Heteroleptic Copper (I) Complexes as Photosensitizers in Dye-Sensitized Solar Cells

Modern civilization highly depends on energy and the sun is the most abundant clean and available energy on Earth. Being able to exploit it efficiently would result in a great environmental and economic breakthrough. Among the photovoltaic devices, dye-sensitized solar cells (DSCs) emerged for their tremendous commercial potential deriving from a combination of low-cost production and attractive features, such as flexibility and transparency, for indoor and outdoor applications. In the DSCs, a dye anchored to a semiconductor layer (typically TiO2) is responsible for capturing the sunlight and converting it into electricity. Nevertheless, existing commercialized dyes for DSCs are based on a very rare metal, ruthenium, and its replacement with a cheap, abundant metal is desirable for spreading this technology. A good alternative to ruthenium is copper, which possesses similar photophysical properties in coordination with diimine ligands, and it is quite cheap and relatively earth-abundant. A particular “on-surface self-assembly” strategy was employed to form, on the surface of TiO2, heteroleptic copper(I) complexes with a push-pull design which facilitates the electron transfer from the copper(I) complex into the conduction band of TiO2 and enhances the performance of the photovoltaic devices. This thesis focuses on the investigation of the properties of five new heteroleptic copper(I) complexes bearing the same anchoring ligand but different ancillary ligands. Because of the method employed for their synthesis, a solid-state characterization of the optical and electrochemical properties on TiO2 was performed employing tools such as UV-Vis spectroscopy, cyclic voltammetry (CV), and differential pulse voltammetry (DPV). As “internal benchmark” through the entire characterization, the homoleptic copper (I) complex with the anchoring ligand was also studied. Some patterns between the heteroleptic complexes on TiO2 and their respective homoleptic complexes in solution were found, opening the possibility to predict the behaviour of the heteroleptic complexes starting from their corresponding homoleptic. Furthermore, the characterization was necessary to ensure that the complexes were fulfilling the requirements to be employed as dyes. The performances of the heteroleptic, and of the anchoring-ligand homoleptic, copper (I) complexes were then investigated as photosensitizers in DSC devices mainly by measuring the current density-voltage (J-V) characteristic at different light intensities and in the dark, the incident-photon-to-current efficiency (IPCE), and electron lifetimes. As “benchmark” for the degree of effectiveness of the device, the state-of-the-art ruthenium (II) complex N719 was also studied. All the copper (I) complexes showed an overall similar behaviour. The J-V characteristics showed a power conversion efficiency up to 2,05% for the best performing device, which is 25% of the efficiency of DSCs based on N719. On the other hand, the least performing heteroleptic copper (I) complex studied showed an efficiency of 1,23%. From a general analysis combining all the results obtained, it may be concluded that a reason for the limited photocurrent measured through these devices may be due to incomplete dye coverage of TiO2. Despite the lower performance compared to the standard dye N719, the simplicity of the system is promising, and its considerable economic advantage could pave the way to the use of DSCs in everyday life applications.