Abstract
Recently, disordered blends of semiconducting and insulating polymers have been used to prepare light-emitting diodes with increased luminous efficiency. Because the thermodynamic stability of the disordered phase in blends is limited, equivalent diblock copolymers (BCPs) could be an alternative. However, the choice between disordered blends and BCPs requires understanding structural differences and their effect on charge carrier transport. Using a hybrid mesoscopic model, we simulate blends and equivalent BCPs of two representative semiconducting and insulating polymers: poly(p-phenylene vinylene) (PPV) and polyacrylate. The immiscibility is varied to mimic annealing at different temperatures. We find stable or metastable disordered morphologies until we reach the mean-field (MF) spinodal. Disordered morphologies are heterogeneous because of thermal fluctuations and local segregation. Near the MF spinodal, segregation is stronger in BCPs than in the blends, even though the immiscibility, normalized by the MF spinodal, is the same. We link the spatial distribution of PPV with electric conductance. We predict that the immiscibility (temperature at which the layer is annealed) affects electrical percolation much stronger in BCPs than in blends. Differences in the local structure and percolation between blends and BCPs are enhanced at a high insulator content.
Highlights
Organic light-emitting diode technology is an active research area owing to its compatibility with lighting and display applications
This approach is motivated by the theoretical prediction of Mark and Helfrich (MH)[5] that the trap-limited current density scales as J ∼ N/Ntr, where N and Nt are the densities of transport and trapping sites, respectively
Locating the phase transition for each blend and BCP is outside the scope of our work. Such calculations would require advanced sampling techniques beyond the simple Monte Carlo (MC) pseudo-dynamics realized in this study, as well as sophisticated finite-size scaling methods specific to first-order phase transitions.[73−75] we need to estimate a region of χ-values where we can assume that the disordered phase is stable in our simulations
Summary
Organic light-emitting diode technology is an active research area owing to its compatibility with lighting and display applications. For polymeric active materials with their excellent film-forming properties, device fabrication based on cost-effective solution processing becomes feasible Such a polymer light-emitting diode (PLED) typically comprises[1] a thin ∼100 nm layer of a light-emitting semiconducting polymer situated between two electrodes, of which at least one is transparent. A PLED generates light through[2] radiative decay of excitons, formed in the semiconductor upon recombination of holes and electrons, injected at the anode and cathode. Balanced transport of these oppositely charged carriers is of paramount importance to reach optimal device performance. It has been shown that diluting the semiconductor with 90% of an insulator doubles the luminous efficiency of the PLED.[4,6]
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