ARTICLES 408 nature materials | VOL 2 | JUNE 2003 | www.nature.com/naturematerials S olid blends of polymers can exhibit mechanical, optical and electro-optical properties not attainable with a single polymer, especially if the blend morphology is formed at submicrometre scales. Moreover, many biological, optical and electro-optical devices include thin layers of polymer blends, which are usually deposited from a solution of all polymer components in a common solvent. For example, highly efficient organic solar cells have been made from thin layers containing a blend of an electron-donating and an electron- accepting polymer 1–3 . In this case, the dimension of phase separation must be in the range of the exciton diffusion length, commonly a few tens of nanometres, and the overall layer thickness should not greatly exceed the penetration depth of the incident light. However, because the entropy of mixing is generally low for polymers, solid polymer blends tend to phase-separate at the macroscopic scale. Moreover, when a thin layer of immiscible polymers is deposited from solution, the resulting morphology strongly depends on various parameters, such as the individual solubility of the polymers in the solvent used, the interaction with the substrate surface, the layer thickness and the method of deposition, drying and annealing 4–10 . Therefore, the adjustment of the lengths of phase separation in thin layers is often arbitrary and based on trial-and-error. Several strategies have therefore been developed to form well- defined and predictable multicomponent polymer structures with phase-separation at the nanometre scale. The most straight-forward approach is to use linear block copolymers 11,12 . However, the drawback of this approach is that both components, A and B, with their different chemical and electronic structures, have to be connected by a covalent bond, which limits the availability of possible A–B pairs. In fact, only few examples of block copolymers containing two semiconducting polymers have been reported 13,14 .AB diblock copolymers have also been used as compatibilizers in bulk blends of the corresponding homopolymers A and B 15,16 . Finally, co-continuous nanostructured polymer morphologies have been prepared by reactive blending 17 ; in this approach, one component bears reactive groups along the backbone and the second component possesses complementary reactive moieties only at one end. Even though this novel strategy is Polymer layers can exhibit significantly improved performances if they possess a multicomponent phase- separated morphology. We present two approaches to control the dimensions of phase separation in thin polymer- blend layers; both rely on polymer nanospheres prepared by the miniemulsion process. In the first approach, heterophase solid layers are prepared from an aqueous dispersion containing nanoparticles of two polymers, whereas in the second approach, both polymers are already contained in each individual nanoparticle. 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. We also show that the efficiencies of solar cells using two- component particles are comparable to those of devices prepared from solution at comparable illumination conditions, and that they are not affected by the choice of solvent used in the miniemulsion process. Novel approaches to polymer blends based on polymer nanoparticles THOMAS KIETZKE 1 , DIETER NEHER* 1 , KATHARINA LANDFESTER* 2 , RIVELINO MONTENEGRO 2 , ROLAND GÜNTNER 3 AND ULLRICH SCHERF* 3 1 University of Potsdam, Institute of Physics, Am Neuen Palais 10, D-14469 Potsdam, Germany 2 Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany 3 University of Wuppertal, Department of Chemistry, Gaußstrasse 20, D-42097 Wuppertal, Germany *e-mail: neher@rz.uni-potsdam.de; landfester@mpikg-golm.mpg.de; scherf@uni-wuppertal.de Published online: 11 May 2003; doi:10.1038/nmat889 © 2003 Nature Publishing Group