Dye-sensitized solar cells on alternative substrates

Abstract

Dye-sensitized solar cells (DSC) could become a potential alternative for the traditional silicon and thin film panels in the near future, due to the DSC's for the most part cheap materials and simple manufacturing methods. One of the challenges of this technology is, however, the heavy, expensive and inflexible glass substrate typically used in the cells. To address this problem, this thesis concentrates on transfer of the DSC technology from glass substrates to light weight, cost-efficient, and flexible plastic foils and metal sheets. Flexible solar cell would be well suited for industrial-scale mass production, for example with roll-to-roll methods and when integrated on building materials it could work as a functional coating, enabling electricity-producing roofing or façade structures. In the course of this thesis, DSCs were prepared on ITO-PET and ITO-PEN plastics, stainless steel (StS), and optical fibers. Due to the low temperature tolerance of the plastics, development and characterization of room temperature processable counter electrode materials suitable for these substrates was a part of this work. Powder suspension based on carbon nanoparticles proved to be an easily depositable, cost-efficient material with catalytic activity as high as that of platinum. With metal materials, the main problem is the corrosive, iodine-based electrolyte conventionally used in the DSC. This is why, in the beginning of this work, the corrosion resistance of some widely used building materials such as zinc-coated carbon steels, copper, and StS was studied with soaking tests in the electrolyte. StS passed the soaking tests and was chosen for further research. StS has also other benefits such as good electrical conductivity and mechanical sturdiness. Substrate-mediated leakage current is also smaller from StS than from glass substrates. With a DSC configuration where the StS sheet worked as the photoelectrode substrate efficiencies comparable to all-glass cells, near 5 %, were obtained so this configuration was chosen also for the cell size upscaling tests. The largest StS photoelectrode cells prepared in the course of this thesis were 6 cm × 6 cm and their efficiencies over 3 % at their best. This is already a promising value considering the ohmic losses bound to happen at the counter electrode, due to the sheet resistance of the counter electrode substrate. To minimize these losses, additional current collector structures were integrated on the counter electrode substrate with inkjet-printing with silver nanoparticle ink. 50 % reduction in the total ohmic losses of the cell was achieved with the current collector structures and 80 % with replacing the photoelectrode glass substrate with the StS sheet. StS-based DSC would seem like a feasible concept even for industrial-scale mass production but special emphasis should be put, in the future research, on the long term stability of the cells and its improvement. Room for improvement still exists in efficiencies also - a research challenge in which for example some recently developed carbon nanomaterials might provide progress.