Conjugation length polydispersity and its effect on charge transport in conjugated polymer based light-emitting diodes

Conjugated polymers have immense potential for optoelectronic and display applications such as light emitting diodes (LEDs). Molecular weight distribution is a parameter intrinsic to all polymers. Yet it's effect on the LED application is previously unexplored. To study this effect, we synthesized a pentamer and nonamer of dialkoxy (p-phenylenevinylene) as well as dialkoxy poly(p-phenylenevinylene (PPV). By blending these materials we were able to simulate the polydispersity in conjugation length inherent in all polymers and systematically probe it's importance to LED performance.; By mixing a low molecular weight dialkoxy-PPV or a nonamer of this PPV into a matrix of the pentamer, we were able to simulate the effect that a small concentration of low bandgap, highly conjugated segments, would have on PL and EL efficiency. The PL quantum efficiency was unaffected by the low bandgap fraction. EL was, however, dramatically impacted, being reduced by 50 to 70%. Based on this knowledge, we modified the synthesis of the dialkoxy PPV to yield a narrow polydispersity sample. When fabricated into a single layer PLED with an aluminum cathode, the EL efficiency was almost two orders of magnitude higher than that of the conventional higher polydispersity PPV.; A clear understanding of the photophysics involved in excited state migration in the previous experiments was deemed necessary. We observe that the photophysics of conjugated polymers used in PLEDs can be modeled as the sum of contributions from two species, isolated chain segments and aggregated chain segments in a mixed solvent system. This system also provides an important vehicle for probing the interaction between long and short conjugated segments in the blends of PPV and pentamer. The absorption and emission spectra of the solution mixtures under different concentration of good solvent (dioxane) and bad solvent (water) mimic a film like condition and subsequent quenching in luminescence efficiency and energy transfer to low bandgap polymer chains which are surrounded by the high bandgap pentamer.; In the solutions of pristine components, we observe that isolated segments exhibit solution like behavior, i.e. the photoluminescence decay is exponential, the spectrum reveals spectral dynamics and the photoluminescence (PL) quantum efficiency is high. The aggregated segments have non-exponential decay, a red-shifted spectrum with no spectral dynamics and poor PL quantum efficiencies. NMR confirms that phenyl ring motion is severely limited in the aggregated segments. The low PL quantum efficiency in the aggregates can be attributed to two phenomena. The first is efficient Förster energy transfer from isolated segments to aggregated segments. The second is that the aggregated segments form interchain polaron pairs, with a propensity for nonradiative decay. This finding has many important implications for PLEDs and the design of electroactive polymers. Key among these is that the electroluminescent efficiency (EL) of a polymer can be more than currently accepted limit of 25% of the PL efficiency.