Singlet exciton diffusion length in organic light-emitting diodes

We present a simple and accurate method to determine the singlet diffusion length in an operating organic light-emitting device (OLED). By using electrical rather than optical excitation, the method ensures that excitons are formed in a tightly confined generation zone, from which they can diffuse towards a quenching material. For a series of devices with varying distance between generation and quenching region, different emission intensities are found, and the experimentally obtained emission spectra of these devices can be used to determine the singlet diffusion length in the emissive layer of the device. By carefully choosing OLED layer materials and thicknesses, we can ensure well-defined quenching and blocking boundary conditions and exclude cavity effects as well as emission from the quenching material. An analytical model is developed to analyze the emission intensity found experimentally. We show that disregarding the fact that the generation zone has a nonzero width leads to an overestimation of the diffusion length. Furthermore, the current, i.e., the excitation density dependency of the singlet diffusion length, is investigated. At low current density (0.15 mA/cm${}^{2}$), a singlet diffusion length of $4.6\ifmmode\pm\else\textpm\fi{}0.5$ nm is obtained in N,N${}^{\ensuremath{'}}$-di-1-naphthalenyl-N,N${}^{\ensuremath{'}}$-diphenyl-[1,1${}^{\ensuremath{'}}$:4${}^{\ensuremath{'}}$,1${}^{\ensuremath{'}\ensuremath{'}}$:4${}^{\ensuremath{'}\ensuremath{'}}$,1${}^{\ensuremath{'}\ensuremath{'}\phantom{\rule{0.16em}{0ex}}\ensuremath{'}}$-quaterphenyl]-4,4${}^{\ensuremath{'}\ensuremath{'}\phantom{\rule{0.16em}{0ex}}\ensuremath{'}}$-diamine (4P-NPD). The singlet diffusion length decreases to $4.0\ifmmode\pm\else\textpm\fi{}0.5$ nm at 154.08 mA/cm${}^{2}$.