An alternative blocking layer for titanium dioxide (TiO 2) solar cell applications

In hybrid organic solar cells a blocking layer between transparent electrode and nanocrystalline titania particles is essential to prevent short-circuiting and current loss through recombination at the electrode interface. Here the preparation of a uniform hybrid blocking layer which is composed of conducting titania nanoparticles embedded in an insulating polymer derived ceramic is presented. This blocking layer is prepared by sol-gel chemistry where an amphiphilic block copolymer is used as a templating agent. A novel poly(dimethylsiloxane) containing amphiphilic block copolymer poly(ethyleneglycol)methylethermethacrylate-block-poly(dimethylsiloxane)-block-poly(ethyleneglycol)methylethermethacrylate has been synthesized to act as the templating agent. Plasma treatment uncovered titania surface from any polymer. Annealing at 450°C under nitrogen resulted in anatase titania with polymer derived silicon oxycarbide ceramic. Electrical characterization by conductive scanning probe microscopy experiments revealed a percolating titania network separated by an insulating ceramic matrix. Scanning Kelvin probe force microscopy showed predominant presence of titania particles on the surface creating a large surface area for dye absorption. The uniformity of the percolating structures was proven by microbeam grazing incidence small angle x-ray scattering. First applications in hybrid organic solar cells in comparison with conventional titanium dioxide blocking layer containing devices revealed 15 fold increases in corresponding efficiencies. Poly(dimethylsiloxane)-block-poly(ethyleneglycol)methylethermethacrylate and poly(ethyleneoxide)-poly(dimethylsiloxane)methylmethacrylate diblock copolymers were also synthesized. Their titania nanocomposite films were compared with the integrated blocking layer. Liner poly(ethyleneoxide) containing diblock copolymer resulted in highly ordered foam like structures. The effect of heating temperature rise to 600°C and 1000°C on titania morphology was investigated by scanning electron and force microscopy and x-ray scattering. Sol-gel contents, hydrochloric acid, titania precursor and amphiphilic triblock copolymer were altered to see their effect on titania morphology. Increase in block copolymer content resulted in titania particles of diameter 15-20 nm.

Application of a hybrid blocking layer in dye-sensitized solar cells

In dye-sensitized solar cells a blocking layer between the transparent electrode and the mesoporous titanium dioxide film is used to prevent short-circuits between the hole-conductor and the front electrode. The conventional approach is to use a compact layer of titanium dioxide prepared by spin coating or spray pyrolysis. The thickness of the blocking layer is critical. On one hand, the layer has to be thick enough to cover the rough substrate completely. On the other hand, the serial resistance increases with increasing film thickness, because the layer acts as an ohmic resistance itself. In this thesis an amphiphilic diblock copolymer is used as a functional template to produce an alternative, hybrid blocking layer. The hybrid blocking layer is thinner than the conventional, compact titanium dioxide film and thereby possesses a higher conductivity. Still, this type of blocking layer covers the rough electrode material completely and avoids current loss through charge recombination. The novel blocking layer is prepared using a tailored, amphiphilic block copolymer in combination with sol-gel chemistry. While the hydrophilic poly(ethylene oxide) part of the polymer coordinates a titanium dioxide precursor to form a percolating network of titania particles, the hydrophobic poly(dimethylsiloxane) part turns into an insulating ceramic layer. With this technique, crack-free films with a thickness down to 24 nm are obtained. The presence of a conductive titanium dioxide network for current flow, which is embedded in an insulating ceramic material, is validated by conductive scanning force microscopy. This is the first time that such a hybrid blocking layer is implemented in a solar cell. With this approach the efficiency could be increased up to 27 % compared to the conventional blocking layer. Thus, it is demonstrated that the hybrid blocking layer represents a competitive alternative to the classical approach.