Toward high-performance polymer solar cells: The importance of morphology control

Polymer solar cells have the potential to become a major electrical power generating tool in the 21st century. R&D endeavors are focusing on continuous roll-to-roll printing of polymeric or organic compounds from solution-like newspapers-to produce flexible and lightweight devices at low cost. It is recognized, though, that besides the functional properties of the compounds the organization of structures on the nanometer level-forced and controlled mainly by the processing conditions applied-determines the performance of state-of-the-art polymer solar cells. In such devices the photoactive layer is composed of at least two functional materials that form nanoscale interpenetrating phases with specific functionalities, a so-called bulk heterojunction. In this perspective article, our current knowledge on the main factors determining the morphology formation and evolution is introduced, and gaps of our understanding on nanoscale structure-property relations in the field of high-performance polymer solar cells are addressed. Finally, promising routes toward formation of tailored morphologies are presented.

Crystallization of syndiotactic polypropylene (sPP) from oriented melts

The influence of the syndiotacticity on the crystallization behaviour of syndiotactic polypropylene (sPP) has been investigated. The syndiotacticity has been measured by C-13-NMR spectroscopy and the phase formation has been observed by electron diffraction of oriented samples. It is shown that the crystal phase formation depends strongly on the perfection of the tacticity of the macromolecules.

EPITAXIAL CRYSTALLIZATION OF TRANS-1,4-POLYBUTADIENE ON HIGHLY ORIENTED ISOTACTIC POLY(PROPYLENE) FILMS

Epitaxial crystallization of trans-1,4-polybutadiene (PBD) on highly oriented isotactic poly(propylene) (iPP) has been investigated at different crystallization temperatures and rates. From electron microscopy and electron diffraction, it is confirmed that epitactic growth of the low-temperature modification (monoclinic) with microcrystals of PBD on the iPP films takes place with their chain directions about +/- 50-degrees apart. No epitaxial relationship occurs between the high-temperature modification (hexagonal) of PBD and the iPP substrate. Thermal analyses of PBD-PP layered films indicate that the epitaxy has an important effect on the formation of the high-temperature modification of PBD.

EPITAXIAL CRYSTALLIZATION OF SYNDIOTACTIC POLYPROPYLENE ON UNIAXIALLY ORIENTED POLYETHYLENE

The epitaxial crystallization of syndiotactic polypropylene (sPP) on uniaxially oriented polyethylene (PE) has been investigated by electron microscopy and electron diffraction. It is found that the sPP lamellae grow epitaxially on the PE substrate film with the preference in sPP for the b axis as the fast growth direction. Instead of 50-degrees as in the system of isotactic polypropylene with PE, the molecular chains of the sPP crystals are approximately +/- 37-degrees inclined to the chain direction of PE.