Organic light-emitting diodes (OLEDs) are a rapidly developing technology with high innovation potential for displays, lighting, and further applications. Critical issues for current research remain the extension of lifetimes and, related to this aim, a deeper understanding of degradation processes in OLEDs. In the present work, we used the charge-based deep level transient spectroscopy technique to investigate changes in the charge transport in OLEDs induced by electrical aging. Both trap states capturing electrons and holes, respectively, were detected. Temperature-dependent measurements enabled estimating the depth of the trap states for electrons. Comparison of spectra of electrically aged and undriven devices revealed that aging seemed to increase the density of traps but did not lead to the occurrence of new types of trap states. Furthermore, experiments with different voltage pulses to fill the charge carrier trap states allowed conclusions on age-induced changes of the injection barrier for holes.
Electrical aging can increase non-radiative recombination processes in organic light-emitting diodes (OLEDs) which leads to a reduction in device efficiency. We investigated aging-related changes of the opto-electronic properties of OLEDs by a combination of photoluminescence, electroluminescence (EL), and photoluminescence excitation spectroscopy as well as measurements of the incident photon-to-current efficiency (IPCE). The EL measurements revealed a pronounced decrease in radiative recombination after electrical aging, and it is demonstrated here that IPCE measurements can make a useful contribution to identify the materials and processes that are affected most by electrical aging.
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