Quantum Efficiency Increasing of a Pristine Polymer by Curbing Picosecond Self-Trapping via Segmental Stretching

Abstract

The quantum efficiencies of conjugated polymers, which have long been a bottleneck barring their broad optoelectronic applications, are found to be increasing dramatically when the chain-like molecules are stretched into a state of confinement over molecular motions. For pristine MEH-PPV molecules, the stretching-induced confinement becomes effective as the segmental stress (σ) has passed a threshold of ∼100 MPa, overcoming the interventions from molecular aggregates, to cause thereafter an increase of quantum efficiency from 6%, almost linearly with σ, to 55% at σ ∼ 215 MPa. By using ultrafast time-resolved confocal spectroscopy, the efficiency increase is revealed arising from suppression of backbone torsion-mediated self-trapping that is normally occurring within ∼2 picosecond following excitation. The dependence on backbone stresses strongly suggests a significant role of lattice strain energy during photoexcitation, implicating the basic understanding of the energy transformation as well as applications of conjugated polymers.

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

“…We stretched the CP molecules into local deformation zones (crazes) where they were subjected to very large local strains (ε’s) − and examined the corresponding changes of QEs within and outside the zones. , During a typical local deformation in a glassy polymer, ,, such as crazing in polystyrene (PS) under an applied strain ( e ) exceeding the threshold of e ≈ 1%, the molecules drawn into the local deformation zones (the crazes) via micronecking are stretched to very large strains comparable to the deformation limit of the chain entanglements network (ε ≈ 300% for PS), ,− while those left outside are subjected to a strain only modestly above the micronecking threshold (ε ≈ 1–4%). ,,− Unlike PS, however, pristine CP generally deforms uniformly without developing local deformation (as shown later), rendering the maximum strains achievable in a standard uniaxial stretching limited by the extensibility of the supporting grids of e ≈ 20% (Experimental Section). To circumvent these problems, we employed two methods in this work: (1) mixing the CP into a PS matrix where the host PS molecules can drag, upon stretching, the dispersed CP strands into the crazes of ε ≈ 300% and (2) juxtaposing the pristine CP film on a substantially thicker PS layer so that the thin CP layer, when stretched, is forced to develop local deformation zones, achieving ε ≈ 500% and a local stress σ ≈ 220 MPa, following the PS-dominated micronecking process (Supporting Information C).…”

“…To analyze the stretching effects on the QE, we define the QE enhancement factors, ξ c and ξ f , respectively for CP molecules in crazes and the regions outside, the two regions under very different stretching conditions with the former drawn to a very large strain ε ≈ 300% under a stress σ ≈ 210 MPa while the latter only to an average of ε ≈ 2.5% with σ ≈ 20–80 MPa (Supporting Information C and G). − The QE enhancement factors ξ c and ξ f are related to γ ( e ) as (stepwise derivations in the Supporting Information G), from which ξ c and ξ f can be determined in simultaneous equations from the slope and intercept in the plot of γ vs e (Figure b and Figures S7a-c in Supporting Information G). For the highly diluted CP of c = 0.1 wt %, the ξ c ’s in crazes were determined to be ∼12.15 (P3HT-rr) and ∼3.60 (for both PFO and MEH-PPV) (Figures c and b, and Figure S8 in Supporting Information G), revealing the very large QE enhancements induced by mechanical stretching on these well-dispersed CP molecules.…”

“…Semiconducting polymers have attracted broad interest for their potentials in opto- and microelectronic applications, but the low quantum efficiencies (QEs, η’s) in thin solid films and other related drawbacks have impeded their prevalence despite the edges on cost, processing ease, and resilient mechanical properties. − The fatal weakness is generally assigned to molecular packing defects that could spawn possibilities of quenching or nonradiative decay. , Recently, an increasing body of evidence has risen demonstrating dramatic QE increases upon mechanic stretching. − The QE increases have been assigned to the stress-rigidified backbones that curb torsional relaxations that may otherwise lead to trapping of the excited states within ∼2 ps after photon absorption . Similar magnitudes of QE increases were also reported when hydrogen bonding was introduced to cause supramolecular confinement restricting polymer segmental motions and to yield more than one order of magnitude enhancements .…”

“…After dodging the in situ self-trapping, the surviving excited states may still risk quenching elsewhere, via Forster or Dexter energy transfer, or hopping, through trapping at the “arresting sites”. , Although their identifications are not entirely clear, these arresting sites are generally attributed to chain packing defects, e.g., molecular aggregates, where trapping may proceed via operations of the local low-energy interchain species , or the induced deformations at the loose or twisted chain segments. To mitigate the effects of molecular aggregation, different practices have commonly been employed, e.g., blending, copolymerization, or adding branches or bulky side groups, in order to improve the QEs, − although in some cases molecular aggregates can enhance fluorescence by hindering chain motions (reminiscent of a molecular confinement effect). − From the perspective of polymer stretching, molecular aggregates can obstruct effective stretching of the individual chain segments and may even create intertwined “chain knots” serving as new arresting sites . However, under an enormously large stretching, molecular aggregates may unravel given a suitable combination of the intrachain rigidity and interchain bonding …”

“…To mitigate the effects of molecular aggregation, different practices have commonly been employed, e.g., blending, copolymerization, or adding branches or bulky side groups, in order to improve the QEs, − although in some cases molecular aggregates can enhance fluorescence by hindering chain motions (reminiscent of a molecular confinement effect). − From the perspective of polymer stretching, molecular aggregates can obstruct effective stretching of the individual chain segments and may even create intertwined “chain knots” serving as new arresting sites . However, under an enormously large stretching, molecular aggregates may unravel given a suitable combination of the intrachain rigidity and interchain bonding …”