Comprehensive efficiency analysis of organic light-emitting diodes featuring emitter orientation and triplet-to-singlet up-conversion

Abstract

We present a method to achieve a consistent, comprehensive efficiency analysis of fluorescent organic light-emitting diodes (OLEDs) showing non-isotropic emitter orientation and triplet-to-singlet up-conversion. Combining photoluminescence lifetime and external quantum efficiency measurements on OLEDs with varying cavity length allows for an independent determination of the radiative emitter efficiency under optical as well as electrical excitation. The difference clearly shows a significant enhancement of the singlet exciton fraction to more than 25% under electrical operation. Furthermore, the presented method does not require detailed information about the emitting system and is generally applicable for a comprehensive efficiency analysis of bottom-emitting OLEDs

Analyzing degradation effects of organic light-emitting diodes via transient optical and electrical measurements

“…Thus, g r becomes unity for phosphorescent emitters [25][26][27] and can clearly exceed the spin-statistical limit of 25% (Refs. [28][29][30][31][32][33][34][35][36] in the fluorescent case. The third factor stands for the effective radiative quantum efficiency of the emitting system and depends on its intrinsic value q, which is modified by the Purcell effect taking into account the microcavity-like structure.…”

“…This factor is strongly influenced by the refractive indices and thicknesses of all used organic and inorganic layers and the orientation of the transition dipole moments of the emitting species. 35,[39][40][41][42][43] The first three factors can be combined to the so-called internal quantum efficiency g int .…”

Analytical estimation of emission zone mean position and width in organic light-emitting diodes from emission pattern image-source interference fringes

“…3,14 Indeed, in the last couple of years numerous authors have presented methods to recover EZ properties from a variety of optical measurements, such as electroluminescence (EL) spectra, emission patterns, external efficiencies, and photoluminescence lifetimes. 12,[15][16][17][18][19][20][21] These methods heavily utilize fitting procedures, which may yield highly resolved evaluation, however, usually require extensive data sets, and naturally rely on advanced numerical techniques, which tend to obscure the underlying physical phenomena. 19,22,23 In recent work, we have presented a different approach to this problem, developing analytical closed-form formulae to extract the emission zone location from measured emission pattern extrema, assuming the excitons are concentrated in a very narrow region.…”

“…12,[15][16][17][18][19][20][21][22] First, for some OLED engineering tasks, the complexity involved in employing the numerical methods is not very cost effective. For initial design stages and routine verification processes, for instance, it seems that a more intuitive, computationally efficient, approach, as the one presented in this paper, would be a better choice.…”

“…4,5 It is well established by now that the optical properties of OLEDs, in particular, the outcoupling efficiency, are strongly dependent on the spatial distribution of the radiating excitons in the active layer and their orientation preference (if exists). [6][7][8][9][10][11][12] Consequently, accessing these emission zone (EZ) properties by optical means has become highly desirable, as it offers an efficient way to evaluate these important parameters during the research and development process, or as quality control at the manufacturing line. Moreover, as the emission zone profile is determined by the electrical characteristics of the device (charge carrier mobilities, diffusion coefficients, injection barriers, etc.…”