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

Reaching Nearly 100% Quantum Efficiencies in Thin Solid Films of Semiconducting Polymers via Molecular Confinements under Large Segmental Stresses

Quantum efficiencies remain a critical issue for general applications of semiconducting polymers in optoelectronics and others. In this work, we demonstrate that nearly 100% quantum efficiencies (η's) in thin solid films can be reached when the polymer molecules are mechanically stretched into molecular confinement. We selected three conjugated polymers of varied backbone stiffness and interchain coupling, prepared in both diluted and pristine states. All of the polymers when highly diluted (c = 0.1 wt %) exhibited massive η increases after stretching to very large strains (∼300-500%) via micronecking, with the rigid polyfluorene (PFO) and semirigid MEH-PPV both manifesting η ≈ 90%, while the most flexible yet regioregular polythiophene (P3HT-rr) exhibited a 10-fold increase to ∼21%. In the pristine state, molecular aggregation and interchain coupling curtail development of the molecular confinement, but the large-strain deformation still enhances η's significantly, to ∼90% (PFO) and ∼55% (MEH-PPV) despite no increases for the crystalline P3HT-rr. Moreover, upon substitution by a bulkier side-group to reduce interchain coupling, the pristine films of polythiophene (P3EHT) exhibited a ∼3-fold increase of η after the stretching. The nearly 100% of η's in fully stretched molecules indicates that the in situ self-trapping occurring via sub-picosecond backbone interactions can be mostly responsible for energy dissipations and quite suppressible by segmental stress control. The mechanical confinement effects also indicate the fundamental role of molecular mechanics during stabilization and migration of photoexcited charges.

Photobleaching Kinetics of MEH-PPV in Solution: The Role of Conformational Disorder

Semiconductor polymers are the foundation of organic electronics due to their remarkable optical features, ability to form a thin film, and low cost compared to silicon. However, some of them have intense photobleaching under UV-blue radiation, compromising several applications. In this context, we have investigated the conformational disorder effect on the real-time photobleaching kinetics of a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/chloroform solution under deep-blue radiation. Our results pointed out that a 405 nm diode laser initially causes a significant conformational disorder in the π-conjugated backbone of MEH-PPV as revealed by the Huang-Rhys factor. As a result, a new vibrational mode arises with an energy separation of 230 meV, indicating the substitution of the vinyl (C═C) by carbonyl (C═O) bonds. Then, the conformational disorder reaches a maximum value at some tens of minutes, which is inversely proportional to the polymer concentration, and after that, a random chain scission occurs. Consequently, the effective conjugation length of MEH-PPV in chloroform decreases from nine to three coplanar repetitive units after 1 h of excitation, producing a drastic drop in photoluminescence. Finally, we show that the photobleaching steps are mapped through the conformational disorder.