"Our work shows that we have to be more careful about how we use nanotubes in polymer photovoltaics," Robin Nicholas of Oxford University's Clarendon laboratory told nanotechweb.org. "If nanotubes harvest excitons then they might be detrimental to the performance of solar cells if the material is in the wrong part of the device."

The team made its observation using a spectroscopic technique known as photoluminescence excitation mapping. By recording the near-infrared luminescence intensity from the nanotubes as a function of the polymer excitation wavelength, the researchers uncovered clear evidence of direct energy transfer between the two materials.

Nicholas and his colleagues found that emission occurred from all of the nanotube species present in their test polymer when excitation wavelengths in the range 450–550 nm were used. It's important to note that this spectral range closely matches the optical absorbance of the polymer material, but does not correspond to any electronic transitions in the carbon nanotubes.

Creating electron-hole pairs
To explain the result, the researchers conclude that the photoexcited polymer is transferring energy to the SWNT, which results in the creation of an electron-hole pair that recombines at the nanotube's bandgap. This behaviour is contrary to the proposed role of SWNTs in organic photovoltaics, which is to facilitate charge separation and transport.

Nicholas is keen to point out that this is by no means the end of the road for SWNTs and organic photovoltaics. "It might be a case of choosing the right combination of nanotube diameters and polymers to produce charge separation," he explained. "It is also possible that the charge separation behaviour could be significantly different in the electric field generated inside a cell."

Regardless of the role that SWNTs play in the internal workings of the device, Nicholas thinks that carbon nanotubes will still be very important when it comes to developing the cell's transparent contacting layers. He reveals that the group's long term goal is to make an "all-carbon based" photovoltaic chip based on a combination of carbon nanotubes and polymer semiconductors.

The researchers presented their work in Nanotechnology.