Suresh Gubbala Defense

The global energy demand is expected to triple by the end of the century. This demand of energy can be easily met with solar energy as the Sun provides more than 10,000 times this energy. In order to tap this energy, large area solar harvesting is required. For this to be feasible, the solar cells must have high efficiencies and low cost. One of the most promising technologies that can meet these demands is the dye sensitized solar cell. However, the dye sensitized solar cells currently in use are made of networks of nanoparticles, in which the electron recombination with the electrolyte is high, leading to loss in efficiency and reliability.

Nanowire based architectures provide unique advantages over nanoparticle and thin film based technologies due to their potentially better charge transport, recombination and electronic properties. Similar opportunities also exist in other devices such as Electrochromic smart windows. The major challenges in these devices are the lack of sufficient contrast in the bleached and colored stated and long switching times on large area windows.

In this work, nanowire based architectures for electrochromic and dye sensitized solar cells were investigated. In the case of electrochromics, the use of WO3 nanowires resulted in high contrast ratios (of ~0% to 70% transmission) between the colored and bleached states. The bleaching time in these devices was found to be slower than the coloration times. However, these timescales were faster than nanoparticle based devices. Similar results were also obtained with nanowires with mat like configuration. It was concluded that even higher performance Electrochromic devices can be made by optimizing the nanowire density, diameters and aspect ratios.

Nanowire based dye sensitized solar cells were demonstrated with SnO2 nanowires as the model system. Although all the previous work on SnO2 nanoparticle based cells showed low open circuit voltages (Voc) of less than 400 mV, the use of SnO2 nanowires significantly improves the Voc of these devices to upto 560 mV. A variety of techniques were used to characterize these devices. Based on these studies, it was seen that nanowire based DSSCs showed faster electron transport, slower electron recombination kinetics, smaller work function and shallow average trap depths, all of which contributes to high Voc of these cells. These characteristics arise due to the high crystallinity of nanowires compared to nanoparticles, which are highly polycrystalline.

Further, the SnO2 nanowires were modified to engineer their band edges by coating them with TiO2 nanoparticles. Dye injected electrons from TiO2 quickly transfer into the SnO2 nanowires which then prevent any recombination reaction, thus improving the solar cell efficiency even further. SnO2 nanowires, due to their excellent properties and their ability to support other materials for band edge engineering, thus emerge as an excellent choice of material for dye sensitized solar cells. These nanowires can be incorporated into other DSSC anodes also, to further improve their performance.

The possibility of the use of vertically oriented nanowires for DSSCs was also explored. He we show some preliminary results on Nb2O5 nanowire based DSSCs and the need to engineer the interface between nanowires and the substrate to improve their efficiency.