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Organic semiconducting materials are typically subject to energetic and positional disorder and localization of electronic states. The electronic behavior of these materials are therefore strongly influenced by thermalization
of charge carriers in the localized density of states (DOS). Consequently, non-equilibrium processes play an important role in the operation of devices made of such materials.
We show as an example that measurements of recombination dynamics, conducted under
transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay
of thermalization and recombination is missing. To enable adequate measurement analysis, we solve
the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady
excitation paradigm for different DOS distributions.
We show that recombination rates measured after pulsed excitation are inherently time-dependent, since
recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order
after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes
on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS
distributions, this asymptotic value equals the recombination order under continuous
excitation. For a more general DOS distribution, the recombination order can also depend on the carrier
density, under both transient and steady-state conditions.
However, we show that there are cases where thermal equilibrium is never attained in the device.
We conclude that transient experiments can provide rich information about recombination in and out of
equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.
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