Abstract
Charge carrier mobilities of organic semiconductors are often characterized using steady-state measurements of space charge limited diodes. These measurements assume that charge carriers are in a steady-state equilibrium. In reality, however, energetically hot carriers are introduces by photo-excitation and injection into highly energetic sites from the electrodes. These carriers perturb the equilibrium density of occupied states, and therefore change the overall charge transport properties. In this paper, we look into the effect of energetically hot carriers on the charge transport in organic semiconductors using steady state kinetic Monte Carlo simulations. For injected hot carriers in a typical organic semiconductor, rapid energetic relaxation occurs in the order of tens of nanoseconds, which is much faster than the typical transit time of a charge carrier throught the device. Furthermore, we investigate the impact of photo-generated carriers on the steady-state mobility. For a typical organic voltaic material, an increase in mobility of a factor of 1.1 is found. Therefore, we conclude that the impact of energetically hot carriers on normal device operation is limited.
Highlights
Charge carrier mobilities of organic semiconductors are often characterized using steady-state measurements of space charge limited diodes
In thermal equilibrium and for vanishing charge densities, charge carriers will on average reside at the equilibrium level finite charge of σ2 below the center conkbcTentrations and no of the density of states (DOS), where kb is the Bolzmann electric field, the particles will arrange constant and according to
These mobilities can be obtained by analysis of space charge limited currents (SCLC) in diodes consisting of an organic layer that is sandwiched between two electrodes[9]
Summary
One dimensional drift-diffusion simulations are a computationally cheap method for determining the charge transport characteristics in (organic) semiconducting devices. If hot-carrier relaxation was not negligible, the KMC simulation should give much higher values for the current density, because the drift-diffusion simulations do not include carrier dispersion This outcome was further supported by the fast relaxation of charge carriers near the injecting electrode: within a few nanometers, charges behave according to the Fermi-Dirac distribution again. Photovoltaic devices usually operate at steady-state conditions under continuous illumination, with the presence of background charges that are in thermal equilibrium This may change the relaxation dynamics of hot carriers, and lead to different charge transport characteristics. This explains why SCLC analyses is still a good method for analyzing OPV devices: the error in experimentally measuring the charge carrier mobility is much smaller than the enhancement due to hot carrier relaxation
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