A new photo-crosslinked polymer electrolyte for dye-sensitized solar cells optimized by design of experiment

As a feasible option for photovoltaic technology to meet the growing energy demand, dye-sensitized solar cells (DSSCs) have attracted much attention due to their low cost, ease of fabrication and good performance. Standard DSSCs use 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 poor long-term stability, difficulty in robust and hermetic sealing, electrolyte evaporation/leakage, permeability to H2O/O2. A very promising and common skill used to solve these problems is the attempt to replace the liquid electrolyte with a quasi-solid or solid electrolyte. Indeed, several efforts have been made to replace the liquid redox mediator by polymer electrolytes, including physically cross-linked gels and gel-polymer electrolytes. Compared with other kinds of materials, gel-polymer electrolytes present some advantages, such as high ionic conductivities, good contacting and manageability, achieved by trapping a liquid electrolyte in polymer cages formed in a host matrix. Moreover, it is also of considerable importance to overcome the drawbacks of conventional polymerization techniques (long reaction times, use of solvents, solubilization/separation steps, addition of catalyst). For all these reasons, we chose photoinduced polymerization. It was previously demonstrated the suitability of this technique to fabricate plasticized polymer membranes for Li-ion batteries application. In this study, we fabricated cross-linked thin films starting from bisphenol A ethoxylate dimethacrylate (BEMA) and polyethylene glycol methyl ether methacrylate (PEGMA). Self standing membranes (100 µm thick) were obtained by UV irradiating the mixtures for 4 min and, then, activated by soaking into a liquid electrolyte solution (NaI/I2/acetonitrile). Finally, membranes were sealed between cathode and photoanode of DSSC and photovoltaic performances were evaluated. With the goal of obtaining high light-to-electricity conversion efficiencies from the DSSCs made with the gel-polymer electrolyte described above, we carried out the optimization of the experimental conditions by means of response surface methodology (RSM). Two designs of experiments (DoE) were planned. At first, two-level fractional factorial design (resolution V) with five factors was used to eliminate unimportant factors. From the data analysis, BEMA:PEGMA ratio and NaI/I2 concentration were proved to be significant factors on DSSC performance; conversely, swelling time, propylene carbonate (as plasticizer) and LiClO4 salt (as additive) were found to be not relevant. Afterwards, central composite face centered design (CCF) was employed to investigate the simultaneous effect of the two relevant factors. The set of experiments we carried out allowed us to produce a quasi-solid DSSC with durable excellent efficiency as high as 5.41 %. In addition, an accurate characterization of the photo-cured membranes and the photo-electrochemical device was performed and will be here thoroughly discussed.