Deposition Polymerization. In Encyclopedia of Chemical Processing (ed. S. Lee). Taylor & Francis, 2009.