Photoluminescence of MEH-PPV Brushes, Pancakes, and Free Molecules in Solutions and Dry States

Photoluminescence (PL) of a conjugated polymer MEH-PPV, poly­[2-methoxy-5-(2′-ethylhexyl)­oxy)-1,4-phenylenevinylene], grafted on a silicon wafer with controlled tether spacing was studied to reveal the effects of molecular conformation, chain packing, and mechanical stress. In the solvent-swollen state, the PL of the densely grafted polymer (denoted “brushes”) was blue-shifted substantially relative to the lightly grafted (denoted “pancakes”) and free polymers. As solvent evaporated, while for the brushes the changes in PL were insignificant, the PL spectra of the pancakes underwent large blue shifts and exhibited significant efficiency enhancements up to ∼175-fold. The solvent evaporation effects were attributed to molecular deformations resulting from coil contraction on the substrate, which gave rise to conjugation-disruptive kinks (blue shift) and segmental stretching (PL enhancement) in the dried molecules. Moreover, heterojunctional quenching was found significantly suppressed by the mechanical stresses. Similar behavior was observed in dried free single molecules. These results unveil the fundamental role of mechanical stresses, not only indirectly through their influence on molecular conformations, but directly via alterations of the excitonic behavior

Massive Enhancement of Photoluminescence through Nanofilm Dewetting

Due to the rather low efficiencies of conjugated polymers in solid films, their successful applications are scarce. However, recently several experiments indicated that a proper control of molecular conformations and stresses acting on the polymers may provide constructive ways to boost efficiency. Here, we report an amazingly large enhancement of photoluminescence as a consequence of strong shear forces acting on the polymer chains during nanofilm dewetting. Such sheared chains exhibited an emission probability many times higher than the nonsheared chains within a nondewetted film. This increase in emission probability was accompanied by the emergence of an additional blue-shifted emission peak, suggesting reductions in conjugation length induced by the dewetting-driven mass redistribution. Intriguingly, exciton quenching on narrow-band-gap substrates was also reduced, indicating suppression of vibronic interactions of excitons. Dewetting and related shearing processes resulting in enhanced photoluminescence efficiency are compatible with existing fabrication methods of polymer-based diodes and solar cells