Fluorinated materials for improved efficiency and stability of flexible OPVs

The aim of this work is the development of advanced fluorinated materials to be employed as functional coatings to improve efficiency and stability of polymer solar cells (PSCs). A preliminary analysis on the main parameters affecting the efficiency in inverted PSC devices based on standard materials was carried out, taking into account both rigid and flexible substrates. In particular, the effect of annealing temperature on the functional properties of the ZnO buffer layer was investigated, focusing onto three different annealing temperatures (100°C, 140°C, 200°C). Subsequently, eight different fluorinated coatings were prepared and characterized based on a commercial functional fluoropolymer (Lumiflon). In particular, the following systems were developed: Lumeta, product of the functionalization of Lumiflon with isocyanatoethyl methacrylate to make it photo-curable; FEVEsil, result of the functionalization with triethoxysilyl propyl isocyanate, yielding a thermo-curable resin; LumetaSil, a resin combining both isocyanates, which exploited the exothermicity of the photo-curing process in order to provide the heat necessary for the thermally-activated crosslinking. FEVEsil and LumetaSil were used for hybrid organic-inorganic sol-gel coatings formulated into different compositions, and a total of 7 different systems were developed, in which the organic-to-inorganic ratio was systematically varied. The inorganic component consisted of silica or zirconia. Coatings were characterized with differential scanning calorimetry, thermogravimetric analysis, UV-vis spectroscopy, and optical contact angle measurements. A first use of such coatings consisted in their application as luminescent downshifting (LDS) layers on PSC devices. For the LDS study, five luminescent species were considered: Lumogen F Violet 570 and SR12 (organic dyes), EuT4DEA (a lanthanide complex), and finally SKA4 and GARO28 (Ir complexes). The luminophores were analyzed by measuring their absorption and emission spectra. Then, the most promising candidates (Lumogen Violet and SR12) were used at increasing concentrations for the LDS experiments, using LumetaSil with low silica content as dye-doped fluoropolymeric carrier. The resulting power conversion efficiency improvements showed a trend with a maximum gain attained for intermediate dye concentrations in both cases. The investigation of the effect of fluorinated hybrid coatings (LumetaSil, high silica) on the PSC stability was performed by means of a UV-A degradation experiment carried out on coated samples and on uncoated reference devices. The UV-A degradation experiment provided the evidence of the effectiveness of LumetaSil in enhancing the PSC stability, as the coated substrates retained 60% of their normalized efficiency (compared to the 40% preserved in the uncoated reference), with performances comparable to those of standard systems encapsulated with a commercial barrier foil, which was taken as a benchmark reference. Overall, this thesis work demonstrates that the fluoropolymeric coatings developed here represent a promising strategy to improve PSC device efficiency and stability under harsh environments in a relatively straightforward fashion.