One of the greatest challenges we face is that of finding a way to provide energy without releasing carbon dioxide into the atmosphere. If the cost of photovoltaic cells could be reduced by a factor of five, they would compete favorably with conventional sources of electricity and could provide a significant fraction of our energy without generating carbon dioxide. Since organic semiconductors and inorganic nanocrystals can be deposited at low cost they provide a means to lower the costs of photovoltaic cells. The key to making efficient cells with these materials is to interpenetrate two semiconductors with offset energy levels at the 10-nm length scale so that photogenerated excitons can be split by electron transfer before they recombine. The network must also be designed to enable efficient collection of the charge by the electrodes.
We have developed a technique for making high-quality thin films of titania with well ordered arrays of pores that have a diameter in the range of 4-10 nm using a titania sol-gel precursor and a structure-directing amphiphilic block copolymer. We have filled the pores with regioregular poly(3-hexyl thiophene) by spin casting the polymer on top of the titania and then heating the polymer at temperatures between 100 and 200 °C. We find that 32 % of the volume of the film can be filled with polymer in just a few minutes at 200 °C. At lower temperatures, the infiltration process takes longer and the amount of polymer that can be incorporated is less. We hypothesize that the polymer coats the walls of the pores and that the coating is thicker when the polymer is incorporated at higher temperatures. Using absorption and photoluminescence spectroscopy we have determined that the polymer chains take on a coiled conformation in the pores and that there is little or no polymer crystallization. The photoluminescence measurements also show that photoinduced electron transfer takes place from the polymer to the titania, but that not all of the excitons are quenched by electron transfer. Since the pores are less than 10 nm in diameter and excitons can diffuse over this distance before emitting a photon in a spin cast polymer film, we think that exciton diffusion is hindered inside the pores. We attribute this to the altered polymer chain morphology. Despite the hindered exciton diffusion, we have been able to make photovoltaic cells with an energy conversion efficiency of 1.3 %. We will present an analysis of the factors that limit the efficiency of the device and plans for increasing the efficiency to more than 10 %. We will also show that these photovoltaic cells could potentially be manufactured at low cost.
A scanning electron microscope image of a self-organized mesoporous titania film.