Excitation transfer processes in a phosphor-doped poly(p-phenylene vinylene) light-emitting diode

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Abstract
We present experimental measurements and theoretical calculations of the electrical and optical properties of phosphor-doped poly (p-phenylene vinylene) light-emitting diodes to determine the excitation processes that lead to radiative recombination from the phosphor molecule. Three possible phosphor excitation processes are considered: (1) sequential electron and hole capture by the phosphor, (2) energy transfer from the polymer triplet exciton (Dexter transfer), and (3) energy transfer from the polymer singlet exciton (Förster transfer). The properties of the doped polymer are investigated for doping levels up to about 20 wt %. At the highest doping density, all radiative recombination occurs in the phosphor molecule and the observed electroluminescence decay time increases significantly compared to the undoped polymer. Built-in potential and current-voltage measurements indicate that the electron and hole energy levels of the phosphor are outside the energy gap of the polymer, and that the phosphor molecule does not capture either individual electrons or holes. Measurements of triplet optical absorption show that the triplet population in the polymer is not affected by the presence of the phosphor, indicating that Dexter transfer processes are weak. Calculations of the triplet optical-absorption cross section combined with the measurements of the triplet optical absorption determine the triplet exciton density in the device. In an analogous chemically substituted polymer, no significant excitation transfer occurs when there is no overlap between the emission spectrum of the polymer and the absorption spectrum of the phosphor. These results demonstrate that the dominant excitation transfer path from the polymer to the phosphor is dipole-dipole (Förster) coupling. Calculations of the charged and neutral electronic excitation energies of the polymer and phosphor are performed using hybrid and time-dependent, density-functional theory. The results of these calculations show why Förster transfer is strong in this system, and why the other two transfer processes do not take place.