According to Dieter Neher of the University of Potsdam, many optoelectronic devices perform better when using layers prepared from polymer blends. But there's a snag. Homogeneous blends of most polymers are thermodynamically unstable and will undergo a phase separation. It's often very difficult to control the length scale of this phase separation, even though it affects the device properties.

"We offered, for the first time, a universal method for preparing polymer blend layers with control over the phase separation," Neher told nanotechweb.org. "In contrast to earlier approaches, our strategy for creating nanostructured polymer blends does not require any chemical modification of the polymer structure. Moreover, it should be applicable to virtually any material that is soluble in an organic solvent immiscible with water and that forms a solid phase after solvent extraction."

To carry out the technique, the scientists dissolved the polymer in a solvent that was immiscible with water. They then added this solution to an aqueous solution containing a surfactant and ultrasonicated the mixture. This resulted in a stable mini-emulsion containing small droplets of the polymer solution. Finally, evaporating the solvent left a stable dispersion of solid polymer nanoparticles in water, and spin-coating the aqueous dispersion onto a glass or silicon substrate laid down a homogeneous layer of the nanospheres.

Neher and colleagues used two variations of the technique. In the first, they mixed two dispersions of single-component nanospheres and laid down thin layers. The second approach saw them prepare nanospheres containing two polymers by dissolving two polymers in a suitable solvent at the start of the mini-emulsion process.

"In both cases, the upper limit for the dimension of phase separation is determined by the size of the individual nanoparticles, which can be adjusted down to a few tens of nanometres," added Neher.

The smallest nanoparticles the scientists have made so far are about 30 nm in diameter, although for solar-cell applications it's desirable to have particles comparable in size to the exciton diffusion length, which is estimated to be about 10-20 nm.

To make an organic solar cell, the team made nanospheres from a mixture of PBT and F8BT polymers using the mini-emulsion process and either xylene or chloroform as a solvent. The resulting two-component polymer nanoparticles had diameters of 49 and 53 nm, respectively. The scientists spin-coated the nanoparticles onto glass covered with a transparent indium tin oxide electrode and then added a Ca/Al cathode. This created a solar cell with a peak quantum efficiency of about 1.7%, a value that is among the best reported for devices using 1:1 PBT/F8BT layers spin-coated from xylene.

"Possible applications we are thinking about include polymeric electronic devices, such as solar cells, which exhibit a pronounced correlation between device performance and phase-separated layer morphology," said Neher. "Furthermore, since our process allows us to deposit polymer layers from an aqueous solution rather than from organic solvents, the preparation of multilayer devices should be possible, even when the components forming the individual layers are soluble in the same solvent. We are also trying to extend the variety of blends, for example to polymer-inorganic hybrid systems."

The researchers reported their work in Nature Materials.