Layer-by-layer self-assembly of porphyrin heterogeneous metal-organic thin films on titanium oxide for photocurrent generation

Dye-sensitized solar cells (DSSCs) are a class of solar cells of the third generation. DSSCs are cheap and simple to fabricate, exhibit flexible properties, and are not limited by the Shockley-Queisser limit of semiconductor junction solar cells. The efficiency of DSSCs has reached a very promising 12% and DSSCs are currently developed for niche applications under low light conditions. Their performances are limited by undesired electron and hole back transfer and quenching of the photoexcited dyes due to dye-dye and dye-substrate interactions. Overcoming these limitations is the subject of a large research activity worldwide. It is our main hypotheses that a self-assembly scheme based on a metal-organic coordination construction is a valid alternative for the preparation of dye films and that the flexibility in achieving heterogeneous assemblies can be exploited to mitigate the various quenching and current hindering factors. This thesis focuses thus on the development of a layer-by-layer self-assembly method for wet deposition of metal-organic coordination thin films of porphyrin-based dyes and HKUST-1 (Honk Kong University of Science and Technology) metal-organic framework (MOF) on flat titanium oxide substrates. The bottom-up growth protocol has been verified on the basis of X-ray photoelectron spectroscopy (XPS) and visible absorption spectroscopy, and allows formation of arbitrary sequences of these molecular components in remarkably uniform and homogeneous layers. A home-made photoelectrochemical cell was used to measure the short-circuit photocurrents generated by the metal-coordinated films. A detailed discussion of the mechanisms reinforcing or hindering the photocurrent generation for various molecular sequences is reported. In particular, it is demonstrated that the HKUST-1, trimesic acid-based MOF, can be used as a spacer between the porphyrin dyes and the titanium oxide substrate, without sacrificing the dye layer uniformity and density, and that a bi-layer of trimesic acid maximized the photocurrent, balancing the substrate quenching and the HKUST-1 conductivity. Metal nanoparticles (NPs) that support surface plasmon resonances (SPR) matching with the dye absorption bands can enhance the absorption cross-section in dye films and reinforce the photocurrents. Citrate-stabilised gold and silver NPs were thus electrostatically assembled on aminosilane-modified titanium oxide films, and functionalized for attachment of the metal-organic coordinated dye and spacer sequences. AuNPs of diameter 17 ± 3 nm exhibit a saturation coverage of 30%, estimated by Scanning Electron Microscopy (SEM) and lead to a decrease in the photocurrent attributed to the quenching of the dye with the Au interband transitions. A saturation coverage of 2-3% was found for AgNPs of diameter 42 ± 6 nm, and a global SPR enhancement of the photocurrent by a factor 4 was measured. The latter corresponds to a local enhancement of 200. Layer-by-layer self-assembly of heterogeneous metal-organic films is thus well worth considering as alternative dye film in DSSCs, since they afford a high flexibility in tailoring the layered sequence for optimization of the photocurrent generation and further allow introduction of NPs for SPR enhancement.