In Situ Synthesis and Integration of Polymer Electrolytes in Nanostructured Electrodes for Photovoltaic Applications
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In Situ Synthesis and Integration of Polymer Electrolytes in Nanostructured Electrodes for Photovoltaic Applications Siamak Nejati and Kenneth K. S. Lau Department of Chemical and Biological Engineering, Drexel University Philadelphia, PA 19104, U.S.A. ABSTRACT The conventional dye sensitized solar cell (DSSC) is limited by the use of a liquid electrolyte that is prone to leakage and evaporation. Efforts to replace the liquid with a solid equivalent have been met with difficulties in penetrating the mesoporous TiO2 nanostructured photoanode with liquid processing, particularly for photoanode layer thickness greater than 2 µm. Here, initiated chemical vapor deposition (iCVD) is successfully applied to directly synthesize and fill the pores of the mesoporous TiO2 network of up to 12 µm thickness with poly(2-hydroxyethyl methacrylate) (PHEMA) polymer electrolyte. Comparing with equivalent liquid electrolyte cells, DSSCs integrated with PHEMA polymer electrolyte showed consistently higher open circuit voltage, which is attributed to a decrease in electron recombination with the redox couple at the electrode-electrolyte interface. INTRODUCTION The dye sensitized solar cell (DSSC) first proposed by O’Regan and Gratzel makes use of a photosensitizer dye for absorbing sunlight and injecting electrons into a titanium dioxide (TiO2) anode consisting of a mesoporous network of TiO2 nanoparticles [1]. The photo-oxidized dye is regenerated by the presence of an iodide/triiodide redox couple in a liquid electrolyte that shuttles between the photoanode and the platinized counter electrode. However, the use of a liquid electrolyte makes the cell prone to leakage and evaporation, reducing its operability and durability [2-4]. Thus, there is significant effort in trying to replace the liquid electrolyte with solid materials, including hole transport materials [5-7], conjugated polymers [8,9], and solid and gel polymer electrolytes [10-13]. To utilize a solid material however requires suitable processing that can provide effective infiltration into the mesoscopic pore network of the TiO2 nanostructure. Methods such as drop casting, spin coating, and vacuum infiltration from solutions of these materials are ineffective in penetrating the tiny pores, especially with mesoporous layers thicker than 2 µm, due to viscous and steric effects [14-16]. In addition, wet processing typically leaves residual solvent which are not entirely removed and will most likely reduce cell performance. We therefore present a liquid-free initiated chemical vapor deposition (iCVD) process for enabling a one-step synthesis and deposition of polymer electrolyte within mesoporous TiO2 that is effective in achieving pore filling up to 12 µm in layer thickness. By enabling tight integration of the polymer electrolyte, in this case poly(2-hydroxyethyl methacrylate) (PHEMA), with the nanostructured TiO2 photoanode, we were able to obtain higher open circuit voltages in fabricated DSSCs that yielded higher power conversion efficiencies compared to equivalent liquid el
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