To make the devices, the scientists combined infrared-sensitive nanocrystals of lead sulphide (PbS) with a conjugated polymer - poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV). They used around 90% nanocrystals by weight, dissolving the two components in chloroform before spin-coating the material onto a substrate to create a film that was 100-150 nm thick.
By changing the size of the nanocrystals, the team was able to tune their wavelength sensitivity. The researchers tailored the nanocrystals to be sensitive to infrared wavelengths of around 980 nm, 1200 nm or 1355 nm. In the absence of nanocrystals, the MEH-PPV polymer reacted to wavelengths between around 400 and 600 nm - i.e. visible light. Other photoconductive polymer devices haven't shown sensitivity beyond about 800 nm.
"We made particles from semiconductor crystals that were exactly 2, 3 or 4 nm in size," said Ted Sargent of the University of Toronto. "The nanoparticles were so small they remained dispersed in everyday solvents, just like the particles in paint."
The devices consisted of a glass substrate, an indium tin oxide layer, a poly(p-phenylenevinylene) (PPV) coating, an MEH-PPV/PbS nanocrystal blend and an upper magnesium contact. They exhibited an infrared photovoltaic effect.
The scientists reckon that since the nanocrystals they used have absorption peaks tunable from 800 to about 2000 nm, they could make nanocomposites that are sensitive at these values.
In the future, the materials might be able to provide a coating for "smart" walls that can respond to the environment in a room, a digital camera able to work in the dark, or clothing that can turn the sun's energy into electrical power.
The researchers reported their work in Nature Materials.