Mechanism of Delocalization-Enhanced Exciton Transport in Disordered Organic Semiconductors

Abstract

Large exciton diffusion lengths generally improve the performance of organic semiconductor devices, because they enable energy to be transported farther during the exciton lifetime. However, the physics of exciton motion in disordered organic materials is not fully understood, and modeling the transport of quantum-mechanically delocalized excitons in disordered organic semiconductors is a computational challenge. Here, we describe delocalized kinetic Monte Carlo (dKMC), the first model of three-dimensional exciton transport in organic semiconductors that includes delocalization, disorder, and polaron formation. We find that delocalization can dramatically increase exciton transport; for example, delocalization across less than two molecules in each direction can increase the exciton diffusion coefficient by over an order of magnitude. The mechanism for the enhancement is 2-fold: delocalization enables excitons to hop both more frequently and further in each hop. We also quantify the effect of transient delocalization (short-lived periods where excitons become highly delocalized) and show that it depends strongly upon the disorder and transition dipole moments.

Towards a fast machine-learning-assisted prediction of the mechanoelectric response in organic crystals

“…Eventually, it will be possible to adopt kMC flavours more in line with the physical mechanism underpinning charge transport in the intermediate regime. 103,104…”

“…Eventually, it will be possible to adopt kMC flavours more in line with the physical mechanism underpinning charge transport in the intermediate regime. 103,104 The protocol is both fast and reliable, although it requires parameterisation of a QMD-FF and training an ML model, steps that apparently prevent its transferability to new materials. We are currently working on the automatic parameterisation of QMD-FFs 108 and on the development of transferable and differentiable ML models, 95,107 which will allow overcoming these limitations.…”

Surface hopping modeling of charge and energy transfer in active environments

“…Thus, the quantum mechanical description may be restricted to single chromophores with classical interactions between them. 229 This simplification allows incorporating fragment approaches in surface hopping, as discussed in this section later. FRET simulations often use hopping algorithms based on Frenkel excitonic Hamiltonians to propagate exciton diffusion.…”

“…FRET simulations often use hopping algorithms based on Frenkel excitonic Hamiltonians to propagate exciton diffusion. 229 Those hopping algorithms should not be mistaken for surface hopping between adiabatic surfaces. The excitation jumps from one site to another, but the nuclear dynamics may even be restricted to a single adiabatic surface.…”

Coexistence of Incoherent and Ultrafast Coherent Exciton Transport in a Two-Dimensional Superatomic Semiconductor

“…These diffusivities are remarkably high for thermally activated exciton transport, which is typically limited to much less than 1 cm 2 /s. ,,, A plausible mechanism for the excellent transport properties of incoherent excitons in CsRe 6 Se 8 I 3 is transient exciton delocalization, whereby weakly localized excitons temporarily access delocalized states through thermal fluctuations. This process enables Frenkel-type excitons in intermediate-coupling systems to reach diffusivities exceeding 1 cm 2 /s. ,− Energetic and long-range structural order are both considered crucial prerequisites to enable such transient delocalization . The extended crystalline framework, low static disorder, and intermediate regime of excitons in CsRe 6 Se 8 I 3 make the latter an ideal candidate to observe and study this intriguing transport regime.…”