Invited: iCVD Synthesis of Polymer Electrolytes for Application in Dye Sensitized Solar Cells (DSSCs)

Initiated chemical vapor deposition (iCVD) is used for the synthesis and integration of polymer electrolytes, such as poly(2‐hydroxyethyl methacrylate) (PHEMA), poly(glycidyl methacrylate)  (PGMA), poly(glycidol) (PGL), poly(4-vinylpyridine) (P4VP) and polyvinylpyrrolidone (PVP), within the mesoporous TiO₂ photoanode of dye sensitized solar cells (DSSCs). DSSCs with the conventional liquid-electrolyte are prone to leakage and evaporation—hindering DSSC applications, durability, and thermal stability. Polymer electrolytes do not suffer from these disadvantages and can also enhance the cell I-V behavior.  However, spin coating and drop casting to deposit the polymer electrolytes leads to incomplete pore filling of the polymer inside the mesoporous TiO₂photoanode, leading to poor electrical contact and lower efficiency; this is one of the main factors limiting solid-state DSSC performance.

To alleviate these pore filling concerns, we fabricated polymer electrolytes using the novel, solvent-free technique of initiated chemical vapor deposition (iCVD)^1,2. iCVD is generally an adsorption limited technique where the reagents for polymerization, the monomer and initiator, are heated to a vapor that can easily penetrate into the mesoscale voids of the TiO­₂ photoanode. The initiator is activated by a hot filament (250-350 C) and polymerization occurs inside small crevices and on surfaces (Figure 1). By understanding the competition between surface reaction kinetics and mass transport into the mesovoids, iCVD polymerization allows for uniform coatings for nanoscale geometries and is an exceptional technique for allowing polymerization in the mesoporous structure of TiO₂. The surface reaction kinetics and mass transport have been found to depend on the relative pressure of monomer, z=P_M/P_M,sat, which is an adsorption parameter that provides a measure of the surface availability of the monomer. By carefully controlling , nearly 100% pore filling of the TiO₂ structure has been achieved as estimated by thermogravimetric analysis (TGA)³. This is significantly higher than that achievable with liquid techniques like spin coating or drop casting of polymer solutions and allows pore penetration for layer thicknesses up to 12 μm, which is much greater than the 2 μm limit with liquid‐based methods. FTIR, NMR, and XPS confirmed the polymers formed are stoichiometric in composition expected of linear homopolymers and identical to ones formed with liquid solution methods.

In this paper, we show that iCVD polymer electrolytes can be effectively integrated within TiO₂ mesoporous photoanodes to produce enhanced DSSCs. By varying the polymer electrolyte chemistry, we show that DSSC cell characteristics, including open circuit voltage, short circuit current density and fill factor, can be tuned (Figure 2).  To gain a better understanding on the photochemical processes inside the DSSC, we have integrated our experimental work with first principles macroscopic modeling⁴ to examine the influence of different polymer chemistries. TiO₂ exhibits a Nernstian response to variation in pH, leading to a 0.059V positive shift (vs NHE) of the conduction band energy per pH unit decrease⁵. As suggested by our first principles model, we believe a similar mechanism is occurring when the TiO₂ surface is in intimate contact with these polymer electrolytes.  For instance, PHEMA and PGMA have relatively more acidic character compared to the more basic P4VP with its pyridine group, therefore PHEMA and PGMA have higher V_oc than P4VP.  The mathematical model also suggests that the primary improvements of J_sc and minor improvements of V_oc are due to the suppression of the back electron transfer at the dye-TiO₂-electrolyte interface. For example, the basic pyridine group of P4VP absorbs readily onto the bare TiO₂ surface due to its Lewis acidity, thus preventing the invasion of triodide and decreasing the rate of undesirable electron transfer from the TiO₂ to triiodide.  PGMA and PHEMA do not have this basic character, and therefore have a lower J_sc than P4VP.