engleski

Signature of the electronic density of states in the dark injection transient record

Dark injection transient spectroscopy (DITS), a method primarily used for measuring charge mobility and electrode injection efficiency, recently became an object of interest and scrutiny, upon observations of pronounced memory effects in the transient record. These effects are an indication of charge trapping – capture of carriers in deep HOMO/LUMO states. Another signature of this phenomenon can be found in the significant attenuation of the transient current prior to reaching the steady-state value, which is observed in many polymer samples. What can the appearance of this attenuation, and the features of the transient response at later times, tell us about the energetic disorder of the HOMO/LUMO levels in organics? We examine how the dark-injection transient response depends on the energetic disorder in an organic material and its density of states (DOS). We do this by recreating the transient behaviour of a thin film device during a DITS measurement procedure, using a three-dimensional device simulation that observes the hopping carrier motion on the microscopic scale. Our results show that the, commonly used, Gaussian DOS models cannot produce the experimentally seen attenuation of the current, in contrast to the composite Gaussian-exponential DOS models. The DITS record therefore shows signature of long (exponential) tails in the electronic DOS. The attenuation of current in films with this type of DOS exhibits no characteristic time-scale, so the transient profile is affected at early times as well. The time and magnitude of the current maximum shift, influencing the way how the charge mobility and electrode injection efficiency should be evaluated. We note that this distinction between different DOS models visible in the DITS response, does not extend to transient response in a time-of-flight setup, making DITS an unexpectedly effective DOS test-suite.

DITS; trapping; disorder; Gaussian DOS; exponential DOS

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