Exciton diffusion controlled quantum efficiency in hybrid dye sensitized solar cells

Well-ordered and uniform titania nanoparticle arrays were synthesized using diblock copolymers as structure directing agents. High molecular weight copolymers of polystyrene-b-polyethylene oxide and poly(methylmethacrylate)-b-polyethylene oxide were used to control the distance between titania nanoparticles in the range of 20-60 nm. Using these titania nanoparticle arrays and regioregular poly(3-hexylthiophene), models for a dye sensitized photovoltaic cell were assembled, in which the interparticle spacing was systematically varied. In these simplified solar cells, the titania nanocrystals were surrounded by a continuous regioregular poly(3-hexylthiophene) phase. The spacing between the titania nanoparticles was chosen as to provide enough space for the hole transfer material-regioregular poly(3-hexylthiophene)-to assemble as pi stacks. The external quantum efficiency showed a clear dependence on the distance between titania nanoparticles and reached 12% at an excitation wavelength of 515 nm in the best case. This demonstrates that the regioregular poly(3-hexylthiophene) phase acts as the locus of excition generation while the dye layer prevents charge recombination at the heterointerface. Thus control of the exciton diffusion is a key issue for present solid-state dye sensitized solar cells.