Strategies towards flexible solid state solar cells based on nanocrystalline titanium oxide and organic hole conductor were investigated. For the flexible cell geometry a metal foil was used as substrate and a semi-transparent gold layer as counter electrode which allows light transmission (back illumination). The device performance of solid state cells based on SnO<sub>2</sub>:F coated glass on the one hand and a metal foil on the other hand were characterized and compared by measuring the current voltage curves on back and front illumination.
The photovoltaic performance of solid-state dye-sensitized solar cells based on spiro-MeOTAD (2,2'7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene) has been improved to 3.2% overall conversion efficiency under air mass AM 1.5 illumination by performing the dye adsorption in the presence of silver ions in the dye solution. Different spectroscopic methods, such as X-ray photoelectron, Fourier-transform infrared and UV-visible spectroscopy have been employed to scrutinize the impact of the silver on the dye-sensitized device. From spectroscopic evidence it is inferred that the silver binds to the sensitizer mainly via the amphidentate thiocyanate, allowing the formation of ligand-bridged dye complexes. The enhancement in overall device efficiency is a result of increased open circuit potential and short circuit current. The increased open circuit voltage was explained by the blocking of the dark current as a result of a closer packed dye layer and/or the partial formation of a dye double layer upon silver coordination. The increased short circuit current corresponds to the higher amount of ruthenium dye units adsorbed to the TiO<sub>2</sub> surface.
Solar cells based on dye-sensitized mesoporous films of TiO<SUB>2</SUB> are low cost alternatives to inorganic semiconductor devices. Solar energy conversion efficiencies of up to 10% have been achieved with such films when used in conjunction with liquid electrolytes. Practical advantage may be gained by the replacement of the liquid electrolyte with a solid charge transport material and various concepts have been proposed in literature to realize such solid-state dye-sensitized heterojunctions. Recently high incident photon to electric current conversion efficiencies have been achieved with a cell consisting of a dye-derivatized mesoporous TiO<SUB>2</SUB> film contacted by a new organic hole conductor. Photoinduced charge carrier generation shows to be very efficient in such devices, while interfacial charge recombination during charge collection can be revealed as the major loss mechanism. Surface treatments with pyridine derivatives proved to significantly improve the energy conversion efficiency of the device.