(167f) Enhancing Dye Sensitized Solar Cell J-V Behavior By Integrating Nanoscale Polymer Films

Dye sensitized solar cells (DSSCs) are inexpensive, are easy to fabricate, have tunable optical properties such as color and transparency, and can be integrated into buildings and apparel easily. A DSSC is composed of three main components: (1) a photosensitive dye which converts sunlight into electrons, (2) a mesoporous TiO₂ photoanode which provides a large surface area for dye adsorption and a pathway for electron transport, and (3) a liquid electrolyte comprised of an iodide-triiodide redox couple which is used to regenerate the dye and acts as a hole transport medium. A major limitation of the current DSSC design is the liquid electrolyte, which is prone to leakage and evaporation along with being corrosive to metal contacts such as silver. Furthermore, electron recombination loss is significant at the TiO₂-electrolyte interface. Polymer electrolyte DSSCs can overcome these disadvantages and can potentially enhance the cell's photocurrent-voltage behavior [1]. However, due to the nanostructured, mesoporous nature of the TiO₂ photoanode and the effects of viscosity on fluid transport in such a structure, conventional polymer deposition techniques such as spin casting and dip coating lead to ineffective pore penetration. Often only the top 2 µm out of a >10 µm photoanode layer is in contact with the polymer with the remaining photoanode below completely void of polymer. This severely reduces device performance because a large portion of the adsorbed dye is not accessible and cannot be regenerated. To ensure optimal performing DSSCs, there needs to be intimate contact between the polymer electrolyte and the TiO₂ throughout the entire photoanode layer to ensure the redox couple can reach the adsorbed dye.

To overcome the limitations of liquid methods for incorporating polymer electrolytes into the mesoporous TiO₂, we employ a novel polymerization technique called initiated chemical vapor deposition (iCVD). iCVD is a solvent-free polymerization method that relies on gas-to-surface reactions at low pressures in which the reagents for polymerization, the monomer and initiator, are heated to vapors that can easily penetrate into the mesoscale voids of the TiO­₂ photoanode. The initiator is activated by a heated filament (250-350 °C) and polymerization as thin films occurs conformally inside the pore surfaces. This makes it especially useful for cases that require penetration of sub-micron and nanometer-sized pores as issues of wettability and surface tension with liquid processing are absent. In addition, the lack of any solvent medium removes the possibility of liquids getting trapped in the photoanode, which may degrade performance, and are difficult to remove [2].

In this work, iCVD is used for the synthesis and integration of polymer electrolyte thin films within the mesoporous TiO₂ photoanode of DSSCs. Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and X-ray photoelectron spectroscopy (XPS) analyses confirm that the polymers formed are stoichiometric in composition expected of linear homopolymers and chemically identical to ones formed with liquid solution methods. The surface reaction kinetics and mass transport have been found to depend on the fractional surface saturation of monomer, , which is an adsorption parameter that provides a measure of the surface availability of the monomer. By carefully controlling , conformal coatings throughout the TiO₂ photoanode have been achieved. To gain a better understanding on the effect of the polymer electrolyte on DSSC behavior, current-voltage, UV-VIS absorption, incident photon-to-current efficiency (IPCE) and electrochemical impedance spectroscopy (EIS) measurements have been employed. Experimental results indicate that DSSCs with ultrathin polymer electrolyte thin films integrated on the photoanode pore surface can enhance cell performance. For example, by applying poly(1-vinylimidazole) (PVIZ), power conversion efficiencies are found to be 27% higher than liquid-electrolyte DSSCs due to a simultaneous increase in short circuit current J_sc and open circuit voltage V_oc [3]. To our knowledge, this is the first reported demonstration of PVIZ as a polymer electrolyte in DSSCs.

1. Smolin, Y. Y.; Nejati, S.; Bavarian, M.; Lee, D.; Lau, K. K. S.; Soroush, M. Journal of Power Sources 2015, 274, 156-164.

2. Nejati, S.; Lau, K. K. Nano letters 2010, 11, (2), 419-423.

3. Kuba*, A. G.; Smolin*, Y. Y.; Soroush, M.; Lau, K. K. S. Chemical Engineering Science. Special Issue: Energy Conversion and Storage 2016, (in press.) http://dx.doi.org/10.1016/j.ces.2016.05.007 (*equal contribution)