Engineering Bandgap by Doping
For the last few years, S Fabbiyola has been experimenting with co-precipitation to synthesise metal-doped zinc oxide nanoparticles. She managed to synthesise zinc oxide doped with cobalt, iron, nickel, copper… Getting skilled in the art of making such doped zinc oxide nano particles was important: doped zinc oxide has lower resistance, transmits electrons more easily and is chemically more stable than pure zinc oxide. So it is a useful material for making dye sensitised solar cells.
Her guide, L John Kennedy at the Vellore Institute of Technology has been playing around with different methods to produce nanoparticles for more than two decades. And from his experience, they knew that a microwave assisted method to create nanoparticles tends to create nanoparticles with more complex structures with wide variations in properties than those derived from the co-precipitation method of doping zinc oxide.
Doping, or partially replacing zinc in the oxide with smaller sized metal atoms, creates a contraction of the nanoparticle structure and reduces the band gap, the gap between the valence band and the conduction band. That means that the transfer of electrons by the nanoparticle is made easier – a property that is important for making dye-sensitised solar cells with zinc oxide as anode.
Dye-sensitised solar cells are often called third generation solar cells. But unlike 3G in mobile communication, they have not become popular because electrons get trapped in zinc oxide. But in the nano forms of zinc oxide, this is reduced. Once the band gap also is reduced the third generation technology is bound to take a leap forward. The question, of course, was what is the amount of doping needed and which metal should we use for doping?
So Fabbiyola put her skills to work. And made zinc oxide doped with different amounts of copper, nickel, cobalt and iron. She and John rigged up a system of dye-sensitised solar cells. In their experiments, Fabiola and John used Rhodamine B as dye sensitiser and an iodide/triiodide redox couple as electrolyte.
Photons from sunlight excite the electrons of the dye. The excited electrons from the lower unoccupied molecular orbital are transferred to the nanoparticles. From the conduction band of the nanoparticles the electrons are transferred to the external circuit and is collected at the counter electrode made of aluminium. The oxidised form of the electrolyte gets reduced at the counter electrode. The dye that had lost electrons regains them from the electrolyte. The electrolyte regains electrons from the counter electrode. And the circuit is complete, setting up a flow of electricity…
By testing zinc oxide doped with different metals and in different concentrations, the team came to understand that a five per cent doping with copper is the best option for improving short-circuit current density and power conversion efficiency. Copper occupies a friendly seat, next to zinc in the periodic table – nearer than the other metals used for doping in this series of experiments.
The paper the duo published in the Journal of Nanoscience and Nanotechnology recently, marks an accomplishment for Fabbiyola as researcher. The theoretical understanding and practical skills she has gained will now have to be put to use to tackle other questions. Which is the best dye for copper doped zinc oxide nanoparticles? And which is the best redox couple to leverage on the bandgap engineered nanoparticles?
If these are also solved, doped zinc oxide can replace titania in dye sensitised solar cells and bring down the costs further.
and Udham P K, Pune