Increasing the efficiency of a hybrid solar cell simply by placing it near a source of ambient noise or vibration would be a boon for photovoltaics in urban areas, in the military, or on machinery or transportation. Hybrid organic/inorganic solar cells are already a tempting option over silicon because of their lower cost, but they suffer from their own drawbacks of efficiency. However, new research demonstrates that the piezoelectric qualities of the cells' inorganic layer can be used to boost the overall efficiency of hybrid systems, which is promising for wherever sound and sun are together.
Piezoelectric materials generate electricity when exposed to vibrations, sound, and even touch. While it’s known that the zinc oxide (ZnO) nanotubes studied in this research have piezoelectric properties, few studies have addressed it. Additionally, focus has been on the electricity generated by the piezoelectric effect itself, a negligible percentage of the overall output, rather than how this layer interacts in its hybrid sandwich.
Various aspect ratios of ZnO nanotubes created for this research, where higher ratios had greater efficiencies (Photo: Professor James Durrant)
However, scientists at Queen Mary University and Imperial College London found that sound levels as low as 75 decibels (quieter than city traffic) improved the efficiency of their photovoltaic device up to 50 percent. In particular, efficiency peaked at 10 kHz, the resonant frequency of the ZnO nanotubes and right about where a lot of high notes of pop music hit. Music was tested for fun, with pop having the most effect, and classical the least (increased efficiencies or not, however, currently researchers have found no excuse for dubstep).
By vibrating the system in the dark, they were able to eliminate the possibility that the increased voltage was merely due to the piezoelectric layer generating its own electricity. The small amount of electricity generated was not comparable to the increase attainable with both light and vibration.
They concluded that the electrical field set up by the vibrating ZnO nanotubes interacts with electrons migrating from the organic polymer layer. This process decreases the likelihood of recombination, in which electrons are energized but settle back into a hole instead of migrating to the electron-accepting ZnO layer.
The physical mechanisms of this are explored in more detail in the paper published in Advanced Materials.
In the video below, Dr. Steve Dunn discusses the process of conducting this research.
Source: Queen Mary University of London