On the Glass Transition of Polymer Semiconductors and Its Impact on Polymer Solar Cell Stability

The glass transition temperature is a critical processing parameter that governs the kinetics of molecular organization of polymer semiconductors during solidification. Yet, little attention is paid to the resulting structure-processing-property relationships that lead to optimal optoelectronic performance, which is usually obtained with nonequilibrium nanostructures. This review elucidates the interplay of molecular design and glass transition phenomena that are common to the most well-studied families of conjugated polymers, including polyfluorenes, polythiophenes, and poly(p-phenylenevinylene)s. The influence of key structural factors known from classical polymer science-such as molecular weight, chain rigidity, side-chain architecture, and intermolecular pi-pi interactions, is explored in order to :provide rationals that can guide the synthesis of new polymer semiconductors with tailored glass transition temperatures. Moreover, the discussion is anchored in an overview of the main measurement techniques with emphasis on rate-dependency and sub-glass transition phenomena as well as differences between bulk and thin films. The second half of the review focuses on the glass transition temperature(s) of polymer:fullerene bulk-heterojunction blends, Which represent the most promising active layer architecture for organic solar cells, and highlights the relevance of fullerene diffusion. A challenging perspective is provided with regard to the thermal stability of the,blend nanostructure vs. the mechanical robustness and ductility of the active layer material. Conflicting demands on the blend glass transition temperature, i.e., higher vs lower than the processing and operating temperature, require a satisfactory compromise that must be achieved before truly flexible polymer solar cells with, a high light-harvesting efficiency can be realized.