It is not just humans who respond to motivational music. Researchers in the UK have discovered that blasting beats at zinc-oxide solar cells makes them perform up to 50% better. According to the researchers, pop and rock music gets the cells going more than classical music, but they suggest that any noise with a broad range of frequencies will produce similar effects. The discovery might be exploited by placing the devices on top of buses, air-conditioning units and in other noisy spots.

Crystalline-silicon photovoltaic cells have been around for more than 50 years but they are fragile and expensive, and producing them is labour intensive. Much attention has been focused on investigating alternative materials, such as flexible, transparent and cheap zinc-oxide devices, but these remain woefully inefficient compared with silicon. In an attempt to stretch its limits, functional-nanomaterials expert Steve Dunn of Queen Mary University of London and Imperial College photochemist James Durrant wondered whether they could combine two of zinc-oxide's special physical properties to make solar cells more efficient.

Extracting excitons

"One of the biggest problems with polymer or organic materials when they're used in solar cells is recombination," explains Dunn. When a photon of light interacts with a solar cell, it generates an exciton – an electron–hole pair. To harvest the photon's energy, the electron and hole must be extracted into an external circuit before they manage to recombine and waste the energy as light or heat within the cell. Zinc oxide – a semiconductor – is known to hinder recombination, and nanorods of the material can be embedded in photovoltaic cells to funnel electrons out of the active layer.

Zinc oxide is also piezoelectronic (piezo comes from the Greek "to squeeze"), meaning that when it is subject to mechanical strain, the symmetry of its component crystals is distorted and a polarization charge appears along the length of the structure. Dunn and Durrant hypothesized that by using acoustic vibrations to wobble – and induce tiny piezoelectric currents in – zinc-oxide nanorods, they could boost their photovoltaic cells' electricity output. To test their idea, the researchers used computer speakers to play the device music from their mobile phones and individual frequencies from a signal generator at volumes of about 75 dB – equivalent to a lively office.

AC from AC/DC

The team found that the device was 40–50% more efficient when particular types of music were played. "It quite liked Adele and AC/DC," says Dunn, "but then Safa [Shoaee] played some Persian funk at it and it was really loving that!" Classical music apparently did little for the device though, having almost no effect on its power-conversion efficiency. "[In classical music] there's a lot less going on in terms of all the additional overtones that come from synthesized or rock music...The device responds to the larger variety of frequencies in the rock music, and also to the fact that there's just a lot more energy available in it," says Dunn.

The more energy delivered by the sound, the more the pressure wave deforms the rods as it ploughs through the device, and the more polarized the rods become. As they deform and relax with each successive wave, an oscillating electric field is generated within them, dragging the free electrons and holes back and forth at the zinc-oxide–polymer interface and hampering their chances of recombining before they can be harvested.

Absolute numbers

If playing music to solar cells sounds like a somewhat inefficient way to produce electricity, the researchers suggest that a similarly broad range of frequencies arise from myriad types of everyday noise, marking the devices out for use on laptops, public transport and near airports. That is a long way off yet, though. Even with the 50% increase, this type of cell only achieves 1.8% power-conversion efficiency and lags way behind crystalline silicon's 10–20% standard.

Jenny Nelson, a physicist at Imperial College who was not involved with the research, calls it a "nice example of how nanomaterials can do things better than bulk materials, and in this case without needing an expensive fabrication processes". But she points out that the effect is especially impressive partly because zinc-oxide cells are currently so inefficient: "In a very efficient device, the losses to recombination would be less and the relative advantage of the acoustic waves would be less."

To find out more, take a look at the video above presented by Dunn and his colleague Joe Briscoe.

The research is published in Advanced Materials.