Impact of Tortuosity on Charge-Carrier Transport in Organic Bulk Heterojunction Blends

Abstract

The impact of the tortuosity of the charge transport pathways through a bulk heterojunction film on the charge carrier mobility are theoretically investigated using model morphologies and kinetic Monte Carlo simulations. The tortuosity descriptor provides a quantitative metric to characterize the quality of the charge transport pathways, and model morphologies with controlled domain size and tortuosity are created using a new anisotropic domain growth procedure. The tortuosity is found to be dependent on the anisotropy of the domain structure and is highly tunable. Time-offlight charge transport simulations on morphologies with a range of tortuosity values reveal that tortuosity can significantly reduce the magnitude of the mobility and the electric field dependence relative to a neat material. These reductions are found to be further controlled by the energetic disorder and temperature. Most significantly, the sensitivity of the electric field dependence to the tortuosity can explain the different experimental relationships previously reported, and exploiting this sensitivity could lead to simpler methods for characterizing and optimizing charge transport in organic solar cells.

Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors

“…41 Alternatively, morphological dead ends may act as charge traps that suppress the current (increase) at higher fields. 42,43 Although not visible in Fig. 5, the PBDB-T:IEICO-4F system has a clear propensity to form various types of traps, especially for electrons, cf.…”

“…A factor that can hamper the accurate parameter extraction from SCLC curves are charge-carrier traps that may, for instance, be induced by nanoscopic water clusters and that show up as a "hump" in the j -V curve, which cannot, without further additions, be reproduced by a driftdiffusion model with either of the investigated mobility expressions [41]. Alternatively, morphological dead ends may act as charge traps that suppress the current (increase) at higher fields [42,43]. Although not visible in Fig.…”

Slip‐Stacked J‐Aggregate Materials for Organic Solar Cells and Photodetectors

“…In this packing structure, ADA1 ( ITIC ) exhibits rather isotropic electron transfer integrals ranging from 3.9 to 11.4 meV and forms an extended 2D percolation network, making it an especially promising candidate for BHJs with a high tortuosity, so a high ratio between the path length for a charge carrier through the BHJ and an ideal straight path to the respective electrode. [ 112,113 ] Going over from the molecular solution to the aggregated solid state, the absorption spectrum of ADA1 (ITIC) shows only a modest red‐shift of about 32 nm (669 cm −1 ) along with spectral broadening (Figure 8b). This is in line with the small J Coulomb derived from the simple point‐dipole moment approximation of the Kasha theory.…”

Device Physics in Organic Solar Cells and Drift-Diffusion Simulations

“…[81] At low charge carrier densities, blend mobility is found to decrease with increasing field. Further calculation from Heiber et al [82] using the KMC approach showed that the electric-field dependence of the charge-carrier mobility in a BHJ blend is profoundly affected by the tortuosity.…”