(233e) ZnO Nano Forest As Electrode Material for DSSC: Where Is the Bottleneck?

The efficiency of dye sensitised solar cells has not improved substantially over the last decade. To resolve this problem, different electrode materials are being tried. One of the promising electrode material is ZnO which has orders of magnitude higher electron mobility than TiO₂. Another advantage of ZnO is that it can produce a variety of morphologies such as nano-rods, wires, forests, flower, tetrapods easily. Such structures, especially the nano-forest, reduces the chances of electron scattering to minimum by providing a network of electron paths of gradually changing hierarchy.

Although synthesis of ZnO nano forest structure has been demonstrated by hydrothermal process, the growth rate is painstakingly slow for this process and may not be suitable for practical purpose. In this talk we shall discuss a microwave based process for quick production of the nano forest and hence resolving this production bottleneck. The new strategy of force seeding for quick and efficient branch growth will also be discussed.

Even when electron freeways and avenues practically eliminates electron scattering at the particle grain boundaries, the efficiency of ZnO solar cells are much lower than those of TiO₂. In this work, we will show that the reason for this bottleneck is limited dye adsorption. The most prevalent Ruthenium based dye N719 is acidic enough to corrode the electrode surface and hence result in poor dye uptake. Microscopic investigation of the electrode surface reveal a much severe corrosion to the nano-forest structure due to small size of the branches.

It will be shown that soaking the electrode in a very concentrated dye solution for very short period may lead to some adsorption. However, huge dye loss occurs in subsequent washing steps. Nevertheless, within these constraints, the dye adsorption has been optimised to keep the efficiency of the cell to reasonable value. EDX and microscopy reveals that a very small fraction of the surface is actually covered with dye and hence dyes specifically suitable for ZnO should be used for better efficiency. Results for less acidic dyes will also be discussed.