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Delocalisation explains efficient transport and charge generation in neat Y6 organic photovoltaics

Daniel Balzer, Paul A. Hume, Geoffrey R. Weal, Justin M. Hodgkiss, Ivan Kassal

TL;DR

This paper addresses how neat Y6 organic photovoltaics achieve efficient charge generation without interfacial energy offsets. It introduces delocalised kinetic Monte Carlo (dKMC), parameterised from atomistic calculations to capture delocalisation, disorder, and polaron formation in a mesoscopic lattice of Y6 molecules. The study shows that delocalisation significantly enhances electron, hole, and exciton transport and raises the internal quantum efficiency (IQE) of neat Y6 to levels closer to experimental observations, with 2D simulations predicting IQEs around 20% and diffusion coefficients aligning with measured exciton diffusion. Overall, dKMC provides a realistic, predictive tool for understanding and screening next-generation OPVs, demonstrating that efficient charge generation in neat Y6 can arise from delocalised states rather than interfacial energy gradients. These findings highlight the importance of considering higher-dimensional delocalised transport in modelling anisotropic organic semiconductors and pave the way for scalable, atomistically informed simulations of complex OPV materials and devices.

Abstract

Non-fullerene acceptors (NFA), such as Y6, have significantly improved the efficiency of organic photovoltaic devices (OPVs). However, the fundamental processes behind the high efficiencies of NFA devices have remained incompletely understood, with the high efficiencies persisting without the large energetic offsets often thought to be required for charge separation. Even more surprising has been the efficient charge generation in neat Y6 devices, where there is no energetic offset at all. Here, we simulate charge transport and separation in Y6 using delocalised kinetic Monte Carlo (dKMC) parameterised using atomistic calculations, thus taking into account the often-neglected ingredients of delocalisation, disorder, and polaron formation. Including delocalisation predicts higher carrier mobilities and exciton diffusion coefficients than is possible with classical simulations, bringing them into agreement with experimental values. Delocalisation also predicts higher charge-generation efficiencies in neat Y6, in agreement with experimental measurements. Finally, this work establishes dKMC as a realistic, predictive tool for understanding next-generation OPVs.

Delocalisation explains efficient transport and charge generation in neat Y6 organic photovoltaics

TL;DR

This paper addresses how neat Y6 organic photovoltaics achieve efficient charge generation without interfacial energy offsets. It introduces delocalised kinetic Monte Carlo (dKMC), parameterised from atomistic calculations to capture delocalisation, disorder, and polaron formation in a mesoscopic lattice of Y6 molecules. The study shows that delocalisation significantly enhances electron, hole, and exciton transport and raises the internal quantum efficiency (IQE) of neat Y6 to levels closer to experimental observations, with 2D simulations predicting IQEs around 20% and diffusion coefficients aligning with measured exciton diffusion. Overall, dKMC provides a realistic, predictive tool for understanding and screening next-generation OPVs, demonstrating that efficient charge generation in neat Y6 can arise from delocalised states rather than interfacial energy gradients. These findings highlight the importance of considering higher-dimensional delocalised transport in modelling anisotropic organic semiconductors and pave the way for scalable, atomistically informed simulations of complex OPV materials and devices.

Abstract

Non-fullerene acceptors (NFA), such as Y6, have significantly improved the efficiency of organic photovoltaic devices (OPVs). However, the fundamental processes behind the high efficiencies of NFA devices have remained incompletely understood, with the high efficiencies persisting without the large energetic offsets often thought to be required for charge separation. Even more surprising has been the efficient charge generation in neat Y6 devices, where there is no energetic offset at all. Here, we simulate charge transport and separation in Y6 using delocalised kinetic Monte Carlo (dKMC) parameterised using atomistic calculations, thus taking into account the often-neglected ingredients of delocalisation, disorder, and polaron formation. Including delocalisation predicts higher carrier mobilities and exciton diffusion coefficients than is possible with classical simulations, bringing them into agreement with experimental values. Delocalisation also predicts higher charge-generation efficiencies in neat Y6, in agreement with experimental measurements. Finally, this work establishes dKMC as a realistic, predictive tool for understanding next-generation OPVs.
Paper Structure (8 sections, 11 equations, 4 figures, 2 tables)

This paper contains 8 sections, 11 equations, 4 figures, 2 tables.

Table of Contents

  1. Methods
  2. Atomistic Calculations
  3. Parameterising dKMC
  4. dKMC dynamics
  5. Results
  6. Transport
  7. Charge generation
  8. Discussion

Figures (4)

  • Figure 1: Model of transport and separation processes in Y6.(a) Neighbouring pairs of molecules from the Y6 crystal structure are used to calculate energies of quasi-diabatic exciton and charge-transfer states, as well as couplings between them. Properties such as reorganisation energies are calculated from single-molecule geometry optimisations. (b) The atomistic calculations are used to parameterise the dKMC Hamiltonian, in which each molecule is represented as a site located at the centroid of the molecule. The lattice is made by replicating the unit cell, each containing four sites. (c) The dynamics proceeds through the eigenstates of the polaron-transformed Hamiltonian. For transport simulations, these represent the location of the delocalised carrier, while for charge-generation simulations they represent the simultaneous position of the delocalised electron and hole. (d) Transport and separation processes are modelled using dKMC, which tracks the movement of carriers through the polaron states.
  • Figure 2: Transport properties of Y6 calculated using classical KMC and dKMC. (a) Electron mobilities, (b) hole mobilities, and (c) exciton diffusion coefficients. Included are results for lattices with different dimensionality where the unit cell is replicated in the direction of one or more of the primitive lattice vectors a, b, and c. In all cases, when delocalisation is included using dKMC, the transport is significantly faster compared to classical KMC results.
  • Figure 3: Internal quantum efficiency (IQE) of charge generation in neat Y6, calculated using classical KMC and dKMC. Included are results for lattices with different dimensionality where the unit cell is replicated in the direction of one or more of the primitive lattice vectors a, b, and c. In all cases, when delocalisation is included using dKMC, the charge generation is significantly more efficient compared to that calculated using classical KMC.
  • Figure 4: Characteristics of delocalised polaron states. Relationship between the energy $E$, inverse participation ratio $\mathrm{IPR}$, and electron-hole separation $r_\mathrm{eh}$ of polaron states of Y6. The states are found by diagonalising a representative 2D $\tilde{H}_\mathrm{S}$ of Y6 with the unit cell replicated in the b and c directions. Some classes of polaron states are highlighted: exciton states are those with >75% of their population on exciton site-pairs; CT states are those that have a CT recombination rate of >10% of the maximum CT recombination rate; and hybridised states are those that have between 25% and 75% of their population on exciton site-pairs, with the remainder on site-pairs with greater electron-hole separation.