• Energy Research

AbstractCharge transport in organic semiconductors is notoriously extremely sensitive to the presence of disorder, both internal and external (i.e., related to interactions with the dielectric layer), especially for n‐type materials. Internal dynamic disorder stems from large thermal fluctuations both in intermolecular transfer integrals and (molecular) site energies in weakly interacting van der Waals solids and sources transient localization of the charge carriers. The molecular vibrations that drive transient localization typically operate at low‐frequency (<a‐few‐hundred cm−1), which makes it difficult to assess them experimentally. Hitherto, this has prevented the identification of clear molecular design rules to control and reduce dynamic disorder. In addition, the disorder can also be external, being controlled by the gate insulator dielectric properties. Here a comprehensive study of charge transport in two closely related n‐type molecular organic semiconductors using a combination of temperature‐dependent inelastic neutron scattering and photoelectron spectroscopy corroborated by electrical measurements, theory, and simulations is reported. Unambiguous evidence that ad hoc molecular design enables the electron charge carriers to be freed from both internal and external disorder to ultimately reach band‐like electron transport is provided.

AbstractCharge transport in organic semiconductors is notoriously extremely sensitive to the presence of disorder, both internal and external (i.e., related to interactions with the dielectric layer), especially for n‐type materials. Internal dynamic disorder stems from large thermal fluctuations both in intermolecular transfer integrals and (molecular) site energies in weakly interacting van der Waals solids and sources transient localization of the charge carriers. The molecular vibrations that drive transient localization typically operate at low‐frequency (<a‐few‐hundred cm−1), which makes it difficult to assess them experimentally. Hitherto, this has prevented the identification of clear molecular design rules to control and reduce dynamic disorder. In addition, the disorder can also be external, being controlled by the gate insulator dielectric properties. Here a comprehensive study of charge transport in two closely related n‐type molecular organic semiconductors using a combination of temperature‐dependent inelastic neutron scattering and photoelectron spectroscopy corroborated by electrical measurements, theory, and simulations is reported. Unambiguous evidence that ad hoc molecular design enables the electron charge carriers to be freed from both internal and external disorder to ultimately reach band‐like electron transport is provided.

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

AbstractThe great progress in organic photovoltaics (OPV) over the past few years has been largely achieved by the development of non‐fullerene acceptors (NFAs), with power conversion efficiencies now approaching 20%. To further improve device performance, loss mechanisms must be identified and minimized. Triplet states are known to adversely affect device performance, since they can form energetically trapped excitons on low‐lying states that are responsible for non‐radiative losses or even device degradation. Halogenation of OPV materials has long been employed to tailor energy levels and to enhance open circuit voltage. Yet, the influence on recombination to triplet excitons has been largely unexplored. Using the complementary spin‐sensitive methods of photoluminescence detected magnetic resonance and transient electron paramagnetic resonance corroborated by transient absorption and quantum‐chemical calculations, exciton pathways in OPV blends are unravelled employing the polymer donors PBDB‐T, PM6, and PM7 together with NFAs Y6 and Y7. All blends reveal triplet excitons on the NFA populated via non‐geminate hole back transfer and, in blends with halogenated donors, also by spin‐orbit coupling driven intersystem crossing. Identifying these triplet formation pathways in all tested solar cell absorber films highlights the untapped potential for improved charge generation to further increase plateauing OPV efficiencies.