A novel UV-cured quasi-solid polymer electrolte for dye-sensitized solar cell

As a feasible photovoltaic technology to meet the growing energy demand, dye-sensitized solar cells (DSCs) have attracted much attention due to their low cost, easy fabrication and good performance. As well known, the standard DSC uses a liquid electrolyte, usually based on the I-/I3- redox couple in an organic solvent. This setting leads to high efficiencies, but it can result in relevant technological drawbacks associated with less long-term stability, difficulty in robust and hermetic sealing, evaporation and leakage of electrolyte, water and oxygen permeability. Replacing the liquid electrolyte with a quasi-solid or solid electrolyte is expected to help solving these problems. Several attempts have been reported to replace the liquid redox mediator by polymer electrolytes, liquid electrolytes solidified with physically cross-linked gelators and polymer-gel electrolytes. In particular, to overcome the drawbacks of conventional polymerization techniques (long reaction times, use of solvents, solubilization and separation steps, catalysts based on heavy metals), a great possibility is provided by the radical photo-polymerization (UV-curing). In fact, this technology is rapid and inexpensive, it is consistent with the main canons of the green-chemistry and could be easily transferred to industrial scale. In this study, we fabricated cross-linked thin films from the two monomers bisphenol A ethoxylate dimethacrylate (BEMA) and polyethylene glycol methyl ether methacrylate (PEGMA). Self standing membranes (100 µm thick) were obtained in only 4 minutes under UV-radiation and then activated by soaking into a liquid electrolyte solution (swelling effect). In order to optimize the experimental conditions, a chemometric approach has been selected. Two designs of experiments (DoE) have been planned. At first, two-level fractional factorial design (resolution V) with five factors has been used in order to eliminate unimportant factors before investing time and money in a more elaborate study. From the analysis of data, BEMA:PEGMA ratio and NaI/I2 concentration have significant impact on DSC efficiency; conversely, swelling time, propylene carbonate (as plasticizer) and lithium perchlorate (as salt additive) are not beneficial or relevant factors. Afterwards, central composite face centered design (CCF) has been employed for the investigation of the simultaneous effect of the two relevant factors. The set of experiments we carried out allowed us to obtain a durable high-efficient (5.41 %) quasi-solid DSC, in the presence of a 35:65 BEMA:PEGMA ratio and a 0.38/0.038 mol L-1 NaI/I2 liquid electrolyte. In addition, an accurate characterization of photo-cured membranes and photoelectrochemical device was performed.