OPVs offer an affordable route to harnessing the sun's energy. Polymer-based photovoltaics are typically less efficient than silicon photovoltaics, but the material is more versatile and can be applied to flexible substrates. Device degradation remains an issue for OPVs as oxidation, exposure to moisture and photochemical reactions can greatly reduce the working life of the unit.

"The ultraviolet part of the spectrum can be devastating to polymer device performance," David Carroll, director of WFU's Center for Nanotechnology and Molecular Materials, told nanotechweb.org. "However, by removing it through scattering and absorption, devices live longer."

Boron-nitride materials are well matched to ultraviolet frequencies and act as a scattering centre for the incoming radiation. As Carroll explains, particle shape plays a key role in the process. "The high aspect ratio of the nanotubes provides increased oscillator strength, which allows better antenna behaviour," he said. "It means that this method of removing ultraviolet radiation is more effective than simply having the boron-nitride in the form of a thin film."

The group makes its nanotubes by first placing a quartz crucible filled with boron, iron oxide and magnesium oxide in a horizontal alumina tube furnace. Next, the reactor is held at 1200 °C for 30 minutes in the presence of NH3 and then cooled to give a fine powder. Catalyst particles are removed from the resulting multiwalled boron-nitride nanotubes by washing in concentrated nitric acid.

Wrapping up the device

Commercial grade Saran, popular in the food packaging sector, was chosen as the polymer host. It turns out that many of the properties required for food packaging are also important when it comes to encapsulating OPV devices – transparency in the visible range; a barrier to oxygen and moisture ingression; and good mechanical and thermal stability.

The degree of device protection was assessed by monitoring the spectral signature of P3HT (a prototype organic electronic material) firstly in its original state and then coated with the composite film. Uncoated samples showed signs of degradation after just 24 hours of exposure to the atmosphere.

Despite the addition of the nanomaterial, the transmission of the ecapsulant was shown be more than 93% over the entire visible range – well above the minimum required value of 90% for photovoltaic devices.

Finally, to gauge the thermal stability of the encapsulant, the group put various samples through differential thermal analysis over the range 50–850 °C. The nanotube-polymer composite significantly outperformed the unloaded polymer sample at a boron-nitride concentration of 1.5%.