Dark injection transient spectroscopy and density of states in amorphous organics

• We examine how the shape of DOS influences the dark-injection transient response.
• Gaussian DOS cannot produce the attenuated response frequently seen in polymer films.
• However, a mixed Gaussian-exponential DOS can.
• Attenuation of the response in Gaussian-exponential DOS is timescale free.
• The standard formula overestimates the carrier mobility at low fields in this case.

We simulate the process of a dark-injection transient spectroscopy (DITS) measurement on an amorphous organic thin film, by modeling the charge transport on a ‘microscopic’ level, with carriers hopping through a three-dimensional network of energetically disordered sites. Our aim is to see what restrictions have to be placed on the form of the energetic disorder to obtain the kind of a DITS response observed in many polymer films, which features significant current attenuation following a distinguishable transient current maximum. We find that the popular models of energetic disorder, with a Gaussian density of states (DOS), cannot account for the observed DITS response, no matter the strength of disorder. A modified DOS, which is sometimes suggested, possessing a Gaussian ‘body’ and an exponential ‘tail’, can explain the transient response. Attenuation of the current in systems with such DOS is of a power-law type, a quality that we connect with relaxation of carriers into deeper states in the exponential tail. We note that such a response, with timescale-free attenuation, should be interpreted with care when extracting the carrier mobility, as the standard procedure significantly underestimates the transit time at low applied voltages. Further, the efficiency of the injecting electrode in this case cannot be unambiguously evaluated from the response.

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