Approximate formulae describing modulated photocurrents in a
sandwich configuration of sample electrodes are derived. The
carrier transport is described in terms of the multiple-trapping
model. The formulae refer to arbitrary spatial distribution of
generated carriers. The case of exponential light absorption in the
sample is considered in detail. It is shown that for limiting low
and high modulation frequencies, both the amplitude and the phase
shift of the photocurrent are almost independent of the light
absorption coefficient. In the intermediate-frequency range, for
the case of surface carrier generation, the phase shift of the
photocurrent usually shows oscillations. This makes it possible to
obtain the energetic density of states from a simple formula.
Examples of calculated frequency dependencies of the photocurrent
phase shift and amplitude are given for exponential and Gaussian
trap distributions as well as for a single trap level.
The electroluminescence quantum efficiency roll-off in iridium(III)-based complexes, namely Ir(iqbt) 2 (dpm) and Ir(iqbt) 3 (iqbt = 1 (benzo[b]thiophen-2-yl)-isoquinolinate, dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) utilized as near-infrared emitters in organic light emitting diodes with remarkable external quantum efficiencies, up to circa 3%, 1.5% and 1%, are measured and analyzed. With a 5-6 weight% of emitters embedded in a host matrix, the double-layer solution-processed structure as well as analogous three-layer one extended by a hole-conducting film are investigated. The triplet-polaron, the Onsager electron-hole pair dissociation and the triplet-triplet annihilation approaches were used to reproduce the experimental data. The mutual annihilation of triplets in iridium emitters was identified as prevailingly controlling the moderate roll-off, with the interaction between those of iridium emitters and host matrixes found as being less probable. Following the fitting procedure, the relevant rate constant was estimated to be (0.5 − 12) × 10 −12 cm 3 /s, values considered to be rather too high for disordered organic systems, which was assigned to the simplicity of the applied model. A coexistence of some other mechanisms is therefore inferred, ones, however, with a less significant contribution to the overall emission quenching.
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