Large quantum efficiency enhancements of pristine conjugated polymer MEH-PPV by interlayer polymer diffusion

Photoluminescence (PL) efficiency of a neat conjugated polymer (CP) increases substantially when the CP molecular segments are subjected to increased tensions resulting from tight packaging of neighboring chains established by interdiffusion under solvent annealing. The enhancements become even greater on low-bandgap substrate, such as Si-wafer, due to reduced heterojunction quenching (HJQ) by the increased stress. After infusing polystyrene (PS) into a MEH-PPV film a depth of ~3.5 nm by solvent annealing, we found that the PL emissions at ~580 nm of the pristine non-permeated portion increased ~6 folds on Si-wafer. For the same diffusion depth, the enhancement reduced to ~50% on a non-quenching substrate (glass cover slip). The observation was in agreement with previous findings that PL efficiencies of CPs can be enhanced dramatically by molecular stretching, which can also produce an effect of reduced HJQ. Owing to PS/MEH-PPV intermixing at the interface, a new peak also emerged, but blue shifted substantially to ~520 nm owing to chain separation and conformation distortions of the molecular segments confined within the tiny mixing zone. After extended annealing, the intermixing zone in the bilayer system may undergo dewetting, which creates further increased PL to the sheared pristine MEH-PPV molecules. The results suggest a close relationship between segmental stress states and the optoelectronic behavior of conjugated polymers.