One of the main limitations of current solid state dye sensitized solar cells (ss-DSC) is incomplete pore-filling of solid state hole conductors (e.g., spiro-OMeTAD) into the mesoporous TiO₂ films. Good pore-filling is essential, since it provides the required intimate interfacial contact between the dye molecules, attached to the TiO₂ surface, and the hole conductor phase. Pore-filling becomes increasingly more problematic with thicker films, which is why the best conversion efficiencies (4-5%) are currently still reached when TiO₂ films of sub-optimized thickness (2 um) are employed.
We address the issue of pore-filling by both advanced hole conductor deposition techniques and the application of ordered TiO₂ morphologies (nanotubes). The hole conductor of choice in our current work is spiro-OMeTAD but the concepts presented here can also be applicable to other hole conductors, including metallic ones.
For the deposition of the spiro-OMeTAD, we applied post-annealing at temperatures up to 150 ºC as well as variations in spin-coating conditions and vacuum application. Transparent TiO₂ nanotube layers on SnO2:F coated glass substrates (including a dense TiO₂ underlayer of ~ 50-100 nm thickness) were developed and investigated with respect to their potential to improve the pore-filling and/or slow down the interfacial recombination kinetics.
We will present a comparative study of a series of solid state DSC devices, prepared with the concepts outlined above. The photovoltaic performance of the devices will be rationalized in terms of pore-filling and interfacial recombination by using characterization methods, such as photoinduced absorption spectroscopy (PIA), photovoltage transient measurements and impedance spectroscopy.