Interface engineering in solid-state dye-sensitized solar cells

Dye-sensitised nanocrystalline solar cells are currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. In particular dye-sensitised cells based on spiro-MeOTAD have gained attention as promising approach towards an organic solid-state solar cell. However, the efficiency in such dye-sensitised solid-state solar devices (SSD) was so far only ca. 10 % of the values reported at AM1.5 for the classical dye-sensitised solar cell with an electrolyte hole transporting medium (DSSC). The objective of the present work is to study the limitations that emerge from the exchange of the electrolyte by the solid-state system and that oppose photovoltaic photon-to-electron conversion as high as for the DSSC. Interfacial charge recombination is an important loss mechanism in dye-sensitised solar cells. This is particularly true for SSD, as the solid hole-transporting medium is less efficient in screening of internal fields which assist recombination. A variety of strategies were tested in the SSD to minimise interfacial charge recombination. The most promising approach was the blending of the hole-transporting medium with tert.-butylpyridine (tBP) and lithium ions. Optical and electrochemical techniques, such as nanosecond laser spectroscopy, impedance spectroscopy and photovoltaic characterisation measurements, were used to study the impact of the additives on the SSD. Both lithium ions as well as tBP were found to increase the open circuit potential of the SSD. At the same time tBP was found to considerably lower the current output. The interaction of the additives was studied and their concentration in the spiro-MeOTAD medium optimised. The doping of the spiro-MeOTAD film, which was intended to support the hole transport, was found to enhance interfacial recombination significantly. The morphological properties of the TiO2, in particular layer thickness, particle size and film porosity, play a more important role in the SSD than in the DSSC. Penetration of the hole conductor into the TiO2 pores and electron diffusion length are coupled to these properties. As a result the light harvesting cannot be controlled at will via the TiO2 film thickness and the active surface area for dye adsorption. An enhanced light harvesting for thin TiO2 layers offers advantages for the charge transport and the formation of the interpenetrating network. The dye uptake in presence of silver ions was found to increase the dye loading and to significantly improve the device performance of thin TiO2 layer devices. The mechanism of this simple dye modification technique was studied by a variety of spectroscopic techniques. From spectroscopic evidence it is inferred that the silver is binding to the sensitiser via the ambidentate thiocyanate, allowing the formation of ligand-bridged dye complexes. The beneficial influence of the silver ions on the photovoltaic performance was not limited to the application of the standard N3 dye nor