Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: Numerical modeling of time-resolved photocurrent

Abstract

We present a model that describes nanosecond (ns) time-scale photocurrent dynamics in functionalized anthradithiophene (ADT) films and ADT-based donor-acceptor (D/A) composites. By fitting numerically simulated photocurrents to experimental data, we quantify contributions of multiple pathways of charge carrier photogeneration to the photocurrent, as well as extract parameters that characterize charge transport (CT) in organic films including charge carrier mobilities, trap densities, hole trap depth, and trapping and recombination rates. In pristine ADT films, simulations revealed two competing charge photogeneration pathways: fast, occurring on picosecond (ps) or sub-ps time scales with efficiencies below 10%, and slow, which proceeds at the time scale of tens of nanoseconds, with efficiencies of about 11%–12%, at the applied electric fields of 40–80 kV/cm. The relative contribution of these pathways to the photocurrent was electric field dependent, with the contribution of the fast process increasing with applied electric field. However, the total charge photogeneration efficiency was weakly electric field dependent exhibiting values of 14%–20% of the absorbed photons. The remaining 80%–86% of the photoexcitation did not contribute to charge carrier generation at these time scales. In ADT-based D/A composites with 2 wt.% acceptor concentration, an additional pathway of charge photogeneration that proceeds via CT exciton dissociation contributed to the total charge photogeneration. In the composite with the functionalized pentacene (Pn) acceptor, which exhibits strong exciplex emission from a tightly bound D/A CT exciton, the contribution of the CT state to charge generation was small, ∼8%–12% of the total number of photogenerated charge carriers, dependent on the electric field. In contrast, in the composite with PCBM acceptor, the CT state contributed about a half of all photogenerated charge carriers. In both D/A composites, the charge carrier mobilities were reduced and trap densities and average trap depths were increased, as compared to a pristine ADT donor film. A considerably slower recombination of free holes with trapped electrons was found in the composite with the PCBM acceptor, which led to slower decays of the transient photocurrent and considerably higher charge retention, as compared to a pristine ADT donor film and the composite with the functionalized Pn acceptor.

Numerical modeling of time-resolved photocurrent in organic semiconductor films

“…16 The same process has been used to fit data from an ADT-TES-F film spin cast on PFBT-treated substrates, the results of which we now present.…”

“…16 The first, denoted here as "Spatially Separated Carriers" (SSC) pathway, describes charge carriers created on ps or sub-ps time-scales, 23 most likely via hot exciton dissociation. The second pathway is a Frenkel exciton (FE) that can dissociate to free carriers with a rate k diss,F E .…”

“…16 We considered as our boundary conditions thermionic injection with image charge effects at the electrode-organic interface. 26,27 The injection barriers φ p B and φ n B for holes and electrons, respectively, were taken to be 0.25 eV and 2.05 eV.…”

“…16 Briefly, due to the large size (L = 25 µm) of our devices, we have assumed that the electric field and current densities can be averaged over the device. This reduces the common drift-diffusion model 11 to a drift-only current model.…”

“…Additionally, we obtained parameters pertaining to subsequent transport of photoexcited charge carriers such as charge carrier mobilities, charge trapping, and recombination properties. 16,20,21 …”